Complexation of Sm3+ and pamidronate: A DFT study

We reported a quantum mechanical study of the complexes formed between Sm3+ and the bisphosphonate ligand pamidronate in aqueous solution. According to available experimental pKa values pamidronate was expected to exist in aqueous solution, at physiologically relevant pH, in its di- and tri-protonated forms (denoted by H3L and H2L). The most stable structures of the ligands and Sm3+ complexes were found by using a detailed analysis of the conformational space with semiempirical and DFT methods. The results showed that both H2L and H3L bisphosphonates acted as a tridentate ligands in their complexes with Sm3+. The addition of explicit water molecules to the coordination sphere of the metal not only gave different coordination numbers for H2L and H3L complexes (CN=9 and 10), but also provided different trends in stabilization energies. The results highlighted the importance of considering not only an explicit first coordination shell, but also a second hydration shell, for an adequate description of this type of complexes in aqueous solution.

Graphical AbstractThe addition of an explicit aqueous coordination shell shows that six and seven water molecules can be bonded to the metal in [Sm(H2L)]+ and [Sm(H3L)]2+, respectivelyFigure optionsDownload as PowerPoint slide