Publication List

Web of Science Researcher ID: E 5701-2011
229 publications

A Divalent Pentastable Redox-Switchable Donor-Acceptor Rotaxane

H.V. Schröder, H. Hupatz, A.J. Achazi, S. Sobottka, B. Sarkar, B. Paulus, C.A. Schalley

Chem. Eur. J. 2017, in press

Full TextSupporting InformationCoordinates of Calculated Structures

Donor-acceptor materials with small HOMO-LUMO gaps are important in molecular electronics, but often difficult to synthesise. A simple and efficient way to position tetrathiafulvalene (TTF) as the donor and naphthalene diamide (NDI) as the acceptor in close proximity to each other in a divalent crown/ammonium pseudo[2]rotaxane is presented. The divalent design provides high chelate cooperativity and much stronger binding compared to a monovalent analogue. The pseudo[2]rotaxane is then doubly interlocked by stoppering it in a catalyst-free 1,3-dipolar cycloaddition. UV/Vis and cyclic voltammetry experiments with the resulting [2]rotaxane reveal the optoelectronic properties of the intramolecular charge-transfer with a small HOMO-LUMO energy gap. Redox-switching experiments show the rotaxane to be pentastable. DFT calculations provide insight into the electronic structures of the five redox-states.

PolyWhips: Directional particle transport by gradient-directed growth and stiffening of supramolecular assemblies

L. Cera, L. Chiappisi, C. Böttcher, A. Schulz, S. Schoder, M. Gradzielski, C.A. Schalley

Adv. Mater. 2016, in press

Full TextSupporting InformationVideo 1Video 2Video 3Video 4Video 5Video 6Video 7Video 8Video 9Highlighted on Advanced Science News

Growth of rigid rods occurs via supramolecular assembly of a non-conjugated pi-donor pi-acceptor monomer and is triggered by a NaCl gradient. The mechanical stiffness of this material is controlled by the local salt concentration. The continuous and well-controlled growth process is exploited to power the directional transport of submillimeter polymer particles.

Halogen-Bonded Supramolecular Capsules in the Solid State, in Solution, and in the Gas Phase

O. Dumele, B. Schreib, U. Warzok, N. Trapp, C.A. Schalley, F. Diederich

Angew. Chem. Int. Ed.2016, 56, 1152-1157; Angew. Chem. 2017, 128, 1172-1177

Full TextSupporting Information

Supramolecular capsules were assembled by neutral halogen bonding (XB) and studied in the solid state, in solution, and in the gas phase. The geometry of the highly organized capsule is evidenced by an X-ray crystal structure which features the assembly of two XB hemispheres, geometrically rigidified by H-bonding to eight MeOH molecules and encapsulation of two benzene guests. Tuning the XB donor is more efficient for enhancing capsular association strength than tuning the XB acceptor, due to desolvation penalties in protic solvents, as shown for a tetraquinuclidine XB acceptor hemisphere. With a tetra(iodoethynyl) XB donor and a tetralutidine XB acceptor, the association in deuterated benzene/acetone/methanol 70:30:1 at 283 K reaches Ka = (2.11±0.39)×105 M–1 (deltaG = –6.9±0.1 kcal mol–1). The stability of the XB capsules in the gas phase was confirmed by electrospray ionization mass spectrometry (ESI-MS). A new guest binding site was uncovered within the elongated iodoethynyl capsule.

The delicate balance of preorganisation and adaptability in multiply bonded host–guest complexes

L.K.S. von Krbek, A.J. Achazi, S. Schoder, M. Gaedke, T. Biberger, B. Paulus, C.A. Schalley

Chem. Eur. J. 2016, in press

Full TextSupporting InformationCoordinates of Calculated Structures

Rigidity and preorganisation are believed to be required for high affinity in multiply bonded supramolecular complexes as they help reducing the entropic penalty of the binding event. This comes at the price that such rigid complexes are sensitive to small geometric mismatches. In marked contrast, nature uses more flexible building blocks. Thus, one might consider putting the rigidity-high affinity notion to the test. Multivalent crown/ammonium complexes are ideal for this purpose as the monovalent interaction is well understood. A series of divalent complexes with different spacer lengths and rigidities has thus been analysed to correlate chelate cooperativities and spacer properties. Too long spacers reduce chelate cooperativity compared to exactly matching ones. However, in contrast to expectation, flexible guests bind with chelate cooperativities clearly exceeding those of rigid structures. Flexible spacers adapt to small geometric host/guest mismatches. Spacer-spacer interactions help overcoming the entropic penalty of conformational fixation during binding and a delicate balance of preorganisation and adaptability is at play in multivalent complexes.

A photoswitchable rotaxane operating in monolayers on solid support

F.B. Schwarz, T. Heinrich, A. Lippitz, W.E.S. Unger, C.A. Schalley

Chem. Commun. 2016, 52, 14458-14461

Full TextSupporting Information

A novel photoswitchable rotaxane was synthesised and its switching behaviour in solution was analysed with NMR and UV-VIS. A monolayer of rotaxanes was deposited on glass surfaces and the on-surface photoswitching was investigated. Angle-resolved NEXAFS spectra revealed a preferential orientation that reversibly changes upon switching.

Investigations on the growth of bismuth oxido clusters and the nucleation to give metastable bismuth oxide modifications

M. Weber, M- Schlesinger, M. Walther, D. Zahn, C.A. Schalley, M. Mehring

Z. Kristallogr. 2016, 232, 185-207

Full Text

Investigations on bismuth oxido clusters are focused on the nucleation and growth processes towards large cluster motifs and their stability in the gas phase, which has been studied by electrospray ionization mass spectrometry (ESI-MS), molecular dynamics (MD) simulations and X-ray scattering experiments evaluated by pair distribution function (PDF) analysis. The formation of metastable bismuth(III) oxides was obtained by hydrolysis of polynuclear bismuth oxido clusters and subsequent thermal treatment under non-equilibrium conditions. Temperature dependent PXRD and Raman spectroscopic experiments gave insight into the formation process of metastable β-Bi2O3 starting from the amorphous hydrolysis products as-obtained from polynuclear bismuth oxido clusters. Furthermore, PXRD as well as energy-dispersive X-ray (EDX) spectroscopy confirmed the formation of several new ternary bismuth(III) rich oxides such as Bi14O20(MO4) (M = S, Se) as-obtained by hydrolysis of bismuth oxido clusters in the presence of diverse additives.

[N•••I+•••N] Halogen-Bonded Dimeric Capsules from Tetrakis(3-pyridyl)ethylenecavitands

L. Turunen, U. Warzok, R. Puttreddy, N.K. Beyeh, C.A.Schalley, K. Rissanen

Angew. Chem. Int. Ed. 2016, 55, 14033-14036

Full TextSupporting Information

Two [N···I+···N] halogen-bonded dimeric capsules using tetrakis(3-pyridyl)ethylenecavitands with different lower rim alkyl chains are synthesized and analyzed in solution and the gas phase. These first examples of symmetrical dimeric capsules making use of the iodonium ion (I+) as the main connecting module are characterized by 1H NMR spectroscopy, diffusion ordered NMR spectroscopy (DOSY), electrospray ionization mass spectrometry (ESI-MS) and ion mobility-mass spectrometry (TW-IMS) experiments. The synthesis and effective halogen-bonded dimerization proceeds through analogous dimeric capsules with [N···Ag+···N] binding motifs as the intermediates as evidenced by the X-ray structures of (CH2Cl2)2@[3a2∙Ag4∙(H2O)2∙OTs4] and (CH2Cl2)2@[3a2∙Ag4∙(H2O)4∙OTs4], two structurally different capsules.

Allosteric and Chelate Cooperativity in Divalent Crown Ether–Ammonium Complexes with Strong Binding Enhancements

L.K.S. von Krbek, A.J. Achazi, M. Solleder, M. Weber, B. Paulus, C.A. Schalley

Chem. Eur. J. 2016, 22, 15475-15484

Full TextSupporting InformationSupporting Video

A thorough thermodynamic isothermal titration calorimetric analysis of allosteric and chelate cooperativity effects in divalent crown ether–ammonium complexes is combined with density functional theory calculations including implicit solvent on one hand and large-scale molecular dynamics simulations with explicit solvent molecules on the other. The complexes studied exhibit binding constants up to 2 • 106 M−1 with large multivalent binding enhancements and thus strong chelate cooperativity effects. Slight structural changes in the spacers, i.e. the exchange of two ether oxygen atoms by two isoelectronic methylene groups, cause significantly stronger binding and substantially increased chelate cooperativity. The analysis is rounded up by the examination of solvent effects and allosteric cooperativity. Such a detailed understanding of the binding processes will help efficiently designing and constructing larger supramolecular architecture with multiple, multivalent building blocks.

Photocontrolled On-Surface Pseudorotaxane Formation with Well-Ordered Macrocycle Multilayers

F.B. Schwarz, T. Heinrich, J.O. Kaufmann, A. Lippitz, R. Puttreddy, K. Rissanen, W.E.S. Unger, C.A. Schalley

Chem. Eur. J. 2016, 22, 14383-14389

Full TextSupporting Information

The photoinduced pseudorotaxane formation between a photoresponsive axle and a tetralactam macrocycle was investigated in solution and on glass surfaces with immobilized multilayers of macrocycles. In the course of this reaction, a novel photoswitchable binding station with azobenzene as the photoswitchable unit and diketopiperazine as the binding station was synthesized and studied by NMR and UV-Vis spectroscopy. Glass surfaces have been functionalized with pyridine-terminated SAMs and subsequently with multilayers of macrocycles through layer-by-layer self assembly. A preferred orientation of the macrocycles could be confirmed by NEXAFS spectroscopy. The photocontrolled deposition of the axle into the surface-bound macrocycle-multilayers was monitored by UV Vis spectroscopy and led to an increase of the molecular order, as indicated by more substantial linear dichroism effects in angle-resolved NEXAFS spectra.

Small beautiful and magnetically exotic: {V4W2}- and {V4W4}-type polyoxometallates

M. Rasmussen, C. Näther, J. van Leusen, U. Warzok, C.A. Schalley, P. Kögerler, W. Bensch

Dalton Trans. 2016, 45, 10519-10522

Full TextSupporting InformationCrystal Structure Data

Minimal-nuclearity vanadato-tungstate clusters in [{VIV(dien)}4WVI2O14]·4H2O (1) and [{VIV(dien)}4WVI4O20]·6H2O (2) feature cores of edge-sharing WO6 octahedra, surrounded by a ring of four vanadyl groups. Surprisingly, the V(IV) centers in both 1 and 2 are ferromagnetically coupled, in contrast to all other known vanadato-polyoxotungstates featuring the ubiquituos V–O- W-O–V exchange pathways.

Lower critical solution temperature (LCST) phase behaviour of an ionic liquid and its control by supramolecular host-guest interactions

S. Dong, J. Heyda, J. Yuan, C.A. Schalley

Chem. Commun. 2016, 52, 7970-7973

Full TextSupporting Information

Lower critical solution temperature (LCST) phase behaviour of an imidazolium-based ionic liquid is reported, which can be controlled by concentration, the choice of cation, anion and solvent, and by supramolecular host-guest complex formation. Molecular dynamics simulations provide insight into the molecular basis of this LCST phenomenon. This thermo-responsive system has potential applications in cloud point extraction processes.

Photooxygenation and gas-phase reactivity of multiply threaded pseudorotaxanes

K. Nowosinski, S. Warnke, K. Pagel, D. Komáromy, W. Jiang, C.A. Schalley

J. Mass Spectrom. 2016, 51, 269-281

Full TextSupporting Information

The solution-phase photooxygenation of multiply threaded crown/ammonium pseudorotaxanes containing anthracene spacers is monitored by electrospray ionization Fourier-transform ion-cyclotron-resonance (ESI-FTICR) mass spectrometry. The oxygenated pseudorotaxanes are mass-selected and fragmented by infrared multiphoton dissociation (IRMPD) and/or collision-induced dissociation (CID) experiments and compared to the non-oxygenated precursors. [4+2]Cycloreversion reactions lead to the loss of O2, when no other reaction channel with competitive energy-demand is available. Thus, the oxygen molecule can serve as a reference reaction for the energy demand of other fragmentation reactions such as the dissociation of the crown/ammonium binding motifs. The photooxygenation induces curvature into the initially planar anthracene and thus significantly changes the geometry of the divalent, anthracene-spacered wheel. This is reflected in ion-mobility data. Coulomb repulsion in multiply charged pseudorotaxanes assists the oxygen loss as the re-planarization of the anthracene increases the distance between the two charges.

Catenation and encapsulation induce distinct reconstitutions within a dynamic library of mixed-ligand Zn4L6 cages

S. Black, D.M. Wood, F.B. Schwarz, T. Ronson, J.J. Holstein, A.R. Stefankiewicz, C.A. Schalley, J.K.M. Sanders, J.R. Nitschke

Chem. Sci. 2016, 7, 2614-2620

Full TextSupporting InformationCrystal Structure Data

Two new Zn4L6 cages composed of diamine subcomponents containing either naphthalene diimide (NDI) or porphyrin moieties are described. Their structural differences allow these cages to exhibit distinct interactions with different chemical stimuli, yielding different supramolecular products. The electron-poor NDIs of the first cage were observed to thread through electron-rich aromatic crown-ether macrocycles, forming mechanically-interlocked species up to a [3]catenane, whereas the porphyrin ligands of the second cage interacted favourably with C70, causing it to be bound as a guest. When mixed, the two cages were observed to form a dynamic combinatorial library (DCL) of seven constitutionally distinct mixed-ligand Zn4L6 cages. The DCL was observed to reconstitute in opposing ways when treated with either the crown ether or C70: the electron-rich macrocycle templated the formation of heteroleptic catenanes, whereas C70 caused the DCL to self-sort into homoleptic structures.

Catalysis of “outer-phase” oxygen atom exchange reactions by encapsulated “inner-phase” water in {V15Sb6}-type polyoxovanadates

M. Wendt, U. Warzok, C. Näther, J. van Leusen, P. Kögerler, C.A. Schalley, W. Bensch

Chem. Sci. 2016, 7, 2684-2694

Full TextSupporting InformationCrystal Structure Data

Four new heteroatom-modified polyoxovanadate (POV) clusters {M(en)3}3[V15Sb6O42(H2O)x] (M = FeII, CoII, NiII) have been prepared under solvothermal conditions. Depending on the reaction conditions, two pseudopolymorphs of the nickel compound are obtained. In contrast to other POVs, these compounds are well soluble in water and thus permit post-functionalisation in solution. The clusters’ magnetic characteristics reflect the individual contributions of the frustrated {V15} spin polytope and the {M(en)3}2+ complexes, with very weak coupling between these groups. Electrospray ionisation mass spectrometry provides evidence (i) for a water molecule encapsulated inside the cavities of a fraction of the clusters, (ii) for a post-functionalisation in water, namely a slow exchange of VO against Sb2O, (iii) for the inner-phase reactivity of the encapsulated water that is capable of opening an oxo-bridge, and (iv) for a significant acceleration of the 16O/18O exchange reactions of oxygen atoms in the cluster periphery with surrounding H218O, when encapsulated water is present. To the best of our knowledge, this is the first example in polyoxovanadate chemistry for the transduction of inner-phase reactivity of an encapsulated guest molecule into changes in the outer-phase reactivity of the cluster.

Thermodynamic Analysis of Allosteric and Chelate Cooperativity in Di- and Trivalent Ammonium/Crown-Ether Pseudorotaxanes

K. Nowosinski, L.K.S. von Krbek, N.L. Traulsen, C.A. Schalley

Org. Lett. 2015, 17, 5076-5079

Full TextSupporting Information

A detailed thermodynamic analysis of the axle-wheel binding in di- and trivalent secondary ammonium/[24]crown-8 pseudorotaxanes is presented. Isothermal titration calorimetry (ITC) data and double mutant cycle analyses reveal an interesting interplay of positive as well as negative allosteric and positive chelate cooperativity thus providing profound insight into the effects governing multivalent binding in these pseudorotaxanes.

Chelate cooperativity effects on the formation of di- and trivalent pseudo[2]rotaxanes with diketopiperazine threads and tetralactam wheels

N.L. Traulsen, C.H.-H. Traulsen, P.M. Deutinger, S. Müller, D. Schmidt, I. Linder,C.A. Schalley

Org. Biomol. Chem. 2015, 13, 10881-10887

Full TextSupporting Information

The formation of singly, doubly and triply threaded pseudo[2]rotaxanes with diketopiperazine threads and tetralactam wheels is investigated with respect to chelate cooperativity effects on multivalent binding. Two series of guest molecules are prepared which differ with respect to their spacers, one with preorganised centrepieces with di- or tripodal roof-like structures, one with more flexible spacers. The thermodynamics of pseudorotaxane formation is examined using isothermal titration calorimetry and 1H NMR spectroscopy. Force-field calculations provide more detailed structural insight and help rationalizing the thermodynamic data. All di- and trivalent pseudorotaxanes exhibit positive chelate cooperativity presumably arising from spacer-spacer interactions. Higher cooperativity factors are observed for the more preorganised threads.

Enzyme-responsive polymer-substituted pillar[5]arene amphiphiles: synthesis, self-assembly in water, and application in controlled drug release

L. Gao, B. Zheng, W. Chen, C.A. Schalley

Chem. Commun. 2015, 51, 14901-14904

Full TextSupporting Information

An enzyme-responsive drug delivery system was constructed from a pillar[5]arene-based polyethyleneglycol-substituted amphiphile which self-assembles into micelles in water. These micelles exhibit superior drug encapsulation capability, and display drug release behaviour in response to enzyme catalysis, in partiular to L-asparaginase. Doxorubicin-loaded micelles show significant cytotoxicity against MCF-7 cancer cells.

Efficient Self-Assembly of Di-, Tri-, Tetra- and Hexavalent Hosts with Predefined Geometries for the Investigation of Multivalency


I. Linder, S. Leisering, R. Puttreddy, N. Rades, K. Rissanen, C.A. Schalley

Chem. Eur. J. 2015, 21, 13035-13044

Full Text ('hot paper")Supporting InformationInside CoverHighlight in Chemistry Views

Coordination-driven self-assembly of differently shaped di- to hexavalent crown ether host molecules is described. A series of [21]crown-7- and [24]crown-8-substituted bipyridine and terpyridine ligands was synthetized in a "toolbox" approach. Subsequent coordination to 3d transition metal and ruthenium(II) ions provides an easy and fast access to host assemblies with variable valency and predefined orientations of the crown ether moieties. Preliminary isothermal calorimetry (ITC) titrations provided promising results, which indicate the host complexes under study to be suitable for the future investigation of multivalent and cooperative binding. The hosts described here will also be suitable for the construction of various multiply threaded mechanically interlocked molecules.

Multi-Stimuli Responsive Materials


Z. Qi, C.A. Schalley

"Multi-Stimuli Responsive Materials" in H.-J. Schneider, "Chemoresponsive Materials: Stimulation by Chemical and Biological Signals", RSoC, London 2015, pp. 98-135

Full Text

Multi-stimuli responsive soft materials have shown great potential for the implementation of function such as selective drug delivery, logic gating, and directional transport. Starting with an overview of multi-stimuli responsive gels, different stimuli-responsive elements and strategies that are employed to implement multi-responsiveness are discussed. As two other examples for typical materials, multi stimuli-responsive thin films on surfaces and nanocarriers are also reviewed. The last part of the chapter discusses more sophisticated function of these materials and provides a perspective for potential future developments.

Supramolecular hydrophobic guest transport system based on pillar[5]arene

Z. Qi, K. Achazi, R. Haag, S. Dong, C.A. Schalley

Chem. Commun. 2015, 51, 10326-10329

Full TextSupporting Information

A pillar[5]arene-based bioactive guest loading system has been developed, which can increases the solubility of the drug norharmane in aqueous medium, and also enables its pH-stimulated release. Furthermore, this supramolecular transport system reduces the toxicity of loaded guest.

Discrete Multiporphyrin Pseudorotaxane Assemblies from Di- and Tetravalent Porphyrin Building Blocks

M. Lohse, L.K.S. von Krbek, S. Radunz, S. Moorthy, C.A. Schalley, S. Hecht

Beilstein J. Org. Chem. 2015, 11, 748-762

Full TextSupporting Information

Two pairs of divalent and tetravalent porphyrin building blocks carrying the complementary supramolecular crown ether / secondary ammonium ion binding motif have been synthesized and their derived pseudorotaxanes have been studied by a combination of NMR spectroscopy in solution and ESI mass spectrometry in the gas phase. By simple mixing of the components the formation of discrete dimeric and trimeric (metallo)porphyrin complexes predominates, in accordance to binding stoichiometry, while the amount of alternative structures can be neglected. Our results illustrate the power of multivalency to program the multicomponent self-assembly of specific entities into discrete functional nanostructures.

Gating the photochromism of an azobenzene by strong host-guest interactions in a divalent pseudo[2]rotaxane

M. Lohse, K. Nowosinski, N.L. Traulsen, A.J. Achazi, L.K.S. von Krbek, B. Paulus, C.A. Schalley, S. Hecht

Chem. Commun. 2015, 51, 9777-9780

Full TextSupporting Information

The ability of an E-configured azobenzene guest to undergo photoisomerisation is controlled by the presence of a complementary host. Addition of base/acid allowed for a weakening/strengthening of the interactions in the divalent pseudo[2]rotaxane complex and hence could switch on/off photochromic activity.

Formation and transmetalation mechanisms of homo- and heterometallic (Fe/Zn) trinuclear triple-stranded side-by-side helicates

B. Akhuli, L. Cera, B. Jana, S. Saha, C.A. Schalley, P. Ghosh

Inorg. Chem., 2015, 4231-4242

Full TextSupporting Information

A novel linear hybrid tris-bidentate neutral ligand bearing 2,2'-bipyridine and two terminal triazolylpyridine coordination sites (L) is efficiently synthesised and explored in the synthesis of trinuclear triple-stranded homometallic side-by.side helicates L3Fe3(OTf)6 (1) and L3Zn3(OTf)6 (2), in which the three metal centres display alternating  and Δ configurations. The selective formation of the analogous heterometallic side-by-side helicate L3Fe2Zn(OTf)6 (3) is achieved from a mixture of L, Fe(OTf)2 and Zn(OTf)2 (1:1:1) in acetonitrile. Various analytical techniques, i.e. single crystal X-ray diffraction as well as NMR and UV/vis spectroscopy, elucidate the sequence of the metal atoms within the heterometallic helicate, with the Zn2+ in the central position. Formation of 3 is also achieved either starting from L3Zn3(OTf)6 or L3Fe3(OTf)6 by adding Fe(OTf)2 or Zn(OTf)2 respectively. ESI-MS and 1H NMR studies elucidate different transmetalation mechanisms for the two cases: While a Zn2+-to-Fe2+ transmetalation occurs by the step-wise exchange of single ions on the helicate L3Zn3(OTf)6 at room temperature, this mechanism is almost inoperative for the Fe-to-Zn transmetalation in L3Fe3(OTf)6 which is kinetically trapped at room temperature. In contrast, a dissociation of L3Fe3(OTf)6 at higher temperature is required and followed by reassembly of L3Fe2Zn(OTf)6. The reassembly follows an interesting mechanistic pathway when an excess of Zn(OTf)2 is present in solution: First, L3Zn3(OTf)6 forms as the high-temperature thermodynamic product and is then slowly converted at room temperature into the thermodynamic heterometallic L3Fe2Zn(OTf)6 product. The temperature-dependent equilibrium shift is traced back to significant entropy differences resulting from an enhancement of the thermal motion of the ligands at high temperature which destabilise the octahedral iron terminal complex and selects zinc in a more stable tetrahedral geometry.

Theoretical and Experimental Investigation of Crown/Ammonium Complexes in Solution

A. Achazi, L.K.S. von Krbek, C.A. Schalley, B. Paulus

J. Comput. Chem., 2015, 37, 18-24

Full TextSupporting Information

The Gibbs energies of association ΔG(sol) between primary alkyl ammonium ions and crown ethers in solution are measured and calculated. Measurements are carried out by isothermal titration calorimetry (ITC) and revealed a strong solvent-dependent ion pair effect. Calculations are performed with density functional theory (DFT) including Grimme’s dispersion correction D3(BJ). The translational, rotational and vibrational contributions to the Gibbs energy of association ΔG(sol) are taken into account by a rigid-rotor-harmonic-oscillator approximation with a free-rotor approximation for low lying vibrational modes. Solvation effects are taken into account by applying the continuum solvation model COSMO-RS. Our study aims at finding a suitable theoretical method for the evaluation of the host guest interaction in crown/ammonium complexes as well as the observed ion pair effects. A good agreement of theory and experiment is only achieved, when solvation and the effects of the counterions are explicitly taken into account.

Coupled Molecular Switching Processes in Ordered Mono- and Multilayers of Stimuli-Responsive Rotaxanes on Gold Surfaces

T. Heinrich, C.H.-H. Traulsen, M. Holzweber, S. Richter, V. Kunz, S.K. Kastner, S.O. Krabbenborg, J. Huskens, W.E.S. Unger, C.A. Schalley

J. Am. Chem. Soc., 2015, 137, 4382-4390

Full TextSupporting Information

Interfaces provide the structural basis for function as, for example, encountered in nature in the membrane-embedded photosystem or in technology in solar cells. Synthetic functional multilayers of molecules cooperating in a coupled manner can be fabricated on surfaces through layer-by-layer self-assembly. Ordered arrays of stimuli-responsive rotaxanes undergoing well-controlled axle shuttling are excellent candidates for coupled mechanical motion. Such stimuli-responsive surfaces may help integrating synthetic molecular machines in larger systems exhibiting even macroscopic effects or generating mechanical work from chemical energy through cooperative action. The present work demonstrates the successful deposition of ordered mono- and multilayers of chemically switchable rotaxanes on gold surfaces. Rotaxane mono- and multilayers are shown to reversibly switch in a coupled manner between two ordered states as revealed by linear dichroism effects in angle-resolved NEXAFS spectra. Such a concerted switching process is observed only when the surfaces are well packed, while less densely packed surfaces lacking lateral order do not exhibit such effects.

Understanding multivalent interactions


C.A. Schalley, B. Koksch, R. Haag

Int. Innov., 2015, 169, 99-101

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An ongoing interdisciplinary research project on multivalency at the Freie Universität Berlin is breaking new ground in its design and synthesis of novel therapeutics for a range of viral diseases

Integrative self-sorting: a versatile strategy for the construction of complex supramolecular architecture

Z. He, W. Jiang, C. A. Schalley

Chem. Soc. Rev., 2015, 44, 779-789

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Large protein-sized synthetic supramolecular architecture is rare and certainly does not yet achieve the structural and functional complexity of biomolecules. As multiple, identical copies of few building blocks are repetitively used, highly symmetrical architecture results with limitations in function. In marked contrast, functional structures in nature are often assembled with high geometric precision from many different building blocks. They cooperate in a complex way realizing energy conversion, mechanical motion or transport phenomena. Beyond self-assembly, the structurally and functionally complex biomolecular machines rely on self-sorting to correctly position all subunits through orthogonal recognition sites. Mimicking such self-sorting processes is a promising strategy for supramolecular synthesis – resulting in higher structural complexity and promising access to more sophisticated function. The term “integrative self-sorting” was coined to describe a strategy to well-defined assemblies with well-controlled subunit positions. The key is the incorporation of two or more orthogonal binding motifs into at least some of the subunits. Modularity and programmability based on orthogonal yet similar binding motifs generate diversity and complexity. Integrative self-sorting is thus inherently related to systems chemistry. Depending on the individual binding motifs, (multi-)stimuli responsiveness can be achieved. When different recognition events en route to the final assembly occur on significantly different time scales, kinetic pathway selection is observed. In this account, we review the modularity, programmability, and emergent properties of integrative self-sorting, emphasizing its utility and perspective for complex supramolecular architecture.

Supramolecular Polymers as Surface Coatings: Rapid Fabrication of Healable Superhydrophobic and Slippery Surfaces

Q. Wei, C. Schlaich, S. Prévost, A. Schulz, C. Böttcher, M. Gradzielski, Z. Qi, R. Haag, C. A. Schalley,

Adv. Mater., 2014, 26, 7358-7364

Full TextSupporting Information

Supramolecular polymerization for non-wetting surface coatings. Self-assembly of low-molecular-weight gelators (LMWGs) with perfluorinated side chains can be utilized to rapidly construct superhydrophobic as well as liquid-infused slippery surfaces within minutes. The lubricated slippery surface exhibits impressive repellency for biological liquids such as human serum and blood and very fast self-healing.

Salicylate-Functionalized Bismuth Oxido Clusters: Hydrolysis Processes and Microbiological Activity

M. Schlesinger, A. Pathak, S. Richter, D. Sattler, A, Seifert, T. Rüffer, P. C. Andrews, C. A. Schalley, H. Lang, M. Mehring

Eur. J. Inorg. Chem., 2014, 4218-4227

Full TextSupporting Information

Hydrolysis of either Bi(HSal4Me)3 (A) or [Bi22O26(HSal4Me)14] (B) in DMSO gave the 4-methylsalicylate-substituted bismuth oxido cluster [Bi38O45(HSal4Me)24(DMSO)14(H2O)2]∙4H2O (1∙4H2O) which crystallizes in the triclinic space group P1¯ with cell parameters of a = 20.7214(5) Å, b = 20.9654(6) Å, c = 22.2128(6) Å, α = 100.867(2)°, β = 114.108(2)°, γ = 107.895(2)° and V = 7815.1(4) ų. Studies on the hydrolysis of A were performed using 1H DOSY NMR spectroscopy and electrospray ionisation Fourier-transform ion-cyclotron-resonance (ESI-FTICR) mass spectrometry which both show the formation of several larger species under ambient conditions in the presence of moisture. Furthermore, ESI- FTICR MS of cluster B shows the formation of mainly {Bi23O26} species which indicates partial dissociation of cluster B in solution. Additionally, the three different bismuth species containing one (A), 22 (B) or 38 (1∙4H2O) bismuth atoms per molecular formula were tested for their microbiological activity against three strains of Helicobacter pylori (251, 26695 and B128). A minimum inhibitory concentration (MIC) value of 6.25 µg/mL for the mononuclear bismuth complex A was obtained whereas the bismuth oxido cluster B shows a lower activity with MIC values between 25 μg/mL and 50.0 μg/mL. The activity of 1∙4H2O is comparable to commercial remedies based on antimicrobial bismuth subsalicylate (BSS; 12.5 ug/mol).

Gas-Phase Chemistry of Molecular Containers

Z. Qi, T. Heinrich, S. Moorthy, C. A. Schalley

Chem. Soc. Rev. 2015, 44, 515-531

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The remarkable technical advances in mass spectrometry during the last decades, including soft ionisation techniques, the coupling of electrospray ionisation to flow reactors, and the broad scope of tandem mass spectrometric experiments applicable to mass-selected ions allow investigating the chemistry of molecular capsules in solution as well as in the absence of any environment. With these methods, mass spectrometry is capable of answering many questions starting from providing analytical characterisation data (elemental composition, stoichiometry, etc.) to structural aspects (connectivities, positions of building blocks in supramolecular complexes) and to the examination of solution and gas-phase reactivity including reactions inside molecular containers. The present article reviews this work with a focus rather on the chemical questions that can be answered than on the technical specialities of (tandem) mass spectrometry.

Exploring Macrocycles in Functional Supramolecular Gels: From Stimuli-Responsiveness to Systems Chemistry

Z. Qi, C. A. Schalley

Acc. Chem. Res. 2014, 47, 2222-2233

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Supramolecular gels are ideal candidates for soft, stimuli-responsive materials, as they combine the elastic behavior of solids with the micro-viscous properties of fluids. The dynamic networks of fibers in supramolecular gels are reminiscent of the cytoskeleton of a cell and provide scaffolds to implement function. When gels are made responsive to stimuli, these mechanical properties can be controlled. Gel-sol transitions also open opportunities to immobilize molecules inside the gel’s cavities and to release them on demand. To establish selective responsiveness, suitable recognition sites are required influencing the properties of the fiber network depending on the presence of the stimulus. Supramolecular gels are expected to be stimuli-responsive per se, for example to temperature, mechanical stress, or an environment that is competitive with the non-covalent interactions connecting the low-molecular weight gelators. Nevertheless, the opportunities for controlling the mechanical properties are rather limited, if one merely relies on interfering with these interactions. It would be much more promising to equip the gel with additional receptor sites that offer selectivity for a broader variety of chemical stimuli. Macrocycles often exhibit a distinct host-guest chemistry and thus are excellent candidates for this purpose. A broad variety of macrocycles differing with respect to structure, topology, solubility, or biocompatibility has been incorporated in gels and endows gels with responsiveness and function. Macrocycles can have different roles: They offer rather rigid scaffolds for the construction of structurally well-defined gelator molecules. Furthermore, their host-guest interactions can be integral to gel formation, if these interactions are required to build the gel fibers. Finally, macrocycles can also be functional groups with which gelators are equipped that would also form gels in the absence of the macrocycle. Here, the macrocycle can be used as a binding site to allow additional stimuli-control. To combine different stimuli for triggering gel-sol transitions certainly expands the options for establishing stimuli-responsiveness. If, for example, an agent trapped inside the gel is only liberated, when two different stimuli are present simultaneously, its release can be controlled with much higher precision and selectivity as compared to a gel that responds to one stimulus only. Here, the recent progress in the construction of functional macrocycle-containing supramolecular gels is summarized. First, recent strategies to engineer responsiveness into macrocycle-containing gels are discussed. In the next chapter, different functions are presented including applications as responsive reaction media, for controlled drug-delivery or tissue engineering, and as self-healing materials. Finally, we highlight the recent progress in designing macrocycle-containing supramolecular gel materials exhibiting complex behavior. This field is part of systems chemistry and still in its infancy, but appears to be one of the most promising routes to smart responsive materials.

Principal Component Analysis assisted Time of Flight Secondary Ion Mass Spectrometry: A Versatile Method for the Investigation of Self-Assembled Mono- and Multilayers as Precursors for the Bottom-up Approach of Nanoscaled Devices

M. Holzweber, T. Heinrich, V. Kunz, S. Richter, C. H.-H. Traulsen, C. A. Schalley, W. E. S. Unger

Anal Chem. 2014, 86, 5740-5748

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The production of high-quality self-assembled monolayers (SAMs) followed by Llayer-by-Llayer (LbL) self-assembly of macrocycles is essential for nanotechnology applications based on functional surface films. To help interpret of the large amount of data generated by a standard ToF-SIMS measurement, principal component analysis was used. For two examples, the advantages of a combination of ToF-SIMS and PCA for quality control and for the optimization of layer-by-layer self-assembly are shown. The first example investigates how different cleaning methods influence the quality of SAM template formation. The second example focuses on the layer-by-layer self-assembly of macrocycles and the corresponding stepwise surface modification.

Self-sorting of crown ether/secondary ammonium ion hetero-[c2]daisy chain pseudorotaxanes

B. Zheng, F. Klautzsch, M. Xue, F. Huang, C. A. Schalley

Org. Chem. Frontiers 2014, 1, 532-540

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Four monomeric building blocks equipped with one crown ether and one secondary ammonium ion are synthesized and studied with respect to their ability to form daisy chain dimers. Two crown ethers with different cavity sizes – i.e. [21]crown-7 and [24]crown-8 – and two ammonium ions substituted with either a thin alkyl group or a more bulky benzyl group are used as the binding motifs. Self-sorting behaviour can be expected as the [21]crown-7/alkyl ammonium and [24]crown-8/benzyl ammonium binding motifs are orthogonal. Three homodimers are characterized by NMR, X-ray crystallo-graphy and ESI mass spectrometry. They are recognizable by the presence of signals for diastereotopic protons in the 1H NMR spectra. The formation of hetero-[c2]daisy chain dimers can be monitored by NMR spectroscopy and ESI mass spectrometry and show the expected self-sorting behaviour.

Evaluation of Multivalency as an Organization Principle for the Efficient Synthesis of Doubly and Triply Threaded Amide Rotaxanes

L. Kaufmann, N. L. Traulsen, A. Springer, H. V. Schröder, T. Mäkelä, K. Rissanen, C. A. Schalley

Org. Chem. Frontiers 2014, 1, 521-531

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Mono-, di- and trivalent pseudorotaxanes with tetralactam macrocycle hosts and axles containing diamide binding stations as the guests have been synthesised. Their threading behaviour was analyzed in detail by NMR experiments and isothermal titration calorimetry. An X-ray crystal structure of the monovalent pseudorotaxane confirms the binding motif. Double mutant cycle analysis provides the effective molarities and insight into the chelate cooperativity of multivalent binding. While the second binding event in a trivalent pseudorotaxane exhibits a slightly positive cooperativity, the third binding is nearly non-cooperative. Nevertheless, the enhanced binding affinities resulting from the multivalent interaction are the basis for a highly efficient synthesis of di- and trivalent rotaxanes through stoppering the axle termini by “click” chemistry. Evidence for the multiply threaded geometry comes from NMR spectroscopy as well as tandem mass-spectrometric fragmentation experiments of mass-selected rotaxane ions in the gas phase. Furthermore, the trivalent rotaxane can be controlled by external stimuli (chloride addition and removal) which lead to an elevator-type movement of the wheel along the axle.

Stimuli-Induced Folding Cascade of a Linear Oligomeric Guest Chain Programmed through Cucurbit[n]uril Self-Sorting (n = 6, 7, 8)

L. Cera, C. A. Schalley

Chem. Sci. 2014, 5, 2560-2567

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A six-station linear guest for cucurbit[7]uril and cucurbit[8]uril has been synthesized in order to implement a cascade of transformations driven by external stimuli. The guest chain is sequence-programmed with electron-deficient viologen and electron-rich naphthalene stations linked by either flexible or rigid spacers that affect the chain’s folding properties. Together with the orthogonal guest selectivity of the two cucurbiturils, these properties result in self-sorted cucurbituril pseudorotaxane foldamers. Each transformation is controlled by suitable chemical and redox inputs and leads not only to refolding of the guest chain, but also to the liberation of secondary messenger molecules which render the system presented here reminiscent of natural signaling cascades. The steps of the cascade are analyzed by UV/Vis, 1H NMR and electrospray (tandem) mass spectrometry to investigate the different pseudorotaxane structures in detail. With one guest oligomer, three different cucurbiturils, and several different chemical and redox inputs, a chemical system is created which exhibits complex behavior beyond the chemist’s paradigm of the pure chemical compound.

The versatility of “Click” reactions: Molecular recognition at interfaces

T. Heinrich, C. H.-H. Traulsen, E. Darlatt, S. Richter, J. Poppenberg, N. L. Traulsen, I. Linder, A. Lippitz, P. M. Dietrich, B. Dib, W. E. S. Unger, C. A. Schalley

RSC Adv. 2014, 4, 17694 - 17702

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In order to investigate molecular recognition on surfaces, an azide-functionalized monolayer was deposited on gold. The monolayer was characterized by x-ray photoelectron spectroscopy (XPS) and angle-resolved near-edge X-ray absorption fine structure (NEXAFS) experiments and the decomposition of the azide upon irradiation with X-ray beams was investigated. Subsequently, various alkyne-functionalized host and guest molecules were attached to the azide by 1,3-dipolar cycloaddition. These modified surfaces and their host/guest chemistry was analysed as well by XPS and angle-resolved NEXAFS. The reversibility of guest binding was shown for one example as a proof of principle.

Supramolecular reactivity in the gas phase: Investigating the intrinsic properties of non-covalent complexes

L. Cera, C. A. Schalley

Chem. Soc. Rev. 2014, 43, 1800-1812

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The high vacuum inside a mass spectrometer offers unique conditions to broaden our view on the reactivity of supramolecules. Because dynamic exchange processes between complexes are efficiently suppressed, the intrinsic and intramolecular reactivity of the complexes of interest is observed. Besides this, the significantly higher strength of non-covalent interactions in the absence of competing solvent allows processes to occur that are unable to compete in solution. The present review highlights a series of examples illustrating different aspects of supramolec-ular gas-phase reactivity ranging from the dissociation and formation of covalent bonds in non-covalent complexes through the reactivity in the restricted inner phase of container molecules and step-by-step mechanistic studies of organocatalytic reaction cycles to cage contraction reactions, processes induced by electron capture, and finally dynamic molecular motion within non-covalent complexes as unravelled by hydrogen-deuterium exchange processes performed in the gas phase.

Self-recovering stimuli-responsive macrocycle-equipped supramolecular ionogels with unusual mechanical properties

Z. Qi, N. L. Traulsen , P. Malo de Molina , C. Schlaich, M. Gradzielski, C. A. Schalley

Org. Biomol. Chem. 2014, 12, 503-510

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A chiral, crown-ether-functionalized bisurea gelator forms supramolecular gels in ionic liquids. The resulting ionogels show a remarkably high thermal stability with gel-sol transition temperatures (Tgs) reaching more than 100 °C. The mechanical strength of these ionogels is surprisingly high and even comparable to that of cross-linked protein fibres. Furthermore, the ionogels exhibit rapid self-recovery properties after structural damage caused by deformation. Pseudorotaxanes form from the gelators’ benzo[21]crown-7 ethers as the wheels and secondary ammonium ions as the axles despite of the competition between that cation and the imidazolium ions of the ionic liquid for crown ether binding. Pseudorotaxane formation as an external chemical stimulus triggers the gel-sol transition of the ionogels.

Synthesis and Coordinative Layer-by-Layer Deposition of Pyridine-Functionalized Gold Nanoparticles and Tetralactam Macrocycles on Silicon Substrates

C. H.-H. Traulsen , V. Kunz , T. Heinrich , S. Richter , M. Holzweber , A. Schulz , L. K. S. von Krbek, J. Poppenberg, U. T. J. Scheuschner, W. E. S. Unger, C. A. Schalley

Langmuir 2013, 29, 14284-14292

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Coordination chemistry was applied to deposit pyridine-functionalized gold nanoparticles on silicon substrates. The particles were synthesized through the Brust/Schiffrin route with a subsequent ligand exchange reaction yielding well-defined particles of two different sizes. Multilayer deposition was carried out on a pyridine-terminated SAM, anchored on a hydroxyl-terminated silicon surface. Analogously, Hunter/Vögtle-type tetralactam macrocycle multilayers were deposited as well as mixed layers containing both either in an alternating sequence or as a macrocycle multilayer with a terminating nanoparticle layer. These composite layers were examined with respect to their ability to bind squaraine axles in the macrocycle cavities. The amount of guest bound is higher for the composite layer with alternating macrocycles and nanoparticles.

The synergetic interplay of weak interactions in the ion pair recognition of quaternary and diquaternary ammonium salts by halogenated resorcinarenes

N. K. Beyeh, M. Göth, L. Kaufmann, C.A. Schalley, K. Rissanen

Eur. J. Org. Chem. 2014, 80-85

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The influence of halogens on the non-covalent interactions of different upper-rim-substituted hexyl resorcinarenes with quaternary and diquaternary ammonium iodide salts were investigated in the gas phase by electrospray ionization Fourier-transform ion-cyclotron-resonance (ESI-FTICR) mass spectrometry and in solution by 1H NMR titrations studies. The electronic nature of the substituents on the upper rim of the resorcinarene is directly reflected in the order of binding strengths of the hosts towards the quaternary and diquaternary ammonium cations in the gas phase. In solution, the opposite trend of increasing binding constants with increasing electronegativity of the halogens was observed with generally higher binding constants for the diquaternary over the quaternary salts. This phenomenon is explained by the synergetic effect originating from the interaction of the halogenated resorcinarenes with the counter anions through enhanced hydrogen bonding leading to ion pair binding in solution. The electronegativity of the halogens renders the hydrogen of the hydroxyl group more acidic and hence enhances hydrogen bonding in solution. The collective effect of several weak interactions is manifested in solution thereby emphasizing the benefits of comparing solution and gas phase studies in recognition processes.

Multivalency in the Gas Phase: H/D-Exchange Reactions Unravel the Dynamic 'Rock 'n Roll' Motion in Dendrimer-Dendrimer Complexes

Z. Qi, C. Schlaich, C. A. Schalley

Chem. Eur. J. 2013, 19, 14867-14875

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Non-covalent dendrimer-dendrimer complexes were successfully ionized by electrospray ionization (ESI) of partly protonated amino-terminated polypropylene amin (POPAM) and POPAM dendrimers fully functionalized with benzo[21]crown-7 on all branches. Hydrogen/deuterium exchange (HDX) experiments conducted on dendrimer-dendrimer complexes in the high vacuum of a mass spectrometer give rise to a complete exchange of all labile N-H hydrogen atoms. As crown ethers represent non-covalent protective groups against HDX reactions on the ammonium group they coordinate to, this result provides evidence for a very dynamic binding situation: Each crown is mobile enough to move from one ammonium binding site to another one. Schematically, one might compare this motion with two rock ‘n roll dancers that swirl around each other without completely losing all contact at any time. Although the multivalent attachment certainly increases the overall affinity, the “microdynamics” of individual site binding and dissociation remains fast.

Sequence-Programmable Multi-Component Multilayers of Nanometer-Sized Tetralactam Macrocycles on Gold Surfaces

S. Richter, C. H.-H. Traulsen, T. Heinrich, J. Poppenberg, C. Leppich, M. Holzweber, W. E. S. Unger, C. A. Schalley

J. Phys. Chem. C 2013, 117, 18980–18985

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Multi component multilayers have been deposited on gold surfaces by metal ion-mediated layer by layer self assembly of differently functionalized tetralactam macrocycles. The layer stack can be programmed with respect to the sequences of metal ions and macrocycles by the deposition sequence.

Fibrous Networks with Incorporated Macrocycles: A Chiral Stimuli-Responsive Supramolecular Supergelator and its Application to Biocatalysis in Organic Media

Pickering Emulsions

Z. Qi, C. Wu, P. Malo de Molina, H. Sun, A. Schulz, C. Griesinger, M. Gradzielski, R. Haag, M. B. Ansorge-Schumacher, C. A. Schalley

Chem. Eur. J. 2013, 19, 10150-10159

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A new and versatile, crown ether appended, chiral supergelator has been designed and synthesized based on the bis-urea motif. The introduction of a stereogenic center improved its gelation ability significantly relative to its achiral analogue. This low-molecular-weight gelator forms supramolecular gels in a variety of organic solvents. It is sensitive to multiple chemical stimuli and the sol–gel phase transitions can be reversibly triggered by host–guest interactions. The gel can be used to trap enzymes and release them on demand by chemical stimuli. It stabilizes the microparticles in Pickering emulsions so that enzyme-catalyzed organic reactions can take place in the polar phase inside the microparticles, the organic reactants diffusing through the biphasic interface from the surrounding organic phase. Because of the higher interface area between the organic and polar phases, enzyme activity is enhanced in comparison with simple biphasic systems.

Mass Spectrometry and Gas-Phase Chemistry of Bismuth Oxido Clusters

D. Sattler, M. Schlesinger, M. Mehring, C. A. Schalley

ChemPlusChem 2013, 78, 1005-1014

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The mass spectrometric characterisation of bismuth–oxido clusters is described together with tandem mass spectrometric experiments aimed at investigating their gas-phase chemistry. Clusters with different ligand shells (nitrate, salicylates, and methacrylate) and different sizes ({Bi6O8}, {Bi22O26}, and {Bi38O45}) were selected for this study. The following aspects were addressed by (tandem) electrospray ionisation Fourier-transform ion-cyclotron-resonance (ESI-FTICR) mass spectrometry: 1) Electrospray ionisation was used successfully for the generation of intact cluster ions, but led to rather complicated distributions of charge states and solvent adducts. The cluster core remained intact. 2) Mass spectrometry was used to follow the cluster growth from {Bi6O8} to {Bi22O27} and finally {Bi38O45} clusters. These experiments revealed that careful tuning of the conditions is required to avoid precipitates forming that hamper ESI ionisation. The stages of growth strongly depend on the experimental conditions. 3) Fragmentation experiments in the gas phase performed with the {Bi22O27} clusters obtained in the growth experiments resulted in the preferential formation of {Bi6O8} fragments and point to the particular stability of this cluster and its role in cluster growth. 4) Solution-phase ligand-exchange reactions on {Bi22O26} salicylate clusters were qualitatively monitored by mass spectrometry, but were limited by the complicated ion series formed that led to overlapping isotope patterns. 5) A number of fragmentation reactions occurred within the ligand shell when the clusters surrounded by an organic ligand shell were subjected to infrared multiphoton dissociation experiments. 6) For the nitrate clusters, fragmentation within the core was observed yielding {Bi6O8} fragments together with the corresponding clusters of reduced size.

Pseudorotaxanes with Self-Sorted Sequence and Stereochemistry

C. Talotta, C. Gaeta, Z. Qi, C. A. Schalley, P. Neri

Angew. Chem. Int. Ed. 2013, 52, 7437-7441; Angew. Chem. 2013, 125, 7585-7589

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Partner preferences in pseudorotaxane formation were exploited to establish an integrative self-sorting system able to discriminate simultaneously at the sequence and stereochemical level (see picture). It was found that calix[6]arenes were threaded selectively with a preferred orientation onto bisammonium axles, even when the structural differences between the possible building blocks were small and located remote from the binding sites.

Programmable Multilayers of Nanometer-Sized Macrocycles on Solid Support and Stimuli-Controlled On-Surface Pseudorotaxane Formation

J. Poppenberg, S. Richter, C. H.-H. Traulsen, E. Darlatt, B. Baytekin, T. Heinrich, P. M. Deutinger, K. Huth, W. E. S. Unger, C. A. Schalley

Chem. Sci. 2013, 4, 3131-3139

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Mechanically interlocked molecules (MIMs) such as rotaxanes and catenanes are capable of mechanical motion on the nanoscale and are therefore promising prototypes for molecular machines in recent nanotechnology. However, most of the existing examples are isotropically distributed in solution, which prohibits concerted movement and with it the generation of macroscopic effects. Thus, arranging them in ordered arrays is of huge interest in recent research. We report the deposition of quite densely packed multilayers of tetralactam macrocycles on gold surfaces by metal-coordinated layer-by-layer self-assembly. Linear dichroism effects in angle-resolved NEXAFS spectra indicate a preferential orientation of the macrocycles. The sequence of the metal ions can be programmed by the use of different transition metal ions at each deposition step. Additionally, reversible on-surface pseudorotaxane formation was successfully realized by repeated uptake and release of axle molecules inside the macrocycles cavities.

Generation of a Dynamic System of Three-Dimensional Tetrahedral Polycatenanes

S. P. Black, A. R. Stefankiewicz, M. M. J. Smulders, D. Sattler, C. A. Schalley, J. R. Nitschke, J. K. M. Sanders

Angew. Chem. Int. Ed. 2013, 52, 5749-5752; Angew. Chem. 2013, 125, 5861-5864

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Seven of the best: A dynamic combinatorial library of polycatenated tetrahedra was prepared by complexation between a dynamic Fe4L6 tetrahedral cage, constructed from ligands containing an electron-deficient naphthalenediimide core, and an electron-rich aromatic crown ether, 1,5-dinaphtho[38]crown-10. The highest order species in the library is the tetrahedral [7]catenane.

Synthesis and Characterization of Polynuclear Oxidobismuth Sulfonates

L. Miersch, T. Rüffer, D. Schaarschmidt, H. Lang, R. W. Troff, C. A. Schalley, M. Mehring

Eur. J. Inorg. Chem. 2013, 1427-1433

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The hydrolysis of [Bi6O4(OH)4(NO3)6]·H2O (1) in the presence of sodium alkylsulfonates gave the oxidobismuth sulfonates [{Bi38O45(C4H7SO3)8(NO3)14(dmso)19.5(H2O)2}][{Bi38O45(C4H7SO3)10(NO3)16(dmso)16(H2O)2}]·3H2O·3dmso (2, dmso = dimethyl sulfoxide), [Bi38O45(C6H13SO3)14(NO3)10(dmso)28] (3), and [Bi38O45(C8H17SO3)16(NO3)8(dmso)12] (4) upon crystallization from dmso. Compound 2 is the first example of a nanoscaled oxidobismuth cluster that is composed of both an anionic (charge 2–) and a cationic (charge 2+) fragment in the solid state. The clusters were characterized by elemental analysis, ESI mass spectrometry, IR and NMR spectroscopy, as well as thermal analysis. Crystals obtained for 2 were suitable for single-crystal structure analysis.

Interpretation of experimental N K NEXAFS of azide, 1,2,3-triazole and terpyridyl groups by DFT spectrum simulations

E. Darlatt, A. Nefedov, C. H.-H. Traulsen, J. Poppenberg, S. Richter, P. M. Dietrich, A. Lippitz, R. Illgen, J. Kühn, C. A. Schalley, C. Wöll, W. E. S. Unger

J. Electr. Spectr. Rel. Phen. 2012, 185, 621-624

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Experimental N K-edge NEXAFS data of surface immobilized azide, 1,2,3-triazole and terpyridyl groups are interpreted with the help of DFT spectrum simulations. Assignments of π* resonances in experimental N K-edge NEXAFS spectra to nitrogen atoms within these functional groups have been made. The azide was immobilized on gold as the head group of a thiol SAM, 1,2,3-triazole was formed on this SAM by click reaction and terpyridyl groups were introduced as substituents of the acetylene used for the click reaction. For azide-terminated molecules, DFT spectrum simulations are found to be useful to find measurement conditions delivering experimental N K-edge NEXAFS data with negligible X-ray damage. The 1,2,3-triazole group is found to be rather stable under X-ray irradiation.

Exploring the palladium and platinum bis(pyridyl) complex motif – do substituents affect NMR spectroscopic, X-ray crystallographic, (tandem) mass spectrometric, and isothermal titration calorimetry data following chemical intuition?

T. Weilandt, N. L. Löw, G. Schnakenburg, J. Daniels, M. Nieger, C. A. Schalley, A. Lützen

Chem. Eur. J. 2012, 18, 16665-16676

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A series of ten palladium-bis(pyridine) complexes, as well as their corresponding platinum complexes, have been synthesized. The pyridine ligands in each series carried different σ-donor and/or π-acceptor/donor substituents at the para-position of their pyridine rings. These complexes were analysed by NMR spectroscopy, X-ray crystallography, (tandem) MS, and isothermal titration calorimetry (ITC) to validate whether these methods allowed us to obtain a concise and systematic picture of the relative and absolute thermodynamic stabilities of the complexes, as determined by the electronic effects of the substituents. Interestingly, the NMR spectroscopic data hardly correlated with the expected substituent effects but the heteronuclear platinum-phosphorus coupling constants did. Crystallographic data were found to be blurred by packing effects. Instead, tandem MS and ITC data were in line with each other and followed the expected trends.

Encapsulation of Luminescent Homoleptic [Ru(dpp)3]2+-Type Chromophores within an Amphiphilic Dendritic Environment

U. Hahn, H. Luelf, H. D. F. Winkler, C. A. Schalley, F. Vögtle, L. De Cola

Chem. Eur. J. 2012, 18, 15424-15432

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A new series of homoleptic metallodendrimers has been synthesized through ruthenium-metal complexation by dendritically modified bathophenanthroline ligands. The presence of hydrophilic oligo(ethylene glycol) groups on the surface of the monodisperse metal complexes enabled the solubilization of all of the fractal species in a wide range of solvents, including water. The specific properties of all of these compounds have been systematically investigated by using photophysical techniques as a function of the generation number. Accordingly, the encapsulation of the highly luminescent [Ru(dpp)3]2+-type (dpp=4,7-diphenyl-1,10-phenanthroline) core unit within a dendritic microenvironment creates a powerful means to shield the center from dioxygen quenching. This shielding effect, as exerted on the phosphorescent ruthenium-derived center, is reflected by enhanced emission intensities and extended excited-state lifetimes that are close to the highest values reported so far, even in an air-equilibrated aqueous medium. Interestingly, when inspecting the largest dendritic assembly, that is, the third-generation assembly, significant drops in emission quantum yields and lifetimes are observed. This anomalous behavior has been attributed to the folding of the branches towards the luminescent core.

Unexpected One-Step Formation of Iodo[1,3]dioxolo[4,5-c]pyridine Derivatives by a Hofmann-Löffler-Freytag Reaction – Studies towards the Synthesis of a Pyridine Containing Macrocycle

T. Lechel, G. Podolan, B. Brusilowskij, C. A. Schalley, H.-U. Reissig

Eur. J. Org. Chem. 2012, 5685-5692

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During attempts to prepare functionalized 5-iodopyridine derivatives the unexpected formation of iodo[1,3]dioxolo[4,5-c]pyridines was discovered. The conversion of 3-alkoxypyridin-4-ols into the corresponding 5-iodo compounds was achieved by reaction with one equivalent of iodine or tetramethylammonium dichloroiodate under basic conditions. When three equivalents of iodine were used in chlorinated solvents, after 5-iodination, subsequent reaction of the 3-alkoxy group took place to form a 1,3-dioxolane ring with the 4-hydroxyl group. Generation of the resulting iodo[1,3]dioxolo[4,5-c]pyridines is explained by a radical process known as the Hofmann–Löffler–Freytag reaction. Two 6-ethynylpyridine derivatives were examined in the iodination process to establish a route to pyridine-containing macrocycles. The pentasubstituted 5-iodopyridine derivative 21 could be prepared; however, attempts to achieve cyclotrimerization of this building block under different conditions were not successful. Reaction of 21 with copper chloride allowed isolation of a copper acetylide 22, which aggregates to a triangular trimeric complex containing four copper(I) ions such as [23·Cu]+ as monitored by ESI mass spectrometry.

Multivalency as a Chemical Organization and Action Principle

Multivalency Review Angewandte Chemie

C. Fasting, C. A. Schalley, M. Weber, O. Seitz, S. Hecht, B. Koksch, J. Dernedde, C. Graf, E.-W. Knapp, R. Haag

Angew. Chem. Int Ed. 2012, 51, 10472-10498; Angew. Chem. 2012, 124, 10622-10650

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Multivalent interactions can be applied universally for a targeted strengthening of an interaction between different interfaces or molecules. The binding partners form cooperative, multiple receptor–ligand interactions that are based on individually weak, noncovalent bonds and are thus generally reversible. Hence, multi- and polyvalent interactions play a decisive role in biological systems for recognition, adhesion, and signal processes. The scientific and practical realization of this principle will be demonstrated by the development of simple artificial and theoretical models, from natural systems to functional, application-oriented systems. In a systematic review of scaffold architectures, the underlying effects and control options will be demonstrated, and suggestions will be given for designing effective multivalent binding systems, as well as for polyvalent therapeutics.

Deposition of Ordered Layers of Tetralactam Macrocycles and Ether Rotaxanes on Pyridine-Terminated SAMs on Gold

S. Richter, J. Poppenberg, C. H.-H. Traulsen, E. Darlatt, A. Sokolowski, D. Sattler, W. E. S. Unger, C. A. Schalley

J. Am. Chem. Soc. 2012, 134, 16289–16297

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The deposition of tetralactam macrocycles and the corresponding benzyl ether rotaxanes on gold substrates is investigated for the first time exploiting metallo-supramolecular chemistry. Two pyridine-terminated self-assembled monolayers (SAMs) are developed that are used as well-ordered template layers. The two SAMs differ with respect to the rigidity of the terminal pyridines as shown by angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The template layers are then used for the metal-mediated self-assembly of macrocylces and rotaxanes on solid supports. The SAM with the more rigid terminal pyridine shows a higher coverage with the macrocycles and is therefore preferable. Angle-resolved NEXAFS spectroscopy also shows the deposited supramolecules to be oriented preferentially upright. This order is only achieved for the macrocycles through the deposition on the more rigid SAM template, whereas rotaxanes form oriented layers on both SAMs. Time-of-flight secondary-ion mass spectrometry analysis was used to determine the deposition time required for the self-assembly process.

Intermixed Terpyridine-functionalized Monolayers on Gold: Non-Linear Relationship between Terpyridyl Density and Metal Ion Coordination Properties

C. H.-H. Traulsen, E. Darlatt, S. Richter, J. Poppenberg, S. Hoof, W. E. S. Unger, C. A. Schalley

Langmuir 2012, 28, 10755-10763

Full TextSupporting Information

Aiming at the functionalization of surfaces with terpyridine anchors for the coordinative deposition of additional layers, mixed self-assembled monolayers (SAMs) were prepared from binary solutions of 12-(2,2′:6′,2″-terpyridine-4′-yl)dodecane-1-thiol (TDT) and 1-decanethiol (DT). The SAMs and the order of the constituting molecules were analyzed by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and time-of-flight-secondary ion mass spectrometry (ToF-SIMS). The composition of the (TDT/DT)-SAMs and with it the surface density of terpyridyl groups correlates linearly with the relative concentrations of the two compounds in the solution used for depositing them. In marked contrast, the amount of terpyridine-coordinated PdII ions significantly deviates from this trend with an optimum at a 1:3 ratio of TDT/DT. This indicates a major fraction of the terpyridines in TDT-rich SAMs not to be accessible for PdII ion coordination. In agreement, NEXAFS spectroscopy reveals the alkyl backbones in TDT-rich SAMs not to be ordered, while they are preferentially upright oriented in the optimal 1:3-(TDT/DT)-SAMs. We interpret this in terms of terpyridine backfolding in TDT-rich SAMs, while they are located in accessible positions on top of the SAM in the 1:3-(TDT/DT)-SAM. While the alkyl backbones in the 1:3-(TDT/DT)-SAM are ordered, NEXAFS spectroscopy shows the terpyridyl groups not to have a preferential orientation in this SAM and thus retain enough flexibility to adjust to molecules that are deposited on top of the mixed SAM. In conclusion, the novel SAM does not undergo phase separation and consists predominantly of intermixed phases with adjustable surface density of quite flexible terpyridine anchor groups. The terpyridine–PdII anchors are not only available for a future deposition of the next layer, but the metal ions also represent a sensitive probe for the accessibility of the terpyridyl groups.

Equipping metallo-supramolecular macrocycles with functional groups: Assemblies of pyridine-substituted urea ligands

R. W. Troff, R. Hovorka, T. Weilandt, A. Lützen, M. Cetina, M. Nieger, D. Lentz, K. Rissanen, C. A. Schalley

Dalton Trans. 2012, 41, 8410-8420

Full TextSupporting InformationCrystal Structure Data

A series of di-(m-pyridyl)-urea ligands were prepared and characterized with respect to their conformations by NOESY experiments and crystallography. Methyl substitution in different positions of the pyridine rings provides control over the position of the pyridine N atoms relative to the urea carbonyl group. The ligands were used to self-assemble metallo-supramolecular M2L2 and M3L3 macrocycles which are generated in a finely balanced equilibrium in DMSO and DMF according to DOSY NMR experiments and ESI FTICR mass spectrometry. Again, crystallography was used to characterize the assemblies. Methyl substitution in positions next to the pyridine nitrogen prevents coordination, while the other ligands form small metallo-supramolecular macrocycles. The incorporated urea carbonyl groups provide hydrogen bonding sites which converge towards the center of the assemblies.

Mass Spectrometry and Gas-Phase Chemistry of Supramolecules: A Primer

Supramolecular Chemistry

E. V. Dzyuba, J. Poppenberg, S. Richter, R. W. Troff, C. A. Schalley

Wiley, Chichester/UK | 2012-04-10

Appeared In: Supramolecular Chemistry - From Molecules to Nanomaterials, J.W. Steed, P. Gale (eds.), pp. 347-378

Book HomeLink to Chapter

Mass spectrometry offers unique tools to examine supermolecules in the gas phase. In this chapter, gas-phase experiments that aim at determining the structures of supermolecules in solution and the gas phase, their reactivity in condensed phase and under environment-free conditions, and their thermochemistry in the gas phase are discussed. A comparison of the gas-phase properties with solution properties provides insight into the effects of solvation.

Ion Pair Recognition of Tetramethyl Ammonium Salts by Halogenated Resorcinarenes

N. K. Beyeh, D. P. Weimann, L. Kaufmann, C. A. Schalley, K. Rissanen

Chem. Eur. J. 2012, 18, 5552-5557

Full TextSupporting Information

The non-covalent interactions of different upper-rim-substituted C2-resorcinarenes with tetramethylammonium salts were analyzed in the gas phase in an Electrospray Ionization Fourier-transform ion-cyclotron-resonance (ESI-FTICR) mass spectrometer and by 1H NMR titrations. The order of binding strengths of the hosts towards the tetramethylammonium cation in the gas phase reflects the electronic nature of the substituents on the upper rim of the resorcinarene. In solution, however, a different trend with particularly high binding constants for halogenated resorcinarenes has been observed. This trend can be explained by a synergetic effect originating from the interaction of the halogenated resorcinarenes with the counter anions through hydrogen bonding. This study highlights the importance of weak interactions in recognition processes and points out the benefits of comparing the gas-phase data with results obtained from solution experiments.

Evidence of click and coordination reactions on a self-assembled monolayer by synchrotron radiation based XPS and NEXAFS

E. Darlatt, C. H.-H. Traulsen, J. Poppenberg, S. Richter, J. Kühn, C. A. Schalley, W. E. S. Unger

J. Electr. Spectr. Rel. Phen. 2012, 185, 85-89

Full Text

An ethynylterpyridine was “clicked” to an azide-terminated self-assembled monolayer on gold and characterized by synchrotron radiation based surface analysis as NEXAFS and XPS. The detection of azide and terpyridine signatures confirmed a partial click reaction at room temperature. The absence of the azides after reaction at 50 °C indicates an almost complete conversion. For the latter case successful Pd(II) coordination has been proven. The Au–S interface of the SAMs has been characterized by S 1s and S 2p XPS.

Analytical Methods in Supramolecular Chemistry, 2nd edition

Analytical Methods, 2nd ed.

The second edition of "Analytical Methods in Supramolecular Chemistry" comes in two volumes and covers a broad range of modern methods and techniques now used for investigating supramolecular systems, e. g. NMR spectroscopy, mass spectrometry, extraction methods, crystallography, single molecule spectroscopy, electrochemisty, and many more. In this second edition, tutorial inserts have been introduced, making the book also suitable as supplementary reading for courses on supramolecular chemistry. All chapters have been revised and updated and four new chapters have been added.

Substituent Effects on Axle Binding in Amide Pseudorotaxanes: Comparison of NMR Titration and ITC Data with DFT Calculations

L. Kaufmann, E. V. Dzyuba, F. Malberg, N. L. Löw, M. Groschke, B. Brusilowskij, J. Huuskonen, K. Rissanen, B. Kirchner, C. A. Schalley

Org. Biomol. Chem. 2012, 10, 5954-5964

Full TextSupporting InformationCrystal Structure Data

The binding behaviour of differently substituted diamide axle molecules to Hunter/Vögtle tetralactam macrocycles was studied with a combination of NMR titration, isothermal titration calorimetry (ITC) experiments and calculations employing density functional theory (DFT), along with dispersion-corrected exchange-correlation functionals. Guests with alkyl or alkenyl chains attached to the diamide carbonyl groups have a significantly higher binding affinity to the macrocycle than guests with benzoyl amides and their substituted analogues. While the binding of the benzoyl and alkenyl substituted axles is enthalpically driven, the alkyl-substituted guest binds mainly because of a positive binding entropy. The electronic effects of para-substituents at the benzoyl moieties have an influence on the binding affinities. Electron donating substituents increase, while electron-withdrawing substituents decrease the binding energies. The binding affinities obtained from both NMR titration and ITC experiments correlate well with each other. The substituent effects observed in the experimental data are reflected in adiabatic interaction energies calculated with density functional methods. The calculated structures also agree well with pseudorotaxane crystal structures.

Systems Chemistry: Logic gates based on the stimuli-responsive gel-sol transition of a crown-ether functionalized bis-urea-gelator

Z. Qi, P. Malo de Molina, W. Jiang, Q. Wang, K. Nowosinski, A. Schulz, M. Gradzielski, C. A. Schalley

Chem. Sci. 2012, 3, 2073-2082

Full TextSupporting InformationHighlighted on the Chemical Science Blog

A quite simple, achiral benzo-21-crown-7-substituted bis(urea) low-molecular weight gelator hierarchically assembles into helical fibrils, which further develop into bundles and finally form a stable gel in acetonitrile. The gel–sol transition can be controlled by three different molecular recognition events: K+ binding to the crown ethers, pseudorotaxane formation with secondary ammonium ions and Cl− binding to the urea units. Addition of a cryptand that scavenges the K+ ions and Ag+ addition to remove the chloride and bases/acids, which mediate pseudorotaxane formation, can reverse this process. With the gelator, and these chemical stimuli, a number of different systems can be designed that behave as logic gates. Depending on the choice of components, OR, AND, XOR, NOT, NOR, XNOR and INHIBIT gates have been realized. Thus, the gel–sol transition as a property of the system as a whole is influenced in a complex manner. For some cases, the type of logic gate is defined by input signal concentration so that an even more complex reaction of the gel towards the two input signals is achieved.

Supramolecular M4L4 tetrahedra based on triangular acylhydrazone catechol ligands

M. Albrecht, Y. Shang, T. Rhyssen, J. Stubenrauch, H. D. F. Winkler, C. A. Schalley

Eur. J. Org. Chem. 2012, 2422-2427

Full TextSupporting Information

The ligands 1-H6 and 2-H6 are prepared by condensation of the triangular triamines 5 and 2,3-dihydroxybenzoic acid hydrazide 7. The ligands form container-type tetrahedral coordination compounds Mx [(1 or 2)4M′4] (M = Li, Na, K; M′ = Ti, x = 8; M′ = Ga, x = 12). The complexes are characterized by NMR spectroscopy and ESI mass spectrometry. Despite the relatively labile acyl hydrazone unit, the complexes show high stability in water. Ligand 3-H6 is prepared from the trisacyl hydrazide 8 and 2,3-dihydroxybenzaldehyde 9 but does not form well-defined coordination compounds with gallium(III) or titanium(IV) ions.

Synthesis of Multivalent Host and Guest Molecules for the Construction of Multithreaded Diamide Pseudorotaxanes

N. L. Löw, E. V. Dzyuba, B. Brusilowskij, L. Kaufmann, E. Franzmann, W. Maison, E. Brandt, D. Aicher, A. Wiehe, C. A. Schalley

Beilstein J. Org. Chem. 2012, 8, 234-245

Full TextSupporting Information

A series of di-, tri- and tetravalent axles and wheels for the synthesis of pseudorotaxanes bearing the tetralactam macrocycle/diamide axle binding motif was prepared. Starting from iodinated monovalent precursors, Sonogashira cross-coupling reactions were utilized to couple the binding sites to appropriate spacer groups. Through this “Lego” or “toolbox” approach, the convergent synthesis of host and guests with a well-defined number of the binding sites is possible. In addition, the spatial arrangement of the binding sites can be controlled through the quite rigid connections between linker and binding sites. Although a quantitative assessment of binding strengths was not possible by NMR titration experiments, typical and significant shifts of the signals of the diamide moiety indicate qualitatively the formation of pseudorotaxanes from the axle and wheel precursors.

Successive Coordination of Palladium(II)-Ions and Terpyridine-Ligands to a Pyridyl-Terminated Self-Assembled Monolayer on Gold

J. Poppenberg, S. Richter, E. Darlatt, C. H.-H. Traulsen, H. Min, W. E. S. Unger, C. A. Schalley

Surf. Sci. 2012, 606, 367-377

Full TextSupporting Information

The deposition of palladium on a novel, reversibly protonatable, pyridyl-terminated self-assembled monolayer on gold substrates has been studied by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS spectroscopy) and time of flight-secondary ion mass spectrometry (ToF-SIMS). For this purpose, 12-(pyridin-4-yl)dodecane-1-thiol, consisting of a surface-active head group, an unfunctionalized hydrocarbon backbone and a terminal pyridyl group, has been synthesized and deposited on gold surfaces. Coordination of Pd(II) ions to the pyridyl group was examined. Furthermore, a reversible protonation/deprotonation cycle has been applied, and the relation between protonation and the amount of complexed palladium was studied. Investigation of the SAM by angle-resolved NEXAFS spectroscopy revealed the aliphatic backbone to be preferentially upright oriented with the aromatic head group being not preferentially oriented. The palladium layer was further coordinated with a CF3-labeled terpyridine ligand in order to prove the accessibility of the Pd(II) ions to further complexation and the platform useful for deposition of further layers toward a multi-layered system.

Phenanthroline- and Terpyridine-Substituted Tetralactam Macrocycles: A Facile Route to Rigid Di- and Trivalent Receptors and Interlocked Molecules

E. V. Dzyuba, B. Baytekin, D. Sattler, C. A. Schalley

Eur. J. Org. Chem. 2012, 1171-1178

Full Text

Bromo-substituted Hunter/Vögtle-type tetralactam macrocycles (TLMs) represent key intermediates for the attachment of terpyridyl and phenanthroline metal binding sites through cross-coupling reactions. From these monovalent precursors, metal complexes can easily be obtained that present the macrocycles in a multivalent fashion. Depending on the nature of the metal ion, the properties of the complexes can be tuned with respect to valency (e.g., phen-TLM + CuI: divalent, phen-TLM + FeII: trivalent) and lability against TLM ligand exchange (e.g., CuI: slow, but reversible exchange, RuIICl2: kinetically inert).

Chelate Cooperativity and Spacer Length Effects on the Assembly Thermodynamics and Kinetics of Divalent Pseudorotaxanes

W. Jiang, K. Nowosinski, N. L. Löw, E. V. Dzyuba, F. Klautzsch, A. Schäfer, J. Huuskonen, K. Rissanen, C. A. Schalley

J. Am. Chem. Soc. 2012, 134, 1860-1868

Full TextSupporting InformationCrystal Structure Data

Homo- and heterodivalent crown-ammonium pseudorotaxanes with different spacers connecting the two axle ammonium binding sites have been synthesized and characterized by NMR spectroscopy and ESI mass spectrometry. The homodivalent pseudorotaxanes are investigated with respect to the thermodynamics of divalent binding and to chelate cooperativity. The shortest spacer exhibits a chelate cooperativity much stronger than that of the longer spacers. On the basis of crystal structure, this can be explained by a noninnocent spacer, which contributes to the binding strength in addition to the two binding sites. Already very subtle changes in the spacer length, i.e., the introduction of an additional methylene group, cause substantial changes in the magnitude of cooperative binding as expressed in the large differences in effective molarity. With a similar series of heterodivalent pseudorotaxanes, the spacer effects on the barrier for the intramolecular threading step has been examined with the result that the shortest spacer causes a strained transition structure and thus the second binding event occurs slower than that of the longer spacers. The activation enthalpies and entropies show clear trends. While the longer spacers reduce the enthalpic strain that is present in the transition state for the shortest member of the series, the longer spacers become entropically slightly more unfavorable because of conformational fixation of the spacer chain during the second binding event. These results clearly show the noninnocent spacers to complicate the analysis of multivalent binding. An approximate description which considers the binding sites to be connected just by a flexible chain turns out to be more a rough approximation than a good model. The second conclusion from the results presented here is that multivalency is expressed in both the thermodynamics and the kinetics in different ways. A spacer optimized for strong binding is suboptimal for fast pseudorotaxane formation.

Gas-phase organocatalysis with crown ethers

H. D. F. Winkler, E. V. Dzyuba, A. Springer, L. Losensky, C. A. Schalley

Chem. Sci. 2012, 3, 1111-1120

Full Text

Catalytic cycles can be investigated in detail in the gas phase in a step-by-step manner by tandem mass spectrometric experiments that allow for the mass-selection of each of the intermediates prior to the subsequent step. In the present article, we describe large crown ethers (dibenzo-24-crown-8 to dibenzo-30-crown-10) to mediate the E2-elimination of propene from crown ether-propylammonium complexes giving rise to NH4+/crown ether complexes. A back exchange of the ammonia against propyl amine completes the catalytic cycle. To the best of our knowledge, this reaction cycle represents the first example of organocatalysis in the gas-phase. In addition, the larger crown ethers significantly accelerate the H/D-exchange reaction of the five NH hydrogen atoms in singly protonated ethylenediamine as compared to the analogous smaller crown ether complexes. This effect, which is not observed for the propylammonium complexes, can be rationalized by a favorable pre-organization of the protonated ethylenediamine ion within the cavity of the crown ether through hydrogen bonding to both the ammonium and the amine groups. Within this complex, the H/D-exchange can proceed through an efficient “relay” mechanism. A comparison of the H/D-exchange behavior of ammonium complexes with differently sized crown ethers reveals a distinct reactivity pattern that strongly depends on the crown ether size. While 18-crown-6 acts as a non-covalent protective group suppressing the H/D-exchange almost completely, smaller and larger crown ethers exhibit higher exchange rates.

Light-Harvesting in Multichromophoric Rotaxanes

M. E. Gallina, B. Baytekin, C. A. Schalley, P. Ceroni

Chem. Eur. J. 2012, 18, 1528-1535

Full Text

Two rotaxanes with benzyl ether axles and tetralactam wheels were synthesized through an anion template effect. They carry naphthalene chromophores attached to the stopper groups and a pyrene chromophore attached to the wheel. The difference between the two rotaxanes is represented by the connecting unit of the naphthyl chromophore to the rotaxane axle: a triazole or an alkynyl group. Both rotaxanes exhibit excellent light-harvesting properties: excitation of the naphthalene chromophores is followed by energy transfer to the pyrene unit with efficiency higher than 90 % in both cases. This represents an example of light-harvesting function among chromophores belonging to mechanically interlocked components, that is, the axle and the wheel of the rotaxanes.

From {Bi22O26} to Chiral Ligand-Protected {Bi38O45}-Based Bismuth Oxido Clusters

D. Mansfeld, L. Miersch, T. Rüffer, D. Schaarschmidt, H. Lang, T. Böhle, R. W. Troff, C. A. Schalley, J. Müller, M. Mehring

Chem. Eur. J. 2011, 17, 14805-14810

Full TextSupporting Information

The reaction of [Bi22O26(OSiMe2tBu)14] (1) in THF with salicylic acid gave [Bi22O24(HSal)14] (2) first, which was converted into [Bi38O45(HSal)22(OH)2(DMSO)16.5] ⋅DMSO⋅H2O (3⋅DMSO⋅H2O) after dissolution and crystallization from DMSO. Single-crystal X-ray diffraction analysis and ESI mass spectrometry associated with infrared multi-photon dissociation (IRMPD) tandem MS experiments confirm the formation of the large and quite stable bismuth oxido cluster 3. The reaction of compound 2 with the butoxycarbonyl(BOC)-protected amino acids phenylalanine and valine (BOC-PheOH and BOC-ValOH), respectively, resulted in the formation of chiral [Bi38O45(BOC-AA)22(OH)2] (AA=deprotonated amino acid), as shown by a combination of different analytical techniques such as elemental analysis, dynamic light scattering, circular dichroism spectroscopy, and ESI mass spectrometry.

Thematic Series in Supramolecular Chemistry II

C. A. Schalley (ed.)

Beilstein J. Org. Chem. 2011

Thematic Series ContentsEditorial

Supramolecular chemistry is a rapidly growing field, which has had remarkable impact on the life sciences on one hand and on materials sciences on the other. In the life sciences, the networks of noncovalent interactions between the constituents of cells, for example, have shifted into the current focus. Self-assembly, templation, self-sorting and multivalent binding all contribute to setting up the extremely complex architecture of a cell. But the same concepts are useful for generating materials with function, when for example the building blocks are programmed appropriately to find their places in a larger, noncovalent architecture. The basis for all these concepts is molecular recognition. Recently, many studies have been devoted to quantifying host–guest interactions, aiming at a more profound understanding of the subtle entropic and enthalpic effects that govern the interactions between host and guest. A first Thematic Series devoted to supramolecular chemistry was assembled about two years ago and published by the Beilstein Journal of Organic Chemistry [1]. This first series of articles had quite a broad scope ranging from encapsulation and carbohydrate, peptide, anion and ammonium ion binding, through chiral recognition, the formation of pseudorotaxanes and template effects, all the way to allosteric binding to synthetic receptors, crystallographic studies of halogen bonding and the use of polymers for protein binding. The second series again has a broad scope, as you will discover in the coming months as the series develops. With the second Thematic Series on supramolecular chemistry, we wish to contribute to the endeavor to investigate noncovalently bound complexes and aggregates of every possible kind, thus highlighting the importance of the above-mentioned concepts. With the now well-known Thematic Series, the Beilstein Journal of Organic Chemistry provides an excellent platform for this aim, in particular since it is a true open access journal. I would like to thank warmly all authors who have accepted the invitation to contribute to this series and sincerely hope that the readers will enjoy reading the articles that are published within this Thematic Series.

CH•••O Hydrogen Bonds in "Clicked" Diketopiperazine-Based Amide Rotaxanes

E. V. Dzyuba, L. Kaufmann, N. L. Löw, A. K. Meyer, H. D. F. Winkler, K. Rissanen, C. A. Schalley

Org. Lett. 2011, 13, 4838-4841

Full TextSupporting InformationCrystal Structure Data

Two amide [2]rotaxanes were synthesized in high yields using a novel N,N′-dipropargyl diketopiperazine axle centerpiece as the template to which the stoppers are attached through “click chemistry”. 1H and 2D NMR spectra provide evidence for two different H-bonding motifs, in one of which the triazole CH groups form C–H···O═C bonds with the wheel carbonyl O atoms. This motif can be controlled and switched reversibly by competitive anion binding.

[4]Pseudorotaxanes with Remarkable Self-Sorting Selectivities

W. Jiang, D. Sattler, K. Rissanen, C. A. Schalley

Org. Lett. 2011, 13, 4502-4505

Full TextSupporting InformationCrystal Structure Data

The synthesis and characterization of several self-assembled [4]pseudorotaxanes is reported, some of which form in a programmed way based on two similar yet orthogonal crown ether/secondary ammonium ion binding motifs. A preference for the formation of a [4]pseudorotaxane with an antiparallel rather than parallel alignment of crown ether building blocks is observed even in the absence of such orthogonal binding sites, when a homodivalent axle is used.

Effects of subtle differences in ligand constitution and conformation in metallo-supramolecular self-assembled polygons

B. Brusilowskij, E. V. Dzyuba, R. W. Troff, C. A. Schalley

Dalton Trans. 2011, 40, 12089-12096

Full TextSupporting Information

3,3′-Bis(pyridin-[n]-ylethynyl)biphenyl (n = 3, 4) and the corresponding 2,2′-bipyridines assemble with (dppp)PtII triflate into metallo-supramolecular polygons. Depending on the position of the terminal pyridine N atoms, the assembly reaction leads to different equilibrium products. With the slow ligand exchange on PtII complexes, the equilibrium is reached on a many-hour time-scale. During the assembly process, larger polygons form under kinetic control. This was confirmed by time-dependent 1H and 31P NMR spectroscopy in line with complementary ESI mass spectrometric experiments. The constitutional difference in the pyridine N-atom position is reflected in the tandem mass spectra of the complex ions. In addition, a highly specific fragmentation process of mass-selected M3L3 ions was observed, which proceeds through a ring contraction yielding smaller M2L2 ions.

Synthesis of 5-Acetyloxazoles and 1,2-Diketones from ß-Alkoxy-ß-ketoenamides and their Subsequent Transformations

T. Lechel, M. Gerhard, D. Trawny, B. Brusilowskij, L. Schefzig, R. Zimmer, J. P. Rabe, D. Lentz, C. A. Schalley, H.-U. Reißig

Chem. Eur. J. 2011, 17, 7480-7491

Full TextSupporting Information

Lithiated alkoxyallenes, nitriles, and carboxylic acids have been employed as precursors in a three-component reaction leading to highly substituted β-alkoxy-β-ketoenamides. Upon treatment with trifluoroacetic acid, these enamides could be easily cyclized to 5-acetyloxazole derivatives. The synthesis is very flexible with respect to the substitution pattern at C-2 and C-4 of the oxazole core. A mechanistic suggestion for the oxazole formation is presented on the basis of 18O-labeled compounds and their mass spectrometric analysis. In several cases, 1,2-diketones are formed as side products or even as major components. The acetyl moiety at C-5 of the oxazole derivatives can efficiently be converted into alkenyl or alkynyl moieties, which allows a multitude of subsequent reactions. Condensation reactions of the acetyl group provided the expected oxime or hydrazone. By applying a Fischer reaction, the phenylhydrazone could be transferred into an indole, which emphasizes the potential of 5-acetyloxazoles for the preparation of highly substituted (poly)heterocyclic systems. The alkynyl group at C-2 is prone to addition reactions, providing an enamine with interesting photophysical properties. Sonogashira couplings were performed with 5-alkynyl-substituted oxazoles, furnishing the expected aryl-substituted products. This alkynyl unit was employed for the preparation of a new, star-shaped trisoxazole derivative. The ability of this multivalent compound to form self-assembled monolayers between the basal plane of highly oriented pyrolytic graphite and 1-phenyloctane was demonstrated by scanning tunneling microscopy (STM). The star-shaped compound seems to prefer the C3-symmetric arrangement in this two-dimensional crystal. Two 1,2-diketones were smoothly converted into functionalized quinoxaline derivatives.

Hydrolysis of a Basic Bismuth Nitrate - Formation and Stability of Novel Bismuth Oxido Clusters

L. Miersch, M. Schlesinger, R. W. Troff, C. A. Schalley, T. Rüffer, H. Lang, D. Zahn, M. Mehring

Chem. Eur. J. 2011, 17, 6985-6990

Full TextSupporting Information

The synthesis of the nanoscaled bismuth oxido clusters [Bi38O45- (NO3)20(DMSO)28](NO3)4⋅4 DMSO (1 a) and [Bi38O45(OH)2(pTsO)8- (NO3)12(DMSO)24](NO3)2⋅4 DMSO⋅ 2 H2O (2) starting from the basic bismuth nitrate [Bi6O4(OH)4](NO3)6⋅H2O is reported herein. Single-crystal X-ray diffraction analysis, ESI mass spectrometry, thermogravimetric analysis, and molecular dynamics simulation were used to study the formation, structure, and stability of these large metal oxido clusters. Compounds 1 a and 2 are based on a [Bi38O45]24+ core, which is structurally related to δ-Bi2O3. Examination of the fragmentation pathways of 1 a and 2 by infrared multi-photon dissociation (IRMPD) tandem MS experiments allows the identification of novel bismuth oxido cluster species in the gas phase.

A neutral Pt3 stack unsupported by any bridging ligand

L. Holland, W.-Z. Shen, P. von Grebe, D. Gupta, P. J. Sanz Miguel, Y. Huang, F. Pichierri, A. Springer, C. A. Schalley, B. Lippert

Dalton Trans. 2011, 40, 5159-5161

Full TextSupporting InformationCrystal Structure Data

Pt...Pt...Pt interactions via their d8 orbitals, combined with π–π stacking of deprotonated, chelating 2-(3′-pyrazolyl)pyridine (pyzpy) ligands, are responsible for trans-Pt(pyzpy)2 (2) crystallization in a stack of three molecules unsupported by any bridging ligand.

Self-Sorting of Water-soluble Cucurbituril Pseudorotaxanes

W. Jiang, Q. Wang, I. Linder, F. Klautzsch, C. A. Schalley

Chem. Eur. J. 2011, 17, 2344-2348

Full TextSupporting Information

Social self-sorting: A binary and a ternary pseudorotaxane, which both contain cucurbituril homologues and share methylviologen as the guest, can self-sort. These two self-sorting pseudorotaxanes were further integrated into one well-defined hetero[3]pseudorotaxane with one symmetrical axle that exclusively selects a pair of different cucurbiturils in a social self-sorting process. Vice versa, a “necklace” of three cucurbit[8]urils is formed, which bind two complementary axles inside their cavities with high fidelity.

Gas-phase H/D-exchange reactions on resorcinarene and pyrogallarene capsules: Proton transport through a one-dimensional Grotthuss mechanism


H. D. F. Winkler, E. V. Dzyuba, J. A. W. Sklorz, N. K. Beyeh, K. Rissanen, C. A. Schalley

Chem. Sci. 2011, 2, 615-624

Full Text

Hydrogen/deuterium exchange (HDX) experiments can be used to examine the gas-phase structure of hydrogen-bonded dimeric resorcinarene and pyrogallarene capsules. Already the qualitative comparison of the isotope exchange rates of different host–guest complexes with Cs+, tetramethyl ammonium (TMA+) and tetraethyl ammonium (TEA+) as the guest cations provides insight into the H/D-exchange mechanisms and with it, into the capsules' gas-phase ion structures. The smaller Cs+ cations bind inside dimeric capsules with an intact seam of hydrogen bonds between the two monomers. Larger cations such as TEA+ lead to capsules with partially disrupted seams of hydrogen bonds. A fast isotope exchange is only observed, when the H-bonding seam between the two monomers is intact. In these cases, the H/D-exchange proceeds by a concerted mechanism reminiscent of the Grotthuss mechanism of proton transfer through water. Since it can only proceed along the seam, we refer to this exchange mechanism as a one dimensional Grotthuss mechanism.

Gas-phase H/D-exchange experiments in supramolecular chemistry

H. D. F. Winkler, E. V. Dzyuba, C. A. Schalley

New J. Chem. 2011, 35, 529-541

Full Text

This review discusses the potential of gas-phase H/D-exchange (HDX) reactions for supramolecular chemistry. The exchange of labile H against D atoms can help unravel structural details of supramolecules—in particular when combined with other gas-phase experiments, e.g. ion-mobility MS. The presence of different, non-interconverting structures in an ion population can lead to bimodal exchange distributions. Since hydrogen bonding has an effect on the exchange rates, the number and positions of hydrogen bonds can often be determined. Zwitterionic and charge-solvated structures of amino acids and peptides can be distinguished. Beyond structure, dynamic features such as the mobility of building blocks within complexes can be investigated.

Thermodynamically controlled self-sorting of hetero-bimetallic metallo-supramolecular macrocycles: What a difference a methylene group makes!

B. Brusilowskij, E. V. Dzyuba, R. W. Troff, C. A. Schalley

Chem. Commun. 2011, 47, 1830-1832

Full TextSupporting Information

31P NMR and tandem MS experiments provide clear evidence for the thermodynamically controlled self-assembly of hetero-bimetallic metallo-supramolecular macrocycles through self-sorting caused by different ancillary ligands.

Multidentate Pyridyl-based Ligands in the Coordination-Driven Self-Assembly of Palladium Metallo-Macrocycles

Self-Assembly Soup

B. Brusilowskij, C. A. Schalley

Eur. J. Org. Chem. 2011, 469-477

Full TextSupporting Information

In the current study, a convenient way is presented to synthesize one ditopic and two tetratopic pyridine-based ligands, which are then used to construct metallo-supramolecular polygons. The tetratopic ligands offer two different metal-binding sites, one central 2,2′-bipyridine, which can act as a chelate ligand, and two separate pyridine rings, which mediate assembly formation. The three ligands differ with respect to their conformational flexibility. While a biphenyl core allows the ligand to adjust its conformation as needed, a bipyridine core strongly prefers a divergent arrangement of the additional pyridine binding sites, but can be fixed in a cisoid conformation by metal complexation to the bipyridine.Instead, a phenanthroline core already fixes the pyridinylethynyl substituents in a cisoid structure without any metal coordinated to it. Upon mixing each one of the ligands separately with the appropriate amount of dpppPdII triflate, discrete self-assembled metallo-macrocycles are formed which are characterized by 1H and 31P NMR spectroscopy and mass spectrometry. Mixing all three ligands simultaneously with the metal complex leads to the formation of a statistical dynamic combinatorial library (DCL) of all possible homo- and heterodimeric metallo-supramolecular assemblies. This underlines the conformational differences between the three ligands not to impact significantly on the self-assembly process.

Vinylogous Aza-Enamines as Neutral d3 Nucleophiles: Aminomethylations of N,N-Dimethylhydrazones of alpha,beta-Unsaturated Aldehydes

M. T. Grabowski, K. Nowosinski, D. Lentz, R. Brehme, C. A. Schalley

Synthesis 2010, 3556-3568

Full Text

N,N-Dimethylhydrazones of propenal- and 2-methyl­propenal and their derivatives and homologues (vinylogous aza-enamines) were allowed to react with N,N-dimethylformiminium chloride in moisture-free dimethylformamide to yield singly, doubly, and even triply aminomethylated products. They can be easily separated and characterized as crystalline hydrochlorides. The reaction takes place at the ω-position of the π-system. This is a consequence of the conjugative interaction of the electron-donating aminohydrazone group with the double bond system in analogy to the enamines. The formation of dialkylhydrazones from unsaturated aldehydes thus causes the umpolung of the formerly electrophilic d³-building blocks into a nucleophile. Depending on the reaction conditions and confirmed by crystal structures and 2D NMR experiments, control can be exerted over the degree of substitution: Up to trisubstituted products were obtained for the 2-methylpropenal derivative. The hydrochlorides can be easily deprotonated to yield the free aminohydrazone bases. The back-conversion of the aminohydrazones into the corresponding amino aldehydes is possible under acidic conditions.

Hierarchical self-assembly of metallo-dendrimers

M. Albrecht, M. Baumert, H. D. F. Winkler, C. A. Schalley, R. Fröhlich

Dalton Trans. 2010, 39, 7220-7222

Full TextSupporting InformationCrystal Structure Data

2,3-Dihydroxybenzylic esters with Frechet-type dendritic branches as the alcohol component form, in a hierarchical self-assembly process, disk shaped dendrimers when titanium(IV) and lithium ions are added.

Tandem Mass Spectrometry for the Analysis of Self-Sorted Pseudorotaxanes: The Effects of Coulomb Interactions

W. Jiang, C. A. Schalley

J. Mass Spectrom. 2010, 45, 788-798

Full TextSupporting Information

The increasing complexity of self-assembled supramolecules generates the need for analytical techniques that can accurately elucidate their structures. Here, we explore the ability of tandem mass spectrometry to deliver structural information on a series of self-sorted crown ether/ammonium pseudorotaxanes. Of these intertwined molecules, different charge states are accessible and the effects of Coulomb interactions on the fragmentation pattern can be examined. Three different cases can be distinguished: (1) one or more counterions are present in the complex and compete with the crown for binding to the ammonium ion. This destabilizes the supramolecular bond. (2) In multiply charged complexes, charge repulsion significantly alters the fragmentation behavior as compared with singly charged ions. (3) If guest and host are both charged, the supramolecular bond becomes very weak. The different charge states provide different pieces of information about the supramolecules under study. Although singly charged complexes provide data on the building block connectivity, the doubly charged analogs are more reliable with respect to complex stoichiometry. As there are several factors which may cause differences in the gas phase and solution behavior of supramolecules (the presence and absence of solvation, changes in the strength of non-covalent interactions upon ionization), it is important to establish well understood correlations between the complexes' gas-phase behavior and their solution structures. A more detailed understanding will help to characterize the structures of even more complex supramolecular architectures by mass spectrometry.

Host-Guest Chemistry of Dendrimers in the Gas Phase

Z. Qi, C.A. Schalley

Supramol. Chem. 2010, 22, 672-682

Full Text

Since the early days of dendrimer chemistry, mass spectrometry has been an important analytical method for determining the purity and the detection of defects in dendrimers. Meanwhile, growing evidence demonstrates the great potential of mass spectrometry for the investigation of non-covalent dendritic host–guest complexes. Mass spectrometry provides an efficient means to isolate them in the high vacuum inside a mass spectrometer under environment-free conditions. Gas-phase chemistry is particularly beneficial for exploring the intrinsic properties which cannot easily be studied in solution. This mini-review highlights the versatility of gas-phase chemistry for (1) screening the specificity and stability of multivalent dendritic host–guest complexes depending on the nature of the guests, (2) revealing a dendritic effect during dendrimer–tweezer complex fragmentation and (3) monitoring an intra complex movement of small guests along the dendrimer periphery.

Experimental evidence for the functional relevance of anion-pi interactions

Anion-pi Interactions

R. E. Dawson, A. Hennig, D. P. Weimann, D. Emery, V. Ravikumar, J. Montenegro, T. Takeuchi, S. Gabutti, M. Mayor, J. Mareda, C. A. Schalley, S. Matile

Nature Chem. 2010, 2, 533-538

Full TextSupporting InformationHighlight by Jeffrey T. DavisHighlighted on RSC Chemistry WorldHighlighted in Chemistry Views

Attractive in theory and confirmed to exist, anion–π interactions have never really been seen at work. To catch them in action, we prepared a collection of monomeric, cyclic and rod-shaped naphthalenediimide transporters. Their ability to exert anion–π interactions was demonstrated by electrospray tandem mass spectrometry in combination with theoretical calculations. To relate this structural evidence to transport activity in bilayer membranes, affinity and selectivity sequences were recorded. π-acidification and active-site decrowding increased binding, transport and chloride > bromide > iodide selectivity, and supramolecular organization inverted acetate > nitrate to nitrate > acetate selectivity. We conclude that anion–π interactions on monomeric surfaces are ideal for chloride recognition, whereas their supramolecular enhancement by π,π-interactions appears perfect to target nitrate. Chloride transporters are relevant to treat channelopathies, and nitrate sensors to monitor cellular signaling and cardiovascular diseases. A big impact on organocatalysis can be expected from the stabilization of anionic transition states on chiral π-acidic surfaces.

Tuning the polarity of hierarchically assembled helicates

M. Baumert, M. Albrecht, H. D. F. Winkler, C. A. Schalley

Synthesis 2010, 953-958

Full Text

Catechol ligands bearing ester groups in 3-position (1a,b, 2) are building blocks for the formation of triply lithium-bridged dinuclear helicate-type complexes [Li3(1a,b,2)6Ti2]-. Attachment of appropriate functionalities at the ester unit allows the fine tuning of the polarity of the compounds to afford solubility in highly nonpolar as well as highly polar solvents. The investigations show that the internal dinuclear core is stable in a broad variety of solvents, even in water.

Templated Versus Non-Templated Synthesis of Benzo-21-Crown-7 and the Influence of Substituents on Its Complexing Properties

W. Jiang, C. A. Schalley

Beilstein J. Org. Chem. 2010, 6, no. 14

Full TextSupporting Information

Two procedures for the synthesis of benzo-21-crown-7 have been explored. The [1+1] macrocyclization with KBF4 as the template was found to be more efficient than the intramolecular macrocyclization without template. Pseudorotaxanes form with secondary ammonium ions bearing at least one alkyl chain narrow enough to slip into the crown ether. Substitution on benzo-21-crown-7 or on the secondary ammonium axle alters the binding affinity and binding mode. Compared to dibenzo-24-crown-8, the complexing properties of benzo-21-crown-7 turn out to be more susceptible to modifications at the crown periphery.

Monitoring Self-Sorting by Electrospray Ionization Mass Spectrometry: Formation Intermediates and Error-Correction during the Self-Assembly of Multiply Threaded Pseudorotaxanes

W. Jiang, P. C. Mohr, A. Schäfer, C. A. Schalley

J. Am. Chem. Soc. 2010, 132, 2309-2320

Full TextSupporting Information

Three binary pseudorotaxanes, which are based on two different secondary ammonium/crown ether binding motifs, have been studied by 1H NMR and 1H,1H EXSY NMR experiments with respect to their thermodynamic stabilities and their axle exchange kinetics. The stability ranking does not follow the order of axle exchange rates, and the thermodynamically most stable axle−wheel combinations assemble only slowly. On the basis of these binding motifs, a series of self-sorting systems have been studied ranging from simple four-component mixtures through sequence-specific pseudorotaxanes to multiply threaded complexes. Because of the mismatch of kinetic and thermodynamic order, wrongly assembled structures are unavoidable, which require error-correction steps to yield the final thermodynamically controlled self-sorted products. These error-correction steps can easily be monitored by electrospray mass spectrometry, when a mixed-flow microreactor is coupled to the ion source to cover second time scales. Self-assembly intermediates, wrongly assembled structures, and the final thermodynamic products can be simultaneously identified. The determination of preferred assembly pathways as well as the formation of dead-end structures provides a clear picture of a rich kinetic behavior of the self-sorting systems under study.

Nickel(II) and Iron(III) Selective Off-On-Type Fluorescence Probes Based on Perylene Tetracarboxylic Diimide

H. Wang, D. Wang, Q. Wang, B. Liang, L. Xue, H. Wu, C. A. Schalley, X.-y. Li

Org. Biomol. Chem. 2010, 8, 1017-1026

Full TextSupporting Information

Two novel “turn-on” fluorescent probes with perylene tetracarboxylic diimide (PDI) as the fluorophore and two different di-(2-picolyl)-amine (DPA) groups as the metal ion receptor (PDI-1 and PDI-2) were successfully synthesized with satisfactory yields. PDI-1 exhibited high selectivity toward Ni2+ in the presence of various other metal cations including Zn2+, Cd2+ and Cu2+ which were expected to interfere significantly. A 1 : 2 stoichiometry was found for the complex formed by PDI-1 and Ni2+ by a Job's plot and by non-linear least square fitting of the fluorescence titration curves. By introducing an extra diamino ethylene group between DPA and the phenyl bridge, the receptor was modified and the high selectivity of the sensor toward Ni2+ shifted to Fe3+. The enhancement factor of the fluorescence response of PDI-2 to Fe3+ was as high as 138. The binding behavior of the receptors in these two compounds is affected significantly by the PDI fluorophores. Most interestingly, both Ni2+ and Fe3+ are paramagnetic metal ions, which are known as fluorescence quenchers and are rarely targeted with “turn-on” fluorescence probes. This result suggests that PDIs are favorable fluorophores for a “turn-on” fluorescence probe for paramagnetic transition metal ions because of their high oxidation potential.

Uncovering Individual Hydrogen Bonds in Rotaxanes by Frequency Shifts

B. Kirchner, C. Spickermann, W. Reckien, C. A. Schalley

J. Am. Chem. Soc. 2010, 132, 484-494

Full TextSupporting Information

We present a theoretical investigation of amide pseudorotaxane IR spectra in the harmonic approximation. In particular, we focus on the effect of axle substitution on the hydrogen bonds that are formed between axle and wheel. Two types of pseudorotaxanes are studied: one with the substituent affecting mostly the axle’s carbonyl group and one with the effect influencing primarily the amide NH group. Sizeable red shifts are predicted for the carbonyl stretching frequencies, and large red shifts for the NH stretching frequencies. For the wheel amide groups involved in hydrogen bonding merely with their NH hydrogens, a small shift is observed for the carbonyl stretch mode. A clear relation is observed between the NH stretch shifts and individual hydrogen bond energies. This is confirmed by correlations of the shared electron number with the NH stretch shift showing that this quantity can be taken as an indicator for individual hydrogen bond energies. Axle substitution influences the strengths of the individual hydrogen bonds which is again reflected in the NH stretch frequency shifts. A linear relationship of Hammett’s substituent parameters with the NH frequency shifts can be established.

Thematic series on supramolecular chemistry I

C. A. Schalley (ed.)

Beilstein J. Org. Chem. 2009

Thematic Series ContentsEditorial

The discovery of crown ethers in the 1960s awakened interest in studying weak and non-covalent molecular interactions, which are typical for supramolecular chemistry. Since then supramolecular chemistry has matured into a research field in its own right. Nowadays, important concepts that have been demonstrated by supramolecular chemistry include molecular self-assembly, self-sorting and recognition, templated synthesis or host-guest chemistry. These concepts have made supramolecular synthesis a powerful tool to construct large and complex chemical architecture from simple building blocks or to develop functional supramolecules. Among these are molecular switches, logic gates, molecular containers, elevators, valves and springs, supramolecular catalysts and many more. This exciting progress has been accompanied by a development of new methods which are able to monitor the sometimes quite fast dynamics of supramolecular systems.

Resorcinarene Podand with Amine Functionalized Side Arms - Synthesis, Structure and Binding Properties of a Neutral Anion Receptor

K. Salorinne, D. P. Weimann, C. A. Schalley, M. Nissinen

Eur. J. Org. Chem. 2009, 6151-6159

Full TextSupporting Information

The synthesis and structure of a neutral resorcinarene host bearing four amine-functionalized side arms is described. The anion binding properties were investigated in solution by 1H NMR spectroscopic titration and diffusion experiments and in the gas phase by mass spectrometric studies. It was observed that in solution 1:2 (host/guest) complexes were formed between the resorcinarene host and the basic fluoride and acetate anions, whereas in the gas phase 1:1 complexes with other anions (Cl–, HCOO–, NO3–, and BF4–) were detected additionally.

Mass Spectrometry and Gas-Phase Chemistry of Non-Covalent Complexes

Gas-Phase Chemistry

A. Springer, C. A. Schalley

Wiley, Hoboken/NJ | 2009-09-08

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Except as a method for the most basic measurements, mass spectrometry (MS) has long been considered incompatible with supramolecular chemistry. Yet, with today's methods, the disconnect between these two fields is not warranted. Mass Spectrometry and Gas-Phase Chemistry of Non-Covalent Complexes provides a convincing look at how modern MS techniques offer supramolecular chemists a powerful investigatory toolset. Bringing the two fields together in an interdisciplinary manner, this reference details the many different topics associated with the study of non-covalent complexes in the gas phase.

Highly dynamic motion of crown ethers along oligolysine peptide chains

D. P. Weimann, H. D. F. Winkler, J. A. Falenski, B. Koksch, C. A. Schalley

Nature Chem. 2009, 1 573-577

Full TextSupporting InformationHighlight by Jennifer S. Brodbelt

Molecular mobility has attracted considerable attention in supramolecular chemistry and biochemistry, but the simple question of whether a small molecule can move directly between different binding sites of a multitopic host without intermediate dissociation has not been addressed so far. To study such processes, we consider hydrogen/deuterium exchange experiments on a model system comprising complexes formed between 18-crown-6 and oligolysine peptides. Because direct binding-site hopping is indistinguishable in solution from a dissociation/reassociation mechanism, here we show that the high vacuum of a mass spectrometer offers a unique environment for probing such processes. The highly dynamic motion of crown ethers along oligolysine peptide chains proceeds mechanistically by a simultaneous transfer of the crown ether from its ammonium ion binding site to a nearby amino group together with a proton. Furthermore, the exchange experiments unambiguously reveal the zwitterionic structure of the 18-crown-6/oligolysine complexes, highlighting the versatility and potential of gas-phase experiments for investigating non-covalent interactions.

Dynamic Motion in Crown Ether-Dendrimer Complexes: A "Spacewalk" on the Molecular Scale

H. D. F. Winkler, D. P. Weimann, A. Springer, C. A. Schalley

Angew. Chem. Int. Ed. 2009, 48, 7246-7250; Angew. Chem. 2009, 121, 7382-7386

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Walking on the edge: Mass spectrometry aided by H/D exchange experiments provides evidence for a surprisingly quick movement of [18]crown-6 units across the periphery of POMAM dendrimers (see picture). In the diluted gas phase, dissociation/reassociation equilibria do not play any role. The movement of the crown ether units resembles that of astronauts on the outside of a space station.

The Halide Binding Behavior of 2-carbamoyl-7-ureido-1H-indoles: Conformational aspects

D. Makuc, Triyanti, M. Albrecht, J. Plavec, K. Rissanen, A. Valkonen, C. A. Schalley

Eur. J. Org. Chem. 2009, 4854-4866

Full TextSupporting Information

Indole-based anion receptors with an carboxamide unit in 2- and an urea in 7-position were prepared and found to bind halides (as well as acetate and nitrate) in chloroform solutions at room temperature. Investigations of the binding behaviour show that the receptor is selective for chloride. Surprisingly, the truncated receptor 3 without the 2-carbamoyl substituent shows the highest affinity for Cl–. Thorough 1H, 13C and 15N NMR investigations indicate different binding modes for acetate, nitrate and halides to the receptor 2. The observation of a major conformational change of this receptor during the binding of the halide ions leads to an understanding of the relative binding affinities of 3 > 4 > 2 for chloride. The results of the NMR study are supported by ab initio calculations. In addition, ESI FTICR MS competition experiments of the indole 2 and the quinoline 1 reveal the “self-aggregation” of the receptors and show that halides have a higher affinity to 2 than to 1.

Mass Spectrometric Study of Oligourea Macrocycles and of Their Anion Binding Behavior

T. Becherer, D. Meshcheryakov, A. Springer, V. Böhmer, C. A. Schalley

J. Mass Spectrom. 2009, 44 1338-1347

Full Text

Two series, one of tris-urea macrocycles and another of hexakis-urea macrocycles, are examined by (tandem) Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry with respect to their fragmentation patterns and anion binding properties. All macrocycles are based on two different building blocks, one of which is a very rigid xanthene unit and the other one is a more flexible diphenyl ether. The composition and the sequence of these units thus determine their flexibility. During the fragmentation of deprotonated oligourea macrocycles in the gas phase, one urea N[BOND]CO bond is cleaved followed by a scrambling reaction within the macrocycle structure. Consequently, fragments are observed that deviate from those that would be expected from the sequence of the subunits. Interesting anion binding properties involve the simultaneous recognition of two chloride anions by one of the hexakis-urea macrocycles, whose flexibility allows this host to form a double-helical structure. Flexibility also determines which of the hexameric receptors bears a high sulfate affinity. The interaction energy between some of the macrocycles and sulfate is high enough to even stabilize the intrinsically unstable sulfate dianion.

Giant Cyclo[n]thiophenes with Extended pi-Conjugation

F. Zhang, G. Götz, H. D. F. Winkler, C. A. Schalley, P. Bäuerle

Angew. Chem. Int. Ed. 2009, 48, 6632-6635; Angew. Chem. 2009, 121, 6758-6762

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Dual roles and nanorings: A new family of cyclic oligothiophenes with up to 35 thiophene units is synthesized by using platinum complexes in a dual role as template and as reactive center. The ease of ionization and aggregation of the new compounds indicates that they have promising properties for use as organic electronic materials of increased structural complexity.

Molecular Recognition between the Constituents of a Pseudorotaxane Studied by Scanning Tunneling Microscopy

M. Sokolowski, I. Kossev, W. Reckien, T. Felder, M. R. Kishan, C. A. Schalley

J. Phys. Chem. C 2009, 113, 12870-12877

Full Text

The formation of intermolecular complexes of two large molecules—a macrocycle and a semiaxle, which have been used in templated syntheses of amide rotaxanes—was studied by scanning tunneling microscopy (STM) and density functional theory (DFT). These experiments mimic the so-called “threading process”, which is based on intermolecular recognition and which is essential for the rotaxane synthesis in solution. First, ordered monolayers of a tetralactam macrocycle (TLM), i.e. the rotaxane wheel, are prepared on a Au(111) surface. Then, semiaxles (SA) are deposited on top of these ordered TLM layers at ca. 140 K. In solution, the SA molecule threads into the TLM cavity by formation of three hydrogen bonds between the amide groups of both molecules. On the Au(111) surface, the scenario is similar, although different in detail due to geometric restrictions given by the underlying Au(111) surface and conformational energy barriers due to the confinement of the TLM geometry in the ordered monolayer structure. Three distinct and defined adsorption sites of the SA molecules with respect to the TLM molecules exist. Notably, the population of these sites is assisted by interaction with the STM tip. Two sites are compatible with a structural model, in which the tail of the SA molecule binds into the TLM cavity, in one case with three H bonds, one to the terminal NH2 group of the SA and two to the central amide group. This SA—TLM adsorption complex formed at low temperatures is metastable and dissociates at higher temperatures. These results demonstrate the possibility to study intermolecular complex formation by STM.

Metallo-Supramolecular Nano-Spheres via Hierarchical Self-Assembly

H. T. Baytekin, B. Baytekin, A. Schulz, A. Springer, T. Gross, W. Unger, M. Artamonova, S. Schlecht, D. Lentz, C. A. Schalley

Chem. Mater. 2009, 21, 2980-2992

Full TextSupporting Information

A novel coordination oligo/polymer is synthesized by metal-directed self-assembly from equimolar amounts of the (dppp)M(OTf)2 precursor complexes (dppp = bis-(diphenylphosphino)-propane, OTf = triflate; M = PdII or M = PtII) and banana-shaped bidentate dipyridyl ligands. The assemblies were characterized by ESI mass spectrometry and NMR spectroscopy. The analysis of the cloudy suspension prepared by dissolving the coordination polymer in aqueous methanol solutions indicates nanosized spherical objects to form. Evidence for vesicle formation from these metallo-supramolecular oligomers comes from (cryogenic) transmission electron microscopy (TEM, cryo-TEM). Atomic force microscopy revealed stable nanospheres on hydrophilic mica and monolayer formation on hydrophobic highly oriented pyrolitic graphite (HOPG) substrates. On mica, also torus-shaped object were observed, which are rationalized by vesicles that opened during the drying procedure and released the internal solvent. Elemental analysis of the nanoassemblies by X-ray photoelectron spectroscopy (XPS) indicates uncoordinated and coordinated pyridines in the coordination polymers that form the nanospheres. Various control experiments using different metal centers and modified ligands support the conclusions.

Dendrimer Disassembly in the Gas Phase: A Cascade Fragmentation Reaction of Fréchet Dendrons

B. Baytekin, H. T. Baytekin, U. Hahn, W. Reckien, B. Kirchner, C. A. Schalley

Chem. Eur. J. 2009, 15, 7139-7149

Full TextSupporting Information

The mass spectrometric characterization of Fréchet-type dendrons is reported. In order to provide the charges necessary for electrospray ionization, dendrons bearing an OH group at the focal point can be deprotonated and observed in the negative ion mode. Alternatively, the corresponding bromides can be converted to quaternary ammonium ions that can easily be detected in the positive mode. If the latter ions are subjected to collision-induced dissociation experiments, a fragmentation cascade begins with the dissociation of the focal amine. The focal benzyl cation quickly decomposes in a fragmentation cascade from the focal point to the periphery until the peripheral benzyl (or naphthylmethyl) cations are formed. Five different mechanisms are discussed in detail, three of which can be excluded based on experimental evidence. The cascade fragmentation is reminiscent of self-immolative dendrimers.

Conformational Flexibility of Tetralactam Macrocycles and Their Intermolecular Hydrogen-Bonding Patterns in the Solid State

S. S. Zhu, M. Nieger, J. Daniels, T. Felder, I. Kossev, T. Schmidt, M. Sokolowski, F. Vögtle, C. A. Schalley

Chem. Eur. J. 2009, 15, 5040-5046

Full Text

Flexible rigidity: Tetralactam macrocycles of the Hunter type bear a rigid scaffold (see space-filling representation), but can still widely adapt to the properties of a guest molecule inside their cavities. X-ray crystal structures of a series of differently substituted macrocycles reveal a remarkably broad variety of intermolecular hydrogen-bonding patterns organizing the macrocycles in the crystals in intriguingly different ways. Despite their rigid scaffold, tetralactam macrocycles (TLMs) display a remarkable degree of conformational flexibility, as revealed by analysis of the corresponding X-ray crystal structures. This flexibility is not limited to the rotatability of the TLM amide groups but also applies to the m-xylene rings, and it thus has a great impact on the overall shape of the macrocycle cavity. The conformational properties of the TLMs give rise to a broad variety of intermolecular hydrogen-bonding patterns, including infinite ladders, an interesting catemer motif, and short C[BOND]H⋅⋅⋅O[DOUBLE BOND]C hydrogen bonds. These results are in accord with previous theoretical calculations, support a structural model proposed earlier for an interpretation of scanning tunneling microscopy images, and substantially contribute to the understanding of the adaptability of macrocyclic scaffolds, which is crucial for guest binding or templated syntheses with TLMs.

A Self-Sorting Scheme Based on Tetra-Urea Calix[4]arenes

Y. Rudzevich, V. Rudzevich, F. Klautzsch, C. A. Schalley, V. Böhmer

Angew. Chem. Int. Ed. 2009, 48, 3867-3871; Angew. Chem. 2009, 121, 3925-3929

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Size and shape do matter: When dimerized in nonpolar solvents, an equimolar mixture of eleven tetra-urea calix[4]arenes with different wide-rim substituents self-sorts into only six out of 35 different homo- and heterodimers (see picture). Since the calixarene scaffold and the four urea units are the same in all cases, the self-sorting process is driven only by the cooperative action of steric requirements and stoichiometry.

Integrative Self-Sorting Is A Programming Language for High Level Self-Assembly

W. Jiang, C. A. Schalley

Proc. Natl. Acad. Sci. USA 2009, 106, 10425-10429

Full TextSupporting Information

Starting from the basis of a simple 4-component self-sorting system of crown ethers and ammonium ions, we design 6 building blocks in which 2 identical or different binding sites are incorporated. These building blocks can be mixed in many different ways to yield quite distinctly different pseudorotaxane assemblies. The self-sorting process integrates all building blocks in specific places so that this approach permits us to exert positional control and can widely influence the resulting assemblies with respect to the details of their structures. At maximum, we report quadruply interlocked species with up to 5 subunits that form specific assemblies. Although NMR methods are limited to the analysis of simpler complexes, ESI-MS and, in particular, tandem mass spectrometry is highly useful to analyze the assemblies' connectivities.

Formation of Chiral Aggregates of Tetralactam Macrocycles on the Au(111) Surface

I. Kossev, T. Felder, C. A. Schalley, F. Vögtle, M. Sokolowski

Springer, Heidelberg/Germany | 2009-03-03

Appeared In: Physics and Engineering of New Materials, D. T. Cat, A. Pucci, K. R. Wandelt (eds.)

Springer Proc. Phys. 2009, 127, 235-245

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Monolayers of a large tetralactam macrocycle were prepared by vacuum sublimation on the Au(111) surface and investigated by scanning tunnelling microscopy. The macrocycles form three different highly ordered monolayer structures α, β, and η. The α and β structure are stable at room temperature and can be understood as two dimensional networks which are held together by hydrogen bonds between the next neighbour molecules. These structures were described in detail before [Kossev et al., Adv. Mat. 17, 513 (2007)]. The third structure, which is described here, is only observed after heating at 400 K and rapid cooling to low temperatures. It consists of chiral aggregates, composed of three molecules. These aggregates are either left or right handed. The surface is covered by a racemic mixture of long range ordered domains with either left or right handed aggregates.

Self-assembly of heterodinuclear triple-stranded helicates: Control by coordination number and charge

M. Albrecht, Y. Liu, S. S. Zhu, C. A. Schalley, R. Fröhlich

Chem. Commun. 2009, 1195-1197

Full TextSupporting InformationCrystal Structure DataSelected as "Hot Paper"

Acylhydrazones of 2,3-dihydroxybenzaldehyde are easily accessible and afford heterodinuclear helicates; the self-assembly is specific when the resulting coordination compounds are neutral in charge.

A double intramolecular cage contraction within a self-assembled metallo-supramolecular bowl

B. Brusilowskij, S. Neubacher, C. A. Schalley

Chem. Commun. 2009, 785-787

Full TextSupporting InformationSelected as "Hot Paper"

When isolated in the high vacuum of an ESI-FTICR mass spectrometer, bowl-shaped metallo-supramolecular M6L4 assemblies undergo a surprising and mechanistically interesting intramolecular double cage contraction to yield smaller M3L2 cages.

Templated Synthesis of Interlocked Molecules

Bottom Up Nanofabrication

C. A. Schalley, J. Illigen

American Scientific Publishers, Valencia/USA | 2009-01-02

Appeared In: Bottom-up Nanofabrication: Supramolecules, Self-Assemblies, and Organized Films, K. Ariga, H. S. Nalwa (eds.)

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Bottom-up Nanofabrication is the first and only reference work ever published to cover all aspects of the bottom-up nanofabrication. The bottom-up nanofabrication approaches are biology inspired which include building up functional nanostructures starting from basic atoms or molecules. The key point is the identification and manipulation of potential atoms and molecules and forming their based well-defined nanostructures in order to tailor and improve desired properties of used components through fabrication strategies. Process also involves control of molecular structures on the surface. This 6-Volume book brings together under a single cover all aspects of the bottom-up nanofabrication including all types of processing and fabrication methodologies, chemical and electrochemical synthesis, physical methods, nucleation and growth phenomena, functional supramolecular nanostructures, molecular self-assemblies and nanostructured films, spectroscopic characterization, electronic, optical, nonlinear optical, and magnetic properties, dynamic processes, molecular interactions, molecular design and computational modeling, and commercial applications of self-assembled nanostructures. This book covers interdisciplinary topics associated with the chemistry, physics, engineering and biology of supramolecular nanostruuctures, self-assemblies, and organized films. The six-volume set contains over 110 state-of-the-art review chapters written by internationally renowned experts from around the world.

Templates in Chemistry III

Templates in Chemistry III

The first two volumes in this “Templates in Chemistry” series have focused on templates that control solution-phase reactions. Among the templates discussed in these two volumes were convex and concave templates that mediate the formation of (macro)cyclic molecules and mechanically bound molecules with their intriguing intertwined topology. Also, three-dimensional templates that are used to imprint polymers and that organize compounds in the solid state for predestined reactions have been included in the earlier volumes. In the present volume, we extend the template topology to surfaces that act as matrices for the controlled growth of two-dimensional arrays. Naturally, the typical methods for the characterization of surfaces such as scanning probe microscopy are prominently represented in this volume. Different lateral interactions such as coordinative bonds or hydrogen bonding play a major role in assembling the 2D networks on surfaces in addition to the interaction of the samples with the underlying substrates. Many phenomena that are also encountered in solution can be directly visualized on surfaces: Reversible self-assembly processes lead to the formation of large structures through multiple recognition of small building blocks and culminate in the engineering of crystals in two dimensions. Self-sorting processes drive the formation of highly ordered arrays through the geometric fit of the available components. Either the surface itself is the template, for example, when clusters grow on metal oxide films, or colloidal templates control the formation of macroporous networks on the substrate. This volume highlights a selection of actual complementary aspects of surface templates.

Hierarchical Self-Assembly of Metallo-Supramolecular Nano-Spheres

H. T. Baytekin, B. Baytekin, A. Schulz, C. A. Schalley

Small 2009, 5, 194-197

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Supramolecular co-ordination polymers are formed in organic solvents by metal-directed self-assembly using 1:1 mixtures of (dppp)M(OTf)2 complexes (M = Pd(II) or Pt(II), dppp = bis-(diphenylphosphino)- propane, OTf = triflate) and a non-linear bidentate dipyridyl-substituted ligand. In aqueous methanol solutions, hollow metallo-supramolecular vesicles and smaller, completely filled nanospheres form.

A Modular "Toolbox" Approach to Flexible Branched Multi-Macrocyclic Hosts as Precursors for Multiply Interlocked Architectures

B. Baytekin, S. S. Zhu, B. Brusilowskij, J. Illigen, J. Ranta, J. Huuskonen, K. Rissanen, L. Kaufmann, C. A. Schalley

Chem. Eur. J. 2008, 14, 10012-10028

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Tetralactam macrocycles can be functionalized by a variety of cross-coupling reactions. A modular “toolbox” strategy is presented that allows 1) several tetralactam macrocycles to be covalently connected with each other or with a central spacer, 2) the macrocycles to be substituted with or connected to different chromophores, and 3) metal-coordination sites to be attached to the macrocycles. With this approach a series of different oligo-macrocyclic hosts was obtained with great structural diversity and enormous potential for further functionalization. Rotaxanes made on the basis of these macrocycles have been synthesized to demonstrate their utility in building more complex supramolecular architectures.

Integrative Self-Sorting: Construction of a Cascade-Stoppered Hetero[3]Rotaxane

W. Jiang, H. D. F. Winkler, C. A. Schalley

J. Am. Chem. Soc. 2008, 130, 13852-13853

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In this Communication, a competing self-sorting system containing benzo-21-crown-7, dibenzo-24-crown-8 and two secondary ammonium salts is constructed, which is then modified to achieve a hetero[3]pseudorotaxane with a specific sequence of wheels. With these two systems, we successfully demonstrate the concept of integrative self-sorting, and their relation. Furthermore, based on this self-sorting scheme, a hetero[3]rotaxane with an efficient stopper cascade has been synthesized.

Characterization of Self-Assembled Metallodendrimers in Solution, in the Gas Phase, and at Air/Solid Interfaces

H. T. Baytekin, M. Sahre, A. Rang, M. Engeser, A. Schulz, C. A. Schalley

Small 2008, 4, 1823-1834

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It is found that 4,4′-bipyridines functionalized in their 3,3′-positions with Fréchet dendrons of 0th to 3rd generation self-assemble with (dppp)M(II) triflates (dppp: bis-(diphenylphosphino)propane; M = Pd, Pt) into metallo–supramolecular squares. They bear a nanometer-sized cavity inside an unpolar dendritic shell. A total of eight amide groups decorate the rims of the cavity connecting the dendrons to the square. Evidence for their formation up to the third generation comes from ESI-FTICR mass spectrometry and NMR experiments. Based on these results, the presence of significant amounts of other polygons or open-chain oligomers can be excluded. Exchange processes have been studied by variable-temperature NMR spectroscopy and by following the ligand exchanges between different squares by mass spectrometry. The ligand exchange is much slower for the Pt(II) squares as compared to their Pd(II) analogs. Visualization of films of these dendrimers using atomic force microscopy (AFM) provides information on their molecular dimensions. After deposition of a square monolayer on the surface, a slow reorganization within this layer is observed which leads to the formation of “tower-like” aggregates and multi-layer formation. The interplay of interactions between the dendrimers and the surface and interactions between different dendrimers are invoked to rationalize the observations.

Self-assembling squares with amino acid-decorated bipyridines: heterochiral self-sorting of dynamically interconverting diastereomers

A. Rang, M. Nieger, M. Engeser, A. Lützen, C. A. Schalley

Chem. Commun. 2008, 4789-4791

Full TextSupporting InformationCrystal Structure Data

Heterochiral self-sorting into only one stereoisomer is observed when metallo-supramolecular squares self-assemble from amino acid-substituted, dynamically interconverting bipyridine stereoisomers and dpppM(OTf)2 corners (M = Pd, Pt).

Metallo-Supramolecular Self-Assembly: the Case of Triangle-Square Equilibria

T. Weilandt, R. W. Troff, H. Saxell, K. Rissanen, C. A. Schalley

Inorg. Chem. 2008, 47, 7588-7598

Full TextSupporting InformationCrystal Structure Data

For the efficient self-assembly of metallo-supramolecular complexes, not only reversibility is required but also two other properties have to be controlled as well: (i) The right binding sites need to be programmed into the building blocks at the appropriate positions. (ii) The building blocks must be rigid enough to support the geometrical arrangement and to avoid the unfavorable entropy effects connected with the conformational fixation of flexible molecules. A series of different bis-pyridyl ligands is reported which self-assemble with (dppp)M(OTf)2 complexes (dppp = 1,3-bis-(diphenylphosphino)propane; M = PdII, PtII) to yield squares and/or triangles as the products. Enthalpic contributions (higher strain in the triangle) and entropic contributions (higher number of triangles from the same building blocks) determine the equilibrium. The effects of concentration, temperature, and solvent properties on the equilibrium have been studied. To characterize the complexes under study, a combination of 1H, 31P, and diffusion-ordered NMR spectroscopy, electrospray-ionization Fourier-transform ion-cyclotron-resonance mass spectrometry, and X-ray crystallography is needed. Variable-temperature NMR spectroscopy provides evidence for fast ligand-exchange processes occurring for the Pd complexes, while the Pt complexes exchange ligands much more slowly.

A Topolgical View on Isomeric Dendrimers

D. Schubert, M. Corda, O. Lukin, B. Brusilowskij, E. Fiskin, C. A. Schalley

Eur. J. Org. Chem. 2008, 4148-4156

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A topological approach to the analysis of isomeric dendrimers by considering their molecular graphs has been applied to correlate various properties of a series of new sulfonimide-based isomeric dendrimers with their structures. According to this approach isomeric dendrimers are referred to as either isographic, that is, having the same graph, or non-isographic, that is, having different graphs. Six sets of non-isographic isomeric dendrimers with four to ten peripheral groups and one set of three isographic isomers with eight peripheral groups have been designed, synthesized and investigated with regard to their melting, solubility, separation, NMR spectroscopic and mass spectrometric characteristics. The results are discussed in the light of the graph-dependent and-independent properties of the isomers.

Host-Guest Chemistry of Self-Assembling Supramolecular Capsules in the Gas Phase

D. P. Weimann, C. A. Schalley

Supramol. Chem. 2008, 20, 117-128

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Supramolecular capsules composed of two or more self-complementary monomers held together by hydrogen bonds and other weak interactions such as cation–π and C–H–π interactions are able to encapsulate neutral as well as positively charged guests. This mini review will highlight work on transferring such non-covalently bound aggregates to the gas phase by soft ionisation methods, such as electrospray ionisation, and investigating structure and encapsulation properties under solvent-free conditions. These analyses reveal exact information about complex stoichiometry as well as about structure and stability of capsules composed of multiple building blocks. The review is organised such that each type of capsule introduced contributes a new aspect to the overall picture.

Synthesis of Axially Chiral 4,4'-Bipyridines and their Remarkably Selective Self-Assembly into Chiral Metallo-Supramolecular Squares

A. Rang, M. Engeser, N. M. Maier, M. Nieger, W. Lindner, C. A. Schalley

Chem. Eur. J. 2008, 14, 3855-3859

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A preferred choice: Prochiral 3,3′,5,5′-tetramethyl-4,4′-bipyridine can be converted into two types of axially chiral 4,4′-bipyridine compounds, separable into enantiomers by chiral HPLC. The obtained enantiopure bipyridines were sufficiently stable in solution to be used in the self-assembly of chiral metallo-supramolecular squares, which reveal a remarkable preference for one of ten possible structures

Phosphazene vs. diazaphospholene PN-bond cleavage in spirocyclic cyclodiphosphazenes

S. Burck, D. Gudat, M. Nieger, C. A. Schalley, T. Weilandt

Dalton Trans. 2008, 3478-3485

Full TextCrystal Structure Data

Thermolysis of 2-azido-1,3,2-diazaphospholenes offers access to novel and rare spirocyclic cyclodiphosphazenes. The spectroscopic data and X-ray structure of one representative of the 2-azido-1,3,2-diazaphospholenes reveals an ionic bonding situation explaining sufficiently its rather high thermal stability. The cyclodiphosphazenes were characterised by NMR, mass spectrometry, and X-ray diffractometry. The results of ESI-FT-ICR studies demonstrate the potential of these compounds to undergo reductive elimination at a phosphazene unit via [1,4]-cycloreversion of a λ5-diazaphospholene ring, as well as symmetrical cleavage of the P2N2-unit. The unexpected inclusion of benzene in the crystal of one of the cyclodiphosphazenes was interpreted in terms of molecular recognition. Chemical reaction studies comprise the proof of double N-protonation at a phosphazene ring, and hydrolytic degradation via selective cleavage of a phosphazene P–N bond.

Fragmentation reactions of singly and doubly protonated thiourea- and sugar-substituted cyclams and their transition-metal complexes

T. Felder, A. Röhrich, H. Stephan, C. A. Schalley

J. Mass Spectrom. 2008, 43, 651-663

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Cyclam macrocycles tetrasubstituted with amino-, thiourea-, and sugar-terminated side chains are ionized by electrospray ionization mass spectrometry (ESI-MS) as singly or doubly protonated species or as transition-metal complexes. Their fragmentation behavior is examined in a Fourier-transform ion-cyclotron-resonance (FT-ICR) mass spectrometer by collision-induced dissociation (CID) experiments. Typically, fragmentation occurs within the side chains through a number of different 1,2-elimination reactions irrespective of the absence or presence of a transition metal ion such as Co2+, Ni2+, or Zn2+. A remarkable exception is Cu2+, which induces ring cleavage reactions. This is traced back to an electron transfer from the cyclam nitrogen atoms to the Cu2+ ion. The electron transfer creates a cation-radical within the macrocycle, which induces typical fragmentation reactions such as α-cleavages that lead to fragmentation within the macrocycle. This interpretation is in line with fragmentation experiments on unsubstituted cyclam and its complexes.

Anion Binding to Resorcinarene-Based Cavitands: The Importance of C-H•••Anion Interactions

S. S. Zhu, H. Staats, K. Brandhorst, J. Grunenberg, F. Gruppi, E. Dalcanale, A. Lützen, K. Rissanen, C. A. Schalley

Angew. Chem. Int. Ed. 2008, 47, 788-792; Angew. Chem. 2008, 120, 800-804

Full Text (International Edition)Full Text (German Edition)Supporting InformationHighlighted in SynFacts

Anions rather than cations: While resorcinarenes bind cations through cation–π interactions, methylene-bridged cavitands surprisingly bind anions through C[BOND]H⋅⋅⋅anion interactions with the acetal protons at the wider rim (see model of the complex in the scheme). Mass spectrometry, in agreement with theory, provides evidence for anion binding on the concave side of the cavitand bowl.

How Can Rotaxanes be Modified by Varying Functional Groups at the Axle? A Combined Theoretical and Experimental Analysis of Thermochemistry and Electronic Effects

C. Spickermann, T. Felder, C. A. Schalley, B. Kirchner

Chem. Eur. J. 2008, 14, 1216-1227

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We present theoretically as well as experimentally determined thermochemical data of the non-covalent interactions in different axle-substituted pseudorotaxanes. The overall interaction energy lies in the region of 35 kJ mol−1, independent of the substitution pattern at the axle. Because rearrangement energies of 7 and 3 kJ mol−1 are required for wheel and axle, respectively, the sum of the net interactions of individual non-covalent bonds must exceed 10 kJ mol−1 to achieve a successful host–guest interaction. The geometrical analysis shows three hydrogen bonds, and the close inspection of the individual dipole moments as well as the individual hydrogen bonds reveals trends according to the different functional groups at the axle. The individual trends for the different hydrogen bonds almost lead to a cancellation of the substitution effects. From solvent-effect considerations it can be predicted that the pseudorotaxane is stable in CHCl3 and CH2Cl2, whereas it would dethread in water. Comparing experimentally and theoretically calculated Gibbs free enthalpies, we find reasonable agreement if an exchange reaction of one solvent molecule instead of the direct formation reaction is considered.

The Wittig Reaction as a Key Step in the Preparation of Triangular Ligands for the Self-Assembly of Molecular M4L4 Tetrahedra

M. Albrecht, S. Burk, P. Weis, C. A. Schalley, M. Kogej

Synthesis 2007, 23, 3736-3740

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Triangular ligands based on a central C3-symmetric backbone, ethylene linkages, and catechol ligand units, are prepared by a Wittig approach. The ligands form, in metal-directed self-assembly­ processes, tetrahedral coordination compounds of the composition M4L4.

Synthesis of Fréchet-type Tetramethylated Resorcarene Dendrimers

M. Luostarinen, M. Nissinen, H. Lähteenmäki, H. Mansikkamäki, K. Salorinne, C. A. Schalley, K. Rissanen

J. Inclusion Phenom. Macrocyc. Chem. 2007, 58, 71-80

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Tetramethylated resorcarenes are good core molecules for the synthesis of Fréchet-type dendrimers. Fréchet-type dendron bromides were synthesised on a gram scale, and were easily attached under phase-transfer conditions to the tetramethylated resorcarene core to obtain pure products in high yields.

Mixed-Metal (Platinum, Palladium), Mixed Pyrimidine (Uracil, Cytosine) Self-Assembling Metallacalix[n]arenes: Dynamic Combinatorial Chemistry with Nucleobases and Metal Species

E. G. Baradjí, E. Freisinger, B. Costisella, C. A. Schalley, W. Brüning, M. Sabat, B. Lippert

Chem. Eur. J. 2007, 13, 6019-6039

Full TextSupporting Information

Reactions between the mononuclear mixed-nucleobase complex [Pt(en)(UH-N1)(CH2-N3)]+ (1; en: ethylenediamine; UH-N1: uracil monoanion bonded through the N1 atom; CH2-N3: neutral cytosine bonded through the N3 atom) and [PdII(en)] or [PdII(2,2′-bpy)] (2,2′-bpy: 2,2′-bipyridine) lead to libraries of compounds of different stoichiometries and different connectivities. In these compounds, the palladium entity binds to or cross-links either the N3 sites of uracil and/or the N1 sites of cytosine, following deprotonation of these positions to give uracil dianions (U) and cytosine monoanions (CH). Cyclic species, which can be considered as metallacalix[n]arenes, have been detected in several cases, with n being 4 and 8. The complexity of the compounds formed not only results from the possibility of the two different nucleobases in building block 1 engaging in different connectivities with the Pd entities, but also from the potential for the formation of oligomers of different sizes and different conformations; in the case of cyclic tetranuclear Pt2Pd2 species, this can, in principle, lead to the various arrangements (cone, partial cone, 1,2-alternate, 1,3-alternate) known from calix[4]arene chemistry. A further complication arises from the fact that, depending on the mutual orientation of the exocyclic groups of the two nucleobases (O2 and O4 of uracil, O2 and N4 of cytosine), these sites can be engaged in additional chelation of [PdII(en)] and [PdII(2,2′-bpy)]. Thus, penta-, hexa-, and octanuclear complexes, Pt2Pd3, Pt2Pd4, and Pt2Pd6, derived from cyclic Pt2Pd2 tetramers have been isolated and characterized.

Protonation of Tris(iminocatecholate) Complexes of Gallium(III) and Titanium(IV)

M. Albrecht, S. Burk, R. Stoffel, A. Lüchow, R. Fröhlich, M. Kogej, C. A. Schalley

Eur. J. Inorg. Chem. 2007, 1361-1372

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The coordination behaviour of the chiral tren-type tripodal ligand 3-H6 with gallium(III) and titanium(IV) ions was investigated and it was found that only mononuclear complexes are formed. In the presence of Na+, one of the cations acts as a template and is bound inside the “cap” of [Na(3)M]n– (M = Ga, n = 2; M = Ti, n = 1). If no such cation is present, three protons are bound to the iminocatechol units in order to prevent repulsion between lone pairs at nitrogen and oxygen. To minimize the charge repulsion between the protonated imines of [H3(3)M]n+ (M = Ga, n = 0; M = Ti, n = 1), an enaminone/chinomethine mesomeric structure becomes important. The adoption of this unusual mesomeric form by the iminocatecholates was supported by computational methods. In further investigations, we found, that the addition of protons (H2SO4) to the dinuclear (catecholimine)titanium helicate K4[(4)3Ti2] leads to protonation of the imine part of the iminocatechol. An excess of acid results in the hydrolysis of the complexes as well as of the imine units.

Highly Ordered 2D Hydrogen-Bonded Structures of a Tetralactam Macrocycle on the Au(111) Surface

I. Kossev, W. Reckien, B. Kirchner, T. Felder, M. Nieger, C. A. Schalley, F. Vögtle, M. Sokolowski

Adv. Funct. Mater. 2007, 17, 513-519

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Monolayers of a tetralactam macrocycle, which are commonly used as building blocks in the synthesis of rotaxanes or catenanes, are deposited on a Au(111) surface by using vapor deposition. Due to self-organization, 2D highly ordered supramolecular networks form. From scanning tunneling microscopy (STM) and concomitant density-function theory calculations, two structurally different phases are found. In both phases, pairs of hydrogen bonds between the amide groups of next-neighbor macrocycles are responsible for the structural arrangement of the macrocycles. The structure of both phases differs from that of bulk lattice planes, which reveals that the Au(111) surface acts as a template for the growth of the specific 2D structures. These networks of tetralactam macrocycles possibly open a route to study mechanical interlocking processes or guest/host interactions of the molecules in further detail by using STM.

Analytical Methods in Supramolecular Chemistry, 1st edition

Analytical Methods, 1st ed.

An overview of the techniques used to examine supramolecular aggregates from a methodological point of view, it is a survey of useful modern analytical methods for understanding supramolecular chemistry, from NMR to single-molecule spectroscopy, from electron microscopy to extraction methods. A definitive study of this field touching many interdisciplinary areas such as molecular devices, biology, bioorganic chemistry, material science, and nanotechnology.

Synthesis of Chiral Resorcinarene-based Hosts and a Mass Spectrometric Study of their Chemistry in Solution and the Gas Phase

N. K. Beyeh, D. Féher, M. Luostarinen, C. A. Schalley, K. Rissanen

J. Inclusion Phenom. Macrocyc. Chem. 2006, 56, 381-394

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The syntheses and characterization of new chiral tetrabenzoxazine and tetrakis-(dialkylaminomethyl) resorcinarenes can be achieved through the reaction of resorcinarene with chiral amines and formaldehyde. In order to examine their host–guest chemistry, chiral quaternary ammonium guests were synthesized by methylation of different amines and amino acid methyl esters through a reductive methylation followed by addition of methyl iodide. Subsequent anion exchange of the iodide against tetraphenylborate helps to improve solubility of the salts in organic solvents. After characterization in solution, mass spectrometry is used to examine the resorcinarenes’ chemistry in the gas phase. Interesting implications of the fragmentation behavior for their solution phase chemistry arise, for which a first example is presented. Ammonium ion binding is indicated by mass spectrometry. Nevertheless, chiral recognition between the chiral hosts and pseudoracemic 1:1 mixtures of appropriately deuterium-labeled chiral guest cations is however not observed.

First gravimetric detection of ethene utilizing metallo-supramolecular macrocycles as sensor-active substances

B. Graewe, A. Rang, C. A. Schalley, J. Haubrich, J. Bargon

Sensors Act. B 2006, 119, 302-307

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For the first time, the real-time gravimetric detection of the light-weight analyte ethene with quartz microbalance sensors becomes possible in the range of 3–600 ppm, when metallo-supramolecular macrocycles are used as sensor-active layers. Thermodynamic and kinetic measurements as well as theoretical simulations indicate a weak and reversible binding of ethene at the metal centers.

Oligothiophene-Based Catenanes: Synthesis and Electronic Properties of a Novel Conjugated Topological Structure

M. Ammann, M. Wilde, G. Götz, E. Mena-Osteritz, A. Rang, C. A. Schalley, P. Bäuerle

Angew. Chem. Int. Ed. 2007, 46, 363-368; Angew. Chem. 2007, 119, 367-372

Full Text (International Edition)Full Text (German Edition)

Make it a double: A double metal template strategy has been used to form a “π-conjugated catenane” consisting of intertwined macrocycles with oligothienyl and phenanthroline units (see calculated structure). The optical and redox properties as well as the structural and conformational analyses give clear evidence that the two macrocycles in the catenane influence each other by through-space donor–acceptor interactions.

Reactivity of self-assembled supramolecular complexes in the gas phase: A supramolecular neighbor group effect

M. Engeser, A. Rang, M. Ferrer, A. Gutiérrez, H. T. Baytekin, C. A. Schalley

Int. J. Mass Spectrom. 2006, 255, 185-194

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Infrared multi-photon dissociation (IRMPD) spectra of mass-selected, self-assembled supramolecular squares and their gas-phase fragments have been recorded in a Fourier-transform ion-cyclotron resonance (FT-ICR) mass spectrometer. The squares have been transferred into the gas phase by electrospray ionization (ESI) under very soft ionization conditions resulting in a series of signals for intact squares in different charge states by stripping off two or more counter ions. The fragmentation patterns of the squares strongly depend on the parent ion's charge state. For species with a lower number of charges, expulsions of edge ligands prevail, whereas charge separation pathways dominate the dissociation pathways of more highly charged species.

Mass Spectrometry as a Tool in Dendrimer Chemistry: From Self-Assembling Dendrimers to Dendrimer Gas-Phase Host-Guest Chemistry

C. A. Schalley, B. Baytekin, H. T. Baytekin, M. Engeser, T. Felder, A. Rang

J. Phys. Org. Chem. 2006, 19, 479-490

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Mass spectrometry has played a significant role in dendrimer chemistry, because it serves as an excellent analytical means to determine the purity and analyze the nature of defects even for higher generations. However, a mass spectrometer can also be used as a laboratory to study isolated dendrimer molecules in the gas phase or their host–guest complexes. Since the properties of molecules under environment-free conditions are often quite different from those in solution, their gas-phase chemistry provides valuable new insight into properties which cannot easily be studied in solution. This article summarizes some of our work on characterizing self-assembling metallo-supramolecular dendrimers, on analyzing ionization artifacts, on the differentiation between several, sometimes even isomeric defects through tandem MS experiments, and finally on the analysis of a surprisingly clear dendritic effect occurring in the fragmentation of dendritic host–guest complexes.

Dicatechol cis-dioxomolybdenum(VI): a building block for a lithium cation templated monomer-dimer equilibrium

M. Albrecht, M. Baumert, J. Klankermayer, M. Kogej, C. A. Schalley, R. Fröhlich

Dalton Trans. 2006, 4395-4400

Full TextCrystal Structure Data

Dinuclear helicate-type complexes form from 3-carbonyl catechol derivatives and MoO2 units. The two monomeric subunits are non-covalently bound through templating Li+ ions. The monomeric and dimeric complexes as well as a hydrolysis product have been investigated in the solid state by X-ray crystallography, in solution by NMR experiments, and in the gas phase by ESI mass spectrometry. Dimerization constants can easily be extracted from NMR experiments. A particular property of these complexes is the mutual homochiral recognition between the two halves of the dimers which only occurs between homochiral monomers.

Flying Capsules: Mass Spectrometric Detection of Pyrogallarene and Resorcinarene Hexamers

N. K. Beyeh, M. Kogej, A. Åhman, K. Rissanen, C. A. Schalley

Angew. Chem. Int. Ed. 2006, 45, 5214-5218;Angew. Chem. 2006, 118, 5339-5342

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Survival of the fittest: Only those hexameric pyrogallarene and resorcinarene capsules which contain a guest with the right size, shape, and symmetry (see picture) survive the electrospray ionization process that makes them detectable by mass spectrometry. Tandem MS experiments with mass-selected ions provide evidence that the hexamers retain their capsular structure even in the gas phase.

Mass spectrometric studies of non-covalent compounds: why supramolecular chemistry in the gas phase?

B. Baytekin, H. T. Baytekin, C. A. Schalley

Org. Biomol. Chem. 2006, 4, 2825-2841

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Supramolecular chemistry has progressed quite a long way in recent decades. The examination of non-covalent bonds became the focus of research once the paradigm that the observed properties of a molecule are due to the molecule itself was revised, and researchers became aware of the often quite significant influence of the environment. Mass spectrometry and gas-phase chemistry are ideally suited to study the intrinsic properties of a molecule or a complex without interfering effects from the environment, such as solvation and the effects of counterions present in solution. A comparison of data from the gas phase, i.e. the intrinsic properties, with results from condensed phase, i.e. the properties influenced by the surroundings of the molecule, can consequently contribute significantly to the understanding of non-covalent bonds. This review provides insight into the often-underestimated power of mass spectrometry for the investigation of supramolecules. Through example studies, several aspects are discussed, including determination of structure in solution and the gas phase, ion mobility studies to reveal the formation of zwitterionic structures, stereochemical issues, analysis of reactivity of supramolecular compounds in the condensed and in the gas phase, and the determination of thermochemical data.

Designer Dendrimers: Branched Oligosulfonimides with Controllable Molecular Architectures

O. Lukin, V. Gramlich, R. Kandre, I. Zhun, T. Felder, C. A. Schalley, G. Dolgonos

J. Am. Chem. Soc. 2006, 128, 8964-8974

Full TextSupporting InformationCrystal Structure DataCrystal Structure Data

The synthesis of “designer” dendrimers and dendrons with sulfonimide units at every branching point is reported. The synthesis is based on a series of (regio)selective functionalization reactions of amines and sulfonamides allowing precise control of the dendrimers' shape, the number of branches in each generation, and their peripheral decoration with functional groups. In principle, structurally different branches can be incorporated at any position within the dendrimer structure at will. Structurally perfect symmetrical and two-faced “Janus”-type dendrimers, as well as dendrimers and dendrons with intended interstices were synthesized on a preparative scale and fully characterized. Oligosulfonimide dendrons of various generations bearing an aryl bromide functional group at their focal points were attached to a p-phenylene core with the aid of Suzuki cross-coupling reactions resulting in dendrimers with photoactive terphenyl cores. The structure and the high purity of all dendritic sulfonimides were confirmed by means of 1H and 13C NMR, electrospray ionization mass spectrometry (ESI-MS), and elemental analysis. The utility of MALDI-TOF mass spectrometry for the analytical characterization of these dendrimers was evaluated in comparison to electrospray ionization. Two model branched oligosulfonimides were characterized in the solid state by single-crystal X-ray analysis. Reaction selectivities and conformation of sulfonimide branching points were rationalized by DFT calculations.

Helicate, Macrocycle, or Catenate: Dynamic Topological Control over Subcomponent Self-Assembly

M. Hutin, C. A. Schalley, G. Bernardinelli, J. R. Nitschke

Chem. Eur. J. 2006, 12, 4069-4076

Full TextSupporting Information

The aqueous reaction between equimolar amounts of 2-(2-(2-aminoethoxy)ethoxy)ethanamine, 1,10-phenanthroline-2,9-dialdehyde and copper(I) produced a dimeric helical macrocycle in quantitative yield. This ring could also be generated by the addition of two equivalents of the diamine to an acyclic helicate containing four mono-imine residues: A transimination occurred, the chelate effect being implicated as a driving force. In the case of a helicate containing mono-imines derived from anilines, the substitution of diamine for monoamine was reversible upon lowering the pH. The aliphatic diamine was protonated at a higher pH than the arylamine, which left the arylamine free for incorporation instead of the alkyl diamine. This reaction thus opened the possibility of switching between closed macrocyclic and open helicate topologies by changing the pH. An additional closed topology became accessible through the use of a diamine that incorporates two rigid phenylene spacer groups between a flexible chain and the imine-forming nitrogen atoms. The resulting catenate consists of a pair of topologically interlinked macrocycles. The presence of the phenylene groups appeared to dictate the topology of the final product, making the formation of a single macrocycle energetically disfavoured.

How useful is mass spectrometry for the characterization of dendrimers? "Fake defects" in the ESI and MALDI mass spectra of dendritic compounds

B. Baytekin, N. Werner, F. Luppertz, M. Engeser, J. Brüggemann, S. Bitter, R. Henkel, T. Felder, C. A. Schalley

Int. J. Mass Spectrom. 2006, 249, 138-148

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Mass spectrometry is usually considered to be one of the very few methods, if not the only reliable one, to investigate dendrimers with respect to the presence and the nature of structural defects. In particular, MALDI mass spectrometry is used routinely for dendrimer characterization. The results reported here emerge from a comparison of ESI and MALDI mass spectra of different dendritic molecules. Two examples are presented for the fact that both methods have their limitations. In the first example, ESI mass spectra of POPAM dendrimers indicate the presence of a high abundance of a new type of defects which are not found in neither the MALDI mass spectra nor 1H or 13C NMR spectra. The second example deals with dendrimers bearing sulfonamide groups in their periphery. Here, the ESI mass spectra provide evidence for sample purity, while MALDI produces signals for defects, which seem to be generated during the synthesis. However, thermal reactions occurring during the ionization within the matrix are responsible for these defects, while synthesis proceeds cleanly to the desired products. When substituted in their periphery with dansyl groups, which absorb light at the wavelength of the MALDI laser, additional fragments are generated through light-induced cleavages of dansyl groups. Consequently, mass spectral data on dendrimer purity needs to be interpreted with care and may be misleading in the sense that falsely negative results are obtained.

A Synthetic Approach Towards Interlocked pi-Conjugated Macrocycles

M. Amman, P. Bäuerle, A. Rang, C. A. Schalley

Eur. J. Org. Chem. 2006, 8, 1940-1948

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A method for the synthesis of interlocked π-conjugated macrocycles is described. Starting from 2,9-bis(oligothienyl)[1,10]phenanthrolines, (trimethylsilyl)acetylene groups were introduced at the terminal thiophene rings by selective iodination and subsequent Sonogashira–Hagihara coupling. Subsequently, we applied our recently developed metal-template approach to macrocyclization reactions by treating the deprotected acetylenes with cis-[Pt(dppp)Cl2] to yield a platina-macrocycle. Based on this synthetic knowledge, by a multiple transition-metal-template protocol we accomplished the synthesis of a copper(I)–catenate consisting of two interlocked π-conjugated macrocycles which contain oligothiophene, diacetylene and phenanthroline units. Conclusive evidence for the new structures came from a detailed characterization of the platina-macrocycles and catenates by ESI-FT-ICR and tandem mass spectrometry experiments.

Towards Functional Macrocycles: Self-Assembly and Template Strategies

Macrocyclic Chemistry

C. A. Schalley, H. T. Baytekin, B. Baytekin

Springer, Heidelberg/Germany | 2005-12-01

Appeared In: Macrocyclic Chemistry: Current Trends and Future, K. Gloe (Ed.), p. 37-52

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Synthesis of Chiral Self-Assembling Rhombs and Their Characterization in Solution, in the Gas Phase, and at the Liquid-Solid Interface

K. S. Jeong, S. Y. Kim, U.-S. Shin, M. Kogej, N. T. M. Hai, P. Broekmann, N. Jeong, B. Kirchner, M. Reiher, C. A. Schalley

J. Am. Chem. Soc. 2005, 127, 17672-17685

Full TextSupporting Information

Chiral, enantiopure metallo-supramolecular rhombs self-assemble in solution through coordination of bis-pyridyl-substituted ligands with (en)M(NO3)2 (en = ethylenediamine, M = PdII, PtII). Characterization by NMR and CD spectroscopy in solution and by ESI-FT-ICR mass spectrometry in the gas phase suggests that an equilibrium exists in water/methanol of a major 2:2 complex and a minor 3:3 complex of ligands and metal corners. In the gas phase, doubly charged 2:2 complexes fragment into two identical singly charged halves followed by metal-mediated C−H and C−C bond activation reactions within the ethylenediamine ligands. Electrochemical scanning tunneling microscopy (EC-STM) provides in situ imaging of the complexes even with submolecular resolution. Flat-lying rhombs are deposited under potential control from an aqueous electrolyte on a Cu(100) electrode surface precovered by a tetragonal pattern of chloride anions from the supporting electrolyte. Chirality induces the formation of only one domain orientation. Density functional calculations help to interpret the STM images.

Formation of 2D supramolecular architectures at electrochemical solid/liquid interfaces

C. Safarowsky, A. Rang, C. A. Schalley, K. Wandelt, P. Broekmann

Electrochim. Acta 2005, 50, 4257-4268

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The controlled formation of supramolecular architectures on chloride pre-covered Cu(1 0 0) has been studied by means of in situ scanning tunneling microscopy (STM) in an electrochemical environment. On top of the c(2 × 2)-Cl layer, ordered arrays of supramolecular cavitand structures could be obtained either by a surface assisted assembly of monomer building-blocks (1,1′-dibenzyl-4,4′-bipyridinium molecules) or by a direct adsorption of supramolecular assemblies (metallo-supramolecular squares) from the solution phase. Besides the omnipresent van-der-Waals-like interactions additional electrostatic interactions between the anionic chloride layer and the positively charged (metallo)-organic molecules are supposed to have strong impact on the 2D phase behavior in both cases. The obtained supramolecular entities with their cavities oriented towards the solution phase can be regarded as potential host assemblies for the specific inclusion of guest molecules.

A Combined ESI- and MALDI-MS(/MS) Study of Peripherally Persulfonylated Dendrimers: False Negative Results by MALDI-MS and Analysis of Defects

T. Felder, C. A. Schalley, H. Fakhrnabavi, O. Lukin

Chem. Eur. J. 2005, 11, 5625-5636

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Mass spectrometry, in particular MALDI-MS, has often been used as a valuable means to characterize dendritic molecules with respect to their molecular masses. Also, it is a valuable tool for analyzing potential defects in their structure which result from incomplete synthetic steps. This article presents a comparison of ESI and MALDI mass spectrometric experiments on dendrimers persulfonylated at their periphery. While the ESI mass spectra easily permit impurities and defects to be identified and thus provide evidence for sample purity, reactions with acidic matrices occur during the MALDI process. The resulting defects are identical to those expected from incomplete substitution. Thus, in these cases, MALDI-MS yields false negative results. With mass-selected, ESI-generated ions, collision experiments were performed in an FT-ICR mass spectrometer cell to provide detailed insight into the fragmentation patterns of the various dendrimers. Different fragmentation patterns are observed depending on the exact structure of the dendrimer. Also, the nature of the charge is important. The fragmentation reactions for protonated species differ much from those binding a sodium or potassium ion. These differences can be traced back to different sites for binding H+ versus Na+ or K+. Tandem MS experiments on mass-selected dendrimer ions with defects can be used to distinguish different types of defects. A concise structural assignment can thus be made on the basis of these experiments. Even mixtures of two isobaric defect variants with the same elemental composition can be identified.

Controlling the rate of shuttling motions in [2]rotaxanes by electrostatic interactions: a cation as solvent-tunable brake

P. Ghosh, G. Federwisch, M. Kogej, C. A. Schalley, D. Haase, W. Saak, A. Lützen, R. Gschwind

Org. Biomol. Chem. 2005, 2691-2700

Full TextSupporting InformationCrystal Structure Data

A series of rotaxanes, with phenolic axle centerpieces and tetralactam macrocycles as the wheels, has been prepared in good yields. The threaded rotaxane structure is confirmed in the gas phase by tandem mass spectrometric experiments through a detailed fragmentation pattern analysis, in solution by NMR spectroscopy, and in the solid state through X-ray crystallography. A close inspection of the 1H,1H NOESY and 1H,1H ROESY NMR data reveals the wheel to travel along the axle between two degenerate diamide “stations” close to the two stoppers. By deprotonation of a phenolic OH group in the axle centerpiece with Schwesinger's P1 base, surprisingly no additional shuttling station is generated at the axle center, although the wheel could form rather strong hydrogen bonds with the phenolate. Instead, the wheel continues to travel between the two diamide stations. Experimental data from 1H,1H NOESY spectra, together with theoretical calculations, show that strong electrostatic interactions between the phenolate moiety and the P1 cation displace the wheel from the “phenolate station”. The cation acts as a “brake” for the shuttling movement. Instead of suppressing the shuttling motion completely, as observed in other rotaxanes, our rotaxane is the first system in which electrostatic interactions modulate the speed of the mechanical motion between a fast and a slow motion state as a response to a reversible external stimulus. By tuning these electrostatic interactions through solvent effects, the rate of movement can be influenced significantly, when for example different amounts of DMSO are added to dichloromethane. Besides the shuttling motion, circumrotation of the wheel around the axle is observed and analyzed by variable temperature NMR spectroscopy. Force field and AM1 calculations are in good agreement with the experimental findings.

Hierarchical Assembly of Helicate-Type Dinuclear Titanium(IV) Complexes

M. Albrecht, S. Mirtschin, M. de Groot, I. Janser, J. Runsink, G. Raabe, M. Kogej, C. A. Schalley, R. Fröhlich

J. Am. Chem. Soc. 2005, 127, 10371-10387

Full TextSupporting InformationCrystal Structure Data

The ligands 4−7-H2 were used in coordination studies with titanium(IV) and gallium(III) ions to obtain dimeric complexes Li4[(4−7)6Ti2] and Li6[(4/5a)6Ga2]. The X-ray crystal structures of Li4[(4)6Ti2], Li4[(5b)6Ti2], and Li4[(7a)6Ti2] could be obtained. While these complexes are triply lithium-bridged dimers in the solid state, a monomer/dimer equilibrium is observed in solution by NMR spectroscopy and ESI FT-ICR MS. The stability of the dimer is enhanced by high negative charges (Ti(IV) versus Ga(III)) of the monomers, when the carbonyl units are good donors (aldehydes versus ketones and esters), when the solvent does not efficiently solvate the bridging lithium ions (DMSO versus acetone), and when sterical hindrance is minimized (methyl versus primary and secondary carbon substituents). The dimer is thermodynamically favored by enthalpy as well as entropy. ESI FT-ICR mass spectrometry provides detailed insight into the mechanisms with which monomeric triscatecholate complexes as well as single catechol ligands exchange in the dimers. Tandem mass spectrometric experiments in the gas phase show the dimers to decompose either in a symmetric (Ti) or in an unsymmetric (Ga) fashion when collisionally activated. The differences between the Ti and Ga complexes can be attributed to different electronic properties and a charge-controlled reactivity of the ions in the gas phase. The complexes represent an excellent example for hierarchical self-assembly, in which two different noncovalent interactions of well balanced strengths bring together eleven individual components into one well-defined aggregate.

Derivatization of Pyrogallarenes

A. Åhman, M. Luostarinen, C. A. Schalley, M. Nissinen, K. Rissanen

Eur. J. Org. Chem. 2005, 2793-2801

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Derivatisation of upper-rim hydroxy groups of pyrogallarenes produced completely acylated and tosylated pyrogallarene derivatives. Mesitylation of pyrogallarene, however, resulted in a regioselective derivatisation of hydroxy groups, i.e. eight OH groups out of 12 were mesitylated. Crystal structures of the synthesised pyrogallarene derivatives indicate that completely substituted pyrogallarenes exist in a distorted crown conformation despite of the lack of stabilising intramolecular hydrogen bonds. In contrast, the partially substituted pyrogallarene adopts a boat conformation and has an open cavity for the inclusion of small guest molecules.

Mass Spectrometric Detection and Fragmentation Patterns of Synthetically Useful Chromium and Tungsten Carbene Complexes

T. Weilandt, E. Gräff, J. Schneider, A. Koch, F. Zschoche, K. H. Dötz, C. A. Schalley

Organometallics 2005, 24, 3671-3678

Full TextSupporting Information

A novel method for the detection of synthetically useful pentacarbonyl chromium and tungsten carbene complexes by electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry is presented which utilizes the addition of CN- anions to the electrophilic carbene carbon. This method is nondestructive, provided the ionization proceeds under soft conditions. The scope and limitations of this method are examined by utilizing different types of carbene complexes. Depending on the α-CH acidity of the carbene complexes, [M − H]- anions can be observed either in competition with the CN- adducts or as the only ions, even in the presence of cyanide. Secondary aminocarbene complexes exclusively form [M − H]- anions by deprotonation of the NH moiety. The ions under study have been mass-selected and subjected to collision-induced decay (CID) experiments in order to examine their fragmentation reactions. CO loss is not the only observable process. Most prominently, 1,2-eliminations and a retro-aldol reaction within a sugar carbene complex compete with decarbonylation and thus have rather low activation barriers. Some reactions can be attributed to coordinatively unsaturated chromium atoms formed through preceding CO losses.

Rational Design of Tightly Closed Coordination Tetrahedra that are Stable in the Solid State, in Solution, and in the Gas Phase

I. Müller, D. Möller, C. A. Schalley

Angew. Chem. Int. Ed. 2005, 44, 480-484; Angew. Chem. 2005, 117, 485-488

Full Text (International Edition)Full Text (German Edition)Supporting Information

No way out: Small guest cations, such as [Et4N]+ template the formation of stable and tightly closed octaanionic metallosupramolecular tetrahedra (see picture). Since [Et3NH]+ is co-encapsulated with exactly one water molecule, the filling of space seems to be a more pivotal factor for the templation than the symmetry of the cation. X-ray crystallography excellently agrees with NMR spectroscopy and ESI-FT-ICR mass spectrometry data.

Gas-Phase Host-Guest Chemistry of Dendritic Viologens and Molecular Tweezers: A Remarkably Strong Effect on Dication Stability

C. A. Schalley, C. Verhaelen, F.-G. Klärner, U. Hahn, F. Vögtle

Angew. Chem. Int. Ed. 2005, 44, 477-480; Angew. Chem. 2005, 117, 481-485

Full Text (International Edition)Full Text (German Edition)Supporting Information

Size does matter: Viologen dications (blue) substituted with dendrons form strong host–guest complexes with molecular tweezers (yellow) in solution and in the gas phase (see picture). Complexation even stabilizes dications which are intrinsically unstable as “naked” species in the high vacuum of a mass spectrometer. A remarkably pronounced effect of dendrimer size on the stability of these dications switches their fragmentation between two mechanisms.

Rotaxanes and Pseudorotaxanes


P. Linnartz, C. A. Schalley

Appeared In: Encyclopedia of Supramolecular Chemistry, J. L. Atwood, J. W. Steed (eds.), pp. 1194-1201

Dekker, New York 2004

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Catenanes and Interlinked Molecules


A. Rang, C. A. Schalley

Appeared In: Encyclopedia of Supramolecular Chemistry, J. L. Atwood, J. W. Steed (eds.), pp. 206-213

Dekker, New York 2004

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Weak interactions between resorcinarenes and diquaternary alkyl ammonium cations

H. Mansikkamäki, C. A. Schalley, M. Nissinen, K. Rissanen

New J. Chem. 2005, 29, 116-127

Full TextCrystal Structure Data

The interactions of resorcin[4]arenes 1 with alkyl ammonium cations bearing a 1,4-diazabicyclo[2.2.2]octane (DABCO) scaffold (32+, 42+ and 52+) were analyzed in the solid state by X-ray crystallography, in solution by 1H NMR spectroscopy, and in the gas phase by ESI-TOF mass spectrometry. The results are complemented with AM1 calculations and compared to previous reports on complexation studies of resorcinarenes with quaternary alkyl ammonium cations. The NMR titration results indicate that there are hardly any differences in the binding of the quaternary tetramethyl ammonium cation 2+ and the diquaternary N,N’-dimethyl DABCO dication 42+. The large N,N’-dibenzyl DABCO dication 52+ has two potential sites for inclusion, that is, the aryl groups and the central cationic part, and the complexation and interactions of both sites with 1 were verified in the NMR studies as well as in the solid state structures.

STM investigations on a tetralactam macrocycle adsorbed on Au(111) and Cu(111) surfaces

I. Kossev, S. Fahrenholz, A. Gö”rling, W. Hieringer, C. A. Schalley, M. Sokolowski

Synth. Metals 2004, 147, 159-164

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A large tetralactam macrocycle (961.3 amu), which is a frequently used building block for catenanes and rotaxanes, was deposited onto Au(1 1 1) and Cu(1 1 1) surfaces by vacuum sublimation under UHV. STM images were recorded for different coverages at room temperature. The macrocycles are found to be very mobile on both surfaces, which leads to pronounced STM tip effects. At low coverage, the macrocycles preferentially form small clusters at step edges and surface defects, leaving large terraces uncovered. At a higher coverage of about one monolayer, disordered closed films are observed on Au(1 1 1), which remain disordered after annealing to 423 K. Presumably, the reason for these observations is a week interaction between the Au and Cu surfaces and the molecules, in combination with formation of hydrogen bonds between the molecules.

Solid-Phase Synthesis of a Double 4-Pyridinyl Terminated Leu-Ala-Leu Tripeptide and Macrocyclization by Palladium(II) Coordination

M. Albrecht, P. Stortz, M. Engeser, C. A. Schalley

Synlett 2004, 2821-2823

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A Leu-Ala-Leu tripeptide, which bears pyridinyl units at the N- as well as C-terminus, is prepared by combination of solid- and solution-phase synthesis. With the Pd(en)2+ fragment it forms a mixture of 1:1 and 2:2 complexes, with the mononuclear complex probably being the dominating species.

Towards a Selective Functionalization of Amino-Terminated Dendrimers

F. V”ögtle, H. Fakhrnabavi, O. Lukin, S. Müller, J. Friedhofen, C. A. Schalley

Eur. J. Org. Chem. 2004, 4717-4724

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Selective functionalization of the periphery of commercial polypropyleneamine (POPAM) and polyamidoamine (PAMAM) dendrimers has been investigated in preparative scale. The first generation (G1) POPAM dendrimer was for the first time selectively N,N-bis(sulfonylated) with tosyl chloride and the corresponding mono-, di-, tri-, and tetra-N-tosylsulfonamides were isolated and fully characterized. Unexpectedly, similar persulfonylation of G2 POPAM results in splitting of a central C-N bond and only fully and partially sulfonylated halves of the initial dendrimer could be isolated. Higher generations of POPAM are also split during the persulfonylation yielding complex mixtures of persulfonylated dendritic fragments which could hardly be identified. A plausible mechanism of the POPAM decomposition on the basis of the reaction product analysis is proposed. N-Sulfonylation of a peripheral octasulfonamide of G2 POPAM with tosyl chloride also leads to the destruction of the dendrimer, while its N-alkylation with benzyl bromide proved to be not selective yielding a completely alkylated derivative. Unlike POPAM dendrimers, PAMAM dendrimers were shown to be more stable during their sulfonylation and no decomposition of the dendritic backbone was detected. In contrast to the POPAM dendrimers, PAMAM dendrimers were shown to be rather inert with respect to the formation of N-tosylsulfonamides since they could only be N-monosulfonylated at all peripheral amino groups. The combination of MALDI-TOF and ESI-FT-ICR tandem mass spectrometry has been shown to be an effective method for structure assignment and purity check of selectively or fully persulfonylated dendritic oligoamines.

Elements for the construction of molecular devices: template effects and self-assembly

C. A. Schalley

J. Phys. Org. Chem. 2004, 17, 967-972

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Four elements which may prove useful for the construction of molecular shuttles and other devices at the molecular level are discussed. (i) A novel anion-mediated template effect allows functionality to be introduced into the axle center pieces of rotaxanes so that molecular motions of the axle and wheel can be controlled by external stimuli. (ii) The deslipping kinetics of rotaxanes provide valuable insight into large changes of the deslipping rate caused by minimal structural changes. These effects, once understood, could be exploited for an optimization of the functionality of molecular devices. (iii) A wheel with an exocyclic metal coordination site provides access to self-assembled species that combine more than one macrocycle or rotaxane. This is a step towards a controlled growth of more complex species and in future may help to reduce the synthetic efforts needed for the generation of large and complex systems through self-assembly rather than covalent synthesis. (iv) In particular, for self-assembled systems with their often high degree of symmetry, mass spectrometry provides a valuable tool for the determination of their sizes. Beyond characterization, mass spectrometry provides an insight into structural details and the intrinsic reactivity of supramolecular species and thus can be expected to play an increasingly important role in the examination of molecular devices.

Theory and Experiment in Concert: Templated Synthesis of Amide Rotaxanes, Catenanes, and Knots

C. A. Schalley, W. Reckien, S. Peyerimhoff, B. Baytekin, F. Vö”gtle

Chem. Eur. J. 2004, 10, 4777-4789

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The synthesis of amide rotaxanes, amide catenanes, and trefoil amide knots is based on template effects mediated by hydrogen bonds. While a large body of experimental data is available, in-depth theoretical studies of these template syntheses are virtually unavailable, although they would provide a more profound insight into the exact details of the hydrogen-bonding patterns involved in the formation of these mechanically interlocked species. In this article we present a density functional study of the conformational properties of tetralactam macrocycles and the threading mechanism that produces the immediate precursor for rotaxane and catenane formation. Predictions of the geometries and relative energies made on the basis of semi-empirical AM1 calculations are compared with these results in order to judge the reliability of the simpler approach. Since these calculations yield good agreement with the structural features, they have been used to extend the calculations in order to understand the mechanism of formation of a trefoil dodecaamide knot that has recently been synthesized. The inherent topological chirality of the knot is reflected in the intermediates generated during its formation; these involve helical loops. These loops parallel the rotaxane and catenane wheels with respect to the arrangement of the functional groups that mediate the template effect and may well serve as wheel analogues through which one of the precursor molecules can be threaded. This threading step finally results in the knotted structure. Good agreement between the results of the calculations presented here and experimental findings is achieved.

Quartz Microbalance Sensor for the Detection of Acrylamide

G. Kleefisch, C. Kreutz, J. Bargon, G. Silva, C. A. Schalley

Sensors 2004, 4, 136-146

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Several macrocycles of the Hunter-V”gtle type have been identified as superior host compounds for the detection of small amounts of acrylamide. When coated onto the surface of a quartz microbalance, these compounds serve as highly sensitive and selective sensor-active layers for their use in electronic noses. In this study, differently substituted macrocycles were investigated including an open-chain analogue and a catenane. Their structure and functional groups are correlated with their observed affinities to acrylamide and related acids and amides. The much smaller response of the open-chain compound and the almost absent sensor response of the catenane suggest that binding occurs within the cavity of the macrocycle. Theoretical calculations agree well with the experimental data even though they do not yet take into account the arrangement of the macrocycles in the sensor-active layer. The lower detection limit of acrylamide is 10 parts per billion (ppb), which is impressively low for this type of sensor. Other related compounds such as acrylic acid, propionamide, or propionic acid show no or significantly lower affinities to the macrocycles in these concentration ranges.

How to Thread a String into the Eye of a Molecular Needle: Template-Directed Synthesis of Mechanically Interlocked Molecules

Strategies & Tactics

M. Kogej, C. A. Schalley

Appeared In: Strategies and Tactics in Organic Synthesis, Vol. 4, M. Harmata (ed.), pp. 171-210

Elsevier, Amsterdam/The Netherlands 2004

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Borromean rings: A one-pot synthesis

C. A. Schalley

Angew. Chem. Int. Ed. 2004, 43, 4399-4401; Angew. Chem. 2004, 116, 4499-4501

Full Text (International Edition)Full Text (German Edition)

Elegance and precision are shown in the self-assembly strategy developed by Stoddart, Atwood, and co-workers for the synthesis of molecular Borromean rings. Metal-ion template effects direct the dynamic formation of covalent bonds, which result in the energetically favorable product having the highly symbolic ring motif.

Deslipping of Rotaxanes with Axle Center Pieces of Different Lengths

P. Linnartz, C. A. Schalley

Supramol. Chem. 2004, 16, 263-267

Full Text

A series of rotaxanes equipped with tetralactam wheels and di-t-butyl stopper groups, which differ only with respect to the length of the alkyl chain serving as the axle center piece, are examined with respect to their deslipping behavior at elevated temperatures. 1H NMR experiments are used to follow the deslipping reactions kinetically, and it is found that the axle length does not have a significant influence on the rate of deslipping. In accordance with expectation, this result confirms that the rate-determining step of the reaction is the passage of the wheel over the stopper group. It also sheds new light on earlier results on rotaxanes bearing ester groups in the axle center piece. The deslipping reaction rate for these rotaxanes was extremely dependent on the choice of solvent and the orientation of the ester groups. The rotaxanes presented here do not contain functional groups in their center pieces and no such effects are observed. For these rotaxanes, the simple model of a thin thread inside a macrocycle mechanically trapped by bulky stopper groups is valid.

Artificial Rotary Motors at the Molecular Level

Bioorganic Chemistry

T. Felder, C. A. Schalley

Appeared In: Highlights in Bioorganic Chemistry, C. Schmuck, H. Wennemers (eds.), pp. 526-539

Wiley-VCH, Weinheim/Germany 2004

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Hydrogen-bond-mediated template synthesis of rotaxanes, catenanes, and knotanes

C. A. Schalley, T. Weilandt, J. Brüggemann, F. V”ögtle

Top. Curr. Chem. 2004, 248, 141-200

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The synthesis of rotaxanes, catenanes, and trefoil knotanes can only be efficiently accomplished, when a suitable template effect is used that provides a means either to thread a string-like molecule through a macrocycle or to wrap an open macrocycle around a string. This overview is devoted to the template effects for the synthesis of mechanically bound species which are mediated by hydrogen bonding. Three major classes can be distinguished by the charges involved: (i) catien-mediated template effects utilizing the formation of crown ether complexes with secondary ammonium ions, (ii) recognition of neutral amides by oligolactam macrocycles, and (iii) threading through hydrogen bonding to anions. The basic idea which we put forward in this chapter is that none of the template effects found so far and discussed here is ""traceless"" in the sense that they would not leave functional groups behind in the final rotaxane, catenane, or knotane structure. Consequently, the variability of the structures available through these template syntheses is limited in this respect. Also, the properties of the mechanically bound species are intimately interconnected with the template effect used for their synthesis. However, what appears here as a drawback may also be an advantage, when the functional groups used for templating can be utilized to control the properties of the final product which hopefully can in future be exploited for the development of new materials. We aim at providing sufficient examples in order to strengthen this point in throughout the article. Among the properties and functions discussed are the realization of molecular machines, topological chirality; and the stability of rotaxanes as determined from deslipping experiments.

Mass spectrometric evidence for catenanes and rotaxanes from negative-ESI FT-ICR tandem-MS-experiments

C. A. Schalley, P. Ghosh, M. Engeser

Int. J. Mass Spectrom. 2004, 232-233, 249-258

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Often, it is a non-trivial task to distinguish intertwined topologies as realized in rotaxanes and catenanes from non-intertwined isomers, such as simple host-guest complexes held together by non-covalent bonds or larger mono-macrocyclic analogues. This article discusses in detail the collision-induced fragmentations of the anions of catenanes, rotaxanes, and analogous compounds bearing tetralactam macrocycles. With these experiments, it is possible to distinguish intertwined from non-intertwined complexes through differences in the fragmentation pattern.

Approaching supramolecular functionality

C. A. Schalley, A. Lützen, M. Albrecht

Chem. Eur. J. 2004, 10, 1072-1080

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Functional molecules require a high degree of complexity which is difficult to achieve by covalent synthesis. This article discusses supramolecular approaches to the creation of larger architectures through non-covalent bonds, self-assembly, and template strategies. It highlights selected examples for the structural and conformational control of function and attempts to identify difficulties and challenges which may arise in future.

Second-order templation: Ordered deposition of supramolecular squares on a chloride-covered Cu(100) surface

C. Safarowsky, L. Merz, A. Rang, P. Broekmann, B. A. Hermann, C. A. Schalley

Angew. Chem. Int. Ed. 2004, 43, 1291-1294; Angew. Chem. 2004, 116, 1311-1314

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Beneath the surface lies the answer: Metallosupramolecular cations are deposited in an ordered manner on a copper surface, which had previously been covered with chloride ions. The anion layer acts as a second-order template, whose interactions with the cation determine the adsorption process.

Synthesis of Fr‚chet-Type Resorcarene Tetrabenzoxazine Dendrimers

M. Luostarinen, T. Laitinen, C. A. Schalley, K. Rissanen

Synthesis 2004, 255-262

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First, second, and third generation Frech‚t-type dendrimer amines were prepared on a gram scale and these dendritic wedges were attached successfully to the resorcarene cores by Mannich reaction.

Deslipping of Ester Rotaxanes: A Cooperative Interplay of Hydrogen Bonding with Rotational Barriers

P. Linnartz, S. Bitter, C. A. Schalley

Eur. J. Org. Chem. 2003, 4819-4829

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A series of rotaxanes has been synthesized which contain two ester groups in their axles. All rotaxanes bear the same tetralactam wheel. The kinetics of the de-slipping reaction of these rotaxanes were monitored in tetrachloroethane (TCE) and dimethyl sulfoxide (DMSO) resulting in the observation of a significant solvent effect. In TCE, two isomeric rotaxanes that differ merely with respect to the orientation of the ester groups show a remarkable difference in their deslipping behavior. When the ester carbonyl group is directly attached to the axle center piece, the rotaxane decomposes with a half life of ca. 10ÿh at 100 øC. The reverse orientation with the carbonyl group attached to the stopper blocks deslipping almost completely and a lower limit for the half life at 100 øC of 25,000ÿh was obtained. These results can be interpreted by inferring a cooperative action of hydrogen bonding between wheel and axle and differences in rotational barriers. Molecular modeling and AM1 calculations support this interpretation. The implications of these results for the determination of steric size and the optimization of molecular machines are discussed.

Determination of the ripening state of Emmental cheese via quartz microbalances

J. Bargon, S. Braschoß, J. Flö”rke, U. Herrmann, L. Klein, J. W. Lö”rgen, M. Lopez, S. Maric, A. H. Parham, P. Piacenza, H. Sch„äfgen, C. A. Schalley, G. Silva, M. Schlupp, H. Schwierz, F. V”ögtle, G. Windscheif

Sens. Act. B 2003, 95, 6-19

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Electronic noses based on an array of differently coated quartz microbalances (QMBs) yield information about the components of food aromas, which can be differentiated upon analyzing the data using contemporary pattern-recognition programs. Even single lead substances can be determined quantitatively against the background of similar carbonyl components. Once properly calibrated, even complicated odors can be analyzed in this fashion with considerable accuracy, whereby it is typically sufficient to focus on one or a few lead components. In this study, this QMB-based method has been applied to monitor the ripening process of Emmental cheeses, during which the concentration of 2-heptanone changes characteristically. Following the development of the 2-heptanone concentration over time not only allows us to distinguish different states of ripening process of Emmental cheese.

Distinguishing the topology of macrocyclic compounds and catenanes

C. A. Schalley, J. Hoernschemeyer, X. Y. Li, G. Silva, P. Weis

Int. J. Mass Spectrom. 2003, 228, 373-388

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The paper explores different possibilities to distinguish macrocyclic oligolactam species with different molecular topologies by ESI mass spectrometry. Collision-induced fragmentation provides unambiguous fragmentation patterns: Monocyclic rings upon single and double protonation predominantly lose water molecules, while catenanes, i.e., two chain-like interlocked macrocycles, undergo ring cleavage followed by loss of a whole ring. At higher collision energies and under multiple collision conditions, a series of different fragments is observed which can be categorized into six different types. These fragments are structure indicative and provide evidence for the connectivities of building blocks within the macrocycles. Gas-phase ion mobility experiments on the other hand result in a good agreement between experimental collision cross-sections and structure predictions from theory. This method does not provide unequivocal differences between monocyclic rings and catenanes due to almost identical cross-sections, but it provides a tool to distinguish between knots (twisted monocyclic rings) and.(untwisted) rings.

Secondary isotope effects on the deslipping reaction of rotaxanes: High-precision measurement of steric size

T. Felder, C. A. Schalley

Angew. Chem. Int. Ed. 2003, 42, 2258-2260; Angew. Chem. 2003, 115, 2360-2363

Full Text (International Edition)Full Text (German Edition)Supporting Information

A supramolecular measuring instrument for determining steric size: The deslipping reaction of the rotaxane shown with deuterated stopper groups senses the smaller steric size of the deuterium atom and proceeds with a 10?% higher reaction rate than the reaction of the unlabeled rotaxane. This result sheds new light from a supramolecular point of view on the classical problem of determining steric demand.

Tetra- and octalactam macrocycles and catenanes with exocyclic metal coordination sites: Versatile building blocks for supramolecular chemistry

X.-Y. Li, J. Illigen, M. Nieger, S. Michel, C. A. Schalley

Chem. Eur. J. 2003, 9, 1332-1347

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The synthesis of a new tetralactam macrocycle and the simultaneous formation of catenanes and larger octalactam macrocycles is reported. These species bear 2,2'-biquinoline moieties suitably positioned to bind a metal center at the outer periphery of the macrocycles. H-1 NMR chemical shifts permit the unambiguous distinction of transoid and cisoid conformations of the biquinoline moiety, thereby allowing an unequivocal identification of the catenane and octalactam structures, despite the fact that both have the same elemental composition and bear identical structural subunits. With the aid of an anion template effect, rotaxanes can be prepared from the smaller tetralactam macrocycle. These reveal significantly altered requirements in terms of the stopper size as compared to previously reported tetralactam wheels. Several copper((I))-mediated dimers and a (bpy)(2)Ru-II complex (bpy = 2,2'-bipyridine) have been synthesized from the tetralactam macrocycle and the rotaxanes. The anion binding abilities of the tetralactam macrocycle and its (bpy)(2)-Ru-II complex in DMSO have been compared by H-1 NMR titration experiments, which revealed significantly enhanced binding by the metal complex. Mass spectrometry has been used to study the potential formation of larger assemblies of copper((I)) and the catenane built-up from two tetralactam macrocycles. Indeed, a 2:2 complex was identified. In contrast, the octalactam macrocycle of the same elemental composition yields only 1:1 complexes, with the Cu-I ion connecting its two biquinoline moieties in the center of a figure-eight-shaped molecule. Molecular modeling studies support the structural assignments made.

Self-assembling resorcinarene capsules: solid and gas phase studies on encapsulation of small alkyl ammonium cations

H. Mansikkamä„ki, M. Nissinen, C. A. Schalley, K. Rissanen

New. J. Chem. 2003, 27, 88-97

Full TextCrystal Structure Data

The self-assembling process of unsubstituted resorcinarenes (1, 2 and 3) to dimeric capsules encapsulating small tetra-alkyl ammonium cations 4-7 has been studied in solid and gaseous states by X-ray crystallographic and mass spectrometric methods. Hydrogen bonding and cation-[small pi] interaction as well as the proper encapsulation in the empty cavity of the capsule appear to be the most important interactions in the capsule formation process. Competitive mass spectrometric studies clearly indicated preference of the tetramethyl ammonium cation (4) over tetraethyl ammonium cation (6) and especially tetrabutyl ammonium cation (7). The crystal structures of altogether eight dimeric capsules of resorcinarenes 1-3 with cations 4 and 5 were determined. In the solid state, the alkyl chain length of the host affects the crystal packing significantly. However, ethyl resorcinarene (2) is the only host, which binds the spherical halide anion (Cl- or Br-) in between the lower rim alkyl chains.

Unusually stable magic number clusters of serine with a surprising preference for homochirality

C. A. Schalley, P. Weis

Int. J. Mass Spectrom. 2002, 221, 9-19

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The amino acid serine forms unusually stable octameric clusters upon electrospray ionization of relatively concentrated solutions. A strong preference for homochiral clusters is observed, while mixed clusters of D- and L-serine are significantly underrepresented. The present essay reviews the experimental results as well as the structures suggested so far. Surprisingly, there is good agreement on the experimental facts among different research groups, but a vivid discussion with respect to the octamer structure is going on. In order to provide a firm basis for a discussion of these issues, we briefly address the questions of chiral recognition of amino acids and cluster formation in more general terms. Another important issue in this respect is the generation of zwitterionic amino acids in the gas phase. This paper suggests a new structure that seems to be in good agreement with the experimental evidence gathered so far. The review terminates with some thoughts on the implications of the homochirality of serine octamers for homochirogenesis and the origin of life.

Novel template effect for the preparation of [2]rotaxanes with functionalised centre pieces

P. Ghosh, O. Mermagen, C. A. Schalley

Chem. Commun. 2002, 2628-2629

Full TextSupporting Information

A novel anion-templated synthesis permits the preparation of [2]rotaxanes with a tetralactam wheel through which axles are threaded that are functionalised at their center pieces; the wheel protects these groups efficiently against modifications.

Mass spectrometric characterization and gas-phase chemistry of self-assembling supramolecular squares and triangles

C. A. Schalley, T. Müller, P. Linnartz, M. Witt, M. Schäfer, A. Lützen

Chem. Eur. J. 2002, 8, 3538-3551

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A detailed mass spectrometric characterization of self-assembling polynuclear metal complexes is described. The complexes can only be ionized as intact species under a surprisingly narrow range of conditions by electrospray ionization. Comparison with the results from NMR experiments shows that several solution-phase features of these squares and triangles (such as trends in bond energies, ligand-exchange reactions, or square-triangle equilibria) are qualitatively reflected in the gas-phase data. Consequently, mass spectrometry represents a valuable method for the characterization of these compounds. Nevertheless, the formation of unspecific aggregates during the ionization process occurs and its implications are discussed. Beyond the chemistry in solution, the fragmentation pathways of these complexes in the gas phase have been studied by infrared multiphoton dissociation (IRMPD) experiments. The results of IRMPD studies allow us to draw conclusions with respect to the structure and energetics of fragmentation products. In this tandem MS experiment, reaction pathways can be observed directly which can hardly be analyzed in solution. According to these results, the equilibration of triangles and squares involves the supramolecular analogue of a neighboring-group effect.

Analysis and improvement of an anion-templated rotaxane synthesis

C. A. Schalley, G. Silva, C.-F. Nising, P. Linnartz

Helv. Chim. Acta 2002, 85, 1578-1596

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A series of new rotaxanes with axles different in length was prepared. Following the synthetic protocol utilizing a known anion template effect (Scheme 1), surprisingly low yields in the order of 2 - 5% were obtained (Scheme 3), which furthermore significantly depended on the nature of the stopper (Fig. 1). Variations in the synthetic procedures and computational results from Monte Carlo simulations allowed us to analyze the origin of these findings: The rotaxane wheel 3 acts as a noncovalently bound 'protecting group' for the stopper nucleophile. The protection of the nucleophilic phenolate O-atom depends much on the steric demands of the stoppers (see 2 vs. 10) which induce different conformations of the wheel. Based on this model. an improved synthetic scheme is suggested.

Of Molecular Gyroscopes, Matroshka Dolls, and Other "Nano"-Toys

C. A. Schalley

Angew. Chem. Int. Ed. 2002, 41, 1513-1515; Angew. Chem. 2002, 114, 1583-1586

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A spinner in a cage, or put simply, a gyroscope. A ?molecular gyroscope?, known as gyroscane (see picture), was recently synthesized. Often such systems are referred to as molecular machines?why not as molecular toys? In this contribution the author illustrates that this view does not have to have negative connotations.

Monitoring apple flavor by use of quartz microbalances

U. Herrmann, T. Jonischkeit, J. Bargon, U. Hahn, Q. Y. Li, C. A. Schalley, E. Vogel, F. V”ögtle

Anal. Bioanal. Chem. 2002, 372, 611-614

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Electronic noses, i.e. arrays of differently coated quartz microbalances (QMB), have been used for selective detection of, and discrimination between, volatile organic compounds (VOC) formed during the post-harvest ripening of apples. The flavor components to be differentiated are chemically rather similar carbonyl compounds, chiefly aldehydes and esters. Because their relative ratios change during the post-harvest ripening period, appropriately selected sensor-active layers lead to characteristic patterns of the sensor responses which can be analyzed via pattern-recognition methods. This enables qualitative and quantitative identification of individual components whereby the post-harvest ripening of apples and other fruits can be monitored. Different kinds of apple differ in type and concentration of individual carbonyl compounds.

Flexiball toolkit: A modular approach to self-assembling capsules

B. M. O'Leary, T. Szabo, N. Svenstrup, C. A. Schalley, A. Lützen, M. Schä„fer, J. Rebek, Jr.

J. Am. Chem. Soc. 2001, 123, 11519-11533

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We report the synthesis and characterization of new, self-assembling molecular capsules. The modular strategy makes use of glycoluril building blocks available in multigram amounts combined with aromatic spacer elements. The lengthy syntheses encountered with earlier generations of capsules are avoided, and several capsules of nanometer dimensions are now accessible, Single bond attachments between spacers and glycoluril modules result in monomers as dimeric capsules that are less rigid than their earlier counterparts. The host-guest properties of the homo- and heterodimeric capsules were studied using a combination of NMR and ESI-mass spectrometry. They show a less pronounced selectivity for guests of different sizes, and their increased flexibility prevents self-assembly when no rigidifying elements are present on the central spacer unit. Some of the new capsules bear inwardly directed, secondary amide N-H protons. These can be further functionalized, as shown by their methylation to give tertiary analogues. The structures hold broader implications for the placement of functional groups on concave molecular surfaces.

Molecular recognition and supramolecular chemistry in the gas phase

C. A. Schalley

Mass Spectrom. Rev. 2001, 20, 253-309

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Supramolecular chemistry, in particular, the fields of molecular recognition and self-assembly profit much from the development of soft ionization techniques and advanced methods for mass analysis and gas-phase chemistry. Vice versa, weakly bonded architectures and host-guest complexes represent a veritable challenge for the mass spectrometrist, leading to further development of methods and techniques. This review describes the state-of-the-art in this field, and includes topics such as the effects of solvation on meta binding to crown ethers, chiral discrimination of guests by chiral hosts, the elucidation of the secondary structure of self assembled complexes, and the mechanistic path ways of self assembly or the fragmentations of supramolecular complexes in the gas phase.

Rotaxane or pseudorotaxane? Effects of small structural variations on the deslipping kinetics of rotaxanes with stopper groups of intermediate size

A. Affeld, G. M. Hübner, C. Seel, C. A. Schalley

Eur. J. Org. Chem. 2001, 2877-2890

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Three series of rotaxanes have been synthesized variously by slipping synthesis, in which axis and wheel are melted in admixture, by recognition of amide groups inside the macrocyclic wheel, or by an anionic template method, in which the stoppering phenolates are hydrogen bonded to the wheel and then joined by reaction with a semi-axle. The 3,5-di-tert-butylphenyl stopper used for most of these rotaxanes is large enough to permit their isolation, but still allows the wheel to deslip from the axle under appropriate conditions. The deslipping activation parameters for all rotaxanes are derived from H-1 NMR kinetic measurements and have been evaluated from the Arrhenius equation as well as according to Eyring theory. Small structural variations give rise to surprising effects on the activation parameters. Firstly, in some examples, the axle length affects the deslipping barrier, although the size complementarity of stopper and wheel remain unchanged. Secondly, stopper flexibility has an important influence on the deslipping rate. Thirdly, exchange of a carbonamide for a sulfonamide in the wheel significantly reduces the entropic costs of the deslipping, resulting in a pronounced deslipping rate enhancement. Fourthly, intramolecular hydrogen bonding within the wheel decelerates deslipping by a factor of more than 10(4).

Selbstorganisation von molekularen Tennisb„llen: Molekulare Erkennung

C. A. Schalley

Chem. unserer Zeit 2001, 35, 166-174

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Mehrfache Kopien einfacher Strukturelemente lassen sich zu einer gr”ößeren, geordneten Architektur mit komplexeren Funktionen zusammensetzen. Die Gültigkeit dieses Prinzips auf der molekularen Ebene wurde am Beispiel wasserstoffverbrückter Kapseln illustriert, die sich - anfangs wenig mehr als eine Laborkuriosit„ät - inzwischen vom Spielball der Grundlagenforschung zum Studienobjekt der Materialwissenschaften gemausert haben. Zwei identische Kopien eines selbstkomplement„ren Monomerbausteins reichen, um Funktionen zu implementieren, die sich von der Bildung geschlossener Hohlrä„ume mit der Fä„higkeit zum Einschluss von Gastmolekülen über die chirale Erkennung bis hin zur Katalyse erstrecken. Multiple Kopien aufgereiht zu wasserstoffverbrückten Polymeren zeigen eine ganze Hierarchie weitaus komplexerer Eigenschaften, indem sie Flüssigkristalle bilden und das Ziehen von Fä„den mit einiger Tragkraft erlauben.

On the way to rotaxane-based molecular motors: Studies in molecular mobility and topological chirality

C. A. Schalley, K. Beizai, F. V”ögtle

Acc. Chem. Res. 2001, 34, 465-476

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ATP synthase represents a machine at the molecular level which couples the rotation of an axle in a wheel with the endergonic production of ATP, the general source of chemical energy in the cell. The natural system prototypically bears all features of a macroscopic motor: a rotor within a stator held by a membrane and fueled by a difference in chemical potential in the form of a proton gradient combined with a machine for ATP production. The assembly of axle and wheel to a rotor device reminds one very much of a rotaxane. In this Account, we discuss some important features of motors and their (potential) realization in simpler artificial model systems, that is, the molecular mobility of mechanically bound molecules, the importance of chirality for unidirectional motion, the sources of energy for driving the rotation, and the potential of using membranes and surfaces for ordering a large number of devices to achieve macroscopic effects.

Metal-Ion Chemistry of Peroxides

Peroxide Chemistry

D. Schr”öder, C. A. Schalley, H. Schwarz

Appeared In: Peroxide Chemistry: Mechanistic and Preparative Aspects of Oxygen Transfer, W. Adam (ed.), pp. 640-664

Wiley-VCH, Weinheim/Germany 2000

First artificial receptor for caffeine - A new concept for the complexation of alkylated oxopurines

S. R. Waldvogel, R. Fr”öhlich, C. A. Schalley

Angew. Chem. Int. Ed. 2000, 39, 2472-2475; Angew. Chem. 2000, 112, 2580-2584

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Addicted to caffeine? The caffeine molecule is strongly held in a 1:1 complex with the first artificial C3-symmetrical caffeine receptor 1, which is based on functionalized triphenylene ketals. Molecular recognition by this receptor involves hydrogen bonds and ?-donor??-acceptor stacking, as shown by 1H NMR spectroscopic experiments and a crystal structure analysis.

Chemical Encapsulation in Self-Assembling Capsules

Stimulating Concepts

C. A. Schalley, J. Rebek, Jr.

Appeared In: Stimulating Concepts in Chemistry (Eds.: F. V”ögtle, J. F. Stoddart, M. Shibasaki), pp. 199-210

Wiley-VCH, Weinheim/Germany 2000

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Supramolecular chemistry goes gas phase: the mass spectrometric examination of noncovalent interactions in host-guest chemistry and molecular recognition

C. A. Schalley

Int. J. Mass Spectrom. 2000, 194, 11-39

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This review deals with the examination of supramolecular architectures by mass spectrometric methodology. Aspects of ion generation, structure determination, thermochemical data determination, the analysis of stereochemical features, and the mechanistic pathways of ion generation and fragmentation in the gas phase are discussed.

Self-assembling capsules: From molecular recognition to hydrogen-bonded liquid crystals and fibers

C. A. Schalley

Adv. Mater. 1999, 11, 1535-1537

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Tennis balls, "softballs", and "footballs" can all be self-assembled from appropriately shaped molecules via hydrogen bonding. The assembly and uses of these capsules as hosts and catalysts are surveyed as are the liquid-crystalline and fibrous properties of hydrogen-bonded capsule polymers, or polycaps - promising alternatives to covalent polymers. The Figure shows the mechanism of guest exchange in a capsule.

Encapsulation of ion-molecule complexes: Second-sphere supramolecular chemistry

A. Lützen, A. R. Renslo, C. A. Schalley, B. M. O'Leary, J. Rebek, Jr.

J. Am. Chem. Soc. 1999, 121, 7455-7456

Full TextSupporting Information

Russian-doll like molecule-in-molecule-in-molecule complexes are formed based on large hydrogen-bonded capsules

Structural examination of supramolecular architectures by electrospray ionization mass spectrometry

C. A. Schalley, J. M. Rivera, T. Martin, J. Santamaria, G. Siuzdak, J. Rebek, Jr.

Eur. J. Org. Chem. 1999, 1325-1331

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Reversibly self-assembling host-guest complexes of the softball type are characterized by electrospray ionization mass spectrometry. Quaternary ammonium ions serve simultaneously as guests and ion labels. Isotope pattern analysis, inclusion of labeled guests, heterodimer experiments, size and shape dependence, and collision-induced fragmentation reveal that the capsular structure of these hydrogen-bonded complexes is retained in the gas phase with the guests inside their cavity. The results parallel findings from solution-phase NMR experiments and show that the guests are encapsulated inside the cavity. These results also demonstrate the value of mass spectrometry for high-order structural examination of supramolecular architecture.

Synthesis and characterization of a unimolecular capsule

M. S. Brody, C. A. Schalley, D. M. Rudkevich, J. Rebek, Jr.

Angew. Chem. Int. Ed. 1999, 38, 1640-1644; Angew. Chem. 1999, 111, 1738-1742

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There is more than one way to assemble the two halves of a tethered, urea-substituted calix[4]arene dimer (shown schematically): formation of unimolecular capsules (far left), dimers, or oligomers. By combination of NMR spectroscopy and electrospray mass spectrometry, a hexamethylene spacer was shown to be exactly right to permit the preferential formation of a unimolecular capsule under inclusion of solvent or other guest molecules.

Investigating molecular recognition by mass spectrometry: Characterization of calixarene-based self-assembling capsule hosts with charged guests

C. A. Schalley, R. K. Castellano, M. S. Brody, D. M. Rudkevich, G. Siuzdak, J. Rebek, Jr.

J. Am. Chem. Soc. 1999, 121, 4568-4579

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Capsules derived from the reversible assembly of calixarene tetraureas have been characterized by electrospray ionization mass spectrometry. Ion labeling was achieved through the encapsulation of ammonium ions as guest molecules. The gas-phase ion structure of the parent calixarene was determined by isotope pattern analysis, inclusion of labeled guests, and collision experiments. Competition experiments with different guest ions revealed a clear dependence of the; encapsulation process on the size and shape of the guest ions. The formation of several different heterodimeric capsules, a covalently bridged capsule, and larger dumbbell-like aggregates containing two and three charged guests has been observed. The results parallel previous findings from NMR experiments in solution and thus support the validity of the MS method for characterization of these complexes in the gas phase.

On the formation of the carbon dioxide anion radical CO2-. in the gas phase

D. Schr”öder, C. A. Schalley, J. N. Harvey, H. Schwarz

Int. J. Mass Spectrom. 1999, 185-187, 25-35

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Although carbon dioxide is well known to have a negative electron affinity, the CO2-. anion radical can be generated in several types of mass spectrometric experiments, such as collisional activation of carboxylate ions or double electron transfer to CO2+.. In particular, it is shown that vibrational excitation of the precursor cations increases the yield of the bound anion radicals in charge inversion experiments. Combined application of experimental and theoretical means indicates that for bent geometries, the (2)A(1) state of the carbon dioxide anion radical is stable against electron detachment in the mu s timescale of the experiments. In a chemical sense, the CO2-. anion radical can be regarded as an activated carbon dioxide unit in which the C-O bonds are weakened and the carbon center exhibits distinct radical character. Thus, CO2-. constitutes a new type of distonic anion in which the charge and the unpaired electron are located in different symmetry planes.

Characterization of Self-Assembling Encapsulation Complexes in the Gas Phase and Solution

C. A. Schalley, T. Martin, U. Obst, J. Rebek, Jr.

J. Am. Chem. Soc. 1999, 121, 2133-2138

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The interaction of quaternary ammonium ion guests with self-assembling hosts was examined by 1H NMR and ESI-MS experiments. The hosts consist of four identical, self-complementary subunits, reversibly joined in a capsular assembly through hydrogen bonding. Both approaches show the ammonium ions to be encapsulated, held within the hollow shell of the capsule. Competition experiments with a series of different guest ions reveal a characteristic size selectivity. The NMR and MS methods are complementary: MS easily reveals the formation of heterotetramers from different subunits that could not be determined by NMR, while NMR allowed competitive encapsulations of neutral and ionic guests that were not possible to detect with MS. These competition experiments gave a lower limit of ca. 3.6 kcal/mol for the contribution of cation- interactions involved in the encapsulation of the ionic guests.

Mass-spectrometric experiments together with electronic structure calculations support the existence of the elusive ammonia oxide molecule and its radical cation

M. Br”önstrup, D. Schrö”der, I. Kretzschmar, C. A. Schalley, H. Schwarz

Eur. J. Inorg. Chem. 1998, 1529-1538

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Mass-spectrometric experiments were combined with ab initio calculations to explore the cationic and neutral [H-3,N,O](.+/0) potential energy surfaces and relevant anionic species. The calculations predict the existence of three stable cationic and neutral [H-3,N,O](.+/0) isomers, i.e, ammonia oxide H3NO.+/0 (1(.+/0)), hydroxylamine HNOH2.+/0 (3(.+/0)) and the imine-water complex HNOH2.+/0 (3(.+/0)). Hydroxylamine 2 represents the most stable isomer on the neutral surface (E-rel = 0), and the metastable isomers 1 (E-rel = 24.8 kcal mol(-1)) and 3 (E-rel = 61.4 kcal mol(-1)) are separated by barriers of 49.5 kcal mol(-1) and 64.2 kcal mol(-1), respectively. Adiabatic ionization of 2 (IEa = 9.15 eV) yields 2(.+), which is 21.4 kcal mol(-1) more stable than 1(.+) and 36.4 kcal mol(-1) more stable than 3(.+). The barriers associated with the isomerizations of the cations are 58.6 kcal mol(-1) for 2(.+) --> 1(.+) and 71.4 kcal mol(-1) for 2(.+) --> 3(.+). Collisional activation (CA) and unimolecular decomposition (MI) experiments allow for a clear distinction of 1(.+) from 2(.+). Besides, neutralization/reionization (NR) experiments strongly support the gasphase existence of the long-sought neutral ammonia oxide.

A combined neutralization-reionization mass spectrometric and theoretical study of oxyallyl and other elusive [C3,N4,O] neutrals

C. A. Schalley, S. Blanksby, J. N. Harvey, D. Schr”öder, W. Zummack, J. H. Bowie, H. Schwarz

Eur. J. Org. Chem. 1998, 987-1009

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Five different anionic [C-3,H-4,O](.-) isomers, i.e., the radical anions of acrolein, acetyl carbene, formyl methyl carbene, methoxy vinylidene, and oxyallyl are generated in an ion beam mass spectrometer and subjected to neutralization-reionization (NR) mass spectrometric experiments including neutral and ion decomposition difference (NIDD) mass spectrometry; the latter allows for the examination of the neutrals' unimolecular reactivity. Further, the anionic, the singlet and triplet neutral, and the cationic [C-3,H-4,O](.-/0/.+) potential-energy surfaces are calculated at the B3LYP/6-311++G(d,p) level of theory. For some species, notably the singlet state of oxyallyl, the theoretical treatment is complemented by G2, CASSCF and MR-CI calculations. Theory and experiment are in good agreement in that at the neutral stage (i) acrolein does not react within the mu sec timescale, (ii) acetyl and formyl methyl carbenes isomerize to methyl ketene, (iii) methoxy vinylidene rearranges to methoxy acetylene, (iv) singlet (1)A(1) oxyallyl undergoes ring closure to cyclopropanone, and (v) triplet B-3(2) oxyallyl may have a Lifetime sufficient to survive a NR experiment.

Reappraisal of the spin-forbidden unimolecular decay of the methoxy cation

M. Aschi, J. N. Harvey, C. A. Schalley, D. Schr”öder, H. Schwarz

Chem. Commun. 1998, 531-532

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The mechanism of the unimolecular loss of H-2 from triplet methoxy cations ((CH3O+)-C-3) is revised.

Mass spectrometric approaches to the reactivity of transient neutrals

C. A. Schalley, G. Hornung, D. Schröder, H. Schwarz

Chem. Soc. Rev. 1998, 27, 91-104

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During the past few years, Neutralisation-Reionisation Mass Spectrometry (NRMS) has developed from a method for the generation and structural characterisation of elusive and highly reactive neutral molecules to a useful tool for probing their chemical reactivity. Three major principles can be distinguished: (i) peak shape analysis, (ii) activation of the neutrals by collisions or light, and (iii) variation of the neutrals' lifetimes. Several methodological approaches are discussed in conjunction with illustrating examples for the chemical reactivity of transient neutrals.

Mass spectrometry as a tool to probe the gas-phase reactivity of neutral molecules

C. A. Schalley, G. Hornung, D. Schr”öder, H. Schwarz

Int. J. Mass Spectrom. Ion Processes 1998, 172, 181-208

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Neutralization-reionization (NR) and charge-reversal (CR) mass spectrometric experiments can be combined to investigate the reactivity of neutrals generated in high energy collisions. Provided that the species under study exists as anion, neutral, and cation, the reactions of neutral molecules can be distinguished from those of projectile and recovery ions by taking neutral and ion decomposition difference (NIDD) mass spectra. The scope and limitation of this approach are discussed in detail for several diatomic species and selected polyatomic molecules. The NIDD spectra of tightly bound diatomics exhibit only minor signals which can be interpreted within the framework of vertical electron transfer processes and their Franck-Condon factors. More significant features arise for systems with weaker bonds in either the neutral or one of the charged states, for example, the different oxidation states of chlorine and hypochlorite, i.e. Cl-2(+.)/Cl-2/Cl-2(-.) and ClO+/(ClOClO-)-Cl-., respectively. The NIDD spectra of some polyatomic compounds demonstrate the performance of the method for the elucidation of the neutrals' structures and reactivities. The [C,H,O-2](-/./+) system is studied in detail and may serve as a model system for prototype carboxylate and acylium ions, HCOO- and HOCOO+, respectively. The NIDD spectra of peroxide molecular cations reveal the intrinsic features of peroxidic systems in their different oxidation states, i.e. preferential O-O bond rupture in the neutral species as compared to a favorable O-O bond cleavage in the ionic systems. The gas-phase reactivity of small alkoxy radicals is dominated by alpha-cleavages, and Barton-type 1,5-hydrogen migrations are observed for larger alkoxy radicals. Application of NIDD to probe the neutral species generated by electron detachment from the (CH2COO-)-C-. distonic ion reveals the potential of the method to study diradicals. Finally, two C2N3 isomers are discussed as extreme examples in which the neutrals do not exhibit any distinct reactivity under the experimental conditions chosen.

Ether oxides: A new class of stable ylides? A theoretical study of methanol oxide and dimethyl ether oxide

C. A. Schalley, J. N. Harvey, D. Schr”öder, H. Schwarz

J. Phys. Chem. A 1998, 102, 1021-1035

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The potential energy surfaces of neutral methanol oxide and dimethyl ether oxide and their anion and cation radicals have been calculated at the BECKE3LYP/6-311++G(d,p) level of theory. Both neutral singlet methanol and dimethyl ether oxides are predicted to correspond to local minima on their potential energy surfaces. Natural bonding orbital (NBO) population analysis reveals a distinct ylidic character for these species. Upon increasing methyl substitution, the R2O+-O- ylide structure is stabilized energetically because of the better charge distribution of the formally positive central oxygen atom; thus, the energy differences relative to the R2O + O-3 exit channel decrease significantly. The barrier for the 1,2-hydrogen migration in methanol oxide to yield methyl hydroperoxide amounts to only 5 kcal/mol, whereas the methyl shift in dimethyl ether oxide to afford dimethyl peroxide demands >40 kcal/mol and can proceed by retention or inversion of the configuration at the migrating carbon. The kinetic stabilization of the latter is instead determined either by the loss of a methyl radical or by spin crossing to the triplet surface followed by O atom loss. For this process, the minimal-energy crossing point of the two neutral surfaces was located. The corresponding cation radicals of methanol and dimethyl ether oxide rest in rather deep wells, and their geometries are not too different from those of the neutrals, Therefore, neutralization-reionization mass spectrometry may allow generation and identification of the neutral species, provided that the cation-radical precursors can be made. Furthermore, the kinetic stability of neutral dimethyl ether oxide may be sufficient for its detection in matrix isolation experiments.

A study of the gas-phase reactivity of neutral alkoxy radicals by mass spectrometry: alpha-cleavages and Barton-type hydrogen migrations

G. Hornung, C. A. Schalley, M. Dieterle, D. Schr”öder, H. Schwarz

Chem. Eur. J. 1997, 3, 1866-1883

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The reactivity of neutral alkoxy radicals in the absence of any interfering intermolecular interactions is investigated by means of the recently introduced method of neutral and ion decomposition difference (NIDD) spectra. These are obtained from quantitative analysis of the corresponding neutralization - reionization (NR) and charge reversal (CR) mass spectra. The following trends emerge: alkoxy radicals with short (C-1 - C-3) or branched alkyl chains give rise to alpha-cleav age products, whereas longer-chained alkoxy radicals undergo 1,5-hydrogen migrations from carbon to oxygen, that is, Barton-type chemistry. This facile rearrangement has been studied in detail for n-pentoxy radicals by isotopic labeling experiments and computation at the Becke 3LYP/6-31G* level of theory. Further, the NIDD spectra of 3-methylpentoxy radicals permit for the first time the identification of the diastereoselectivity of the gas-phase hydrogen migrations. The results from the NIDD method are compared to those from earlier studies in the condensed phase. This new mass spectrometric approach is suggested as a tool for the examination of intramolecular reactions of free alkoxy radicals which can usefully complement theoretical studies.

On the oxenoid character of alkylperoxy anions and their lithium compounds: A combined mass-spectrometric and theoretical investigation

C. A. Schalley, D. Schr”öder, H. Schwarz, K. Mö”bus, G. Boche

Chem. Ber./Recl. 1997, 130, 1085-1097

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A combined mass-spectrometric and theoretical approach has been used for an investigation of the gas-phase chemistry of two representative alkylperoxy anions. Metastable CH3OO- ions undergo unimolecular loss of molecular hydrogen and formaldehyde yielding HCO2- and OH-, respectively. The observed reactivity is in pleasing agreement with calculations of the [C,H-3,O-2](-) and [C,H,O-2](-) potential-energy surfaces at the BECKE3LYP/6-311++G** level of theory. Upon exhaustive methylation of the alpha-position as in t-C4H9OO- anions, the reactivity switches completely to an elimination of (CH3)(2)C=CH2 giving rise to the formation of HOO-. The results obtained for the ''bare'' alkylperoxy anions are used for the analysis of the EI mass spectrum of (t-C4H9OOLi)(12) dodecamers, which thermally decompose in the inlet system at a probe temperature of ca. 130 degrees C. The decomposition is rationalized by a mechanism involving nucleophilic attach of one 1-C4H9OOLi subunit on the oxenoid oxygen atom of a second t-C4H9OOLi moiety. This reaction may produce t-C4H9OOOLi trioxy species as intermediates, which rapidly decompose to singlet dioxygen and t-C4H9OLi.

Substituent effects on Fe+-mediated [4+2] cycloadditions in the gas phase

K. Schroeter, C. A. Schalley, D. Schr”öder, H. Schwarz

Helv. Chim. Acta 1997, 80, 1205-1220

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The Fe+-mediated [4+2] cycloaddition of dienes with alkynes has been examined by four-sector ion-beam and ion cyclotron resonance mass spectrometry. Prospects and limitations of this reaction were evaluated by investigating several Me-substituted ligands. Me Substitution at C(2) and C(3) of the diene, i.e., 2-methylbuta-1,3-diene, 2,3-dimethylbuta-1,3-diene, hardly disturbs the cycloaddition. Similarly, variation of the alkyne by use of propyne and but-2-yne does not affect the [4 + 2] cycloaddition step, but allows for H/D exchange processes prior to cyclization. In contrast, Me substituents in the terminal positions of the diene moiety (e.g., penta-1,3-diene, hexa-2,4-diene) induce side reactions, namely double-bond migration followed by [3+2] and [5+2] cycloadditions, up to almost complete suppression of the [4+2] cycloaddition for 2,4-dimethylhexa-2,4-diene. Similarly, alkynes with larger alkyl substituents (pent-1-yne, 3,3-dimethylbut-1-yne) suppress the [4+2] cycloaddition route. Stereochemical effects have been observed for the (E)- and (Z)-penta-1,3-diene ligands as well as for (E, E)- and (E,Z)-hexa-2,4-diene. A mechanistic explanation for the different behavior of the stereoisomers in the cyclization reaction is developed. Further,the regiochemical aspects operative in the systems ethoxyacetylene/pentadiene/Fe+ and ethoxyacetylene/isoprene/Fe+ indicate that substituents avoid proximity.

Gas-phase ion chemistry of peroxides

C. A. Schalley

Ph. D. Thesis, TU Berlin D83, Shaker, Herzogenrath/Germany 1997

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Dimethyl peroxide radical cation: A new theoretical and experimental approach to the C2H6O.+ potential energy surface

C. A. Schalley, A. Fiedler, G. Hornung, R. Wesendrup, D. Schr”öder, H. Schwarz

Chem. Eur. J. 1997, 3, 626-638

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The structure and the unimolecular fragmentations of the metastable dimethyl peroxide radical cation have been investigated by mass spectrometric and isotopic labeling methods as well as high-level ab initio calculations. In line with the theoretical results, neutralization-reionization and charge reversal experiments suggest that ionized dimethyl peroxide bears a CH3OOCH3.+ connectivity. In the cation the O-O bond dissociation energy is larger than that of the neutral counterpart; in contrast, the C-O bond strength is slightly and that of the C-H bond significantly reduced upon ionization. These energetic changes upon one-electron oxidation of CH3OOCH3 are also reflected in the NR and CR mass spectra of CH3OOCH3.+. Further, for metastable CH3OOCH3.+ two major fragmentation pathways are observed: 1) Loss of a hydrogen atom by cleavage of a C-H bond is associated with a skeletal reorganization, which gives rise to a proton-bound formaldehyde dimer. 2) The expulsion of a CH3O. radical leads to protonated formaldehyde in a surprisingly specific double hydrogen transfer involving a [CH3OH/CH2O](.+) ion/dipole complex as central intermediate; this complex also accounts for other minor fragmentation channels. The structures of intermediates and transition states are calculated with the BECKE 3LYP density-functional method employing a 6-311++G** basis.

On the cleavage of the peroxide O-O bond in methyl hydroperoxide and dimethyl peroxide upon protonation

C. A. Schalley, M. Dieterle, D. Schr”öder, H. Schwarz, E. Uggerud

Int. J. Mass Spectrom. Ion Processes 1997, 163, 101-119

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The unimolecular decays of protonated methyl hydroperoxide and dimethyl peroxide have been studied by tandem mass spectrometric techniques in combination with isotopic labeling as well as computational methods. The potential-energy surfaces calculated at the BECKE3LYP/6-311++G** level of theory are in good agreement with the experimental findings. The decomposition of the protonated peroxides can be described by a general mechanistic scheme which involves rearrangement to proton-bridged complexes, i.e. [CH2O-H-OH2](+) and [CH2O-H-O(H)CH3](+), respectively. When formed unimolecularly via rearrangement of the protonated peroxides, these complexes are rovibrationally highly excited; consequently, their fragmentations are affected remarkably as compared to proton-bound complexes of lower internal energy which are independently generated from the corresponding alcohol and carbonyl compounds in a chemical ionization plasma. For methyl hydroperoxide, both oxygen atoms can be protonated, giving rise to two isomeric cations with rather similar heats of formation but entirely different fragmentation behaviors. Cleavage of the O-O bond in dimethyl peroxide upon protonation results in proton- as well as methyl-cation-bridged intermediates, e.g. [CH2O-H-O(H)CH3](+) and [CH2O-CH3-OH2](+).

Covalent assistance in metal-mediated [4+2]cycloadditions of butadiene and acetylene in the gas phase

K. Schroeter, C. A. Schalley, R. Wesendrup, D. Schr”öder, H. Schwarz

Organometallics 1997, 16, 986-994

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The reactions of butadiene and acetylene, both complexed to atomic metal cations M(+) (M Cr, Mn, Fe, Co, Cu), have been investigated using ion-beam four-sector and Fourier transform ion cyclotron resonance mass spectrometry. For all metal ions studied, with the exception of Cu+, the reaction proceeds as a formal [4 + 2] cycloaddition involving 1,4-cyclohexadiene/M(+) as an intermediate. This subsequently eliminates molecular hydrogen to generate the corresponding benzene/M(+) complexes or bare M(+) and C6H6. The Fe+-mediated reaction has been analyzed in detail, and isotopic labeling reveals that the cyclization step is rate-determining and that dehydrogenation of the six-membered ring occurs specifically from the C(1)/C(4)-positions of the butadiene building block. In conjunction with literature thermochemistry, qualitative potential-energy surfaces for the [4 + 2] cycloaddition are derived for M = Cr, Mn, Fe, and Co. The reactions are very efficient for Fe+ and Co+, while Cr+ and Mn+ are less capable of inducing C-C bond formation. Finally, Cu+ with its closed-shell s(0)d(10) electronic ground state does not mediate the [4 + 2] cycloaddition at all. These differences are explained in terms of a model which invokes the active participation of the transition metal's d orbitals (covalent assistance), rather than mere Lewis-acid catalysis, which is known to catalyze many Diels-Alder reactions in the condensed phase.

Gas-phase experiments aimed at probing the existence of the elusive water oxide molecule

D. Schr”öder, C. A. Schalley, N. Goldberg, J. Hrusak, H. Schwarz

Chem. Eur. J. 1996, 2, 1235-1242

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Collisional activation, charge reversal, and six different neutralization-reionization mass spectrometric experiments with [H-2,O-2](.+) radical cations and [H-2,O-2](.-) radical anions were performed in order to probe the predicted existence of neutral water oxide, H2OO, the long sought after tautomer of hydrogen peroxide, HOOH. The experiments together with ab initio calculations indicate that H2OO is a local minimum on the [H-2,O-2] potential-energy surface, and the elusive molecule seems to be formed as a transient upon neutralization of the corresponding radical cation H2OO.+ in the gas phase.

A neutralization-reionization mass spectrometric study of alkyl hydroperoxide cation radicals and four distinguishable [C,H3,O2]+ isomers

C. A. Schalley, D. Schr”öder, H. Schwarz

Int. J. Mass Spectrom. Ion Processes 1996, 153, 173-199

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The cation radicals of simple alkyl hydroperoxides ROOH(+.) (R = CH3, C2H5, iso-C3H7, tert-C4H9) have been studied by various mass spectrometric techniques, and neutralization-reionization (NR) methods in particular. Collisional activation (CA) of the beam of fast neutrals before reionization (NCR) and collision experiments with the mass-selected survivor ions (NR/CA) demonstrate that the peroxide O-O bond remains intact in ionized methyl hydroperoxide. In the series of hydroperoxides, the abundances of the survivor ions decrease with a-substitution, which can be traced back to increasing contributions of [R(+)/(OOH)-O-.] ion/dipole complexes to the parent ion beam. For the elucidation of ion structures, (+)NR(-) experiments are shown to be particularly helpful, in addition, they also provide insight into the bonding situations in hydroperoxides. Unimolecular loss of H-. from metastable ions (MI) of CH3OOH+. leads to hydroperoxy methyl cations, CH2OOH+, which are characterized by their MI/CA mass spectra. For comparison, four distinguishable [C,H-3,O-2](+) isomers (CH2OOH+, CH3OO+, HC(OH)(2)(+), and H3O+. CO) have been generated and examined by MI, CA, and NR experiments. With the exception of the proton-bound species H3O+. CO, the corresponding neutral [C,H-3,O-2](.) radicals exist as well and do not interconvert into each other. In addition, HOCH2O. radicals can be probed by (-)NR(+) experiments with HOCH2O- anions. Unimolecular and collision-induced decomposition of CH2OOH+ gives rise inter alia to a composite [C,H-2,O](+) peak: which consists of a narrow gaussian and a broad, dish-topped component. The narrow component vanishes in the (+)NR(+) experiment.

Sequential iron(I)-mediated homologization of olefins by methanol and methyl halides in the gas phase

R. Wesendrup, C. A. Schalley, D. Schr”öder, H. Schwarz

Organometallics 1996, 15, 1435-1440

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The gas-phase chemistry of simple Fe(olefin)(+) complexes with CH(3)X (X = OH, F, Cl, Br, I) has been studied by Fourier-transform ion cyclotron resonance mass spectrometry. C-C bond formation between the alkene and CH(3)X occurs via initial insertion of Fe+ into the C-X bond, followed by a migratory insertion of the olefin into the iron-carbon bond of(X)-Fe(CH3)(+). This step constitutes a gas-phase analog of the initial stage in the Ziegler-Natta type C-C bond formation; a combination of subsequent beta-H shift and reductive elimination of HX completes the reaction. In the case of Fe(propene)(+) a remarkable regioselectivity is observed in that the addition results in the exclusive formation of an unbranched Fe(butene)(+) complex. Starting from Fe(ethene)(+), up to two consecutive methylations occur using CH3-. OH as a reactant; with methyl halides the number of methylations varies from 2 (for X = I) to 4 (for X = F). For alkyl halides RX bearing a beta-hydrogen, Fe+-mediated dehydrohalogenation of RX competes efficiently with the C-C coupling of RX and Fe(olefin)(+).

Gas-Phase Reactions of Aliphatic Alcohols with 'Bare' FeO+

D. Schr”öder, R. Wesendrup, C. A. Schalley, W. Zummack, H. Schwarz

Helv. Chim. Acta 1996, 79, 123-132

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Ion/molecule reactions of ?bare? FeO+ with linear and branched aliphatic alcohols have been examined by Fourier-transform ion-cyclotron resonance mass spectrometry. Depending on the chain length of the alcohol, three different types of reactions can be distinguished: (i) Oxidation of the alcohols in the ?-positions, to yield the corresponding carbonyl-Fe+ complexes, involves an initial O&bond;H bond activation of the alcohol resulting in the formation of RO&bond;Fe+&bond;OH as the central intermediate. (ii) The formation of Fe(OH)2+, concomitant by loss of the corresponding neutral alkenes, competes with the generation of neutral OFeOH and a carbocation R+. These couples point to the existence of an intracomplex acid-base equilibrium and are connected with each other by a proton transfer from either acid to the other, e.g. i-C3H7+ + OFeOH&rlarr2;C3H6 + Fe(OH)2+. The process is driven by the Lewis acidity of FeO+ and starts with the abstraction of a hydroxide anion from the alcohol. (iii) For longer alcohols, e.g. pentanol, functionalization of non-activated C&bond;H bonds which are remote from the O functionality is observed. Here, the OH group of the alcohol serves as an anchor, which directs the reactive metal-oxide cation toward a particular site of the hydrocarbon chain.

Gas-phase chemistry of hydrogen peroxide and alkyl hydroperoxides with ''bare'' first-row transition-metal cations Cr+-Co+. Generation of MO+ cations using H2O2 as oxidant

C. A. Schalley, R. Wesendrup, D. Schr”öder, H. Schwarz

Organometallics 1996, 15, 678-683

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The unimolecular reactions of ''bare'' first-row transition-metal cations Cr+-Co+ complexed to small alkyl hydroperoxides and hydrogen peroxide have been examined by means of tandem mass spectrometry. In the metal-mediated reactions of the methyl, ethyl, and isopropyl hydroperoxides disproportionations of the peroxides dominate which involve insertion of the metal ion into the weak O-O bond followed by beta-hydrogen shifts, These processes lead to competitive losses of water, aldehydes, or ketones as thermochemically favored neutral products. In addition, for CH3OOH/M(+) unimolecular dehydrogenation to yield M(OH2)(CO)(+) takes place, The metal complexes of tert-butyl hydroperoxide give rise to methanol as a side product via an analogous beta-methyl migration and subsequent reductive elimination of methanol, However, for this complex the major reaction channel corresponds to water loss, which is rationalized by a multistep rearrangement, Interestingly, the observed reactivity patterns remain essentially unchanged irrespective of the nature of the metal, This may be attributed to the high thermodynamic stability of the products and the absence of other competing reaction channels, In contrast to the alkyl hydroperoxides, the hydrogen peroxide/M(+) complexes permit oxygen-atom transfer to the metal center to yield inter alia MO(OH)(+), M(OH)(+), and MO(+) cations. As shown previously, the latter can further be used as oxidants in the oxygenation of hydrocarbons.

Intermediacy of proton-bound dimers and ion/dipole complexes in the unimolecular decompositions of dialkyl-peroxide radical cations: Evidence for a coupled proton and hydrogen-atom transfer

C. A. Schalley, D. Schr”öder, H. Schwarz

Helv. Chim. Acta 1995, 78, 1999-2010

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The unimolecular fragmentation reactions of the radical cations of diethyl, diisopropyl, dipropyl, isopropyl propyl, and di(tert-butyl) peroxide have been investigated by mass spectrometric and isotopic labeling techniques. Two competing pathways for unimolecular decomposition in the Cls time regime (metastable ions) are observed: i) A combination of an alpha-C-C bond cleavage and a H migration gives rise to proton-bound dimers of two ketone or aldehyde molecules. ii) Ion/dipole complexes of alkyl cations and alkylperoxy radicals are generated by C-O bond cleavage. These complexes either exhibit direct losses of alkylperoxy radicals, or they rearrange via a coupled proton and H-atom transfer, this sequence of unprecedented isomerizations is completed by losses of alkyl radicals. Collisional activation experiments confirm that the ionic products of the latter process correspond to RR'C=OOH+; these ions can be regarded as protonated carbonyl oxides. In addition, we observe the elimination of alkenes leading to hydroperoxide radical cations and the expulsion of HO2. radicals. The latter process implies a C-C bond formation step between the two alkyl fragments leading to higher alkyl cations.

Bimolecular gas-phase reactions of d-block transition-metal cations with dimethyl peroxide: Trends across the periodic table

R. Wesendrup, C. A. Schalley, D. Schr”öder, H. Schwarz

Chem. Eur. J. 1995, 1, 608-613

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The bimolecular gas-phase reactions of d-block transition-metal cations M(+) with dimethyl peroxide were screened by means of Fourier transform ion cyclotron resonance mass spectrometry. The rich chemistry can be classified into four types of reactions: i) Oxygen-atom transfer to generate MO(+), ii) elimination of radicals, mostly CH3O., iii) intramolecular redox reaction of dimethyl peroxide to form CH3OH, CH2O and CO, and iv) charge transfer from the metal cation to produce CH3OOCH3+. Some general trends became apparent from this study. For example, the ''early'' transition metals almost exclusively induce oxygen transfer to generate MO(+), in line with the notoriously high oxophilicities of these metals, and electron transfer is only observed for Zn+ and Hg+. Both the radical loss and the disproportionation reaction emerge from a rovibrationally highly excited insertion intermediate (CH3O)(2)M(+), and for the first-row metals the branching ratio of the competing processes seems to be affected by the M(+)-OR bond strengths as well as the electronic groundstate configurations of M(+). For the 4d and 5 d cations Ru+-Ag+ and Pt+-Au+, respectively, products resulting from intramolecular redox reactions dominate; this probably reflects the higher propensity of these metal ions to facilitate beta-hydrogen atom shifts.

Structure/Reactivity Relationships in the Unimolecular Gas-Phase Chemistry of Dialkyl Peroxide/Fe+ Complexes

C. A. Schalley, R. Wesendrup, D. Schr”öder, K. Schroeter, H. Schwarz

J. Am. Chem. Soc. 1995, 117, 12235-12242

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Unimolecular and collision-induced fragmentations of dialkyl peroxide/Fe+ complexes, ROOR'/Fe+ (R, R' = methyl, ethyl, isopropyl, tert-butyl), were examined by means of tandem mass spectrometry. In the initial reaction step, iron bis(alkoxide) ions (RO)Fe(OR')(+) are generated by insertion of the metal cation into the weak O-O bond of the peroxide. Depending on the nature of R and R', five major reaction channels have been observed, corresponding to the generation of a methyl radical, methane, and water. Kinetic isotope effects are determined and interpreted in terms of rate-determining steps of the multistep processes. The observed reactivity can be described as a result of the balance of the energetics of beta-hydrogen versus beta-methyl shifts together with product stabilities. A scheme is presented, which correlates the unimolecular reactivity of ROOR'/Fe+ complexes with the substitution pattern in the alpha- and beta-positions of the peroxidic alkyl groups.

Gas-Phase Ion Chemistry of Dimethyl Peroxide with the Bare Transition-Metal Cations Cr+, Mn+, Fe+, and Co+

C. A. Schalley, R. Wesendrup, D. Schr”öder, T. Weiske, H. Schwarz

J. Am. Chem. Soc. 1995, 117, 7711-7718

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Unimolecular and collision-induced fragmentations of dimethyl peroxide complexes (CH3OOCH3)M(+), with M(+) = Cr+, Mn+, Fe+, and Co+, were examined by means of tandem mass spectrometry and compared to results from bimolecular gas-phase reactions of the corresponding ''bare'' metal cations with dimethyl peroxide as studied by Fourier transform ion cyclotron resonance (FT-ICR) experiments. Bismethoxide metal ions, (CH3O)(2)M(+), are generated in the initial reaction step by insertion of the metal cation into the weak O-O bond of the peroxide. In the ion-beam apparatus these intermediates undergo a variety of processes involving beta-H shifts, intramolecular redox reactions, and radical losses, depending on the respective metal cations. In contrast, the insertion products (CH3O)(2)M(+) formed in the ion/molecule reactions of dimethyl peroxide with bare metal cations in the FT-ICR mass spectrometer decompose to complexes M(OCH3)(+) with concomitant loss of a methoxy radical, a process which is not observed in the unimolecular dissociation of metastable (CH3OOCH3)M(+) ions. The distinct differences between the unimolecular chemistry of dimethyl peroxide/M(+) complexes in the ion-beam apparatus and the bimolecular reactions of M(+) with dimethyl peroxide in the FT-ICR are explained in terms of internal energy effects, which result from the method of ion formation. This analysis is further supported by ligand-exchange reactions and collisional-activation experiments. In addition, analogies and differences of the gas-phase chemistry of dimethyl peroxide with M(+) as compared to metal-catalyzed decomposition of dialkyl peroxides in the condensed phase are discussed.

Activation of C-C and C-H Bonds of cis- and trans-1-Acetyl-2-methylcyclopropane by Bare Metal(I) Cations in the Gas Phase: Comparative Study of the First-Row Transition-Metal Ions Cr+-Cu+

C. A. Schalley, D. Schröder, H. Schwarz

Organometallics 1995, 14, 317-326

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Stereochemical effects on transition-metal-mediated C-H and C-C bond activation are probed by examining unimolecular fragmentation reactions of 1-acetyl-2-methylcyclopropane/M+ complexes (M = Cr+-Cu+) in the gas phase. Basically, three general reaction types can be distinguished: (i) metal-induced ring cleavage leading to losses of molecular hydrogen, ethene, and acetaldehyde (these C-H and C-C bond activation processes can be ascribed to remote functionalization of an acyclic hexenone/M+ intermediate), (ii) insertion of the metal in an a-C-C bond and subsequent decarbonylation to yield the corresponding olefin/M+ complexes, and (iii) geminal C-C bond activation leading to the formation of metal carbene complexes as intermediates for M = Cr, Mn, Co, and Ni. These carbene complexes eventually give rise to unimolecular losses of ethene or propene. Direct C-H bond activation of the terminal methyl group is observed for none of the metals. Stereochemical differences for cis and trans isomers are only observed when the rate-determining steps for C-C bond activation leading to different products, i.e. remote functionalization and decarbonylation, compete directly with each other, and distinct stereochemical effects are observed for the Mn+, Fe+, and Cu+ complexes. The experimental data and the trends within the late firstrow transition metals are compared with previous findings and discussed in terms of activation barriers associated with C-C and C-H bond activation processes and thermodynamics.

Iron(I) Mediated Activation of C-C and C-H Bonds of cis-1-Acetyl-2-Methylcyclopropanes and trans-1-Acetyl-2-Methylcyclopropanes in the Gas-Phase - Competition between Ring-Cleavage and a-CC-Bond Insertion Reactions

C. A. Schalley, D. Schr”öder, H. Schwarz

J. Am. Chem. Soc. 1994, 116, 11089-11097

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Unimolecular and collision induced fragmentations of cis- and trans-1-acetyl-2-methylcyclopropanes, complexed to bare Fe+ cations, were examined by means of tandem mass spectrometry. Stereoselective isotopic labeling data as well as comparative studies with acylic isomers reveal that two reaction pathways are in competition with each other. On the one hand, dehydrogenation, loss of ethene, and loss of acetaldehyde proceed via a common intermediate which is formed via ring cleavage of 1-acetyl-2-methylcyclopropane/Fe+ to yield the corresponding 3-hexen-2-one/Fe+ complexes; these undergo subsequently CH- and CC-bond activation via the remote functionalization mechanism. These ring cleavages are associated with a complete loss of stereochemical features for the cis and trans isomers. In contrast, unimolecular decarbonylation occurs stereoselectively and is favored for the trans isomers; this is rationalized via a stereoselective formation of cis- and trans-eta(3)-allyl complexes in the course of the electrocyclic ring-opening process. Furthermore, interconversion reactions, thermodynamic and kinetic aspects, and isotope effects of the CH- and CC-bond activation processes are discussed.