Kinetics and mechanism of oxidation of a ternary complex involving dipicolinatochromium(III) and DL-aspartic acid by N-bromosuccinimide

The kinetics of oxidation of ICrIII (Dpc)(Asp) (H2O)(2)] (Dpc = dipicolinic acid and Asp = DL-aspartic acid) by N-bromosuccinimide (NBS) in aqueous solution have been found to obey the equation: d\Cr-VI\/dt = {9k(1)K(2) + k(2)K(1)K(3)/\H+\)\NBS\\Cr-III\(T)}/{1 + {K-1/\H+\) divided by(K-2 + K1K3/\H+\)\NBS\} where k(2) is the rate constant for the electron transfer process, K, is the equilibrium constant for deprotonation of \Cr-III(Dpc)(Asp)(H2O)(2)], K-2 and K-3 are the preequilibrium formation constants of precursor complexes \Cr-III(Dpc)(Asp)(H2O)(NBS)] and \Cr-III(Dpc)(Asp)(H2O)(OH)(NBS)](-). Values of: k(2) = 4.85 x 10(-2) s(-1), K-1 = 1.85 x 10(-7) mol dm(-3), and K-2 = 78.2 mol(-1) dm(3) have been obtained at 30degreesC and I = 0.1 mol dm(-3). The experimental rate law is consistent with a mechanism in which the deprotonated \Cr-III(Dpc)(Asp)(H2O)(OH)](-) is considered to be the most reactive species compared to its conjugate acid. it is assumed that electron transfer takes place via an inner-sphere mechanism. (C) 2004 Wiley Periodicals, Inc.