Alcohol or Guanidine

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Switchable Solvents Consisting of Amidine/Alcohol or Guanidine/Alcohol Mixtures Lam Phan,† Daniel Chiu,† David J. Heldebrant,† Hillary Huttenhower,‡ Ejae John,‡ Xiaowang Li,† Pamela Pollet,‡ Ruiyao Wang,† Charles A. Eckert,‡,§ Charles L. Liotta,‡,§ and Philip G. Jessop*,† Department of Chemistry, Queen’s UniVersity, Kingston, Ontario, Canada, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0100, and School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100

Liquids that consist of a mixture of an alcohol and either an amidine or a guanidine have been developed to switch from a low-polarity form to a high-polarity ionic liquid upon treatment with CO2 at atmospheric pressure. Treatment with N2 and/or mild heat (50-60 °C) reverses the process. These liquids can be used as switchable solvents to dissolve and then precipitate a solute or to dissolve reagents for a chemical synthesis and then precipitate the product. Introduction The inflexibility of the physical properties of conventional solvents can lead to significant limitations in their use as media for reaction and separations. Many chemical production processes involve multiple steps (reactions, extractions, and/or separations), and the type of solvent that is optimum for any one step often is different from that which is optimal for the next. The common practice of adding/removing solvent for each step greatly adds to the economic cost and environmental impact of a process. If it were possible to trigger a drastic change in the properties of a solvent, then one could use the same solvent for several consecutive process steps. Unfortunately, moderate changes in temperature and pressure cause only minor changes in solvent properties and therefore cannot be used as a method for dramatically changing solvent properties. It is known that the properties of supercritical fluids (such as CHF3)1 and CO2expanded liquids (i.e., subcritical mixtures of CO2 and an organic liquid)2 can be continuously and reversibly changed by variation of the pressure, but only at pressures of >40 bar. What is needed, then, is a switchable solvent that is capable of reversibly changing its nature and properties under mild conditions and preferably with very mild reagents. As described in our preliminary communication,3 we reasoned that switching a normal (neutral) liquid to an ionic liquid (molten salt) would inherently cause a change in most of the properties of the liquid. Ionic liquids (salts that are liquid at or near room temperature) are often viscous and typically quite polar. In contrast, neutral solvents are usually nonviscous and are available in a wide range of polarities. We found that the exposure of a 1:1 mixture of the two neutral liquids 1,8diazabicyclo-[5.4.0]-undec-7-ene (abbreviated hereafter as DBU) and 1-hexanol to gaseous CO2, at 1 atm, causes an exothermic conversion of the liquid phase to an ionic liquid (eq 1), which is akin to going from chloroform to DMF in polarity. Furthermore, one can readily convert the viscous ionic liquid back to a neutral liquid by bubbling N2 or argon through the liquid at room temperature, or, for a more rapid reaction, at 50-60 °C. * To whom correspondence should be addressed. Phone: (613) 5333212. Fax: (613) 533-6669. E-mail address: [email protected]. † Queen’s University. ‡ School of Chemistry and Biochemistry, Georgia Institute of Technology. § School of Chemical and Biomolecular Engineering, Georgia Institute of Technology.

The number of known switchable solvents is increasing rapidly.4-7 We now report many more such liquids, each of which can be converted to an ionic liquid upon exposure to an atmosphere of CO2 gas and then converted back to a neutral organic liquid upon exposure to N2 or argon gas, or heat. We describe switchable solvents based on mixtures of alcohols with DBU or 2-butyl-1,1,3,3-tetramethylguanidine (abbreviated hereafter as TMBG; see eq 2).

Results and Discussion Synthesis of Switchable Solvents and Demonstration of Solvent Switching. DBU alkylcarbonate salts, prepared by bubbling CO2 through equimolar mixtures of DBU and ethanol, methanol,8,9 or water,10-12 are solids at room temperature. However, those prepared from DBU and 1-propanol, 1-butanol, 1-hexanol, 1-octanol, or 1-decanol are viscous liquids at or near room temperature (see eq 1, where R ) H or an alkyl group). The melting points are a function of the number of carbons in the linear alkyl chain (see Figure 1); these melting points are only approximate, because they are affected by the extent of conversion to the ionic liquid, which can change during the melting point measurement. The viscosity of a DBU-1-propanol mixture under N2 is 5.5 cP at 23 °C but increases by orders of magnitude during CO2 treatment. The viscosity of the ionic liquid form is 410 cP, but with a large standard deviation ((77 cP). The alkylcarbonate salts have also been characterized spectroscopically, including the detection of the hexylcarbonate anion by mass spectroscopy (see Experimental Section). After the reaction of an equimolar DBU-1-hexanol mixture with CO2, the oxygen-bound methylene of the hexyl group in the sample shifted from the normal position for hexanol (3.58 ppm in CDCl3) to 3.90 ppm, which is closer to the chemical shifts of CH3C(O)O(CH2)5CH3 (4.05 ppm)13 or dihexyl carbonate (4.13 ppm in CCl4).14 The 13C NMR spectrum of [DBUH][O2CO(CH2)5CH3] in CDCl3 shows a carbonate carbon at 158.7 ppm

10.1021/ie070552r CCC: $40.75 © 2008 American Chemical Society Published on Web 09/05/2007

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Figure 3. Melting points of 2-butyl-1,1,3,3-tetramethylguanidinium alkylcarbonate salts, as a function of the alkyl group chain length.

Figure 1. Melting point of the [DBUH][O2COR] ionic liquids prepared from DBU, linear alcohols, and CO2.

Figure 2. Molecular structure of [DBUH][O2COMe]. Displacement ellipsoids for non-hydrogen atoms are shown at the 50% probability level and H atoms are represented by circles of arbitrary size.

and a peak for the central carbon of [DBUH]+ at 164.9 ppm. The former signal is comparable to that of [NBu4][O2COMe]15 at 158.2 and [PMePh3][O2COH]16 at 160.3 ppm in the same solvent. The 164.9 ppm signal is comparable to that of [DBUH][O2COH] (162.6 ppm), a 1:1 salt of DBU and Cl3CCO2H (162.8 ppm), and a 1:1 salt of DBU and H2SO4 (166.3 ppm).12 The infrared (IR) spectrum of [DBUH][O2CO(CH2)5CH3] shows a strong band at 1648 cm-1, which is comparable to the ν(CdN) frequency for [DBUH]Cl (1645 cm-1).12 The νas(CO2) vibration of the anion is expected to be in the same region but is not detected as a separate band, most likely due to overlap. As further evidence, the structure of the solid DBU methylcarbonate salt has been determined by crystallography (Figure 2). The structure differs from that of the DBU bicarbonate salt in that the latter is a hydrogen-bonded dimer, [DBUH]2[O2COH]2, held together by hydrogen bonding between DBUH and bicarbonate and between one bicarbonate anion and the other.11 In the structure of [DBUH][O2COCH3], there is no dimerization, because the methylcarbonate anion lacks an acidic proton with which to hydrogen bond to another anion. The reaction in eq 1 has been reported previously for the case of DBU and ethanol in toluene solution;10 however, only 80% conversion was obtained in that solution. Our nuclear magnetic resonance (NMR) data show that the reaction proceeds to greater conversion in CDCl3. Specifically, an equimolar mixture of hexanol and DBU in CDCl3, after CO2 was bubbled through the solution, showed an integration of C5H11CH2OC(O)O- that is 93% of that of the DBU C6H2 (i.e., the methylene next to the bridgehead carbon). With 1-propanol, the same ratio was 92%. Considering the moderate accuracy of 1H NMR integration, these results indicate either complete or almost-complete conversion to the alkylcarbonate salt in this

deuterated solvent. The conversion in neat DBU-ROH-CO2 mixtures would probably be higher, given the higher polarity, but NMR measurements were not possible, because of the viscosity-induced broadness of the peaks. The conversion was monitored gravimetrically. Bubbling CO2 through a neat equimolar mixture of DBU and 1-hexanol caused a rapid mass increase that leveled off at a value consistent with 1.0 mol CO2 chemically reacted (per mol of DBU) plus 0.3 mol of CO2 (4 wt %) dissolved in the ionic liquid. In contrast to primary alcohols, secondary and tertiary alcohols give inferior conversion at 1 bar of CO2, as measured in CDCl3. Reconversion of [DBUH][O2COR] ionic liquids back into a neutral liquid can be achieved by bubbling N2 or argon through the ionic liquid at room temperature, or, for a faster reaction, at 50-60 °C. The conversion back to the DBU and free alcohol was confirmed by comparison of the 1H NMR spectrum to that of the same mixture of DBU and alcohol before CO2 treatment. Conductivity tests of a neat, equimolar DBU-1-hexanol mixture were performed to further confirm the reversibility of the conversion from neutral to ionic liquid. The conductivity was measured while CO2 was bubbled through the liquid at room temperature and then N2 was bubbled through the liquid at 55 °C. The conductivity changes over three cycles of CO2 and N2 treatments showed that the change is reversible and repeatable. The initial conductivity was 7 µS/cm, whereas the conductivity after the CO2 treatment was 187-189 S/cm each time. The N2 treatment returned the conductivity to the 7 µS/ cm starting point. The 2-butyl-1,1,3,3-tetramethylguanidinium alkylcarbonate salts prepared from bubbling CO2 through equimolar solutions of 2-butyl-1,1,3,3-tetramethylguanidine (TMBG) and methanol, 1-butanol, 1-hexanol, 1-octanol, or 1-dodecanol (eq 2) are also viscous liquids at room temperature. Figure 3 shows the melting points of the ionic liquids, as a function of the chain length of the alcohol. 1H and 13C NMR data of the neat ionic liquids indicate the reaction of TMBG/alcohols mixture with CO2 (eq 2) proceeds to completion in all cases. Formation of 2-butyl1,1,3,3-tetramethylguanidinium methylcarbonate is evidenced in 1H NMR by an upfield shift in the aliphatic protons and by the collapse of the two N-Me peaks into one (two singlets at 2.53 and 2.62 ppm becomes one singlet at 3.1 ppm). The 13C NMR spectra exhibit the appearance of a characteristic carbonate peak at 161 ppm. Furthermore, the chemical shifts of the methoxyl peak, aliphatic carbons, and the change in N-Me peaks are clearly observed. Unreacted CO2 is not detected by NMR, because no peaks are present at 120 ppm. The 2-butyl1,1,3,3-tetramethylguanidinium methylcarbonate ionic liquid was also further characterized with elemental analysis: the elemental analysis was consistent with methyl carbonate and 2-butyl-1,1,3,3-tetramethylguanidinium in a 1:1 ratio. Reconversion of the ionic liquid back to a neutral liquid can be achieved either by bubbling N2 through the ionic liquid at room temperature or by heating at temperatures above 60 °C. Conductivity tests were also performed to further confirm the

Ind. Eng. Chem. Res., Vol. 47, No. 3, 2008 541 Table 1. Polarities of Selected Nonswitchable Liquids, as Indicated by Solvatochromic Dyes

solvent

ET(30) ( kcal/mol)

ether TMBG CHCl3 DBU quinoline CH2Cl2 DMF 1-octanol 1-propanol [bmim]PF6b

34.5 naa 39.1 39.4 39.4 40.7 43.2 48.1 50.7 52.3

methanol [eim]BF4c

55.4 naa

reference Reichardt19 Reichardt19 this work Reichardt19 Reichardt19 Reichardt19 Reichardt19 Reichardt19 Muldoon et al.21 Reichardt19

λmax (Nile Red) (nm) 504.4 524.8 537.6 545.8 548.4 535.2 541.2 544.0 545.6 547.5 549.6 562.9

reference Deye et al.20 this work Deye et al.20 this work Deye et al.20 Deye et al.20 Deye et al.20 Deye et al.20 Deye et al.20 Carmichael and Seddon22 Deye et al.20 Ogihara et al.23

a Not accessible spectroscopically. b “bmim” ) N-butyl-N′-methylimidazolium. c “eim” ) N-ethylimidazolium.

reversibility of the conversion from neutral to ionic species. In this case, the experiment had to be conducted in solvent (chloroform), because the ionic liquid is too viscous for neat conductivity measurements. At the beginning of the experiment, the equimolar mixture of methanol and 2-butyl-1,1,3,3-tetramethylguanindine in chloroform does not conduct electricity (0 µS/cm). However, as CO2 is bubbled into the mixture, the conductivity increases, up to 246 µS/cm. This is consistent with the formation of the ionic species 2-butyl-1,1,3,3-tetramethylguanidinium and methyl carbonate. Upon heating, the conductivity decreases to 19 µS/cm. This is consistent with the 2-butyl-1,1,3,3-tetramethylguanidinium methylcarbonate reversing back to the starting materials, 2-butyl-1,1,3,3-tetramethylguanidine and methanol, respectively. This cycle was repeated three times, showing similar observations. The Polarity of Switchable Solvents. The polarities of DBU, TMBG, and the alcohol/base mixtures have been measured through the use of the solvatochromic dye Nile Red, where λmax, the wavelength of maximum absorbance, is indicative of the solvent polarity. The more common solvatochromic dye Reichardt’s dye, or “ET(30)”, was also used for DBU but not for TMBG or the switchable solvents, because the dye seemed to be bleached to yellow when dissolved in DBU-1-hexanol under CO2, probably because of protonation by (or hydrogen bonding with) the acidic proton on the [DBUH]+ cation. Although similar bleaching was observed when Reichardt’s dye was dissolved in neat TMBG, the reason for that bleaching is unknown. It is clear from the Nile Red data (Table 1) that pure TMBG is significantly less polar than pure DBU, although Nile Red does seem to overestimate the polarity of cyclic amines.17 Table 1 also presents data for nonswitchable solvents for comparison purposes. Mixing TMBG with an equimolar amount of alcohol causes a slight increase in polarity, relative to TMBG alone (compare the data points represented by hollow circles in Figure 4 to the data in Table 1). In contrast, DBU/ROH equimolar mixtures seem to be significantly less polar than either DBU alone or alcohol alone. This unusual behavior may be associated with factors other than pure polarity, such as basicity (for the DBU) or hydrogen-bond donating ability (for the alcohol); the formation of a hydrogen-bonded DBU:ROH adduct would effectively hide the basicity and hydrogen-bond donating ability of these liquids and thereby make the equimolar mixture have a lower Nile Red λmax value than the amine or alcohol alone. A study by Kipkemboi and Easteal18 showed that the Nile Red λmax value of water/t-butylamine mixtures is a strongly nonlinear function of the composition of the mixture.

Figure 4. Polarity of the neutral and ionic forms of the DBU/1-alkanol and TMBG/1-alkanol liquids as a function of the alkanol chain length.

Alcohol-DBU or alcohol-TMBG equimolar mixtures become significantly more polar when exposed to CO2, as shown by the shift of the λmax value to longer wavelengths (see Figure 4). For example, the λmax value of the 2-butyl-1,1,3,3-tetramethylguanidine/methanol mixture is 538.0 nm, whereas that of the corresponding ionic liquid (2-butyl-1,1,3,3-tetramethylguanidinium methylcarbonate) is 554.0 nm, corresponding to a shift of 16 nm. Such a shift in λmax value quantifies a polarity switch akin to going from chloroform to acetic acid. The polarity of the ionic liquids is greatly dependent on the length of the alkyl chain, with the λmax values falling well within the range found for nonswitchable ionic liquids. Note that ionic liquids that contain acidic protons often have λmax values higher than those without acidic protons.23 The many possible combinations of base and alcohol give us a wide selection of solvent switches. Using shorter alcohols gives a greater difference between the polarities of the ionic and neutral forms of the solvent, while using TMBG gives both forms lower polarities than are obtained with DBU. Furthermore, switchable solvents based on TMBG have a larger switch in polarity, compared to those based on DBU; the ionic forms of the two systems are almost comparable, whereas the neutral form with TMBG is significantly less polar than that with DBU. Solubility and Miscibility of Species in Switchable Solvents. Conversion between the neutral liquid and the ionic liquid results in changes in the properties of the new solvent. For example, the mixture of 1-hexanol and DBU under N2 (neutral form) is miscible with the nonpolar solvent decane, whereas the ionic liquid form (under CO2) is not. Thus, CO2 and N2 at 1 bar can be used as triggers of miscibility and immiscibility. Equimolar mixtures of DBU with 1-butanol, 1-hexanol, or 1-octanol (but not 1-decanol) exhibit the same behavior with hexane; they are miscible while under N2, but immiscible after exposure to CO2 (see Table 2). Miscibility is restored when N2 is bubbled through the mixture. However, a 1:1 mixture of DBU and 1-decanol is miscible with hexane, even when exposed to CO2. The 1-ethylcarbonate salt of DBU differs significantly from the higher alkylcarbonates. It is immiscible with hexane, toluene, and ethyl acetate, forming a separate liquid (not solid) phase, although it is a solid when pure. The polarity of the liquid [DBUH][O2COEt] is expected to be much higher than that of the liquid [DBUH][O2COC6H13], based on the trend that is clear in Figure 4. Switchable solvents based on TMBG, being generally less polar than those with DBU, have greater miscibility with low polarity solvents, although the TMBG/methanol combination is an exception. TMBG/methanol equimolar mixture in its neutral form is miscible with the nonpolar solvents pentane, hexane, heptane, and octane, whereas the ionic liquid form is

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Figure 6. Numbering system for positions in the structure of DBU. Table 3. Polymerization of Styrene in Neutral DBU/1-Propanol with Solvent Switching, Filtration, and Recycling of the Solventa Molecular Weight cycle

Figure 5. Method for polymerization of styrene in a switchable solvent consisting of DBU and 1-propanol. Green indicates the solvent in its neutral form, whereas pink indicates the ionic form. Table 2. Miscibility of the Ionic Liquids with Nonpolar Solvents Miscible? base

R of alcohol

hexane

toluene

ethyl acetate

Conditions: 0.2 mL of Nonpolar Solvent Combined with the Ionic Liquid Created by Bubbling CO2 through 1 mL of an Equimolar Mixture of DBU and Alcohol, at Room Temperature DBU ethyl no no no DBU 1-butyl no yes yes DBU 1-hexyl no yes yes DBU 1-octyl no yes yes DBU 1-decyl yes yes yes Conditions: 0.15 mL of Nonpolar Solvent, Combined with the Ionic Liquid Created by Bubbling CO2 through 0.15-0.21 g of TMBG Mixed with an Equimolar Amount of Alcohol, at Room Temperature TMBG methyl no yes yes TMBG 1-butyl yes yes yes TMBG 1-hexyl yes yes yes TMBG 1-octyl yes yes yes TMBG 1-dodecyl yes yes yes

not. Furthermore, GC-MS and NMR data indicated no crosscontamination between the ionic liquid phase and the octane, heptane, or pentane phase. The other TMBG/alcohol ILs were soluble in all tested solvents. Table 2 summarizes the miscibility of the ionic liquids with hexane, toluene, and ethyl acetate. The solubility of several solids in the DBU-1-propanol mixture was qualitatively measured. At a loading of 50 mg solute in 2.22 mL of 1:1 solvent mixture, polystyrene (2000 and 100 000 MW) was determined to be soluble under N2 but insoluble after CO2 was bubbled through the mixture. Benzyltriethylammonium chloride, in contrast, was insoluble under N2 and became soluble after the CO2 treatment. Glucose and tetraethylammonium tosylate were insoluble under either gas. Chemical Synthesis in a Switchable Solvent. Polymerization of styrene in the switchable solvent was performed to demonstrate the utility of the solvent switching (see Figure 5). The solvent was a 1:2.5 molar ratio mixture of DBU and 1-propanol. The use of this nonequimolar ratio makes the ionic form of the solvent less viscous and, therefore, easier to filter, presumably because of the unreacted 1-propanol in the ionic liquid; the viscosity of the ionic form is 75 ( 7 cP (cf. 3.8 ( 0.2 cP in the nonionic form), which is much less than that of the equimolar ionic liquid. Styrene was polymerized with K2S2O8 initiator in the neutral solvent at 50 °C. After polymerization, the solvent was switched to its ionic form, in which polystyrene has little solubility. The precipitated polymer was collected by filtration, and the filtrate solvent was reconverted to its neutral form and used again for another polymerization. The solvent was used

1 2 3 4

volume of styrene (mL)

yield (%)

number-average, Mn

weight-average, Mw

PDI

1.5 1.0 1.0 1.0

42a

21 142 11 941 16 775 12 457

40 800 17 561 28 457 18 235

1.93 1.47 1.70 1.46

172 50 151

a Note that the first run started with a larger volume of styrene than did subsequent runs.

for a total of four cycles, although some makeup solvent had to be added in each cycle to compensate for solvent losses in the filtration step. To facilitate the filtration step, the compensating propanol was added immediately before the filtration and the compensating DBU was added immediately after the filtration. The overall yield of polystyrene over the four cycles was 97% (see Table 3). Conclusions We have demonstrated the first examples of a class of switchable solvents that are readily convertible, under an atmosphere of CO2, to ionic liquids and that can be returned to their original neutral states by the application of N2 gas or mild heat. Although the reactivity of amidines and guanidines will limit their applications as solvents, we hope that the publication of these first switchable solvents will encourage researchers to identify many other switchable solvents that have less reactivity and are based on a variety of triggering mechanisms. Experimental Methods DBU (Aldrich, 98% grade) was refluxed over CaH2 and distilled under vacuum into 4 Å molecular sieves and then deoxygenated by repeated freeze/vacuum/thaw cycles. 1-Hexanol (99+%, anhydrous), 3-ethyl-3-pentanol (98%), and 1-octanol (99+%, anhydrous) were used as received from Aldrich. 2-Octanol was dried over 4 Å molecular sieves and then deoxygenated by repeated freeze/vacuum/thaw cycles. Decane (Aldrich, 99+% grade) was degassed. Supercritical-grade CO2 (99.999%,