Synthesis, spectral and electrochemical studies of mixed-ligand oxovanadium(IV) and oxovanadium(V) complexes incorporating the tridentate ONO donor Schiff base derived from acetylacetone and benzoylhydrazine

Mixed-ligand oxovanadium(IV) and oxovanadium(V) complexes with a tridentate dinegative ONO donor Schiff base ligand [viz., 4-(1-hydroxybenzylidenehydrazono)-2-penten-2-ol (H2L)] and bidentate NN [ viz., 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen): complexes (1) and (2), respectively] or OO- [viz., ethylene glycol (H(2)gol), salicylaldehyde (Hsal) and vanillin (Hvan): complexes (3) -(5), respectively] donor ligands have been prepared and characterized by elemental analyses and by i.r., e.p.r., n.m.r., and u.v.-vis. spectroscopies. The complexes with NN donor ligands are one electron paramagnetic, displaying axial e.p.r. spectra and exhibiting two ligand-field transitions in the visible region, whereas the complexes with OO- donor ligands are diamagnetic and display only LMCT bands. H-1 n.m.r. spectral data indicate that the pentavalent complexes (4) and (5) exist in two isomeric forms [(4A), (4B) and (5A), (5B) in 1: 1 and 4: 1 ratios, respectively]. The vanadyl( IV) complexes display an irreversible oxidation peak near + 0.67 V while among the vanadyl( V) complexes, ( 3), displays an irreversible reduction peak near -0.20 V and (4) and (5) display a quasi-reversible one electron reduction peak near + 0.25 V versus s.c.e. The trend in redox potential values and the selective stabilization of VO2+ and VO3+ motifs have been explained on the basis of the basicity of the bidentate auxiliary ligands.