Ring-size effects of macrocyclic ligands on complex formation: the copper(II) complex of the 15-membered cyclic tetra-amine 1,4,8,12-tetra-azacyclopentadecane

Abstract

The thermodynamics and kinetics of complex formation between Cu^II and 1,4,8,12-tetra-azacyclopentadecane (L⁴) have been studied polarographically. The stability constant of 10^{24.4 ± 0.2}(I= 0.20 mol dm^–3, 25 °C) for the 1 : l complex containing 5,6,6,6-membered chelates is 10⁷- and 10³-fold greater than those for complexes of the corresponding open-chain tetra-amines 4,8.diazaundecane-1.11-diamine (L⁵) containing 6,6,6-chelates and 4,7-diazadecane-1,10-diamine (L⁵) containing 5,6,6-chelates, respectively. Depending on the choice of the reference complexes, either the ΔH term (cf the L⁵ complex) or the ΔS term (cf, the L⁶ complex) makes a major contribution to the macrocyclic effects. A significant effectof the macrocyclic ring expansion is a facile protonation of the complexed ligand in acetate buffer solution, K_Cu(HL)^H=[CuL²⁺][H⁺]/[Cu(HL)³⁺]=(3.0 ± 0.2)× 10^–6,ol dm^–3. The rate law for the formation of the complex of L⁴ in acetate buffer is of the form d[Cu(HL)³⁺]/dt=k₅-[Cu(O₂CMe)⁺][HL⁺], where k₅=(4.0 ± 0.5)× 10⁸ dm³ mol^–1s^–1 at 25 °C and the activation parameters are ΔH^‡= 13.7 ± 0.3 kcal mol^–1 and ΔS^‡= 26.7 ± 1.0 cal K^–1 mol^–1. The larger formation and dissociation rate constants suggest that the 15-membered macrocycle is more flexible relative to the smaller analogues.