Coordination Environment of Highly Concentrated Solutions of Cu^II in Ionic Liquids through a Multidisciplinary Approach

Abstract

The coordination environment around Cu^II in highly concentrated solutions of copper(II) salts (CuCl₂ and Cu(Tf₂N)₂) in two pure ionic liquids bearing the same anion, namely, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf₂N]) and 1-butyl-3-methylimidazolium chloride ([bmim]Cl), is investigated by X-ray photoelectron spectroscopy (XPS), UV/Vis spectroscopy, EPR spectroscopy and DFT calculations. Moreover, the electrochemical behavior of these mixtures is studied. Whereas reversible reduction of Cu^II to copper metal can be observed in the 1:1 [bmim][Tf₂N]:Cu(Tf₂N)₂ solution, 2:1 and 1:1 [bmim]Cl:CuCl₂ mixtures showed one-electron reduction of Cu^II to Cu^I with formation of a permanent deposit of CuCl. XPS, UV/Vis and EPR spectra as well as DFT calculations suggest the formation in [bmim]Cl of dynamic coordination complexes arising from the interaction between CuCl₂ and [bmim]⁺Cl⁻ . The two long-lived situations are probably trigonal and deformed tetrahedral copper(II) chloride coordination complexes ([CuCl₃]⁻ and [CuCl₄]²⁻, respectively).