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 [V^V(O₂)HL^1–2] (1, 2) in acetonitrile, two oxidomethoxido complexes [V^VO(L^1–2)(OMe)(OHMe)] (3, 4) in methanol and the corresponding dinuclear μ-oxidodivanadium complexes [{V^VOL^1–2}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)V^IV–O–V^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.