Diketopiperazine: the Anisotropic Compression of NâH

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High-Pressure-Induced Phase Transition in 2,5-Diketopiperazine: the Anisotropic Compression of N-H···O Hydrogen-Bonded Tapes Yuxiang Dai, and Yang Qi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03931 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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The Journal of Physical Chemistry

High-Pressure-Induced Phase Transition in 2,5Diketopiperazine: the Anisotropic Compression of N−H···O Hydrogen-Bonded Tapes

Yuxiang Dai,†,‡, and Yang Qi*,†,‡

†

Institute of Materials Physics and Chemistry, School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, China.

‡

Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang, 110819, China.

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Abstract 2,5-Diketopiperazine was found to undergo a high-pressure phase transition at about 11.0 GPa through in situ high-pressure synchrotron X-ray diffraction experiments. The anisotropic compression of the samples before the phase transition was discussed. The subsequent results of in situ high-pressure Raman scattering experiments illustrated the wrinkle of the N−H···O hydrogen-bonded tapes was the major mechanism that drove this phase transition. The firstprinciple calculations and Hirshfeld surfaces further confirmed that the anisotropic compression of N−H···O hydrogen-bonded tapes was derived from the reduction of molecular interlayer spacing. This study demonstrated the evolutions of low dimensional intermolecular interactions under continuous compression, which would help to understand the self-assembly of supramolecular materials.

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Introduction Layered materials have presented unique conductivity, lubricity, catalytic performance and other properties duo to their low dimensionality.1-2 The actual and potential applications of layered supramolecular materials have attracted considerable attentions in past decades, because these materials have shown remarkable physicochemical properties, which are based on the lowdimensional motif of self-assembly.3-4 Intermolecular noncovalent interactions, especially the hydrogen-bonded networks can govern the manner of molecular arrays.5-6 The one-dimensional hydrogen-bonded chains or cyclic rosettes can stabilize, direct and control solid-state structure.713

The two-dimensional hydrogen bonds can stabilize of the monolayers, construct functional

surfaces and build unique channels with different physicochemical characteristics.14-18 These low-dimensional self-assembly processes will help design new functional materials or induce new chemical reactions. Therefore, the stability of low-dimensional hydrogen-bonded systems is of important significance. Whitesides et al. have assessed the low-dimensional N−H···O hydrogen bonds in two typical styles: tapes and ribbons.19 2,5-Diketopiperazine (glycine anhydride) crystal is the model of the N−H … O hydrogen-bonded tape which can act as a template to build supramolecular structures.20-21 The 2,5-diketopiperazine crystal has a monoclinic structure in the P21/a space group at ambient conditions.22-23 Either pair of one-dimensional N−H···O hydrogen bonds links the adjacent two 2,5-diketopiperazine molecules which extend in two different non-coplanar layers respectively, as shown in Figure 1 and S1 in the Supporting Information. This robust structural motif is usually used for crystal engineering as well as the design of molecular arrays.24-25 Moreover, many low-dimensional supramolecular assemblies are based on the formation of N−H…O hydrogen bonds in the derivatives of 2,5-diketopiperazine.26-27

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According to reported literatures, pressure has efficient ability to alter hydrogen bonds which can lead to the changes of molecular arrangement and crystal structures.28-34 It will be interesting to apply pressure to in-depth investigate one-dimensional N−H···O hydrogen-bonded interactions in the 2,5-diketopiperazine crystal as a model to analyze low-dimensional hydrogen bonds under stress stimulus. On the heroic assumption, the dimension of the molecular array can be altered by compression. In this study, we explored the high-pressure structural stability of the 2,5-diketopiperazine crystal and the hydrogen-bonded tapes in it by conducting in situ synchrotron angle-dispersive X-ray diffraction (ADXRD) and Raman spectroscopy experiments. The ab initio calculations and Hirshfeld surfaces were utilized to confirm the evolutions of molecular aggregation under high pressure. Since 2,5-diketopiperazine and its derivatives are often found in foods, lives and drugs, the results of this research can gain insight into lowdimensional hydrogen bonds in the nature and pharmacy industry as well as crystal engineering.35

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Experimental Section Commercial 2,5-diketopiperazine crystals (purity is 99%) were purchased for high-pressure Raman and ADXRD experiments. A T301 steel gasket of a pre-indented thickness about 40 µm was placed between a pair of parallel 400 µm diameter culet diamonds in a symmetric diamond anvil cell (DAC). The sample compartment with the diameter of 130 µm was manually drilled in the center of the gasket. Subsequently, 2,5-diketopiperazine powder crystals were loaded and sealed in the sample compartment together with a small ruby ball, which was utilized to measure the pressure through the standard ruby-fluorescence method.36 Argon was used as the pressuretransmitting medium (PTM) for Raman experiments and silicon oil was used as the PTM for ADXRD experiments. The temperature conditions in all of the high-pressure experiments were kept at room temperature. A spectrometer (Acton SpectraPro 2500i) and a liquid nitrogen cooled CCD camera (Pylon, 100B) formed the in situ high-pressure Raman spectra test system. The excitation source was 10 mW laser line (532 nm) from the diode pumped solid state (DPSS). Each Raman spectra was measured at 30 s to have enough intensity. In situ high-pressure ADXRD experiments were conducted at beamline 12.2.2 at the Advanced Light Source. The wavelength of monochromatic beam used for data collection was 0.496775 Å. The CeO2 standard was used to calibrate the distance and geometric parameters from the sample to the detector. Mar345 detector was utilized to collect the typical Bragg diffraction rings. Enough average exposure time for each data was kept to maintain sufficient intensity. The Fit2D software was further utilized to convert the twodimensional (2D) data to the plots of intensity versus 2θ.37 More detailed information of the 2,5diketopiperazine crystal under high pressure was analyzed by using Materials Studio 5.0.

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First-principles density functional theory (DFT) computations of the high-pressure crystal structure were performed by using the pseudopotential plane wave method in the CASTEP package. The generalized gradient approximation of Perdew-Burke-Ernzerh38-40 exchange correlation was used in the geometric optimization with a cutoff energy of 340 eV. These calculations were performed with a 3×1×4 Monkhorst-Pack k-point mesh.

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Results and Discussion To investigate the stability of 2,5-diketopiperazine crystals under compression, in situ highpressure synchrotron ADXRD experiments were carried out. Selected ADXRD patterns are shown in Figure 2. As the pressure increased up to 10.0 GPa, all the ADXRD peaks moved to higher angles regularly. This phenomenon indicate the lattice space was squeezed throughout the compression process.41 When the pressure reached 11.0 GPa, the ADXRD pattern changed abruptly. The initial (110) peak splited into two new peaks, and another new peak emerged at a higher angle. These three new peaks are marked by red asterisks in Figure 2. These phenomena indicate the crystal structure was changed during a high-pressure phase transition. The subsequent Pawley refinement of the diffraction pattern collected at 11.0 GPa indicate the new phase belongs to P2/m symmetry possibly (Figure 3). The lattice parameters of 2,5diketopiperazine at 11.0 GPa according to Figure 3 are shown in Table S1 in the Supporting Information. Meanwhile the crystal structure showed anisotropic compression before the phase transition. The reduced lattice constants in Figure 4(a) indicate the compressibility of c-axis is bigger than that of a, b-axes. As shown in Figure 1, the N−H···O hydrogen bonds extend almost in AB plane. Since the N−H···O hydrogen bonds are relatively strong noncovalent interactions, the distance between molecular layers (almost along c-axis) is easy to be reduced. Thus, the anisotropic compression of N−H···O hydrogen-bonded tapes will give rise to the different compressibility of a, b, c-axes. The symmetry of the crystal structure was reduced and the volume of unit cell was collapsed by about 7.6% (Figure 4(b)) during the phase transition. When the pressure increased continuously, the crystal structure of the high-pressure new phase was stable up to 18.5 GPa. The results of the third-order Birch−Murnaghan equation of states fitting

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on the unit cell volume with compression are shown in Figure 4(b): V0 = 242.30(2) Å3, B0 = 25.51(1) GPa, B1 = 4 (fixed) in the initial phase (phase I); V0 = 208.18(2) Å3, B0 = 32.82(5) GPa, B1 = 4 (fixed) in the high-pressure phase (phase II). P (V ) =

3B0 V0 73 V0 35 V 2 3 [( ) − ( ) ]{1 + ( B1 − 4)[( 0 ) 3 − 1]} 2 V V 4 V

P, V, V0, B0 and B1 represent the applied pressure, volume under pressure P, volume at ambient pressure, the bulk modulus and its first pressure derivative, respectively. The changes of these parameters reflect the reduction in the compressibility of 2,5-diketopiperazine crystals after this high-pressure phase transition. In addition, the released ADXRD pattern of 2,5-diketopiperazine powder crystals remains the same as the original one, which indicate this high-pressure phase transition is reversible. In order to further analyze the evolutions of the hydrogen-bonded tapes under high pressure, experiments of in situ Raman spectroscopy were performed. The assignments for the Raman modes of 2,5-diketopiperazine shown in Table S2 in the Supporting Information are based on the reported literature.42-46 Selected Raman spectra at different pressures are shown in Figure 5(a). Five modes (initial positions at 63, 77, 142, 151, 172 cm-1) are assigned as the external modes, which are corresponding to the lattice vibrations. All these external modes showed regular blue shifts at different speeds up to 10.9 GPa as shown in Figure 5(b), which indicates the lattice space was squeezed progressively. When the pressure exceeded 10.9 GPa, a new external mode marked by a red asterisk emerged and then enhanced as the pressure increased. This phenomenon confirms the occurrence of high-pressure phase transition and indicates the intermolecular interactions including the hydrogen-bonded networks were changed during the process of this phase transition. The new series of external modes moved regularly in the

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direction to high frequency without further abrupt changes up to 20.1 GPa, indicating the highpressure phase was stably compressed throughout this pressure region. The internal modes corresponding to the above pressure range are depicted in Figure 6, and the pressure dependence of these Raman shifts is shown in Figure 7. The ring in-plane bending, CO out-of-plane bending, ring stretching, CH2 twisting, CH2 bending, NH in-plane bending, CO stretching, CH2 sym-stretching, CH2 antisym-stretching NH2 stretching modes all showed regular blue shifts up to 10.9 GPa. These phenomena indicate the vibrational frequency of each chemical moiety increased during the compression duo to the enhancement of intermolecular interactions.47-49 These blue shifts also indicate that the C−H, N−H and C=O bonds were shortened in the contraction of the crystal volume,50-52 which were mainly driven by the interlayered compression of the N−H…O hydrogen-bonded tapes. A new CO in-plane bending mode (marked by red asterisk at about 642 cm-1) emerged in the lower frequency region beside the initial mode at 9.1 GPa, and the relative intensity of these two peaks were exchanged over 11.9 GPa (Figures 6(a) and 7(a)). This abrupt change indicates the chemical environment around the C=O group was altered and the C=O bond elongated in the high-pressure phase transition. Thus, the N−H…O hydrogen bonds were enhanced and distorted in the new phase. Additionally, two new ring stretching modes as well as a new mode between the CH2 bending and NH in-plane bending modes emerged at 11.9 GPa (Figures 6(a) and (b)). Meanwhile a new CH2 antisymstretching mode was also observed beyond 11.9 GPa in Figure 6(c). All these new peaks are marked by red asterisks. The dramatic changes of these internal modes indicate the intermolecular interactions around the ring kept being enhanced during the compression. The ring shank and bended obviously when the high-pressure phase transition occurred. Although the intensity of NH2 stretching modes is too weak to be analyzed, the abrupt changes of CO in-plane

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bending, ring stretching modes reflect the distortion of N−H···O hydrogen bonds over 10.9 GPa.30, 53 In order to further analyze the mechanism of the high-pressure phase transition, DFT computations of the high-pressure crystal structure were performed. The calculated lattice constants in Figure 8(a) showed the similar anisotropic compression as the experimental results in Figure 4(a) and the unit cell volume before the phase transition up to 9.0 GPa in Figure 8(b) was also corresponding to the ADXRD results. The results of the third-order Birch−Murnaghan equation of states fitting on the calculated unit cell volume in the initial phase are shown in Figure 8(b): V0 = 234.34(4) Å3, B0 = 24.48(3) GPa, B1 = 4 (fixed). At ambient conditions, the shortest distance values of N···N and N···O between adjacent molecular layers are longer than 3.700 Å, while these values are reduced significantly at high pressures. According to the calculated results, the adjacent distance of N···N between molecular layers is 3.107 Å and the adjacent distance of N···O between molecular layers 3.195 Å (Figure 9). Therefore the interlayered N−H···O and N−H···N contacts are enhanced as strong intermolecular interactions such as the hydrogen bonds, and the wrinkle of the original N−H···O hydrogen-bonded tapes in the molecular layers is inevitable under higher pressures.54 Figures 10(b) and (c) depict the Hirshfeld surfaces for 2,5-diketopiperazine at ambient pressure and 9.0 GPa, respectively. The blue regions on the Hirshfeld surfaces are relative with long interactions, and the red regions represent short interactions.55 Fingerprint plots (Figure 11) are also used to analyze the evolution of intermolecular interactions. Pressure-induced shortening of the long contacts is corresponding to the decreased maximum values of de from ambient pressure (2.153 Å) to 9.0 GPa (1.997 Å). The two “spikes” in these two plots marked by red arrows represent N−H···O hydrogen bonds. In Figure 11(b), the single N−H···O hydrogen bond extends

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down to (di, de)=(0.640 Å, 1.000 Å), which is corresponding to the distance (1.640 Å) of the N−H···O hydrogen-bonded interaction. Because the N−H···O hydrogen-bonded tape is a robust motif, the distance of N−H···O hydrogen bond was reduced slowly. Nevertheless the N−H···O hydrogen-bonded contact was reduced obviously up to 9.0 GPa, which is very possible to induce the distortion of the N−H···O hydrogen-bonded tapes. Additionally, the red color of the N−H part also enhanced at 9.0 GPa (Figure 10(c)) which indicated enhancement of the interlayered N···O and N···N contacts corresponding to the analysis of Figure 9. The significant increase of red color in the part of the ring at 9.0 GPa (Figure 10(c)) indicates the C···C contacts on or between molecular rings of adjacent layers are enhanced obviously.56 Meanwhile all the plots in Figure 11(b), especially the diffused points marked with a red circle between the two spikes, move toward the origin obviously, which arises from the enhancement of C−H···H−C contacts.57 Thus the vibration of C−H bonds should be strengthened under high pressures, which is corresponding to the blue shifts of the C−H stretching modes in Figure 6(c). These phenomena all indicate the anisotropic compression of the N−H···O hydrogen-bonded tapes and the crystal structure. Considering the results of ADXRD and Raman experiments, the abrupt changes of the 2,5-diketopiperazine crystal structure and the intermolecular interactions in it were confirmed. The original one-dimensional N−H···O hydrogen-bonds in each molecular layer were robust so that the distance between the molecular layers was easy to be compressed. Intermolecular interactions especially the interactions between adjacent molecular layers enhanced continuously throughout the compression. When the pressure increased sufficiently, the free energy of the system was getting overwhelming until it destroyed initial molecular arrangement, which led to the transition of the crystal structure. When the excess energy was released by the distortion of

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the hydrogen-bonded networks, the stability of the high-pressure phase was maintained up to nearly 20 GPa.

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Conclusion In summary, we have studied the structural stability and N−H···O hydrogen bonds of 2,5diketopiperazine by collecting in situ high-pressure Raman spectra and ADXRD patterns. The lattice space and the volume of the unit cell were contracted when the samples were subjected to high pressures. A reversible phase transition occurred at about 11.0 GPa, which was related with the anisotropic compression and the deformation of the initial hydrogen-bonded tapes. The reduction of molecular interlayered spacing and distortion of the hydrogen-bonded networks were confirmed by the first-principle calculations and Hirshfeld surfaces subsequently. These results improve the understanding of this common and robust intermolecular interaction (i.e. the N−H···O hydrogen bond) in supramolecular self-assembly.

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Figure 1. Crystal structure of 2,5-diketopiperazine under ambient conditions viewed along the caxis. The dash lines represent the N-H···O hydrogen bonds.

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Figure 2. ADXRD patterns of 2,5-diketopiperazine at pressures from 0.2 GPa to 18.5 GPa. The red asterisks mark the new peaks. The ADXRD pattern of the released samples are essentially the same as the initial one.

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Figure 3. Pawley refinements of the diffraction patterns collected at 11.0 GPa. Solid lines and asterisks represent the simulated and observed profiles, respectively.

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Figure 4. (a) Reduced lattice constants and (b) Unit cell volume as a function of pressure from ADXRD experiments.

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Figure 5. (a) Raman spectra of 2,5-diketopiperazine at selected pressures ranging from 50 to 300 cm-1, (b) Pressure dependence of Raman shifts of 2,5-diketopiperazine at selected pressures ranging from 50 to 300 cm-1.

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Figure 6. Raman spectra of 2,5-diketopiperazine at selected pressures: (a) from 300 to 1300 cm1

; (b) from 1400 to 2000 cm-1; (c) from 2800 to 3300 cm-1.

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Figure 7. Pressure dependence of Raman shifts of 2,5-diketopiperazine at selected pressures ranging : (a) from 200 to 1300 cm-1; (b) from 1400 to 1700 cm-1; (c) from 2900 to 3150 cm-1.

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Figure 8. (a) Reduced lattice constants and (b) unit cell volume as a function of pressure from calculated results.

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Figure 9. The shortest distance of N···N and N···O between adjacent molecular layers are 3.107 Å and the 3.195 Å at 9.0 GPa based on the calculated results. The white shadow represents for one molecular layer.

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Figure 10. (a) Molecular structure of 2,5-diketopiperazine viewed along c-axis. (b) Hirshfeld surface for the structure of 2,5-diketopiperazine at 1 atm mapped with dnorm. (c) Hirshfeld surface for the calculated structure of 2,5-diketopiperazine at 9.0 GPa mapped with dnorm.

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Figure 11. Fingerprint plots for 2,5-diketopiperazine at (a) ambient pressure and (b) 9.0 GPa.

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Supporting Information Detail information about the crystal structure and assignments of the major Raman bands cited in manuscript. Corresponding Author *E−mail: [email protected]. Notes The authors declare no competing financial interests. Acknowledgments This work is supported by “the Fundamental Research Funds for the Central Universities” (No. N170203007). This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. In addition, we would like to thank Martin Kunz for his help in the high-pressure ADXRD measurements at beamline 12.2.2.

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Table of Contents Graph

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Figure 1. Crystal structure of 2,5-diketopiperazine under ambient conditions viewed along the c-axis. The dash lines represent the N-H···O hydrogen bonds. 76x50mm (300 x 300 DPI)



Figure 2. ADXRD patterns of 2,5-diketopiperazine at pressures from 0.2 GPa to 18.5 GPa. The red asterisks mark the new peaks. The ADXRD pattern of the released samples are essentially the same as the initial one. 76x101mm (300 x 300 DPI)


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Figure 3. Pawley refinements of the diffraction patterns collected at 11.0 GPa. Solid lines and asterisks represent the simulated and observed profiles, respectively. 76x38mm (300 x 300 DPI)



Figure 4. (a) Reduced lattice constants and (b) Unit cell volume as a function of pressure from ADXRD experiments. 76x40mm (300 x 300 DPI)


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Figure 5. (a) Raman spectra of 2,5-diketopiperazine at selected pressures ranging from 50 to 300 cm-1, (b) Pressure dependence of Raman shifts of 2,5-diketopiperazine at selected pressures ranging from 50 to 300 cm-1. 76x56mm (300 x 300 DPI)



Figure 6. Raman spectra of 2,5-diketopiperazine at selected pressures: (a) from 300 to 1300 cm-1; (b) from 1400 to 2000 cm-1; (c) from 2800 to 3300 cm-1. 152x63mm (300 x 300 DPI)


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Figure 7. Pressure dependence of Raman shifts of 2,5-diketopiperazine at selected pressures ranging : (a) from 200 to 1300 cm-1; (b) from 1400 to 1700 cm-1; (c) from 2900 to 3150 cm-1. 152x61mm (300 x 300 DPI)



Figure 8. (a) Reduced lattice constants and (b) unit cell volume as a function of pressure from calculated results. 76x40mm (300 x 300 DPI)


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Figure 9. The shortest distance of N•••N and N•••O between adjacent molecular layers are 3.107 Å and the 3.195 Å at 9.0 GPa based on the calculated results. The white shadow represents for one molecular layer. 76x50mm (300 x 300 DPI)



Figure 10. (a) Molecular structure of 2,5-diketopiperazine viewed along c-axis. (b) Hirshfeld surface for the structure of 2,5-diketopiperazine at 1 atm mapped with dnorm. (c) Hirshfeld surface for the calculated structure of 2,5-diketopiperazine at 9.0 GPa mapped with dnorm. 76x31mm (300 x 300 DPI)


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Figure 11. Fingerprint plots for 2,5-diketopiperazine at (a) ambient pressure and (b) 9.0 GPa. 76x38mm (300 x 300 DPI)



Table of Contents Graph 82x30mm (300 x 300 DPI)


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Diketopiperazine: the Anisotropic Compression of NâH

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