Kinetics and mechanism of the oxidation of some carboxylates by a nickel(III) oxime-imine complex

The kinetics of the oxidation of formate, oxalate, and malonate by [Ni-III(L(1))](2+) (where HL(1) = 1 5-amino-7-methyl-4,7,10,13-tetraazapentadec-3-en-2-one oxime) were carried out over the regions pH 3.0-5.75, 2.80-5.50, and 2.50-7.58, respectively, at constant ionic strength and temperature 40 degrees C. All the reactions are overall second-order with first-order on both the oxidant and reductant. A general rate law is given as - d/dt[ Ni-III(L(1))(2+)] = k(obs)[Ni-III(L(1))(2+)] = (k(d) + nk(s)[R])[Ni-III(L(1))(2+)], where k(d) is the auto-decomposition rate constant of the complex, k(s) is the electron transfer rate constant, M is the stoichiometric factor, and R is either formate, oxalate, or malonate. The reactivity of all the reacting species of the reductants in solution were evaluated choosing suitable pH regions. The reactivity orders are: k(HCOOH) > k(HCOO-) > k(H2OX) > k(OX2-) and k(H2mal) > k(Hmal-) < (mal2-) for the oxidation of formate, oxalate, and malonate, respectively, and these trends were explained considering the effect of hydrogen bonded adduct formation and thermodynamic potential. (C) 1996 John Wiley & Sons, Inc.