The asymmetric bis(sulfonyl) imide, CH3SO2N(Li)SO2CF3 (3), was synthesized in high yield by refluxing (CH3)3SiN(Li)SO2CF3 (5) in CH3SO2F at ∼125°C. Equimolar combination of 3 with BrCH2SiMe3 and ClSiMe3 with presence of a base (nBuLi) gave rise to Me3SiCH2CH2SO2N(Li)SO2CF3 and Me3SiCH2SO2N(Li)SO2CF3, respectively, which were isolated subsequently in the acid form as Me3SiCH2CH2SO2N(H)SO2CF3 (6) and Me3SiCH2SO2N(H)SO2CF3 (7). The new compounds, 3, 6, and 7, were characterized by NMR spectroscopies including 1H, 13C, and 19F.
Lithium salts of bis(alkylsulfonyl) imide anions have attracted much attention due to the discovery of promising properties as electrolyte solutes [1–4]. In particular, bis(trifluoromethane sulfonyl) imide (1) and bis (perfluoroethane sulfonyl) imide (2) are two of the most studied lithium imides as electrolyte solutes. While these lithium imides have shown promising electrochemical properties, such as higher ionic conductivity and improved stability, they lack controllable reactivity for further chemical modifications. Hence, they cannot be chemically grafted onto a polymer backbone to form single-ion conducting solid polymer electrolytes (SPEs).
Single-ion conducting polymer electrolytes have several advantages over typical bi-ion based SPEs [5]. For example, during discharge or charge in bi-ion based SPEs, mobile anions and cations migrate toward the oppositely charged electrodes, thereby polarizing the electrolytes and increasing the resi-stivity. As a result, discharge of Li-ion cells results in less power and energy, while charge requires a greater potential, and more energy and time. This problem can be readily solved by employing single-ion conducting polymer electrolytes. We report here the synthesis and reactivity of an asymmetric bis(alkylsulfonyl) imide (3), which would allow a facile attachment of the imide to a polymer backbone to form a single-ion polymer electrolyte.
Experimental
General information
All reactions were carried out in an inert atmosphere unless otherwise noted. Tetrahydrofuran (THF), ether and hexane were dried by distillation over sodium/benzophenone [6]. NMR spectra were recorded on a Bruker AC-300 spectrometer using tetramethylsilane (1H and 13C) and CH3F (19F) as references. nBuLi, HN(Si(CH3)3)2, (CF3SO2)2O, CF3SO2F, BrCH2SiMe3, and ClSiMe3 were purchased from Aldrich and used without further purification.
Synthesis and characterization of (CH3)3SiN(H)SO2CF3 (4)
A 250 mL, 3-necked flask, fitted with a reflux condenser and an addition funnel, was charged with HN(Si(CH3)3)2 (86.0 g, 0.533 mol). A sample of (CF3SO2)2O (150.3 g, 0.533 mol) was added slowly with stirring to the amine via the addition funnel, and the reaction mixture was stirred at room temperature for 4 hrs. The by-product CF3SO3Si(CH3)3 (1H NMR (CDCl3), δ= 0.472 ppm) was distilled off under vacuum at 23°C. Vacuum distillation (41°C/0.01 torr) of the residue gave a clear liquid 4. Yield: 84.3 g, 71.5%). 1H NMR (CDCl3, ppm): δ 0.339 (s, (CH3)3Si). 13C NMR (CDCl3, ppm): δ -0.195 (s, (CH3)3Si), 119.4 (q, JC - F = 320 Hz, CF3). 19F NMR (CDCl3, ppm): δ -77.3 (s, CF3).
Synthesis of (CH3)3SiN(Li)SO2CF3 (5)
A 250 mL Schlenk flask charged with a sample of 4 (11.1 g, 50.2 mmol) and THF (∼50 mL) was cooled to –78°C. An aliquot of nBuLi (20.1 mL, 2.5 M, 50.2 mmol) solution was added via an air-tight syringe. The reaction mixture was stirred at –78°C for 1 hr and 23°C for 3 hrs. Removal of volatiles under vacuum gave an off-white powder, which was washed with dry hexane, and 5 was isolated by filtration. Yield: 10.9 g, 92.3%.
Synthesis and characterization of CH3SO2N(Li)SO2CF3 (3)
A 100 mL flask fitted with a condenser was charged with a sample of 5 (11.4 g, 50.0 mmol) and CF3SO2F (∼25 g, 255 mmol). The reaction mixture was heated under reflux for 4 hrs until no more gas ((CH3)3SiF) evolved, and the resulting mixture was cooled to room temperature. Unreacted CF3SO2F was recovered by vacuum distillation, leaving an off-white solid, that was washed with dry hexane. Yield: 11.2 g, 95.7%. 1H NMR (acetone-d6, ppm): δ 2.928 (s, (CH3)3Si) (Fig. 1). 13C1H NMR (acetone-d6, ppm): δ 43.2 (s, (CH3)3Si), 121.2 (q, J C - F = 322 Hz, CF3) (Fig. 2). 19F NMR (acetone-d6, ppm):δ -74.5, (s, CF3).
Synthesis and characterization of (CH3)3SiCH2CH2SO2N(H)SO2CF3 (6)
A solution of 3 (1.0 g, 4.3 mmol) in THF (30 mL) was cooled to –78°C. A solution of nBuLi in hexane (2.5 M, 1.7 mL, 4.3 mmol) was added slowly to the cooled solution via an air-tight syringe. The reaction mixture was slowly warmed to room temperature and stirred for another hr. The reaction mixture was cooled to –78°C again, and a solution of BrCH2SiMe3 (0.72 g, 4.3 mmol) in THF (10 mL) was added. The reaction mixture was stirred at –78°C for 1 hr and 23°C for 5 hrs. Aqueous HCl (1N, ∼30 mL) was added to the reaction mixture, and the organic phase was extracted with ether (2×20 mL). The organic phases were combined and dried over MgSO4. Removal of volatiles under reduced pressure resulted in a pale yellow, viscous solids. Recrystallization of the crude solids from cold pentane gave an off-white crystalline solid, 6. Yield: 0.75 g, 55.7%. 1H NMR (CDCl3, ppm): δ 0.08 (s, Si(CH3)3, 9H), 1.10 (m, CH2CH2Si(CH3)3, 2H), 3.41 (m, CH2CH2Si(CH3)3, 2H), 7.96 (s, broad, NH, 1H) (Fig. 3). 13C1H NMR (CDCl3, ppm): δ -2.1 (s, Si(CH3)3), 10.4 (s, CH2CH2Si(CH3)3), 54.0 (s, CH2CH2Si(CH3)3, 118.9 (q, CF3, J C - F = 322 Hz) (Fig. 4). 19F NMR (CDCl3, ppm): δ -75.9 (s, CF3).
Synthesis and characterization of (CH3)3SiCH2SO2N(H)SO2CF3 (7)
A solution of 3 (1.0 g, 4.3 mmol) in THF (30 mL) was cooled to –78°C. A solution of nBuLi in hexane (2.5 M, 1.7 mL, 4.3 mmol) was added slowly to the cooled solution via an air-tight syringe. The reaction mixture was slowly warmed to room temperature and stirred for another hr. The reaction mixture was cooled to –78°C again, to which a solution of ClSiMe3 (0.47 g, 4.3 mmol) in ∼10 mL THF was added. The reaction mixture was stirred at –78°C for 1 hr and 23°C for 5 hrs. Aqueous HCl (1N, ∼30 mL) was added to the reaction mixture and the aqueous phase was extracted with ether (2×20 mL). The organic phases were combined and dried over MgSO4. Removal of volatiles under reduced pressure resulted in pale yellow, viscous solids. Recrystallization of the solids from cold pentane gave an off-white crystalline solid, 7. Yield: 0.26 g, 20.3%. 1H NMR (CDCl3, ppm): δ 0.29 (s, Si(CH3)3, 9H), 3.25 (s, CH2Si(CH3)3, 2H), 8.22 (s, broad, NH) (Fig. 5). 13C1H NMR (CDCl3, ppm): δ -1.2 (s, Si(CH3)3), 49.3 (s, CH2Si(CH3)3), 118.9 (q, CF3, J C - F = 322 Hz) (Fig. 6). 19F NMR (CDCl3, ppm): δ - 76.3(s, CF3).
Discussions
The synthesis of the amine (CH3)3SiN(H)SO2CF3 (4) was first described by H. W. Roesky [7]. For the present study, compound 4 was prepared in the same fashion with minor modification resulting in a higher yield (71% vs 42%) (Scheme 1). The low yield reported previously was probably due to the air-sensitive nature of the starting materials and product. The reaction itself appears to be quantitative based upon 1H NMR analysis of the reaction mixture. The reduced isolated yield is likely due to loss during purification. The compound was characterized by NMR spectroscopies (1H, 13C, and 19F), and the data agree with the literature data. Compound 4 was converted in almost quantitative yield to the lithium form (5) by reaction with nBuLi in THF, and 5 was isolated as an off-white solid. 1H NMR analysis indicated that no solvent (THF) was coordinated to the salt.
The asymmetric target compound CH3SO2N(Li)SO2CF3 (3) was synthesized by refluxing 5 in CH3SO2F at ∼125°C. Compound 3 is a hygroscopic, but air-stable solid. Attempts failed to synthesize CH3SO2N(H)SO2CF3, the acid form of the imide, by direct refluxing of (CH3)3SiN(H)SO2CF3 with CH3SO2F, which would eliminate one synthetic step. However, CH3SO2N(H)SO2CF3 can be obtained readily by acidifying 3. It appears metallization of the imide, e.g. lithiation or sodiation, is necessary to increase the reactivity. The compound was characterized by NMR spectroscopies (1H, 13C, and 19F) and the data agree with the structure. The 1H and 19F NMR spectra of the compound both showed a single resonance at 2.93 ppm and –74.5 ppm, respectively. The 13C NMR spectrum showed two resonance peaks, one at 43.2 ppm and the other at 121.2 ppm. The later displayed a quadruplet splitting pattern (J C - F = 322 Hz) due to one bond 13C-19F coupling (Fig. 2).
The driving force for the synthesis of 3 is the formation of stable and volatile Me3SiF. Similar approach was previously used to prepare a symmetric sodium imide, (CF3SO2)2N(Na) [1].(Equation 1)
The reactivity of 3 was examined. Substitution of one H atom on – SO2CH3 with CH2SiMe3 and SiMe3 lead to formation of two new asymmetric bis(sulfonyl) imides 6 and 7 (Equations 2 and 3).
Both 6 and 7 were characterized by NMR spectroscopies (1H, 13C, and 19F) and the data agree with the structures. The 19F NMR spectra both showed a single resonance at –75.9 ppm for 6 and –76.3 ppm for 7 for the CF3 groups. The 1H NMR spectrum of 6 showed a trimethyl silyl resonance at 0.08 ppm, two methylene resonances at 1.10 and 3.41 ppm respectively, and a broad NH resonance at 7.96 ppm (Fig. 3). The 1H NMR spectrum of 7 showed a trimethyl silyl resonance at 0.29 ppm, one methylene resonance at 3.25 ppm, and a broad NH resonance at 8.22 ppm (Fig. 5). The 13C NMR spectrum of 6 (Fig. 4) showed four resonance peaks, the most downfield quadruplet at 118.9 ppm was assigned to CF3SO2 group with a 1JC - F = 322 Hz, and the most upfield singlet at –2.1 ppm was assigned to the Si(CH3)3 group. The other two resonance peaks were assigned to the two methylene groups, the one at 10.4 ppm to – CH2-SiMe3 and the other at 54.0 ppm to -CH2–CH2-SiMe3. The 13C NMR spectrum of 7 (Fig. 6) showed three resonance peaks, the most downfield quadruplet at 118.9 ppm was assigned to CF3SO2 group with a 1JC - F = 322 Hz, and the most upfield singlet at –1.2 ppm was assigned to the Si(CH3)3 group. The resonance peak at 49.0 ppm was assigned to the methylene group, -SO2–CH2-SiMe3.
The successful synthesis of 6 and 7 indicated that the asymmetric, chemically labile imide 3 can be readily grafted to the backbone of a silicon based polymer. Work is in progress in synthesis of single-ion polymer electrolytes via this synthetic approach.
Conclusions
The successful synthesis of the asymmetric, chemically labile bis(sulfonyl) imide 3, and it’s derivatives 6, and 7 suggests that the approach applied in the synthesis may be general and more chemically labile bis(sulfonyl) imides may be synthesized. The reactivity study of 3 indicates that it is possible to chemically attach the imide to a polymer backbone to form single-ion conducting polymers. Efforts continue in the area to include more asymmetric bis(sulfonyl) imides, and to attach them onto varieties of polymer backbones for solid polymer electrolyte applications.
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