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

Abstract

Equilibria and kinetics of complex formation between Cu^II 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 K_CuL 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=k_2H[Cu(O₂CMe)⁺][H₂L²⁺]+k_3H[Cu(O₂CMe)⁺][H₃L³⁺], and in unbuffered acid solution as d[CuL]/dt =k_2H′[Cu²⁺][H₂L²⁺]+k_3H′[Cu²⁺][H₃L³⁺]. (The macrocyclic complexes formed are a mixture of [CuL]²⁺ and [Cu(HL)]³⁺ depending on the acidity.) The rate constant k_2H′ 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 k_3H′ 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.