Magnesium(II) d-Gluconate Complexes Relevant to Radioactive Waste Disposals: Metal-Ion-Induced Ligand Deprotonation or Ligand-Promoted Metal-Ion Hydrolysis?

Bence Kutus; Csilla Dudás; Eszter Orbán; Alexandru Lupan; Amr A A Attia; István Pálinkó; Pál Sipos; Gábor Peintler

doi:10.1021/acs.inorgchem.9b00289

Magnesium(II) d-Gluconate Complexes Relevant to Radioactive Waste Disposals: Metal-Ion-Induced Ligand Deprotonation or Ligand-Promoted Metal-Ion Hydrolysis?

Inorg Chem. 2019 May 20;58(10):6832-6844. doi: 10.1021/acs.inorgchem.9b00289. Epub 2019 May 8.

Authors

Bence Kutus^{1

2}, Csilla Dudás², Eszter Orbán², Alexandru Lupan³, Amr A A Attia³, István Pálinkó², Pál Sipos², Gábor Peintler²

Affiliations

¹ Department of Molecular Spectroscopy , Max Planck Institute for Polymer Research , D-55128 Mainz , Germany.
² Institute of Chemistry , University of Szeged , H-6720 Szeged , Hungary.
³ Department of Chemistry , Babeş-Bolyai University , RO-400028 Cluj-Napoca , Romania.

Abstract

The complexation equilibria between Mg²⁺ and d-gluconate (Gluc^-) ions are of particular importance in modeling the chemical speciation in low- and intermediate-level radioactive waste repositories. NMR measurements and potentiometric titrations conducted at 25 °C and 4 M ionic strength revealed the formation of the MgGluc⁺, MgGlucOH⁰, MgGluc(OH)₂^-, and Mg₃Gluc₂(OH)₄⁰ complexes. The trinuclear species provides indirect evidence for the existence of multinuclear magnesium(II) hydroxido complexes, whose formation was proposed earlier but has not been confirmed yet. Additionally, speciation calculations demonstrated that MgCl₂ can markedly decrease the solubility of thorium(IV) at low ligand concentrations. Regarding the structure of MgGluc⁺, both IR spectra and density functional theory (DFT) calculations indicate the monodentate coordination of Gluc^-. By the potentiometric data, the acidity of the water molecules is higher in the MgGluc⁺ and MgGlucOH⁰ species than in the Mg(H₂O)₆²⁺ aqua ion. On the basis of DFT calculations, this ligand-promoted hydrolysis is caused by strong hydrogen bonds forming between Gluc^- and Mg(H₂O)₆²⁺. Conversely, metal-ion-induced ligand deprotonation takes place in the case of calcium(II) complexes, giving rise to salient variations on the NMR spectra in a strongly alkaline medium.