Reversible Carbene Formation in the Ionic Liquid 1-Ethyl-3-Methylimidazolium Acetate by Vaporization and Condensation

The role of N-heterocyclic carbenes in the chemistry of ionic liquids based on imidazolium salts has long been discussed. Here, we present experimental evidence that 1-ethyl-3-methylimidazolium-2-ylidene (EMIm) can coexist with its protonated imidazolium cation (EMImH(+) ) at low temperatures. If the vapor of the ionic liquid [EMImH(+) ][AcO(-) ] is trapped in solid argon or nitrogen at 9 K, only acetic acid (AcOH) and the carbene, but no ionic species, are found by IR spectroscopy. This indicates that during the evaporation of [EMImH(+) ][AcO(-) ] proton transfer occurs to form the neutral species. If the vapor of [EMImH(+) ][AcO(-) ] is trapped at 9 K as film in the absence of a host matrix, a solid consisting of EMImH(+) , EMIm, AcO(-) , and AcOH is formed. During warming to room temperature the proton transfer in the solid to form back the IL [EMImH(+) ][AcO(-) ] can be monitored by IR spectroscopy. This clearly demonstrates that evaporation and condensation of the IL [EMImH(+) ][AcO(-) ] results in a double proton transfer, and the carbene EMIm is only metastable even at low temperatures.