科研之友
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摘要
Unlike the most of alpha-alkoxy coordination in alpha-hydroxycarboxylates to vanadium, novel alpha-hydroxy coordination to vanadium(IV) has been observed for a series of chiral and achiral monomeric alpha-hydroxycarboxylato vanadyl complexes [VO(H(2)cit)(bpy)]center dot 2H(2)O (1), [VO(Hmal)(bpy)]center dot H2O (2), [VO(H(2)cit)(phen)]center dot 1.5H(2)O (3), [VO(Hmal)(phen)]center dot H2O (4), and [(VO)-V-Delta(S-Hcitmal)(bpy)]center dot 2H(2)O (5), [VO(H(2)cit)(phen)](2)center dot 6.5H(2)O (6), which were isolated from the reactions of vanadyl sulfate with alpha-hydroxycarboxylates and N-heterocycle ligands in acidic solution. The complexes feature a tridentate citrate, malate or citramalate that chelates to vanadium atom through their alpha-hydroxy, alpha-carboxy and beta-carboxy groups; while the other beta-carboxylic acidic group of citrate is free to participate strong hydrogen bonds with lattice water molecule. The neutral alpha-hydroxy group also forms strong intermolecular hydrogen bonds with water molecule and the negatively-charged alpha-carboxy group in the environment. The inclusion of a hydrogen ion in alpha-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 alpha-carboxy are 2.196 and 2.003 angstrom respectively, which are comparable to the V-O distance (2.15 angstrom) of homocitrate in FeV-cofactor of V-nitrogenase. A new structural model is suggested for R-homocitrato iron vanadium cofactor as VFe7S9C(R-Hhomocit) (H(4)homocit = homocitric acid) with one more proton in homocitrate ligand.
