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.