Complexation kinetics of copper(II) and nickel(II) with macrocycles: identification of an outer-sphere chelate effect

Ligand-protonation constants and complexation kinetics of differently protonated forms of 10-methyl-1,4,8,12-tetraazacyclopentadecan-10-amine, 1,4,8,12-tetraazacyclopentadecane, and 1,4,8,12-tetraazacyclopentadecan-10-carboxylic acid with Cu(II) and Ni(II) in aqueous solution have been determined. These results are combined with a wide range of published complexation rate constants of partially protonated macrocyclic ligands with the same metal ions. Insight is gained into the electrostatic effects and the outer-sphere interaction of partially protonated ligands with aquated metal ions. An outer-sphere chelate effect has been ascertained.