13578
J. Phys. Chem. B 2007, 111, 13578-13582
Hydrides in Liquid Chloroaluminates David F. Wassell,† Keith E. Johnson,* and Lynn M. Mihichuk Department of Chemistry and Biochemistry, UniVersity of Regina, Regina, Saskatchewan, Canada S4S 0A2 ReceiVed: June 21, 2007; In Final Form: August 7, 2007
Infrared spectroscopy shows that CaH2 in liquid 1-ethyl-3-methyl-1H-imidazolium chloroaluminates reduces the cation in Lewis basic systems, does not affect the neutral salt, and forms AlCl3H- and eventually allanes in Lewis acidic systems. The AlCl3H- species can be determined by UV-vis spectroscopy through its reaction with 2,3-dichloro-1,4-naphthoquinone to form an alkoxide. While AlCl3H- reacts with protic species to give H2, its electrochemical oxidation proceeds through H(ads), some of which is oxidized to H+, which can be electrochemically reduced.
Introduction The hydrides of the alkali and alkaline-earth metals are ionic in the solid state, and lithium hydride yields hydrogen upon electrolysis of the pure liquid1 or of its solutions in a LiClKCl eutectic melt.2 Complex hydrides such as LiAlH4 are wellknown for their reducing role in organic chemistry. In preliminary studies3 we observed that hydride, added as CaH2, reacted with the cation in basic EmimCl/ACl3 liquid and with the Al2Cl7- anion in acidic EmimCl/AlCl3 liquid. Tsuda et al.,4 using LiAlH4 as the solute, studied the H2/H- electrode reaction in acidic and LiCl-buffered EmimCl/AlCl3 liquids. The formation of a carbene and H2 from an EmimCl liquid and potassium tert-butoxide5 has also been reported, and carbene complexes have been obtained from liquid dialkylimidazolium salts and transition-metal salts provided a halide ion was present.6 Hydride chemistry in acidic haloaluminates with sulfonium cations has been investigated7 and the acidity of the C(2) hydrogen of dialkylimidazolium ions estimated.8 In this paper we present details of hydride chemistry in the EmimCl/AlCl3 ionic liquid system and attempt to explain the behavior of different hydride sources. Experimental Section Aluminum chloride (Fluka puriss, >99%) was purified by sublimation under reduced pressure as described previously.9 1-Ethyl-3-methyl-1H-imidazolium chloride (EmimCl), prepared as previously described, was recrystallized from acetonitrile, vacuum-dried above 50 °C, and stored under nitrogen as large colorless crystals.9 Calcium hydride (Anachemia, 90-95%) was packed under argon. 2,3-Dichloronaphthoquinone (Aldrich) was sublimed under vacuum at 220 °C to a constant melting point (197-198 °C). HCl gas was from Matheson, semiconductor grade, 99.5%. All syntheses and most manipulations of the liquids were carried out in an inert atmosphere glovebox (MBraun Labmaster 100): the working gas was dry nitrogen. Centrifugation was performed with an International Equipment Co. clinical centrifuge: the liquids were placed in threaded centrifuge tubes, sealed * To whom correspondence should be addressed. E-mail: Keith.
[email protected]. † Present address: QUILL, David Keir Building, Stranmillis Rd., Belfast BTG 5A9, U.K.
with Teflon tape, capped, sealed with Parafilm, and then transferred from the glovebox to the centrifuge for a minimum of 2 h of centrifugation. AlCl3/EmimCl liquids were prepared as previously described9 except that in some of the later experiments phosgene was not used to remove hydrolytic impurities but protons were removed with small additions of hydridic melt. Hydridic liquids were prepared by adding calcium hydride to AlCl3/EmimCl in an amount just in excess of one hydride ion per chloride ion. This caused the evolution of some gas and complete decoloration of the neutral and acidic liquids, which were stirred for 12 h and filtered either through a fritted glass funnel or by centrifugation in a sealed tube. The acidic liquids appeared cloudy and were thixotropic. An EmimCl/HCl melt was prepared from solid EmimCl and high-purity HCl as described elsewhere and its HCl content determined from the intensity and location of the anionic 1H NMR signal.9 Infrared spectra were obtained with a Perkin-Elmer 1600 series FTIR instrument, with samples being placed as thin films between NaCl plates in the glovebox before rapid removal and scanning to limit exposure to air. It was necessary to polish the plates with an ethanol-soaked paper towel between uses. UV-vis spectra were obtained with a Hewlett-Packard 8452A diode array spectrophotometer controlled by standard HP operating software on an HP Vectra 486s/20 computer. To obtain the spectra of solutions of 2,3-dichloro-1,4-naphthoquinone (DCNQ), known volumes of a 5.89 × 10-4 M solution of DCNQ in HPLC-grade chloroform were evaporated to dryness in a stream of argon and the solid was transferred to the glovebox for addition to a known weight of 55% AlCl3/ EmimCl. Samples of the red solution were transferred by Pasteur pipet to a 1 mm quartz cuvette which was sealed with Parafilm and removed to the spectrophotometer. 1H NMR spectra were obtained with a Bruker AC 200 QNP NMR spectrometer and chemical shifts recorded relative to the peak for internal TMS but using a sealed capillary of DMSOd6 as the internal standard. Cyclic and square wave voltammetry were performed with a Bioanalytical Systems Inc. BAS 100A electrochemical analyzer utilizing the BAS 100W software package and a Gateway 2000 computer. Experiments were carried out in the glovebox with a 10 mL cell; the working electrode was a 0.21 mm2 platinum
10.1021/jp0748563 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/10/2007
Hydrides in Liquid Chloroaluminates
J. Phys. Chem. B, Vol. 111, No. 48, 2007 13579
Figure 1. Infrared spectrum of the 55% AlCl3/EmimCl liquid (a) alone, (b) after CaH2 addition, (c) after LiH addition, and (d) after CaH2 addition followed by HCl addition. Reprinted in part with permission from ref 3. Copyright 1999 The Electrochemical Society.
disk, the auxiliary electrode a coiled aluminum wire, and the reference electrode an aluminum wire in a saturated (66 mol %) AlCl3/EmimCl liquid separated from the bulk by a glass frit. Results and Discussion Infrared Spectroscopy. Figure 1 shows the infrared spectrum of a 55:45 AlCl3/EmimCl liquid alone, after treatment with calcium hydride, and after subsequent treatment with HCl. The hydride generated new peakssa broad strong absorption at 1870 cm-1, a very broad peak centered at 1700 cm-1, and a small peak in the 700-600 cm-1 region. HCl addition, concomitant with gas evolution, eliminated the 1870 cm-1 peak and the weak peak, while dramatically reducing the 1700 cm-1 peak. The addition of lithium hydride to the original liquid produced somewhat weaker peaks at 1890 and 1700 cm-1. Calcium hydride itself shows a sharp peak at 3650 cm-1 and a weak broad peak below 1500 cm-1,10 while lithium aluminum hydride shows broad absorption between 1300 and 2100 cm-1.10 Al-H stretching and bending modes for a variety of compounds occur in the 1700-1900 and 700-850 cm-1 regions, respectively.11 Thus, it appears that an aluminum hydride species is generated in the Lewis acidic chloroaluminate:
Al2Cl7- + H- f AlCl4- + AlCl3H-
(1)
AlCl3H- + HCl f AlCl4- + H2
(2)
The addition of calcium hydride to liquids richer in AlCl3 (Figure 2) gave rise to spectra where the 1870 cm-1 band shifted
Figure 2. Infrared spectra of AlCl3/EmimCl + CaH2 liquids. [AlCl3] (mol %) ) (a) 45, (b) 50, (c) 55, (d) 60, and (e) 65. Reprinted with permission from ref 3. Copyright 1999 The Electrochemical Society.
to 1900 cm-1 and the 1700 cm-1 band increased in size such that it dominated the spectrum of the 65% AlCl3/EmimCl liquid. This suggests that reaction 1 is indeed the source of the
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Figure 3. Infrared spectra of (a) 45% AlCl3/EmimCl + CaH2 and (b) 50% AlCl3/EmimCl + CaH2.
Figure 4. UV-vis spectrum of DCNQ in 55% AlCl3/EmimCl. Insets: (A) Beer’s law plot for the 360 nm band; (B) Beer’s law plot for the 462 nm band.
Figure 6. (a) Anion fractions in AlCl3/EmimCl. (b) Anion fractions in AlCl3/EmimCl + CaH2.
Figure 5. Spectra of DCNQ after hydridic liquid additions of (a) 0 g, (b) 0.0198 g, (c) 0.0189 g, (d) 0.0216 g, and (e) 0.1199 g. Inset: Amount (mol) of DCNQ remaining vs the weight of hydride added (360 nm). Reprinted in part with permission from ref 3. Copyright 1999 The Electrochemical Society.
aluminum hydride species. The Raman spectrum of AlCl3H- 12 was reported to contain one Al-H band at 1860 cm-1, in agreement with the distinct infrared band. The development of the 1700 cm-1 band can be associated with Al-H-Al bridging structures akin to Al2Cl7-. The anion fractions of the initial and hydridic liquids are shown in Figure 4. They were calculated using data from ref 13. A complete spreadsheet is available.14
When calcium hydride was added to a 45% AlCl3 liquid, the latter darkened and additional peaks appeared in the C-H stretching region but no Al-H peak at 1870 cm-1 was observed. On addition of calcium hydride to the 50% AlCl3 liquid, no change in the infrared spectrum occurred (Figure 3). This implies that hydride reacts with the imidazolium cation in the presence of chloride but not in its absence; i.e., the removal of the weakly acidic H at C(2) (pKa > 20 in DMSO8) requires a base with the strength of a halide, and hydride is not adequate in this regard. Further, hydride in the presence of any chloroaluminate species (AlCl4-, Al2Cl7-, Al3Cl10-) alone will not attack the imidazolium cation. This result is consistent with the requirement of halide ion presence in such reactions as the Pd carbene complex formation from a dialkylimidazolium tetrafluoroborate.6,15,16 These results do not match the relative proton affinities, which differ by ∼300 kJ mol-1.17 An alternative explanation may be that the halide compounds are more ionic and thus open to hydride attack. Ready carbene formation from KH and a diphenylimidazolium salt5 can be ascribed to the increased acidity of the C(2)H.8
Hydrides in Liquid Chloroaluminates
J. Phys. Chem. B, Vol. 111, No. 48, 2007 13581 additions to a pure DCNQ solution. From Figure 6b the content of a given sample was found from the slope to be 6.78 × 10-4 mol g-1 hydride. From mass balance arguments a value of 7.07 × 10-4 mol g-1 was obtained. The value using the 462 nm band was 8.7 × 10-4 mol g-1. Overall these results suggest that the colorimetric determination of hydride with DCNQ is quite satisfactory. This can be exploited for certain systems in that excess hydride can be added to remove protons and the excess determined colorimetrically by addition of a standard solution of DCNQ in the ionic liquid. Electrochemistry. Square wave voltammetry of the 55% AlCl3/EmimCl liquid indicates the presence of a small quantity of impurity with peaks at 0.97 and 1.72 V vs the Al(III)/Al reference. The addition of hydride leads first to the evolution of gas, presumably hydrogen, and then to the development of large peaks at 1.07 and 1.6 V. Cyclic voltammetry of the original liquid shows the 0.97 V oxidation peak and small reduction peaks with a high sweep rate. The addition of hydride produces oxidation peaks at 1.2 and 1.8 V and a reduction peak at 1.0 V, with the impurity oxidation peak at 1.0 V just discernible at a sweep rate of 200 mV s-1 (Figure 7); the impurity peak is very distinct at 1000 mV s-1. The reduction peak is a consequence of the prior oxidation of hydride (Figure 7c) and is clearly smaller than the oxidation peak at 1.20 V. It is also apparent that oxidation continues over a wide potential range after sweep reversal. The prior reductive sweep also gives rise to a higher hydride oxidation current, presumably through “cleaning” the electrode. For apparently similar conditions, currents increase with sweep rate but a possible explanation for the hydride behavior is the sequence
Figure 7. Cyclic voltammograms of 55% AlCl3/EmimCl + CaH2 (a) at a scan rate of 200 mV s-1, (b) at a scan rate of 10 mV s-1, and (c) at a scan rate of 100 mV s-1 beginning with a cathodic sweep. (d) Cyclic voltammogram of the system before addition of CaH2 at a scan rate of 100 mV s-1. Reprinted in part with permission from ref 3. Copyright 1999 The Electrochemical Society.
UV-Vis Spectroscopy. Solutions of DCNQ in a 55% AlCl3/ EmimCl liquid are deep red in color. The spectrum (Figure 5) shows peaks at 360 and 462 nm, the latter shifting somewhat with concentration. Beer’s law plots for nine concentrations appear as insets: the molar absorptivities obtained were 11200 L cm-1 mol-1 for the 360 nm peak and 3990 L cm-1 mol-1 for the 462 nm peak (correlation coefficients of 0.9926 and 0.9791, respectively). The addition of hydride to a DCNQ solution in 55% AlCl3/ EmimCl results in the suppression of the principal peaks (Figure 6), presumably due to the formation of the alkoxide:7 The
reaction can be used to determine the hydride in a given liquid sample by calculating the DCNQ remaining after successive
H+ + AlCl3H- f H2 + AlCl3
(4)
AlCl3H- - e- f H(ads) + AlCl3
(5)
H(ads) - e- f H+
(6)
2H(ads) f H2
(7)
H+ + e- f H(ads)
(8)
AlCl3 + AlCl4- f Al2Cl7-
(9)
Reaction 5 can be identified with the oxidation at 1.2 V and reaction 6 with that at 1.8 V. Reaction 8, proton reduction, then occurs at the potential of ClHAl2Cl7- reduction found previously:9
ClHAl2Cl7- + e- f 1/2H2 + 2AlCl4-
(10)
The reduction of ClHAl2Cl7- and the other hydrogenate species, ClHAlCl4-, present in HCl solutions in Lewis acidic chloroaluminates is also complex, showing evidence of adsorption9,10 but not giving rise to hydride which can be oxidized. The behavior of AlCl3H- differs from that of AlH4-. Tsuda et al.4 observed a clear oxidation wave at 0.6 V but no reduction current when LiAlH4 was added to 67% AlCl3/EmimCl, while LiAlH4 in a LiCl-buffered AlCl3/EmimCl liquid yielded an oxidation wave at -0.1 V and two reduction waves at -0.3 to -0.45 V, the reduction waves being much more sensitive to the sweep rate. Tsuda et al.4 did confirm by gas chromatography that the evolved gas was H2. They also observed the formation of aluminum metal when LiH was added to a Lewis acidic liquid (presumably with a 1AlCl3:g2EmimCl mole ratio).
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TABLE 1: Electrode Potentials of Chloroaluminates electrode reaction
E (V) vs Al(III)/Al
Emim+ + e- f Emim HCl2- + e- f 1/2H2 + 2ClClHAlCl4- + e- f 1/2H2 + Cl- + AlCl4AlCl4- - e- + AlH4- f H(ads) + Al2Cl4H3H(ads) - e- f H+ AlH4- + 3e- f Al + 4HAl2Cl7- + 3e- f Al + AlCl4- + 3ClAl2Cl7- + AlH4- - e- f H(ads) + Al3Cl7H3AlCl4- + AlCl3H- - e- f H(ads) + Al2Cl7H(ads) - e- f H+ HAl2Cl8- + e- f 1/2H2 + 2AlCl4-
-2 -0.4 0 -0.1 -0.35 -0.7 0 0.6 1.2 1.8 1.0
system Emim/AlCl4 Emim/AlCl4 + HCl Emim/AlCl4 + LiCl + LiAlH4 Emim/Al2Cl7 Emim/Al2Cl7 + LiAlH4 Emim/Al2Cl7 + CaH2 + Emim/AlCl4
The salient electrode potentials are collected in Table 1. The well-known influence of Lewis acidity must be included in any attempt at rationalization. However, while the reducing power of added hydride appears to follow the sequence
LiAlH4 < CaH2 < LiH the ease of oxidation of hydride species in solution is
AlH4- in neutral buffer < AlH4- in 67% acidic buffer < AlCl3H- in 55% acidic buffer Conclusions Hydride, added as the calcium salt, reduces the cation in a Lewis basic EmimCl/AlCl3 liquid but does not affect the neutral liquid. Infared spectroscopy indicates that it attacks the Al2Cl7anion in acidic liquids, forming AlCl3H- in the 45:55 EmimCl/ AlCl3 system and more complex species at higher AlCl3 ratios. The hydridic entities react with added gaseous HCl to give hydrogen and AlCl3H-, which can be determined by spectrophotometric titration with 2,3-dichloro-1,4-naphthoquinone. The electrochemical behavior of AlCl3H-, like that of chloroaluminate-HCl adducts, is complex. The prime reaction is H- anodic oxidation but in two stages, presumably to H and H+ on the platinum electrode, since a distinct reduction wave ascribable to the H+/1/2H2 process is seen on reversal of the polarization direction. Hydride should be useful as a determinant of protic impurities through its reaction with select quinones. It will also serve as a reductant of ketones to alkoxides of Emim or like cations.
ref 17, 9 9 4 15 4 this work
Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for partial support of this work. References and Notes (1) Moers, K. Anorg. Z. Chem. 1920, 113, 179. (2) Plambeck, J. A.; Elder, J. P.; Laitinen, H. A. J. Electrochem Soc. 1966, 113, 931. (3) Wassell, D. F.; Johnson, K. E.; Mihichuk, L. M. Proc.s Electrochem. Soc. 1999, 99-41, 132. (4) Tsuda, T.; Hassey, C. L.; Nohira, T.; Ikoma, Y.; Yamauchi, K.; Hagiware, R.; Ito, Y. Electrochemistry. 2005, 73, 644. (5) Earle, M.; Seddon, K. R. World Patent WO 01/77081 Al, 2001. (6) Xu, L.; Chen, W.; Xiao, J. Organometallics 2000, 19, 1123. (7) Xiao, L.; Johnson, K. E. Can. J. Chem. 2004, 82, 491. (8) Kim, Y. -J.; Streitweiser, A., Jr. J. Am. Chem. Soc. 2002, 124, 1267. (9) Campbell, J. L. E.; Johnson, K. E. J. Am. Chem. Soc. 1995, 117, 7791. (10) Nyquist, R. A.; Putzig, C. L.; Leugeurs, M. A. The Handbook of Infrared and Raman Spectra of Inorganic Compounds and Organic Salts; Academic Press: San Diego, 1997. (11) Wehmschulte, R. J.; Grigsby, W. J.; Schiemenz, B.; Bartlett, R. A.; Power, P. P. Inorg. Chem. 1996, 35, 6694. (12) Graef, M. W. M. J. Electrochem Soc. 1985, 132, 1038. (13) Melton, T. J.; Joyce, J.; Maloy, J. T.; Boon, J. A.; Wilkes, J. S. J. Electrochem. Soc. 1990, 137, 3865. (14) Wassell, D. F. M.Sc. Thesis, University of Regina, 1999. (15) Matthews, C. J.; Smith, P. J.; Welton, T.; White, A. J. P. Organometallics 2001, 20, 3848. (16) Chianese, A. R.; Crabtree, R. H. ACS Symposium Series 885; Goldberg, K. J., Goldman, A. S., Eds.; American Chemical Society: Washington, DC, 2004; p 169. (17) Huheey, J. E.; Keiter, E. A.; Keiter, R. J. Inorganic Chemistry; Harper Collins: New York, 1993; p 332. (18) Wilkes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg. Chem. 1982, 20, 1263.
