Potential biochromium sources; kinetic studies on acid- and base-catalyzed aquation of [Cr(ox)2(2-(aminomethyl)pyridine

Abstract

Acid- and base-catalyzed hydrolysis of [Cr(ampy)(ox)2]−, where ampy = 2-(aminomethyl)pyridine, leads to successive dissociation of the ligands via concurrent reaction paths, whereas at pH 1–9 only ampy is liberated as a result of spontaneous processes. The first ligand dissociation proceeds via aqua intermediates with one-end bonded ampy (1) or ox ligands (2), respectively, which in alkaline media undergo rapid deprotonation to give the appropriate hydroxo-forms. The kinetics of two reaction stages, namely the chelate ring opening and the ligand liberation, were studied spectrophotometrically. In acidic media, the first stage is much faster than the second, whereas in alkaline solutions, both the stages are characterized by similar rate constants. The dependences of kobs on [H+] are as follows: kobs1,H = a1 + b1/[H+], kobs2,H = a2 + b2[H+]. At pH > 13, rate constants kobs1,OH and kobs2,OH are [OH−] independent. The effect of pH on the complex reactivity was rationalized based on proposed mechanisms.