New VVO2, VVO, VVOVVO and electrogenerated VIVOVVO systems of valproic acid hydrazones: a study of catalytic activity.

The reaction between [VO(acac)2] and two valproic acid hydrazide ligands, H2L1 and H2L2, resulted in the formation of two mononuclear dioxido complexes [VV(O2)HL1-2] (1, 2) in acetonitrile, two oxidomethoxido complexes [VVO(L1-2)(OMe)(OHMe)] (3, 4) in methanol and the corresponding dinuclear μ-oxidodivanadium complexes [{VVOL1-2}2 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, e.g. 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)VIV-O-VV(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.