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J. Phys. Chem. B 2009, 113, 8878–8886
Crystal Structure, Vibrational Spectroscopy and ab Initio Density Functional Theory Calculations on the Ionic Liquid forming 1,1,3,3-Tetramethylguanidinium bis{(trifluoromethyl)sulfonyl}amide Rolf W. Berg,*,† Anders Riisager,†,‡ Olivier N. Van Buu,†,‡ Rasmus Fehrmann,†,‡ Pernille Harris,† Alina A. Tomaszowska,§ and Kenneth R. Seddon§ Department of Chemistry, Technical UniVersity of Denmark, Building 207, KemitorVet, DK-2800 Kgs. Lyngby, Denmark, Centre for Catalysis and Sustainable Chemistry, Technical UniVersity of Denmark, Building 206, KemitorVet, DK-2800 Kgs. Lyngby, Denmark, and The QUILL Centre, The Queen’s UniVersity of Belfast, Stranmillis Road, BT9 5AG, Belfast, Northern Ireland, U.K. ReceiVed: March 20, 2009; ReVised Manuscript ReceiVed: April 29, 2009
The salt 1,1,3,3-tetramethylguanidinium bis{(trifluoromethyl)sulfonyl}amide, [((CH3)2N)2C)NH2]+[N(SO2CF3)2]- or [tmgH][NTf2], easily forms an ionic liquid with high SO2 absorbing capacity. The crystal structure of the salt was determined at 120(2) K by X-ray diffraction. The structure was found to be monoclinic, space group P21/n with a ) 11.349(2), b ) 11.631(2), c ) 11.887(2) Å, and β ) 90.44(3)°. Raman and IR spectra are presented and interpreted. The results are interpreted using ab initio quantum mechanics calculations that also predicted vibrational spectra. The relationship between the transoid (C2 symmetry) structure of the [NTf2]ion and the conformationally sensitive bands is discussed. TABLE 1: Crystal Data of [tmgH][NTf2]
Introduction Room temperature ionic liquids are salts composed of organic or inorganic ions that are fluids at or close to room (ambient) temperature.1,2 The stability, nonflammability, amphiphilicity, low volatility, and other characteristics of ionic liquids make them useful, inter alia, as solvents or alternative reaction media. In addition, they offer the intriguing possibility to tune their physical properties simply by changing the anion and/or cation, though comprehensive studies have been relatively scarce. One rather new class of ionic liquids is based on the 1,1,3,3- or N,N,N′,N′-tetramethylguanidinium (C5H14N3)+ (or [((CH3)2N)2C)NH2]+ or [tmgH]+) cation. The ionic liquid of that cation and the bis{(trifluoromethyl)sulfonyl}amide anion, [C2F6NO4S2]- (or [N(SO2CF3)2]- or [NTf2]-), have been studied. (Alternative abbreviations used in the literature include [(CF3SO2)2N]-, [Tf2N]-, [NTf2]-, [TFSI]-, [TFSA]-, [BTA]-. In this paper, we use the most commonly accepted [NTf2]-.) This ionic liquid with its air and moisture stability, is of considerable interest because of its use in battery electrolytes and as an extraction medium for the recovery of SO2 and CO2 from stack gases.3,4 In our previous study, it was demonstrated that [tmgH][NTf2] can physically absorb large amounts of gaseous SO2 at ambient temperature and pressure. The absorbed SO2 gas remains in the molecular state and can readily be desorbed from the ionic liquid. To get a better understanding of the reasons for the apparent affinity for this liquid to absorb the gases, a detailed study of [tmgH][NTf2] applying methods such as X-ray crystallography was undertaken, combined with vibrational Raman spectroscopy and DFT-type ab initio molecular orbital calculations. These methods are known to be very powerful * To whom correspondence should be addressed. E-mail: rwb@ kemi.dtu.dk. Tel: +45 45 25 24 12. Fax: +45 45 88 31 36. † Department of Chemistry, Technical University of Denmark. ‡ Centre for Catalysis and Sustainable Chemistry, Technical University of Denmark. § The Queen’s University of Belfast.
Formula Mw/g mol-1 crystal size/mm crystal system space group a/Å b/Å c/Å β/° V/Å3 Dc/g cm-3 Dm/g cm-3 temperature/K Z F(000) µ (Mo-KR)/mm-1 wavelength/Å absorption correction θ range for data collection/° no. of reflections reflections collected R(int) reflections, total and with I > 2σ(I) no. of parameters R1 ) ∑|F0|-|Fc|/∑|F0| for all reflections for those with F02 > 2σ(F02) wR2 ) [∑w(F02 - Fc2)2/∑wF04]1/2 for all reflections for those with F02 > 2σ(F02) weight function, where P ) (F02 +2Fc2)/3 goodness of fit residual charge density/e- Å-3
C7H14F6N4O4S2, or [C5H14N3]+[C2F6NO4S2]396.34 0.1 × 0.1 × 0.1 monoclinic P21/n (No. 14, C2h5) 11.349(2) 11.631(2) 11.887(2) 90.44(3) 1569.0(5) 1.678 1.6(2) 120(2) 4 808 0.426 0.71073 none 2.45 to 27.96 20408 (7125 independent) -14 e h e 14 -15 e k e 15 -15 e l e 15 0.0208 3742 and 3512 212 0.0315 0.0298 0.0828 0.0812 w-1 ) σ2(F02) + (0.0562P)2 + 0.7789P 0.908 -0.294 < F < 0.358
tools in the study of molecular structures and intermolecular interactions among ions.1 The structure and spectra yield important information on spatial relationships and interactions, and it was hoped to gain a better understanding of the solution chemistry of the combination of [tmgH]+ with [NTf2]-.
10.1021/jp902745j CCC: $40.75 2009 American Chemical Society Published on Web 06/09/2009
Ionic Liquid forming [(CH3)2N)2C)NH2]+[N(CF3SO2)2]-
J. Phys. Chem. B, Vol. 113, No. 26, 2009 8879
TABLE 2: Selected Bond Distances and Angles in the [tmgH]+ Cation, in the Structure of [tmgH][NTf2], and Comparable Data distance/angle N1-C1/Å N3-C1/Å N2-C1/Å N1-H1A/Å N1-H1B/Å N2-C2/Å N2-C3/Å C2-H to C5-H/Å N3-C4/Å N3-C5/Å N2-C1-N1/° N2-C1-N3/° N1-C1-N3/° C1-N1-H1A/° C1-N1-H1B/° H1A-N1-H1B/° C1-N3-C4/° C1-N3-C5/° C4-N3-C5/° C1-N2-C3/° C1-N2-C2/° C3-N2-C2/° angles around C2, C3, C4, C5/°
X-ray, this work at 120(2) K ab initioa calculated16 1.3408(15) 1.3433(15) 1.3393(15) 0.8600 0.8600 1.4686(16) 1.4684(16) 0.9600 1.4635(16) 1.4693(15) 119.46(11) 121.25(11) 119.28(11) 120.0 120.0 120.0 120.96(10) 122.42(10) 114.63(10) 122.21(11) 120.49(10) 115.64(10) 109.5
1.3475 1.3440 1.3440 1.0073 1.0073 1.4717 1.4702 1.0900 1.4702 1.4717 119.13 121.74 119.3 121.29 121.29 117.4 121.44 122.33 115.27 121.44 122.33 115.27 109(2)
[tmgH]Cl13
[tmgH][H2PO4]14
1.3304(15) 1.3417(15) 1.3370(14) 0.918(17) 0.886(17) 1.4630(17) 1.4543(15) 0.98(1) 1.4580(15) 1.4621(14) 120.91(10) 119.36(10) 119.70(10) 118.3(9) 120.8(11) 119.5(14) 122.60(9) 122.79(10) 114.45(9) 121.63(10) 121.51(10) 115.76(10) 109.5
1.3198(13) 1.3451(14) 1.3447(15) 0.88 0.88 1.466(2) 1.455(2) 0.98 1.461(2) 1.458(2) 120.22(10) 119.01(10) 120.78(11) 120.0 120.0 120.0 120.91(10) 121.53(12) 114.50(12) 121.27(11) 121.02(12) 115.34(12) 109.5
tmg · GaH315 1.316(3) 1.347(3) 1.360(3) 0.78(3) 1.455(3) 1.457(3) 0.92(4) to 1.01(4) 1.458(3) 1.448(3) 120.21(19) 117.64(19) 122.15(19) 112.6(23) 121.9(2) 120.8(2) 114.9(2) 122.5(2) 120.8(2) 115.9(2) 104.0(3) to 113.2(2)
a Gaussian 03W DFT/B3LYP/6-311+G(d,p), energy for [tmgH]+ ) -363.10735630 au, dipole moment ) 1.4001 D, and no imaginary frequencies.
Experimental Section Synthesis. [tmgH][NTf2] was prepared from a solution of tmg (0.10 mol 1,1,3,3-tetramethylguanidine, 99%, Aldrich) in ethanol (100 mL) and an aqueous acidic solution of HNTf2 (0.10 mol) in water (50 mL), obtained by cation exchange of Li[NTf2] (Aldrich), on hydrogenated Dowex 50 W × 2 resin (Aldrich). Caution: The direct neutralization of the strong base with the strong acid is highly exothermic and should be performed carefully, wearing eye protection, and stirring and cooling the solution in an ice bath. After continuous stirring (25 °C, 24 h), the solvent was removed under reduced pressure (1 mbar) at 70 °C, followed by heating in vacuo (0.1 mbar), leaving a nearly quantitative yield.4 The raw product was recrystallized from ethanol/diethyl ether (1:3 v/v) mixture, followed by drying in vacuo (50 °C, 0.1 mbar) to produce a white solid. The melting point (Tm ) 50 °C) was determined by differential scanning calorimetry using a TA-2620 DSC equipped with cryostat cooling (ca. 10 mg sample, 5 °C min-1 heating and cooling rates, see Supporting Information). By visual observation, the melting point was ca. 42-43 °C. Crystals suitable for X-ray measurements were grown from the melt in a freezer at -18 °C. NMR spectra (CDCl3, 25 °C) were recorded on a Bruker AM360 NMR spectrometer to characterize the compound, giving peak positions at 1H NMR (300 MHz): δ/ppm ) 3.01 (s, 12H; CH3), 6.20 (s, 2H; NH2). 13C NMR (75.5 MHz): δ/ppm ) 39.62 (N-CH3), 122.08 (CF3), 161.38 (NdC). Peak positions were determined relative to the solvent (CDCl3: δH ) 7.26 ppm, δC ) 77.0 ppm). X-ray Diffraction. Because of the low melting point, the crystals were mounted in a cold room using a cryo loop based on liquid dinitrogen. The crystal was transferred directly to liquid dinitrogen, where it was stored until it was mounted under the cryo stream at the diffractometer. Profile data from ω-2θ scans were collected at 120 K on a Bruker SMART CCD platform diffractometer5 using graphite monochromated Mo KR radiation. Data collection and reduction was performed using the SMART and SAINT systems.5 SHELXTL software6 was used to solve
the structure by direct methods and refine it by weighted fullmatrix least-squares fitting to F2 using all data. Hydrogen atoms were observed and modeled at ideal positions (at distances 0.96 and 0.86 Å for CH and NH, respectively). Data collection and refinement details are given in Table 1. Positional and equivalent isotropic thermal parameters are listed in Supporting Information. Some selected bond lengths and angles are included in Tables 2 and 3. Raman Spectroscopy. Dispersive DILOR-XY and BrukerIFS66 FRA-106 Fourier-Transform Raman spectrometers were used as described elsewhere.7-10 The FT-Raman spectrum for the solid sample and liquids were obtained on the Bruker spectrometer using a 1064 nm NIR Nd:YAG laser (
