Heterometal Cuboidal Clusters Mfe4s6(pet3)4cl (m = V, Mo) - Synthesis, Structural-analysis By Crystallography and Exafs, and Relevance To the Core Structure of the Iron Molybdenum Cofactor of Nitrogenase

The cluster self-assembly system MCl3(THF)3/2FeCl2(PEt3)2/4-7(Me3Si)2S affords, after separation of cluster coproducts, the isomorphous compounds MFe4S6(PEt3)4Cl (M = V, Mo) in modest yields. These species were identified by spectroscopic methods and X-ray diffraction. The compounds crystallize in trigonal space group R3cBAR; the cluster molecules are isostructural and essentially isometric. They consist of a cuboidal Fe4S3 fragment that is bridged by three mu2-S atoms to a M-PEt3 unit whose M-P bond is coincident with an imposed C3 axis. Coordination is completed at the three symmetry-related Fe sites by PEt3 ligands and at the Fe site on the C3 axis by chloride. The trigonal pyramidal coordination units FeS3P and MoS3P are infrequently observed and unprecedented, respectively, and the cuboidal fragment has been observed previously only in several Fe-S-nitrosyl clusters. The cluster structure presents atoms at a series of distances from the M or Fe atoms and has been utilized in a detailed test of distance determinations in a new approach to the analysis of EXAFS data using theoretical phases and amplitudes (the GNXAS approach). The Mo and Fe EXAFS of the cluster MoFe4S6(PEt3)4Cl have been analyzed in detail and first coordination shell distances determined with high accuracy. The contribution of multiple scattering pathways in the EXAFS of the cluster has been evaluated. The relevance of the results to the FeMo-cofactor structure as recently deduced from protein crystallography is considered.