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

Abstract

The stability and kinetics of formation of NiII complexes with the macrocyclic ligand 3,6,9,17,20,23-hexaazatricyclo[23.3.1.111,15]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 NiII 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 NiII 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.