Coordination Environment of Highly Concentrated Solutions of CuII in Ionic Liquids through a Multidisciplinary Approach

• Published in: Chemphyschem Vol. 13; no. 7; pp. 1885 - 1892
• Main Authors: Caporali, Stefano, Chiappe, Cinzia, Ghilardi, Tiziana, Pomelli, Christian Silvio, Pinzino, Calogero
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 14.05.2012; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: complexation; Copper; density functional calculations; electrochemistry; ionic liquids; Ions; Liquids; Spectrum analysis; X-rays
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201100876

The coordination environment around CuII 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 CuII 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 CuII to CuI 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). Counteranion complexation ability determines the nature of the complexes in concentrated solutions of CuII salts in imidazolium‐based ionic liquids (ILs), whereas the salt/IL molar ratio plays only a marginal role, according to spectroscopic and electrochemical studies. DFT calculations suggest that [CuCl3]− and [CuCl4]2− are the dominant CuII species when the counteranion is Cl− (see picture).