Pyridine and phosphonate containing ligands for stable lanthanide complexation. An experimental and theoretical study to assess the solution structure

Abstract

We report an experimental and theoretical study of the stability and solution structure of lanthanide complexes with two novel ligands containing pyridine units and phosphonate pendant arms on either ethane-1,2-diamine (L²) or cyclohexane-1,2-diamine (L³) backbones. Potentiometric studies have been carried out to determine the protonation constants of the ligands and the stability constants of the complexes with Gd^III and the endogenous metal ions Zn^II and Cu^II. While the stability constant of the GdL² complex is too high to be determined by direct pH-potentiometric titrations, the cyclohexyl derivative GdL³ has a lower and assessable stability (log K_GdL³ = 17.62). Due to the presence of the phosphonate groups, various protonated species can be detected up to pH ≈ 8 for both ligands and all metal ions studied. The molecular clusters [Ln(L)(H₂O)]³⁻·19H₂O (Ln = La, Nd, Ho or Lu; L = L² or L³) were characterized by theoretical calculations at the HF level. Our calculations provide two minimum energy geometries where the ligand adopts different conformations: twist-wrap (tw), in which the ligand wraps around the metal ion by twisting the pyridyl units relative to each other, and twist-fold (tf), where the slight twisting of the pyridyl units is accompanied by an overall folding of the two pyridine units towards one of the phosphonate groups. The relative free energies of the tw and tf conformations of [Ln(L)(H₂O)]³⁻ (L = L², L³) complexes calculated in aqueous solution (C-PCM) by using the B3LYP model indicate that the tw form is the most stable one along the whole lanthanide series for the complexes of L³, while for those of L² only the Gd^III complex is more stable in the tf conformation by ca. 0.5 kcal mol⁻¹. ¹H NMR studies of the Eu^III complex of L³ show the initial formation of the tf complex in aqueous solution, which slowly converts to the thermodynamically stable tw form. The structures calculated for the Nd^III complexes are in reasonably good agreement with the experimental solution structures, as demonstrated by Nd^III-induced relaxation rate enhancement effects in the ¹H NMR spectra.