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

Reaction between [VO(acac)₂] and two valproic acid hydrazide ligands, H₂L¹ and H₂L² leads to 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 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. neutral amido form in dioxoido complex 1 and dianionic iminolato form the oxidomethoxido complex 3. The redox behaviour of all the complexes were investigated using a combination of experimental and theoretical approaches. Partial reduction of 5, 6 by constant potential electrolysis (CPE) conforming to a Robin–Day type II mixed-valence species 5a and 6a of general formula (L)(O)V^IV−O−V^V(O)(L). The Complexes catalytically oxidized pyrogallol to purpurogallin and catechol like system to quinone under ambient condition. Catecholase like activity was found to follow a very rare semiquinone radical intermediate for vanadium model system.