New VO, VO, VOVO and electrogenerated VOVO systems of valproic acid hydrazones: a study of catalytic activity.

The reaction between [VO(acac)] and two valproic acid hydrazide ligands, H2L1 and H2L2, resulted in the formation of two mononuclear dioxido complexes [V(O)HL] (1, 2) in acetonitrile, two oxidomethoxido complexes [VO(L)(OMe)(OHMe)] (3, 4) in methanol and the corresponding dinuclear μ-oxidodivanadium complexes [{VOL} μ-O] (5, 6) in dichloromethane. Here, H2L1 is the valproic acid hydrazone of salicylaldehyde and H2L2 is that of 2-hydroxy naphthaldehyde. X-ray crystallographic studies revealed the dual binding mode of the ligands, the neutral amido form in the dioxido complex 1 and the dianionic iminolato form in the oxidomethoxido complex 3. The redox behaviour of all the complexes was investigated using a combination of experimental and theoretical approaches. The partial reduction of 5 and 6 by constant-potential electrolysis (CPE) resulted in Robin-Day type II mixed-valence species 5a and 6a with a general formula of (L)(O)V-O-V(O)(L). The complexes catalytically oxidized pyrogallol to purpurogallin and a catechol-like system to quinone under ambient conditions. The catecholase-like activity was found to be facilitated by a very rare semiquinone radical intermediate in the vanadium model systems.