Quantum chemical modeling of zirconium complexes: methanesulfonate, carbonate, chloride, and sulfate

The work carried out quantum chemical modeling of zirconium complexes with various ligands (methanesulfonate, chloride, carbonate and sulfate) in an aqueous medium using DFT/B3LYP methods and QTAIM theory. The aim of the study was to determine the influence of the nature of the ligands on the structure, ionicity/covalence of bonds in zirconium complexes and to analyze how these features affect the precipitation of zirconium hydroxide from aqueous solutions of various salts. The effective charges, binding energies and geometric parameters of the complexes were calculated. The influence was shown of the nature of the ligands on the nature of the ionic or covalent bond in zirconium complexes. It was shown that zirconyl sulfate complexes form electron density cycles involving four atoms, while the [ZrO(CH3SO3)2] complex forms a six-membered cycle due to weak hydrogen bonding. The effect of ligands on the binding energy of the zirconyl oxygen atom was analyzed, which decreased in the series [ZrOCl2] > [ZrO(MS)2] > [ZrO(СО3)] > [ZrO(SО4)2]2–, which indicates a competing effect between the Zr=O and Zr–L bonds. A decrease was found in bond ionicity in the series [ZrO(MS)2] ? [ZrO(СО3)] ? [ZrOCl2] ? [ZrO(SO4)2]2–. The obtained results demonstrate that the minimal 3-21G basis is insufficient for an adequate description of the electronic structure of zirconyl complexes, as it underestimates the effective charge of the metal and slightly lengthens the Zr=O bond. The main contribution to improving the description is provided by the broadening of the basis for the ligands, while the addition of polarization functions on zirconia is of a secondary nature. The process of precipitation of zirconium hydroxide from solutions of zirconium oxychloride and zirconium methanesulfonate in the pH range from 1 to 12 was experimentally investigated. It has been found that the dependence of the volume of the precipitate on pH is extreme, and for zirconium methanesulfonate a significant volume of the precipitate is formed already at pH 2. The change in the volume of the precipitate is explained by the formation of various zirconium hydroxocomplexes at different pH values. The maximum volume of the precipitate for methanesulfonate was observed at pH 4, and for oxychloride – at pH 6, which corresponds to the formation of clusters of the Zr4(OH)16 type. A further increase in pH leads to a decrease in the volume of the precipitate due to the formation of denser structures – Zr4O2(OH)12 and ZrO(OH)2. Thus, zirconium methanesulfonate demonstrates high efficiency as a precursor for the synthesis of nanodispersed forms of zirconium hydroxide and potentially stabilized ZrO2.