Stoichiometry of lanthanide(iii) complexes with tripodal aminophosphonic ligands – a new solution to an old problem

Abstract

The Eu³⁺ and Gd³⁺ complexes with an N-(methylene-2-pyridine)-N,N-di(methylenephosphonate) ligand (H₄NP₂py), an analogue of nitrilotri(methylphosphonic) acid (H₆NTP), were synthesized and structurally characterized by X-ray single crystal diffraction. The determined crystal structures ([C(NH₂)₃]₅[Ln(NP₂py)₂]·12H₂O) are the first example of a monomeric Ln³⁺ complex encapsulated by two tripodal aminophosphonic ligands. Each of the NP₂py anions coordinates to Ln³⁺ through two oxygen atoms from each monodentate phosphonic group, amine nitrogen and pyridine nitrogen atoms, filling thus 8 coordination sites of Ln³⁺. The luminescence properties of [C(NH₂)₃]₅[Eu(NP₂py)₂]·12H₂O crystals were studied and compared with those of Eu–NP₂py complexes in solution. Speciation analysis of Ln–NP₂py complexes (Ln : NP₂py = 1 : 2), performed by luminescence and potentiometric methods, showed that both [Ln(NP₂py)]⁻ and [Ln(NP₂py)₂]⁵⁻ species may exist in solution. However, the formation of the latter one occurs in alkaline solutions at pH as high as 8. By implementing the Specific Ion Interaction Theory (SIT) it was possible to calculate the thermodynamic stability constants of the [Eu(NP₂py)]⁻ and [Eu(NP₂py)₂]⁵⁻ complexes. The corresponding log β⁰_Eul and values are 16.3 ± 0.11 and 19.5 ± 0.15, respectively.