Synthesis, structure and solution chemistry of quaternary oxovanadium(V) complexes incorporating hydrazone ligands

摘要

[V~(IV)O(acac)_2] reacts with an equimolar amount of benzoyl hydrazone of 2-hydroxyacetophenone (H_2L~1) or 5-chloro-2-hydroxyacetophenone (H_2L~2) in the presence of excess pyridine (py) in methanol to pro_duce the quaternary [V~vO(L~1)(OCH_3)(py)] (1) and [V~VO(L~2)(OCH_3)(py)] (2) complexes, respectively, while under similar condition, the benzoyl hydrazones of 2-hydroxy-5-methylacetophenone (H_2L~3) and 2_hydroxy-5-methoxyacetophenone (H_2L~4) afforded only the methoxy bridged dimeric [V~VO(L~3/ L~4)(OCH_3)]_2 complexes. The X-ray structural analysis of 1 and 2 indicates that the geometry around the metal is distorted octahedral where the three equatorial positions are occupied by the phenolate-O, enolate-O and the imine-N of the fully deprotonated hydrazone ligand in its enolic form and the fourth one by a methoxide-O atom. An oxo-O and a pyridine-N atom occupy two axial positions. Quaternary complexes exhibit one quasi-reversible one-electron reduction peak near 0.25 V versus SCE in CH_2Cl_2 and they decompose appreciably to the corresponding methoxy bridged dimeric complex in CDCI_3 solu_tion as indicated by their ~1H NMR spectra. These quaternary VO~(3+) complexes are converted to the corresponding V_20_3~4 -complexes simply on refluxing them in acetone and to the VO_2 -complexes on reaction with KOH in methanol. An equimolar amount of 8-hydroxyquinoline (Hhq) converts these quaternary complexes to the ternary [V~VO(L)(hq)] complexes in CHCl_3.