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 Ln³⁺ ions of L1 than EGTA. On the other hand, the selectivity of L1 for Ca²⁺ 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 ¹H- and ¹³C NMR spectra of the diamagnetic La³⁺, Y³⁺ and Lu³⁺ complexes in aqueous media show pronounced stereochemical rigidity and the occurrence of a structural change across the lanthanide series. Finally, a complete ¹H and ¹⁷O 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 (²⁹⁸k_ex = 59 × 10⁶ s^-1).