Rapid reduction and complexation of vanadium by 1-phenyl-3-methyl-4-toluoyl-5-pyrazolone: Spectroscopic characterization and structure modelling

Abstract

The reduction of oxovanadium(V) by 1-phenyl-3-methyl-4-toluoyl-5-pyrazolone (Hpmtp) over the pH range 1.0–7.0 has been assessed by cyclicvoltammetry, electron paramagnetic resonance spectroscopy, magnetic susceptibility, FT-IR, electronic spectroscopy, high-resolution mass spectroscopy and elemental analyses. The reaction proceeds by the transfer of one electron from Hpmtp to oxovanadium(V) via the formation of a free-radical intermediate, and subsequently the reduced oxovanadium(IV) species rapidly complexes with the available free ligand. An indirect evidence for the generation of the free-radical intermediate in the process of the reduction has been confirmed by acrylonitrile polymerization. The free-radical intermediates thus generated in the process couple to form a dimer, which also has been confirmed by various spectroscopic techniques. The electronic spectrum of VO(pmtp)2 displays characteristic absorption peaks corresponding to transitions from the dxy orbital to dxz (794nm), dyz (643nm), dx2-y2 (501nm) and dz2 (424nm) of the oxovanadium(IV) complex with a distorted square pyramidal geometry. The slightly distorted square pyramidal geometry was obtained from a DFT calculation with the four oxygen atoms from the two pmtp− ligands at the base and a VO double bond of length 1.584Å pointing towards the apex of the pyramid.