The importance of hydrogen bonding in the complexation of lanthanide ions by polymer-bound malonamide-type ligands

Polymer-bound malonate ligands modified with diethylenetriamine (DETA-MAm) are prepared and the lanthanide ion affinities from solutions of 0.001-8 M HCl are quantified. A mechanism is proposed. The affinities are not due to the triamine ligand alone or the adjacent carbonyl sites alone: protonation of the carbonyl oxygen triggers formation of an iminium ion and it acts as an ion-exchange site. Two competing reactions occur during binding: electrostatic attraction of [Ln(H₂O)_xCl₄]⁻ by the protonated ligand and (partial) loss of the waters of hydration. The affinity and selectivity are affected by substituents on the iminium (CNRR′(+)) ion. Research with tetramethylmalonamide showed that its two methyl groups at the iminium site weaken the positive charge and decrease its affinity for the chlorocomplexes of the later lanthanides; DETA-MAm has at its amide nitrogen only one −CH₂− (and one H) moiety and therefore is a stronger but less selective ligand since electrostatic attraction is more dominant in the overall mechanism. The higher affinities of malonate monoamidated with ethylenediamine (EDA-MAm) and decreased affinities for those amidated with ethanolamine (EA-MAm) suggest that the protonated -NH- stabilizes the lanthanide chlorocomplex.