Thermodynamic and kinetic effects of 13-membered macrocycles. Polarographic studies of 1,4,7,10-tetra-azacyclotridecanecopper(II)

Abstract

Thermodynamics and kinetics of complex formation between Cu²⁺ and 1,4,7,10-tetra-azacyclotridecane(L²) in acetate-buffered solution have been studied polarographically. The stability constant (log K 29.1 at 25 °C) for the 1 : 1 complex with 5,5,5,6-membered chelate rings is ca. 10⁵- and 10⁹-fold greater than those for the corresponding open-chain tetra-amine complexes with 5,6,5- and 5,5,5-membered chelate rings, respectively. The enhanced stability with the 13-membered cyclic ligand is largely due to a more favourable entropy change, as found for 12-membered macrocycles. However, there is a significant, although small, contribution from the enthalpy term. These trends may link the differences between the 12- and 14-membered macrocyclic effects. The rate law (pH < 4, 25 °C) is d[(CuL²)²⁺]/dt=k₅[Cu(O₂CMe)⁺][(HL²)⁺]+k₆[Cu(O₂CMe)⁺][(H₂L²)²⁺], where k₅=(5.6 ± 0.5)× 10⁶ dm³ mol^–1s^–1and k₆= 10.1 ± 1 dm³ mol^–1 s^–1. The activation parameters for [HL²]⁺ reaction are ΔH^‡= 16.1 kcal mol^–1 and ΔS^‡=+26.9 cal K^–1 mol^–1, and for [H₂L²]²⁺ reaction are ΔH^‡= 11.6 kcal mol^–1 and ΔS^‡=–15.1 cal K^–1 mol^–1. The strikingly different activation parameters between [HL²]⁺ and [H₂L²]²⁺ reactions suggest a shift of the reaction mechanism from dissociative to associative. The kinetic behaviours indicate that the 13-membered macrocycles have intermediate properties between 12-membered cyclic and open-chain tetra-amines.