Copper(II) complexes of tetraaza macrocycles bearing pendant arms: syntheses, structures and properties

Abstract

A new difunctionalized tetraaza macrocycle, 1,8-bis(2-hydroxyethyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, H₂L² has been synthesized in one step by the reaction of ethylene oxide and C-rac-5,5,7,12,12, 14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (L¹). The crystal structure of [Cu(H₂L²)][ClO₄]₂ 1 shows the metal ion in tetragonally elongated octahedral geometry, co-ordinated by the oxygen atoms of the pendant arms in the trans-axial position. Copper(II) complexes were also synthesized and characterized from other 1,8-disubstituted macrocyclic ligands L³, L⁴, HL⁵, H₂L⁶, L⁸ and H₂L⁹. The molecular structure of [CuL³][ClO₄]₂ 2 (R¹ = R² = CH₂CH₂CN) and [CuL⁵]ClO₄, 4 (R¹ = H, R² = CH₂CO₂^–) shows the macrocycle in trans-I configuration with square planar and distorted square pyramidal geometries respectively at the metal centers. The structures of complexes [CuL⁸][ClO₄]₂ 6 [L⁸ = 1,8-bis(2-cyanoethyl)-4,7,11,14-tetramethyl-1,4,8,11-tetraazacyclotetradecane] and [Cu(H₂L⁹)][ClO₄]₂ 7 (corresponding CH₂CO₂H derivative) show the macrocyclic ligand in the trans-III configuration. Interestingly the two pendant acetic acid groups of H₂L⁹ in 7 remain protonated and co-ordinate through the carbonyl oxygen atoms in trans-octahedral geometry. The cyclic voltammetric studies of the complexes revealed that the copper(I) oxidation state is highly stabilized by the ligands L³ and L⁸. The pH dependent co-ordination geometry changes of complexes 4, [CuL⁶] 5 (R¹ = R² = CH₂CO₂^–) and 7 based on the changes in their electronic spectra are discussed. The kinetics of acid promoted dissociation reactions of complexes 2 and 6 has been studied in HCl–NaCl solutions (I = 5.0 M). The possible mechanisms of the reactions, the factors influencing the rate and the relative importance of the solvent separation and protonation pathways are discussed.