Effects of cyclization and ring size on complex formation between penta-amine ligands and copper(II)

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Equilibria and kinetics of complex formation between CuII and the macrocyclic penta-amines 1,4,7,10,13-penta-azacyclopentadecane (L1), 1,4,7,10,13-penta-azacyclohexadecane (L2), and 1,4,7,11,14-penta-azacycloheptadecane (L3) have been investigated. Thermodynamic functions for formation of the 1 : 1 complex have been determined polarographically: for L1, L2, and L3, respectively, log KCuL 28.3, 27.1, and 23.8, –ΔH 32.9, 32.7, and 27.2 kcal mol–1, and ΔS 22, 14, and 18 cal K–1 mol–1(at / 0.2 mol dm–3 and 25 °C). The large stability enhancements caused by the cyclization of penta-amines are due to a favourable enthalpy factor, in contrast to the entropy-favoured macrocyclic effects of tetra-amines. Kinetic studies have established the rate laws for the complex formation in acetate buffer as d[CuL]/dt=k2H[Cu(O2CMe)+][H2L2+]+k3H[Cu(O2CMe)+][H3L3+], and in unbuffered acid solution as d[CuL]/dt =k2H′[Cu2+][H2L2+]+k3H′[Cu2+][H3L3+]. (The macrocyclic complexes formed are a mixture of [CuL]2+ and [Cu(HL)]3+ depending on the acidity.) The rate constant k2H′ for the diprotonated ligand species increases from 105 to 107 dm3 mol–1 s–1 as the macrocyclic ring opens, approaching the value found for 3,6,9-triazaundecane- 1, 11 -diamine (L4). The rate constant k3H′ for the triprotonated species increases similarly from zero to 102 dm3 mol–1 s–1, although falling below the corresponding value for L4. The presence of acetate anions as a buffer significantly accelerates the reactions involving proton-congested species. The implications for substitution reactions of protonated macrocyclic ligands are discussed.