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 k_obs.=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 k_D2O/k_H2O= 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.