Zinc(II) complexation with bis(1,4,7-triazacyclononane) ligands containing polymethylene linker groups. An equilibrium and structural study

The stoichiometry and stability of a series of zinc(II) complexes of bis(1,4,7-triazacyclononane) ligands containing from two- to eight-membered polymethylene bridging chains have been studied by pH potentiometry. Equilibrium modeling of the data indicates that stable 1:1 complexes form when the linker group contains only two or three C atoms. Ligands comprising longer bridging chains form stable 1:1 and 2:1 Zn(II)–L complexes. The stability trends observed for the 1:1 complexes follow those of the Cu(II) complexes where the ligands with the shorter bridging groups form the most stable 1:1 complexes. The 2:1 complexes show virtually no variation in stability throughout the series. The X-ray structures of two monomeric complexes, [Zn(EM2)][ZnCl3ONO2] (1) and [Zn(EM3)][ZnBr4] (2), and a dimeric complex, [Zn2(EM5)(μ-Cl)3]2(ClO4)2·4H2O (3), containing a face shared bioctahedral Zn(μ-Cl)3Zn cluster are determined. The structure of the [ZnCl3ONO2]2− anion has not previously been reported.