Kinetics and mechanism of the hydrolysis of cis-β-carbonato(3,7-diazanonane-1,9-diamine)cobalt(III) and cis-β-carbonato(4,7-diazadecane-1,10-diamine)cobalt(III) cations in acidic aqueous solution

Abstract

The acid-catalysed hydrolyses of the complex cations cis-β-[CoL(CO₃)]⁺[L = 3,7-diazanonane-1,9-diamine (L¹) or 4,7-diazadecane-1,10-diamine (L²)] have been studied over the ranges 30.0 ⩽θ⩽ 40.0 °C and 1 ⩽ pH ⩽ 5 (for L², 1 ⩽ pH ⩽ 2) at an ionic strength of 0.5 mol dm^–3. The kinetic results exhibit a rate law of the form –d(ln [complex])/dt=k₀+k₁[H⁺] where the values of k₀(25 °C), k₁(25 °C), ΔH^‡₀, ΔS^‡₀, ΔH^‡₁, and ΔS^‡₁ are 1.0 × 10^–4 s^–1, 0.17 dm³ mol^–1 s^–1, 95 ± 15 kJ mol^–1, –4 ± 4 J K^–1 mol^–1, 37 ± 2 kJ mol^–1, and –137 ± 20 J K^–1 mol^–1 for the L¹ complex and 3.9 × 10^–5 s^–1, 2.7 × 10^–2 dm³ mol^–1 s^–1, 72 ± 10 kJ mol^–1, –84 ± 25 J K^–1 mol^–1, 72 ± 4 kJ mol^–1, and –34 ± 8 J K^–1 mol^–1 for the L² complex. The acid-catalysed reactions are subject to a deuterium solvent isotope effect, consistent with a mechanism involving a rapid protonation equilibrium followed by slow ring opening of the carbonate ring. The diaqua product of the acid hydrolysis of the L² complex undergoes cis-to-trans isomerization at rates comparable to the hydrolysis reaction under certain conditions. The values of k_iso at 25 °C (I= 0.5 mol dm^–3 HClO₄) and corresponding activation parameters, ΔH^‡ and ΔS^‡, are 1.1 ± 0.05 s^–1, 115.8 ± 2.5 kJ mol^–1, and 67 ± 21 J K^–1 mol^–1 respectively.