Single- and Double-Cubane Clusters in the Multiple Oxidation States [VFe₃S₄]^3+,2+,1+

A new series of cubane-type [VFe3S4]z clusters (z = 1+, 2+, 3+) has been prepared as possible precursor species for clusters related to those present in vanadium-containing nitrogenase. Treatment of [(HBpz3)VFe3S4Cl3]2- (2, z = 2+), protected from further reaction at the vanadium site by the tris(pyrazolyl)hydroborate ligand, with ferrocenium ion affords the oxidized cluster [(HBpz3)VFe3S4Cl3]1- (3, z = 3+). Reaction of 2 with Et3P results in chloride substitution to give [(HBpz3)VFe3S4(PEt3)3]1+ (4, z = 2+). Reaction of 4 with cobaltocene reduced the cluster with formation of the edge-bridged double-cubane [(HBpz3)2V2Fe6S8(PEt3)4] (5, z = 1+, 1+), which with excess chloride underwent ligand substitution to afford [(HBpz3)2V2Fe6S8Cl4]4- (6, z = 1+, 1+). X-ray structures of (Me4N)[3], [4](PF6), 5, and (Et4N)4[6]·2MeCN are described. Cluster 5 is isostructural with previously reported [(Cl4cat)2(Et3P)2Mo2Fe6S8(PEt3)4] and contains two VFe3S4 cubanes connected across edges by a Fe2S2 rhomb in which the bridging Fe-S distances are shorter than intracubane Fe−S distances. Mössbauer (2−5), magnetic (2−5), and EPR (2, 4) data are reported and demonstrate an S = 3/2 ground state for 2 and 4 and a diamagnetic ground state for 3. Analysis of 57Fe isomer shifts based on an empirical correlation between shift and oxidation state and appropriate reference shifts results in two conclusions. (i) The oxidation 2 → 3 + e- results in a change in electron density localized largely or completely on the Fe3 subcluster and associated sulfur atoms. (ii) The most appropriate charge distributions are [V3+Fe3+Fe2+2S4]2+ (Fe2.33+) for 1, 2, and 4 and [V3+Fe3+2Fe2+S4]3+ (Fe2.67+) for 3 and [V2Fe6S8(SEt)9]3+. Conclusion i applies to every MFe3S4 cubane-type cluster thus far examined in different redox states at parity of cluster ligation. The formalistic charge distributions are regarded as the best current approximations to electron distributions in these delocalized species. The isomer shifts require that iron atoms are mixed-valence in each cluster.