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

A new N-carboxymethyl derivative of the oxa-tetraaza macrocyle, 4,7,10,13-tetrakis-(carboxymethyl)-1-oxa-4,7,10,13-tetraazacyclopentadecane, has been synthesised. The protonation constants of this compound and the stability constants of its complexes with several di- and trivalent metal ions were determined by potentiometric or spectrophotometric methods, at 25°C and ionic strength 0.10 M in tetramethyl ammonium nitrate. The ligand exhibits 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 complexes 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 the dinuclear complexes. This was interpreted, in the case of mononuclear complexes, 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 or 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 sphere, while in the other three nitrogen donor atoms of the macrocyclic framework 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 carboxymethyl arms and the relatively large size of the macrocycle severely constrains the geometric arrangement of the nitrogen donor atoms of the macrocyclic backbone around the metal centre decreasing the co-ordination number or leading to a preferred co-ordination with oxygen atoms. Another consequence of these structural features, is their easy ability to form dinuclear complexes, as found in the equilibria studies in solution and also by EPR spectroscopy of the Cu²⁺ complexes where the presence of two types of signals in the ΔM_s=1 and ΔM_s=2 regions, clearly reveals the presence of the dinuclear complex.