3-Methylimidazolium bromohydrogenates(I): a room-temperature ionic liquid for ether cleavage† G. Driver and K. E. Johnson* University of Regina, 3737 Wascana Pkwy, Regina, Saskatchewan, Canada S4S 0A2 Received 22nd November 2002 First published as an Advance Article on the web 21st February 2003 1 H- and 13 C{ 1 H}-NMR indicate that the liquid formed at room temperature from 1 mole of 1-methylimidazole and 2 moles of anhydrous HBr contains the 3-methylimidazolium cation (Hmim + ) and an equilibrium anionic mixture of Br 2 , HBr 2 2 , and H 2 Br 3 2 . This liquid cleaves ethers efficiently, producing the corresponding bromides and alcohols from cyclic and straight-chain dialkyl ethers and phenols and alkyl bromides from aryl alkyl ethers. During the course of the reaction, the single anion proton signal moves several ppm downfield as the [HBr 2 ]/[H 2 Br 3 2 ] ratio increases. The nitrogenic proton of Hmim + is unaffected, confirming that it is not labile and nor is it involved in the reaction, in accord with the large proton affinity of 1-methylimidazole. Introduction Ionic liquids (ILs) containing the 3-methylimidazolium cation are characterized for the first time. The only reference found in the literature for any liquid containing this cation was a European patent application 1 made in the year 2000 which described the formation of a liquid at 100 °C by combination of aluminium chloride with 1-methylimidazole under a dry hydrogen chloride atmosphere. That systems containing the Hmim + cation have not been studied is likely due to the fact that initially, ILs were largely prepared for their excellent electro- chemical uses. The choice of a protic cation for these purposes would have quickly become unpopular due to the unavoidable hydrogen ion reduction potential. Additionally, systems of this type are often highly associated from hydrogen bonding. The main interest in this work was to explore the nature of the pure liquid so as to allow for confident use of it as a reaction medium. All too often ILs are used with the emphasis placed on the reaction products and not on the reaction medium. Additionally, reports in the literature, have often over-emphasized the chemistry of the anion and understated that of the cation, when in general, changing the cation is what distinguishes the chemical properties of one liquid to another. 2 The combination of 1-methylimidazole with anhydrous hydrogen bromide, in a 1+2 mole ratio, produced the 3-methylimidazolium bromohy- drogenates(I) ionic liquid (HmimBr–HBr) at room temperature. Preliminary results indicate that this liquid cleaves ethers efficiently at room temperature. Interest in this reaction has arisen from the need for an economical and environmentally friendly process for the cracking of the polymeric ether linkages found in lignin. In this way, lignin could potentially serve as a natural source of fuel additives for the petroleum industry. 3a,b It will be shown from 1 H- and 13 C{ 1 H}-NMR studies that the title room-temperature ionic liquid contains the fused ‘onium’ cation, 3-methylimidazolium (Hmim + ). Nitrogenic protons of fused ‘onium’ cations are typically considered to be acidic 4–6 but evidence found in this work indicates that the nitrogenic proton of the 3-methylimidazolium cation (Hmim + ) was neither the source of the acidic proton nor labile to any observable extent. Further characterization of the IL by IR spectrophoto- metry and cyclic voltammetry have been included in this report, along with the results some physico-chemical measurements. Experimental Techniques and equipment All reactions were carried out either using standard Schlenk tube techniques under inert conditions or in an M-Braun † This work was presented at the Green Solvents for Catalysis Meeting, held in Bruchsal, Germany, 13–16th October 2002. Gordon Driver is a MSc candidate at University of Regina. Keith E. Johnson is Professor Emeritus of University of Regina. He holds BSc, PhD and DSc degrees from University of London and has published > 100 papers, mostly on molten salts and ionic liquids. Gordon Driver Keith E. Johnson Green Context The use of ionic liquids is ever-increasing. This paper deals with a novel aspect of these liquids. Instead of forming the quaternary centre with an alkyl group, the ionic liquids here are formed by reaction of alkyl imidazoles with HBr. The resultant liquid is a potent Brønsted acid, and is very active in the cleavage of ethers. In particular, the cleavage of anisole takes place under very mild conditions, and is unusually selective. DJM This journal is © The Royal Society of Chemistry 2003 DOI: 10.1039/b211548d Green Chemistry, 2003, 5, 163–169 163