New EXAFS models for the iron sites of the iron-molybdenum cofactor in nitrogenase: the [(p-CH3C6H4S)2FeS2FeS2MoS2]3- trianion and the [(C6H5O)2FeS2MoS2]2- dianion

This report examines the single-crystal X-ray diffraction structure determinations and Fe EXAFS spectra of a linear trinuclear cluster anion, ((p-CH/sub 3/C/sub 6/H/sub 4/S)/sub 2/FeS/sub 2/FeS/sub 2/MoS/sub 2/)/sup 3 -/(1), and a binuclear cluster anion, ((C/sub 6/H/sub 5/O)/sub 2/FeS/sub 2/MoS/sub 2/)/sup 2 -/(2). It is noted that (1) is the first structurally characterized Mo-Fe-S cluster to contain iron in two distinct sites with localized oxidation states, whereas (2) is the first structurally characterized Mo-Fe-S cluster with terminal oxygen ligation to iron. The structures of (Et/sub 4/N)/sub 3/()pCH/sub 3/C/sub 6/H/sub 4/S)/sub 2/FeS/sub 2/FeS/sub 2/MoS/sub 2/) (3) and (Et/sub 4/N)/sub 2/((C/sub 6/H/sub 5/O)/sub 2/FeS/sub 2/MoS/sub 2/) (4) were solved by conventional Patterson and Fourier techniques. In (3), two out of three Et/sub 4/N/sup +/cations are disordered. Refinement by full-matrix least-squares methods gave rise to a conventional R value of 8.10%, with anisotropic isotropic thermal parameters for the trianion and ordered cation and isotropic thermal parameters for all other atoms. Least-squares refinement of (4) resulted in an R value of 14.35%, with anisotropic thermal parameters for Mo, Fe, S1, and S2 and isotropic thermal parameters for all other atoms. The pseudo mirror plane at y=1/8 relating the two halves of the molecule (with Mo,more » Fe, S1, and S2 residing on the plane) causes severe parameter correlation, thereby preventing further anisotropic refinement. Somewhat specialized refinement procedures must be used to fully refine the structure (in progress).« less