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
E. Nordlander et al., 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, Journal of the American Chemical Society, 115(13), 1993, pp. 5549-5558
The cluster self-assembly system MCl3(THF)3/2FeCl2(PEt3)2/4-7(Me3Si)2S
affords, after separation of cluster coproducts, the isomorphous comp
ounds MFe4S6(PEt3)4Cl (M = V, Mo) in modest yields. These species were
identified by spectroscopic methods and X-ray diffraction. The compou
nds crystallize in trigonal space group R3cBAR; the cluster molecules
are isostructural and essentially isometric. They consist of a cuboida
l 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 coor
dination units FeS3P and MoS3P are infrequently observed and unprecede
nted, respectively, and the cuboidal fragment has been observed previo
usly only in several Fe-S-nitrosyl clusters. The cluster structure pre
sents atoms at a series of distances from the M or Fe atoms and has be
en utilized in a detailed test of distance determinations in a new app
roach to the analysis of EXAFS data using theoretical phases and ampli
tudes (the GNXAS approach). The Mo and Fe EXAFS of the cluster MoFe4S6
(PEt3)4Cl have been analyzed in detail and first coordination shell di
stances determined with high accuracy. The contribution of multiple sc
attering pathways in the EXAFS of the cluster has been evaluated. The
relevance of the results to the FeMo-cofactor structure as recently de
duced from protein crystallography is considered.