4,7,10,13-Tetrakis(carboxymethyl)-1-oxa-4,7,10,13-tetraazacyclopentadecaneand properties of its metal complexes

A new N-carboxymethyl derivative of the oxa-tetraaza macrocyle, 4,7,10,13-t etrakis-(carboxymethyl)-1-oxa-4,7,10,13-tetraazacyclopentadecane, has been synthesised. The protonation constants of this compound and the stability c onstants of its complexes with several di- and trivalent metal ions were de termined by potentiometric or spectrophotometric methods, at 25 degrees C a nd ionic strength 0.10 M in tetramethyl ammonium nitrate. The ligand exhibi ts two high or fairly high values of protonation constants and two low ones , and its overall basicity is about 27 in log units. Mono- and dinuclear co mplexes were found. The stability constant values of the 1:1 complexes with most of the metal ions studied are lower than expected, but not those of t he dinuclear complexes. This was interpreted, in the case of mononuclear co mplexes, as the non-involvement in the co-ordination to these metal ions of two nitrogen atoms of the macrocycle backbone and, probably also, of one o r two carboxylate groups. The Cu2+ ion has an exceptional behaviour, its 1: 1 complex exhibits a high stability constant value. Spectroscopic data have indicated, for the last complex, the presence of two octahedral isomers in solution, one of them having only two nitrogens in the co-ordination spher e, while in the other three nitrogen donor atoms of the macrocyclic framewo rk are co-ordinated in the equatorial plane. A third species appears at pH values higher than 7. These features suggest that the presence of four carb oxymethyl arms and the relatively large size of the macrocycle severely con strains the geometric arrangement of the nitrogen donor atoms of the macroc yclic backbone around the metal centre decreasing the co-ordination number or leading to a preferred co-ordination with oxygen atoms. Another conseque nce of these structural features, is their easy ability to form dinuclear c omplexes, as found in the equilibria studies in solution and also by EPR sp ectroscopy of the Cu2+ complexes where the presence of two types of signals in the Delta M-s = 1 and Delta M-s = 2 regions, clearly reveals the presen ce of the dinuclear complex. (C) 1999 Elsevier Science Ltd. All rights rese rved.