1,4,7,10-tetraazacyclododecane metal complexes as potent promoters of carboxyester hydrolysis under physiological conditions.

New 1,4,7,10-tetrazacyclododecane ([12]aneN4 or cyclen) ligands with different heterocyclic spacers (triazine and pyridine) of various lengths (bi- and tripyridine) or an azacrown pendant and their mono- and dinuclear Zn(II), Cu(II), and Ni(II) complexes have been synthesized and characterized. The pKa values of water molecules coordinated to the complexed metal ions were determined by potentiometric pH titrations and vary from 7.7 to 11.