Kinetics and mechanism of the acid-catalysed decarboxylation of the cis-carbonato(5,12-dimethyl-1,4,8,11-tetra-azacyclotetradeca-4,11-diene)cobalt(III) ion

Abstract

The acid-catalysed decarboxylation of the title compound has been studied over the range of perchloric acid concentrations 0.1–0.5 mol dm–3 at l= 0.5 mol dm–3(Na[ClO4]), and 25, 34.8, and 45.4 °C. Over this acidity range kobs.=k0+k1[H+]; however, the k0 term does not make a significant contribution. The values of the activation parameters (for k1) are ΔH‡= 82.9 kJ mol–1 and ΔS‡298=–0.4 J K–1mol–1. The reaction exhibits a solvent deuterium-isotope effect kD2O/kH2O= 2.6 at 25 °C; the solvent deuterium-isotope effect is consistent with a rapid pre-equilibrium protonation followed by rate-determining ring opening, and excludes a mechanism involving concerted attack by [H3O]+. The value of ΔS‡ close to zero, and the magnitude of the solvent isotope effect, suggests an A-1 type reaction to give a five-co-ordinate intermediate.