Synthesis, characterisation and catalytic potential of hydrazonato-vanadium(V) model complexes with [VO]3+ and [VO2]+ cores

Reaction between [ VO( acac)(2)] andH(2)L( H2L are the hydrazones H(2)sal- nah I or H(2)sal- fah II; sal = salicylaldehyde, nah = nicotinic acid hydrazide and fah = 2- furoic acid hydrazide) in methanol leads to the formation of oxovanadium( IV) complexes [ VOL . H2O] ( H2L = I : 1, H2L = II : 4). Aerial oxidation of the methanolic solutions of 1 and 4 yields the dinuclear oxo- bridged monooxovanadium( V) complexes [ {VOL}(2)mu - O] ( H2L = I : 2, H2L = II : 5). These dinuclear complexes slowly convert, in excess methanol, to [ VO( OMe) ( MeOH) L] ( H2L = I : 9, H2L = II : 10), the crystal and molecular structures of which have been determined, confirming the ONO binding mode of the dianionic ligands in their enolate form. Reaction of aqueous K [ VO3] with the ligands at pH ca. 7.5 results in the formation of [ K( H2O)] [ VO2L] ( H2L = I : 3, H2L = II : 6). Treatment of 3 and 6 with H2O2 yields ( unstable) oxoperoxovanadium( V) complexes K [ VO( O-2) L], the formation of which has been monitored spectrophotometrically. Acidification of methanolic solutions of 3 and 6 with HCl affords oxohydroxo complexes, while the neutral complexes [ VO2( Hsal- nah)] 7 and [ VO2( Hsal- fah)] 8 were isolated on treatment of aqueous solutions of 3 and 6 with HClO4. These complexes slowly transform into 9 and 10 in methanol, as confirmed by H-1, C-13 and V-51 NMR. The anionic complexes 3 and 6 catalyse the oxidative bromination of salicylaldehyde in water in the presence of H2O2/ KBr to 5- bromosalicylaldehyde and 3, 5- dibromosalicylaldehyde, a reaction similar to that exhibited by vanadate- dependent haloperoxidases. They are also catalytically active for the oxidation of benzene to phenol and phenol to catechol and p - hydroquinone.