HETEROMETAL CUBANE-TYPE CLUSTERS - A REFE3S4 SINGLE-CUBANE CLUSTER BY CLEAVAGE OF AN IRON-BRIDGED DOUBLE CUBANE AND THE SITE-VOIDED CUBANE [FE3S4] AS A CLUSTER LIGAND

The reaction of the iron(II)-bridged double-cubane cluster compound (Et4N)2[Re2Fe7S8(SEt)12] ((Et4N)2[3]) with 4 equiv of 1,2-bis(dimethylphosphino)ethane (dpme) in acetonitrile affords the single-cubane cluster (Et4N)]ReFe3S4(SEt)4(dpme)] ((Et4N)[4]) in 72% yield as a black crystalline solid. (Et4N)[4] crystallizes in orthorhombic space group Pbc2(1) with a = 12.835 (2) angstrom, b = 21.260 (4) angstrom, c = 28.473 (5) angstrom, and Z = 8. The structure of cluster 4, which was refined to R (R(w)) = 5.22 (5.66)%, reveals the presence of the [ReFe3S4] cubane-type core as contained in cluster 3, with the dmpe ligand bound in a chelating fashion to the Re atom, whose distorted octahedral coordination geometry is completed by an ethanethiolate ligand. The Fe atoms reside in the typical distorted-tetrahedral coordination geometry common to these types of clusters. In the process of bridge cleavage from 3 to 4, the core oxidation state has been reduced by thiolate from [ReFe3S4]4+ to [ReFe3S4]3+. Cluster 4 undergoes two electrochemically reversible processes in acetonitrile solution, which define the three-member core series [ReFe3S4]2+,3+,4+. The S = 2 ground state of 4 was established by magnetic susceptibility and magnetization measurements. Zero-field Mossbauer spectra were fit with two quadrupole doublets, whose isomer shifts indicate an iron mean oxidation state near Fe2.5+, in accord with structural data. Heterometal cubane clusters [MFe3S4] of proven structures now include those with M = V, Mo, W, Re, and Ni. Structurally unproven but probable cubane clusters with M = Co, Ni, Zn, and Cd are formed by the reaction of divalent metal ions with protein-bound Fe3S4 clusters. These collective demonstrate an already extensive family of MFe3S4 clusters and lead to the concept of the site-voided cuboidal cluster Fe3S4 as a semirigid cluster ligand.