HETEROMETAL DEPENDENCE OF ELECTRON DISTRIBUTIONS AND REDOX POTENTIALS IN CLUSTERS WITH CUBANE-TYPE [MFE3S4]Z CORES

The self-assembly systems [MS4]3-/FeCl2/NaSEt (M = V, Nb) in acetonitrile afford the clusters [V2Fe6S8(SEt)9]3-, [Nb2Fe6S8(SEt)9]3-, and [Nb2Fe6S8(SEt)9]5-, which were isolated as Et4N+ salts. These clusters have the triply-bridged double cubane stereochemistry [(MFe3S4(SEt)3)2(mu-SEt)3]3-, as demonstrated for the M = V cluster by X-ray diffraction. The compound (Et4N)3[V2Fe6S8(SEt)9] crystallizes in hexagonal point group P6(3)/m with a = 17.223(3) angstrom, c = 16.212(3) angstrom, and Z = 2. The cluster anion has imposed D3h symmetry; the individual clusters are separated by a V...V distance of 3.601 angstrom. These clusters, together with others previously reported, form a set [M2Fe6S8(SEt)9]3- whose members (M = V, Nb, Mo, W, Re) possess the same charge, identical ligands, and essentialy congruent structures with only small metric differences. This condition permits the examination of changes in heterometal on cluster properties. Redox potentials for the reversible couples [M2Fe6S8(SEt)9]3-/4- and [M2Fe6S8(SEt)9]4-/5- decrease in the order M = V > Nb > Mo > W > Re and parallel the trend in Fe-57 isomer shifts, M = V < Nb almost-equal-to Mo < W almost-equal-to Re. Reductions become more difficult as the ferrous character of the core increases. Redox reactions are accompanied by changes in electron density that largely occur in Fe3S4 Cluster portions. The concept of cuboidal Fe3S4 as a cluster ligand is considered in terms of isomer shifts of heterometal clusters. The shifts of the majority of such clusters are close to those protein-bound [Fe3S4]0,1-, providing an apparent definition of oxidation states. Isotropic shifts of ligands at iron sites provide a convenient indicator of the ground spin state of the clusters. Cubane-type MFe3S4 clusters are of renewed interest in view of the occurrence of the cuboidal cluster fragment MoFe3S3 in the cofactor of nitrogenase. The possible relationship of these results to the FeMo-cofactor and the FeV-cofactor of nitrogenase is noted. If the cofactor of vanadium-containing nitrogenase has an analogous structure, properties that derive from the heterometal cuboidal fragment may be influenced by vanadium vs molybdenum in a manner similar to that for the cubane-type clusters.