Synthesis, structure, solution chemistry and the electronic effect of para substituents on the vanadium center in a family of mixed-ligand [V VO(ONO)(ON)] complexes

Abstract

Four tridentate dibasic ONO donor hydrazone ligands derived from the condensation of benzoylhydrazine with either 2-hydroxyacetophenone or its para substituted derivatives (H 2L 1–4, general abbreviation H 2L) have been used as primary ligands and 8-hydroxyquinoline (Hhq, a bidentate monobasic ON donor species) has been used as auxiliary ligand. The reaction of [V IVO(acac) 2] with H 2L in methanol followed by the addition of Hhq in equimolar ratio under aerobic condition afforded the mixed-ligand oxovanadium(V) complexes of the type [V VO(L)(hq)] ( 1– 4) in excellent yield. The X-ray structure of the compound [V VO(L 4)(hq)] ( 4) indicates that the H 2L 4 ligand is bonded with vanadium meridionally in a tridentate dinegative fashion through its deprotonated phenolic-O, deprotonated enolic-O and imine-N atoms. The V–O bond length order is: oxo < phenolato < enolato. 1H NMR spectra of 4 in CDCl 3 solution indicates that it’s solid-state structure is retained in solution. Complexes are diamagnetic and exhibit only ligand to metal charge transfer (LMCT) transition band near 530 nm in CH 2Cl 2 solution in addition to intra-ligand π → π ∗ transition band near 335 nm and they display quasi-reversible one electron reduction peak near − 0.10 V versus SCE in CH 2Cl 2 solution. λ max (for LMCT transition) and the reduction peak potential ( E p c ) values of the complexes are found to be linearly related with the Hammett ( σ) constants of the substituents in the aryloxy ring of the hydrazone ligands. λ max and E p c values show large dependence d λ max/d σ = 32.54 nm and d E p c / d σ = 0.19 V, respectively, on the Hammett constant.