Zinc complexes of four tetra-aza macrocycles: enthalpies of formation in acetonitrile solution

Abstract

The reactions of four tetra-aza macrocycles, 1,4,8,12-tetra-azacyclotetradecane, 1,4,8,12-tetra-azacyclopentadecane, 1,5,9,13-tetra-azacyclohexadecane, and NN′N″N‴-tetramethyl-1,4,8,11-tetra-azacyclotetradecane, with ZnBr₂ and [ZnBr₂(py)₂](py = pyridine) in acetonitrile solution have been studied conductimetrically and calorimetrically and a variety of complexes isolated. With equivalent amounts of base and ZnBr₂ or [ZnBr₂(py)₂] the reaction product is an equilibrium mixture of [ZnBr₂(base)] and [ZnBr(base)] Br, but is completely ionised when the four N atoms of the base are methylated. In the presence of an excess of ZnBr₂ all the bases form [ZnBr(base)]-[ZnBr₃]. Enthalpies of formation of the complexes are smaller for the 14-membered macrocycle (‘cyclam’) than for the 15- and 16-membered and are similar for methylated and unmethylated cyclam. The results support five-co-ordinated structures for the complex ions, in which the size of the macrocyclic cavity is not critical. Thermodynamic data are also reported for the formation of ZnBr₃^– and ZnBr₄^2– from ZnBr₂ and Br^– in acetonitrile solution.