Letter pubs.acs.org/JPCL
High-Pressure Effects on Hofmann-Type Clathrates: Promoted Release and Restricted Insertion of Guest Molecules Qian Li,# Xiaojing Sha,§ Shourui Li,¶ Kai Wang,† Zewei Quan,# Yue Meng,‡ and Bo Zou*,† #
Department of Chemistry, South University of Science and Technology of China, Shenzhen, 518055, China State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China § Materials Genome Center, Beijing Institute of Aeronautical Materials, Beijing 100095, China ‡ High Pressure Collaborative Access Team (HPCAT), Geophysical Laboratory, CIW, Argonne, Illinois 60439, United States ¶ National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China †
S Supporting Information *
ABSTRACT: The search for effective methods to accurately control host−guest relationship is the central theme of host−guest chemistry. In this work, high pressure successfully promotes guest release in the Hofmann-type clathrate of [Ni(NH3)2Ni(CN)4]·2C6H6 (Ni−Bz) and restricts guest insertion into Ni(NH3)2Ni(CN)4 (Ni−Ni). Because of the weak host−guest interactions of Ni−Bz, external force gradually removes guest benzene from the host framework, leading to puckered layers. Further theoretical calculations reveal the positive pressure contribution to breaking the energy barrier between Ni−Bz and Ni−Ni, explaining guest release from an energy standpoint. Inversely, guest insertion is restricted in the synthesized host of Ni−Ni because of the steric hindrance effect at high pressure. This study not only reveals structural effects on host−guest behaviors but also proves the role of pressure in controlling host−guest interactions. This unique observation is also crucial for the further application of host− guest materials in sustained and intelligent drug release, molecular separation, and transportation.
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In nature, the host−guest formation and decomposition are completely spontaneous and uncontrollable. In host−guest chemistry, thermodynamic, electric, photo, magnetic, surfacetension, and ultrasound techniques have all been established in recent years to control the host−guest variations and make these processes more effective.7−10 However, it remains to be further explored how pressure, as a green “catalyst” in chemical reactions and one of the most essential thermal parameters, affects the host−guest relationships in host−guest systems11,12 and what the structural contribution to host−guest behaviors is at high pressure. Previous high-pressure studies of host−guest relationships were mainly focused on separate host frameworks and guest molecules. Guest insertion has been observed in several open structures of porous coordination polymers during compression.13,14 For MOF-5 (Zn4O(BDC)3, BDC = 1,4-benzenedicarboxylate), for example, high pressure forces the guest of hydrostatic medium to enter the pores of the host framework, expanding the unit cell and making the host more resilient to external forces.15 For typical host−guest systems, normally, guest molecules are stabilized in host cavities at high pressure
ost−guest chemistry describes the dynamics and formation of supramolecular complexes in which the guest is selectively and noncovalently bonded to the host through unique relationships.1 The host in this case is the artificial receptor, which offers the available cavities that incorporate the guest units. Guest units include a wide range of building blocks, such as simple inorganic ions, small organic molecules, and even complex molecular clusters.2 Formation of host−guest systems calls for excellent matches between host and guest in terms of both space and interaction.3 Inversely, the host−guest decomposition inevitably requires proper methodology and energy assistance. In basic research, unique structures with specific targeted properties can be obtained through controlling host−guest relationships.4 Molecular separation, storage, and recognition applications are just the latest utilization of guest insertion and release processes within host−guest systems. The search for an effective method to accurately tune the host−guest relationship has long been the central theme of host−guest chemistry. Additionally, deducing the structural effects on host−guest behaviors is also of significant importance for the further design and synthesis of host−guest systems.5 Connection and separation between host and guest moieties are ubiquitous in real life, acting as the most elegant approach to the complex and functional chemical and biological systems.6 © XXXX American Chemical Society
Received: May 1, 2017 Accepted: June 5, 2017 Published: June 5, 2017 2745
DOI: 10.1021/acs.jpclett.7b01057 J. Phys. Chem. Lett. 2017, 8, 2745−2750
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Figure 1. Structure of (a) guest benzene molecules, (b) layered host framework, and (c, d) host−guest Ni−Bz at ambient conditions. Hydrogen atoms within the Ni−Bz framework are omitted for clarity. (e, f) Unit cell of Ni−Ni at ambient conditions. H2O molecules are omitted for clarity.
because of the enhanced host−guest interactions.14 External forces only induce molecular conformation changes and phase transitions of host−guest systems. However, as with guest insertion, guest release is also crucial for practical applications, such as sustained and intelligent drug release and molecular transportation, protection, and solubilization.16,17 Consequently, unique host−guest structures were chosen in this work to explore the process of guest release at high pressure, proving the pressure function on host−guest structures. Following the strategy of combining high-pressure techniques and host−guest chemistry, we choose typical Hofmann clathrate host−guest structures, [Ni(NH3)2Ni(CN)4]·2C6H6 (Ni−Bz) and Ni(NH3)2Ni(CN)4 (Ni−Ni) host framework, as model systems for the investigation of pressure effects on host−guest relationships. Ni−Bz is one of the most classical clathrates, which is famous for its reversible absorption of small guest molecules.18 This unique property of Ni−Bz makes it widely used in the fields of molecular separation, exchange, and storage. Meanwhile, the structural peculiarities of Ni−Bz, such as the host−guest interactions, NH3 free rotors, and guest molecule motions, also have attracted tremendous interest in previous reports.19−21 Ni−Bz is the first synthesized Hofmann clathrate with the most regular and typical host−guest structures.22 At ambient conditions, it belongs to tetragonal P4/m symmetry, with the lattice parameters of a = b = 7.2196 Å, c = 8.1007 Å.23 As shown in Figure 1, the host framework of Ni−Bz is a perfectly layered structure, in which the crossshaped linkers of [Ni(CN)4]2− are bridged by the [Ni(NH3)2]2+ cations. Between the adjacent layers, the protruding NH3 groups form cavities that contain the guest benzene molecules (Figure 1a,b). The host and guest are connected together by van der Waals and weak hydrogen bonding interactions between the hydrogen of NH3 and the π electron cloud of benzene (Figure 1c,d). The weak host−guest
interactions in Ni−Bz make the host−guest connection extremely unstable so that simply grinding, heating, or evaporation can remove the guest molecules at ambient conditions.24 It is expected that the extraordinary weak host− guest interactions in Ni−Bz may induce some anomalous host−guest behaviors at high pressure. Furthermore, without guest molecules, the synthesized Ni−Ni converts into the puckered layered structure with 0.25 water molecules left for each chemical formula.25 The offset stacking of Ni−Ni layers belongs to orthorhombic Imma space group with the lattice parameters a = 7.1031 Å, b = 14.2371 Å, and c = 8.8304 Å. In our work, the ambient structure of Ni−Ni is confirmed through the Rietveld refinement of X-ray diffraction pattern, based on the previously reported structure (Figure S1 and Table S1).25 A comprehensive study of the host−guest relationships in Ni−Bz and Ni−Ni is performed at high pressure, using Raman and X-ray diffraction techniques. We successfully achieve the high-pressure responses of host framework and guest molecules. The promoted release and restricted insertion of guest molecules observed at high pressure proves the function of external forces on host−guest tuning. The structural effects and pressure contribution on the host−guest behaviors are deduced and are of pivotal importance for creating new materials with targeted structures and physicochemical properties as well as in the further application of host−guest materials in molecular separation, storage, and recognition. In this work, we apply diamond anvil cell (DAC) and standard ruby fluorescence techniques to generate a high-pressure environment and calibrate the pressure.26 High-pressure experiments were performed with different pressure-transmitting medium (PTM). High-pressure Raman spectra were detected by the ARC-SP 2558 spectrograph (Princeton Instruments). In situ angle dispersive X-ray diffraction (ADXRD) experiments were conducted at 4W2 High Pressure Station of Beijing 2746
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of ν(NiN) in this pressure range imply that the phase transition is not associated with the rupture of the bonds in the polymeric sheets.30 During further compression, the Raman patterns display continuous changes in both intensity and positions, suggesting the stability of the high-pressure phase. Internal Raman modes provide structural clues for specific groups. Benzene and NH3 related modes are illustrated in Figure 3. For guest benzene related vibrations, one new π(CH) mode appears at 2.0 GPa (Figure 3a), and ν(CC) splits into two at 3.0 GPa (Figure 3b), which are indicative of the slight distortion of benzene molecules.32 Between 5.0 and 6.0 GPa, there is an obvious discontinuity in the Raman shifts (Figure 3d,e), and two new peaks of ν(CCC) and ν(CH) emerge at approximately 615 and 3110 cm−1, respectively (Figure 3c), both of which are in agreement with the proposed phase transition. After the phase transition, another new ν(CH) mode marked by an asterisk is observed at 6.6 GPa, and the ν(CH) mode with the lowest frequency displays continuous redshift, indicating further distortion of benzene molecules. For NH3 related vibrations, it was proven that the peak positions and relative intensities of ν(NH3) are the indication of Ni−Bz decomposition ratio.30 Release of benzene molecules is accompanied by hydrogen bonding variations between NH3 and π electrons of the benzene ring, revealed as a second ν(NH3) mode that is approximately ∼20 cm−1 higher in frequency than that of the nondecomposed sample. Upon complete removal of benzene, the original ν(NH3) bands disappear completely, leaving the higher-frequency bands of the empty host lattice. At high pressure, a new ν(NH3) mode appears in the Raman pattern (Figures 3c and S2) at 1.1 GPa, confirming the beginning of Ni−Bz decomposition. During further compression, the intensity of this new ν(NH3) peak increases up to 5.0 GPa, at which the original ν(NH3) mode totally disappears. That is, pressure successfully separates the guest benzene from the host framework of Ni−Bz in the pressure range of 1.1−5.0 GPa. Meanwhile, redshifts of two ν(NH3) modes demonstrate the enhancement of weak N− H···π hydrogen bonding between host and guest.33 When the distance between H+ ions and π electron cloud is shortened by pressure, the electrostatic attraction is enhanced, resulting in elongated N−H distances and the eventual redshifts. The phase transition begins as soon as benzene is fully removed. Without the influence of hydrogen bonding, the new ν(NH3) mode continues blueshifting up to ∼10 GPa without any discontinuity, as a result of the sustained contraction of the highpressure phase. All the variations in ν(NH3) reflect that pressure can promote the release of guest molecules, which is the rare observation for high-pressure host−guest systems. Meanwhile, other runs of Raman experiments with different PTM exhibit similar behaviors (Figures S3 and S4). This implies that pressure conditions and surrounding molecules cannot affect the guest release at high pressure. The most important external condition for Ni−Bz decomposition is simply the external force. High-pressure ADXRD experiments offer the most conclusive structural information on Ni−Bz (Figure S6). Combining Raman and ADXRD results, it can be concluded that between ∼5 and 6 GPa, as soon as all the guest molecules are removed from Ni−Bz, the host framework transforms into the new structure with distorted layers and orthorhombic Imma symmetry. On the basis of the unit cell information and the phase transition mechanism, it is inferred that the high-pressure phase of Ni−Bz is likely to be the further atomic adjusting or
Synchrotron Radiation Facility (BSRF) and 16-IDB station of the Advanced Photon Source (APS), Argonne National Laboratory (ANL). The two-dimensional XRD images were integrated into one-dimensional patterns using Fit2D software.27 Further analysis of ADXRD data was performed with the commercial program Materials Studio 5.5. High-pressure enthalpy calculations are carried out using density functional theory, as implemented in the ab initio simulation package (VASP) code.28 More detailed experimental and calculation information is provided in the Supporting Information. High-pressure Raman spectra provide straightforward information on the local structural environment. Raman mode assignments for Ni−Bz are based on previous reports.29,30 Typical spectra of [Ni(CN)4]2− related vibrations and the corresponding Raman shifts at high pressure are displayed in Figure 2. It is observed that below 5.0 GPa most of
Figure 2. (a) Selected high-pressure Raman patterns in the frequency range of 70−380 cm−1 with methanol−ethanol as PTM. The Raman peak marked with an asterisk is located at the lower frequency at ambient conditions and shifts to the detected region above 1.1 GPa. (b) Corresponding Raman shifts as a function of pressure. The rectangular part with shadow represents the phase transition boundary.
the Raman peaks reveal normal blueshifts due to the reduction of interionic distances (Figure 2a). However, the in-plane bending modes of π(NiCN) show obvious redshift upon compression, indicating the gradual distortion of the host framework. Between 5.0 and 6.0 GPa, obvious changes, including the emergence of five new δ(CNiC) peaks and splitting of π(NiCN) mode, are observed in Raman patterns. In combination with the discontinuities in the Raman shifts (Figure 2b), it is inferred that Ni−Bz undergoes a phase transition involving the distortion of the host layers in this pressure range.31 Meanwhile, the unchanged stretching modes 2747
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Figure 3. Selected high-pressure Raman patterns of Ni−Bz in the frequency region of (a) 600−1250 cm−1, (b) 1560−1600 cm−1, and (c) 3020− 3370 cm−1 when methanol−ethanol is applied. (d, e) Corresponding peak positions as a function of pressure.
distortion of the ambient solved Ni−Ni structure (Figure 1), which is also coincident with the guest release during compression.25 Guest removal of benzene may involve the volume collapse of empty cavities within the structure. The ambient Ni−Bz structure contains one empty cavity within the unit cell, and the potential high-pressure Ni−Bz structure expands to contain four empty cavities per unit cell. During the transition, the roughly calculated average volume of Ni−Bz cavity collapses from ∼390 to ∼210 Å3, which also indicates the guest release from the host framework. Furthermore, theoretical enthalpy calculations are also carried out on the host framework of Ni−Bz and Ni−Ni structure. As illustrated in Figure 4, the host framework of Ni−Bz possesses enthalpy that is larger than that of Ni−Ni, demonstrating its metastable structural nature. Even at ambient conditions, the larger enthalpy of Ni−Bz promotes its structural transition to Ni− Ni, as evidenced by the guest release of Ni−Bz upon long-time evaporation and grinding. With increasing pressure, Ni−Bz host framework still keeps the enthalpy larger than that of Ni−Ni,
and the differences become slightly larger after the compression. That is, pressure clearly contributes to the breaking of the energy barrier between the metastable state of Ni−Bz host and the stable phase of Ni−Ni.32 The structural transformation between Ni−Bz and Ni−Ni becomes more facile at high pressure, which promotes guest release under high-pressure treatment. The effects of external force on the inverse process of guest insertion in the Ni−Ni host framework have been previously investigated. Guest insertion induces the stabilization of interlayered space, enlargement of lattice parameters, and expansion of some chemical bonds.12−15 High-pressure ADXRD and Raman experiments are carried out on synthesized Ni−Ni with and without guest molecules, respectively. The methanol−ethanol molecules function as both the PTM and the surrounding guest molecules in highpressure experiments. During compression, no phase transition is observed in the ADXRD patterns up to the pressure of ∼10 GPa (Figures S7 and S8), reflecting the more stable nature of the offset stacking in Ni−Ni than the direct stacking in Ni−Bz. The evolution of lattice parameters in Figure 5 shows that the compression of the c axis (Figure 5c,f) is much more dramatic than for the a (Figure 5a,d) and b (Figure 5b,e) axes, which results from the relatively weak interactions between the layers.34 In the presence of guest molecules, Ni−Ni exhibits less compressibility of the intralayer framework (along a axis) and a more significant contraction of the interlayer space (along c axis), all of which are indicative of the failed guest insertion at high pressure. Such differences between different experimental runs could be induced only by the different pressure conditions. High-pressure Raman spectra also confirm the noninsertion of guest molecules (Figures S9−S12). Even when small gas molecules of neon are applied as guest, no obvious difference is observed in either the Raman pattern or shifts, suggesting the lack of influence of guest molecules on structural evolution. Meanwhile, the splitting ν(NH3) modes at ∼1.5 GPa demonstrate the distortion of NH3 groups, which may be attributed to the compressed interlayer space and enhanced interactions.14 Although guest insertion can be realized through
Figure 4. Enthalpy curves of Ni−Bz host framework and Ni−Ni structures within the given pressure range. The insets are the graphical representation of Ni−Bz and Ni−Ni structures. 2748
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On the basis of the weak host−guest interactions and high enthalpy, pressure successfully promotes the guest release in Ni−Bz and induces the gradual separation between host Ni−Ni framework and guest benzene molecules. Then, without the structural support of guest molecules, the empty Ni−Ni framework undergoes a phase transition during further compression that involves the destruction of the perfectly layered structure. In contrast, guest insertion in the synthesized Ni−Ni network is not observed at high pressure. Pressure restricts the insertion of surrounding guest molecules into the empty space in Ni−Ni because of the steric hindrance of the rapidly contracted interlayered space and the distorted NH3 groups at high pressure. This study expands host−guest chemistry research, proving that application of pressure is a promising technique for the promotion of guest release in host−guest systems and restriction of guest insertion in layered host framework. Additionally, it reveals the structural effects on host−guest behaviors of Ni−Bz and Ni−Ni. An understanding of this phenomenon is of crucial importance for crystal engineering and the further application of these materials in molecular regionalization, separation, and storage.
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Figure 5. High-pressure evolution of lattice parameters of (a) a axis, (b) b axis, and (c) c axis. (d−f) Compressibility of three lattice axes as a function of pressure.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01057. Experimental and calculation details and more data of Ni−Bz and Ni−Ni (PDF)
long-time mixing of Ni−Ni and guest molecules at ambient conditions, high-pressure treatment restricts such host−guest formation. When all the previous experimental and calculation results are combined, the high-pressure effects on the host−guest relationships of Ni−Bz and Ni−Ni are inferred as follows. In the Ni−Bz host−guest system, the high-pressure condition enhances weak hydrogen bonding and van der Waals force between host and guest and the compression of the structure. To balance the increased free energy, guest benzene is gradually distorted and leaves the host framework between ∼1 and ∼4 GPa, as evidenced by the splitting of benzene-related Raman modes and the emergence and disappearance of ν(NH3) vibrations.14 During this process, the extraordinary weak host− guest interactions are easily destroyed, contributing to guest release under high pressure. The weak host−guest interaction is one of the structural advantages of Ni−Bz decomposition at high pressure. In contrast to the spontaneous decomposition of Ni−Bz in nature, external force is able to control the decomposition ratio and promote the release process of benzene molecules. As soon as all the guest molecules are removed, the empty host framework becomes unstable, resulting in the phase transition in the pressure range of 5.0− 6.0 GPa. During this transition, the perfectly layered Ni−Ni framework is distorted without significant destruction, as evidenced by the unchanged Ni−N stretching modes and the significantly modified host-related bending Raman vibrations and ADXRD patterns. In addition, high pressure induces the dramatic approach between adjacent layers and the distortion of NH3 groups in the synthesized Ni−Ni host framework, which enhances the blocking of interlayer space. With such steric hindrance effects, the surrounding small molecules cannot be squeezed into the Ni−Ni empty layers at high pressure. That is, high pressure successfully restricts the guest insertion into the host Ni−Ni framework. In summary, we applied high pressure to tune the host−guest relationships in Hofmann-type clathrate of Ni−Bz and Ni−Ni.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kai Wang: 0000-0003-4721-6717 Bo Zou: 0000-0002-3215-1255 Author Contributions
Q.L. and B.Z. designed and performed experiments, analyzed data, and wrote the manuscript. X.S., S.L., K.W., Y.M., and Z.Q. assisted in performing experiments and calculations. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Science Foundation of China (NSFC) (Nos. 21673100, 91227202, 11604141), Shenzhen fundamental research programs (No. JCYJ20160530190717385), Chang Jiang Scholars Program (No. T2016051), the Changbai Mountain Scholars Program (No. 2013007), and Program for Innovative Research Team (in Science and Technology) in University of Jilin Province. ADXRD measurements were performed at 4W2 HP-Station, Beijing Synchrotron Radiation Facility (BSRF), which is supported by Chinese Academy of Sciences (Grants KJCX2SW-N20 and KJCX2-SW-N03). Portions of work were performed at HPCAT’s beamline facility (Sector 16), of the Advanced Photon Source at Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974 and DOE-BES under Award No. DE-FG02-99ER45775, with partial instrumentation funding by NSF. The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the 2749
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DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The support provided by China Scholarship Council (CSC) during a visit of Qian Li to HPCAT is also acknowledged.
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