Syntheses, crystal structures and biological relevance of glycolato and S-lactato molybdates*1

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 K2[MoO2(glyc)2] · H2O (H2glyc=glycolic acid, C2H4O3) (1) and {Na2[MoO2(S-lact)2]}3 · 13H2O (H2lact=lactic acid, C3H6O3) (2) chelate through their α-alkoxyl and α-carboxyl oxygen atoms. In contrast, octanuclear K6[(MoO2)8(glyc)6(Hglyc)2] · 10H2O (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).