High-Pressure and Low-Temperature Polymorphism in C–H···F–C

Article pubs.acs.org/crystal

High-Pressure and Low-Temperature Polymorphism in C−H···F−C Hydrogen Bonded Monofluorotoluenes Joe Ridout and Michael R. Probert* Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, United Kingdom S Supporting Information *

ABSTRACT: Three fluorotoluene isomers, all liquids under ambient conditions, have been crystallized using two distinct in situ methods: cryo-crystallization (by decreasing the temperature below the freezing point) and through the application of high-pressure. In all three isomers, crystallization from highpressure results in different polymorphic forms to those observed at low-temperature: cryo-crystallization and compression of the pure liquid results in the formation of two distinct polymorphs of both 3- and 4-fluorotoluene, but only one polymorph of 2-fluorotoluene. However, compression of a 1:1 mixture of 2- and 3-fluorotoluenes results in a different polymorph of 2-fluorotoluene to that seen when crystallizing the pure liquid. The effects of temperature and pressure on the crystallization process in systems that have only weak C−H···F−C hydrogen bonds are confirmed to be very different.

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INTRODUCTION

chains are formed in preference to C−H···F−C hydrogen bonds.14 To probe this very weak interaction, only molecules containing C, H, and F should be examined to avoid competing with other supramolecular synthons that would likely dominate the weak C−H···F−C hydrogen bond.11 As a consequence of the weakness of the C−H···F−C interaction, materials whose crystal packing is driven by these very weak hydrogen bonds are often liquids at ambient conditions. The role of in situ crystallization of liquids in informing our perspective on intermolecular interactions has, in recent years, been increasingly realized. 15 The use of high-pressure crystallization as an alternative to cooling has been known for many years.16−21 Some liquids give the same structure when crystallized at high-pressure to that at low-temperature, for example, CS2 and 1,2-dichloroethane.22,23 However, it has been suggested that it is more common than not for different polymorphs to be produced24 under high-pressure and lowtemperature growth conditions from pure liquids (such as materials being disordered at low-temperature but ordered at high-pressure),25 yet the reason behind this phenomenon is poorly understood.26−32 Furthermore, it is possible for materials synthesized through a combination of pressure and temperature to have a different form to that synthesized purely through compression.33 Key differences between cryo- and high-pressure crystallization of pure liquids can be found in the kinetics and thermodynamics of the crystallization process. The densities of the systems are typically much higher when at

Polymorphism, defined by Dunitz as the existence of different crystal structures that melt to give identical liquid and vapor states,1,2 is of critical importance in such fields as materials chemistry, biosciences, and pharmaceuticals. For example, the pharmaceutical properties of different drug polymorphs, such as dissolution rates or bioavailability, have been shown to vary widely.3 In this study, we will examine the polymorphic structures of 2-, 3-, and 4-fluorotoluene generated via cryocrystallization or high-pressure crystallization. All three regioisomers are liquids under ambient conditions with melting points of −62, −87, and −56 °C, respectively, and their solid state structures are hitherto unknown. In the fluorotoluenes studied, polymorphism results from formation of distinct C−H···F−C hydrogen bonded networks. C−H···F−C bonds represent the weakest point on the hydrogen bond spectrum, on the basis of their long length and deviation from linearity, as shown by several extensive database and theoretical studies.4−7 This behavior has been attributed to the low polarizability of fluorine relative to its molecule volume.4 The bond lengths observed are typically high compared to the traditional hydrogen bond lengths, usually in the range 2.5−2.7 Å,8−12 although they do decrease with increasing s hybridization of the carbon atom, with a more traditional hydrogen bond length of 2.26 Å observed by Boese et al. for the sp hybridized C−H group in 4-fluoroethynlbenzene.13 The C−H···F−C hydrogen bond is sufficiently weak, on occasion even being outcompeted by interactions as subtle as C−H···π. Examples of this are to be found in 2fluoroethynlbenzene and 3-fluoroethynlbenzene. In one of each of the two polymorphs of both compounds, C−H···π © 2013 American Chemical Society

Received: December 3, 2012 Revised: March 18, 2013 Published: March 20, 2013 1943

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elevated pressures, and different crystal structures may represent the most thermodynamically favorable form at different pressures. Furthermore, the high rate of compression in high-pressure crystallization and any overpressurization will result in a faster nucleation time. This may result in polymorphism through metastable nuclei not having sufficient time to relax toward the thermodynamically most stable crystal form. As such, pressure-induced polymorphism of liquids gives insight into the flaws of the commonly held belief that increasing pressure is in some ways equivalent to decreasing temperature. Despite the wealth of literature examining C−H···F−C bonds using cryo-crystallography, examination of the role of this interaction at high-pressure is sparse. There are only 88 fluorine-containing structures in the CSD, of which many are near-duplicate structures observed at multiple pressures, that contain only inorganic fluorine, or where the role of the fluorine atom is not expected to be significant in molecular aggregation. Taking this into account, there are only a few examples where the role of organic fluorine could be considered to be significant in structure evolution.34−38 In all of these cases, there are still stronger supramolecular synthons present, with none of these structures containing only C, H, and F atoms. Herein, we explore and contrast the high pressure and low temperature solid state structures of the monofluorotoluenes, in an attempt to isolate weak driving forces of C− H···F−C interactions.

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Table 1. Data Collection Conditions (Close to the Liquid− Solid Phase Boundary) compound

low-Temperature (K)

high-pressure (kbar)

2-fluorotoluene 3-fluorotoluene 4-fluorotoluene

207 (2) 179 (2) 217 (2)

5.3 (2) 10.0 (2) 5.0 (2)

facility43 at Durham University. This system employs the Incoatec Ag IμS system,44 which uses multilayer focusing optics to allow a high flux density at the sample position. The choice of Ag X-radiation for highpressure work was made because the reduced wavelength (0.5608 Å), compared to Mo Kα, allows more data to be collected within the geometrical constraints of the DAC. Furthermore, the short wavelength results in lower absorption by the diamonds of the cell. Low-temperature data were collected on a Bruker SMART 6K CCD area-detector diffractometer using Mo Kα radiation (0.7107 Å). The program ECLIPSE45 was used to generate mask files correlating to areas occluded by the diamond anvil cell and hence excluded from the integration of high-pressure data. Integration, cell refinement, and scaling were carried out with SAINT46 and SADABS,47 respectively, as part of the Bruker Apex248 software suite. Structural solutions and least-squares refinements were carried out using the Olex249 interface to the SHELX50 suite of programs. Table 2 summarizes the crystallographic data for the various crystal forms of the three materials.

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RESULTS AND DISCUSSION Both 3- and 4- fluorotoluene crystallized into distinct forms at low-temperature and high-pressure, while structures of 2fluorotoluene were initially found to be isomorphous under both crystallization conditions. 2-Fluorotoluene. 2-Fluorotoluene was found to crystallize in the same polymorph (form I, Pbca) at both low-temperature and high-pressure from the pure liquid. Despite being isomorphous, the unit cell dimensions and hence densities of the low-temperature and high-pressure forms are very different (see Table 2). It is apparent that the ordering down the c axis follows a relatively complicated ABCDABCD packing arrangement, while there are head to tail 1D infinite chains formed along the b axis involving two adjacent c axis layers (AD or BC) (see Figure 1). There are two C−H···F−C hydrogen bonds per molecule between these adjacent layers that form a hydrogen bonding network in the bc plane (shown as light dotted lines in Figure 1). Two slightly shorter hydrogen bonds per molecule link neighboring molecules along the b axis in a 1D chain (shown as bold dotted lines in Figure 1). The principal intermolecular contacts at both low-temperature and highpressure are summarized in Table 3. 3-Fluorotoluene. 3-Fluorotoluene was found to crystallize in two distinct forms: low-temperature (P21/n, polymorph I) and high pressure (Pbca, polymorph II). The high-pressure form was grown using a modified crystallization protocol, as attempts to crystallize the liquid using pressure alone produced a glass. This method involved pressurizing the liquid to just below the liquid-glass phase boundary and then suspending the diamond anvil cell in liquid nitrogen (see experimental section for further detail). This process should be of great use in any case where simple pressurization and depressurization cycles does not form crystals. The low-temperature structure (shown in Figure 2) adopts a relatively complicated structure for such a simple molecule, ordering ABCDABCD in two dimensions, with infinite 2D sheets of molecules linked by isolated hydrogen bonded quadrilaterals formed by two distinct, fairly long hydrogen

EXPERIMENTAL SECTION

Low-temperature structures were obtained from in situ cryocrystallization on the diffractometer of a sample loaded into a Lindemann tube. The samples were flash-frozen, followed by temperature cycling around the melting point in order to grow crystals sufficiently large for study using single-crystal X-ray diffraction. A modified mount described by Yufit et al.39,40 attached to the ω-circle of the diffractometer was used in order to avoid icing around the Lindemann tube. The high-pressure structures were grown in a diamond anvil cell (DAC) of cutlet size 0.8 mm using a stainless steel gasket. The gasket was prepared to provide a chamber of approximately 0.3 mm diameter and depth 0.15 mm. The sample and a ruby chip were subsequently loaded into this chamber and then pressurized until the formation of microcrystals was observed. Pressure-cycling (at constant temperature) was then employed to grow a single crystal to fill the sample chamber. The pressure inside the chamber was measured using the Ruby R1 fluorescence method as described by Piermarini et al.41 In the case of 3-fluorotoluene, pressurization of the pure liquid led to the formation of a glass rather than crystalline material, requiring adjustment to the crystallization protocol: The sample was pressurized to change the phase from liquid to glass and was then immersed in liquid nitrogen. When the formation of a polycrystalline phase was not observed, the sample was warmed, and the pressure was decreased slightly; then, the sample was reimmersed in liquid nitrogen. This process was followed repeatedly until crystallization occurred. An overpressurization was then applied before the crystallites could melt. The sample was then allowed to warm to room temperature. Subsequent pressure cycling was then used to form a single crystal. Data were collected at just beyond the liquid−solid phase boundary rather than exactly at the melting point in order for the crystals to be stable with respect to the liquid phase throughout the diffraction experiment. The data collection conditions are shown in Table 1. One can see that, as expected, the deviation of temperature and pressure from ambient conditions required for crystallization are approximately inversely proportional. High-pressure data were collected on a diffractometer custom-built for high-pressure studies, XIPHOS II,42 at the XIPHOS diffraction 1944

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Table 2. Crystallographic Data compound empirical formula formula weight T (K) P (kbar) crystal system space group a (Å) b (Å) c (Å) β (deg) Z V (Å3) Dcalc (g cm−3) μ (mm−1) reflns collected unique reflns observed reflns completeness % R1 [I > 2σ] wR2 [all] goodness-of-fit

2-fluorotoluene (LT)

207(2) ambient orthorhombic Pbca 5.960(3) 14.019(9) 15.021(11) 90 8 1255.1(14) 1.166 0.087 2787 1064 870 81.77 0.0477 0.1348 1.080

2-fluorotoluene (HP polymorph 1)

2-fluorotoluene (HP polymorph 2)

3fluorotoluene (LT)

3-fluorotoluene (HP)

4fluorotoluene (LT)

4-fluorotoluene (HP)

ambient 5.5(2) monoclinic P21/c 8.773(13) 5.838(8) 10.740(15) 91.45(4) 4 549.9(13) 1.330 0.061 3975 494 236 47.77 0.0747 0.2191 1.056

C7H7F 110.13 179(2) ambient monoclinic P21/n 7.223(9) 7.623(10) 12.098(16) 105.67(3) 4 641.4(14) 1.140 0.085 1033 1051 747 71.08 0.0489 0.1520 1.121

ambient 10.0(2) orthorhombic Pbca 5.4526(12) 13.538(3) 14.735(4) 90 8 1087.7(5) 1.345 0.062 6993 833 593 71.19 0.0558 0.1549 1.093

217(2) ambient monoclinic P21/c 7.452(6) 5.987(6) 14.396(17) 104.00(3) 4 636.1(11) 1.150 0.086 1071 724 534 73.76 0.0474 0.1384 1.031

ambient 5.0(2) orthorhombic Pna21 13.215(4) 4.7872(8) 8.9622(19) 90 4 567.0(2) 1.290 0.059 3013 540 369 68.33 0.0499 0.1692 1.050

ambient 5.3(2) orthorhombic Pbca 5.807(2) 13.549(8) 14.280(6) 90 8 1123.5(9) 1.302 0.060 5772 755 347 45.96 0.0430 0.1316 1.113

Figure 1. Packing in 2-fluorotoluene (polymorph I). C−H···F−C contacts under or only marginally above the combined van der Waals’ radii (≤2.75 Å) have been highlighted in this and subsequent packing diagrams. The heavier and lighter dotted lines indicate shorter and longer C−H···F−C interactions, respectively.

Figure 2. 3-Fluorotoluene low-temperature structure showing the hydrogen bonding motif.

Table 4. Intermolecular Contact Distances for 3Fluorotoluene

Table 3. Intermolecular Contact Distances for 2Fluorotoluene; the First Hydrogen Bond Listed Is That Linking Molecules down the b Axis, the Second Is That Forming the Zig-Zag Motif in the bc Plane contact CF−HC

distance (Å) angle (deg)

low-temperature 2.71(3) 124(2)

2.71(3) 154(2)

contact CF−HC

high-pressure 2.716(13) 128.2(8)

low-temperature

distance (Å) angle (deg)

2.651(3) 138.61(16)

2.682(3) 136.9(2)

high-pressure 2.441(3) 150.6(2)

The hydrogen bonding network is also very different from the low-temperature polymorph. In the high-pressure structure, hydrogen bonding links layers A and B through a double helixtype motif involving relatively short hydrogen bonds (shown as a bold dotted line in Figure 3). Each molecule is linked to the next within the 1D chain along the b axis through a longer C− H···F−C hydrogen bond (shown as a light dotted line in Figure 3). The density is again much higher for the high-pressure polymorph (1.330 g cm−3) than for the low-temperature form (1.140 g cm−3). Intermolecular contacts for 3-fluorotoluene at

2.743(7) 152.6(10)

bonds of 2.651(3) and 2.682(3) Å (see Table 4) and their symmetric equivalents. The high-pressure structure (shown in Figure 3) exhibits little similarity to the low-temperature structure. There is ABCDABCD ordering along the c axis and head-to-tail ordering along the b axis similar to that in 2-fluorotoluene. 1945

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Figure 5. 4-Fluorotoluene high-pressure structure (no CH···F−C hydrogen bonds are observed).

Figure 3. 3-Fluorotoluene high-pressure polymorph, showing duallayered structure. Heavier and weaker dotted lines show shorter and longer C−H···F−C interactions, respectively.

intermolecular contacts for 4-fluorotoluene at both lowtemperature and high-pressure are summarized in Table 5.)

both low-temperature and high-pressure are summarized in Table 4. 4-Fluorotoluene. 4-Fluorotoluene crystallizes in two distinct forms: low temperature (P21/c, polymorph I) and high pressure (Pnma, polymorph II). Further analysis showed that compression of the polymorph I results in the formation of polymorph II. Both polymorphs formed layered structures with infinite 1D chains linked by C−H···F−C contacts. In both cases, the ordering down the infinite chain was layered head to tail, while that perpendicular was found to layer head to head (ABAB). In the low-temperature structure, each layer is aligned with the next, with disorder of the methyl hydrogen atoms. The highpressure form has a denser, fully ordered packing arrangement with the layers offset by approximately half a molecule (see Figures 4 and 5)

Table 5. Shortest C−H···F−C Intermolecular Contact Distances for 4-Fluorotoluene contact CF−HC

distance (Å) angle (deg)

low-temperature

high-pressure

2.589(3) 173.8(3)

2.832(2) 126.54(6)

It is known from theoretical and experimental work that polymorphs, which have high packing efficiencies at the expense of weaker hydrogen bonding motifs51 or halogen− halogen contacts,52 may be energetically stabilized at highpressure. We believe that the polymorphism shown herein demonstrates that interactions such as C−H···F−C are sufficiently weak that they may cease to be a structure-directing factor at high-pressure. In these cases, the maximization of packing efficiency dominates the final structure. In addition to the polymorphs formed by compression of the pure liquids, a new polymorph of 2-fluorotoluene (II) was formed during an attempt to cocrystallize a 1:1 ratio of 2- and 3-fluorotoluenes at high pressure. The observed structure is entirely distinct to that of polymorph I (created by cryocrystallization or compression of the pure liquid). The determined pressure was 5.5(2) kbar (as opposed to 5.2(2) kbar for polymorph I.) The calculated density is appreciably higher than the original polymorph (1.330 g cm−3, cf. 1.302 g cm−3). We subsequently increased the pressure on the form I from 5.2(2) kbar up to 11.6(2) kbar and did not observe a phase transition to form II. Reports of polymorphism achieved through changes in the high-pressure experimental methodology have been previously been reported, i.e., altering the rates of decompression and compression and using a different pressure-transmitting medium have both resulted in the formation of distinct polymorphs. This area of research has been reviewed thoroughly by Boldyreva and references therein.53 However, to the authors knowledge, this is the first report where the minimum application of pressure on a liquid to induce crystallization and subsequent pressure-cycling to form a single crystal has been show to result in two distinct polymorphs, although a similar effect has been previously observed in the cryo-crystallography of liquid mixtures.54 It is thought that this

Figure 4. 4-Fluorotoluene low-temperature structure, showing the formation of isolated C−H···F−C 1D chains.

As with 3-fluorotoluene, the packing efficiency of the two structures are very dissimilar, with the high-pressure polymorph being the denser (1.290 g cm−3, cf. 1.150 g cm−3). Given this observation, it is surprising that the C−H···F−C contacts in the high-pressure form are longer. (The shortest C−H···F−C 1946

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contacts at elevated pressure and C−H···F−C contacts at lowtemperature. Instead, C−H···F−C contacts were seen throughout the study, except in the high-pressure polymorph of 4fluorotoluene where neither type of contact was observed. There appears to be no obvious link between crystallization conditions and hydrogen bond length, with 2-fluorotoluene having similar length hydrogen bonds at high-pressure, those of 3-fluorotoluene being noticeably shorter and those of 4fluorotoluene being noticeably longer. The weakness of the C−H···F−C interaction in fluorotoluenes is thought to contribute to the observed polymorphic behavior as there are a wide variety of possible packing motifs possibly due to the dominance of nondirectional London dispersion forces to the total interaction energy. The structures adopted at high-pressure appear to principally correspond to the requirement for very close packing rather than the creation of particularly favorable C−H···F−C contacts. It is expected that the presence of strong hydrogen or halogen bonding between molecules would reduce the occurrence of high-pressure/low-temperature polymorphism. Work on this area using cryo- and pressure-induced crystallization is ongoing.

phenomenon is likely to be observed as a result of a different nucleation process at the liquid−solid phase boundary, as it seems most unlikely that two isostructural crystals of similar size would subsequently behave differently under a subsequent alteration of pressure. We believe that the most likely explanation for this observation is that the lower purity of the liquid meant that the liquid-to-solid nucleation process was slowed, allowing formation of a more thermodynamic product, which is consistent with the increased density of polymorph II. The packing order of the new structure is ABAB and ABCDABCD (a rotation of the methyl group stopping the ordering being AA and ABAB, respectively) when viewed down the stacking layers (see Figure 7), as opposed to AA and

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ASSOCIATED CONTENT

* Supporting Information S

Crystallographic information files. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 7. 2-Fluorotoluene high-pressure structure obtained from a 1:1 mixture of 2-fluorotoluene and 3-fluorotoluene at high-pressure.

AUTHOR INFORMATION

Corresponding Author

*(M.R.P.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ABCDABCD in the originally observed high-pressure polymorph. The intermolecular C−H···F−C contacts observed are similar in length (2.617(14) Å) to those seen in the low-density polymorph described above (2.61(2) and 2.65(2) Å). We repeated this experiment at low-temperature but found only poor quality crystals of polymorph I, indicating that either the kinetics or the thermodynamics of the high-pressure experiment combined with the presence of the second component causes the appearance of the second structure. Crystallization of a 1:1 ratio of 2- and 4-fluorotoluenes and a 1:1 ratio of 3- and 4-fluorotoluenes were both attempted but led only to the formation of the originally observed highpressure polymorph of 4-fluorotoluene.

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ACKNOWLEDGMENTS We are grateful to Prof. J. A. K. Howard, Prof. J. W. Steed, Dr. E. Pohl, and Dr. D. Yufit for excellent discussions, to Prof A. Beeby for contribution of the fluorescence spectrometer, and to Bruker UK Ltd. and EPRSC for financial support to both J.R. and M.R.P.

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CONCLUSIONS We have shown that altering the crystallization conditions (crystallizing a mixture as opposed to a pure liquid) results in polymorphism in 2-fluorotoluene, with two different forms observed at very similar pressures. This finding highlights the importance of kinetics (in addition to thermodynamics) in the production of different high-pressure polymorphs.53 For 3-fluorotoluene, direct compression results in a glass, whereas cryo-crystallization results in a crystalline form. A highpressure crystalline form can be isolated by a combination of low-temperature and high-pressure, and this form is distinct from the low-temperature form. C−H···F−C bond lengths and angles are, broadly speaking, similar to those found observed by Thalladi et al. for fluorobenzenes.11 Unlike the study by Parsons et al. on 4fluorophenol,34 we did not observe the formation of F···F 1947

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