Abstract
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.
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