On the Colossal and Highly Anisotropic Thermal Expansion Exhibited

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On the colossal and highly anisotropic thermal expansion exhibited by imidazolium salts I. de Pedro, A. Garcia-Saiz, J. Dupont, P. Migowski, O. Vallcorba, J. Junquera, J. Rius, and J. Rodríguez Fernández Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 22 Sep 2015 Downloaded from http://pubs.acs.org on September 25, 2015

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Crystal Growth & Design

On the colossal and highly anisotropic thermal expansion exhibited by imidazolium salts I. de Pedro†*, A. García-Saiz†, J. Dupont¥,§*, P. Migowski¥, O. VallcorbaŦ, J. Junquera†, J. Riusɸ and J. Rodríguez Fernández† †

CITIMAC, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander.

¥

Instituto de Química, Universidade Federal do Rio Grande do Sul, Porto Alegre, 91501-970

Brazil. §

School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, UK.

Ŧ

ALBA Synchrotron Light Source, Cerdanyola del Vallés, Barcelona, Spain.

ɸ

Institut de Ciència de Materials de Barcelona (CSIC) Campus de la UAB, 08193 Bellaterra,

Catalunya (Spain). KEYWORDS: Imidazolium molten salts; Crystal Structure; Density Functional Theory; Hydrogen and π+- π+ Interactions, Anisotropic Positive and Negative Thermal Expansion

ACS Paragon Plus Environment

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ABSTRACT: The imidazolium salts 1-ethyl-2,3-dimethylimidazolium chloride, Edimim[Cl], and bromide, Edimim[Br], salts exhibit negative and positive thermal expansion as determined by variable-temperature synchrotron powder X-ray diffraction experiments. Both compounds crystallize in the same monoclinic centrosymmetric space group, showing an anisotropic H bonding network and imidazolium-imidazolium π+-π+ interactions, which have been corroborated by density functional theory studies. The chloride derivative displays a highly anisotropic thermal expansion with a colossal positive coefficient along one direction. Replacement of Cl- by Br- in the same crystal structure produces an increase of the colossal coefficient and induces a biaxial negative thermal expansion. By studying the molecular vibrations factors and the H-bonding framework in their crystals as a function of temperature, it was possible to rationalize at the molecular level the mechanism for the observed anomalies in thermal expansion.

INTRODUCTION Without doubt, there is increasing interest in thermo-responsive materials in view of their potential applications as thermo-mechanical actuators and sensors.1 In general, upon heating, a material normally experiences positive thermal expansion (PTE) along all three dimensions. However, some materials may experience contraction upon warming and expansion upon cooling i.e. negative thermal expansion (NTE).2 This unusual property has been observed in materials such as metal oxides, metal cyanides, pure silica zeolites and metal–organic frameworks (MOFs).3-12 In this view, some MOFs display anisotropic and exceptionally large thermal expansion on the order of magnitude of metal cyanides.7 The occurrence of NTE in MOFs is attributed to the combination of flexible metal


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coordination polyhedra, elongated molecular linkers and open pore spaces.10 However, there are few known materials with anomalous thermal expansion behavior over a broad range of temperatures. Ionic liquids (ILs) based on imidazolium cation are highly thermal stable, possess negligible vapor pressure, are not flammable and are highly organized 3-D materials in the condensate phase13 and may be regarded as H-bonded “liquid” frameworks. Indeed, the structural organization of ILs is controlled by an intricate interplay of interionic interactions between anion and cations. Ionic, hydrogen bonding (H-bonding) and van der Waals (vdW) forces are the bond strengths used to explain the majority of their interesting features.14, 15 In addition, ILs based on the imidazolium cation are compounds in which π+-π+ stacking interactions among the imidazolium rings14 may be also involved.16-18 The binding energy depends on the different orientations adopted by the imidazolium rings, and possible π–π motifs include: (a) anti-parallel stacked, (b) parallel stacked, (c) rotated stacked and (d) T-shaped.19 Furthermore, a parallel displaced motif also probably contains significant dispersion and electron correlations with large repulsive Coulombic forces exerted by positively charged π-rings.20 Moreover, in some 1alkyl-3-methylimidazolium salts, relatively weak C-H···π interactions via the methyl group and the imidazolium ring-π system21 can also be found. Thus, all these non-bonding interactions should be recognized as key components in the local structure of imidazolium based compounds. Several theoretical22 and experimental studies23 have examined the reorientation dynamics of H-bond interactions in ILs as a function of temperature, with the aim of obtaining the relationship between these bonding interactions and their physico-chemical properties.24 In particular, an overview in the research reported about the crystal structure


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of Cnmim[X] or Cn-dmim[X] (X = Cl, Br) salts reveals a typical tendency: they form an extended tridimensional network in the solid state where one imidazolium cation is surrounded by at least three anions and one halide is linked to at least three cations.25 In principle, one could expect a reduction in H-bonding strength at higher temperatures. However, the thermodynamics of these systems is quite complex. Due to the anisotropy of both the unit cell and H-bonding network with possible π-interactions, some contacts could readily be influenced by temperature and others not. This could suggest an anisotropic thermal expansion process, with a possible strengthening of some van der Waals interactions as one crystallographic direction packing rearranges,11,

26

i.e. it

experiences NTE.12, 27, 28 We report herein that an imidazolium salt may indeed constitute a new class of thermoresponsive materials with anomalous thermal expansion behavior. The crystal structures of Edimim[Cl] and Edimim[Br] were studied using variable-temperature synchrotron X-ray powder diffraction experiments (SXPD)f rom 100 to 350 K and with density functional theory (DFT) calculations at 0 K. A colossal anisotropic PTE for the chloride salt and bi-axial negative thermal expansion for the bromide derivative were observed. Molecular vibration factors and the hydrogen-bonding framework within the crystal structures were determined to propose an atomic scale mechanism for the thermal response of these compounds. EXPERIMENTAL SECTION Methods: Variable temperature synchrotron powder data collection:


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Powder diffraction data were collected at the high resolution end station of beamline BL04 of the ALBA synchrotron using the microstrip Mythen II solid-state detector (six modules, 1280 channels/module, 50 µm/channel, sample-to-detector distance 550 mm). The wavelength used for the experiment was 0.95336 Å for all measurements except for Edimim[Cl] at 100 K, where a wavelength of 0.82621 Å was employed. The specimens were introduced into a 0.7 mm capillary and the temperature was controlled using an Oxford Cryosystems Series 700 Cryostream. Samples were cooled down to 100 K and then heated to 350 K at 5 K/min, continuously collecting the pattern every 30 s from 2 to 45° (2θ). Longer acquisition time patterns (3 min) from 2 to 82° (2θ) were collected at 350 and 100 K. RESULTS AND DISCUSSION The crystal structures of Edimim[Cl] at 100 K and 350 K were refined from SXPD data starting from the previously reported single-crystal structure model.29 Structure solutions from powder diffraction data at 100 and 350 K showed that Edimim[Cl] and Edimim[Br] are isostructural in this temperature range. Final structural parameters and figures of merit are listed in Electronic Supplementary Information (ESI Table S1). The experimental, calculated and difference powder profiles (ESI Figure S1) and atomic coordinates, (ESI Tables S2 and S3) are listed in the Supporting Information as well as the thermal parameters in the corresponding CIF files. Both compounds crystallize in the centrosymmetric space group (P21/a). Hydrogen positions derived from molecular mechanic calculations were introduced and constrained to the respective C atoms in the last refinement cycle. The crystal structure consists of


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alternating layers of Edimim+ cations and Cl- (or Br-) anions normal to the c-axis. The anions form an approximate cubic sublattice (a’ ≈ 6.28 Å; dashed lines in Figure 1) with one imidazolium cation in each subcell. According to this description, each imidazolium is surrounded by eight Cl- (or Br-) and, in addition, is off-center and approaching the nearest cation. The imidazole ring is aromatic and planar. The soft geometrical restraints applied to the molecule during the refinement process had a small impact to the final crystal structure, compared without restraints, (ESI Table S4 and 5) and to the C-C and CN bond lengths, which lie in the expected range of other halide imidazolium-based compounds.30 Similarly to imidazolium salts with the 1-ethyl-3-methylimidazolium cation,17, 31, 32 Edimim+ displays a gauche conformation (non-planar) for the ethyl group. The values of the C2-N1-C1’-C2’ torsion angle for Edimim[Cl] range from 32.9° (100 K) to 33.7° (350 K), whereas for Edimim[Br] the corresponding values are 36.5° and 32.5°.

Figure 1. Crystal packing in the ab plane of the Edimim[Cl] or Edimim[Br] crystal structure. Different atoms are indicated in green (chloride or bromide), grey (carbon), blue (nitrogen) and white (hydrogen). The blue dashed squares represent the unit cell. The yellow dashed squares display an approximate cubic sublattice formed by the halide anions.


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Inspection of the SXPD patterns of both salts from 100-350 K shows that the peak positions of most reflections changed considerably from pattern to pattern (with some values increasing and others decreasing) with no significant peak broadening effects (ESI Figure S2). The experimental temperature-dependent variations in the lattice parameters of both salts are listed in ESI Tables S6 and S7. The principal thermal expansion (TE) coefficients, α (main text Figure 2 and ESI Table S8), as well as their indicatrices (ESI Figure S3) were obtained via linear fits using the PASCal33 program, using orthogonal lattice parameter evolution; see Figure 2. The volume thermal-expansion coefficient, αV, is positive at all temperatures and almost two times larger than in ice [184(3) and 197(3) MK−1 for Cl and Br]. The strong anisotropic positive TE values, αx3 = 187(2) and 301(3)MK−1 for Cl and Br, respectively, are larger than the well-known “colossal” (|αx| ≥ 100) mechanical responses observed in cyanide-based inorganic material Ag3[Co(CN)6] 7,8

and comparable to those recently reported in some MOFs, i.e. MOF-3430 and fu-MOF-

3535. The principal axis X2 is exactly parallel to the crystal b-axis for both salts. For the chloride derivative, the degree of thermal contraction is essentially constant and positive, αx2 = 7.03(2) MK−1, which is similar in order of magnitude to the expansion observed for so called “low expansion” ceramics [+2 ≤ α ≤ +8].36 This value increases up to -64.4(3) MK−1 with the replacement of the halide anion (Br- for Cl-), which is greater than that reported in several metal complexes, i.e. silver (I) 2-methylimidazolate [Ag(mim)] [α = −24.5(8) MK−1]37 and Cd(imidazolate)2 [α = −22.5(5) MK−1],38 and also greater than that calculated for simple halogen bonded complexes, i.e. pyridine–ICl and pyridine–IBr,39 lying within the range of various MOFs based on the hexacyano ion, i.e. KMn[Ag(CN)2]2


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[α = −60(8) MK−1]40 and Zn[Ag(CN)2]2 [α = −57.58(8) MK−1].41 Finally, the uniaxial negative TE coefficients observed in both salts, αx1 = −12.8(6) and -40(2)MK−1 for Cl and Br, lie in the range of various MOFs (-11 MK−1 ≤ αx ≤ -27 MK−1).42 From all these data, two especially anomalous issues become apparent. First, the crystal expansion of both salts is highly anisotropic, displaying significant linearity in all orthogonal axes. Second, the replacement of the halide (Cl- for Br-) leads to an increase in the positive and negative TE coefficients, with a change from positive to negative along the b-axis (X2; see Figure 2).

Figure 2. Changes in lengths of the principal orthogonal axis of (a) chloride and (b) bromide salts as a function of temperature (experimental data are shown in ESI Table S6 and S7). In these graphs, X3 is inclined by 72.5° and 54.8° to the crystallographic c-axis; X2 ≈ the b-axis and X1 is inclined by 162° and 145° to the crystallographic a-axis for Cl and Br, respectively. Continuous lines show the linear fitting for the calculation of the thermal expansion coefficients obtained using the PASCal program.

The anisotropic temperature-dependent variations of the unit cell parameters significantly affect the H-bonding network of these salts (ESI Table S9). Cation-anion interactions in Edimim[Cl] at 100 K are represented by the formation of nine probable hydrogen bonds


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between Cl and the surrounding Edimim+ cations (according to IUPAC definition for H bonds43; ESI Figure S4). The strongest H-bond is between C4-H4···Cl which has a C···Cl distance of 3.412(7) Å (H···Cl of 2.52 Å) [ESI Figure S4(a)] in good agreement with single-crystal results.29 On the other hand, at 350 K, only seven probable H-bonds are left [ESI Figure S4(b)]. Similarly, in Edimim[Br] at 100 and 350 K, there are eight probable H bonds. The shortest one also involves the H4 atom [C···Br contact distances of 3.608(7) and 3.628(7) Å respectively (H···Br of 2.72 Å and 2.74 Å)] and the effect of temperature-dependent variations in H bond contacts is smaller than in the previous compound [ESI Figure S3 (c) and (d)]. There are no π–anion interactions between the imidazole ring and halide ions in the temperature range studied. The cation arrangement along a gives rise to dimers with parallel and slightly shifted aromatic rings (see Figure 3(a)). This is presumably due to the formation of π+−π+ interactions in a displaced-stacked geometry (nearly parallel and angle displaced geometries: 20° < θ < 70°.44 Distances between ring planes do not vary too much with temperature and range from 3.394(7) Å to 3.449(7) Å for Edimim[Cl] and from 3.441(7) to 3.591(5) Å for Edimim[Br] between 100 and 350 K. This distance is in agreement with previous studies about the most frequent stacking distance of two positively charged aromatic rings (between 3.4 and 3.6 Å) in diverse pyridinium and imidazolium dimer complexes45 provided in the Cambridge Data Base (CSD), which apparently form π+−π+ stacking interactions. In order to confirm the existence of π+−π+ interactions in these salts, theoretical models based on the Edimim[Cl] and [Br] structures were optimized by DFT calculations using the SIESTA46 code (ESI Table S10 and S11). The experimental atomic coordinates from Rietveld analysis (at 100 K) were used as starting values. The intramolecular geometries


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and distances of the [Edimim]+ cation obtained by the final refined positional coordinates from DFT calculations at 0 K (ESI Table S10) show an overestimation of the latter of about 4%, which may be regarded a satisfactory match by considering the standard deviations of the functional approach.47,48 To check the presence of π+−π+ interactions in the solid phase, the projected density of states (PDOS) of the imidazolium rings in the anti-parallel projection was analyzed (ESI Figure S5). A wide range of energies between 3.5 and -4.5 eV below the Fermi energy was detected for which the π+−rings PDOS take non-zero values, suggesting the existence of a attractive interaction between them. To visualize the shape of one of these states, we plotted a representative state at the Gamma point, with an energy of −3.79 eV and -3.81 eV for Cl- and Br- below the Fermi energy, respectively. The isosurface for a value of the wavefunction is shown in Figure 3 (b), and displays how the wavefunction can connect two imidazolium cations in both compounds. Remarkably, the wavefunction displays a parallel-displaced motif. Hence, it can avoid the large repulsive columbic forces exerted by the positively charged π–rings, in good agreement with crystallographic studies. Therefore, in addition to the previously mentioned H bonds, cation—cation interactions in the form of π+−π+ contacts could also help to stabilize the presented crystal structures.


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Figure 3. (a) Potential π+−π+ interactions between the imidazolium rings with labeling of the atoms. (b) Representation of the π+−π+ interaction wavefunction with an energy of -3.79 eV below the Fermi level for Edimim[Cl] (-3.81 for Edimim[Br]) at 0 K. The levels of the isosurfaces are 0.015 e/Å-3. Blue and red represent the opposite signs of the wavefunction. It is possible to rationalize the mechanism of thermal expansion for these imidazolium salts at the molecular level by systematically comparing the crystal structures at 350 and 100 K. As inferred from the equilibrium positions of the atoms derived from X-ray diffraction, both compounds show an apparent shortening the N-C and C-C imidazolium distances due to vibrational effects (ESI Table S5). In addition, the onset of dynamic molecular methyl and ethyl moiety reorientation does not substantially change the 3D molecular footprint of the imidazolium cation. Moreover, the distances between the nearest rings increase by about 2% from 100 to 350 K (ESI Table S9). Figure 4 shows the electron density maps calculated from the SXRD data by GFOURIER49 for both compounds at 100 and 350 K. In all the cases, it has been possible to refine two different isotropic displacement parameters, one for the halide atom and one for all the atoms of the imidazolium cation. Both the halide atoms and the imidazolium cation experience systematic decreases of the atomic displacements with decreasing temperature, this being stronger in the case of the bromide salt. Therefore, a decrease in


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temperature causes a reduction in the vibrational motion of these anions, with concomitant strengthening of the van der Waals interactions of the molecules that stabilize the packing arrangement. Through the decreasing of bond vibrations, the average distance between bonded pairs of atoms increases with temperature, and this increase is reflected in the TE process. Focusing on the halide-halide distance of the cubic sublattice shown in Figure 1, the largest variation is located along the a-axis of the unit cell in both salts upon heating [from 7.991(5) to 8.364(5) and from 7.847(1) to 8.356(1) for Cl- and Br-, respectively] (ESI Table S9 and Figure S4 and S6), where is located the colossal positive TE. This gradual shift is also consistent with a decrease in H bond distance along this direction and the change of the H-bond framework in the other directions (see Figure 5). In addition, both compounds display NTE located along the principal orthogonal axis X1, which could be also attributed to molecular vibration factors of the atoms. A mechanism that reflect negative TE originates from transverse atomic and molecular vibrations has been proposed for metal organic framework materials.50 In the case of the network structure of silver(I) 2-methylimidazolate, Ag(mim), the vibrational motion of mim has two largest thermal components, which are along the directions perpendicular to the ligand. These thermal components support the ligand displacement as transverse vibrations, which will contribute to the overall NTE observed along the c-axis in this material. However, for these materials, it has not been possible to obtain single crystals with enough quality to perform single crystal X-ray diffraction and obtain the isotropic equivalent displacements.


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Figure 4. Map of scattering density obtained by synchrotron powder X-ray diffraction inside the unit cell of the crystal structure of Edimim[Cl] at (a) 100 K and (b) 350 K and Edimim[Br] at (c) 100 K and (d) 350 K calculated by the GFOURIER program of the Fullprof suite along the caxis. The levels of the isosurfaces are 1 e/Å-3. The hydrogen atoms were omitted in the picture for clarity and the atoms were imported to the pictures from the CIF files.

The negative TE thermal expansion found in these isostructural imidazolium salts is highly dependent on the halide guest. The increase in the α coefficient by bromide substitution

is

also

consistent

with

weaker

interactions

involving

the

lower

electronegativity of this anion, and results in a higher relative volume coefficient. In addition, the bromide substitution induces a biaxial negative TE within the same framework. Inspection of the interatomic contacts of Edimim[Br] (ESI Table S9) reveals that three probable hydrogen bond, located along b, decreases C−H···Br and C···Br distances, whereas the C−H···Br angle increases (see Figure 5). For comparison, in the chloride salt, which has positive TE along b, only the C1’’-H1A’’···Cl distance and angle slightly decreases, from 3.719(6) to 3.667(6) Å for the C···Cl distance (2.77 to 2.75 Å for H···Cl) and from 170 to 159º for the angle, when increasing the temperature (see Figure


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5). By taking into account the general correlation between the strength of a hydrogen bond and its length and linearity,51 this issue could be one of the responsible for inducing negative TE along b with halide substitution. Consequently, the molecules can then approach more closely to each other along the b direction and so leading to contraction along this axis. A similar trend has been observed in a simple halogen bonded complexes and the nitromethane solvate of hexaoxacyclooctadecane, where a colossal TE is found coupled to a negative TE.52,

53

The authors reported that these changes are driven by a

temperature variations of C–H⋯X (X = Br, Cl and O) hydrogen bonds.

Figure 5. Crystal packing in the ab plane of Edimim[Cl] at (a) 100 K and (b) 350 K and Edimim[Br] at (c) 100 K and (d) 350 K. Hydrogen-bonding network ((H···Cl and H···Br contact up to 3.1 and 3.2 Å, respectively) are marked with red and green stripes. The blue dashed squares represent the unit cell.


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CONCLUSION We have identified via synchrotron powder X-ray diffraction that the imidazolium salt halides Edimim[Cl] and Edimim[Br] display a colossal and highly anisotropic positive TE along the b-axis. Moreover, replacing Cl with Br anions increases the value of the colossal positive TE in response to temperature changes and induces a biaxial negative TE in the others two directions. This phenomenon, which is unprecedented in imidazolium salts and very uncommon in any other type of material, is quite interesting, especially for crystal engineering. Understanding the atomic scale mechanisms responsible for the thermal expansion properties of these salts can be of direct use in other materials with hinge movement of the host framework by anions contained and linked by H bonds. This mechanism results from the interplay of two effects involving molecular vibration factors and H-bonded framework rearrangement. It illustrates how very weak bonding interactions could have a profound effect on crystal thermodynamics. Finally, it can serve to extend the present studies to other imidazolium salts and to explore the piezo-mechanical response of these materials. ASOCIATED CONTENT Electronic Supplementary Information (ESI) available: Detailed experimental procedures description, temperature dependent SXPD data and crystal refinement details and detailed DFT simulation results. Accession Codes. Crystallographic data for structures reported in this paper have been deposited in the Cambridge Crystallographic Data Center under


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reference numbers CCDC 1053981 1053982 1053983 and 1053984. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif. AUTHOR INFORMATION Corresponding authors: [email protected] and [email protected] Notes: The authors declare no competing ACKNOWLEDMENTS Financial support from the Spanish Ministerio de Ciencia e Innovación (Projects MAT2011-27573-C04 and FIS2012-37549-C05-04) and Becas Iberoamericas Jóvenes Profesores Investigadores, 2015, Santander Universidades is acknowledged. The authors gratefully acknowledge the computer resources, technical expertise and assistance provided by the Red Española de Supercomputación. The paper is (partly) based on the results of experiments carried out at the ALBA synchrotron light source in Barcelona.

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On the colossal and highly anisotropic thermal expansion exhibited by imidazolium salts

I. de Pedro, A. García-Saiz, J. Dupont, P. Migowski, O. Vallcorba, J. Junquera, J. Rius and J. Rodríguez Fernández

We show for the first time that simple imidazolium salts (ionic liquids) constitute new family of thermo sensitive materials (with both positive and negative thermal expansion).


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On the Colossal and Highly Anisotropic Thermal Expansion Exhibited

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