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

Four tridentate dibasic ONO donor hydrazone ligands derived from the condensation of benzoylhydrazine with either 2-hydroxyacetophenone or its para substituted derivatives (H2L1–4, general abbreviation H2L) 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 [VIVO(acac)2] with H2L in methanol followed by the addition of Hhq in equimolar ratio under aerobic condition afforded the mixed-ligand oxovanadium(V) complexes of the type [VVO(L)(hq)] (1–4) in excellent yield. The X-ray structure of the compound [VVO(L4)(hq)] (4) indicates that the H2L4 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 CDCl3 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 CH2Cl2 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 CH2Cl2 solution. λ max (for LMCT transition) and the reduction peak potential View the MathML source(Epc) 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 View the MathML sourceEpc values show large dependence dλ max/dσ = 32.54 nm and View the MathML sourcedEpc/dσ=0.19 V, respectively, on the Hammett constant.