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.