Kinetics and mechanism of the acid-catalysed decarboxylation of thecis-?- andcis-?-carbonato(3,6-dimethyl-1,8-diamino-3,6-diazaoctane)-cobalt(III) ions

Abstract

The acid-catalysed decarboxylation of thecis-α- andcis-β-[CoL(CO3)]+ complexes (L = 3,6-dimethyl-1,8-diamino-3,6-diazaoctane) have been studied over a range of HClO4 concentrations and the temperatures 25, 35 and 45° at I = 1.0 mol dm−3 (NaClO4). The rate expression takes the form kobs = k0 + k1[H+] where kobs is the observed first order rate constant at constant hydrogen ion concentration. The k0 term makes only a minor contribution to the overall reaction. Both complexes display solvent deuterium isotope effects ofca. 2.6 for the acid-catalysed decarboxylation, consistent with a rapid proton pre-equilibrium mechanism. Activation parameters have been determined and the mechanism of the reaction discussed. The magnitude of the solvent isotope effect is consistent with an A-1 type mechanism involving formation of a 5-coordinate intermediate.