Conformational Flexibility and CationâAnion Interactions in 1-Butyl-2,3

Article pubs.acs.org/crystal

Conformational Flexibility and Cation−Anion Interactions in 1-Butyl-2,3-dimethylimidazolium Salts Gerhard Laus,† Gino Bentivoglio,† Volker Kahlenberg,‡ Klaus Wurst,† Gerhard Nauer,§ Herwig Schottenberger,*,† Masato Tanaka,# and Hans-Ullrich Siehl*,# †

Faculty of Chemistry and Pharmacy, University of Innsbruck, Innrain 52a, 6020 Innsbruck, Austria Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria § Institute of Physical Chemistry, University of Vienna, Währinger Strasse 42, 1090 Vienna, Austria # Institute of Organic Chemistry, University of Ulm, Albert Einstein Allee 11, 89069 Ulm, Germany ‡

S Supporting Information *

ABSTRACT: The butyl group in 1-butyl-2,3-dimethylimidazolium (BMMI) salts, a common group of low-melting solids, was found to exhibit different conformations in the solid state. Crystal structures of pure BMMI azide, thiocyanate, propynoate, hexachlorocerate(IV), chlorocyanocuprate(I), nonachlorodititanate(IV), and mixed azide/chloride and cyanide/chloride salts were determined by single crystal X-ray diffraction, and their butyl chain conformations were examined. The twist angle of the C(α)−C(β) bond out of the plane of the imidazole ring ranges from 57° to 90°, whereas the torsion angle along the C(α)−C(β) bond determines the overall conformation: 63° to 97° (gauche) and 170° to 179° (trans). The preferred conformations of the butyl group are trans−trans and gauche−trans, but trans−gauche and gauche−gauche were also observed. More than one conformer was present in disordered structures. Numerous polar hydrogen bonds between cations and anions were identified. Five structures exhibit stacking of the aromatic imidazole systems, indicated by parallel alignment of pairs of cations with short centroid−centroid distances due to π−π interactions, which is surprisingly frequent. Not only imidazole ring protons are involved in the formation of short CH···X hydrogen bonds, but also interactions between methylene and methyl groups of the alkyl chain and the anion are visible. Hirshfeld surface analysis revealed that nonpolar H···H contacts represent the majority of interactions. The volume-based lattice potential energy, enthalpy, entropy, and free energy were calculated by density functional theory. Calculated and experimental molecular volumes in the range from 0.27 to 0.70 nm3 agreed favorably, thus facilitating reliable predictions of volume-derived properties.

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interactions in the gas-phase by ab initio calculations.32,33 The acidity of the C(2)−H in imidazolium cations is well-known, allowing the formation of carbenes by deprotonation.34,35 Therefore, the 1-butyl-2,3-dimethylimidazolium (BMMI) cation was introduced which is more stable to alkaline conditions, and its salts exhibit improved thermal stability.36 Again, the existence of two crystalline forms of BMMI chloride37 with different conformations along the C(α)−C(β) bond of the butyl chain, one gauche and one trans (Figure 1), confirmed that the potential energy surface contained at least two local conformational energy minima. However, a number of these low-melting salts have been crystallized and their structures were determined by X-ray diffraction.38,39 In this work, single crystal structures of eight new BMMI salts (Chart 1) were determined, and the butyl group conformations are discussed. In addition, hydrogen bond interactions between the BMMI cations and the anions were observed and

INTRODUCTION Ionic liquids (ILs) are salts exhibiting liquidity below a given temperature, conventionally taken to be 100 °C. Early reports on low-melting pyridinium salts date back to the nineteenth century, although they did not yet use the term IL.1,2 The concept of “molten salts” emerged with the discovery of liquid quaternary ammonium salts.3 In recent years, ILs based on imidazolium,4−6 guanidinium,7 phosphonium,8 pyrrolidinium,9 triazolium cations,10,11 or eutectic mixtures12,13 have been developed. Their physicochemical properties have been reviewed.14−19 Ultimately, N-methylimidazolium-based salts have found the most widespread utilization, mostly containing butyl or, less common, ethyl, hexyl, or octyl substituents. Efficient syntheses have been described.20−22 In the previously reported polymorphic structures of 1-butyl-3-methylimidazolium (BMI) chloride,23−27 different conformations of the butyl group have been observed. Experimental information on these conformers has also been gained by Raman24,28 and IR29 spectroscopy. Cation−cation interactions in the liquid have been established by quantitative NOE measurements.30 Effects of alkyl chain length and anion size have been studied by molecular dynamics simulation,31 and ion pair © 2012 American Chemical Society

Received: October 27, 2011 Revised: January 27, 2012 Published: February 21, 2012 1838

dx.doi.org/10.1021/cg201424m | Cryst. Growth Des. 2012, 12, 1838−1846

Crystal Growth & Design

Article

from subcooled melt at 5 °C; mp 43 °C; IR (neat, cm−1): 3129, 2960, 2934, 2874, 2073, 1601, 1538, 1463, 1318, 1252, 1135, 1036. 1-Butyl-2,3-dimethylimidazolium Nonachlorodititanate(IV), 4. Prepared from BMMI Cl and TiCl4 (2 equiv) in CH2Cl2; crystals were obtained from the solution at −18 °C; IR (neat, cm−1): 3144, 2959, 2871, 1590, 1538, 1464, 1380, 1344, 1269, 1237, 1180, 1137, 743, 667. Bis(1-butyl-2,3-dimethylimidazolium) Hexachlorocerate(IV), 5. Obtained from BMMI Cl and CeCl4 in acetone/EtOH (1:1); crystals were obtained from the solution at −18 °C; IR (neat, cm−1): 3129, 3105, 2962, 1608, 1588, 1537, 1463, 1415, 1374, 1259, 1138, 1088, 1016, 796, 772, 754, 667, 545. 1-Butyl-2,3-dimethylimidazolium Chlorocyanocuprate(I), 6. Prepared from BMMI Cl and CuCN in CH3CN; crystals were obtained from the solution at −18 °C; IR (neat, cm−1): 3076, 2958, 2933, 2872, 2114, 1587, 1538, 1464, 1420, 1380, 1340, 1251, 1136, 756, 668. 1-Butyl-2,3-dimethylimidazolium Chloride/Cyanide (2:1), 7. Obtained from BMMI Cl and NaCN in acetone by incomplete metathesis; crystals were obtained from subcooled melt at ambient temperature. IR spectra showed the presence of cyanide. 1-Butyl-2,3-dimethylimidazolium Azide/Chloride (7:3), 8. Obtained from BMMI Cl and NaN3 in acetone by incomplete metathesis; crystals were obtained from subcooled melt at ambient temperature. IR spectra showed the presence of azide. Quantum Chemical Calculations. Geometry optimization calculations were performed for the 1-butyl-2,3-dimethylimidazolium (BMMI) cation and anions. The DFT (B3LYP) method was used with DZVP2 basis sets.44,45 The Stuttgart relativistic, small-core effective core potential (Stuttgart RSC 1997 ECP)46 was used for the Ce atom. The normal-mode analysis was performed for optimized structures of ions to confirm that these are energy minima structures. Molecular volumes were estimated based on the electron density isosurface with 0.001 au. X-ray structures and optimized structures were used for molecular volume calculations. All quantum chemical calculations were performed with the Gaussian 03 program package.47 The volume-based lattice potential energy Upot (kJ mol−1), enthalpy ΔHL (kJ mol−1), entropy ΔSL (J K−1 mol−1), and free energy ΔGL (kJ mol−1) were calculated using the following eqs 1−4,42,48,49

Figure 1. (a) Synclinal (gauche) and (b) antiperiplanar (trans) conformation along the C(α)−C(β) bond of the butyl chain in Fischer projection; (c) perpendicular and (d) twisted orientation of the C(α)−C(β) bond with respect to the imidazolium ring plane.

Chart 1. 1-Butyl-2,3-dimethylimidazolium Salts

are described in detail. This study aims to provide molecularbased insight into this interplay and to contribute to the increasing understanding of these IL-relevant imidazolium salts.40,41 The volume-based lattice enthalpy and entropy were calculated using the density functional theory (DFT) method as described previously for the hexafluorophosphates of several 1-alkyl-3-methylimidazolium salts.42

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EXPERIMENTAL SECTION

All chemicals and solvents were purchased from Sigma-Aldrich. Optimized procedures for the synthesis of BMMI chloride37,43 allowed the preparation of other BMMI-based ILs by ion metathesis or treatment with Lewis acids as described below. NMR spectra were recorded on a Bruker AC 300 spectrometer using tetramethylsilane as reference, and mass spectra were measured with a Finnigan MAT 95 spectrometer. In all cases, the presence of the BMMI cation was confirmed. IR spectra were measured with a Bruker IFS25 FTIR spectrometer. X-ray diffraction intensity data were collected using a Nonius Kappa CCD or a Stoe IPDS-II diffractometer with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). The structures were solved with direct methods (SHELXS-97, SIR2002) and refined against F2 (SHELXL-97). Hydrogen atoms were added geometrically and, in most cases, refined using a riding model. All non-hydrogen atoms were refined with anisotropic displacement parameters. 1-Butyl-2,3-dimethylimidazolium Azide, 1. This nonexplosive low-melting azide was prepared from BMMI Cl and NaN3 in acetone. Suitable crystals were obtained from the subcooled melt at ambient temperature; mp 41 °C; IR (neat, cm−1): 3296, 3077, 2963, 2931, 2878, 1997, 1587, 1540, 1464, 1419, 1372, 1274, 1135, 790, 754, 667. 1-Butyl-2,3-dimethylimidazolium Thiocyanate, 2. Prepared from BMMI Cl and NaSCN in acetone; crystals were obtained from subcooled melt at 0 °C; mp 40 °C; IR (neat, cm−1): 3134, 3109, 2955, 2933, 2872, 2054, 2045, 1589, 1541, 1463, 1416, 1379, 1252, 1191, 1137, 1034, 767, 754, 736, 662. 1-Butyl-2,3-dimethylimidazolium Propynoate, 3. Prepared from BMMI Cl and Na propynoate in MeOH; crystals were obtained

⎛ χ ⎞ Upot = |z+||z−|(p + q)⎜ 3 + δ⎟ ⎝ V ⎠

(1)

⎡ ⎛n ⎞ ⎛n ⎞⎤ ΔHL = Upot + ⎢p⎜ M − 2⎟ + q⎜ X − 2⎟⎥RT ⎠ ⎝ 2 ⎠⎦ ⎣ ⎝ 2

(2)

ΔSL = 1360V + 15

(3)

ΔG L = ΔHL − T ΔSL

(4)

where z+ and z− are formula charges of the cation and the anion, respectively, and p and q are the numbers of cations and anions in the salt MpXq. V is the molecular volume (nm3) and coefficients χ = 117.3 kJ mol−1 nm and δ = 51.9 kJ mol−1 were used for MX salts; χ = 165.3 kJ mol−1 nm and δ = −29.8 kJ mol−1 were used for M2X salts.48 The parameters nM and nX describe the degrees of freedom. The parameters ni = 3, 5, and 6 (i = M or X) were used for monatomic ions, linear polyatomic ions, and nonlinear polyatomic ions, respectively. In the special cases of 7 and 8, which possess mixed anions, the relationship q = qX1 + qX2 was applied, where x1 and x2 denote components of anions. Hence, in eq 2 the expression qX1(nX1/2 − 2) + qX2(nX2/2 − 2) was used for 7 and 8. The molecular volumes were calculated as the sum of the BMMI cation volume (V+) and the anion volume (V−), i.e., V = pV+ + qV−. On the other hand, experimental molecular (formula unit) volumes were obtained from unit cell parameters by eq 5, Vexp =

abc 1 − cos2 α − cos2 β − cos2 γ + 2 cos α cos β cos γ Z

(5) 1839



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Table 1. Crystallographic Data and Structure Refinement Details of Compounds 1−8

molecular formula CCDC no. Mr color, habit crystal size, mm crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z Dcalc, g cm−3 μ, mm−1 F(000) diffractometer T, K θ range, deg index ranges

reflections collected independent reflns

BMMI N3 (1)

BMMI SCN (2)

BMMI C3HO2 (3)

BMMI Ti2Cl9 (4)

C9H17N5 846832 195.28 colorless prism 0.28 × 0.2 × 0.1 monoclinic P21/n 8.723(2) 11.689(2) 11.205(2) 90 106.56(2) 90 1095.1(4) 4 1.184 0.077 424 Stoe IPDS-II

C10H17N3S 846833 211.33 colorless plate 0.4 × 0.2 × 0.1

C9H17Cl9N2Ti2 281561 568.10 yellow prism 0.2 × 0.1 × 0.08

orthorhombic P212121 10.5732(11) 13.739(2) 16.5878(18) 90 90 90 2409.6(5) 8 1.165 0.238 912 Stoe IPDS-II

C12H18N2O2 846834 222.28 colorless plate 0.4 × 0.3 × 0.02 monoclinic P21/c 7.1066(15) 20.285(3) 9.274(2) 90 110.17(2) 90 1254.9(4) 4 1.177 0.081 480 Stoe IPDS-II

173(2) 2.6−27.0 −11 < h < 10 −14 < k < 14 −14 < l < 14 8089 2323 (Rint = 0.040) 1801 2323/0/131

173(2) 1.0−27.4 −13 < h < 13 −17 < k < 17 −21 < l < 20 18308 5129 (Rint = 0.079) 3622 5129/0/259

reflns with I > 2σ(I) data/restraints/ parameters goodness-of-fit on F2 1.02 final R indices R1 = 0.041 [I > 2σ(I)] wR2 = 0.093 R indices R1 = 0.060 (all data) wR2 = 0.101 Δρmax, Δρmin (e Å−3) 0.13, −0.14

BMMI2 CeCl6 (5)

BMMI CuClCN (6)

BMMI CN/Cl (7)

C10H17ClCuN3 846836 278.26 colorless prism 0.20 × 0.20 × 0.16 orthorhombic Pbca 9.1026(9) 13.6901(17) 20.269(2) 90 90 90 2525.8(5) 8 1.463 1.914 1152 Stoe IPDS-II

C9.33H17Cl0.67N2.33 281560 185.55 colorless prism 0.3 × 0.2 × 0.2

173(2) 1.0−27.4 −9 < h < 9 −25 < k < 23 −11 < l < 11 9160 2659 (Rint = 0.064) 1528 2659/0/153

triclinic P1̅ 8.8562(5) 9.4412(4) 13.3806(6) 75.251(3) 86.185(3) 86.496(3) 1078.40(9) 2 1.750 1.850 564 Nonius KappaCCD 233(2) 1.0−25.0 −10 < h < 10 −11 < k < 11 −15 < l < 14 6291 3790 (Rint = 0.022) 3257 3790/0/293

C18H34CeCl6N4 846835 659.31 yellow plate 0.20 × 0.12 × 0.02 monoclinic P21/c 10.071(5) 15.568(5) 17.737(5) 90 92.127(5) 90 2779.0(18) 4 1.576 2.227 1320 Stoe IPDS-II 173(2) 1.7−27.2 −12 < h < 12 −19 < k < 19 −22 < l < 22 18494 5814 (Rint = 0.089) 2793 5814/21/349

1.07 R1 = 0.067

1.03 R1 = 0.057

1.06 R1 = 0.029

wR2 = 0.124 R1 = 0.104

wR2 = 0.108 R1 = 0.116

wR2 = 0.137 0.34, −0.35

wR2 = 0.131 0.18, −0.20

BMMI N3/Cl (8)

173(2) 2.0−27.3 −10 < h < 11 −17 < k < 17 −25 < l < 25 17946 2701 (Rint = 0.039) 2135 2701/0/140

trigonal R3̅ 28.3294(3) 28.3294(8) 7.2009(5) 90 90 120 5004.9(4) 18 1.108 0.221 1812.2 Nonius KappaCCD 233(2) 1.0−24.0 0 < h < 32 −32 < k < 27 −8 < l < 8 8594 1761 (Rint = 0.033) 1417 1761/0/131

C9H17Cl0.3N4.1 281559 193.30 colorless prism 0.25 × 0.15 × 0.05 trigonal R3̅ 28.2470(10) 28.247(2) 7.301(2) 90 90 120 5045.0(14) 18 1.145 0.142 1886.4 Nonius KappaCCD 233(2) 1.0−22.0 0 < h < 29 −29 < k < 25 −7 < l < 7 6958 1368 (Rint = 0.045) 1016 1368/0/130

1.02 R1 = 0.056

1.05 R1 = 0.042

1.05 R1 = 0.056

1.07 R1 = 0.063

wR2 = 0.061 R1 = 0.037

wR2 = 0.070 R1 = 0.084

wR2 = 0.096 R1 = 0.057

wR2 = 0.147 R1 = 0.072

wR2 = 0.157 R1 = 0.085

wR2 = 0.065 0.29, −0.32

wR2 = 0.142 0.60, −0.64

wR2 = 0.103 0.60, −0.36

wR2 = 0.160 0.37, −0.25

wR2 = 0.174 0.17, −0.15

where, a, b, c are unit cell dimensions, α, β, γ are unit cell angles, and Z is the number of ion pairs per unit cell.48

short centroid−centroid distances due to π−π interactions in 1, 3, 6, 7, and 8, which is surprisingly frequent. Although π-stacking in the BMI and BMMI series is not unheard of (it is observed in BMMI Cl37,38 and BMMI Br51), it is not all too common, either. The conformation of the butyl group in BMMI cations is of interest because the coexistence of conformers26 may impede crystallization.28 Not all possible conformers have been observed so far; some might be too unstable to exist in real crystals. Theoretical calculations of the isolated cation have predicted that two major conformers are to be expected.28,52 In fact, the butyl chain is capable of adopting several different conformations in the solid state, even giving rise to concomitant polymorphs.37 The pertinent torsion angles are not constrained to a narrow range. In particular, the twist angle of the C(α)−C(β) bond out of the plane of the imidazole ring ranges from 57° to 90°, whereas the torsion angle along the C(α)−C(β) bond determines the overall conformation: 63°to 97° (g) and 170° to 179° (t). In contrast, the torsion angle C(α)−C(β)−C(γ)−C(δ) preferably adopts values between 169° and 179° (t). However, in a disordered structure

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RESULTS AND DISCUSSION Molecular Conformation and Supramolecular Aggregation. The crystallization of alkylimidazolium salts, which are often liquid at temperatures below 100 °C, requires patience50 and a certain amount of serendipity as a result of obstructed crystal growth by a flexible alkyl chain. Single crystals were obtained under different conditions, either from cooled solutions or from subcooled melts, sometimes as fortunate incidents. These salts crystallized in triclinic, monoclinic, orthorhombic, and trigonal space groups. The structures reported herein are centrosymmetric, with the exception of compound 2. Crystallographic data and structure refinement details of compounds 1−8 are summarized in Table 1. Packing diagrams and ion interactions are shown in Figure 2. Five structures exhibit stacking of the aromatic imidazole systems, indicated by parallel alignment of pairs of cations with 1840



Article

Figure 2. Packing diagrams of (a) BMMI azide 1, (b) BMMI thiocyanate 2, (c) BMMI propynoate 3, (d) BMMI nonachlorodititanate 4, (e) BMMI2 hexachlorocerate 5, (f) BMMI chlorocyanocuprate 6, and (g) BMMI azide/chloride 8. Short interionic contacts are indicated by dashed lines.

short C−H···N and C−H···S contacts to the anions (Figure 2b). The butyl groups adopt tt (C(α)−C(β) antiperiplanar) conformations.

such as 5, this angle may take up exceptional values covering a wider range than predicted (Table 2). Ion−ion interactions are of relevance for the stabilization of the supramolecular arrangement. For the BMMI cation, only CH···X hydrogen bonds are feasible. In our analysis, distances shorter than the sum of van der Waals radii53,54 were accepted as legitimate hydrogen bonds, although this may be too restrictive a criterion for weak interactions.55,56 Not only are imidazole ring protons involved in the formation of close contacts, but also interactions between methylene and methyl groups of the alkyl chain and the anion are visible. Conformational data of the butyl group in compounds 1−8 are summarized in Table 2, and hydrogen bond parameters (distances and angles) in Table 3. 1-Butyl-2,3-dimethylimidazolium Azide, 1. The azide ion is almost linear with an N−N−N angle of 179°, and N−N bond lengths are 1.186(2) and 1.164(2) Å. An imidazolium ring centroid distance of 3.44 Å (interplanar distance 3.39 Å) was found. Each cation forms eight short contacts to seven azide ions (Figure 2a). The butyl group adopts the g′t (C(α)−C(β) synclinal) conformation. 1-Butyl-2,3-dimethylimidazolium Thiocyanate, 2. The two independent thiocyanate ions are almost linear, S−C−N angles are 178° and 179°. The two independent cations form

Table 2. Conformational Data of BMMI Salts 1−8 torsion angles (deg) compound BMMI N3 (1) BMMI SCN (2)

C(β)oop

82.8 80.7 77.3a BMMI C3HO2 (3) 89.9 BMMI Ti(IV)2Cl9 68.2 (4) (56.7)b BMMI2 Ce(IV)Cl6 80.9 (5) (79.3, 80.6)a,c BMMI Cu(I) 90.0 ClCN (6) BMMI CN/Cl (7) 87.2 BMMI N3/Cl (8) 85.3 a

C(α)−C(β) −64.0 −176.0 178.6a −175.1 84

−175.8 177.7 176.7a −179.2 −178.3

g′t tt tta tt gt

(−97)b −62.8

(−171)b 179.2

(g′t)b g′t

(−175, 69)a,c (−71, 81)a,c −169.6 177.7

(tg′, gg)a,c tt

−176.1 −175.1

tt tt

Two independent molecules. disordered. 1841

C(β)−C(γ) conformation

b

173.2 168.8

Disordered.

c

Second molecule



Article

Table 3. Hydrogen Bond Parameters in BMMI Salts (Å, deg) BMMI Azide 1 D−H···A N−CH3···N4 C(α)−H···N6a C(δ)−H···N4b C(δ)−H···N4c C2−CH3···N4d N−CH3···N4d C4−H···N6e C5−H···N4f D−H···A NA−CH3···S1g C(α)A−H···N4h C(α)A−H···N14i C5A−H···N14i C2A−CH3···S1 C4B−H···N14j NB−CH3···N14 C5B−H···N4i NB−CH3···S2j C2B−CH3···S2k D−H···A C(α)−H···O1 C2−CH3···O1 C4−H···O2l 2−CH3···O2m N−CH3···O2m N−CH3···O2n C5−H···O1o C5−H···O2o C−H···O1p

H···A

BMMI2 Hexachlorocerate 5 D···A

2.71 3.596 2.51 3.452 2.69 3.662 2.65 3.575 2.73 3.696 2.58 3.557 2.45 3.241 2.42 3.264 BMMI Thiocyanate 2 H···A 2.99 2.75 2.59 2.61 2.98 2.70 2.64 2.58 2.86 2.96 BMMI Propynoate H···A

D−H···A

D−H···A

151 158 175 158 169 174 141 148

NB−CH3···Cl5u C2B−CH3···Cl4v C5B−H···Cl5t C4B−H···Cl2t C(γ)B−H···Cl2v

D···A

D−H···A

3.914 3.705 3.431 3.345 3.588 3.634 3.584 3.388 3.837 3.797 3

159 162 143 134 121 167 162 143 173 144

D···A

2.39 3.379 2.66 3.623 2.27 3.215 2.66 3.597 2.31 3.276 2.34 3.293 2.56 3.369 2.62 3.447 2.16 3.100 BMMI Nonachlorodititanate 4

D−H···A

D···A

D−H···A

C2−CH3···Cl4q N−CH3···Cl4r

2.89 2.73

3.749 3.680

149 166

C4−H···Cl2r

2.66

3.568

162

2.88 3.788 BMMI2 Hexachlorocerate 5

154

D−H···A

H···A

D···A

D−H···A

C2A−CH3···Cl4 NA−CH3···Cl1d C5A−H···Cl5t

2.88 2.80 2.94

3.818 3.767 3.770

161 171 147

D···A

2.66 3.558 2.71 3.685 2.91 3.855 2.74 3.707 2.55 3.492 BMMI Chloride/Cyanide 7 H···A

C5−H···Cl C(α)−H···Claa C2−CH3···Clbb C2−CH3···Clcc C2−CH3···Claa N−CH3···Clbb N−CH3···Clcc C4−H···N4dd C5−H···C12 C(α)−H···C12 C(α)−H···N4ee C2−CH3···C12ff C2−CH3···N4ff N−CH3···N4ff N−CH3···C12bb

D···A

2.81 3.780 2.94 3.500 2.77 3.654 2.87 3.728 2.92 3.761 BMMI Chlorocyanocuprate 6 H···A

C5−H···Cl C2−CH3···NCw N−CH3···Clx N−CH3···Cly C4−H···Clz

174 169 172 161 169 164 143 145 170

H···A

C(α)−H···Cl7s

D−H···A

D−H···A

D−H···A

H···A

D···A

2.69 3.534 2.77 3.747 2.87 3.835 2.83 3.588 2.88 3.830 2.76 3.722 2.84 3.626 2.48 3.32 2.62 3.45 2.86 3.71 2.71 3.69 2.82 3.51 2.72 3.48 2.59 3.42 2.82 3.78 BMMI Chloride/Azide 8

D−H···A 170 117 155 150 144 D−H···A 159 176 162 171 171 D−H···A 150 172 175 136 167 173 138 149 147 145 172 129 136 144 173

D−H···A

H···A

D···A

D−H···A

C2−CH3···Cl C(α)−H···Clgg C2−CH3···Clgg C2−CH3···Clhh N−CH3···Clhh C5−H···Clii C2−CH3···N6 N−CH3···N6 C(α)−H···N6gg C2−CH3···N6hh C5−H···N4ii C4−H···N6jj

2.87 2.68 2.85 2.87 2.73 2.82 2.68 2.40 2.74 2.73 2.50 2.19

3.591 3.645 3.804 3.840 3.663 3.670 3.41 3.279 3.607 3.65 3.35 3.05

132 170 170 176 163 151 133 150 149 160 150 152

Symmetry operators: 3/2 − x, 1/2 + y, 3/2 − z. b−1/2 + x, 1/2 − y, 1/2 + z. c2 − x, 1 − y, 2 − z. d2 − x, 1 − y, 1 − z. e−1/2 + x, 1/2 − y, −1/ 2 + z. f−1 + x, y, z. g1/2 + x, 3/2 − y, 1 − z. h−1/2 + x, 3/2 − y, 1 − z. i3/2 − x, 1 − y, −1/2 + z. j−1/2 + x, 1/2 − y, 1 − z. k1/2 + x, 1/2 − y, 1 − z. l1 − x, −y, −z. m−x, −y, −z. nx, y, −1 + z. o1 + x, y, z. px, 1/2 − y, 1/2 + z. q1 − x, 1 − y, 1 − z. r−x, 1 − y, 1 − z. s−x, 2 − y, 1 − z. tx, 1/2 − y, −1/2 + z. u−1 + x, 1/2 − y, −1/2 + z. v1 − x, 1/2 + y, 1/2 − z. w1 − x, −1/2 + y, 3/2 − z. x3/2 − x, −1/2 + y, z. y1 − x, −y, 2 − z. z 1/2 + x, 1/2 − y, 2 − z. aax, y, 1 + z. bb5/3 − x, 4/3 − y, 4/3 − z. ccy, 1 − x + y, 1 − z. dd5/3 − x, 4/3 − y, 1/3 − z. eex, y, 1 + z. ffy, 1 − x + y, 1 − z. ggx − y, x, 1 − z. hh2/3 − x + y, 1/3 − x, 1/3 + z. iix − y, x, −z. jj2/3 − x + y, 1/3 − x, −2/3 + z. a

1-Butyl-2,3-dimethylimidazolium Propynoate, 3. Each BMMI cation coordinates with five anions, whereas each propynoate anion accepts eight hydrogen bonds from the five surrounding cations and acts as a C−H donor to the O atom of another anion (Figure 2c), thus creating a zigzag chain. The butyl group adopts the tt conformation. A ring centroid distance of 3.52 Å (interplanar distance 3.42 Å) was observed.

1-Butyl-2,3-dimethylimidazolium Nonachlorodititanate(IV), 4. The BMMI cation displays positional disorder and was refined with occupancies of 0.6:0.4. The butyl groups adopt gt and g′t conformations, respectively. The major component of the cation coordinates to terminal and bridge Cl atoms of three anions. The Ti−Cl(bridge) bond lengths range from 2.47 to 2.52 Å, and the Ti−Cl(terminal) distances from 2.19 to 2.23 Å. 1842



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Table 4. Comparison of Anion Volumes (V−) by DFT Calculations and References anion −

N3 SCN− HC3O2− Ti(IV)2Cl9− Ce(IV)Cl62− Cu(I)ClCN− CN− Cl−

Table 6. Molecular Volumes of BMMI Salts by Calculation and Experimental Data

V−,cra/nm3

V−,optb/nm3

V−,refc/nm3

BMMI salt

Vcalca/nm3

Vexpb/nm3

0.051 0.075 0.091 0.323 0.219 0.109 0.049 0.046

0.063 0.080 0.093 0.320 0.235 0.103d 0.049 0.046

0.058 ± 0.014 0.071 ± 0.003

BMMI N3 1 BMMI SCN 2 BMMI HC3O2 3 BMMI Ti(IV)2Cl9 4 BMMI2 Ce(IV)Cl6 5 BMMI Cu(I)ClCN 6 BMMI CN/Cl 7c BMMI N3/Cl 8d

0.284 0.300 0.313 0.541 0.676 0.323 0.267 0.279

0.274 0.301 0.314 0.539 0.695 0.316 0.278 0.280

0.255 0.050 ± 0.006 0.047 ± 0.013

a

DFT calculation with crystal structure data. bDFT calculation with optimized structures at same level of theory. cExperimental values from Table 5 of ref 48. dEstimated value from V([Cu(I)Cl(CN)2]2−) − V(CN−).

DFT calculation with optimized structures, V+ = 0.221 nm3 is used for BMMI cation volume. bMolecular volumes from unit cell parameters by eq 5. cqCN = 0.33 and qCl = 0.67 are used for each anion component. d qN3 = 0.7 and qCl = 0.3 are used for each anion component.

Table 5. Relative Energy (ΔE) and Molecular Volume (V+) of BMMI Cation Conformers

cation A displays weak interactions with three anions, the cation B with four anions. Both centrosymmetric anions show short contacts to H atoms of the methyl and butyl groups as well as to the imidazole ring H atoms (Figure 2e). The Ce(IV)−Cl bond lengths range from 2.59 to 2.64 Å, which is in reasonable agreement with the literature57 value of 2.55 Å. 1-Butyl-2,3-dimethylimidazolium Chlorocyanocuprate(I), 6. A centroid distance of 3.51 Å between the cationic rings (interplanar distance 3.46 Å) was found. The butyl group adopts the tt conformation. The anion consists of an infinite polymeric chain with the pertinent angles CN···Cu···CN 134.0(1)°, N···Cu···Cl 110.3(1)°, and C···Cu···Cl 115.7(1)°; bond lengths Cu−Cl 2.3145(10), Cu−C 1.896(3), Cu−N 1.916(3), and C−N 1.155(4) Å. The Cu(I) atom displays distorted trigonal-planar coordination as observed in similar adducts.58 The H atoms of the imidazole ring and the N−CH3 group interact with the chloride ions, and the C−CH3 group with the N atom of the cyanide ion. The interaction of the cation with the coordination polymer is shown in Figure 2f. 1-Butyl-2,3-dimethylimidazolium Cyanide/Chloride (1:2), 7. The structure was refined as space group R3̅ with 18 molecules in the unit cell, conforming to the trigonal polymorph of the pure BMMI chloride.37 The two methyl groups, C(α)H of the butyl group, and the imidazole ring H atoms engage in short contacts with chloride ions. The butyl group adopts the tt conformation. A distance of 3.48 Å between the ring centroids (interplanar distance 3.43 Å) was observed. 1-Butyl-2,3-dimethylimidazolium Chloride/Azide (3:7), 8. The structure is isomorphic to compound 7, with a ring centroid distance of 3.51 Å (interplanar distance 3.50 Å). Although the azide anions are in excess, the salt crystallized in the space group of the trigonal polymorph of the pure BMMI chloride, not in the monoclinic system of the pure BMMI azide 1. Obviously, the tt conformation of the butyl group here compelled this arrangement as in the structure of the chloride. Part of the crystal packing in the domain 0 < z < 1/2 is depicted in Figure 2g. Conformational data of the related salts BMMI Cl, BMMI Br, BMMI I, BMMI Br2I, BMMI HSO4, BMMI BF4, BMMI PF6, BMMI SbF6, BMMI Fe(II,III)Cl4, BMMI (C6F5)2N, and BMMI CH6B11Cl6 are collected in the Supporting Information. Quantum Chemical Calculations. Anion volumes by DFT calculations from crystal structures (Vcr) and optimized structures (Vopt) are listed in Table 4. For comparison, some anion volumes from X-ray data of inorganic salts48 are also included. The calculated anion volumes are in good agreement with experimental data, indicating the validity of the cal-

conformer

ΔEa/kJ mol−1

V+b/nm3

tt g′t gt tg′ tg g′g′ gg gg′ g′g

0.0 2.3 2.6 3.0 3.4 6.2 7.0 12.3 12.7

0.218 0.227 0.216 0.228 0.224 0.206 0.223 0.227 0.215

a

a

The energy is related to the lowest energy conformer (tt) and includes zero-point energy correction. bThe average value is 0.221 ± 0.006 nm3.

Figure 3. Correlation of molecular volumes from unit cell parameters (Vexp) with those from DFT calculations (Vcalc). Full circles and the solid line show DFT calculation with optimized structures (R2 = 0.9974); open squares and dashed line show DFT calculation with coordinates from X-ray structure analysis (R2 = 0.9941).

The structure of the new BMMI nonachlorodititanate(IV) is depicted in Figure 2d. Bis(1-butyl-2,3-dimethylimidazolium) hexachlorocerate(IV), 5. There are two independent cations in the asymmetric unit; one of them is disordered with a 0.5:0.5 ratio. The butyl group of the ordered cation exhibits g′t conformation, whereas tg′ and gg are observed in the disorder components. The 1843



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Table 7. Lattice Potential Energies (Upot), Enthalpies (ΔHL), Entropies (ΔSL), and Free Energies (ΔGL) of BMMI Saltsa from Molecular Volumes with Optimized Structures and X-ray Data Upot/kJ mol−1

a

ΔHL/kJ mol−1

ΔSL/J K−1 mol−1

ΔGL/kJ mol−1

BMMI salt

calc

exp

calc

exp

calc

exp

calc

exp

BMMI N3 1 BMMI SCN 2 BMMI HC3O2 3 BMMI Ti(IV)2Cl9 4 BMMI2 Ce(IV)Cl6 5 BMMI Cu(I)ClCN 6 BMMI CN/Cl 7b BMMI N3/Cl 8c

458.1 451.7 446.9 390.6 944.5 443.4 465.1 460.3

465.1 453.8 449.1 392.0 941.0 448.3 463.2 462.3

461.9 455.5 451.8 395.6 951.9 448.3 467.2 463.3

468.8 457.5 454.0 397.0 948.4 453.3 465.3 465.3

409.7 431.9 449.9 759.3 951.2 463.5 387.2 402.4

387.4 424.6 441.7 748.3 960.0 444.4 393.1 396.2

339.8 326.8 317.7 169.3 668.4 310.2 351.8 343.4

353.4 330.9 322.4 174.0 662.3 320.8 348.1 347.2

At T = 298 K. bqCN = 0.33 and qCl = 0.67 were used for the anion components. cqN3 = 0.7 and qCl = 0.3 were used for the anion components.

Figure 4. Normalized Hirshfeld surface of the BMMI cation and associated fingerprint plot highlighting the CH···N interactions in 1. The full fingerprint appears as a gray shadow.

similar. In the crystal structures, g′t conformation is adopted in the salts 1 and 4, and tt conformation in the salts 2, 3, and 6−8. The salt 5 (where the butyl group is disordered) exhibits the three conformations g′t, tg′, and g′g′. As expected, the most stable conformer tt appears most frequently (Table 5). Figure 3 shows the satisfactory correlation of molecular volumes obtained from X-ray experiments (Vexp) and from DFT calculations (Vcalc). Calculated molecular volumes are smaller than those derived from experimental unit cell parameters. This can be understood as the former method uses the molecular shape, whereas the latter is just the formal volume derived from cell volume divided by the number of ion pairs which includes voids between molecules in addition to the actual molecular volume. Thus, the latter is larger than the former. From this point, the excellent quantitative agreement is somewhat accidental. Of course, calculated molecular volumes may depend on the calculation method and basis sets used. However, this agreement indicates that the calculated molecular volume with optimized structures is quite useful to predict molecular volumes and related energies such as the lattice potential energy. Salts 7 and 8, which have mixed anions, are also in good agreement with experimental formal molecular volumes. A summary of molecular volumes by DFT calculations from optimized structures and crystal data is shown in Table 6. Table 7 compiles calculated lattice potential energies, enthalpies, entropies, and free energies based on these molecular volumes. Hirshfeld Surface Analysis. Electrostatic potentials mapped on Hirshfeld surfaces59 provide direct insight into and quantitative analysis of intermolecular interactions in crystals.60,61

Table 8. Percent Contributions of Selected Cation−Anion Interactions Relative to the Whole Hirshfeld Surface compound

C−H ···H

C−H ···N

1 2A 2B 3 6 7 8

70.7 69.6 68.7 65.3 63.1 74.2 70.2

20.9 7.1 10.1 6.4 7.2 16.8

C−H ···C 5.8 7.8 12.9 8.8 9.2

C−H ···Cl

C−H ···O

C−H ···S 9.9 6.0

13.2 9.7 2.1 4.2

culations. For the BMMI cation, it is expected that the molecular volumes depend on the conformations of the butyl group. The conformers are named according to the two gauche (g and g′) and trans (t) torsion angles, N1−C(α)−C(β)−C(γ) and C(α)−C(β)−C(γ)−C(δ), in the butyl group. Enantiomers were ignored in this study; that is, the conformer with a negative twist angle of the C(α)−C(β) bond with respect to the ring plane was assumed to be equal to the conformer with the positive angle. For the estimation of molecular volumes, the molecular volumes for nine possible conformers of the BMMI cation by the combinations of three conformations of two bonds were calculated (Table 5), and the average value was used as the cation volume. The average value is 0.221 ± 0.006 nm3, and the minimum and maximum values are 0.206 nm3 for the g′g′ conformer and 0.228 nm3 for the tg′ conformer, respectively. Reichert et al. found the BMMI cation volume as Vexp = 0.219 and Vcalc = 0.221,42 and our results are quite 1844



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(13) Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K. J. Am. Chem. Soc. 2004, 126, 9142. (14) Bonhote, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Grätzel, M. Inorg. Chem. 1996, 35, 1168. (15) Xu, W.; Cooper, E. I.; Angell, C. A. J. Phys. Chem. B 2003, 107, 6170. (16) Chiappe, C.; Pieraccini, D. J. Phys. Org. Chem. 2005, 18, 275. (17) Handy, S. Curr. Org. Chem. 2005, 9, 959. (18) Anthony, J. L.; Brennecke, J. F.; Holbrey, J. D.; Maginn, E. J.; Mantz, R. A.; Rogers, R. D.; Trulove, P. C.; Visser, A. E.; Welton, T. Ionic Liquids in Synthesis, 2nd ed.; Wasserscheid, P.; Welton, T., Eds.; Wiley-VCH: Weinheim, 2006; pp 41−126. (19) Zhang, S.; Sun, N.; He, X.; Lu, X.; Zhang, X. J. Phys. Chem. Ref. Data 2006, 35, 1475. (20) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.V; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 1765. (21) Dzyuba, S. V.; Bartsch, R. A. J. Heterocycl. Chem. 2001, 38, 265. (22) Dupont, J.; Consorti, C. S.; Suarez, P. A. Z.; de Souza, R. F. Org. Synth. 2003, 79, 236. (23) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Johnston, S.; Seddon, K. R.; Rogers, R. D. Chem. Commun. 2003, 1636. (24) Hayashi, S.; Ozawa, R.; Hamaguchi, H. Chem. Lett. 2003, 32, 498. (25) Saha, S.; Hayashi, S.; Kobayashi, A.; Hamaguchi, H. Chem. Lett. 2003, 32, 740. (26) Ozawa, R.; Hayashi, S.; Saha, S.; Kobayashi, A.; Hamaguchi, H. Chem. Lett. 2003, 32, 948. (27) Kärkkäinen, J.; Asikkala, J.; Laitinen, R. S.; Lajunen, M. K. Z. Naturforsch. 2004, 59b, 763. (28) Berg, R. W.; Deetlefs, M.; Seddon, K. R.; Shim, I.; Thompson, J. M. J. Phys. Chem. B 2005, 109, 19018. (29) Paulechka, Y. U.; Kabo, G. J.; Blokhin, A. V.; Shaplov, A. S.; Lozinskaya, E. I.; Golovanov, D. G.; Lyssenko, K. A.; Korlyukov, A. A.; Vygodskii, Ya. S. J. Phys. Chem. B 2009, 113, 9538. (30) Mele, A.; Romano, G.; Giannone, M.; Ragg, E.; Fronza, G.; Raos, G.; Marcon, V. Angew. Chem., Int. Ed. 2006, 45, 1123. (31) Urahata, S. M.; Ribeiro, M. C. C. J. Chem. Phys. 2004, 120, 1855. (32) Hunt, P. A.; Gould, I. R.; Kirchner, B. Aust. J. Chem. 2007, 60, 9. (33) Hunt, P. A.; Kirchner, B.; Welton, T. Chem.Eur. J. 2006, 12, 6762. (34) Amyes, T. L.; Diver, S. T.; Richard, J. P.; Rivas, F. M.; Toth, K. J. Am. Chem. Soc. 2004, 126, 4366. (35) Handy, S. T.; Okello, M. J. Org. Chem. 2005, 70, 1915. (36) Fox, D. M.; Awad, W. H.; Gilman, J. W.; Maupin, P. H.; De Long, H. C.; Trulove, P. C. Green Chem. 2003, 5, 724. (37) Andre, M.; Loidl, J.; Laus, G.; Schottenberger, H.; Bentivoglio, G.; Wurst, K.; Ongania, K.-H. Anal. Chem. 2005, 77, 702 , Supporting Information. (38) Kölle, P.; Dronskowski, R. Inorg. Chem. 2004, 43, 2803. (39) Kölle, P.; Dronskowski, R. Eur. J. Inorg. Chem. 2004, 2313. (40) Weingärtner, H. Angew. Chem., Int. Ed. 2008, 47, 654. (41) Dean, P. M.; Pringle, J. M.; MacFarlane, D. R. Phys. Chem. Chem. Phys. 2010, 12, 9144. (42) Reichert, W. M.; Holbrey, J. D.; Swatloski, R. P.; Gutowski, K. E.; Visser, A. E.; Nieuwenhuyzen, M.; Seddon, K. R.; Rogers, R. D. Cryst. Growth Des. 2007, 7, 1106. (43) McCamley, K.; Warner, N. A.; Lamoureux, M. M.; Scammells, P. J.; Singer, R. D. Green Chem. 2004, 6, 341. (44) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Can. J. Chem. 1992, 70, 560. (45) Sosa, C.; Andzelm, J.; Elkin, B. C.; Wimmer, E.; Dobbs, K. D.; Dixon, D. A. J. Phys. Chem. 1992, 96, 6630. (46) Dolg, M.; Stoll, H.; Preuss, H.; Pitzer, R. M. J. Phys. Chem. 1993, 97, 5852. (47) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A. Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;

This method has been successfully applied to imidazolium salts.62−64 Figure 4 shows the Hirshfeld surface of the BMMI cation as an example in 1. Red areas mark the locations of directed interactions. From the associated fingerprint plot, it can be seen that polar C−H···N contacts contribute only one-fifth of the total interactions, and differences between di and de (distance from the surface to the nearest atom inside the surface and to the nearest atom external to the surface) are clearly recognized. The percent contributions of specific types of interactions are listed in Table 8. In all cases, nonpolar H···H contacts represent the majority of interactions. Detailed fingerprint plots for the other compounds are included in the Supporting Information. The analysis was not conducted for 4 and 5 because of disorder.

■

CONCLUSIONS In summary, the crystal structures of eight new 1-butyl-2, 3-dimethylimidazolium salts have been determined. The preferred conformations of the butyl group are tt and gt, but tg and gg were also observed. More than one conformer was present in disordered structures. Numerous hydrogen bonds between cations and anions were identified. Hirshfeld surface analysis revealed that nonpolar H···H contacts represent the majority of interactions. Calculated and experimental molecular volumes in the range from 0.27 to 0.70 nm3 agreed favorably, thus facilitating reliable predictions of volume-derived properties.

■

ASSOCIATED CONTENT

S Supporting Information *

Conformational data of known BMMI salts. Hydrogen bond parameters of BMMI chlorides. Crystallographic information files in CIF format. Hirshfeld fingerprint plots highlighting selected cation−anion interactions. This information is available free of charge via the Internet at http://pubs.acs.org/.

■

AUTHOR INFORMATION

Corresponding Author

*(H.S.) Phone: +43 512 507 5118. Fax: +43 512 507 2934. E-mail: [email protected]; Web: http:// www-c724.uibk.ac.at/aac/ (H-U.S.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Conformational Flexibility and CationâAnion Interactions in 1-Butyl-2,3

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