Structure and spectroscopy of a bidentate bis-homocitrate dioxo-molybdenum (VI) complex: Insights relevant to the structure and properties of the FeMo-cofactor in nitrogenase

Direct reaction of potassium molybdate (with natural abundance Mo or enriched with Mo-92 or Mo-100) with excess hydrolyzed homocitric acid-gamma-lactone in acidic solution resulted in the isolation of a cis-dioxo bis-homocitrato molybdenum(VI) complex, K-2[*MoO2(R,S-H(2)homocit)(2)]center dot 2H(2)O (1) (*Mo=Mo, 1; Mo-92, 2; Mo-100, 3; H(4)homocit=homocitric add-gamma-lactone center dot H2O) and K-2[MoO2(O-18-R,S-H(2)homocit)(2)]center dot 2H(2)O (4). The complex has been characterized by elemental analysis. FT-IR, solid and solution C-13 NMR, and single crystal x-ray diffraction analysis. The molybdenum atom in (1) is quasi-octahedrally coordinated by two cis oxo groups and two bidentate homocitrate ligands. The latter coordinates via its alpha-alkoxy and alpha-carboxy groups, while the beta- and gamma-carboxylic acid groups remain uncomplexed, similar to the coordination mode of homocitrate in the Mo-cofactor of nitrogenase. In the IR spectra, the Mo=O stretching modes near 900 cm(-1) show 2-3 cm(-1) red- and blue-shifts for the Mo-92-complex (2) and Mo-100-complex (3) respectively compared with the natural abundance version (1). At lower frequencies, bands at 553 and 540 cm(-1) are assigned to v(Mo-O) vibrations involving the alkoxide ligand. At higher frequencies, bands in the 1700-1730 cm(-1) region are assigned to stretching modes of protonated carboxylates. In addition, a band at 1675 cm(-1) was observed that may be analogous to a band seen at 1677 cm(-1) in nitrogenase photolysis studies. The solution behavior of (1) in D2O was probed with H-1 and C-13 NMR spectra. An obvious dissociation of homocitrate was found, even though bound to the high valent Mo(VI). This suggests the likely lability of coordinated homocitrate in the FeMo-cofactor with its lower valence Mo(IV). (C) 2012 Elsevier Inc. All rights reserved.