AQUEOUS-SOLUTION EQUILIBRIA INVOLVING THE LIGAND 2,2,4-TRIMETHYL-1,5,9-TRIAZACYCLODODECANE AND NICKEL(II), COPPER(II), AND ZINC(II)

Complexes formed in aqueous solution by the ligand 2,2,4-trimethyl-1,5,9-triazacyclododecane with nickel(II), copper(II) and zinc(II) have been investigated potentiometrically and spectrophotometrically. The nickel(II) complexes are shown to be five-coordinate. Acid dissociation constants (pka values) of the ligand are 2.51, 7.34 and 12.3 at 25.0 °C; I = 0.1 mol dm-3 (NaNO3). Formation of the nickel(II) complex was too slow to be investigated potentiometrically; the formation constants for the copper(II) and zinc(II) complexes are log K = 11.58 and 7.68, respectively. Formation of the copper complex was catalyzed by bromide ions, and bromide complexes were observed in solution. The complexes of all three metals readily lose a proton from a coordinated water molecule; pKs for the reaction ML(H2O)22+ ⇌ ML(OH)(H2O)+ + H+ is 9.81 (Ni), 8.48 (Cu), and 9.56 (Zn). The species [M2L2(OH)2]2+ is also shown to exist for M = Ni and Cu. The equilibrium constants (log K values) for the reaction 2ML(OH)+ ^ M2L2(OH)22+ are 2.44 (Ni) and 2.00 (Cu). At high pH, the species CuL(OH)2 was also formed; pKa = 11.9. Nickel complexes with the ligand 2,4,4-trimethyl-l,5,9-triazacyclododec-l-ene have also been investigated. © 1979, American Chemical Society. All rights reserved.