The effect of ring size variation on the structure and stability of lanthanide(iii) complexes with crown ethers containing picolinate pendants

Abstract

The coordination properties of the macrocyclic receptor N,N′-bis[(6-carboxy-2-pyridyl)methylene]-1,10-diaza-15-crown-5 (H₂bp15c5) towards the lanthanide ions are reported. Thermodynamic stability constants were determined by pH-potentiometric titration at 25 °C in 0.1 M KCl. A smooth decrease in complex stability is observed upon decreasing the ionic radius of the Ln^III ion from La [log K_LaL = 12.52(2)] to Lu [log K_LuL = 10.03(6)]. Luminescence lifetime measurements recorded on solutions of the Eu^III and Tb^III complexes confirm the absence of inner-sphere water molecules in these complexes. ¹H and ¹³C NMR spectra of the complexes formed with the diamagnetic La^III metal ion were obtained in D₂O solution and assigned with the aid of HSQC and HMBC 2D heteronuclear experiments, as well as standard 2D homonuclear COSY and NOESY spectra. The ¹H NMR spectra of the paramagnetic Ce^III, Eu^III and Yb^III complex suggest nonadentate binding of the ligand to the metal ion. The syn conformation of the ligand in [Ln(bp15c5)]⁺ complexes implies the occurrence of two helicities, one associated with the layout of the picolinate pendant arms (absolute configuration Δ or Λ), and the other to the five five-membered chelate rings formed by the binding of the crown moiety (absolute configuration δ or λ). A detailed conformational analysis performed with the aid of DFT calculations (B3LYP model) indicates that the complexes adopt a Λ(λδ)(δδλ) [or Δ(δλ)(λλδ)] conformation in aqueous solution. Our calculations show that the interaction between the Ln^III ion and several donor atoms of the crown moiety is weakened as the ionic radius of the metal ion decreases, in line with the decrease of complex stability observed on proceeding to the right across the lanthanide series.