Syntheses, crystal structures and biological relevance of glycolato and S-lactato molybdates

Abstract

Glycolato and S-lactato complexes containing the dioxomolybdenum(VI) moiety have been synthesized for studies on the role of the α-hydroxycarboxylato anion in the iron molybdenum cofactor of nitrogenase. The ligands in these complexes, vis K 2[MoO 2(glyc) 2] · H 2O (H 2glyc=glycolic acid, C 2H 4O 3) ( 1) and {Na 2[MoO 2( S-lact) 2]} 3 · 13H 2O (H 2lact=lactic acid, C 3H 6O 3) ( 2) chelate through their α-alkoxyl and α-carboxyl oxygen atoms. In contrast, octanuclear K 6[(MoO 2) 8(glyc) 6(Hglyc) 2] · 10H 2O ( 3) formed by the reduction of the glycolato complex ( 1), features three different ligand binding modes: (i) non-bridging and bridging bidentate coordination of α-alkoxyl and α-carboxyl groups, and (ii) bidentate bridging using α-carboxyl group, leaving the α-alkoxyl group free. The octanuclear skeleton shows strong metal–metal interactions. The coordination modes in ( 1) and ( 2) mimic that of homocitrate to the iron molybdenum cofactor (FeMo-co) of nitrogenase. The bidentate coordination of α-alkoxyl and α-carboxyl groups shows that bond of α-carboxyl group to Mo is less susceptible to the oxidation state of molybdenum compared with the Mo-α-alkoxyl bond. This is supported by the dinuclear coordination of α-carboxyl group with free α-alkoxyl group in glycolato molybdate(V) ( 3).