ARTICLE pubs.acs.org/crystal
Influence of the Flexibility of the Diimidazolium Cations on Their Organization into Crystalline Materials Loïc Leclercq† and Andreea R. Schmitzer* Department of Chemistry, Universite de Montreal, C.P. 6128 Succursale Centre-ville, Montreal, Quebec, H3C 3J7, Canada
bS Supporting Information ABSTRACT: Diimidazolium cations self-assemble into molecular networks by electrostatic interactions and hydrogen bonds. To explore the influence of the molecular flexibility on the crystalline networks, three alkylene spacers [(CH2)n, with n = 1, 2, or 3] have been investigated. Semiempirical PM6 methods were used to predict the most stable geometries of the dications and the formation of the H-bonds network. PM6 calculations provided a qualitative agreement with the obtained crystalline structures: The increase of the flexibility results in a more complex H-bond network. Surprisingly, a transition in the crystal organization occurs when n = 2. This transition represents a different organization from the classical packing of imidazolium salts (where the structure is hold together by ionic bonds and H-bonds), toward the formation of a well-defined supramolecular inclusion of bromide anions in the network only on one direction. For n = 3, novel architectures are formed by complementary π-stacking interactions that connect the diimidazolium cations together and by the inclusion of water molecules into the network.
’ INTRODUCTION Imidazolium salts are a novel class of compounds with interesting properties such as wide electrochemical windows, low melting points, negligible vapor pressure, nonvolatile, nonflammable, etc.1 Imidazolium-based ionic liquids have been described as “designed solvents” due to their tunable physicochemical properties by the judicious choice of the combination cations/ anions for a specific application.2 Therefore, imidazolium salts were used for many applications in various fields: for example, organic synthesis and catalysis,3 electrochemistry,4 biochemistry,5 and material engineering.6 To select or design imidazolium salts for a given application, it is essential to understand the structure that they adopt in the solid, liquid, and gas phase. Imidazolium cations possess preorganized structures mainly through H-bonds that induce structural directionality, contrary to classical salts where the aggregates are mainly formed through ionic interactions.7 Moreover, aromatic stacking can also occur as a secondary attractive interaction (i.e., imidazolium cations are electron-poor rings so the interaction between the π-electrons is usually weak; see Figure 1).7 In the solid state, imidazolium salts generally assemble in the same extended network of cations and anions.8 The cation is always surrounded by at least three anions, and each anion is surrounded by three cations (Figure 1a).7 This organization involves the H2, H4, and H5 H-bonding and the aromatic stacking r 2011 American Chemical Society
Figure 1. Intermolecular interactions in the imidazolium salts: (a) H-bonds (purple dotted lines) between H-bond donors (dark orange sphere) and H-bond acceptors (blue sphere) and ionic interactions (orange dotted lines); (b) aromatic stacking interaction (red dotted lines).
of the imidazolium cations (Figure 1b). We have recently reported that methylene or arene diimidazolium cations present interesting properties in solution,9 and they are versatile tectons that can be further decorated with functional groups to obtain various crystalline materials.9a In line with current interest in the development of new supramolecular crystalline structures in the Received: March 24, 2011 Revised: June 22, 2011 Published: June 24, 2011 3828
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Crystal Growth & Design Chart 1. Structure of the Diimidazolium Dibromide Salts Used in This Work and Torsion Angles Used To Illustrate Their Flexibilities
ARTICLE
Table 1. Predicted Bond Distances (Å), Bond Angles ( — , ), and Dihedral Angles (D, )a [M] imidazolium ring
[E]
[P]
A
B
A
B
A
B
N1C2
1.397
1.397
1.397
1.391
1.388
1.388
C2N3
1.372
1.372
1.377
1.377
1.379
1.379
N3C4 C4C5
1.405 1.389
1.405 1.389
1.401 1.392
1.401 1.392
1.400 1.394
1.400 1.393
C5N1
1.409
1.409
1.404
1.404
1.402
1.402
C2H2
1.092
1.092
1.091
1.091
1.091
1.091
C4H4
1.093
1.093
1.091
1.092
1.090
1.090
C5H5
1.087
1.087
1.086
1.086
1.086
1.085
N1CH2
1.481
1.481
1.476
1.476
1.481
1.480
N3CH3
1.466
1.466
1.466
1.466
1.466
— N1C2N3 — N3C4C5
107.83 107.79
107.83 107.78
0.25
DC2N3C4C5 0.22
DN1C2N3C4
107.87 107.57
1.466
107.82 107.67
107.81 107.67
0.21
0.11
0.09
0.13
0.12
0.17
0.13
0.11
0.07
0.06
ϕ1b
100.99
98.54
101.16
ϕ2c
100.05
98.63
103.66
107.86 107.56
Calculated using semiempirical PM6 method (MOPAC 2009). b ϕ1 = DC2N1CH2X of cycle A (X = N1 for [M][Br]2 or CH2 for [E][Br]2 and [P][Br]2. c ϕ2 = DC2N1CH2X of cycle B (X = N1 for [M][Br]2 or CH2 for [E][Br]2 and [P][Br]2. a
Figure 2. Energy as a function of ϕ1 and ϕ2 torsion angles for [M] diimidazolium cation and the two most stable conformers, A and B (calculated with PM6, MOPAC 2009).
field of molecular tectonics,10 we have studied the effect of the length of an alkylene spacer to explore the influence of the molecular flexibility on the H-bond networks. Three alkylene diimidazolium cations were investigated by molecular modeling and X-ray crystallography: N,N0 -(dimethylmethylene) diimidazolium ([M][Br]2), N,N0 -(dimethylethylene)diimidazolium ([E][Br]2), and N,N0 -(dimethyl-propylene)diimidazolium dibromides ([P][Br]2) (Chart 1).
’ RESULTS AND DISCUSSION Prediction of the H-Bond Network by Semiempirical Calculations. Geometries of Single Diimidazolium Cations.
The diimidazolium cations can be described as rigid N-methylimidazolium units linked by flexible alkylene spacers. In the structures investigated here, the increase of the spacer's length may allow the formation of more complex H-bond network. However, higher flexibility within a tecton makes it difficult to predict its self-assembly in the solid state. Theoretical calculations were performed to determine the most stable conformations of the diimidazolium cations. Calculations were performed with Ghemical and MOPAC 2009 computational chemistry packages for Linux workstations using semiempirical PM6 method.11 After a short molecular mechanic optimization (UFF performed with ArgusLab),12 all of the degrees of freedom were relaxed, except the ϕ1 and ϕ2 torsion angles angles (ϕ1 and ϕ2 are the two dihedral angles C2N1CH2N for [M] or C2N1CH2CH2 for [E] and [P]; see Chart 1 and Figure 2) that were varied by a 10 increment from 0 to 360. The 1369 obtained conformers were analyzed by plotting the 2D variation of the total energy as a function of ϕ1 and ϕ2 values. Second, the initial values of the two torsion angles were set as those of the most stable conformation, and all of the degrees of freedom were relaxed; the geometry was fully optimized using
Figure 3. Relative stability as a function of the ϕ1 and ϕ2 torsion angles and the most stable conformer for: (a) [E] and (b) [P] (calculated with PM6, MOPAC 2009).
the PM6 method. The most stable geometry of [M] diimidazolium cation is presented in Figure 2 as the conformer A. The dihedral angle value for the most stable geometry of [M] diimidazolium cation was obtained at ϕ1 = 100.99 and ϕ2 = 100.05 (Table 1). The same analyses were performed to obtain the most stable conformers of [E] and [P] diimidazolium cations (Figure 3). The dihedral angle values in the most stable geometries of [E] and [P] diimidazolium cations corresponded to ϕ1 = 98.54, ϕ2 = 98.63 and ϕ1 = 101.16, ϕ2 = 103.66, respectively. For the three cations, the geometrical parameters are reported in Table 1. For all diimidazolium cations, the imidazolium rings showed a planar structural geometry, although the two dihedral angles DN1C2N3C4 and DC2N3C4C5 were slightly different (see Table 1). The distortions were larger for the methylene or propylene spacers, although the largest variation was 3829
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Figure 4. Binding energy of the [Br] anion to the imidazolium ring of the diimidazolium cation as a function of the Hx 3 3 3 Br distance for (a) [M][Br], (b) [E][Br], and (c) [P][Br] (calculated with PM6, MOPAC 2009).
Figure 5. Most stable geometries of single ion pairs of (a) [M][Br]2, (b) [E][Br]2, and (c) [P][Br]2 (calculated with PM6, MOPAC 2009). Bromide anions are shown as red spheres.
less than 0.3. These results suggest that the effects of the spacer's length on the structures of the imidazolium rings of diimidazolium cations are negligible. H-Bonds of Single Ion Pair. For all of the diimidazolium cations, the interaction of one [Br] with the imidazolium ring through the H2, H4, or H5 protons was analyzed with the PM6 method. First, we have considered the interaction of one anion with one imidazolium ring: the CxHx 3 3 3 Br angle was fixed at 180, and the Hx 3 3 3 Br distance was decreased from 6 to 1 Å. The binding energy of the [Br] anion to one imidazolium ring of the diimidazolium cation was calculated as a function of the Hx 3 3 3 Br distance (Figure 4). The absolute value of the interaction energy was much larger than the expected hydrogen bond energies ( 120) are shown as dotted purple lines.
bond angles were comprised between 126 and 170. The alkyl spacer has a certain influence on the formation of the H-bonds, especially in the case of [M][Br]2 and [E][Br]2 salts. The absolute values of the interaction energies were much larger for these two salts, due to the multiple H-bonds formed between the diimidazolium cation and the two bromides. In this case, a strong ionic contribution may be part of the interaction energy, brought by the diimidazolium/bromides direct interaction. A general trend can be outlined: By increasing the length of the spacer, the H2 3 3 3 Br distances decrease, while the C2H2 distances and the C2H2 3 3 3 Br bond angles increase. The binding energies between bromides and imidazolium cations increase due to the weaker H-bonds of the ion pairs and the decrease of the number of H-bonds. H-Bonds of Multiple Ion Pairs. In a 3D network, the global organization may be directed by the global flexibility of the diimidazolium cations and the number of H-bonds formed. Therefore, we have attempted to predict the organization in the crystalline structure of the three salts. Eight diimidazolium dications and 16 bromide anions were positioned in a cubic box and optimized using the PM6 method in periodic boundary conditions (PBC) to model their 3D packing. The optimized structures for the three salts are shown in Figure 6. It can be observed that there were three [M] or [E] and two [P] diimidazolium dications around one anion. Similarly, there were six bromide anions around one [M] or [E] dication and only two bromides for one [P] dication. The structural properties of the hydrogen bonds were preserved for the system composed of multiple ions. For [M][Br]2 and [E][Br]2, the H-bonds network obtained by PM6 method was compatible with monoclinic lattices: (i) for [M][Br]2, a = 4.7, b = 11.4, and c = 13.1 Å and R = 90, β = 93.5, and γ = 90, and (ii) for [E][Br]2, a = 8.0, b = 8.6, and c = 8.6 Å and R = 90, β = 103.0, and γ = 90). In the case of [P][Br]2 salt, the formation of a more open network was predicted (see dotted red circle in Figure 6). However, the formation of this open network is unlikely in the crystal, and at least other molecules are included in the lattice.
Scheme 1. Synthesis of the N,N0 -Dimethylmethylene-diimidazolium Tectons
Investigations of H-Bonds Networks in the Solid Phase. Synthesis, Crystal Information, and Structure of the Diimidazolium Cations. The diimidazolium salts ([M][Br]2, [E][Br]2 , and
[P][Br]2) were obtained by a double SN2 reaction involving Nmethylimidazole and dibromoalcane (dibromomethane, 1,2-dibromoethane, and 1,3-dibromopropane, respectively) without solvent (see Scheme 1 and ESI).9 To investigate the structures adopted in the solid phase of the three salts, a crystallographic study was performed. The crystalline materials obtained were monoclinic for [M][Br]2 and [E][Br]2 and orthorhombic for [P][Br]2. The space groups obtained were P21/m for [M][Br]2, P21/c for [E][Br]2, and Pccn for [P][Br]2 (Table 3). From a constitutional point of view, [P][Br]2 crystals had an asymmetric lattice. Moreover, [P][Br]2 had an incorporated water molecule in the lattice. The increase of the alkylene spacers between the imidazolium units resulted in a higher flexibility and a larger void space between the two anions. To obtain a crystal packing and to ensure its cohesion, the structure should include structuring water molecules, as they are H-bond donors and acceptors. These experimental results are in good agreement with the predicted molecular modeling results: The [M][Br]2 and [E][Br]2 salts were less flexible than [P][Br]2 analog; thus, the molecules can easily come together and blend into the crystal: that is, water molecules are not necessary to ensure the cohesion in the solid state. For [M][Br]2 and [E][Br]2, we noticed that the crystalline cell length and cell angles were very 3831
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Table 3. Experimental Crystal Data of [M][Br]2, [E][Br]2, and [P][Br]2 Saltsa [M][Br]2b
[E][Br]2c
[P][Br]2b
C9H14N4; Br2
C10H16N4; Br2
C11H18N4; Br2
molecular weight
338.06
359.09
490.25
crystal system
monoclinic
monoclinic
orthorhombic
space group
P21/m
P21/c
Pccn
a (Å)
4.7028(8)
8.4504(2)
18.5648(12)
b (Å)
11.333(2)
8.8991(2)
12.5284(8)
c (Å) R (deg)
11.786(2) 90
9.2217(3) 90
13.5418(8) 90
β (deg)
93.894(2)
108.340(1)
90
γ (deg)
90
90
90
compound cation and anion formula water
H2O
V (Å3)
626.71(19)
658.26(3)
3149.6(3)
Z
2
2
8
color
colorless
colorless
colorless
crystal dim. (mm3)
0.21 0.14 0.14
0.20 0.18 0.13
0.21 0.14 0.14
μ (mm1) temperature (K)
6.443 200 (2)
7.657 150 (2)
5.143 200 (2)
Dcalcd (Mg m3)
1.791
1.776
1.620
F(000)
332
348
1536
θ range for collection (deg)
1.73 to 30.00
5.52 to 72.29
1.96 to 30.00
limiting indices
6 e h e 6
10 e h e 10
26 e h e 26
15 e k e 15
11 e k e 10
17 e k e 17
16 e l e 16
11 e l e 11
19 e l e 19
reflections collected/unique Rint
13197/1912 0.034
8524/1289 0.038
65390/4589 0.061
completeness to θ max. (%)
1.000
1.000
1.000
data/restraints/parameters
1912/0/74
1289/0/75
4589/74/217
final R indices [I > 2σ(I)]
R1
0.0212
0.0426
0.0249
wR2
0.0509
0.0991
0.0468
R indices (all data)
R1
0.0280
0.0426
0.0608
wR2
0.0522
0.0991
0.0510
0.973 0.689; 0.221
1.305 0.613; 1.757
0.814 0.379; 0.383
goodness-of-fit on F2 largest diff. peak and hole (eA3) a
Absorption correction, empirical (SADABS); refinement method, full-matrix least-squares on F2. b Diffractometer: Bruker smart diffractometer equipped with an APEX II CCD Detector and a graphite monochromator (Cu KR radiation, 1.54178 Å). c Diffractometer: Bruker Platform, equipped with a Bruker SMART 4K Charged-Coupled Device (CCD) Area Detector using the program APEX II and a Nonius FR591 rotating anode equipped with Montel 200 optics (Cu KR radiation, 1.54178 Å).
close to those obtained by molecular modeling. For the asymmetric lattice ([P][Br]2), we adopted the atom numbering that uses the first digit of the atom's labeling as in the dication of the corresponding moiety (Figure 7). In Table 4, we report some structural parameters of the diimidazolium cations. Their conformations are similar to those obtained from molecular modeling (Tables 1 and 4). Experimental H-Bonds of Single Ion Pair. In all of the structures, electrostatic interactions and H-bonds were the dominant interactions. In [M][Br]2, the H-bonds occurred between the most acidic proton of the imidazolium rings and the bromide anions.14 As in the previously reported structures, all of the H 3 3 3 X distances are inferior to 3.0 Å, and the CH 3 3 3 X angles vary between 144 and 172 (Table 5).8,15 The observed H-bonds prove that the methylene spacer has a great influence on the crystal stability: The H6 protons are involved in a hydrogen-bonding network. Once again, the principal H-bonds patterns were very close to those predicted by molecular modeling (Tables 2 and 5).
In the case of [E][Br]2, the H-bonds were weaker compared to [M][Br]2, and the obtained the H-bonds were different to those predicted by molecular modeling (Tables 2 and 6). For the [P][Br]2 crystal, the H-bonds occurred between (i) the acidic protons of imidazolium rings, (ii) bromide anions and oxygen atom of water molecules, and (iii) bromide anions and the hydrogen atom of water molecules (Table 7). Water molecules were incorporated in the crystal to ensure its cohesion: The presence of the propylene spacer between the imidazolium rings resulted in direct H-bonds between the most acidic proton of the imidazolium rings and the anions and the insertion of water brought a supplementary connection between the bromide anions. Moreover, the water molecules were also directly H-bonded to the acidic protons of the dication. The H-bonds between the anions and the water molecules were disorganized; no particular pattern can be highlighted. The local H-bonds close to a diimidazolium cation obtained for [M][Br]2, [E][Br]2, and [P][Br]2 are presented in Figure 8. 3832
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Figure 7. ORTEP view of the (a) [M][Br]2, (b) [E][Br]2, and (c) [P][Br]2 with the numbering scheme adopted. Ellipsoids are drawn at the 50% probability level.
Table 4. Experimental Bond Distances (Å), Bond Angles ( — , ), and Dihedral Angles (D, ) of Three Diimidazolium Cations [P] [M]
cation 1
cation 2
1.330 (3) 1.332 (3)
1.328 (2) 1.328 (2)
1.331 (2) 1.320 (2)
1.376 (2)
1.384 (4)
1.373 (2)
1.371 (2)
1.347 (2)
1.351 (4)
1.341 (2)
1.341 (3)
1.385 (2)
1.383 (4)
1.383 (2)
1.378 (2)
Nx1CHx6a
1.457 (2)
1.460 (3)
1.477 (2)
1.473 (2)
Nx3X (X = CHx7b or CHx8c)a
1.459 (2)
1.465 (3)
1.467 (2)
Nx1Cx2a Cx2Nx3a Nx3Cx4a Cx4Cx5a Cx5Nx1a
1.334 (2) 1.327 (2)
[E]
1.471 (2)
— Nx1Cx2Nx3a
107.9 (2)
108.3 (2)
108.5 (2)
108.9 (2)
— Nx3Cx4Cx5a DNx1Cx2Nx3Cx4a
107.4 (2) 0.9 (2)
107.3 (2) 1.1 (3)
107.4 (2) 0.2 (2)
107.2 (2) 0.1 (2)
DCx2Nx3Cx4Cx5a
0.2 (2)
0.2 (3)
0.2 (2)
0.1 (2)
94.8 (2)
103.4 (3)
117.3 (2)
118.8 (2)
DCx2Nx1Cx6X (X = N1d or C6e or Cx7c)a a
For [M] and [E] (x = 0), for [P] cation 1 (x = 1), and for [P] cation 2 (x = 2). b For [M] and [E]. c For [P]. d For [M], with a symmetry operator: x, 3/2 y, z. e For [E], with a symmetry operator: 1 x, y, 1 z.
Table 5. Experimental Distances and Angles of CH 3 3 3 Br Hydrogen Bonds in [M][Br]2a C2H2 3 3 3 Br1 C4H4 3 3 3 Br2 C5H5 3 3 3 Br2 C6H6A 3 3 3 Br1 a
C6H6B 3 3 3 Br2
symmetry operators for Brb
C 3 3 3 H (Å)
H 3 3 3 Br (Å)
C 3 3 3 Br (Å)
CH 3 3 3 Br (deg)
0.95
2.789 (6)
3.608 (8)
144 (6)
1 + x, y, z
0.95 0.95
2.784 (8) 2.812 (4)
3.685 (9) 3.712 (1)
158 (5) 158 (9)
x, 1/2 + y, 1 z x, y, z
0.99
2.787 (4)
3.642 (3)
144 (2)
1 + x, y, z
0.99
2.729 (6)
3.712 (9)
172 (2)
1 + x, y, z
Only H 3 3 3 Br that have significant interactions are reported. b Symmetry operators for the other atoms are x, y, z.
The comparison of the Figures 5 and 8 allows us to point out two observations: (i) for [M][Br]2 and [E][Br]2, the predicted
H-bonds for single diimidazolium cation bromides ion pairs were very close to those obtained experimentally, as well as the 3833
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Table 6. Experimental Distances and Angles of CH 3 3 3 Br Hydrogen Bonds in [E][Br]2a
a
C2H2 3 3 3 Br1 C7H7B 3 3 3 Br1
symmetry operators for Brb
C 3 3 3 H (Å)
H 3 3 3 Br (Å)
C 3 3 3 Br (Å)
CH 3 3 3 Br (deg)
0.95
2.885 (6)
3.553 (4)
128 (3)
1 x, 1/2 + y, 1/2 z
0.98
2.828 (6)
3.733 (4)
153 (5)
1 + x, 1/2 y, 1/2 + z
Only H 3 3 3 Br that have significant interactions are reported. b Symmetry operators for the other atoms are x, y, z.
Table 7. Experimental Distances and Angles of XH 3 3 3 Y Hydrogen Bonds in [P][Br]2a XH 3 3 3 Y (deg)
symmetry operators for Yb
3.607 (2)
148 (4)
1/2 + x, 1 y, 3/2 z
3.750 (6)
143 (2)
1 x, 1 y, 2 z
3.827 (6) 3.721 (6)
154 (3) 154 (4)
1/2 + x, 1 y, 3/2 z 1 x, 1 y, 1 z
3.718 (9)
145 (3)
1/2 + x, 1 y, 3/2 z
3.702 (6)
150 (2)
1 x, 1 y, 1 z
2.902 (3)
3.830 (8)
156 (3)
1/2 + x, 1 y, 3/2 z
0.98
2.675 (5)
3.622 (7)
162 (4)
1 x, 1 y, 2 z
C14H14 3 3 3 O1 C14H14 3 3 3 O2 C16H16A 3 3 3 O4
0.95
2.492 (4)
3.167 (2)
128 (1)
x, y, z
0.95 0.99
2.544 (3) 2.504 (1)
3.214 (3) 3.197 (4)
127 (5) 126 (5)
x, y, z 1/2 + x, 1 y, 3/2 z
C24H24 3 3 C24H24 3 3
3 O2
0.95
2.717 (4)
3.294 (5)
119 (5)
x, y, z
3 O3
0.95
2.242 (2)
2.980 (4)
133 (6)
x, y, z
0.98
2.068 (4)
2.953 (4)
154 (2)
x, y, z
0.98
2.639 (3)
3.408 (5)
135 (7)
x, y, z
C28H28B 3 3 3 O3
0.95
2.610 (4)
3.413 (2)
139 (6)
x, y, z
O1H1A 3 3 3 Br1 O1H1B 3 3 3 Br2 O2H2A 3 3 3 Br1
0.84 0.84
2.67 (3) 2.62 (4)
3.501 (17) 3.445 (17)
171 (13) 166 (11)
x, y, z x, y, z
0.84
2.51 (2)
3.335 (13)
167 (9)
x, y, z
3 Br2
0.84
2.54 (6)
3.288 (13)
148 (10)
x, y, z
3 Br1
0.84
2.38 (4)
3.192 (14)
163 (11)
x, y, z
3 Br2
0.84
2.43 (4)
3.220 (14)
158 (11)
x, y, z
3 Br1
0.84
2.40 (2)
3.237 (11)
174 (10)
x, y, z
0.84
2.57 (6)
3.276 (11)
142 (9)
x, y, z
X 3 3 3 H (Å)
H 3 3 3 Y (Å)
C12H12 3 3 3 Br2 C15H15 3 3 3 Br2 C16H16A 3 3 3 Br2 C18H18C 3 3 3 Br1
0.95
2.764 (3)
0.95
2.945 (6)
0.99 0.98
2.908 (1) 2.814 (3)
C22H22 3 3 3 Br1 C25H25 3 3 3 Br1 C26H26A 3 3 3 Br1
0.95
2.896 (5)
0.95
2.845 (3)
0.99
C28H28C 3 3 3 Br2
X 3 3 3 Y (Å)
H-bonds between cation 1 and bromide
H-bonds between cation 2 and bromide
H-bonds between cation 1 and water
H-bonds between cation 2 and water
C24H24 3 3 3 O4 C28H28B 3 3 3 O2
H-bonds between water and Br
O2H2B 3 3 O3H3A 3 3 O3H3B 3 3 O4H4A 3 3 a
O4H4B 3 3 3 Br2
Only H 3 3 3 Y that have significant interactions are reported. b Symmetry operators for the other atoms are x, y, z.
structure of the dications; (ii) for [P][Br]2, the structure of the diimidazolium cation showed a significant difference between the predicted and the experimental ion pairs, due to the optimization in gas phase of the flexible cation with the propylene spacer. H-Bonds of Multiple Ion Pairs. Crystal organization of the diimidazolium cations was in close agreement with the theoretical organization predicted by molecular modeling, except for [P][Br]2. For this last, the conformation of the dication was responsible for the special organization observed in the solid state: The dimidazoliums were folded to bring closer the two imidazolium rings, within a 3.4 Å distance. In this case, two different dications were connected by aromatic stacking between their imidazolium rings. In this structure, the succession of two imidazolium rings separated by a flexible spacer (propylene) was very favorable to the formation of charge transfer interactions,
contrary to the less flexible structures such as [M][Br]2 (cisoid) or [E][Br]2 (transoid) that allowed the prevalence of directional H-bonds in addition to the electrostatic interactions. The divergence between the calculated and the experimental structure of [P][Br]2 is due to the parameters of the semiempirical methods that do not take into account in an accurate way the aromatic stacking. A careful analysis on the crystal packing of the three salts also revealed an increase in the complexity of the supramolecular organization (Figure 9). The organization of [M][Br]2 can be described as the same as the classical one of imidazolium salts; that is, cations and anions are held together by ionic interactions and H-bonds. The structure observed for [E][Br]2 reveals the formation in one plane of a channeled structure, where the bromide anions are trapped in a complex imidazolium network. These two structures 3834
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Crystal Growth & Design were formed by the maximization of the electrostatic interactions and H-bonds: eight for [M][Br]2 and four for [E][Br]2. Therefore, the increase of the spacer's length decreases the number of H-bonds. The organization of [P][Br]2 reveals an increase of structural complexity: The diimidazolium forms a clathrate around four bromides (i.e., an inclusion complex is formed, where bromides are trapped in the diimidazolium network). The diimidazolium units are connected to each other by π-stacking, and each inclusion complex is connected in the network by disorganized water molecules forming H-bonds between the dications. The increase of flexibility is responsible for the changes highlighted by the shift from the classical self-association of imidazolium salts ([M][Br]2) to the formation of well-defined inclusion complex of bromides ([P][Br]2). [E][Br]2 can be seen as a transition compound between the classical ionic/H-bonds organization of [M][Br]2 and the ionic/π-stacking/H-bonds organization of [P][Br]2. The melting points of these three salts can be directly correlated with the structural organization in the solid state. The melting points are 300, 216, and 162 C for [M][Br]2, [E][Br]2, and [P][Br]2, respectively. The decrease of the melting temperature is due to the decrease of the H-bond
Figure 8. Local H-bonds (dotted purple lines) close to a diimidazolium cation obtained by X-rays diffraction for [M][Br]2, [E][Br]2, and [P][Br]2.
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number and the incorporation of disorganized water molecules for [P][Br]2. Water molecules affect the cohesion in the crystal as they are part of the supramolecular network. We previously reported the same observations in N,N0 -(diphenylmethylene)diimidazolium and N,N0 -(diphenylxylylene)diimidazolium dibromides structural organization, where water molecules were in both cases incorporated in the crystal.9a It is important to note that in that case, the H-bonds were only secondary interactions, as the presence of electron-rich (phenyl) and electron-poor (imidazolium) rings in the dication molecules was maximizing the aromatic stacking interactions. However, again, by increasing the flexibility of the spacer between the imidazolium units, as in the case of xylylene, water molecules became essential to ensure the interconnection of all of the molecules in the crystal. The flexibility can be used as a controlling element and driving force for the crystal organization of these diimidazolium salts, by maximizing the ionic interactions, the H-bonds, and the aromatic stacking. The structure of the anion (not studied here) is also essential in dictating the self-assembly of the dications and anions and eventually the inclusion of water molecules in the solid state. Therefore, diimidazolium cations can be used as versatile tectons, and crystalline materials with predictable properties can be obtained. The self-assembly properties of these salts can be controlled by the incorporation of various supramolecular binding sites (e.g., aromatic residues) or the flexibility of the binding sites or the spacer (e.g., alkyl groups).
’ CONCLUSIONS In summary, the detailed structures of three new alkylene diimidazolium cations with bromide anions have been obtained. Given the precise X-ray crystal data and the molecular modeling results, we are now able to predict the structural organization in the solid state of new diimidazolium salts. The structure and conformation of the dication govern the general packing of these salts in the crystal. As we previously demonstrated, the diimidazolium cations are versatile units that can be incorporated in more complex structures with predictable structures and properties. By judicious choice of the flexibility of the diimidazolium dication, porous ionic channels can be designed and obtained in these crystals.
Figure 9. Crystal packing of [M][Br]2, [E][Br]2, and [P][Br]2 salts obtained by X-ray diffraction. 3835
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Crystal Growth & Design
’ ASSOCIATED CONTENT
bS
Supporting Information. Synthesis, characterization, large ORTEP views, supplementary figures, and crystallographic information. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses †
Universite Lille 1, E.A. 4478 Chimie Moleculaire et Formulation, B^at. C6, F-59655 Villeneuve d'Ascq Cedex, France.
’ ACKNOWLEDGMENT We are grateful to the Natural Sciences and Engineering Research Council of Canada, the Fonds Quebecois de la Recherche sur la Nature et les Technologies, the Canada Foundation for Innovation, and Universite de Montreal for the financial support of this work. We thank Dr. Michel Simard for X-ray acquisitions. We also thank colleagues for careful reading and discussion of this manuscript. ’ REFERENCES (1) (a) Welton, T. Chem. Rev. 1999, 99, 2071–2083. (b) Wassercheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772–3789. (c) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351–356. (d) Hagiwara, R.; Ito, Y. J. Fluorine Chem. 2000, 105, 221–227. (e) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Green Chem. 2001, 3, 156–164. (2) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391–1398. (3) (a) Baudequin, C.; Baudoux, J.; Levillain, J.; Cahard, D.; Gaumont, A.-C.; Plaquevent, J.-C. Tetrahedron: Asymmetry 2003, 14, 3081–3084. (b) Leclercq, L.; Suisse, I.; Agbossou-Niedercorn, F. Chem. Commun. 2008, 311–313. (c) Leclercq, L.; Suisse, I.; Nowogrocki, G.; Agbossou-Niedercorn, F. Eur. J. Org. Chem. 2010, 14, 2696–2700. (d) Leclercq, L.; Lacour, M.; Sanon, S. H.; Schmitzer, A. R. Chem.—Eur. J. 2009, 15, 6327–6331. (e) Leclercq, L.; Schmitzer, A. R. Supramol. Chem. 2009, 21, 245–263. (f) Noujeim, N.; Leclercq, L.; Schmitzer, A. R. Curr. Org. Chem. 2010, 14, 1500–1516. (g) Leclercq, L.; Suisse, I.; Nowogrocki, G.; Agbossou-Niedercorn, F. Green Chem. 2007, 9, 1097–1103. (4) (a) De Souza, R. F.; Padilha, J. C.; Goncalves, R. S.; Dupont J. Electrochem. Commun. 2003, 5, 728–731. (b) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Exnar, I.; Gratzel, M. J. Am. Chem. Soc. 2003, 125, 1166–1167. (5) (a) Dyson, P. J.; McIndoe, J. S.; Zhao, D. B. Chem. Commun. 2003, 508–509. (b) Lozano, P.; de Diego, T.; Carrie, D.; Vaultier, M.; Iborra, J. L. Biotechnol. Prog. 2003, 19, 380–386. (c) Abraham, M. H.; Zissimos, A. M.; Huddleston, J. G.; Willauer, H. D.; Roger, R. D.; Acree, W. E. Ind. Eng. Chem. Res. 2003, 42, 413–418. (6) Mjewski, P.; Pernak, A.; Grzymislawski, M.; Iwanik, K.; Pernak, J. Acta Histochem. 2003, 105, 35–39. (7) (a) Dupont, J. J. Braz. Chem. Soc. 2004, 15, 341–350. (b) Dupont, J.; Suarez, P. A. Z. Phys. Chem. Phys. Chem. 2006, 8, 2441–2452. (8) (a) Gordon, C. M.; Holbrey, J. D.; Kennedy, A. R.; Seddon, K. R. J. Mater. Chem. 1998, 8, 2627–2636. (b) Dupont, J.; Suarez, P. A. Z.; de Souza, R. F.; Burrow, R. A.; Kintzinger, J. P. Chem.—Eur. J. 2000, 6, 2377–2381. (c) van den Broeke, J.; Stam, M.; Lutz, M.; Kooijman, H.; Spek, A. L.; Deelman, B.-J.; van Koten, G. Eur. J. Inorg. Chem. 2003, 2798–2811. (d) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyen, J. S.; Seddon, K. R.; Rogers, R. D. Chem. Commun. 2003, 1636–1837. (e) Saha, S.; Hayashi, S.; Hamaguchi, H. Chem. Lett. 2003, 32, 740–741. (f) Downard, A.; Earle, M. J.; Hardacre, C.; McMath, S. E. J.;
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