Synthesis and characterization of novel oxovanadium(IV) complexes with 4-acyl-5-pyrazolone donor ligands: Evaluation of their catalytic activity for the oxidation of styrene derivatives Original Research Article

Reaction of VOSO4·5H2O with a methanol solution of the HQ proligand (HQ = 1-R1-4-R2(CO)pyrazol-5-one in general, in detail: HQnPe, R1 = phenyl, R2 = neopentyl; HQMe,Me, R1 = R2 = methyl; HQMe,naph, R1 = methyl, R2 = naphthalen-1-yl; HQnaph, R1 = phenyl, R2 = naphthalen-1-yl; HQPh, R1 = R2 = phenyl; HQCF3, R1 = phenyl, R2 = trifluoromethyl; HQpy,CF3, R1 = pyridin-2-yl, R2 = trifluoromethyl) gave seven novel VO(Q)2(H2O) complexes which have been characterized by elemental analyses, IR, ESI-MS, electronic spectroscopy and magnetic susceptibility measurements and, in the case of derivative VO(QCF3)2, also by EPR spectroscopy. The X-ray diffraction study, carried out on derivative VO(QnPe)2(H2O), evidenced a distorted octahedral environment with the two pyrazolonates in anti configuration and the vanadium atom 0.2914(7) Å away from the least-squares plane defined by the four equatorial oxygen atoms. The catalytic activity of these new oxovanadium(IV) complexes has been exhaustively tested for the oxidation of styrene, α-methylstyrene and cis-β-methylstyrene, in the presence of H2O2 as primary oxidant. The effects of oxidant to substrate molar ratio, catalyst amount, solvent and temperature have been studied. Overall, the vanadium complexes showed high activity and high to moderate selectivity toward the benzaldehyde (acetophenone) formation, depending from both the type of starting substrate and experimental reaction times.