Metal complexes of pentadentate macrocyclic ligands containing oxygen and nitrogen as donor atoms

Article abstract of DOI:10.1002/hlca.19940770212

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 = 0.10 M (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⁺.