Solution properties of the Ln(III) complexes of a novel octadentate chelator with rigidified iminodiacetate arms

A novel octadentate ligand derived from the core structure of EGTA (ethyleneglycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid) with two piperidine-cis-dicarboxylic acid moieties spaced by a triethylenedioxy chain (L1) has been synthesised with the aim to assess the effect of the structural rigidity on the thermodynamic and kinetic stabilities of its Ln(III) complexes. The stability constants of [Ln(L1)](-) complexes show a weaker binding affinity for Ln(3+) ions of L1 than EGTA. On the other hand, the selectivity of L1 for Ca(2+) over Mg(2+) is one of the highest ever reported. The kinetic inertness has been also investigated and faster rates of both acid catalysed and metal assisted decomplexation have been measured for [Gd(L1)](-) as compared to [Gd(EGTA)](-). The (1)H- and (13)C NMR spectra of the diamagnetic La(3+), Y(3+) and Lu(3+) complexes in aqueous media show pronounced stereochemical rigidity and the occurrence of a structural change across the lanthanide series. Finally, a complete (1)H and (17)O NMR relaxometric study indicates that [Gd(L1)](-) possesses a nine-coordinate ground state with one inner coordination sphere water molecule characterized by a fast rate of exchange ((298)k(ex) = 59 × 10(6) s(-1)).