Vanadium Complexes Derived from Acetyl Pyrazolone and Hydrazides: Structure, Reactivity, Peroxidase Mimicry and Efficient Catalytic Activity for the Oxidation of 1-Phenylethanol

Schiff bases obtained from the condensation of acetylpyrazolone (H(2)ap) with benzoyl hydrazide (bhz), furoyl hydrazide (fah), nicotinoyl hydrazide (nah) and isonicotinoyl hydrazide (inh) [H(2)ap-bhz (I), H(2)ap-fah (II) H(2)ap-nah, (III) and H(2)ap-inh (IV)], sharing a ONO donor set, upon reaction with [VIVO(acac)(2)] lead to the formation of [VIVO(ap-bhz)(H2O)] (1), [VIVO(apfah)(H2O)] (2), [VIVO(ap-nah)(H2O)] (3) and [VIVO(ap-inh)(H2O)] (4), respectively. These complexes slowly convert to monooxidovanadium(V) complexes [VVO(ap-bhz)(OMe)(MeOH)] (11), [VVO(ap-fah)(OMe)(MeOH)] (12), [VVO(ap-nah)(OMe)(MeOH)] (13) and [VVO(ap-inh)(OMe)(MeOH)] (14) in methanol. The reaction of aqueous K[H2VVO4] with the corresponding potassium salt of the ligands at neutral pH gives dioxidovanadium(V) complexes, K(H2O)[VVO2(ap-bhz)] (5), K(H2O) 0.5[VVO2(ap-fah)] (6), [VVO2(Hap-nah)] (9) and [VVO2(Hap-nah)] (10). Acidification of solutions of 5 and 6 affords the neutral complexes [VVO2(Hap-bhz)] (7) and [VVO2(Hap-fah)] (8), respectively. All complexes were characterized by various spectroscopic techniques: FT-IR, UV/Visible, EPR, NMR (H-1, C-13 and V-51); elemental analysis, thermal studies, cyclic voltammetry (CV) and single-crystal X-ray analysis. X-ray diffraction studies of complexes 6, 7, 9-12 confirm the ligand's coordination to the metal centre through enolic oxygen (of pyrazolone), azomethine nitrogen and enolic oxygen (hydrazide) atoms. The reactivity of the complexes and their catalytic potential was screened towards their peroxidase mimetic activity in the oxidation of pyragallol in aqueous media with H2O2 as oxidant, showing high activity under mild conditions. They were also tested in the catalytic oxidation of 1-phenylethanol with H2O2 that yields acetophenone as main product. Parameters such as catalyst and oxidant amount, time, temperature, and solvent effects were optimised for maximum oxidation of 1-phenylethanol. The complexes show excellent catalytic activity towards oxidation of 1-phenylethanol being structural and functional models of the vanadate-dependent haloperoxidases.