Anion receptors containing coordinatively unsaturated metal ions: copper(II) complexes with cyclam derivatives

Abstract

The two coordinatively unsaturated complexes [Cu(cyclam)]2+ (cyclam: 1,4,8,11-tetraazacyclotetradecane, 1) and [Cu(Bn4cyclam)]2+ (Bn4cyclam: 1,4,8,11-tetrabenzyl-1,4,8,11-tetraazacyclotetradecane, 2) in an MeCN solution uptake an anion X− (halide, pseudohalide) to give square pyramidal ternary complexes, whose stability has been determined through spectrophotometric titration experiments. The formation of a five-cordinate complex is favored by the trans-I configuration of the [Cu(Bn4cyclam)]2+ receptor, but it is contrasted by the severe steric repulsions exerted by the benzyl substituents on the macrocyclic framework. No significant repulsive effects have been observed in anion binding to the [Cu(cyclam)]2+ receptor, exhibiting a trans-III configuration. The existence of a linear relationship between the magnitude of the red shift experienced by the d−d absorption band on anion binding and log K values is discussed.