Crystal Growth, Structure, and Polymorphic Behavior of an Ionic Liquid

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Crystal Growth, Structure and Polymorphic Behavior of an Ionic Liquid: Phthalate Derivative of N-butyl,N-methylimidazolium Hexafluorophosphate Clément Brandel, Gabin Gbabode, Yohann CARTIGNY, Claudette Martin, Géraldine Gouhier, Samuel Petit, and Gérard Coquerel Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm501220x • Publication Date (Web): 02 Jul 2014 Downloaded from http://pubs.acs.org on July 4, 2014

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Chemistry of Materials

Crystal Growth, Structure and Polymorphic Behavior of an Ionic Liquid: Phthalate Derivative of N-butyl,Nmethylimidazolium Hexafluorophosphate Clément Brandel,# Gabin Gbabode,# Yohann Cartigny,# Claudette Martin,§ Géraldine Gouhier,§ Samuel Petit,#* Gérard Coquerel# # Normandie Université, Crystal Genesis Unit, SMS, EA 3233, IMR 4114 Université de Rouen, F-76821 Mont Saint-Aignan Cedex, France § Normandie Université, COBRA, UMR 6014, FR 3038, INSA Rouen, CNRS, IRCOF, rue Tesnière 76821 Mont SaintAignan Cedex, France KEYWORDS . Conformational Polymorphism, Liquid-liquid Demixing, Oiling-out, Crystal Defects, Liquid Inclusions, Supersaturation.

ABSTRACT: After the multistep synthesis of an original imidazolium hexafluorophosphate ionic liquid [pbmim][PF6], two polymorphic forms have been isolated from methanolic solution and characterized by XRPD, DSC and Raman spectroscopy. The stable Form A (mp=90.3 °C) was obtained by conventional crystallization at moderate cooling rate (10 K/min) was applied. Structural analyses carried out by using single crystal (Form A) and powder (Form B) X-ray diffraction revealed a rotational disorder of anionic octahedrons and, more interestingly, large conformational differences between cationic moieties caused by their molecular flexibility. Crystal growth of [pbmim][PF6] (Form A) in methanol often leads to numerous crystal defects and revealed that most of them consist of liquid inclusions. The supersaturation ratio (β) appeared as the predominant factor influencing the crystal growth behavior in isothermal and stagnant conditions. At low β values, a morphological transition from rod-shaped crystals to platelets was observed, presumably caused by a change in the growth mechanism of specific faces. Using high β values promotes the formation of microscopic (< 5 µm) liquid inclusions that become easily detectable upon heating, or the appearance of macroscopic inclusions with an hourglass shape.

Despite the increasing number of crystal structures published during the two last decades, an overview of the literature devoted to ILs reveals that their crystallization behavior has been poorly investigated, usually using experimental protocols consisting of basic solvent evaporations. For these reasons studies of the crystal growth behavior of ILs should be conducted in order to identify their specificities compared to small organic molecules. In particular, the incidence of crystallization parameters20 (e.g. nature of the solvent,21 presence of impurities,22 supersaturation, atmosphere,23 growth temperature, cooling rate, seeding, etc) on the size, habit and quality of the produced IL crystals could be emphasized. This knowledge would be beneficial in terms of both chemical purity (by controlling for instance the formation of crystal defects, such as liquid inclusions24,25) and structural purity26 in case of polymorphic behavior, already encountered for ILs.16,27-30 Ionic liquids are also used as supports for synthesis in organic chemistry.31 Indeed the purification step is then reduced to a simple liquid/liquid extraction avoiding the timeconsuming and expensive column chromatographies. In the course of a multi-step synthesis, the ionic liquid labeled here-

INTRODUCTION Ionic liquids (ILs) are salts exhibiting low melting points (often below 100°C) and are usually composed of the association of an organic cation and an inorganic anion.1 Due to their low vapor pressure in the molten state, this class of compounds has found many applications in the field of green chemistry2 and ILs are also used as non-volatile solvents for organic synthesis, purification, catalysis and electrochemistry.3 The attractive potential of ILs as liquid media has also motivated a large number of studies concerning the physicochemical properties of ILs (dealing e.g. with viscosity, density, surface tension, conductivity measurements,4,5 and molecular dynamics studies6-8). By contrast, the specific properties of ILs in the solid state have been rarely considered and the origins of their low melting points compared to traditional salts remain unclear.9 It was however suggested, based on structural data, that the combined contributions of weak intermolecular contacts and diffuse Coulombic interactions are responsible for the low melting points of ILs (see for instance4,5,10-18). The high chemical stability of ILs in the liquid state was also proved to be beneficial for their chemical purification by zone melting.19

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after [pbmim][PF6] (1-(4-(1,2-bis(methoxycarbonyl)phen-5oxy)butyl)-3-methylimidazolium hexafluorophosphate, Figure 1) was synthesized as an intermediate supported scaffold. It contains a N-alkyl-N-methylimidazolium32,33 cation linked by a flexible alkyl chain of four carbon atoms to a phthalate ester derivative by a stable ether function. After synthesis, [pbmim][PF6] showed a marked capability to crystallize readily as small crystal tablets, but closer inspection of the crystals by optical microscopy revealed the frequent occurrence of crystal defects. These preliminary observations prompted us to further investigate the crystallization behavior of [pbmim][PF6] and to get some insights into the formation of macroscopic defects in single crystals.

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Crystallization Methods and Hot Stage Microscopy (HSM) for in-situ Observations Solvents used for crystallization experiments were of HPLC grade and purchased from Fisher Scientific (USA). Solubility measurements were performed by gravimetric method in 1 mL vials. Crystallizations were also carried out in 1 mL sealed vials starting from 0.35 mL of solution and temperature during cooling ramps was controlled (± 1 °C) using a Julabo F25 cryothermostat, allowing cooling rates ranging from 1 K/min to 0.001 K/min. Faster cooling rates (i.e. 1 to 50 K/min) were achieved by placing crystallizers on the Peltier cooler of a THMS 600 hotstage setup (Linkham) flushed with liquid nitrogen, allowing accurate control of cooling rate and sample temperature (± 1 °C). Cooling rates and nominal temperatures were regulated via the Linksys32 software. For microscopy observations, crystals were loaded into a quartz cell with a cylindrical geometry (d=13 mm, h=1.3 mm) and sampled in the THMS 600 hot-stage setup (Linkam). The setup is coupled with a Nikon Eclipse LV100 microscope (maximum magnification: ×1000) connected to a computer for image capture by using a CCD camera. For direct observations of crystallizations, the quartz cell was filled with ca. 0.17 mL of saturated solution and covered with a glass cover slip. Solvent evaporation during observations was prevented by sealing the cell and the cover slip with silicone gel. Differential Scanning Calorimetry (DSC) DSC experiments were performed using a Netzsch DSC 204 F1 apparatus equipped with an intracooler. Enthalpy and temperature calibrations were performed with biphenyl and indium. Each DSC run was performed with 5 mg of a powdered sample (േ 0.05 mg) in sealed aluminium pans with pierced lids. Measurements were carried out with heating rates in the range 5-15 K/min under a helium atmosphere (constant flow of 40 mL/min) and, thanks to the thermal stability of the compound, repeated at least twice to check the reproducibility of the measurements. The Netzsch-TA Proteus Software v4.8.4 was used for data processing. Second Harmonic Generation (SHG) Spectroscopy SHG spectroscopy was used for the assessment of the centrosymmetry of the crystal lattice in the metastable polymorph of [pbmim][PF6]. The laser was a Nd:YAG Q-switched (Quantel). The signal generated by the sample was collected into an optical fibre and directed onto the entrance slit of a spectrometer (Ocean Optics). According to Kurtz and Perry SHG powder method,34 SHG signal intensities were compared to the signal of a reference compound (quartz, 45 µm average size). The experimental setup was recently described elsewhere.35 Single Crystal X-ray Diffraction (SC-XRD) The selected single crystal was stuck on a glass fiber and mounted on the full three-circle goniometer of a Bruker SMART APEX diffractometer equipped with a CCD area detector and operating with Mo Kα1 (λ=0.71071 Å) as incident beam. The SMART36 software was used to determine the cell parameters and the orientation matrix. Intensities were integrated and corrected for Lorentz polarization and absorption effects using SAINT software.36 The structure was solved by direct methods using the SHELX-97 suite of program,37 and anisotropic displacement parameters were refined for all non-

Figure 1. Molecular structure of the ionic liquid [pbmim][PF6].

EXPERIMENTAL SECTION Materials, Reagents and Solvents for Organic Synthesis Unless otherwise stated, 1H NMR, 13C NMR, 19F NMR spectra were recorded in deuterated solvent using a Bruker AC 300 spectrometer operating at 300, 75 and 282 MHz respectively. IR Spectra were recorded on a Perkin-Elmer 16 PC FTIR and expressed in cm-1. Electrospray mass spectra were recorded with an Esquire–LC ion-trap mass spectrometer (ITMS) (Bruker Daltonics, Wissembourg, France). Highresolution mass spectra were performed on a LCTOF Premier XE (Micromass, Manchester, UK) equipped with an ESI source. The spectra were acquired in W mode in positive mode. Elemental compositions were obtained with a mass accuracy better than 5 ppm. All reactions were monitored by analytical thin-layer chromatography using pre-coated silica gel plates. Visualization was accomplished by UV-light (254 nm). Flash chromatography was performed using silica gel (mesh 230-400). Chemicals were used as received without further purification. Dichloromethane and acetonitrile were distilled over calcium hydride under nitrogen. Cyclohexane and ethyl acetate were distilled prior to use. Metathesis and Analysis of [pbmim][PF6] 6 The complete synthesis route is presented as Supporting Information and summarized in Scheme 1. Compound 5 (1.5 g, 3.16 mmol) and hexafluorophosphorus potassium (0.87 g, 4.74 mmol, 1.5 eq) were reacted in a mixture of CH3CN/water (1/1, 13 mL) at room temperature for 5 hours. Then the solvent was evaporated under reduced pressure and ethyl acetate was added. Water was poured into the solution and the organic layer was separated, washed with brine, dried on MgSO4 and evaporated affording the ionic liquid 6 (1.54 g, 3.13 mmol, 99%). mp: 83-85 °C. 1H NMR (300 MHz, CDCl3) δ 1.74-1.82 (m, 2H), 1.98-2.04 (m, 2H), 3.81 (s, 3H), 3.82 (s, 3H), 3.83 (s, 3H), 4.10 (t, 2H, 6.4 Hz), 4.21 (t, 2H, J = 7.5 Hz), 7.05 (d, 1H, J = 2.6 Hz), 7.09 (d, 1H, J = 1.5 Hz), 7.31 (s, 1H, J = 1.9 Hz), 7.41 (d, 1H, J = 1.9 Hz), 7.78 (d, 1H, J = 7.9 Hz), 8.52 (s, 1H). 13 C NMR (75 MHz, CDCl3) δ 26.1, 27.6, 36.3, 44.9, 50.7, 52.5, 69.5, 110.9, 118.7, 122.9, 123.3, 123.7, 124.3, 124.6, 128.1, 135.3, 136.7, 163.2, 164.5, 166.8. 19F NMR (282.5 MHz, CDCl3) δ -73.6 (d, J = 0.7 MHz). ESI: [M]+: 347.


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hydrogen atoms. All hydrogen atoms were located by Fourierdifference synthesis and fixed geometrically according to their environment with a predefined isotropic thermal factor. X-ray Powder Diffraction (XRPD) and Structure Determination from XRPD Data XRPD analyses were performed using a D8 diffractometer (Bruker, Germany) equipped with a modified goniometer of reverse-geometry (-θ/-θ).38 The incident X-ray beam consisted of Cu Kα radiation (λ=1.5418 Å) with a tube voltage and amperage set at 40 kV and 40 mA, respectively. The diffraction patterns were collected with a Lynx Eyes® linear detector (Bruker, Germany). Routine XRPD analyses were performed with a step of 0.04° (2θ), and a 12 s per step counting time from 3° to 30° (2θ). High-quality XRPD patterns were collected using a step of 0.018°(2θ) with a 40 s per step counting time from 3° to 80° (2θ). The crystal structure determination from X-ray powder diffraction data for the metastable form of [pbmim][PF6] was performed using the Reflex Plus module implemented in the Materials Studio suite of programs.39 The X-Cell program40 was used for indexation of the unit cell. The dimensions of the obtained unit cell and the peak profile parameters (Pearson VII peak shape parameters, Berrar Baldinozzi parameters for peak asymmetry and FWHM parameters) were refined by the Pawley method41 (3° < 2θ < 80°, Rwp = 6.21%). The Powder Solve program42 was used to perform a simulated annealing procedure (in parallel tempering mode), consisting of the random perturbation of an initial model to obtain the best fit between experimental and calculated XRPD patterns. The degrees of freedom used for this procedure were the three rotations and three translations of the two ionic entities, as well as the relevant torsions of the cationic moiety. Subsequent Rietveld refinements of the solved crystal structure were performed by using a global anisotropic factor and repeated until convergence to the lowest Rwp. RESULTS AND DISCUSSION 1. Synthesis, Purification and Characterization of [pbmim][PF6] 6 in the Solid State The original ionic liquid (IL) [pbmim][PF6] was obtained with excellent yields in five steps from the commercial aromatic acid 1 (Scheme 1). The hydroxyphthalic acid 1 was esterified with methanol using activation by thionyl chloride. The 1-chloro-4-iodobutane was introduced to the phenol function of the diester 2 using Williamson reaction. Sodium iodine was used to activate the terminal halide position and to increase the nucleophilic addition of 1-methylimidazole. The IL 5 was formed in acetonitrile under reflux or using microwave activation decreasing the reaction time from three hours to ten minutes. The metathesis using potassium hexafluorophosphate led to IL 6 quantitatively with a chemical purity higher than 99.5%. Complete analytical details can be found as Supporting Information. After the metathesis between compounds 5 and 6, crystallization of [pbmim][PF6] 6 was induced by evaporation under reduced pressure in ethyl acetate solution. The compound was obtained as a yellowish powder. Analysis of this crude sample by XRPD confirmed its satisfactory crystallinity (Figure 2), and DSC analysis (Figure 3) showed the occurrence of two overlapping endothermic events upon melting. Owing to the satisfactory chemical purity of the sample, this melting behavior suggests the presence of two polymorphic forms of

[pbmim][PF6] 6 in the crude sample and illustrates the good sensitivity of this technique for the evaluation of structural purity. Further recrystallization experiments were carried out in order to isolate pure polymorphs of [pbmim][PF6].

Figure 2. XRPD patterns of [pbmim][PF6]: (1) crude powder, (2) Form A and (3) Form B.

Figure 3. DSC analysis of the crude powder of [pbmim][PF6] at a 5 K/min heating rate.

2. Polymorphic Behavior of [pbmim][PF6] 6 The preparation of two different crystal forms was successfully achieved by applying various cooling rates to a saturated solution. Methanol (MeOH hereafter) was identified as the most suitable solvent for all subsequent crystallization experiments since the solubility of [pbmim][PF6] was estimated to 1.2 %wt at 25°C in this solvent. By contrast, [pbmim][PF6] is much more soluble (>10 %wt) in most polar aprotic solvents (such as ethyl acetate) and hardly soluble in water (0.20 %wt at 25°C) or in apolar aprotic solvents. When cooling at 1 K/min from 40 °C to 0 °C a saturated solution in MeOH under stirring, a single solid (Form A, hereafter) was produced, identified by its XRPD pattern shown in Figure 2-2. Conversely, the implementation of a 10 K/min cooling rate in the same temperature range yielded a distinct crystal form (labeled Form B). Despite its lower crystallinity (weaker diffracted intensities), Form B can be readily differentiated from Form A by XRPD (Figure 2-3).



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Scheme 1. Synthesis route of the ionic liquid [pbmim][PF6] 6.

The absence of significant weight loss upon annealing for the two crystal forms at 50°C confirmed the occurrence of true polymorphic forms rather than MeOH solvates. The cooling rate implemented during recrystallization from MeOH solutions appeared therefore as a decisive and sufficient parameter for the preparation of pure polymorphs, at least at the detection threshold of conventional XRPD. Thus, the crude sample collected during the last step of chemical synthesis appears as a physical mixture of both polymorphs. Other crystallization experiments performed in water and usual organic solvents at various cooling rates did not induce the formation of any other crystal form. Further characterization of powdered pure forms of [pbmim][PF6] was carried out by means of Raman spectroscopy and DSC. Raman spectra of Form A and Form B (provided as Supporting Information) revealed that this technique is suitable for unambiguous differentiation of both polymorphs. The thermal behavior of [pbmim][PF6] was studied by DSC and was shown to be sensitive to the heating rate. For Form B, the use of a 5 K/min heating rate induced a partial conversion upon melting (confirmed by HSM). The implementation of a 15 K/min heating rate appeared suitable for the observation of single melting endotherms (Figure 4), and a monotropic relationship could be deduced from onset temperatures and melting enthalpies.43 The high melting entropies (95 and 83 J.K1 .mol-1 for Forms A and B, respectively) appeared to be in the usual range measured for ionic liquids.17 Cross-seeding experiments performed from methanolic suspensions at various temperatures (20 °C, 30 °C and 40 °C) confirmed that Form B has a monotropic character under normal pressure. However, in the absence of solvent and under ambient atmosphere, the conversion of Form B into Form A is slow: the transformation starts within several days and takes weeks for completion.

Figure 4. DSC curves of [pbmim][PF6], FormA (upper) and Form B (lower) at a 15 K/min heating rate.

3. Structural Analyses of [pbmim][PF6] 6 A single crystal (250 × 100 × 20 µm3) of Form A suitable for SC-XRD experiments was prepared by slow evaporation at room temperature of a saturated MeOH solution and was used for structural analysis. Crystallographic data and refinement parameters are given in Table 1. Figure 5a presents the asymmetric unit and Table 2 summarizes relevant torsion angles for the cation. The PF6- anion is disordered, and the fluorine octahedron was modeled in two different orientations with occupancy ratios of 58% and 42%. Only the major PF6- orientation is represented in subsequent figures. A systematic search in the Cambridge Structural Database (vers. Nov. 2012) showed that, for organic compounds (i.e. after excluding compounds containing block d and f elements) comprising a hexafluorophosphate anion, only ca. 30% of PF6- anions are, at least partially, disordered.

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Table 1. Crystallographic data at 298 K for the two polymorphs of [pbmim][PF6] 6, refinement parameters for Form A and agreement indicators of the Rietveld refinement for Form B Polymorphic form

Form A

Form B C18H23F6N2O5P (492.12 g.mol-1)

Chemical formula (mol. weight) Crystal system (Space group)

Triclinic (P-1)

a (Å)

8.9848(9)

13.05(4)

b (Å)

10.0585(10)

11.11(3)

c (Å)

13.0237(13)

8.59(2)

α (°)

82.866(2)

72.486(6)

β (°)

75.195(2)

109.682(6)

γ (°)

78.641(2)

104.919(4)

1112.24

1101.82

Volume (Å3) Z Calculated density (g.cm-3) Measured/Unique data

2

2

1.470

1.48(2)

8712/4471

Rietveld Parameters

Observed data (Fo>4σ(Fo))

3431

Number of restraints/parameters

0/353

Number of refined parameters

20

1.075

2θ range for structure solution

20-80

0.0646/0.1589

2θ range for Rietveld refinement

20-50

0.0844

Rwp

0.123

0.31, -0.17

Rp

0.0845

Goodness-on-fit on F

2

R1/ wR2 (Fo>4σ(Fo)) R1 (all data) Largest difference peak and hole (e-. Å-3)

Owing to the difficulty to prepare single crystals of Form B, a high-quality XRPD pattern was collected and used for crystal structure determination. During the long data collection (48 h), partial conversion of Form B into Form A was revealed by the presence of small characteristic diffraction peaks marked by stars in the XRPD pattern shown in Figure 6. These peaks (not detected by routine XRPD) were omitted for the structure solution of Form B and a triclinic unit cell was found by indexation, indicating that the lattice symmetry could be either P1 or P-1 (since these two space groups give the same extinction conditions). The centrosymmetric character of the crystal lattice was determined by second harmonic generation (SHG) spectroscopy.44 Indeed, a SHG signal can be obtained only when the irradiated material is non-centrosymmetric and the absence of SHG signal for [pbmim][PF6] (Form B) is indicative that P-1 is the most probable space group. For the structure solution, the asymmetric unit obtained in Form A was chosen as the initial model for the simulated annealing procedure. After 2 annealing cycles, the calculation converged towards a Rwp value of 20%. Subsequent Rietveld refinement cycles were performed so as to increase the agreement between observed and calculated XRPD patterns by small changes in the structure, leading to an acceptable Rwp value of 12.3%. Final lattice dimension values and refinement indicators are given in Table 1. The calculated (blue) and experimental (red) XRPD patterns are shown in Figure 6, alongside with the difference between the two diffractograms (in black). The asymmetric unit of Form B is depicted in Figure 5b and shows a slightly more extended conformation of the cation with reference to Form A. The detailed comparison of cationic moieties is presented in Figure 5c and Table 2, revealing large conformational differences related to the aliphatic bridge

Figure 5. ORTEP-like representation of asymmetric units in structures of [pbmim][PF6] Form A (a) and Form B (b). Displacement ellipsoids are shown at the 50% probability level and H atoms are spheres with arbitrary radii. A superimposition of the cations in Form A (blue) and Form B (pink) is given in (c).



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Figure 6. Experimental (red) and calculated (blue) XRPD patterns from the structure solution of [pbmim][PF6] (Form B). Systematic existences calculated from the triclinic unit cell (green ticks), and difference between calculated and observed XRPD (black curve). Stars depict diffraction peaks of Form A.

between cyclic moieties, but also caused by different orientations of acetate groups. Hence, as in the case of 1-butyl-3methylimidazolium chloride IL,27 [pbmim][PF6] constitutes a case of conformational polymorphism,45,46 mainly resulting from the molecular flexibility of the butyl chain bridging the rigid aromatic groups.

[pbmim][PF6] polymorphs exhibit significant differences illustrated in Figure 8. The structure of Form A can be described as the alternation of cationic (shown in green) and anionic layers (shown in red). The phthalate ring is coplanar to the (100) plane and cationic moieties are connected by several C-H⋅⋅⋅O hydrogen bonds, involving mainly acetate functions and imidazolium rings. In Form B, a similar stacking is found along the c direction with phthalate rings roughly parallel to (001) planes, but successive (002) slices (shown in blue) contain both ions and successive (002) layers are related to each other by the inversion center. A number of C-H⋅⋅⋅O hydrogen bonds are established between and inside (002) slices, extensively listed in Supporting Information.

Table 2. Selected torsion angles of cationic moieties in the two crystal structures of [pbmim][PF6] 6 Torsion angle (°)

Form A

Form B

C13-N1-C10-C9

64.12 (0.40)

-154.25

N1-C10-C9-C8

65.83 (0.35)

149.00

C10-C9-C8-C7

-172.51 (0.25)

-87.59

O1-C7-C8-C9

66.37 (0.32)

130.81

C6-O1-C7-C8

-176.09 (0.23)

84.93

C2-C3-C16-O5

76.73 (0.35)

-107.54

C3-C2-C15-O3

-174.60 (0.25)

-23.74

Coordination environments of cationic moieties in the two crystal structures are presented in Figure 7. The C-H⋅⋅⋅F close contacts (shown as green dashed lines) were calculated on the basis of the sum of corresponding van der Waals radii with an additional 0.2 Å for the upper limit. Further details about the geometries of these C-H⋅⋅⋅F intermolecular bonds are given as Supporting Information. In Form A, each cation is connected to 4 anions through the presence of 11 C-H⋅⋅⋅F contacts. By contrast, each cation in Form B is surrounded by 6 anions, of which 3 are linked to the acetates of the phthalate moiety. It should be noted that the exceedingly short contact at 1.62 Å in this structure is probably a consequence of the disordered anion and/or of the lower quality of the refinement method with reference to a single crystal analysis. As a consequence of specific conformational and coordination features in the two structures, crystal packings of

Figure 7. Contact coordination environment of the cations in the crystal structures of [pbmim][PF6], Form A (a) and Form B (b).


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Chemistry of Materials perature was therefore performed by cooling from 55 °C to 0 °C homogeneous solutions of [pbmim][PF6]. The cooling rate was varied from 1 K/min to 50 K/min and the homogeneity of the medium was ensured by magnetic stirring.

Figure 8. Projections along the b axis of the crystal packings in [pbmim][PF6] Form A (a) and Form B (b). C-H⋅⋅⋅O contacts are represented as dashed lines.

From this description of the two crystal structures, it can be inferred that high activation energy should be required for the irreversible Form B → Form A transition. Consistently, DSC experiments revealed that this polymorphic transition does not occur spontaneously in ambient conditions but rather proceeds via the classical scheme involving melting of the metastable polymorph (B) followed by recrystallization of the stable form (A). However a long storage at room temperature induces a sluggish irreversible conversion, possibly triggered and/or facilitated by specific crystal defects.47 Figure 9. (a) Calculated (black) and experimental (green) solubility curves of [pbmim][PF6] (Form A) in methanol. (b) Zoom on the 0-16 %wt region, highlighting stable and metastable equilibria in the [pbmim][PF6] – MeOH system.

4. Stable and Metastable Equilibria in the ([pbmim][PF6] - MeOH) Binary System In order to get an accurate control on crystallization parameters, the solubility curve of [pbmim][PF6] in methanol was determined by gravimetric method in the temperature range 5 °C – 60 °C. The green curve in Figure 9 indicates a moderate increase in solubility (roughly doubled) between 5 °C and 30 °C whereas a dramatic increase (by more than one order of magnitude) of solubility values is depicted from 35 °C to 60 °C. This far-from-linearity behavior was confirmed by the non-linear van’t Hoff plot (ln(solubility) vs 1/T) shown as Supporting Inforrmation. Further investigations consisted in drawing the theoretical solubility curve derived from the Schröder-Van Laar equation.48 The large differences in temperature values between the two curves shown in Figure 9a indicate a strong deviation from ideality in the [pbmim][PF6] – MeOH system suggesting either a stable miscibility gap above the solubility curve or – possibly and – a metastable submerged miscibility gap below this curve. To assess these hypotheses, homogeneous solutions of [pbmim][PF6] in methanol were subjected to heating and cooling treatments at various rates. The existence of a stable miscibility gap in the temperature range 60 °C-80 °C could not be established, whatever the concentration of methanolic solutions. The search for a submerged miscibility gap at low tem-

For solutions of concentrations ranging from 2 %wt to 14 %wt, it was observed that a cooling rate higher than 10 K/min was sufficient to induce the formation of a liquidliquid demixing (also known as oiling out) detected visually by the appearance of tiny droplets as soon as a threshold temperature (shown as red points in Figure 9b) was reached. The resulting binodal curve is poorly sensitive to the cooling rate (10, 20 or 50 K/min) and depicts, for a given concentration, the temperature at which demixing is observed through the nucleation of liquid droplets in supersaturated solutions (Figure 10a). Such phenomenon has already been reported in solutions of organic compounds,49-51 co-crystals, proteins52 and salts, including ILs.53 In the present case, it was observed that liquid-liquid demixing is actually a transient state that evolves spontaneously within 5-10 min towards the formation of a crystal form subsequently identified by XRPD as [pbmim][PF6] (Form B). This crystallization could be observed in-situ by optical microscopy at -20 °C under quiescent conditions (Figure 10) and revealed that crystal growth of Form B is continuously fed by the migration of droplets, thus giving rise to irregular and ill-shaped crystals resulting in branched aggregates.



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Figure 10. (a) Optical microscopy pictures of a 6 %wt solution after quenching from 52 °C to -20 °C under quiescent conditions (required for optical microscopy observations). (b) Migration of droplets feeding the growth of Form B (c) within minutes.

Figure 11. Optical microscopy pictures illustrating crystal defects reproducibly observed in single crystals of Form A: (a) macroscopic cracks, (b) hourglass-shaped liquid inclusions and (c) microscopic parallel crackles.

These observations suggest that liquid-liquid demixing might be a necessary condition for the spontaneous nucleation of [pbmim][PF6] (Form B). Since structural analyses have highlighted that cationic moieties exhibit large conformational differences, it can also be suspected that distributions and/or eventual pre-associations (or molecular clusters) of solvated conformers in distinct media (homogeneous solution vs droplets in an emulsion) might be involved in the nucleation process and might be responsible for the polymorphic behavior of [pbmim][PF6]. However, at the present stage there is no experimental evidence of this assumption and further studies of solvated or pre-associated states are required to assess these hypothetical relationships.54-59 Besides, the implementation of cooling rates lower than 10 K/min for a global composition in the range 3 - 11 %wt induced the formation of [pbmim][PF6] (Form A). The blue points in Figure 9b correspond to the Ostwald limit obtained at 1 K/min, and the use of higher cooling rates induced a shift by ca. 4 °C (2 K/min) to 10 °C (at 5 K/min) of temperatures at which spontaneous crystallization was detected visually. It can be seen from Figure 9b that the close vicinity between the Ostwald limit and the oiling out curves accounts for the versatility of crystallization behavior of this compound, characterized by a frequent concomitant polymorphism60 under uncontrolled conditions and exemplified for instance in Figures 2 and 3. Despite our limited understanding of nucleation mechanisms, the present study demonstrates that the polymorphic form obtained by spontaneous crystallization upon cooling is strongly under the influence of kinetic factors.

5. Crystal Growth Study and Liquid Inclusions in [pbmim][PF6] (Form A) 5.1. Characterization of Crystal Defects The observation by optical microscopy of numerous single crystals of Form A obtained from methanolic solutions revealed the frequent occurrence of crystal defects classified in three main types according to their shapes, sizes and specific features. However it should be noticed that defective particles often exhibit simultaneously several types of defects, without possibility, in most cases, to establish relationships between them. The first type consists of macroscopic cracks presenting a random orientation with reference to crystal faces. As shown in Figure 11a, their size and boundaries can also vary but they usually spread from an internal defective zone to the surface of particles. Owing to these features, it is likely that these cracks actually result from the propagation of distortions or strains initiated during crystal growth that do not self-heal entirely in the course of subsequent particle development. Such cracks may affect the mechanical properties of the material and are probably detrimental to the crystalline integrity of single particles but are not supposed to affect the structural or chemical purity of samples. The second type of macroscopic crystal defects, illustrated in Figure 11b, is made of large and readily recognizable liquid inclusions, forming an hourglass oriented along the main direction of crystal development. NMR analyses in deuterated DMSO of redissolved particles (previously dried) proved the presence of a significant amount of methanol within these defective crystals thus indicating that macroscopic vacuoles are filled with a saturated solution of the mother liquor.


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Figure 12. Photographs of the HSM experiment conducted on a defective single crystal of Form A presenting the behavior of hourglass inclusions upon heating. Heating rate is 1 K/min. Red dashed lines show the initial boundaries of the largest liquid inclusions. Their dissymmetric angular shape (not compatible with P-1 space group) might result from incidental different growth rates along opposite directions.

Our recent investigations devoted to the characterization and formation mechanism of fluid inclusions have shown that the thermal behavior of fluid inclusions and their final evolution toward negative crystals was a consequence of anisotropic dissolution/recrystallization phenomena.23-25 In the present case, it was observed by hot stage microscopy (HSM) that heating at 1 K/min from room temperature to ca. 80 °C induces, in particular from 50 °C upwards a large development of these inclusions that can be easily explained by the evolution of the solubility curve in this temperature range (Figure 12). The roughly isotropic increase in volume usually induces the escape of the solvent as soon as a surface of the particle is reached (Figure 12c, upper part). Although the detailed analysis of the formation mechanism of these macroscopic vacuoles is beyond the scope of the present paper, their specific and reproducible shape suggests that their appearance is triggered by a local growth inhibition during the early development of corresponding crystal faces.23 As demonstrated recently, the growth inhibition might be due to the role of gaseous matter in the mother liquor. Evidence of gas bubbles contained in these cavities (Figure 12c, lower part) is shown in Supporting Information. The third type of crystal defects that were often observed in [pbmim][PF6] (Form A) consists of zones in which the transparency of particles is altered by the presence of numerous, small and oriented cracks that can be described as fine striations or microscopic grooves with a typical size in the range 110 µm (Figure 11c). It could be established by HSM that crackled zones are actually made of numerous small liquid inclusions often aligned along parallel directions. It can be seen in Figure 13 that upon heating up to 60 °C, the volume of individual pre-existing inclusions strongly increases, in consistency with the previously mentioned evolution of solubility. Further heating up to 80 °C leads to the appearance of many new (previously undetected) inclusions, probably by coalescence of these tiny liquid cavities. The corresponding zone becomes cloudy and this evolution is usually irreversible upon cooling (Figures 13c and d). The formation of numerous micro-inclusions upon heating is therefore of general relevance in the crystals of [pbmim][PF6] (Form A), and do not seem to be related to the presence of other macroscopic defects detectable by optical microscopy at room temperature (see the red arrow in Figure 12c).

Figure 13. Photographs of the HSM experiment conducted on a defective single crystal of Form A, presenting the thermal behavior of microscopic crystal cracks. Heating/cooling rate is 1 K/min.

5.2. Crystal Growth Study In order to identify the experimental parameters involved in the formation of macroscopic defects, the crystal growth behavior of [pbmim][PF6] (Form A) in methanol was investigated by using a methodology based on controlled variations of a single parameter in each set of experiments. A prerequisite for this study was the determination of experimental conditions suitable for preparation of single particles presenting a high crystalline purity. It was established empirically that defectfree crystals can be reproducibly obtained by cooling from 52 °C to 25 °C a methanolic solution (2.5 to 3.5 %wt) of [pbmim][PF6] at 5 K/min. The supersaturation ratio β is then comprised between 2 and 3 at 25 °C. In these conditions the system is located in the Ostwald metastable zone so nucleation can be controlled by seeding with finely ground crystals and growth occurs in stagnant and isothermal conditions (25 °C). The representative morphology obtained when using the above protocol corresponds to crystal platelets as illustrated in Figure 14a, from which the determination of Miller indices for the most developed sets of crystal faces could be envisaged. This indexation step was achieved by using the geometrical BFDH



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method61 (implemented in the Materials Studio software39) for a rough estimate of predicted morphology, and the latter was further refined for a satisfactory representation of the experimental morphology. The (hkl) indices of the predominant faces were established by comparisons between calculated and measured dihedral angles. Furthermore, any risk of erroneous indexation was avoided by using single crystal X-ray diffraction in order to confirm unambiguously the identity of the main crystal axis. It can be seen in Figure 14b that the predominant faces consist of {010} faces and that the main growth direction is the crystallographic a axis, implying that the fastest crystal development proceeds by successive additions of (100) molecular slices shown in Figure 8a. The relative development (or morphological index) of {101} and {111} faces can vary from one particle to another, giving rise to diamond-shaped crystal habits (e.g., Figure 11b) or of a hexagonal platelet. The two sides of the crystal shown in Figure 14a actually illustrates a combination of the two habits.

this morphological transition is most likely related to a change of growth mechanism (e.g. from dislocation or spiral growth to bidimensional) since a clearly discontinuous evolution is observed. Indeed the batch of crystals obtained at β = 1.7 contains a mixture of distinct rod-shaped and diamond-shaped particles instead of an intermediate habit (Figure 15b). When relative supersaturation is close to or higher than 3.3, an increasing proportion of crystals exhibits macroscopic defects, ranging from mainly micro-inclusions at β ≈ 3.3-3.5 (Figure 15d) to large liquid inclusions in ca. 10% of particles grown at β ≈ 4 (Figure 15e). Higher β values could not be considered because of spontaneous nucleation occurring before the growth temperature was reached. The decisive incidence of kinetic factors (growth rate or supersaturation) on the formation (or detection) of liquid inclusions was already reported.25,62 Regarding the specific features of inclusions, it could be established in the case of [pbmim][PF6] that microinclusions are usually roughly parallel to {100} or {110} faces and that large inclusions are often produced during the defective growth of the same sets of faces.23 The incidence of temperature on the crystal growth behavior of [pbmim][PF6] (Form A) was assessed, based on the above data obtained at 25°C, by increasing progressively the β ratio at growth temperatures fixed to 30 °C and 35 °C. The results were similar to those obtained at 25 °C but both the morphological transition and the appearance of crystal defects were detected at slightly lower β values. The combined incidence of relative supersaturation and growth temperature on representative crystal habits and presence of defects could therefore be gathered in the diagram shown in Figure 16. It should also be noticed that these two parameters do not strongly impact the mean particle size: the main dimensions of most crystals are usually in the range 100-200 µm long.

Figure 14. Crystals of [pbmim][PF6] (Form A) presenting platelet (a) or rod (c) morphologies obtained at 25 °C. Corresponding simulated morphologies derived from the BFDH method are shown in (b) and (d).

As a complementary contribution to this crystal growth study, the incidence of kinetic factors was further investigated by applying various cooling rates to 4 %wt solutions previously seeded about 4 °C below the saturation point (i.e. at 30 °C) and then cooled down to 22 °C at 0.01, 0.05 and 0.1 K/min. In these conditions, one may expect to evaluate the impact of the rate at which supersaturation (i.e. driving force for crystallization) is applied in the growth medium. However crystallization continuously decreases the supersaturation so it can be stated that increasing the cooling rate actually results in the implementation of a higher global growth rate.20,62 As expected it was observed (Figure 17) that crystals obtained at higher cooling rate present a large proportion of micro- and macroscopic liquid inclusions, as well as a significant amount of macroscopic cracks. These experiments therefore confirm that the latter are most probably formed as a consequence of large proportions of liquid inclusions in highly defective zones of single particles, and that these (small or large) liquid inclusions are actually a consequence of the difficulty for particles to ‘self-heal’ when faster kinetic conditions and/or higher supersaturation are applied.

From this morphological study and using as reference the experimental conditions described above, it became feasible to investigate the incidence of various parameters on the crystal growth behavior and in particular on the formation of macroscopic crystal defects. Spontaneous nucleation was systematically skipped by manual seeding usually performed at the growth temperature. The main experimental parameters that could be considered included the growth temperature, the initial supersaturation ratio and the cooling rate. The global and comparative analysis of our observations proved unambiguously that the supersaturation ratio β (and therefore the composition of the mother liquor before seeding) is the predominant factor affecting the shape and macrocrystalline quality of particles. Indeed, for a crystal growth initiated at 25 °C by seeding, it was evidenced that a progressive increase of initial β from 1.2 to 4 induces no less than two successive changes. For β values lower than 1.7, a macroscopically defect-free rod-shaped morphology was obtained (Figure 14c and Figure 15a) but was replaced, for β value in the range 1.73.3, by the previously described hexagonal or diamond-shaped platelets. This morphological change can be caused by an increase of {001} growth rate (Figure 14d) as soon as β is close to or higher than 1.7, and it should be highlighted that


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Figure 15. Optical microscopy photographs showing the morphological evolution as a function of the supersaturation ratio β at 25 °C. (a) Rod-shaped crystals. (b) Mixture of rods and platelets. (c) Platelet-shaped crystals without inclusions. (d) Platelets with small inclusions. (e) Platelets with large liquid inclusions.

Figure 16. Schematic representation of the macroscopic features of crystals as a function of Tgrowth and β. Green area: mixture of rods and tablets. Blue area: threshold for the appearance of crystal defects. Red line: spontaneous crystallization of Form A.

Figure 17. Influence of the cooling rate from 30 °C to 22 °C on the formation and distribution of liquid inclusions and macroscopic cracks.

of the original ionic liquid [pbmim][PF6] reveals interesting and unusual features that may be used for future studies. In particular, it was established that the nucleation and growth behavior of [pbmim][PF6] in methanol presents a remarkable versatility illustrated by (i) the existence of two polymorphic

CONCLUSION Although the study of a single compound cannot be used to derive general statements applicable to most compounds of the same family, our investigations of the crystallization behavior



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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

forms in monotropic relationship under ambient pressure, with distinct cationic conformations and distinct crystal packings despite the similar nature of intermolecular (or inter-ionic) contacts and interactions; (ii) the formation of a metastable liquid-liquid demixing (oiling out) upon fast cooling that appears as a sufficient (and perhaps necessary) condition for the spontaneous crystallization of the metastable polymorph and (iii) a crystal growth behavior of the stable form characterized by a morphological transition at low supersaturation and by the formation of numerous liquid inclusions at high supersaturation. It can be suggested that this unusual diversity might be explained by two fundamental parameters. The first one is the molecular flexibility of the cationic moiety of [pbmim][PF6] involved in the conformational polymorphism that may also impact the crystal growth behavior and the formation of crystal growth defects if one assumes the existence of multiple solvated conformers in the mother liquor. The second parameter is more specific to ionic liquids since it is related to the relative weakness and poor directionality of intermolecular contacts usually depicted in the crystal packings of ionic liquids. By contrast with most organic crystals in which the crystal cohesion is ensured by well-identified and directional interactions such as hydrogen bonds or π-stacking, the stability of packings in crystalline ionic liquids relies mainly on diffuse, roughly isotropic and loose electrostatic contributions completed by a number of weak (C-H⋅⋅⋅O) hydrogen bonds. The resulting lower proportion of efficient and highly directional periodic bond chains (PBCs) may at least partially account for the versatility of their crystallization behavior. Further studies are however required to assess the validity of this interpretation. Moreover this ionic liquid might also constitute a valuable model compound for the study of nucleation mechanisms since the crystallization of a metastable polymorph from a liquid-liquid demixing might provide new insights in the frame of the two-step nucleation model assuming the existence of an intermediate dense precursor.

Note The authors declare no competing financial interest.

ACKNOWLEDGMENT The French Ministery of Research is acknowledged for financial support to C.B. via the E.D. n°351 (SPMII). Thanks are also due to Dr. Cécile Barbot and Dr. Morgane Sanselme (University of Rouen) for technical assistance in structure determination and to CRIHAN (Région Haute-Normandie, France) for providing access to software Materials Studio (v. 6.0, Accelrys Inc.).

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ASSOCIATED CONTENT Supporting Information. Cif files of the 2 crystal structures (data deposited to the CSD with refcodes CCDC 994846-994847). Synthesis route and analysis of compounds 2-6. Raman spectra of [pbmim][PF6] Form A and Form B. Solubility and metastable curves of [pbmim][PF6] in methanol. van’t Hoff plot (ln(solubility vs. 1/T) for [pbmim][PF6] Form A in methanol. Optical microscopy photographs of fluid inclusions in [pbmim][PF6] Form A, containing liquid and gas bubbles. Hydrogen bonds distances (in Å) and angles (in °) in the crystal structures of [pbmim][PF6] Form A and Form B. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Phone: +33235522428. Fax: +33235522959

Present address Prof. G. Gouhier: Princess Nora Bint Abdulrahman University, PO Box 84428, Riyadh, Saudi Arabia.

Author Contributions


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Crystal Growth, Structure, and Polymorphic Behavior of an Ionic Liquid

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