Synthesis of New Lewis Basic Room-Temperature Ionic Liquids by Monoquaternization of 1,4-Diazabicyclo[2.2.2]octane (DABCO)

Abstract

Sixteen new ionic liquids, including three room temperature ionic liquids, possessing cations derived from monoquaternization of DABCO have been identified through systematic variation of cation alkyl chain length and anion structure. The DABCO salts are obtained in good to excellent yields and show melting point trends, with respect to cation alkyl chain length, consistent with those reported for the imidazolium series of ionic liquids.

alkyl halides - amines - ionic liquids - green chemistry - heterocycles

References and Notes

See:

1a Rogers RD. Seddon KR. Science  2003,  302:  792 

1b Wasserscheid P. Welton T. Ionic Liquids in Synthesis   Wiley-VCH; Weinheim: 2002. and references cited therein

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2b Welton T. Chem. Rev.  1999,  99:  2071 

This designation is widely used but has been challenged recently. See, for example:

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3c Garcia MT. Gathergood N. Scammells PJ. Green Chem.  2005,  7:  9 

3d Dupont J. Spencer J. Angew. Chem. Int. Ed.  2004,  43:  5296 

4

A SciFinder^® search for ‘ionic liquid’ returns six hits for 1996 and 584 hits for 2005.

5 Fei Z. Geldbach TJ. Zhao D. Dyson PJ. Chem. Eur. J.  2006,  13:  2122 

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7

Based on reported pK _a values for protonated DABCO: pK _a1 = 2.97; pK _a2 = 8.82.

8 Quagliano JV. Banerjee AK. Goedken VL. Vallarino LM. J. Am. Chem. Soc.  1970,  92:  482 

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11

Typical Procedure for Synthesis of Halide Salts. A solution of freshly sublimed DABCO (2.01 g, 17.8 mmol) in MeCN (20 mL) was prepared at r.t. under Ar in a flame-dried round-bottomed flask equipped with a reflux condenser. The resulting clear, colorless solution was treated with chloroheptane (1.37 mL, 8.96 mmol), added dropwise via syringe, and the reaction mixture was heated to 80 °C (sand bath temperature) and stirred for 22 h. After being cooled to r.t., the reaction mixture was transferred via cannula into 80 mL of Et₂O, yielding a white slurry which eventually separated into two layers. The whole was transferred to a separatory funnel, and the bottom layer was collected, washed repeatedly with Et₂O and dried under high vacuum at 60 °C for 16 h to yield 1-heptyl-4-aza-1-azaniabicyclo[2.2.2]octane chloride (3e, 1.70 g, 77%) as a white glassy solid, mp 110-111.5 °C. ¹H NMR (300 MHz, CDCl₃): δ = 3.66 (t, J = 7.5 Hz, 6 H), 3.51-3.46 (m, 2 H), 3.24 (t, J = 7.5 Hz, 6 H), 1.75-1.71 (m, 2 H), 1.32-1.22 (m, 8 H), 0.84 (t, J = 6.9 Hz, 3 H). ¹³C NMR (56.25 MHz, CDCl₃): δ = 64.7, 52.6, 45.6, 31.6, 29.0, 26.5, 22.6, 22.3, 14.1. LRMS (ES): positive ion mode, m/z (%) = 211 (100) [C₁₃H₂₇N₂]; negative ion mode, m/z (%) = 35 (100), 37 (44).

12

All compounds were characterized by ¹H NMR, ¹³C NMR, ¹⁹F NMR [for BF₄, PF₆ and N(SO₂CF₃)₂ salts] and ³¹P NMR (for PF₆ salts) and LRMS (ES).

13 Typical Procedure for Synthesis of Tetrafluoroborate Salts. A solution of 1-heptyl-4-aza-1-azaniabicyclo[2.2.2]octane chloride (3e, 0.402 g, 1.63 mmol) in MeCN (1.0 mL), prepared at r.t. under Ar in a flame-dried round-bottomed flask, was transferred via cannula to a slurry of NaBF₄ (0.181 g, 1.65 mmol) in MeCN (1.5 mL). A 0.5 mL MeCN rinse was used to ensure complete transfer. A white precipitate formed immediately and the resulting slurry was stirred at r.t. for 24 h. Filtration followed by concentration of the filtrate in vacuo yielded a colorless, viscous liquid. Drying under high vacuum at 60 °C for 3 d yielded 1-heptyl-4-aza-1-azaniabicyclo[2.2.2]octane tetrafluoroborate (4m, 0.458 g, 94%) as a clear, glassy solid which was shown to be free of chloride contamination by a negative AgNO₃ test; mp 64-66 °C. ¹H NMR (300 MHz, CD₃CN): δ = 3.21 (t, J = 7.2 Hz, 6 H), 3.12-3.05 (m, 8 H), 1.72-1.62 (m, 2 H), 1.37-1.22 (m, 8 H), 0.87 (t, J = 6.8 Hz, 3 H). ¹³C NMR (56.25 MHz, CD₃CN): δ = 65.4 (t, J = 3.3 Hz), 53.2 (t, J = 3.4 Hz), 45.9, 32.3, 29.5, 27.0 (t, J = 1.5 Hz), 23.3, 22.4, 14.5. ¹⁹F NMR (282 MHz, CD₃CN): δ = -146.06, -146.11; LRMS (ES): positive ion mode, m/z (%) = 211 (100) [C₁₃H₂₇N₂]; negative ion mode, m/z (%) = 87 (100) [BF₄]. Note the splitting in the ¹⁹F NMR spectrum resulting from isotope shifts from ¹⁰B and ¹¹B. See: Hofstetter C. Pochapsky TC. Magn. Reson. Chem.  2000,  38:  90 

14

Typical Procedure for Synthesis of Hexafluorophosphate Salts. A solution of 1-heptyl-4-aza-1-azaniabicyclo[2.2.2]octane chloride (3e, 0.398 g, 1.61 mmol) in MeCN (1.0 mL), prepared at r.t. under Ar in a flame-dried round-bottomed flask, was transferred via cannula to a slurry of NaPF₆ (0.275 g, 1.64 mmol) in MeCN (1.0 mL). A 0.5 mL MeCN rinse was used to ensure complete transfer. A white precipitate formed immediately and the resulting slurry was stirred at r.t. for 20 h. Filtration followed by concentration of the filtrate in vacuo yielded an oily, white solid shown to be contaminated with chloride by a positive AgNO₃ test. The crude was partitioned between CH₂Cl₂ and H₂O, and the CH₂Cl₂ layer was washed repeatedly with H₂O until the aqueous layer gave a negative AgNO₃ test. The CH₂Cl₂ extract was dried (Na₂SO₄), subjected to filtration and concentrated in vacuo. Drying under high vacuum at 60 °C for 5 d yielded 1-heptyl-4-aza-1-azaniabicyclo[2.2.2]octane hexafluorophosphate (4n, 0.479 g, 83%) as a white solid, mp 151.5-152.5 °C. ¹H NMR (300 MHz, CD₃CN): δ = 3.17 (t, J = 7.3 Hz, 6 H), 3.10-3.06 (m, 8 H), 1.73-1.62 (m, 2 H), 1.40-1.24 (m, 8 H), 0.89 (t, J = 6.8 Hz, 3 H). ¹³C NMR (56.25 MHz, CD₃CN): δ = 65.6, 53.4, 45.9, 32.3, 29.5, 27.0, 23.3, 22.4, 14.4. ¹⁹F NMR (282 MHz, CD₃CN): δ = -73.0 (d, J _P-F = 707 Hz). ³¹P NMR (121 MHz, CD₃CN): δ = -143.5 (septet, J _P-F = 707 Hz). LRMS (ES): positive ion mode, m/z (%) = 211 (100) [C₁₃H₂₇N₂]; negative ion mode, m/z (%) = 145 (100) [PF₆].

15

Typical Procedure for Synthesis of Bis(trifluoromethylsulfonyl)imide Salts. A solution of LiN(SO₂CF₃)₂ (0.465 g, 1.62 mmol) in MeCN (1.0 mL), prepared at r.t. under Ar in a flame-dried round-bottomed flask, was transferred via cannula to a solution of 1-heptyl-4-aza-1-azaniabicyclo[2.2.2]octane chloride (3e, 0.396 g, 1.60 mmol) in MeCN (1.5 mL). A 0.5-mL MeCN rinse was used to ensure complete transfer. A white precipitate formed within 5 min and the resulting slurry was stirred at r.t. for 47 h. Filtration followed by concentration of the filtrate in vacuo yielded a colorless liquid shown to be contaminated with chloride by a positive AgNO₃ test. The crude was partitioned between CH₂Cl₂ and H₂O, and the CH₂Cl₂ layer was washed repeatedly with H₂O until the aqueous layer gave a negative AgNO₃ test. The CH₂Cl₂ extract was dried (Na₂SO₄), subjected to filtration and concentrated in vacuo. Drying under high vacuum at 60 °C for 5 d yielded 1-heptyl-4-aza-1-azaniabicyclo[2.2.2]octane bis(trifluoromethylsulfonyl)imide (4o, 0.706 g, 89%) as a colorless liquid, mp 14-15 °C. ¹H NMR (300 MHz, CD₃CN): δ = 3.19 (t, J = 6.9 Hz, 6 H), 3.12-3.01 (m, 8 H), 1.73-1.63 (m, 2 H), 1.39-1.24 (m, 8 H), 0.89 (t, J = 6.8 Hz, 3 H). ¹³C NMR (56.25 MHz, CD₃CN): δ = 121.0 (q, J = 321 Hz), 65.6 (t, J = 3.3 Hz), 53.41 (t, J = 3.4 Hz), 45.9, 32.3, 29.5, 27.0, 23.3, 22.4, 14.4. ¹⁹F NMR (282 MHz, CD₃CN): δ = -80.3. LRMS (ES): positive ion mode, m/z (%) = 211 (100) [C₁₃H₂₇N₂]; negative ion mode, m/z (%) = 280 (100) [C₂F₆NS₂O₄], 282 (9).

16

Yields are unoptimized.

17

All BF₄, PF₆ and N(SO₂CF₃)₂ salts were deemed halide-free by negative AgNO₃ tests.

18

All melting points were determined using samples sealed in evacuated tubes after drying in vacuo at 60 °C overnight and are uncorrected. For salts with melting points <25 °C, the sealed tubes and melting point apparatus were precooled to 5 °C in a cold room prior to melting point determination.

19 Interestingly, the charge is delocalized onto the sulfur atoms but not, to a significant extent, onto the oxygen atoms in the bis(trifluoromethylsulfonyl)imide anion. This effectively shields the charge from the cation resulting in weaker interactions and lower melting points. See: Golding JJ. MacFarlane DR. Spiccia L. Forsyth M. Skelton BW. White AH. Chem. Commun.  1998,  1593 

20 Holbrey JD. Seddon KR. J. Chem. Soc., Dalton Trans.  1999,  2133 

21 Boese R. Weiss H.-C. Bläser D. Angew. Chem. Int. Ed.  1999,  38:  988