α-Hydroxy coordination of mononuclear vanadyl citrate, malate and S-citramalate with N-heterocycle ligand, implying a new protonation pathway of iron–vanadium cofactor in nitrogenase.

Unlike the most of α-alkoxy coordination in α-hydroxycarboxylates to vanadium, novel α-hydroxy coordination to vanadium(IV) has been observed for a series of chiral and achiral monomeric α-hydroxycarboxylato vanadyl complexes [VO(H 2 cit)(bpy)]·2H 2 O ( 1 ), [VO(Hmal)(bpy)]·H 2 O ( 2 ), [VO(H 2 cit)(phen)]·1.5H 2 O ( 3 ), [VO(Hmal)(phen)]·H 2 O ( 4 ), and [ Δ VO( S -Hcitmal)(bpy)]·2H 2 O ( 5 ), [VO(H 2 cit)(phen)] 2 ·6.5H 2 O ( 6 ), which were isolated from the reactions of vanadyl sulfate with α-hydroxycarboxylates and N -heterocycle ligands in acidic solution. The complexes feature a tridentate citrate, malate or citramalate that chelates to vanadium atom through their α-hydroxy, α-carboxy and β-carboxy groups; while the other β-carboxylic acidic group of citrate is free to participate strong hydrogen bonds with lattice water molecule. The neutral α-hydroxy group also forms strong intermolecular hydrogen bonds with water molecule and the negatively-charged α-carboxy group in the environment. The inclusion of a hydrogen ion in α-alkoxy group results in the formation of a series of neutral complexes with one less positive charge. There are two different configurations of citrate with respect to the trans -position of axial oxo group, where the complex with trans -hydroxy configuration seems more stable with less hindrance. The average bond distances of V O hydroxy and V O α-carboxy are 2.196 and 2.003 Å respectively, which are comparable to the V O distance (2.15 Å) of homocitrate in FeV-cofactor of V-nitrogenase. A new structural model is suggested for R -homocitrato iron vanadium cofactor as VFe 7 S 9 C( R -Hhomocit) (H 4 homocit = homocitric acid) with one more proton in homocitrate ligand. [ABSTRACT FROM AUTHOR]