Metal Complexes of Pentadentate Macrocyclic Ligands Containing Oxygen and Nitrogen as Donor Atoms

Four macrocyclic ligands have been synthesized: 1-oxa-4,7,10,13-tetraazacyclopentadecane ( 1 ), 1,4-dioxa-7,10,13-triazacyclopentadecane ( 2 ), 1,4-dioxa-7,11,14-triazacyclohexadecane ( 3 ), 1,4-dioxa-7,11,15-triazacycloheptadecane ( 4 ), one of them 3 , for the first time. The protonation constants of the ligands and the stability constants of the complexes formed by the four ligands with some divalent first-series transition-metal ions, Cd²⁺ and Pb²⁺, were determined by potentiometric methods, in aqueous solution, at 25° and I = 0.10M (KNO₃). The sequence of protonation of ligand 1 was studied by ¹H-NMR spectroscopy. The Irving-Williams' order of stability is obeyed for the complexes of all the ligands, and the metal complexes of 1 present the higher values of stability. A drop in the stability of all the metal complexes studied is observed when the metal complexes of 1 are compared with the corresponding complexes of 2 . The effect of the increase of the ring size of the macrocycle can be observed for metal complexes of the series of ligands 2 4 , where, in spite of the slight increase of the overall basicity of the ligands (20.28, 22.25, and 24.96 for 2, 3 , and 4 respectively), small differences in stability are found for the corresponding complexes of 2 and 3 , but a significant drop occurs for all the metal complexes formed with the 17-membered ligand, specially for the larger metal ions like Mn²⁺ and Pb²⁺.