Metal complexes of edta-derived macrocyclic ether bis(lactones). Hydrolysis of the macrocycles and the metal catalysis effect

Abstract

Four edta-derived macrocycles containing an ether moiety of increasing length, (CH 2CH 2O) n CH 2CH 2 with n=0–3, H 2L 1H 2L 4 were prepared. The protonation constants of the compounds and their stability constants with the alkaline-earth metal ions, and Cu 2+, Zn 2+, Cd 2+ and Pb 2+ were determined by potentiometric techniques at 298 K and ionic strength 0.10 mol dm −3 in tetramethyl ammonium nitrate. The overall basicity of those compounds is very low when compared with ethylenediaminotetracetic acid, H 4edta or H 4L 7, as expected, due to the electron-withdrawing ability of the ester functions incorporated on the macrocyclic backbone in close vicinity of the nitrogen sites. The 1H NMR spectra, obtained at three distinguishable pD values of H 2L 2 in D 2O, have suggested that the first protonation occurs at the nitrogen atoms, and the second one in a carboxylate group. The values of stability constants are the expected ones taking into account the basicity of the ligands. Those compounds are not stable in water at pH values higher than 7 (even at low pH on prolonged storage in the case of H 2L 1) but form the open-chain monoester by hydrolysis of one ester group. The presence of metal ions enhances hydrolysis of all compounds, where the hydrolysis rate seem to correlate roughly with the stability constants of the ML species and the size of the metal ion. 1H and 13C NMR spectroscopy of the Zn 2+ and Pb 2+ complexes of H 2L 1H 2L 3 showed that complexation occurs mainly in the edta-derived part of the molecule, but also proved that for the complex [Pb(L 2)] the ether oxygen of H 2L 2 is directly co-ordinated to the metal. EPR spectroscopy of the copper(II) complexes showed two types of signals corresponding to Δ M s=1 and Δ M s=2 transitions, which reveal the presence of dinuclear complexes.