Pressure-Dependent Formation and Decomposition of Thiourea

Article pubs.acs.org/crystal

Pressure-Dependent Formation and Decomposition of Thiourea Hydrates Hanna Tomkowiak, Anna Olejniczak, and Andrzej Katrusiak* Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznań, Poland S Supporting Information *

ABSTRACT: High pressure can favor the formation of either thiourea hydrates or anhydrates. Above 0.60 GPa thiourea crystallizes as monohydrate (NH2)2CS·H2O, while only anhydrous thiourea is obtained from aqueous solution at normal conditions. At 0.70 GPa another hydrate, 3(NH2)2CS·2H2O, is formed, but above 1.20 GPa anhydrous thiourea becomes stable again. The single crystals of both hydrates were grown in situ in a diamond-anvil cell and their structures were determined by X-ray diffraction. The structural factors favoring the formation of hydrates above 0.6 GPa involve new types of hydrogen bonds to water molecules and the more efficient molecular packing. The crystallization of thiourea anhydrate above 1.20 GPa coincides with the stability region of ice VI.

1. INTRODUCTION Formation of solvates can be advantageous in pharmaceutical and agriculture industries, because the solvation of bioactive compounds can improve their properties, such as solubility and bioaccessibility.1,2 Thiourea, diamide of thiocarbonic acid, (NH2)2CS, and its derivatives can be used as insecticides, dyes, plant protection agents, pesticides, corrosion inhibitors and fungicides. It is frequently applied in metal refinery and in organic synthesis as a catalyst and substrate, as a component of fertilizers and explosives.3 It has been extensively studied at normal conditions and at various temperatures and pressures.4−24 Its molecular symmetry was investigated by X-ray diffraction already in 1928,4 but the correct structure was determined in 1932.5 In 1956 the ferroelectric behavior of thiourea was discovered.6 The thermodynamic behavior of thiourea is very different from that of its close structural analogue, urea (NH2)2CO, for which no polymorphs were detected at 0.10 MPa.25 Both urea and thiourea crystallize as anhydrates at 0.10 MPa, and to our knowledge there have been no reports on their hydrates. Both these compounds were intensely studied at high pressure.7−14,26,27 At least five phase transitions were described for thiourea below room temperature. At normal conditions it crystallizes in orthorhombic space group Pnma (phase V); below 202 K9−12 it undergoes a sequence of phase transitions between four modulated phases, and below 169 K it ensures a ferroelectric phase I, space group P21ma,15 related to phase V by the group-subgroup relation mmmF2mm.28 High-pressure studies revealed that at 0.34 GPa phase V transforms to a nonpolar phase VI.7−9,16 Gesi9 reported that there is another phase (VII) above 0.54 GPa. No hydrates have been obtained of urea crystallized at high pressure of aqueous solution,25 and no such crystallizations of thiourea have been reported. However we have found that at high-pressure thiourea can easily form hydrates. Hence our study was aimed at determining the crystal structures of © XXXX American Chemical Society

thiourea hydrates and understanding the mechanism and interactions leading to such a different behavior and properties of the sister compound urea.

2. EXPERIMENTAL SECTION The experiments were performed on the samples in situ crystallized in a modified high-pressure diamond anvil-cell (DAC),29 with the anvils directly supported on the backing plates. Pressure in the DAC chamber was calibrated by the ruby-fluorescence method,30,31 with a Photon Control Inc. spectrometer, with an accuracy of 0.02 GPa; the calibration was repeated before and after each diffraction measurement. The crystallization of thiourea aqueous solution (75:25 vol.) leads either to the anhydrous or hydrated thiourea depending on pressure. Below 0.60 GPa only anhydrous crystals were obtained. Isochoric crystallization above 0.60 GPa resulted in thiourea monohydrate (NH2)2CS·H2O (Figure 1). After performing an X-ray diffraction measurement for this single crystal, it was compressed to 1.20 GPa and the X-ray measurement was repeated. The structure determination showed that the crystal remained in the form of monohydrate (NH2)2CS·H2O, with its unit cell and structure compressed as expected for the applied pressure change.32−35 However, when the in situ crystallization was performed at 0.70 GPa, another hydrate, 3(NH2)2CS·2H2O, was formed. After collecting X-ray data, it was in situ recrystallized at 0.95 GPa and X-ray diffraction data were collected again. The crystal morphology of (NH2)2CS·H2O (Figure 1) is clearly different from that of 3(NH2)2CS·2H2O (Figure 2) and both forms can be easily recognized. After opening the DAC a visible loss of the 3(NH2)2CS·2H2O singlecrystal quality was observed, as illustrated in Figure S3, and the crystal decomposed, indicating that this structure is unstable at ambient conditions. Releasing pressure in the DAC with (NH2)2CS·H2O resulted in the same effect. All attempts to crystallize thiourea hydrates at 0.10 MPa by cooling 75:25 and 50:50 thiourea aqueous solution (vol.) resulted in anhydrous thiourea and ice Ih frozen separately. Received: August 29, 2012 Revised: November 6, 2012

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methods, and refined by full-matrix least-squares.40 Anisotropic temperature factors were generally applied for all non-hydrogen atoms, except for few atoms for which deformed thermal ellipsoids were replaced by isotropic spheres. The H-atoms in the structures were calculated from molecular geometry, with the N−H distance equal to 0.86 Å, and the Uiso factors of H atoms constrained to 1.2 Ueq of the carrier atoms. The distances of H atoms in water molecules were restrained (the DFIX instruction of SHELXL) to O−H of 0.96 Å and H···H of 1.66 Å and positions and orientations of the so-formed rigid molecules were freely refined. The residues of hydrate 3(NH2)2CS·2H2O at 0.70 GPa are high due to the unfavorable position of the sample crystal at the edge of the high-pressure chamber. For this sample location the precise description of the gasket and crystal shapes are critical, and even small deformations (rounding) of the gasket edges considerably affects the coefficients for correcting the effects of the sample absorption and shadowing by the gasket. Nonetheless the structure was solved for these data and refined to reasonable structural dimensions, and hence it has been included into this report. The crystal data and the structure-refinement details are summarized in Table 1 (cf. Table S1 in Supporting Information). Structural drawings were prepared using the X-Seed interface of POVRay.41,42 The crystal structures have been deposited in CIF form as supplementary publications Nos. CCDC 874417−874420 in the Cambridge Crystallographic Database Centre.

Figure 1. Stages of isochoric growth of a (NH2)2CS·H2O single crystal of 75:25 (vol.) thiourea aqueous solution: (a) the crystal at 333 K; (b) at 323 K; (c) 311 K; and (d) at 296 K/0.60 GPa.

3. RESULTS AND DISCUSSION The structures of six unsolvated phases of thiourea are mainly governed by NH···S hydrogen bonds. In thiourea and urea there is an excess of H-donors over H-acceptors. Therefore the O-atom in urea and the S-atom in thiourea exhibit a high Haccepting capacity of four hydrogen bonds. Both urea and thiourea form cocrystals with different compounds; however, there are no reports about their hydrates. Urea crystallized from methanol, water and methanol/ethanol/water mixtures yields only pure nonsolvated forms.25 In thiourea pressure efficiently induces the formation of hydrates, precipitated from aqueous solution. Above 0.60 GPa thiourea forms hydrate (NH2)2CS·H2O; pressure higher than 0.70 GPa leads to the formation of hydrate 3(NH2)2CS·2H2O (Table 1), and above 1.20 GPa pure (NH2)2CS is again more stable than the hydrates. In both hydrates the molecular packing is governed by hydrogen bonds NH···O, NH···S and OH···S. In the (NH2)2CS·H2O structure the double NH···S bonds link molecules into dimers according to the graph notation R22(8).43 They are further NH···O and OH···S bonded to water molecules only, and no hydrogen bonds are formed to other thiourea molecules (Figure 3 and Table S2). The water and thiourea molecules are alternatively NH···O bonded in chains running along the [100] direction (Figures 3 and 4), and OH···S bonded in zigzag chains along [010]. All these hydrogen bonds bind the molecules into a 3-D network. In 3(NH2)2CS·2H2O one thiourea and one water molecules lie in general positions, and the other thiourea molecule lies on a 2-fold axis. The OH···S bonds arrange the molecules into chains along direction [110], bonds NH···O into chains along direction [1̅10] and bonds NH···S into chains along directions [010] and [001]. In this manner the molecules are H-bonded into a three-dimensional network (Figures 4 and S5). Of eight hydrogen bonds of the first thiourea molecule, three are the NH···S bonds (two NH···S and one S···HN), two S···HO bonds and three NH···O bonds; and of the second molecule six are NH···S bonds (four NH···S and two S···HN) and two S···HO bonds. Thus the number of H-bonds to water molecules for one of the thiourea molecules in 3-

Figure 2. The isochoric growth of a 3(NH2)2CS·2H2O single crystal of 75:25 (vol.) thiourea aqueous solution: (a) one seed at 363 K; (b, c) at 343 K; and (d) at 296 K/0.95 GPa. Several other in situ crystallizations below 0.60 GPa were repeated and they all resulted in pure (NH2)2CS in phase VI. In situ crystallizations above 0.60 GPa led to (NH2)2CS·H2O samples, which could be isothermally compressed to 1.20 GPa (Figure S4, Supporting Information). However, all crystallizations above 1.20 GPa, involving heating the sample to about 423 K at isochoric conditions, resulted in the formation of phase VI of pure thiourea. The single-crystal data have been measured with a KUMA KM4CCD diffractometer. The CrysAlis software36 was used for the highpressure data collections37 and preliminary reduction of data. Reflections intensities have been corrected, for the effects of DAC absorption, sample shadowing by the gasket, the sample absorption,38,39 and the reflections overlapping with diamond reflections were eliminated. All structures were solved straightforwardly by direct B

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Table 1. Selected Crystal Data for High-Pressure Thiourea Hydrates pressure (GPa) temperature (K) crystal system space group unit cell (Å,°) a b c β Z/Z′ volume (Å3) Dcalc (g/cm3) final R1/wR2 (I > 2σ1) R1/wR2 (all data)

(NH2)2CS·H2O

(NH2)2CS·H2O

0.60(2) 296(2) monoclinic P21/c

1.20(2) 296(2) monoclinic P21/c

3(NH2)2CS·2H2O 0.70(2) 296(2) monoclinic C2/c

3(NH2)2CS·2H2O 0.95(2) 296(2) monoclinic C2/c

5.995(2) 8.996(10) 7.986(4) 98.63(4) 4/1 425.7(6) 1.469 0.0725/0.1432 0.0799/0.1458

5.9685(8) 8.820(4) 7.9218(13) 98.930(13) 4/1 411.96(19) 1.518 0.0501/0.0957 0.0667/0.0976

8.020(2) 8.649(6) 16.645(11) 90.78(5) 4/0.5 1154.4(11) 1.521 0.1761/0.3419 0.1864/0.3463

7.968(2) 8.5877(13) 16.554(9) 91.01(4) 4/0.5 1132.6(7) 1.551 0.0551/0.1311 0.0591/0.1359

Figure 3. Molecular packing in (NH2)2CS·H2O at 0.60 GPa/296 K. The shortest intermolecular contacts have been marked as dashed lines.

(NH2)2CS·2H2O is reduced to two (compared to four H-bonds to water molecules of the other molecule in 3(NH2)2CS·2H2O and also four such bonds in (NH2)2CS·H2O − see Figure 4). According to this reduced number of H-bonds to water molecules, the 3(NH2)2CS·2H2O crystal can be considered as an intermediate between the monohydrate (NH2)2CS·H2O and anhydrate (NH2)2CS. The water molecules form OH···S and NH···O bonds both in (NH2)2CS·H2O and 3(NH2)2CS·2H2O. Figure 5 shows that NH···O distances are similar in both hydrates. However, the distances NH···S are much longer and OH···S are shorter in (NH2)2CS·H2O than in 3(NH2)2CS·2H2O. The preference for pressure-dependent formation of thiourea hydrates can be due to the formation of new types of intermolecular interactions involving water molecules, but also more compact packing of thiourea and water molecules in the hydrates than in the pure crystals. The pressure dependence of the volume of anhydrous thiourea crystals and hydrates (Figure

Figure 4. The crystal environments of thiourea molecules: (a) in (NH2)2CS·H2O at 0.60 GPa/296 K; and (b) symmetry-independent molecules in 3(NH2)2CS·2H2O at 0.95 GPa/296 K. Hydrogen bonds are indicated by the dashed lines. Symmetry codes of the H-bonded atoms are specified in Table S2.

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above 0.48 GPa, and therefore it is possible that no volume gain of water incorporation would be possible. Also, the NH···O bonds between urea molecules may be too strong to be substituted by bonds OH···O involving water molecules. On the other hand, the formation of urea/H2O2 solvate46,47 suggests that the molecules with increased number of acceptor sites, compared to H2O, are more suited for the solvate formation by improving the balance between the H-donor and H-acceptor sites in the structure.

4. CONCLUSIONS Pressure of 0.60 GPa induces the crystallization of (NH2)2CS·H2O, whereas only pure (NH2)2CS is obtained from aqueous solution at normal conditions. At 0.70 GPa a new hydrate, 3(NH2)2CS·2H2O, is formed. The mechanism of pressure-dependent formation of thiourea hydrates can be due to the more efficient packing of molecules in the hydrates, compared to nonsolvated thiourea and water separately, due to the energetically more stable OH···S and NH···O hydrogen bonds, and a better balance of the H-donor and H-acceptor sites. It is possible that these factors themselves are strongly pressure dependent and their modified contribution changes the direction of hydrates formation between (NH2)2CS·H2O, 3(NH2)2CS·2H2O and finally pure (NH2)2CS. The anhydrate crystallization above 1.20 GPa coincides with freezing pressure of ice VI of pure water at 296 K. Noteworthy, the region of changed crystallization of hydrates (NH2)2CS·H2O and 2(NH2)2CS·2H2O also coincides with the region of the transition between phases VI and VII (postulated by Gesi at 0.54 GPa9) of pure thiourea. To our knowledge till now a similar process of hydrates formation and decomposition was described only for the hydrates of methane48 and 1,4diazabicyclo[2.2.2]octane hydroiodide.49 Clearly, more information about pressure-dependent formation of hydrates and their decomposition in other compounds are needed for better understanding this intriguing phenomenon.

Figure 5. The shortest intermolecular distances marked as circles for NH···O, OH···S and NH···S in hydrates (NH2)2CS·H2O and 3(NH2)2CS·2H2O, as well as anhydrate (NH2)2CS (empty circles), as a function of pressure (cf. Figure S6 in Supporting Information). Sums of van der Waals radii are marked with dashed lines.44 Dotted lines joining the points are for guiding the eye only.

6) illustrates that between 0.60 and below 1.20 GPa thiourea and water molecules in the hydrates occupy less space than in

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ASSOCIATED CONTENT

S Supporting Information *

Detailed crystallographic information on high-pressure hydrates (Table S1), the shortest interionic distances (Table S2). Crystallographic data (CCDC 874417, CCDC 874418, CCDC 874419, and CCDC 874420) in CIF format are available free of charge via the Internet at http://pubs.acs.org.

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Figure 6. Pressure dependence of molecular volume of unsolvated thiourea (black triangles) and its hydrates (red triangles). Blue arrows indicate the volume of water molecules, as assessed for the compression of water and ice VI,45 and blue triangles show the volume of hydrates after subtracting the volume of water molecules. The subtracted volumes of H2O in liquid are 25.49, 16.73, and 15.97 Å3 for 0.60, 0.70, and 0.95 GPa, respectively, and in ice −21.92 Å3.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

the unsolvated thiourea crystals and water/ice separately. The compression of pure thiourea is smaller than that of hydrates, and at about 1.20 GPa the volume of hydrate (NH2)2CS·H2O becomes approximately equal to the summed volumes of (NH2)2CS anhydrate and ice VI. Above this pressure, due to the strong compression of ice, the volume of thiourea and ice separately becomes smaller than the hydrates. This can be the reason for crystallization of pure thiourea above 1.20 GPa. The reasons for only anhydrate crystallization of urea from aqueous solution remain unclear. The urea crystal in phase I contains large voids, which collapse on transition to phases III

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ACKNOWLEDGMENTS

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REFERENCES

This study was supported by Polish Ministry of Science and Higher Education, Grant N N204 086838. A.O. acknowledges the scholarship START from the Foundation for Polish Science in 2012.

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