Electrochemical Studies of Imidazolium Carboxylate Adducts in a Room-Temperature Ionic Liquid

 Introduction

     Recent interest in carbon dioxide sequestration has prompted much research into ways of forming compounds from CO₂  (1).  One possible route for this process is reaction of CO₂ with carbenes, which can be formed from the reduction of imidazolium cations (2).  This paper reports recent results in this regard, carried out in a room-temperature ionic liquid.

Experimental

    All electrochemical experiments were carried out in a Vacuum Atmospheres glovebox.  Cyclic voltammograms were obtained on a PAR 283 potentiostat using PowerSuite software.  Potentials are reported with respect to an Ag/AgCl reference electrode. X-Ray diffraction structure determinations were run on a Bruker AXS Kappa APEX-II. 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMI BF₄ ) and 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate (BMPY TfO) were prepared by literature methods (3, 4).

Results and Discussion

    The electrochemical properties of EMI BF₄ were investigated in the room-temperature ionic liquid BMPY TfO.   The EMI cation undergoes reduction at -2.2 V vs Ag/AgCl, followed by an oxidation process at -0.40 V on the reverse sweep.  Introduction of CO₂ into the solution results in a rather large increase in the reduction process at -0.40 V, suggesting that the EMI⁺ reduction product can interact with the added CO₂ in a catalytic process.  For comparison,  CO₂ reduction by itself in BMPY TfO occurs at  -2.8V, so that the observed current increase at -0.40 V for the EMI⁺ / CO₂ combination is not due to direct reduction of CO₂.   In order to identify the reaction products from the EMI⁺ / CO₂ interaction, voltammograms of the imidazolium – carboxylate adducts, 1,3-dimethylimidazolium-2-carboxylate and 1,3-dimethylimidazolium-4-carboxylate, were run.  The adduct structures were confirmed by X-ray structure determinations.  The reduction of these adducts occurs at -2.6 V and -2.4 V, respectively, with an oxidation process at -0.35 V on the return sweep.  These results support the formation of CO₂ adducts with the EMI⁺ reduction product, probably a carbene. 

References

(1)    “Electrochemical CO₂ Reduction on Metal Electrodes,” Y. Hori, Modern Aspects of Electrochemistry, Number 42, edited by C. Vayenas et al., Springer, New York, (2008).

(2)    A. B. Bocarsly, Q. D. Gibson, A. J. Morris, R. P. L’Esperance, Z. M. Detweiler, P. S. Lakkaraju, E. L. Zeitler, and T. W. Shaw, ACS Catalysis, 2, 1684 (2012).

(3)    S. Park and R. J.  Kazlauskas,  J. Org. Chem. 66, 8395 (2001).

(4)    L. Crowhurst, N. L. Lancaster,  J. M. P. Arlandis, and T. Welton,  J. Am. Chem. Soc., 126, 11549 (2004).