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

Abstract

Direct reaction of potassium molybdate (with natural abundance Mo or enriched with 92Mo or 100Mo) with excess hydrolyzed homocitric acid-γ-lactone in acidic solution resulted in the isolation of a cis-dioxo bis-homocitrato molybdenum(VI) complex, K2[*MoO2(R,S-H2homocit)2]·2H2O (1) (*Mo=Mo, 1; 92Mo, 2; 100Mo, 3; H4homocit=homocitric acid-γ-lactone·H2O) and K2[MoO2(18O-R,S-H2homocit)2]·2H2O (4). The complex has been characterized by elemental analysis, FT-IR, solid and solution 13C 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 α-alkoxy and α-carboxy groups, while the β- and γ-carboxylic acid groups remain uncomplexed, similar to the coordination mode of homocitrate in the Mo-cofactor of nitrogenase. In the IR spectra, the MoO stretching modes near 900cm−1 show 2–3cm−1 red- and blue-shifts for the 92Mo-complex (2) and 100Mo-complex (3) respectively compared with the natural abundance version (1). At lower frequencies, bands at 553 and 540cm−1 are assigned to νMo–O vibrations involving the alkoxide ligand. At higher frequencies, bands in the 1700–1730cm−1 region are assigned to stretching modes of protonated carboxylates. In addition, a band at 1675cm−1 was observed that may be analogous to a band seen at 1677cm−1 in nitrogenase photolysis studies. The solution behavior of (1) in D2O was probed with 1H and 13C 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).