Search

Dimeric mixed-valence oxovanadium citrate [V 2O 3(phen) 3(Hcit)].5H 2O ( 1) (H 4cit = citric acid, phen = 1,10-phenanthroline) was isolated from a weak acidic medium. It could be converted quantitatively into a tetrameric oxovanadium citrate adduct of 1,10-phenanthroline [V 2O 3(phen) 3(Hcit) 2(phen) 3O 3V 2].12H 2O ( 2). This was supported by the trace of infrared spectra and X-ray diffraction patterns. The two compounds feature a bidentate citrate group that chelates only to one vanadium center through their negatively charged alpha-alkoxy and alpha-carboxy oxygen atoms, while the other beta-carboxy and beta-carboxylic acid groups are free to participate in strong intramolecular and intermolecular hydrogen bonding [2.45(1) in 1 and 2.487(2) A in 2], respectively. This is also the case of homocitrato vanadate(V/IV) [V 2O 3(phen) 3( R, S-H 2homocit)]Cl.6H 2O ( 3) (H 4homocit = homocitric acid), which features a binding mode similar to that found in the R-homocitrato iron molybdenum cofactor of Mo-nitrogenase. Moreover, the homocitrato vanadate(V) [VO 2(phen) 2] 2[V 2O 4( R,S-H 2homocit) 2].4H 2O.2C 2H 5OH ( 4) is isolated as a molecular precursor for the formation of mixed-valence complex 3. The V-O alpha-alkoxy and V-O alpha-carboxy bond distances of homocitrate complexes 3 and 4 are 1.858(4) and 1.968(6) av and 2.085(4) and 1.937(5) A, respectively. They are shorter than those of homocitrate to FeVco (2.15 A). The gamma-carboxy groups of coordinated homocitrato complexes 3 and 4, and the free homocitrate salt Na 3(Hhomocit).H 2O ( 5), form strong hydrogen bonds with the chloride ion and the water molecule [2.982(5) in 3, 2.562(9) in 4, and 2.763(1) A in 5], respectively.

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]

Asymmetric citrato dioxovanadates(V), [Hneo]4[V2O4(R-Hcit)(OH)][V2O4(S-Hcit)(OH)] · 4H2O (1) and [Ni(phen)3]2[V2O4(R-Hcit)(OC2H5)][V2O4(S-Hcit)(OC2H5)] · 4H2O (2) and (H4cit = citric acid, neo = 2,9-dimethyl-1,10-phenanthroline, phen = 1,10-phenanthroline) are isolated with the help of large counterions. Structural analyses of complexes 1 and 2 show that vanadium atoms are coordinated by tridentate citrate ligand and hydroxy or ethoxy groups, respectively. The insertions of hydroxy and ethoxy groups give new examples of the mixed RO-bridges for vanadium-citrate complexes. [ABSTRACT FROM AUTHOR]