Interactions of nickel(II) ions in protic ionic liquids comprising N-hexyl(or N-2-ethtylhexyl)ethylenediamines

Abstract

The interactions of nickel(II) ions in a protic ionic liquid (PIL) comprising monoprotic N-alkyl (alkyl=hexyl or 2-ethylhexyl)–ethylenediaminium ions (HHexen+ or HEthexen+, respectively) combined with bis(trifluoromethanesulfonyl)amide (Tf2N−) or trifluoroacetate (TFA−) ions were investigated using visible (VIS) electronic and extended X-ray absorption fine structure (EXAFS) spectroscopy. These were compared to the analogous nickel(II) complexes in molecular solvents, and copper(II) ion in the same PIL systems. A complete extraction of nickel(II) and copper(II) ions from the aqueous phase was achieved by using the above mentioned PILs with the Tf2N− counter anion. Absorption spectra were used to explore the interaction mode of the nickel(II) ions with the chelating diamine PIL by the addition of nickel(II) salts to the PIL. This demonstrated a dominant formation of the tris(alkyl-en) complex at lower nickel(II) concentrations and changes of the complex to a cis-aniono bis(alkyl-en) complex with the addition of the nickel(II) salts. The ethyl-branch in the cationic unit of the PIL was observed to have a larger effect on the solubility of the copper(II) salts than that of the nickel(II) salts. The EXAFS spectra analysis of the nickel(II) ions in the PILs gives the significant information on the fine structure around the nickel(II) ion coordinated by hexylethylenediamine (Hexen) in the PILs.