Equilibrium studies between some transition metal ions and Me₆[14]dieneN₄ ligand

The complex formation equilibria between Me₆[14]dieneN₄ (5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) and metal ions such as Cu²⁺, Ni²⁺, and Zn²⁺ in aqueous 0.1 M NaCl at 298 K were studied by glass electrode potentiometric titrations. The formation constants of the complexes were determined by titration of mixtures of metal ion (2.0 mM) and ligand in concentration ratios of 1:1 and 1:2 (metal/ligand). The stability constants for the species of the general formula M _p H _q L _r (M = Cu²⁺, Ni²⁺, and Zn²⁺ ion, L = Me₆[14]dieneN₄) were calculated using a computational program, taking into account the hydrolysis of metal ions and protonation of the ligand. The stoichiometry and stability constants of the formed complexes are reported, and the concentration distribution of the various complex species was evaluated as a function of pH. All the above metals form mononuclear ([ML] and/or [MHL], [MH₂L]) and hydroxo complexes. The selectivity of the ligand toward metal ions is Cu²⁺ > Ni²⁺ > Zn²⁺. Analysis of the calculated pM values also shows that the Cu²⁺ ion has higher affinity for the Me₆[14]dieneN₄ ligand than Ni²⁺ and Zn²⁺ ions do.