Stabilization of monovalent nickel in aqueous solutions by complexation with the β-isomer of C-5,12-racemic-1,4,5,7,7,8,11,12,14,14-decamethyl-1,4,8,11-tetraazacyclotetradecane

The monovalent nickel complex formed by the reduction of the β-isomer of the complex of C-5,12- racemic-1,4,5,7,7,8,11,12,14,14-decamethyl-1,4,8,11- tetraazacyclotetradecane nickel(II), NiL₁²⁺ in 0.1 M HCO₂Na, pH 7.6, has a half-life longer than 90 h. The redox potential of the couple NiL₁⁺/NiL₁²⁺ is -0.94 V vs. Ag/AgCl. The absorption spectrum of NiL₁⁺ consists of a band with λ_max = 335 nm and ε{lunate}_max = 2200 M^-1 cm^-1. For the analogous complex with C-5,12-racemic-5,7,7,12,14,14-hexamethyl-1,4, 8,11-tetraazacyclotetradecane, L₂, the half-life time of NIL₂⁺ is less than 1 min and the redox potential is -1.44 V vs. Ag/AgCl. These results are similar to those reported earlier for the analogous nickel complexes with the meso-isomers of the ligands. The results thus indicate that both the kinetic and thermodynamic stabilization of monovalent nickel by N-methylation of tetraazamacrocyclic ligands is not significantly affected by the configuration of the ligand.