Kinetic studies on H+-catalyzed aquation and hydrogen peroxide oxidation of tris-asparaginatochromium(III)

Abstract

Tris-asparaginatochromium(III), [Cr(Asn)3]0 (where Asn forms a 5-membered chelate ring via amine nitrogen and α-carboxylate oxygen atoms) and its mono- and diaqua-derivatives were obtained, and their acid-catalyzed aquation was studied. The first reaction for [Cr(Asn)3]0 and [Cr(Asn)2(H2O)2]+ is the chelate ring opening at the Cr-NH2 bond, leading to metastable intermediates. Kinetics of these processes were studied spectrophotometrically in 0.1–1.0 M HClO4 at 303 and 333 K, respectively. A linear dependence of kobs on [H+], kobs = a + b[H+] was determined for both the complexes. Additionally, oxidation of chromium(III) to chromate(VI) by hydrogen peroxide was studied. The process proceeds through a chromium(V) intermediate, which is next transformed, in faster parallel steps into CrO42− and [Cr(O2)2]3− anions. The latter species, a chromium(V)-peroxo complex, is metastable under a large excess of H2O2. Kinetics of oxidation of [Cr(Asn)3]0 were studied at 298 K, at constant [OH−], within 0.2–1.0 M H2O2 range. A linear dependence of kobs on H2O2 was established. A mechanism is proposed, where the rate-determining step is an inner sphere 2-electron transfer within a precursor chromium(III) complex with coordinated O2H− anion of the [Cr(Asn)2(OH)(HO2)]− formula. EPR results provided clear evidence for formation of a relatively stable tetrakis(η2-peroxo)chromate(V) complex, [Cr(O2)4]3−.