Kinetics of hydrolysis of trans-dichloro- and trans-dibromo-(n-meso-5,12- dimethyl-1,4,8,11-tetra-azacyclotetradeca-4,11 -diene)cobalt(III) cations

Abstract

The kinetics of hydrolysis of the title cations trans-[CoX₂(N-meso-L⁴)]⁺(X = Cl or Br; L⁴= 5,12-dimethyl-1,4,8,11-tetra-azacyclotetradeca-4,11 -diene) have been studied at I= 0.1 mol dm^–3 over the range pH 3.0–4.5. Hydrolysis of the first halogen ligand obeys a rate expression of the form k_obs.=k_OH2+k₁^OH[OH^–], where k_obs. is the observed first-order rate constant at constant pH. The values of k₁^OH are 8.5 × 10⁷(X = Cl)and ca.1 × 10⁹ dm³ mol^–1 s^–1(X = Br) at 25 °C. The base-hydrolysis rates are the highest yet reported for halogenocobalt(III) complexes of the [CoX₂(N₄)]²⁺ type. Ths pH-independent process is represented by k_OH2= 5.4 × 10^–3 s^–1 for the chloro-complex. Hydrolysis of the second halogen ligand proceeds more slowly and in this case k_obs.=k₂^OH[OH^–] with k₂^OH= 5.9 × 10⁶(X = Cl) and 1.2 × 10⁷ dm³ mol^–1 s^–1(X = Br) at 25 °C. The thermodynamic activation parameters for the hydrolysis of the second halogen ligand are ΔH^‡= 57 kJ mol^–1 and ΔS₂₉₈^‡= 75 J K^–1 mol^–1(X = Cl) and ΔH^‡= 56 kJ mol^–1 and ΔS^‡= 80 J K^–1 mol^–1(X = Br). These values may indicate rate-determining deprotonation of the substrate in the base-hydrolysis reactions.