Metal Complexes with Macrocyclic Ligands,IV. Synthesis,Properties and Kinetics of Complexation with Three N-methyl Substituted 1,4,8,11-tetraazacyclotetradecanes

The synthesis, properties and complexation of 1-methyl-1,4,8,11-tetraazacyclotetradecane (1-MeCyclam-14), 1,5-dimethyl-1,5,8,12-tetraazacyclotetradecane (2-MeCyclam-14) and 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (4-MeCyclam-14) are described. While the Ni²⁺ and Cu²⁺ complexes of 1-MeCyclam-14 and 2-MeCyclam-14 exhibit square planar geometries, 4-MeCyclam-14 forms Ni²⁺ and Cu²⁺ complexes, whose absorption spectra are best explained by assuming pentaco-ordination of the metal ions. The complexation rate of the three N-methyl substituted macrocycles with Cu²⁺ and Ni²⁺ is slower than can be accounted for by water exchange and little affected by introducing methyl groups at the nitrogens. Both results are in contrast to what is known for open chain amine ligands. A mechanism for the complexation is proposed, which also explains why the products of the reaction of 4-MeCyclam-14 with Cu²⁺ and Ni²⁺ are pentaco-ordinated.