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Issue 45, 2006
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Pyridine and phosphonate containing ligands for stable lanthanide complexation. An experimental and theoretical study to assess the solution structure

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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 GdIII and the endogenous metal ions ZnII and CuII. 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 KGdL3 = 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 GdIII complex is more stable in the tf conformation by ca. 0.5 kcal mol−1. 1H NMR studies of the EuIII 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 NdIII complexes are in reasonably good agreement with the experimental solution structures, as demonstrated by NdIII-induced relaxation rate enhancement effects in the 1H NMR spectra.

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

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Publication details

The article was received on 09 Aug 2006, accepted on 29 Sep 2006 and first published on 13 Oct 2006


Article type: Paper
DOI: 10.1039/B611544F
Citation: Dalton Trans., 2006,0, 5404-5415
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    Pyridine and phosphonate containing ligands for stable lanthanide complexation. An experimental and theoretical study to assess the solution structure

    M. Mato-Iglesias, E. Balogh, C. Platas-Iglesias, É. Tóth, A. de Blas and T. Rodríguez Blas, Dalton Trans., 2006, 0, 5404
    DOI: 10.1039/B611544F

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