Halogen Derivatives of Toluene under High Pressure - ACS Publications

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Halogen Derivatives of Toluene under High Pressure Szymon Sutuła, Roman Gajda, and Krzysztof Woźniak* Biological and Chemical Research Centre, Department of Chemistry, University of Warsaw, Ż wirki i Wigury 101, 02-089 Warszawa, Poland S Supporting Information *

ABSTRACT: Crystal structures of 4-chlorotoluene and 4bromotoluene under high pressure have been determined. In the case of 4-chlorotoluene, two high-pressure forms of this compound exist in the same range of pressure. However, no direct phase transition between them at high pressure has been observed. It has been confirmed that the symmetry of the highpressure structure of 4-iodotoluene is the same as its symmetry determined at low temperature. Finally, we have extended the pressure range in which the high-pressure structure of 4fluorotoluene was investigated. We have found that the space group previously reported (Pna21) as the proper one for the high-pressure form of 4-fluorotoluene is not correct as Pnma seems to be a better choice. 4-Bromotoluene formed only P21/ c space group crystals isostructural with the α form of 4-chlorotoluene. For the single crystals of 4-iodotoluene under high pressure, we only managed to determine the cell parameters. These values correspond with those determined previously at low temperature measurement.

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INTRODUCTION Standard single crystal X-ray diffraction data collection for small organic compounds is carried out mostly under ambient pressure and at low temperature. However, measurements under high pressure are more and more popular due to their profitability.1 Using a simple diamond-anvil cell (DAC), which is built according to the Merrill and Bassett design,2 one can easily obtain pressures up to a few gigapascals. The X-ray experiments at high pressure, which involve compression of single crystals already grown outside a DAC, need usage of a hydrostatic liquid. In this case, a previously prepared single crystal is immersed in such liquid, which can be a solution of many compounds. However, it is also possible to conduct the process of crystallization in situ, inside the DAC. In such a case, the DAC is filled only with a pure investigated compound, and no solvents or hydrostatic fluids are present. In some cases, structures grown from solution differ from those obtained at the same pressure from pure substances.3 Pressure of a few gigapascals is in many cases high enough to crystallize compounds that are liquids under normal conditions. The ability of a substance to exist in multiple forms, or crystal structures, is known as polymorphism. According to a more precise definition by Dunitz, polymorphism is the existence of different crystal structures that after melting always give identical liquid or vapor.4,5 Even if a low-temperature structure of an investigated compound is already known, measurements under high pressure can reveal the existence of a new, up to now unknown, polymorphs.3 It is frequently observed that molecules of a compound investigated under high pressure © XXXX American Chemical Society

are arranged differently depending on the pressure. It is true not only for molecular crystals of organic compounds6−8 but also for pure chemical elements such as nitrogen9 or crystals of inorganic compounds.10 The first step in the process of raising pressure upon a substance loaded in a DAC is isotropic compression of the crystal in each direction, which manifests at the molecular level as shrinking of the unit cell parameters. When potential voids between molecules are already squeezed, the next step is a distortion of the molecular conformation and geometry of intermolecular contacts. When changes are significant and symmetry of a crystal is affected, it ends with a phase transition. In some cases, even chemical transformations are observed.1 Changing external conditions is a way of affecting crystallization processes of chemicals. An impact of temperature changes on the crystal is predictable. The lower the temperature, the less vibrational energy of atoms around their equilibrium positions and the smaller thermal ellipsoids. What is more interesting, during polymorphic phase transitions, lower temperature leads to lower symmetry of crystals.11 The role of pressure is harder to predict and assumptions of its impact on the symmetry should be taken with a great caution. It is well-known that high pressure could have an ordering effect, and some structures of small organic compounds that are disordered at low temperature become ordered at high pressure.12 Received: August 21, 2016 Revised: February 21, 2017 Published: February 22, 2017 A

DOI: 10.1021/acs.cgd.6b01237 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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According to some more specific experiments, the knowledge of consequences of temperature and pressure on the substance behavior is sometimes insufficient to predict its phase diagram. Some transitions depend not only on pressure and temperature but also on the initial form. For example, the phase transitions of glycine have been widely explored, and it has been proven that the form of this amino acid cannot be totally specified just based on knowledge about the external conditions.13−18 Also in the case of paracetamol, a phase transition was observed that required pressurizing and then depressurizing of the initial form.19 Studies of simple halogen derivatives of toluene, such as 4iodotoluene20,21 and 4-fluorotoluene22 proved that in this type of compound both disorder and polymorphism are observed. We have decided to check whether rules concerning molecular aggregation and behavior under high pressure observed in the case of 4-fluorotoluene also apply in the case of toluene substituted by chlorine or bromine atoms. We would also like to answer the question what influence the type of halogen atom has on crystal structure and how significant the impact of the pressure on this structure and its potential ability to exhibit polymorphism is. High-pressure research on these compounds could be challenging because van Miltenburg et al.23 found that 4iodotoluene, 4-bromotoluene, and 4-chlorotoluene can form glasses. As those substances are cooled from their melting point temperatures, first they form a crystalline phase. When cooled further, a glass transition occurs. However, the impact of the pressure on the glass transition has not been studied. The crystal structure of toluene (and its polymorphism) is already well-known.24,25 Even recently, some new high-pressure forms of toluene have been discovered.26 When a molecule or its significant fragment (for example, toluene) shows propensity to create polymorphs under pressure, it is rational to assume that also its halogen derivatives would show a similar tendency. One of goals of our investigations is to find new high-pressure polymorphs of the investigated compounds.

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Figure 1. Selected photographs depicting particular stages of growing single crystals of investigated compounds inside DAC: (a) 4fluorotoluene, (b) 4-chlorotoluene, (c) 4-bromotoluene, and (d) 4iodotoluene. crystal of 4-chlorotoluene is shown in Figure 1b. The shown sequence was taken while cooling the substance within the range of temperature from 170 to 154 °C under the pressure of 0.19(10) GPa, observed using a microscope with polarizers. The steps of growing a crystal of 4bromotoluene within the range of 116−103 °C under the pressure of 0.08(10) GPa are shown in Figure 1c. A sharp shape of the ruby chip is visible on the left upper edge of the chamber. The crystal of 4iodotoluene growing under the pressure of 0.65(10) GPa within the temperature range of 98−90 °C is shown in Figure 1d. After X-ray data collection of a single crystal grown inside the DAC at particular pressure, we slightly increased the pressure and collected the X-ray data for the studied crystal again. At some point, after several measurements, the pressure was so high that melting point was not achievable using our heat gun. Exceeding such pressure led to an inevitable cracking of the crystal which lowered the quality of the collected data. The experiments were stopped when there were too many cracks and the unit cell could not be found. X-ray Measurements. All high-pressure single-crystals described in this paper were obtained inside a DAC of the modified Merrill and Bassett design,2 and our X-ray measurements were conducted by using a SuperNova single source diffractometer (Ag radiation, λ = 0.56085 Å). The CrysAlis PRO program was applied29 for the data collection and its further reduction. The structures were solved by direct methods using the SHELXS program and refined with SHELXL.30 An analytical numeric absorption correction using a multifaceted crystal model based on expressions derived by Clark and Reid was used.31 The program Absorb732,33 was used for correcting the data for the DAC absorption, gasket shadowing, and absorption of the sample itself. Structures were then solved and refined with the use of the Olex2 program.34 Measurements in a Piston−Cylinder High-Pressure Apparatus. Apart from the X-ray measurement, in the case of 4chlorotoluene, a measurement of compressibility in a piston−cylinder apparatus was conducted. The procedure is simple and goes as follows. In the first step, the cylinder is filled with a liquid substance (known volume of 9.8 cm3). Then the piston closes the vent and is pushed to exert the pressure upon the fluid. This reduces the volume that the examined substance occupies. By measuring the space, the substance is confined within and the force acting upon the piston, the single molecule volume dependence on pressure can be plotted. When such a curve exhibits a discontinuity at some pressure, this suggests that a phase transition of the first order occurs, should it be either a crystallization or a polymorphic transition.

EXPERIMENTAL SECTION

Derivatives of toluene substituted at the para position were bought from ABCR and used without further purification. 4-Fluorotoluene and 4-chlorotoluene are liquids under ambient conditions with approximated melting points of −56 and 7 °C, respectively. 4Bromotoluene and 4-iodotoluene are solids with the melting points of 28 and 34 °C, respectively. Crystallization. In each case, the crystallization process was conducted in a similar way. First, liquid halogen derivative of toluene and some ruby chips were loaded into the DAC, and the pressure was increased to the point where crystallization occurred. Mostly, polycrystalline forms are obtained in this way. As a second step, the whole sample is slowly heated using a heat gun. As the temperature approaches the melting point (corresponding to the increased pressure), all crystallites start melting. When there is only one crystallite left, the third step, in which the sample is slowly cooled, begins. During such a procedure the single crystal grows until it fills the whole chamber. After the sample reached room temperature, the pressure inside the DAC was calculated. Calculations are based on measured shift in wavelength of the R1 line peak of ruby fluorescence spectrum and corrected pressure scale by Mao et al.27,28 The sequence of photographs, showing the process of crystallization in the DAC, are presented in Figure 1a−d. The process of growing a single-crystal of 4-fluorotoluene is shown in Figure 1a. The photos were taken under the pressure of 0.53(10) GPa within the range of temperature from 93 to 71 °C. The ruby chip is clearly visible on the left edge of the chamber. The growth of a single

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RESULTS Structure of 4-Fluorotoluene. The crystal structure of 4fluorotoluene has already been determined at low temperature B



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as well as at high pressure by Ridout and Probert.22 However, the upper limit of the pressure achieved in their work was only 0.5 GPa, and our intention was to extend this range to check if there is any phase transition at the higher pressure. We have investigated crystals of 4-fluorotoluene in a wider range of pressures from 0.54(10) to 2.48(10) GPa (Table 1). Results of

function of resolution are in the range from 0.9 to 1.05, which suggests the centrosymmetric space group. The difference between the unit cell parameters in Pna21 and Pnma settings is that b and c are simply switched over (Table 1. In the Pnma, only half of the molecule of 4-fluorotoluene is present in the asymmetric unit, whereas in the asymmetric part of Pna21, the whole molecule is present. Hydrogen atoms of the methyl group can exhibit a disorder as this moiety is allowed to rotate and locate at a few different positions. Projections of packed molecules (drawn for the Pnma space group) of 4fluorotoluene under the pressure of 0.93(10) GPa viewed along the main crystallographic axes are shown in Figure 2a−c.

Table 1. Crystal Data and Details of Data Collection for 4Fluorotoluene empirical formula formula wt [g/mol] pressure [GPa] temp [K] cryst syst space group a [Å] b [Å] c [Å] V [Å3] Z dcalc (g cm−3) μ (mm−1) reflns collected unique reflns obsd reflns completeness % R1 [I > 2σ] wR2 [all] goodness-of-fit largest diff. peak/hole [e Å−3]

0.54(10) ambient orthorhombic Pnma 13.248(1) 8.986(1) 4.793(1) 570.72(6) 4 1.282 0.06 8448 543 387 72.2 0.056 0.141 1.12 0.09, −0.13

C7H7F 110.13 0.93(10) ambient orthorhombic Pnma 13.091(1) 8.854(1) 4.727(1) 548.02(10) 4 1.335 0.06 3959 510 344 72.0 0.071 0.159 1.10 0.15, −0.19

1.55(10) ambient orthorhombic Pnma 12.873(2) 8.729(1) 4.613(2) 518.42(18) 4 1.411 0.07 2973 477 330 73.7 0.070 0.117 1.19 0.13, −0.21

particular measurements have confirmed that within this pressure range the structure remains stable and no phase transition has been observed. As the pressure rises, the unit cell parameters continuously shrink but still correspond to those obtained by Ridout and Probert. However, careful insight into the details of the X-ray results led us to change our opinion about the space group in which 4-fluorotoluene at highpressure does crystallize. Ridout and Probert22 described the space group of the high-pressure structure of 4-fluorotoluene as acentric Pna21, but we have found that it is possible to solve the structure also in the centrosymmetric Pnma space group. Choosing the Pnma instead of Pna21 seems to be a better option not only because this space group has a higher symmetry, but also it is justified by point parameters, for example, |E2 − 1|. To restrict or even exclude the possibility that intensity distribution across the reciprocal space is distorted by low completeness of the data or biased by selecting a particular space group, we have conducted some calculations. For all three measurements (each conducted at different pressure), we have checked the |E2 − 1| statistics, simultaneously changing the resolution of measurements. In general, the mean value |E2 − 1| is different for noncentrosymmetric (0.74) and centrosymmetric (0.97) structures. We varied resolution between 0.75 and 1.1 Å with the increment of 0.05 Å, which gives 8 values of |E2 − 1| for each pressure. We have performed this check for the unmerged data analyzed in the P1 space group of the triclinic crystal system. The plot showing output of these calculations is presented in Supporting Information (SI). Our check shows that the lower resolution of X-ray measurement the lower the value of |E2 − 1| statistics. However, the calculated values of |E2 − 1| as a

Figure 2. Molecular arrangement in the structure of 4-fluorotoluene; (a−c) views along the a, b, and c axes, respectively. Red dotted lines depict C−H···F contacts and blue dotted lines C−H···C contacts.

As predicted, the cell parameters of 4-fluorotoluene shrink as the pressure rises. Also, as the molecules are packed more tightly, the closest contacts tend to shorten as well. The distance between the benzene ring carbon (C2) atom and H2 hydrogen atom in the neighboring molecule shortens from 2.856 Å (p = 0.54(10) GPa) to 2.813 Å (p = 0.93(10) GPa) and to 2.722 Å (p = 1.55(10) GPa). The distance between fluorine and benzene ring hydrogen (H2) in next molecule shortens from 2.853 Å (p = 0.54) to 2.756 Å (p = 0.93(10) GPa) and to 2.667 Å (p = 1.55(10) GPa). Intermolecular contact between fluorine and disordered hydrogen from the C



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parameters (and as a result, the volume per molecule) are systematically underestimated. This may result from overestimation of electrostatic interactions present in such calculations. In the case of the theoretical data fitted to Poirier−Tarantola’s EoS, the difference in comparison with the experimental data is more or less constant within the whole pressure range (ca. 10 Å3). In the case of the other types of EoS, the higher the pressure, the smaller the difference. The difference between experimental data and calculated data fitted to the Murnaghan’s EoS at 2.5 GPA is only ca. 2.5 Å3. Structure of 4-Chlorotoluene. The first attempt to determine the crystal and molecular structure of 4-chlorotoluene was done by Mentzen.37 He sorbed this substance in a zeolitic material of the monoclinic symmetry at room temperature. After sorption of 4-chlorotoluene, those complexes crystallized in the Pnma space group. It is worth underlining that in such conditions the compound exhibited head-to-tail disorder. In our research, when applying the high pressure to the pure sample of 4-chlorotoluene loaded in a DAC, we have obtained two polymorphs. The first polymorph, described further as α, crystallized just at 0.07(10) GPa in the P21/c space group of the monoclinic crystal system. The unit cell contains two independent molecules, both placed at the special positions. The center of symmetry is localized at the center of the benzene ring, thus only half of the molecule is present in the asymmetric unit. As a result, a static positional disorder is observed, which is demonstrated within the molecule by the chlorine atom exchanging position with the methyl group. Projections of packed molecules viewed along the main crystallographic axes for this structure are shown in Figure 5a−c. The α phase of 4-chlorotoluene was observed within the pressure range from 0.07(10) to 2.43(10) GPa. After filling the high-pressure chamber with the liquid 4-chlorotoluene, the pressure was increased extremely slowly to catch the point of crystallization (pressure freezing point). As the pressure was rising, the unit cell parameters were continuously shrinking, but the structure remained stable and no phase transition was observed. In total, during the process of increasing the pressure, eight X-ray measurements were conducted (each at different pressure). Structural results for the α polymorphs of 4chlorotoluene are shown in Table 2. To obtain more data points and build a smooth chart of molecular volume as a function of pressure, another DAC was filled with pure 4-chlorotoluene. However, this time pressure was increased quite rapidly to 0.4 GPa. Surprisingly, the results of the X-ray measurements indicated that a different polymorph of 4-chlorotoluene was crystallized. This second form will further be described as the β phase. It was investigated within the range of pressure from 0.4(10) GPa to 2.2(10) GPa. There are also two molecules in the unit cell (half of the molecule in the asymmetric unit) in the phase β, and the same type of a disorder is observed. The crucial detail, which makes the difference, is the relative orientation of molecules within the unit cell. The packing of molecules viewed along the main crystallographic axes for this structure is shown in Figure 6a−c. The space group P21/n is an alternative setting of P21/c and could be simply transformed into it by different choice of cell axes (see SI). However, in the case of the polymorphs of 4chlorotoluene, the differences between the unit cell parameters of both structures are caused by different molecular arrangement, not by an arbitrary choice of an unconventional cell

methyl group changes from 2.747 Å (p = 0.54) to 2.708 Å (p = 0.93(10) GPa) and to 2.627 Å (p = 1.55(10) GPa). The shortest and the strongest H···π interaction observed in this structure is present between the benzene ring and the methylgroup disordered hydrogen atom from a parallel molecule, and this distance varies from 2.796 Å (p = 0.54(10) GPa) to 2.701 Å (p = 0.93(10) GPa) and to 2.597 Å (p = 1.55(10) GPa). No valence angle change is noticeable. Mentioned shortest hydrogen bonds are shown in Figure 3.

Figure 3. Shortest hydrogen bonds in the structure of 4-fluorotoluene. Symmetry codes: (i) x, y, z; (ii) x, 0.5 − y, z; (iii) x, y, −1 + z; (iv) x, 0.5 − y, −1 + z; (v) 0.5 − x, −y, 0.5 + z; (vi) 0.5 − x, −0.5 + y, −0.5 + z; (vii) 0.5 + x, 0.5 − y, 0.5 − z; (viii) 0.5 + x, y, 0.5 − z.

The crystal structure of 4-fluorotoluene was also investigated with use of the CRYSTAL14 program.35,36 The details of these calculations are described in SI. The results (molecular volume as a function of pressure) fitted to four different equations of state such as Murnaghan’s, third-order isothermal Birch− Murnaghan’s, “universal” logarithmic Poirier−Tarantola’s and exponential Vinet’s, as well as experimental data, are presented in Figure 4. The slope of the experimental trend line correspond with the shapes of those calculated theoretically. However, results of DFT calculations show that the unit cell

Figure 4. Calculated and experimental molecular volume of 4fluorotoluene (in crystal structure) as a function of pressure. D



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Table 2. Crystal Data and Details of Data Collection for the α Phase of 4-Chlorotoluene empirical formula formula weight [g/mol] pressure [GPa] temp [K] cryst syst space group a [Å] b [Å] c [Å] β [deg] V [Å3] Z dcalc (g cm−3) μ (mm−1) reflns collected unique reflns obsd reflns completeness % R1 [I > 2σ] wR2 [all] goodness-of-fit largest diff. peak/hole [e Å−3]

0.07(10) ambient monoclinic P21/c 7.725(1) 7.277(7) 6.028(1) 94.544(7) 337.8(3) 2 1.244 0.24 1563 268 227 34.4 0.073 0.181 1.26 0.07, −0.07

C7H7Cl 126.58 0.19(10) ambient monoclinic P21/c 7.660(1) 7.228(8) 5.978(1) 94.273(11) 330.1(4) 2 1.273 0.24 1879 283 241 41.9 0.077 0.209 1.28 0.17, −0.14

0.47(10) ambient monoclinic P21/c 7.600(1) 7.087(3) 5.938(1) 93.669(13) 319.20(16) 2 1.317 0.25 1467 365 266 50.3 0.137 0.370 1.21 0.31, −0.31

and it identifies crystal packing fragments of similar geometry, so-called, supramolecular constructs (SCs). The level of isostructurality is described by dissimilarity index x, which gives information about how far two structures deviate from perfect geometrical similarity.40 Each molecular crystal structure is described by a cluster build of central molecule and the n molecules that form its first crystal environment. Two different crystal structures are then compared simply by comparing their so defined clusters. Such a comparison gives a set of parameters δa,i and δp,i which are the mean differences for corresponding intermolecular angles and planes. The dissimilarity index x is obtained from all M data points (δa,i, δp,i) that can be generated for the cluster fragments concerned: M

M

x = 1/M ∑ xi = 1/M ∑ (δa, i 2 + δp, i 2)1/2 i=1

i=1

Additionally, a distance similarity parameter can be calculated: n

n

d = 1/n ∑ dj = 1/n ∑ |l1, j − l 2, j| j=1

j=1

where l1,j and l2,j are distances from the center of the central molecule to centers of surrounding molecules in the clusters of two crystal structures and 0 < j ≤ n. In this way, d is a measure of the overall change in intermolecular distances. According to Gelbrich and Hursthouse, x values smaller than 1° are found for SCs with high similarity, where SCs of lowdegree similarity produce the x values of 6° or even higher. In our case, x is equal 6.5°, d varies between 0.29 and 0.37 Å, and in both structures slightly different 1-D chains are observed. The X-ray data collections conducted in a DAC revealed that there is a pressure range in which both polymorphs can occur. Moreover, no symptoms of phase transition were observed when crystals were grown inside the DAC. To determine the exact values of pressure of the phase transition from the liquid to the solid state, and possibly from phase α to phase β, the compressibility of 4-chlorotoluene was

Figure 5. Molecular arrangement in the structure of 4-chlorotoluene (the phase α); (a−c) views along the a, b, and c axes, respectively. Red dotted lines depict Cl···Cl intermolecular contacts.

setting. The differences of molecular arrangements for the both structures of the polymorphs of 4-chlorotoluene are clearly visible when the structures are overlapped (see Figure 7). When two molecules (one from the phase α and one from the phase β) are superimposed, the rest of the structure splits up. Selected structural data for the β polymorph of 4-chlorotoluene are shown in Table 3. Comparison of the Polymorphs using XPac Analysis. We have used the XPac program38,39 to compare both crystal structures. This software allows for comparison of the corresponding internal coordinates of the studied structures, E



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lines show the results for crystals measured in the DAC by using X-ray diffraction for the phases α and β, respectively. The experiment in a cylinder−piston apparatus was carried out at room temperature. As a result, we obtained a plot where a discontinuity at 0.09 GPa (which slightly differs from the pressure of the first crystal in the phase α) clearly corresponds to the phase transition from the liquid to the α-form. However, no discontinuity that may suggest a phase transition from the αform to the β-form was observed in this experiment. Due to the hardware limitations, the experiment was conducted up to 1.2 GPa only. The green dashed line in Figure 8 allows for extrapolation of these results up to higher values of pressure. The comparison of its trace with the trend line for results obtained from the X-ray experiment for polymorph α shows that the slopes of these lines are slightly different, and under pressure higher than ca. 1 GPa, the molecular volumes for the α-form are smaller than predicted by compression in a cylinder−piston apparatus. A comparison of the trend lines depicting molecular volumes for the α and β forms shows that the β form has always visibly smaller volume than form α within the whole pressure range in which these polymorphs exist. An explanation why both phases are observed within the same pressure range could be that one of them is metastable. Possibly, the α phase, which was twice crystallized by a very gentle decreasing of pressure (once in a DAC and the second time in the piston−cylinder apparatus) and has lower density (greater molecular volume) is the metastable one. Probably there is a particular value of pressure, that when exceeded, should lead to a rearrangement of molecules to the more densely packed phase β. However, no such transition has been observed up to 2.4(10) GPa. The molecular arrangement in phase α is noticeably different than the one in phase β. As a result, also intermolecular interactions differ, which can be easily observed within the investigated structures. In phase α, the molecules are arranged into layers, and because of the disorder, relatively short intermolecular Cl···Cl contacts are observed (3.474(17) Å at p = 0.07(10) GPa, 3.389(22) Å at p = 0.47(10) GPa). However, the shortest observed contacts are the H···π-electron ones (2.779 Å at p = 0.07(10) GPa, 2.754 Å at p = 0.19(10) GPa, 2.656 Å at p = 0.47(10) GPa) formed between the hydrogen atom bonded to the benzene ring and the benzene ring of another closest molecule. This is not surprising, because it is well-known that molecular packing of benzyl derivatives is strongly affected by the directing influence of the C−H···π interactions.41 With rising pressure, the mentioned contacts shorten, and the angle formed by carbon, bonded to a chlorine atom and another interacting chlorine changes from 155.2° (p = 0.07(10) GPa) to 152.1° (p = 0.19(10) GPa) and to 147.5° (p = 0.47(10) GPa). However, the angle between planes defined by the benzene rings of the molecules remains rather stable in the range of measured pressures and varies between the values of 51.3° and 52.1°. Mentioned intermolecular contacts are shown in Figure 9. In phase β, the shortest intermolecular halogen contact varies from 3.499(9) Å (p = 0.40(10) GPa) to 3.385(5) Å (p = 1.04(10) GPa) and the shortest type of H···Cl contact is between a disordered hydrogen linked to the methyl group and a disordered chlorine atom (2.999 Å at p = 0.40(10) GPa, 2.867 Å at p = 1.04(10) GPa). The shortest intermolecular contacts are observed between the hydrogen atoms where one of them

Figure 6. Molecular arrangement in the structure of 4-chlorotoluene (phase β); (a−c) views along the a, b, and c axes, respectively. Red dotted lines depict Cl···Cl contacts and blue dotted lines C−H···C contacts.

Figure 7. Effect of overlapping of polymorph α (the red molecules, view along the c axis) and β (the green molecules, view along the a axis). Only molecules embedded within the navy-blue ellipse are perfectly overlapped.

measured. The results of this experiment in the form of a plot are shown in Figure 8. The green line shows the results of compressibility experiment, while the blue and red dots and F



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Table 3. Crystal Data and Details of Data Collection for the β Phase of 4-Chlorotoluene empirical formula formula wt [g/mol] pressure [GPa] temp [K] cryst syst space group a [Å] b [Å] c [Å] β [deg] V [Å3] Z dcalc [g cm−3] μ [mm−1] reflns collected unique reflns obsd reflns completeness goodness-of-fit (F2) R1 [I > 2σ(I)] wR2 [all data] largest diff. peak/hole [e Å−3]

C7H7Cl 126.58 0.40(10) ambient monoclinic P21/n 5.735(1) 5.041(1) 11.227(5) 102.329(17) 317.15(14) 2 1.325 0.25 2492 353 294 48.3 1.21 0.039 0.090 0.08, −0.07

0.76(10) ambient monoclinic P21/n 5.683(1) 4.955(1) 11.029(4) 102.534(15) 303.22(11) 2 1.386 0.26 2561 341 316 48.4 1.21 0.042 0.073 0.08, −0.09



the angle between the planes defined by the benzene rings of the molecules remains rather stable in the range of measured pressures. In phase β, its value varies between 86.5° and 87.5° forming almost a right angle. Mentioned intermolecular interactions within phase β are shown in Figure 10.

Figure 8. Pressure dependence of the volume per molecule of 4chlorotoluene. Results of compressibility experiment (green solid line) and its extrapolation (green dashed line) are combined with molecular volume obtained from X-ray diffraction experiments conducted for both polymorphs (blue and red dots). Vertical black dashed line represents the pressure of phase transition from liquid to solid state.

Figure 10. Intermolecular interactions in the structure of 4chlorotoluene (phase β). Symmetry codes: (i) x, y, z; (ii) 1 − x, 1 − y, 1 − z; (iii) x, 1 + y, z; (iv) 1 − x, − y, 1 − z; (v) 1.5 − x, 0.5 + y, 0.5 − z; (vi) 0.5 + x, 1.5 − y, −0.5 + z; (vii) 0.5 − x, −0.5 + y, 0.5 − z; (viii) −0.5 + x, y − 0.5, −0.5 + z.

Structure of 4-Bromotoluene. Crystals of 4-bromotoluene were grown in the DAC and measured under high pressure of 0.08(10) GPa. During the cooling process, the substance did not form a clear and transparent crystal, and it always exhibited many minor fractures appearing on the surface. Despite this specific reluctance to make visually perfect crystals by this compound, it was still possible to solve and refine the crystal structure. The determined structure is isostructural with the α form of 4-chlorotoluene with cell parameters a = 7.846 (1) Å, b = 7.229 (1) Å, c = 6.068 (4) Å, and β = 93.043 (18)° of P21/c space group. It is worth mentioning that a very similar substance, 4chlorobromobenzene, has already been examined at low

Figure 9. Intermolecular interactions in the structure of 4chlorotoluene (phase α). Symmetry codes: (i) x, y, z; (ii) 1 − x, 1 − y, 1 − z; (iii) 1 + x, y, 1 + z; (iv) 2 − x, 1 − y, 2 − z; (v) 1 + x, 0.5 − y, 0.5 + z; (vi) 2 − x, −0.5 + y, 1.5 − z.

belongs to the benzene ring and the other one to the disordered methyl group (type H···H, 2.573 Å at p = 0.40(10) GPa, 2.333 Å at p = 1.04(10) GPa). There are also relatively close contacts between the disordered chlorine and benzene ring (Cl··· π contacts). They change from 3.680 Å (p = 0.40(10) GPa) to 3.527 Å (p = 1.04(10) GPa). As in phase α, G



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temperature by Klug.42 He determined that this compound also crystallizes in the P21/c space group and exhibits disorder as chlorine and bromine atoms have a 50% occupancy on both sides of the ring. What is more interesting, it has been proven that 4-dichlorobenzene and 4-dibromobenzene also crystallize in P21/c space group.43 In the case of those substances, no disorder is observed as those molecules initially are centrosymmetric. In 1999, 4-chloroiodobenzene was studied by Meriles et al.,44,45 and it was found to crystallize in the structure isomorphic with those already mentioned. Structure of 4-Iodotoluene. The structure of 4iodotoluene has already been examined at ambient conditions after growing a crystal using the melted-zone technique, but no results of high-pressure measurements have been published so far.20 The crystal structure of 4-iodotoluene has also been examined by Serrano-Gonzalez et al.21 They crystallized 4iodotoluene by a slow sublimation and discovered that, in such circumstances, it crystallizes in the orthorhombic P212121 space group with molecules exhibiting head-to-tail disorder. They proved that the disorder does depend on temperature. In their research, at 173 K, only 8% of molecules were disordered, and at 273 K this fraction increased up to 17%. They also claimed that if under specific conditions the disorder reached 50% then some additional inversion centers would be generated and the whole structure would be described as Pbca. We have taken a challenge of growing a single crystal of 4iodotoluene in the DAC and conducting appropriate high pressure X-ray measurement. Several attempts have been taken, but when temperature of the grown single-crystal decreases below ca. 30 °C, the crystal cracks, and many smaller crystals rapidly start growing on its surface. The procedure has been repeated under various pressures, but a single crystal sufficient for structure refinement has never been obtained. However, the quality of the obtained crystals were good enough to determine parameters of the unit cell itself. By comparison of the unit cell parameters, we can infer that 4-iodotoluene crystallized under pressure of 0.03(10) GPa is the same structure as the one solved by Ahn et al. at room temperature.20 The unit cell parameters correspond to each other (data from Ahn et al. in the brackets): a = 7.492 Å (a = 7.46 Å), b = 16.34 Å (b = 16.50 Å), c = 6.146 Å (c = 6.11 Å).

Single crystals of 4-iodotoluene under high pressure were difficult to obtain, but we managed to determine the cell parameters. These values correspond with those determined previously at low temperature measurements.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01237. Description of the DFT calculations in CRYSTAL14, short note on 4-chlorotoluene, polymorph β, and illustration of |E2 − 1| statistics as a function of resolution for 4-fluorotoluene X-ray data (PDF) Accession Codes

CCDC 1499205−1499214 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Krzysztof Woźniak. Mailing address: Chemistry Department, University of Warsaw, ul. Pasteura 1, 02-093 Warszawa, Poland. Phone: +48 22 55 26 391. E-mail: [email protected]. ORCID

Roman Gajda: 0000-0002-9742-2941 Krzysztof Woźniak: 0000-0002-0277-294X Author Contributions

S.S. and R.G. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported in part by PL-Grid Infrastructure and in part by 120000-501/86-DSM-112 700.

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REFERENCES

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CONCLUSIONS We have examined toluene derivatives substituted by halogen atoms at the para position. All of the single crystals were crystallized and measured under high pressure in DAC. Investigation of 4-fluorotoluene indicates that it crystallizes in Pnma space group and up to ca. 2.5 GPa no phase transition is observed, whereas 4-chlorotoluene exhibits polymorphism and can crystallize either in P21/c (phase α) or in P21/n (phase β) space group. Both phases were observed within the same pressure range, but no direct phase transition between them was noticed. Most probably, phase α is only a metastable form, and at a certain pressure it should finally rearrange to phase β, but such pressure was not achieved in current experiments. 4Bromotoluene in our research formed only P21/c space group crystals that are isostructural with the α form of 4chlorotoluene. On the base of our research, we see that inserting different types of halogen atoms into the molecule (chlorine or bromine instead of fluorine) causes not just a simple elongation of unit cell parameters but a significant rearrangement of the molecules. H



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Halogen Derivatives of Toluene under High Pressure - ACS Publications

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