Kinetic and equilibrium studies on copper(II) complexes of a 9-membered macrocyclic triamine, 1,4,7-triazacyclononane, and a 14-membered macrocyclic tetra-amine, 1,4,8,11-tetra-azacyclotetradecane

Abstract

The thermodynamics and kinetics of formation of copper(II) complexes of 1,4,7-triazacyclononane (L¹) and 1,4,8,11-tetra-azacyclotetradecane (L²) have been studied. The stability constants and thermodynamic parameters have been determined by a polarographic method for L¹ and by the potentiometric method of Bjerrum and Nielson for L². The complex [CuL²]²⁺ has log K_CuL 27.2 (ΔH–30.4 kcal mol^–1 and ΔS 22.4 cat K^–1 mol^–1) at I 0.20 mol dm^–3 and 25 °C, which represents stability enhancement over the complex of the corresponding linear tetra-amine by three orders of magnitude due to more favourable entropy and enthalpy contributions. The higher frequency of the maximum of the visible absorption band indicates that the favourable ΔH term derives from an increase in Cu–N bond energy. On the other hand, the cyclization poses severe steric constraints on the complex of the triamine ligand, as manifested by a large enthalpy loss. This destabilization is barely compensated by the favourable entropy term: log K_CuL 16.2, ΔH–13.0 kcal mol^–1, and ΔS 30.5 cal K ^–1 mol^–1. Kinetic studies in acetate buffers using the stopped-flow method have given the following rate laws for the complex formation at 25 °C and I 0.20 mol dm^–3: for L¹, d[CuL²⁺]/dt=k_H[Cu(O₂CMe)⁺][HL⁺], where k_H= 10^6.8 dm³ mol^–1 s^–1; for L², d[CuL²⁺]/dt=k_H[Cu(O₂CMe)⁺][HL⁺]+ k_2H[Cu(O₂CMe)⁺][H₂L²⁺], where k_H= 10^6.7 and k_2H= 10^0.9dm³ mol^–1 s^–1. The rate law for the formation of [CuL²]²⁺ at pH 1.8–2.5 (unbuffered) and 25 °C has been determined by the polarographic method: d[CuL²⁺]/dt=k_H′[Cu²⁺][HL⁺]+k_2H′[Cu²⁺][H₂L²⁺], where k_H′= 10^6.9 and k_2H′= 10^–1-1 dm³ mol^–1 s^–1.