Copper(II) complexes of C-meso-1,5,8,12-tetramethyl-1,4,8,11-tetraazacyclotetradecane: syntheses, structures and properties†>

Abstract

A series of copper(II) complexes of C-meso-1,5,8,12-tetramethyl-1,4,8,11-tetraazacyclotetradecane L¹ have been synthesized and characterized. The crystal structure of [CuL¹(ClO₄)]ClO₄ 1 showed the macrocyclic ligand in the unstable trans-II configuration, while those of [CuL¹(N₃)₂] 2 and [CuL¹(H₂O)₂]X₂ (X = NO₂ 3, Cl 4, Br 5 or I 6) showed the macrocyclic ligand to be in the stable trans-III configuration. The metal ion is in a tetragonally elongated octahedral geometry in all the complexes. In 1 one of the perchlorate groups is free while the other bridges two metal ions with two of its oxygen atoms thus forming a linear chain, while in 2 the axial positions are occupied by the nitrogen atoms of two azide groups. The complexes 3–6 are isomorphous. The electronic spectral and the redox properties of the complexes are discussed. The acid dissociation kinetics of 1 has been studied in the temperature range 25.0 ± 0.1 to 45 ± 0.1 °C in aqueous HNO₃–NaNO₃ solutions (I = 5.0 M). The complex exhibited a first order dependence on the hydrogen ion concentration with k_obs = k_1H[H⁺] + k_1d where k_1H = 2.66 × 10^–5 M^–1 s^–1 and k_1d = 5.54 × 10^–5 s^–1 at 25.0 ± 0.1 °C. The temperature dependence of the rate constant for the reaction gave ΔH ^‡_1H = 64.7 kJ mol^–1 and ΔS ^‡_1H = –117 J K^–1 mol^–1 for k_1H while for k_1d the parameters are ΔH ^‡_1d = 27.4 kJ mol^–1 and ΔS ^‡_1d = –234 J K^–1 mol^–1. A possible mechanism of the reaction is discussed.