VFe3S4 Single and Double Cubane Clusters: Synthesis Structures and Dependence of Redox Potentials and Electron Distribution on Ligation and Heterometal

摘要

Both vanadium and molybdenum cofactor clusters are found in nitrogenase. In biomimetic research, many fewer heterometal MFe3S4 cubane-type clusters have been synthesized with M = V than with M = Mo because of the well-established structural relationship of the latter to the molybdenum coordination unit in the enzyme. In this work, a series of single cubane and edge-bridged double cubane clusters containing the cores [VFe3(μ3-S)4]²⁺ and [V2Fe6(μ3-S)6(μ4-S)2]²⁺ have been prepared by ligand substitution of the phosphine clusters [(Tp)VFe3S4(PEt3)3]¹⁺ and [(Tp)2V₂Fe₆S₈(PEt₃)₄]. The single cubanes [(Tp)VFe₃S₄L₃]²⁻ and double cubanes [(Tp)₂V₂Fe₆S₈L₄]⁴⁻ (L = F⁻, N₃⁻, CN⁻, PhS⁻) are shown by X-ray structures to have trigonal symmetry and centrosymmetry, respectively. Single cubanes form the three-member electron transfer series [(Tp)VFe₃S₄L₃]^{3−,2−,1−}. The ligand dependence of redox potentials and electron distribution in cluster cores as sensed by ⁵⁷Fe isomer shifts (δ) have been determined. Comparison of these results with those previously determined for the analogous molybdenum clusters (Pesavento, Berlinguette, and Holm Inorg. Chem. >2007, 46, 510) allows detection of the influence of heterometal M on the properties. At constant M and variable L, redox potentials are lowest for π-donor ligands and largest for cyanide and relate approximately with decreasing ferrous character in clusters with constant charge z = 2−. At constant L and z and variable M, E_V > E_Mo and δavV<δavMo, demonstrating that M = Mo clusters are more readily oxidized and suggesting a qualitative relation between lower potentials (greater ease of oxidation) and ferrous character.