Anal. Chem. 1994, 66, 3558-3561
Stoichiometry of Latent Acidity in Buffered Chloroaluminate Ionic Liquids I. C. Quarmby, R. A. Mantz, L. M. Goldenberg, and R. A. Osteryoung’ Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204
Chloroaluminate ambient temperature ionic liquids consisting of AlCl3and 1-ethyl-3-methylimidazoliumchloridecan be both neutralized and buffered by the addition of an alkali halide to an acidic melt, one containingexcess AIC13. Such melts possess an electrochemical window of 4.4 V. The ionic composition of a NaC1-buffered neutral melt is modified such that [Im+] + [Na+] = [AlCld-1. We have observed that these buffered melts exhibit a “latent acidity”. Weak Lewis bases, B:, which are uncomplexed by AlC13 in an unbuffered neutral melt, form AICl3 adducts in a buffered melt. This occurs because the equilibrium B: AlC4- + Na+ ==B:AlCl3 + NaCl(s) is driven to the right by the precipitation of NaCl(s); in the absence of Na+, this reaction does not take place. The reaction was observed for the weak bases acetylferrocene, dimethylaniline, and pyrrole with use of electrochemical and spectroscopic techniques and is limited by the amount of Na+ present in solution in the initially buffered neutral melt.
-
+
We previously observed that these buffered neutral melts exhibited a “latent acidity”, generating AlC13 adducts when a weak Lewis base such as acetylferrocene was added.5 In unbuffered neutral melts, no adduct formation occurs. Qualitative results of cyclic voltammetric experiments suggested that speciation of the weak Lewis base acetylferrocene in the buffered melts was controlled by the following equilibrium: B: + AlCl;
+ Na+(1) ==B:AlC13 + NaCl(s)
(2)
where B: is a Lewis base. Cheek has observed that aromatic ketones also form AlC13 adducts in NaC1-buffered melts but did not investigate this further.6 We present here the results of a more complete electrochemical and spectroscopic investigation into the behavior of Lewis bases in NaC1-buffered melts.
where M+ is an alkali metal ~ a t i o n . The ~ , ~Lewis acid, A12C17-, is thereby neutralized and the melt driven to neutrality. The buffering mechanism alters the composition of the melt such that [Im+] [Na+] = [AlCld]. MC1, which is virtually insoluble in a neutral melt, is precipitated if C1- is generated; excess MCl is dissolved if Al2C17- is generated.
EXPERIMENTAL SECTION Preparation of ImCl and purification of AlC13 have been described e l ~ e w h e r e . ~ Melts ? ~ were prepared by slowly mixing weighed amounts of AlC13 and ImCl to produce a slightly basic melt (0.99:l.OO). Oxide and proton impurities were then removed by treatment with phosgene and high vacuum.8 A weighed amount of AlC13 was then added to the basic melt to yield the desired neutral or acidic composition. Alternately the basic melt was titrated with a purified acidic melt until the desired composition was achieved. NaC1-buffered melts were prepared from acidic melts as described by Wilkes et ala4 NaCl (Aldrich, 99.999%) was dried by heating to 350 OC at 0.02 Torr for 7 days. All melt preparation and handling was carried out in a nitrogen-filled Vacuum Atmospheres drybox with oxygen and water levels less than 5 ppm. Acetylferrocene (Aldrich) was sublimed twice at 70 ‘C and 1 Torr and transferred into the drybox. N,N-Dimethylaniline (DMA, Fluka, >99.95%) and pyrrole (Aldrich, 98%) were used without further purification. All electrochemical experiments were carried out in a drybox with a three electrode system. The working electrode was either a platinum disk (area = 0.020 cm2, Bioanalytical Systems) or a glassy carbon disk (area = 0.071 cm2, Bioanalytical Systems) attached to a Pine MSR electrode rotator with a homemade electrode adapter. The reference electrode was an aluminum wire immersed in a 1.5:1.O acidic melt; this was separated from the main solution by a Vycor
(1) Wilkes, J. S.; Levisky, J. A.; Wilson, R. A,; Hussey, C. L. Inorg. Chem. 1982, 21, 1263. (2) Robinson, J.; Osteryoung, R. A. J . Am. Chem. Sor. 1979, 101. 323. (3) Melton, T. J.; Joyce, J.; Maloy, J. T.; Boon,J. A.; Wilkes, J. S . J. Electrochem. SOC.1990, 137, 3865. (4) Scordilis-Kelley, C.; Fuller, J.; Carlin, R. T.; Wilkes, J. S . J . Electrochem. SOC.1992, 139, 694.
(5) Quarmby, I. C.; Osteryoung, R. A. J. Am. Chem. SOC.1994, 116, 2649. (6) Cheek, G. in Proceedings of the 8th International Symposium on Molten Salts; Gale, R. J., Blomgren, G., Kojima, H., Eds.; The Electrochemical Society: Pennington, NJ, 1992; Vol. 92-16, p 426. (7) Gale, R. J.; Osteryoung, R. A. In Molten Salt Techniques; Lovering, D. G., Gale, R. .I.Eds.; , Plenum: New York, 1983; Vol. 1, pp 55-78. (8) Noel, M. A,; Trulove, P. C.; Osteryoung, R. A. Anal. Chem. 1991,63. 2892.
Chloroaluminate ionic compounds formed from aluminum chloride and an organic chloride, such as 1-ethyl-3-methylimidazolium chloride (ImCl) or N-butylpyridinium chloride, These systems are are liquids at room characterized as acidic, basic, or neutral as the mole ratio of the aluminum chloride to the organic chloride is greater than, less than, or equal to 1.OO: 1.OO. Neutral melts, in which the principal ionic species are Im+ and AlCld-, and where [ Im+] = [AlC14-], may be prepared by adding exactly stoichiometric amounts of A1C13to the organic chloride. The concentrations of Lewis acidic species (Al2C17-) and basic species (Cl-) in these neutral melts are very small. However, such systems are not buffered; the addition of excess organic chloride or aluminum chloride shifts the systems respectively into the basic or acidic regime. A method for producing and buffering a neutral melt has been reported. Addition of NaCl or LiCl to an acidic melt generates a neutral melt through the following reaction: MCl(s)
+ A12ClJ
1) + 2AlC1[( 1) + M+( 1)
K >> 1 (1)
+
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Analytical Chemistry, Vol. 66, No. 21, November 1, 1994
0003-2700/94/0366-3558$04.50/0
0 1994 American Chemical Society
frit, and the counter electrode was a platinum wire. Cyclic, pulse, and rotating disk electrode (RDE) voltammetric experiments were carried out using a PARC Model 273 potentiostat with a computer-controlled system, described previo~sly.~ A Pine MSR speed control was used to control the rotation rate of the disk. Chronoamperometric experiments were performed using a PARC Model 273 potentiostat under manual control. NMR spectra were run on a GE Omega 300 N M R spectrometer. Each sample was prepared in the drybox and filtered through a 0.45 pm PVDF syringe filter (Gelman) into a precision coaxial insert (Wilmad Glass Co.), which was then capped and sealed with Parafilm. This was inserted into a 5 mm NMR tube (Wilmad Glass Co.) containing 0.1% TMS in CDC13 (Aldrich). All peak positions were referenced to TMS. UV-visible spectra were recorded on a HewlettPackard 8452A diode array spectrophotometer, using 1 cmz air-tight silica cells. Short path lengths were obtained with precision silica inserts (Wilmad Glass Co.) and calibrated with KzCr04 in 0.05 N aqueous KOH, as described by Smith et a1.10 Safety Considerations. Phosgene is an extremely toxic gas which should be used with extreme caution.ll A phosgene detector, Matheson Toxic Gas Detector Model 8014LA, is used to test for leaks in the apparatus employed to remove oxide; the apparatus is assembled in a laboratory hood.*
RESULTS AND DISCUSSION Acetylferrocene. The chemistry of acetylferrocene is dependent upon melt composition.12 In acidic melts, an aluminum chloride adduct is formed at the carbonyl group: Fc(C0Me) + A12C17-
Fc(C(O:AlCl,)Me)
+ AlC14- (3)
where Fc is (CsHs)Fe(CsH4-). Adduct formation at Lewis basic moieties such as carbonyl, cyanide, and ferrocyanide has been reported.13-15 In basic melts, no adduct formation occurs. The oxidation potential of the adduct is -350 mV more positive than that of free acetylferrocene. We observed that the voltammetric behavior of acetylferrocene in unbuffered neutral melts varied with the use of different batches of melt.5 Typically, a mixture of adduct and free acetylferrocene was observed. Occasionally, only free acetylferrocene was found. The maximum concentration of adduct observed was 2 mM. The presence of the adduct arises because of experimental difficulties encountered when preparing a “neutral” melt. A neutral melt is usually prepared from a slightly basic melt, and the disappearance of the C1oxidation wave is monitored electrochemically as AlC13 is added. However, the levels of C1- and A12C17- that are observableelectrochemicallyare approximately 5 mM. l6 Thus, (9) Brumleve, T. R.; ODea, J. J.; Osteryoung, R. A.; Osteryoung, J. G.Anal. Chem. 1981, 53, 702. (10) Zingg, S. P.; Dworkin, A. S.; Sarlie, M.; Chapman, B. M.; Buchanan, A. C., 111; Smith, G. P. J . Am. Chem. SOC.1989, 111, 525; 1989, 111, 5075. (1 1) TheMerck Index, 1 lth ed.;Budavari,S.,O’Neil, M. J., Smith, A., Heckelman, P. E., Eds.;Merck & Co.: Rahway, NJ, 1989; p 1165. (12) Slocum, D. W.; Edgecombe, A. L.; Fowler, J. S.; Gibbard, H. F.; Phillips, J. Organometallics 1990, 9, 307. (13) Cheek, G.; Osteryoung, R. A. J. Electrochem. SOC.1982, 129, 2488; 2739. (14) Woodcock, C.; Shriver, D. F. Inorg. Chem. 1986, 25, 2137. (15) Das, B.; Carlin, R. T.; Osteryoung, R. A. Inorg. Chem. 1989, 28, 421. (16) Karpinski, 2.J.; Osteryoung, R. A. Inorg. Chem. 1984, 23, 4561.
“neutral” melts may in fact be slightly acidic or basic, and the observed speciation of an added base will vary from batch to batch. In contrast, free acetylferrocene cannot be observed in a NaC1-buffered melt, [Na+] = 0.36 M, even at concentrations of acetylferrocene up to 20 mM. While the formation of the adduct requires a source of A1C13,the concentration of AlzC17in the buffered melts is very low. No deposition/stripping wave was evident at either platinum or glassy carbon electrodes. (A recent report, however, has suggested that buffered melts are slightly Lewis acidic, with the upper limit on the concentration of A1&17- being 4 mM.” ) This, however, is significantly lower than the concentration of adduct that we have observed. We proposed that the source of A1C13is AlC14-, a very weak Lewis acid. In an unbuffered neutral melt, the equilibrium, Fc(C0Me)
+ AlC1,
+ Fc(C(0:AlClJMe)
+ C1-
(4)
lies far to the left. Tetrachloroaluminate is not a sufficiently strong Lewis acid for the reaction to occur. However, in a buffered melt, the equilibrium is modified by the presence of sodium ions: Fc(C0Me)
+ AlCl; + Na+(1) =e Fc(C(O:AiCl,)Me)
+ NaCl(s)
(5)
The generation of chloride ion during adduct formation results in the precipitation of NaCl(s), thus providing a driving force to pull the equilibrium to the right. This modified equilibrium was previously characterized qualitatively using cyclic volt a m m e t r ~ .When ~ [Fc(COMe)] was less than [Na+], only adduct was observed, and when [Fc(COMe)] was greater than [Na+], a mixture of adduct and free specieswas observed. The stoichiometry of the reaction was determined using RDE voltammetry and UV-visible spectroscopy. An acidic melt which was -80 mM in AlC13 (A1 deposition evident) was buffered with NaCl(s), giving a solution with an electrochemical window similar to that of a neutral but unbuffered melt. Acetylferrocene was then added and its electrochemistry examined. RDE voltammograms were run over a range of rotation rates and acetylferrocene concentrations. The adduct was the only species observed at concentrations under 80 mM. On increasing the concentration of acetylferrocene, a second wave, attributed to uncomplexed acetylferrocene, appeared (Figure 1). Limiting currents were measured for each species for a range of rotation rates. Plots of limiting current of each species against u1I2were linear for both waves throughout the rotation range at each concentration. From the Levich equation, Zlim = 0.62nFADz/3J‘/601/zC 0
(6)
the slope of the Z1im vs acetylferrocene concentration plot at a fixed rotation rate is proportional to the concentration, CO, the electroactive species in the bulk of the solution. A plot of Zlim against total acetylferrocene concentration at 2000 rpm in a neutral buffered melt, initially -80 mM in AlzClT-, is shown in Figure 2. It clearly illustrates that when the acetylferrocene concentration is less than the amount of sodium ~ ~ _ _ _ _
(17) Riechel, T. L.; Wilkes, J. S. J. Electrochem. SOC.1993, 140. 3104.
Analytical Chemistry, Vol. 66, No. 21, November 1, 1994
3559
1
1 9
0
20
1
1
5
0 E
c 3
0 5 /
V a
60
40
80
100
121
Acelylhrrocene concn. I mmol
I
20CL t-5
Flgure 1. RDE voltammograms of acetylferrocene at various concentrations In a NaCI-buffered melt. [Na+] = 80 mM; 2000 rpm; Pt dlsk electrode (area = 0.020 Cm2).
Flgure 3. Plot of absorbance at 580 nm vs acetylferrocene concentration In a 0.05 mm path length cell. Table 1. Molar Absorptlvlty of Acetylferrocene In AICl$ImCI
Melts c
(&5%)/dm3mol-' cm-I 518 nm 580 nm
melt
species
454 nm
basic acidic buffered
free adduct adduct
426 872 904
140 2137 2176
12 1083 1106
Scheme 1 B 0
20
60 80 [ AcFc ] / mmol
100
40
1
I Iim free
o I lim adduct
+ Na' +
K'
AICC-
B:AICI3
120
B+
II
Flgure 2. Plot of llmltlng currentsfor free acetylferroceneand Ws AICI3 adduct vs concentratlon of acetylferrocene. Pt dlsk electrode (area = 0.020 cm2); 2000 rpm.
+ ~ a ++ MI,-
+
ion in solution in the buffered melt, which corresponds to the amount of excess AlC13present initially, the adduct is the sole species observed. When the concentration of acetylferrocene exceeds the amount of sodium ion, the adduct concentration becomes constant and equal to the concentration of sodium ions initially present in the buffered melt. The concentration of the free species then increases linearly with total acetylferrocene concentration, as predicted by eq 5 . That is, once all the sodium ions have precipitated, the equilibrium reverts to eq 4, and no further adduct formation occurs. Qualitative information on the stability of the adduct in the neutral and oxidized states can be obtained from the difference in the half-wave potentials of acetylferrocene and the adduct. The chemical and electrochemical reactions are represented by Scheme 1. Nernst equations for the redox processes for the free species and the adduct are given in eqs 7a and 7b, respectively:
+ RT/Fln([B+]/[B])
+
E = E o 2 R T / F ln([B:AlCl,+]/[B:AlCl,l)
(7a) (7b)
The chemical equilibrium constants for adduct formation in the neutral and oxidized states from Scheme 1 are given in eqs 8a and 8b, respectively: K', = [B:AlCl,]/([B] [AlCl;] [Na']) KZw= [B:AlCl,+]/([B+][AlCl;] 3560
[Na'])
(8a) (8b)
AnalyticalChemistry, Vol. 66, No. 21, November 1, 1994
B:AICI~++ ~acys)
Substituting for [B] and [B+] in eq 7a gives eq 9, E = E o l + R T / F ln(K1,/K2,)
E = Eol
+ NaCl(s)
+
R T / F ln([B:AlCl,']/[B:AlCl,l)
(9)
which, by inspection with respect to eq 7b, gives eq 10: Eo2 = E o l
+ R T / F ln(K1,/K2,)
(10)
EO2 - EO1 can be approximated from the difference in halfwave potentials (420 mV) of the free species and the adduct. From this expression we estimate that the ratio K',,@, is approximately 1.2 X lo7; Le., B is much more strongly complexed by AlCl, than B+. The equilibrium in eq 5 was also characterized by UVvisible spectroscopy. The adduct and free acetylferrocene exhibit markedly different spectra (Table l), both species obeying Beer's law. At 580 nm, the molar absorptivity of the adduct is 2 orders of magnitude greater than that of the free species, and thus absorbance at 580 nm gives, to a first approximation, a measure of adduct concentration. A plot of absorbance at 580 nm vs acetylferrrocene concentration in a buffered melt is shown in Figure 3. The absorbance at 580 nm due to the adduct rises linearly until the concentration of acetylferrocene added is equal to the initial concentration of sodium ion in solution. Above this concentration, the absorbance remains constant. The molar absorptivities determined from the experimental data agree with those obtained from a Beer's law calibration plot of acetylferrocene in an acidic melt ([Na+] = 0.5 M). This result is in good agreement with the electrochemical data above. Dimethylaniline (DMA) and Pyrrole. DMA, a stronger Lewis base than acetylferrocene, has been shown to form an
3 ,
I 3
\
\
26
-1 - 0 8 - 0 6 - 0 4 - 0 2
0
b \
0 2 0 4 0.6 0 8
1
[DMAII .[Na] / [OMAIt
Flgure 4. Chemical shift of DMA methyl protons vs ( [DMA],- [Na+]/ [DMAI r). 0
aluminum chloride adduct in an acidic room temperature molten salt1*according to eq 11: DMA
+ A12C1,-
DMA:AlCl,
+ AlC1,-
(1 1)
The equilibrium lies far to the right when A12C17- is in excess over DMA. When DMA is in excess, a mixture of adduct and free DMA results. The 'H N M R spectrum of ths methyl region shows a single line whose chemical shift is proportional to the population weighted average of the spectrum of the adduct and the free DMA. This is the result of a fast exchange process on the N M R time scale between DMA and DMA: AlC13. Similarly, the chemical shift of the methyl protons in a buffered melt also depends on the population weighted average of the adduct and the free DMA (Figure 4). When the stoichiometry of [DMA] / [Na+] is less than 1 the chemical shift is that of the adduct. Above this, the chemical shift decreases linearly with mole fraction of free DMA in exactly the same fashion as in an acidic melt. The electropolymerization of pyrrole is very sensitive to adduct formation. l 9 In unbuffered neutral melts, pyrrole is easily polymerized at potentials close to 1 V. A chronoamperometric curve (Figure 5a) shows the typical nucleation loop, and a thin film is observed on a platinum electrode. In contrast, no nucleation loop (Figure 5b) or polymer peak is observed in acidic or buffered neutral melts. We attribute this behavior to formationof A1C13adducts. As the [pyrrole]/ [Na+] concentration ratio becomes greater than unity, waves for polypyrrole formation appear. A white solid, presumably (18) Trulove, P. C.;Carlin, R. T.; Osteryoung, R. A. J. Am. Chem. SOC.1990,112, 4561. (19) Zawcdzinski, T. A., Jr.; Janiszewska, L.; Osteryoung, R. A. J. Electroanal. Chem. 1988, 255, 11 1 and references therein.
IO
20
30
time/s
Figure 5. Chronoamperometric response of a Pt disk electrode. [pyrrole] = 0.5 M; €@ = 1.0 V vs AI wlre in 1 . 5 1 melt. (a) Neutral unbuffered melt. (b) NaCl buffered melt, [Na+] = 0.5 M.
NaC1, precipitated upon addition of both DMA and pyrrole to a buffered melt.
CONCLUSIONS Our observations are in agreement with eq 12, a general description of the chemistry of a Lewis base, B:, in a buffered melt:
+
B: AlClL Na+( 1) B:AlCl, + NaCl(s) (12) The principal anionic species in NaC1-buffered neutral melts and nonbuffered melts is AlC14-. However, very weak Lewis bases behave differently in buffered and nonbuffered neutral melts. In the former, precipitation of NaCl(s) increases the apparent acidity of the AlC14-ion and provides the driving force for the formation of aluminum chloride adduct. Such adduct formation is not observed in neutral but is in unbuffered melts. Thus, NaC1-buffered neutral melts possess a latent acidity that is controlled by the concentration of sodium ions in solution. These melts are an excellent medium in which to study AlC13 complexation, allowing precise control of the amount of AlC13 available for complexation and possessing a very large electrochemical window in which to observe complexed species.
ACKNOWLEDGMENT This work was supported by the Air Force Office of Scientific Research. Received for review May 13, 1994. Accepted July 15, 1994.' *Abstract published in Aduance ACS Abstracts. September 15, 1994.
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