Synthesis, spectral and electrochemical studies of alkoxo-bonded mixed-ligand oxovanadium(IV) and oxovanadium(V) complexes incorporating tridentate ONO donor azophenolalcoholate/aldiminealcoholates

Mixed-ligand vanadyl(IV) and vanadyl(V) complexes with tridentate ONO donor azophenolalcoholate/aldiminealcoholates [viz., 2-hydroxy-2′-hydroxymethyl-5-methylazobenzene (H2L1), N-(2′-hydroxymethylphenyl)-salicylaldimine (H2L2) and N-(2′-hydroxymethylphenyl)1-hydroxy-2-naphthaldimine (H2L3)] and bidentate NN [viz., 2,2′-bipyridine (bipy) and 1,10-phenanthroline (phen)] or NO [viz., 8-hydroxyquinoline (Hhq)] donor ligands have been prepared and characterized by elemental analyses and by i.r., e.p.r. and u.v.–vis. spectroscopies. The data show that the vanadyl(IV) complexes exist in two isomeric solid forms viz., monomers and polymers, whereas vanadyl(V) complexes exist only in the monomeric form. The polymeric structure results from intermolecular V=OċċċV=O interactions. The complexes with NN donor ligands are one electron paramagnetic and display two ligand-field transitions in the visible region, whereas the complexes with ON donor ligands are diamagnetic and exhibit only LMCT bands. The vanadyl(IV) complexes display an irreversible oxidation peak near +0.30 V for complexes with (L1)2− and near +0.55 V for complexes with (L2)2− and (L3)2− ligands, while the vanadyl(V) complexes display a quasi-reversible one electron reduction couple near −0.23 V versus s.c.e. The trends in the ν(V=O), ε values for the visible region transitions and the redox potential values for the vanadyl(IV) complexes have been scrutinized.