Takudrei

207
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— 2015

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.

Title207
Formation and transmetalation mechanisms of homo- and heterometallic (Fe/Zn) trinuclear triple-stranded side-by-side helicates
AuthorB. Akhuli, L. Cera, B. Jana, S. Saha, C.A. Schalley, P. Ghosh
Date20150416
IdentifierDOI 10.1021/ic502855g
Source(s)
CitationInorg. Chem., 2015, 4231-4242
TypeText