N,N′-diacetate derivatives of some polyoxa-polyaza macrocyclic compounds: Protonation and complexation studies

Abstract

The N,N'-diacetate derivatives of the 12-membered polyoxa-polyaza macrocycles 1-oxa-4,7,10-triazacyclododecane, 1,7-dioxa-4,10-diazacyclododecane and 1,4-dioxa-7,10- diazacyclododecane have been synthesized. Their protonation constants were determined by potentiometry and the sequences of protonation were studied by 1H NMR spectroscopy. These studies have shown that the first two steps of protonation in the three ligands occur on the nitrogen atoms, even when the molecule has the two nitrogen atoms of the ring in contiguous positions. The stability constants of the complexes formed by these ligands with the alkaline-earth, first-series transition metal ions Zn2+, Cd2+ and Pb2+ were determined by automated potentiometry at 25.0°C and 0.10 M ionic strength. In the case of 1,4-dioxa- 7,10-diazacyclododecane-7,10-diacetic acid, the constants for the Cu2+ and Pb2+ complexes had to be determined by a ligand-ligand competition potentiometric method. For the alkaline-earth metal ions the stability constants (log KML) are of the order 5–8 log units; for the other metals they vary from ca 11 to 18 log units. However, the results show that the replacement of one nitrogen by an oxygen atom in the ring leads to a considerable decrease in the stability of the complexes of the transition metal ions Zn2+ and Cd2+ (from 1.7 to 5.0 log units), and to an increase, albeit small, in the stability of the complexes of the alkaline-earth metal ions. The Irving-Williams' order of stability is not obeyed for the metal complexes of any of the ligands, probably due to the steric constraints of the rings. The comparison of the behaviour of the metal complexes formed with the ligands studied in the present work with those formed by 1-oxa-4,8,12-triazacyclotetradecane-4,12-diacetic acid, a 14-membered ligand with the same set of donor atoms in the ring, shows that all the complexes formed with the 12-membered ligands are generally more stable, but the 14- membered ligand has a far more marked selective behaviour towards the same series of metal ions. Hence, whereas the difference in stability of e.g. the copper and lead complexes of the 12-membered ligands varies from 0 to 3 log units, the difference for the complexes of 14-membered ligands formed with the same metals is of the order 10 log units.