Heavy metal ion complexes with a simple phenolic ligand. Solid state and solution studies

The coordination properties of the ligand 2-[bis-(aminoethyl)-aminomethyl]-phenol (L) toward Cd(II) and Pb(II) are reported. The studies were carried out using potentiometric measurements (298.1 K, I/ 0.15 mol dm3), UV/Vis absorption and 1H, 13C, 113Cd NMR spectra both in aqueous and non-aqueous solution. By potentiometric measurements performed in aqueous solution, L was found to form only mononuclear species with both the ions investigated; [MH1L] is the main species formed. A dinuclear species with stoichiometry [M2(H1L)2](ClO4)2 was almost quantitatively isolated in solid state from an aqueous solution in the pH range in which the [MH1L] mononuclear species are present. This behavior is ascribed mainly to the low solubility of the dinuclear complex formed. L does not fulfill the coordination requirement of the ion in the mononuclear species and to saturate it, each metal ion binds the phenolate oxygen of another mononuclear species. The dinuclear species were characterized in organic solution by NMR and ESI mass spectroscopy. The results are confirmed by the crystal structure of [Cd2(H1L)2](ClO4)2, which displays the two Cd(II) ions close to each other and bridged by two phenolic oxygens each belonging to a H1L species.