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

摘要

A new difunctionalized tetraaza macrocycle, 1,8-bis(2-hydroxyethyl)-5,5,7,12,12,14-hexamethyl-1,4,8,1-tetraazacyclotetradecane, H_2L~2 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 (L~1). The crystal structure of [Cu(H_2L~2)][ClO_4]_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 L~3, L~4, H_2L~6, L_8 and H_2L~9. The molecular structure of [CuL_3][ClO_4]_2 2 (R~1 = R~2 = CH_2CH_2CN) and [CuL_5]ClO_4, 4 (R~1 = H, R~2 = CH_2CO_2~-) shows the macrocycle in trans-I configuration with square planar and distorted square pyramidal geometries respectively at the metal centers. The structures of complexes [CuL~8][ClO_4]_2 6 [L_8 = 1,8-bis(2-cyanoethyl)-4,7,11,14-tetramethyl-1,4,8,11-tetraazacyclotetradecane] and [Cu(H_2L_9)][ClO_4]_27 (corresponding CH_2CO_2H derivative) show the macrocyclic ligand in the trans-III configuration. Interestingly the two pendant acetic acid groups of H_2L~9 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 L~3 and L~8. The pH dependent co-ordination geometry changes of complexes 4, [CuL_6]5 (R~1 = R~2 = CH_2CO_2~-) 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.