Chemistry of mixed-ligand complexes with variable valence VOz+ (z = 2, 3) incorporating pentadentate hydrazone ligands

2-Hydroxybenzoylhydrazide reacts with an equimolar amount of [VIVO(acac)2] and [VIVO(bzac)2] [where acac− and bzac− are the monoanionic forms of acetylacetone (Hacac) and benzoylacetone (Hbzac) respectively] in the presence of an equimolar amount of 2,2′-bipyridine (bipy) in methanol to afford the tetravalent complexes [VIVO(HL1)(bipy)] (1) and [VIVO(HL2)(bipy)] (2) respectively [where (HL1)2− and (HL2)2− represent the dianionic forms of the 2-hydroxybenzoylhydrazone of acetylacetone (H3L1) and benzoylacetone (H3L2) respectively]. On replacing bipy by 8-hydroxyquinoline (Hhq) or vanillin (Hvan), the pentavalent complexes [VVO(HL1/HL2)(hq)] (3/4) and [VVO(HL1/HL2)(van)] (5/6) respectively were obtained. These pentadentate hydrazone ligands bind the vanadium in a tridentate dinegative meridional fashion utilizing their enolic-O, one imine-N and amide-O (in the enol form), leaving the other two potential donor sites, viz., phenolic-O and the other imine-N, which are practically H-bonded intramolecularly, as is evident from IR, 1H NMR and X-ray diffraction (of 3 and 4) studies. The EPR active tetravalent complexes 1 and 2 exhibit a quasi-reversible one-electron oxidation peak near +0.90 V, while the pentavalent complexes 3 and 4 exhibit quasi-reversible one-electron reduction peaks near −0.07 V versus SCE in CH2Cl2 solution. The [VVO(HL)(van)] complexes 5 and 6 display two quasi-reversible one-electron reduction peaks near +0.06 V and near +0.40 V versus SCE in CH2Cl2 solution, attributed to the successive reduction of VV → VIV and VIV → VIII respectively.

Graphical abstractMixed-ligand oxovanadium(IV) and oxovanadium(V) complexes incorporating pentadentate 2-hydroxybenzoyl hydrazones have been synthesized and characterized by various techniques including structure determination by X-ray crystallography.Figure optionsDownload full-size imageDownload as PowerPoint slide