Equilibrium and kinetic studies on the formation of mono- and bi-nuclear Niii complexes with a binucleating hexaaza macrocycle

Abstract

The stability and kinetics of formation of Ni^II complexes with the macrocyclic ligand 3,6,9,17,20,23-hexaazatricyclo[23.3.1.1^11,15]triaconta-1(29),11(30),12,14,25(26),27-hexaene (L) that contains two diethylenetriamine sub-units linked by m-xylyl spacers has been studied in water solution. The equilibrium studies indicate the formation of several mono- and bi-nuclear metal complexes. However, only mononuclear species are formed in the presence of ligand excess, whereas the complete conversion of L in binuclear complexes can be achieved in the presence of metal excess. The stability data and the electronic spectra suggest a similar coordination environment of the metal centres in all the complexes, each Ni^II being coordinated to three of the amine groups in the ligand. At pH close to 7, complex formation occurs in a single kinetic step both for the mono- and the bi-nuclear species, which indicates that coordination of both metal ions to the macrocycle occurs with statistically controlled kinetics. The resolved rate constants for reaction of Ni^II with the protonated forms of the ligand are close to those found for related open-chain polyamines of similar steric requirements thus showing that the macrocycle is flexible enough to allow a rapid structural reorganization during the complex formation processes.