Ionic Structure and Interactions in 1-Methyl-3-Ethylimidazoliurn Chloride/AlCl3 Molten Salts

Room temperature chloroaluminate molten salts formed by mixing 1-methyl-3-ethylimidazolium chloride (MEICI) with AIC13 are of interest as electrolytes and nonaqueous reaction media. Results of NMR studies of MEI + in melts of various composition (determined by the mole fraction, N, of AIC13 used in forming a melt) were originally explained by a stack model of the ionic structure. In this model, the anions are located between stacked "parallel" MEI + ring planes. An alternative model has been suggested in which C[ forms ion pairs with MEI + by a H-bonding interaction through the H on the C-2 in the ring. Our IR studies, particularly on deuteriated MEI + , now show that CIinteracts with the hydrogens at the C-2, C-4, and C-5 members of the ME[+ ring. One of the most widely studied room temperature melt systems electrochemical windows and high conductivities. The structure is the l-methyl-3-ethylimidazolium chloride-AIC 3 (MEICIof MEI + is shown below: AICI 3) melt which is liquid at room temperature for compositions between 33 and 67 mol % AICI3 . 2 This melt exhibits acid-base chemistry which is dictated by the following reactions: CH3/ . N -'C2HS ME[+CI + AICI 3 MEI+AICI4 K >> 1 (1) MEI MEI + AIC14+ AIC13 MEI + Al 2CI1K >> 1 (2) A better understanding of the physical and chemical properties of these melts has been sought through investigation of their ionic When the mole fraction of AIC13 (N) used in preparing the melt interactions. The crystal structure of MEICI's iodide analogue, is less than 0.5, it contains Cl-, which acts as a Lewis base, and MEII, has been reported, and based upon the C-2 hydrogen-iodide is basic. For compositions with N > 0.50, the melt is considered distance and position, an interaction involving hydrogen bonding acidic because it contains AI 2CI,-, which acts as a Lewis acid. At through the C-2 hydrogen has been suggested.' Previous inN = 0.50 the melt is neutral, with AIC14the only detectable anion. vestigations have also used this model to explain composition These melts exhibit several attractive features such as large dependent features in the IR spectra of basic MEICI-AICI3 melts. 4 In the work reported here we studied both normal and deuteriated (I) Wilkes, J. S.; Levisky, 1. A.; Wilson, R. A.; Hussey, C. L. Inorg. Chem. 192. 21. 1263. (3) Abdul-Sada, A. K.; Greenway, A. M.; Hitchcock, P. B.; Mohammed, (2) Hussey, C. L. Advan in Molten Salt Chemistry; Mamantov, G., T. J.; Seddon, K. R.; Zora, J. A. J. Chem. Soc., Chem. Commun. 1986, 1753. Mamantov, C., Eds.; Elsevier: New York, 1983; Vol. 5, p 185. (4) Tait, S.; Osteryoung, R. A. Inorg. Chem. 194, 23, 4352. 89 0 A Intera tions in 1Methyl-3-ethvlirnidazolium Chloride/.41I 3 J. Am. Chem. Soc., Vol. 110, No. 9, 1988 2723 MEI* in melts of various composition. We also examined the bromide analogues of these melts, the effects of solvent addition, 0.6 a and the replacement of the C-2 hydrogen with methyl. Seniempirical molecular orbital calculations were used to supplement our experimental observations and explore possible structures L . through which the ions interact in these melts. Z Experimental Section 0 . General Procedures. The purification of AICI5 and the preparation cc . of MEICI from 1-methylimidazole and ethyl chloride have been pres2 0.2. ented earlier Bromide analogues of these melts were prepared with the 03 same techniques. 1.2-Dimethyl-3-ethvlimidazolium chloride (MMEICI) < CA was prepared in the same way with I .2-dimethylimidazole and ethylI chloride. Benzene-d. (Aldrich) and dichloromethane-d, (Aldrich) for 0 solvent studies "-ere used as received. Benzene (Aldrich. HPLC grade) and dichloromethane for the dilution of melts (Baker. HPLC grade) were purified b% refluxing over P:O5 for several days followed by fractional 3400 3200 300