Dioxovanadium(V) Complexes Incorporating Tridentate ONO Donor Hydrazone Ligands Derived from Acetylhydrazide and 2-hydroxybenzaldehyde/2-hydroxyacetophenone. Synthesis, Characterization and Reactivity

Reaction of VOSO₄ with the tridentate ONO donor ligand derived from the condensation of acetylhydrazide with either 2-hydroxybenzaldehyde (H₂L¹) or 2-hydroxyacetophenone (H₂L²) (general abbreviation H₂L) in an equimolar ratio in the presence of two equivalents of sodium acetate in aqueous-methanolic medium in air produces yellow dioxovanadium(V) complexes of the type, [V^vO₂(H⁺-L)], (1) and (2) in good yield. Aerial oxygen is most likely the oxidant (for the oxidation of V^IV→V^V) assisted by the ligand basicity. Complexes are characterized by elemental analyses and by i.r., n.m.r. and u.v.–vis. spectroscopies. I.r. spectra of the complexes indicate the tridentate dinegative forms of the ligands. The ¹H-n.m.r. spectrum of (2) in CD₃CN solvent indicates the presence of a strongly deshielded N–H proton. Conductivity measurements in DMF solution indicate that the complexes are non-electrolytic and so the H⁺ ion is strongly bonded probably with the uncoordinated imine-N nitrogen of the coordinated Schiff base moiety. Both complexes exhibit only the LMCT band in the u.v. region in MeOH and they are electroactive, displaying an irreversible reduction peak near −0.35 V versus s.c.e. in methanol solution. Methanol solutions of the complexes are reversibly reduced by ascorbic acid to the corresponding V^IV analogue as is evident from their u.v.–vis. spectra. The CH₂Cl₂ suspension of these complexes (1) and (2) reacts separately with 8-hydroxyquinoline (Hhq) in air to produce the mixed-ligand complexes of the type [V^vO(L)(hq)].