Dramatic increase of selectivity for heavy lanthanide(III) cations by tuning the flexibility of polydentate chelators

Two novel octadentate ligands have been synthesized by attaching two terminal iminodiacetic groups to either 1,4-diazepane (BCAED) or piperazine (BCAEP) as central scaffold. The introduction of the seven- or six-membered ring into the ligand backbone is expected to modify their overall flexibility and then to affect the stability of the corresponding lanthanide(II) complexes. In this work, thermodynamic stability data are determined for the formation of the complexes of BCAED and BCAEP with La³⁺, Nd³⁺, Eu ³⁺, Gd³⁺, Ho³⁺, and Lu³⁺. The ligand BCAED shows a strong bindingj affinity for Lu³⁺ (log K=20.99), moderate for Gd³⁺ (logK=17.15) and rather weak for La³⁺ (logK=12.77). Thus, the variation of logK across the Ln series assumes the remarkable value of 8.22, the largest so far reported. This points to a predominant role of a suitable size match between the metal ion and the ligand cavity, determined by its structure and flexibility. The ligand BCAEP forms less stable complexes with lanthanide(III) cations although it retains a good selectivity (ΔlogK_La-Lu=5.66). The Gd(III) complexes have been investigated in aqueous solution by measuring their relaxivity as a function of pH, at 20 MHz and 25°C. The results can be interpreted very well in terms of the species distribution curves calculated from the thermodynamic data and indicate that in these complexes Gd³⁺ is octacoordinated, without any bound water molecule. This coordination geometry is maintained in the solid state as shown by the X-ray crystal structure of [Na(H₂O) ₂][Gd(BCAED)] where the metal ¡is at the center of a bicappedtrigonal prism. Finally, the ¹³C NMR spectra (9.4 T, 25°C) of the diamagnetic La³⁺, Y³⁺, and Lu ³⁺ complexes show that a pronounced stereochemical rigidity is associated with the thermodynamically more stable complexes.