Solubilization properties and structural characterization of dissociated HgO and HgCl2 in deep eutectic solvents

A synergic approach combining X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies together with quantum mechanical (QM) calculations on cluster models has been used to investigate the solvation properties of the HgCl2 salt and of HgO dissolved in deep eutectic solvents (DESs). Choline chloride (ChCl)-based DESs were prepared using different hydrogen bond donors, namely 1:2 mixtures of ChCl and either urea, acetylsalicylic acid (ASA) or sesamol (SES) and a 1:1 mixture of ChCl and pyrogallol (PYR). The XANES results show that both HgCl2 and HgO molecules are completely dissociated in all the investigated DESs and the Hg2+ first coordination shell is composed only of Cl− anions in all the systems, with a local structure of Cl− ligands around the Hg2+ ion that is the same independently of the DES solvent and of the nature of the dissolved species. By applying a deconvolution procedure on the Hg L3-edge raw XANES experimental data and carrying out fitting procedures of the deconvolved XANES data using different coordination models for the Cl− anions surrounding Hg2+, we were able to unambigously determine the coordination number and the geometry of the Hg2+ first shell complex, namely a [HgCl4]2− tetrahedral coordination model. The analysis of the EXAFS spectra of HgCl2 salt in ChCl-Urea, ChCl-ASA, ChCl-SES and ChCl-PYR and of HgO dissolved in ChCl-ASA allowed us to obtain a very accurate determination of the Hg[sbnd]Cl first shell average distance (2.47–2.48 Å), in agreement with the XANES determinations. Moreover, ab initio calculations of different [HgCln](2−n)+ clusters carried out both in vacuum and simulating bulk solvent effects by means of the SMD solvation model strongly support the experimental findings that, in DESs, Hg2+ forms a complex with four chloride ions arranged in a tetrahedral geometry. The Cl− ion is thus shown to form such strong interactions with Hg2+ that none of the DES hydrogen bond donors is able to compete with it and act as a ligand for Hg2+. The strong solvation ability of the Cl− anions towards metal ions can be at the origin of the high solubility of metal oxides in chloride-based DESs, which is crucial in several important processes such as metal winning, corrosion remediation and catalyst preparation.