Properties of monovalent nickel complexes with tetraaza-macrocyclic ligands in aqueous solutions

The effects of N-alkylation on the redox potential of the couples NiL(i2+)/NiL(i+), L = tetraaza-14-membered-macrocyclic ligands, and on the properties of the monovalent nickel complexes in aqueous solutions are reported for 14 complexes. The spectra and lifetimes of the NiL(i+) complexes are reported. The self-exchange rates for the couples NiL(i2+)/NiL(i+) were determined. Two of the ligands were synthesized for the first time for this study. Cyclic voltammetry and pulse radiolysis were used. The results point out that: (i) N-alkylation always shifts the redox potential to a less cathodic one; this effect stems to a large degree from the decrease in the solvation energy of the complex caused by the N-alkylation of the ligand. (ii) The lifetime of the monovalent complexes is not linearly related to the redox potential of the NiL(i2+)/NiL(i+) couples. (iii) The NiL(i+) complexes exist in several isomeric forms; the rate of the isomerization depends on the structure of the ligand. (iv) Different isomers of NiL(i+) may be formed when the complex NiL(i2+) is reduced by different reagents; therefore, the pulse-radiolytically formed NiL(i+) complexes might have different properties than those formed electrochemically.