Coordination and extraction of mercury(II) with an ionic liquid-based thione extractant

A neutral thione extractant, 1,3-diethylimidazole-2-thione (C2C2ImT), was prepared from an ionic liquid (IL), 1,3-diethylimidazolium acetate, and used within a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethane)sulfonamide ([C2mim][NTf2]), to extract Hg(II) from aqueous solutions of HgCl2 or Hg(OAc)2. Investigations of the extraction mechanism, spectroscopic analyses of the extracted species, and crystallographic studies of the interactions of C2C2ImT with Hg(II) are reported, including the first structurally characterized Hg-NTf2 coordination compound, Hg(C2C2ImT)2(NTf2)2. Coordination complexes of the thione ligand with Hg(II) show variability in coordination numbers and geometries with stoichiometry, suggesting that the extraction mechanism is dependent on the speciation of mercury in aqueous solution. HgCl2 can form neutral, extractable complexes with the thione in aqueous solution. Hg(OAc)2 dissociates on dissolution in water and Hg(II) is extracted through a cation exchange mechanism involving [Hg(C2C2ImT)2](2+) ions. The precipitation of neutral mercury complexes from the IL following the extraction of excess mercury suggests a simple and unusual way to recycle the IL.