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, Cd 2+ and Pb 2+ , were determined by potentiometric methods, in aqueous solution, at 25° and I = 0.10 M (KNO 3 ). The sequence of protonation of ligand 1 was studied by 1 H‐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 Mn 2+ and Pb 2+ .