Nickel(II) complexation by bis(1,4,7-triazacyclononane) ligands with polymethylene linker groups.: A study of the stability and structure of their mononuclear and binuclear dimeric complexes

The Ni(II) complexes of a series of bis(1,4,7-triazacyclonon-1-yl)alkanes with polymethylene bridges containing two to six C atoms (EMn (n=2–6)) have been studied by pH potentiometric, electrochemical, UV–Vis spectrophotometric, and X-ray crystallographic experiments. Ligands with bridges of two or three C atoms (EM2 and EM3) exclusively form mononuclear complexes and those with longer linker chains form binuclear dimers. The X-ray structures of [Ni2(EM5)2]Cl3NO3·15H2O (3) and [Ni2(EM6)2]Cl3(NO3)2·6H2O (4) establish the existence of the dimeric complexes in which two Ni(II) ions are sandwiched between two ligands. The structure of [NiEM3](ClO4)2·NaClO4·H2O (1) is monomeric with a distorted NiN6 coordination. Equilibrium models of pH potentiometric data from batch titrations of equimolar Ni(II) ligand aqueous solutions provide a rationale for the formation of the various complex species. Electrochemical cyclic and square wave voltammetric and UV–Vis continuous variation experiments on the series of Ni(II) complexes are also reported and interpreted based on the equilibrium models.