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. λmax (for LMCT transition) and the reduction peak potential (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. λmax and Epc values show large dependence dλmax/dσ = 32.54 nm and dEpc/dσ=0.19 V, respectively, on the Hammett constant.

The reaction of methanolic solution of [VIVO(acac)2] with the benzoyl hydrazone of either 2-hydroxyacetophenone or its para substituted derivatives (H2L) and 8-hydroxyquinoline (Hhq) in equimolar ratio produces the pentavalent complexes of the type [VVO(L)(hq)] in excellent yield.Figure optionsDownload as PowerPoint slide