2839
Anal. Chem. 1987, 59, 2639-2640
Removal of Protons from Ambient-Temperature Chloroaluminate Ionic Liquids Sir: Ambient-temperature chloroaluminate ionic liquids have been employed extensively in recent years for a wide range of studies (1).These solvents are formed from mixtures of AlCl, with either 1-methyl-3-ethylimidazoliumchloride (ImCl) or N-1-butylpyridinium chloride (BuPyC1) in various proportions. Purification of the melts has proven a particularly vexing problem. Though some improvements have been made in the workup procedure for the organic chloride component of the melt (2), oxide and protons from water still are present in small quantities in the melt (3,4). These impurities have been shown to affect the chemistry of several compounds in the melt. For example, oxide interacts with Ti(1V) in mixtures containing a molar excess of organic chloride to form the oxytetrachlorotitanate complex (5) and protons undergo oxidative addition to quadruply bonded molybdenum dimers to form hydrido complexes (6). Thus, removal of impurities from these melts is an important problem. In the present work, we describe a simple, effective method for the removal of protons from the melt by addition of EtAlC12.
EXPERIMENTAL SECTION The preparation and purification of melt components and melts are described elsewhere (7). Ethylaluminum chloride (98%) waa used as obtained from Aldrich. The synthesis of HCl and DCl salts of ImCl is also described elsewhere (8). All reagents were handled in a Vacuum Atmospheres Co. drybox under a helium atmosphere maintained at less than 5 ppm oxygen and water. ,H NMR experiments were performed by using a JEOLCO FX-270 NMR spectrometer operating at 41.27 MHz with broad-band proton decoupling. 'H and 27AlNMR experiments were performed on a JEOL-FX-9OQ spectrometer operating at 89.95 and 23.43 MHz, respectively. Samples were prepared in the drybox, capped, and sealed with Parafilm. Electrochemical experiments were carried out in the drybox with a Bioanalytical Systems Pt-disk electrode (1.6-mm diameter) as the working electrode and A1 wires dipped in 1.51.0 melts as the reference and counter electrodes. Measurements were carried out by using an EG&G PARC Model 273 potentiostat controlled by software running on a DEC PDP-S/e computer (9). RESULTS AND DISCUSSION When EtAlCl, is added to a melt containing protons and stirred, bubbles emerge from solution. As a preliminary check on the possibility of proton removal, we added a slight excess (60 mM) of EtAlCl, to a solution of 50 mM imidazolium deuterium dichloride, ImDCl,, which we have been using as a proton/deuterium donor in the melts (8), in a 0.88:l.O A1Cl3/ImC1 melt. In order to ensure that the solution was fully degassed, it was evacuated for 30 min after addition of the EtAlC1,. A control sample that contained 50 mM ImDCl, to which no EtAlCl, was added was also evacuated. No change was observed in the deuterium NMR spectrum of the control sample. The ,H spectra for the solutions before and after EtAlCl, addition are shown in Figure 1. The large peak at 10.2 ppm in the spectrum taken before addition of EtAlCl, (Figure l a ) is due to DC1, which is hydrogen bonded to a second chloride. After addition of EtAlCl,, as shown in Figure l b , the large peak has been removed and only natural abundance peaks from the organic cation are observed. To monitor the proton removal directly, we have performed normal pulse voltammetry on a Pt electrode immersed in the melt. Since the voltammetric response to proton reduction is markedly dependent on the pretreatment of the electrode
surface (2), we have developed a treatment protocol involving potentiostatically conditioning the electrode (8). The diffusion-limited current for the reduction of protons was used to monitor the H+ concentration in the melt. A titration curve for removal of H+by addition of EtAlCl, is shown in Figure 2. The stoichiometry of the reaction between EtAlCl, and HC1 is 1:l. As the proton concentration decreases, the reaction becomes progressively more sluggish. Sufficient time between EtAlCl, addition and monitoring must be allowed. Thus, with the points shown in Figure 2, voltammograms were measured until a constant value of the limiting current for successive measurements was obtained (within a few percent). As an example of a typical reaction time, the final point shown on the plot in Figure 2, was obtained after approximately 2 h. Significantly shorter reaction times were required for more concentrated solutions. The formation of gas upon reaction suggests that the proton removal reaction is EtAlC1,
+ HC1 = C,H,(g) + AlC1,
Since the products are a normal melt component and a removable gas, EtAlCl, can be used as a melt purification reagent. The simplicity of the reaction and the fact that no byproducts extraneous to a normal melt are left in solution are attractive features of this reagent. Additionally, since the background proton impurity level is usually 10 mM or less (81, the change in melt composition upon treatment will generally be insignificant. If very accurate knowledge of the melt composition is crucial, the composition can be corrected for the formation of AlCl,. In order to investigate the behavior of excess EtAlC1, in the melt, 'H and 27AlNMR spectra were measured on 1 M solutions of EtAlCl, in a 0.4:l.O melt. The proton NMR spectrum of EtAlCl, in the melt consists of a quartet at 0.9 ppm (methylene protons) and a triplet at 2.0 ppm (methyl protons). These peaks are shifted slightly upfield from their respective positions in the neat liquid EtAlCl,. Additionally, the 'H peaks from the melt cation shift in such a way as to indicate that the melt is becoming less basic (10). No separate 27Al resonance peak was observed for the EtAlC1, species in solution. However, the peak due to AlC14- in the melt narrowed significantly. The composition of the melt changes as EtAlCl, is added due to uptake of a C1- ion by the EtAlCl, to form EtAlCl,. The decreased Cl- ion concentration results in lower melt viscosity and thus narrows the AlC1, line observed. This also explains the shifted proton peaks observed for both EtAlCl, and the organic melt cation. The formation of such an ion may be responsible for the fairly sluggish reaction between HCl and EtAlCl, since the ionic species may be less reactive than the parent compound. Excess EtAlCl, does not appear to react with the organic melt component. If EtAlC1, does take up a chloride ion, addition of this material has the same effect on melt acidity as addition of AlC1, and thus the total change in melt composition upon addition of a known number of moles of EtAlC1, is the same regardless of whether all of the added material reacts with protons. From the view point of the effective melt composition, over-titration of protons present in the melt does not present a problem since a correction of the melt composition is readily made. In addition to the observations presented above, we have also found that excess EtAlCl, left standing for several days in a basic melt will remove added protons with equal efficacy. This suggests that the material is neither reactive with melt
0003-2700/S7/0359-2639$01.50/00 1987 American Chemical Society
2640
ANALYTICAL CHEMISTRY, VOL. 59, NO. 21, NOVEMBER 1, 1987
investigation. The effect of EtAIClz and of the removal of protons on systems involving proton-solute interactions is of particular interest in this regard. In summary, we have demonstrated that EtAIClz acts as a reagent for removal of protons present in the melt. The reaction probably results in the formation of ethane and A1Cl3 and thus no material other than normal melt components remains in solution after removal of ethane. In basic melts, the reagent remains in solution and is reactive for extended periods. Further work on the extent and limitations of this reaction is in progress.
a
I
b
LITERATURE C I T E D
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Frequency (ppml Flgure 1. 'H NMR spectrum (a) before and (b) after addition of EtACI, to a solution of 50 mM. ImDCI, in a 0.88:l.O melt.
\;: I 2'o
T
2
(1) Hussey, C. L. Adv. Molten Sad Chem. 1983, 5 , 185. (2) Sanders, J.; Hussey, C. L., Department of Chemistry, University of Mississippi, personal communication, 1987. (3) Sahami, S.;Osteryoung, R. A. Anal. Chsm. 1983, 55, 1970. (4) Stojek, 2.;Linge. H.; Osteryoung, R. A. J. Electroanal. Chem. Interfaclal Electrochem. 1981. 119. 365. (5) Linge, H.; Stojek, Z.; Osteryoung, R. A. J. Am. Chem. SOC. 1981, 103, 3754. (6) Carlin, R. T.; Osteryoung, R. A., submitted for publication in Inorg. Chem . (7) Wiikes, J. S.; Levisky, J. A,; Wilson, R. A,; Hussey, C. L. Inorg. Chem. 1982, 21, 1263. (8) Zawodzinski, T. A.; Osteryoung, R. A,, State University of New York, unpublished work. (9) Brumleve, T. R.; O'Dea, J. J.; Osteryoung, R. A.; Osteryoung, J. Anal. Chem. 1981, 53, 702. (10) Fannin, A. A.; King, L. A,; Levisky, J. A.; Wilkes, J. S. J . f h y s . Chem. 1984, 8 8 , 2609.
BOT
dot
0 0
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moles E t A l C 1 2 added (units o f 0 1
mmolesl
Flgure 2. Titration curve for removal of protons by successive a d d i n of EtAICI, to a melt initially containing 0.66 mM of H.'
components nor volatile over extended periods. The reactivity in melts of this compound with reagents whose chemistry is likely to be directly affected by the presence of EtAlCl,, and other aspects of the chemistry of EtA1C12,are currently under
Thomas A. Zawodzinski, Jr. Richard T. Carlin Robert A. Osteryoung* Department of Chemistry State University of New York a t Buffalo Buffalo, New York 14214 RECEIVEDfor review June 11, 1987. Accepted July 24, 1987. This work was supported by the Air Force Office of Scientific Research.
