Oxidation kinetics of the potent insulin mimetic agent bis(maltolato)oxovanadium(IV) (BMOV) in water and in methanol

The kinetics of oxidation of bis(maltolato)oxovanadium(IV), BMOV or VO(ma)(2), by dioxygen have been studied by UV-vis spectroscopy in both MeOH and H2O media. The VO(ma)(2):O-2 stoichiometry was 4:1. In aqueous solution, the pH-dependeut rate of the VO(ma)(2)/O-2 reaction to generate cis-[VO2(ma)(2)](-) is attributed to the deprotonation of coordinated H2O, the deprotonated species [VO(ma)(2)(OH)](-) being more easily oxidized (k(OH) = 0.39 M(-1) s(-1), 25 degrees C) than the neutral form VO(ma)(2)(H2O) (k(H2O) = 0.08 M(-1) s(-1), 25 degrees C). The activation parameters for the two second-order reactions in aqueous solution were deduced from variable temperature kinetic measurements. In MeOH, VO(ma)(2) was oxidized by dioxygen to cis-VO(OMe)(ma)(2), whose structure was characterized by single-crystal X-ray diffraction; the crystals were monoclinic, C2/c, with a = 28.103(1) Angstrom, b = 7.721(2) Angstrom, c = 13.443(2) Angstrom, beta = 94.290(7)degrees, and Z = 8. The structure was solved by Patterson methods and was refined by full-matrix least-squares procedures to R = 0.043 for 1855 reflections with I greater than or equal to 3 sigma(I). The kinetic results are consistent with a mechanism involving an attack of O-2 at the V(IV) center, followed by the formation of radicals and H2O2 as transient intermediates.