Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Guest-Induced Switchable Breathing Behavior in a Flexible Metal− Organic Framework with Pronounced Negative Gas Pressure Yi-Xiang Shi,† Wu-Xiang Li,† Wen-Hua Zhang,*,† and Jian-Ping Lang*,†,‡ †
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, Jiangsu, PR China State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, PR China
‡
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S Supporting Information *
ABSTRACT: Flexible metal−organic frameworks (MOFs) have attracted great interest for their dynamically structural transformability in response to external stimuli. Herein, we report a switchable “breathing” or “gate-opening” behavior associated with the phase transformation between a narrow pore (np) and a large pore (lp) in a flexible pillared-layered MOF, denoted as MOF-1as, which is also confirmed by SCXRD and PXRD. The desolvated phase (MOF-1des) features a unique stepwise adsorption isotherm for N2 coupled with a pronounced negative gas adsorption pressure. For comparison, however, no appreciable CO2 adsorption and gate-opening phenomenon with stepwise sorption can be observed. Furthermore, the polar micropore walls decorated with thiophene groups in MOF-1des reveals the selective sorption of toluene over benzene and p-xylene associated with self-structural adjustment in spite of the markedly similar physicochemical properties of these vapor molecules.
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flipping of organic linkers, or conformational transition of SBUs.48,49 Furthermore, the host−guest and guest−guest interactions of the interpenetrated or interwoven frameworks should also be taken into account.50,51 In addition, the topology of the skeleton and organic linker may be considered as main factors to achieve flexible MOFs.19,52−54 The prototypical flexible MOFs are the so-called MIL-53-series and MIL-88-series (MIL stands for Materials of Institute Lavoisier), which exhibit a large reversible breathing behavior involving a large pore (lp) to narrow pore (np) phase transition upon exposure to external stimuli.55−57 However, the crystalline nature of flexible MOFs is hard to maintain after thermal treatment to remove the guest molecule, and this makes the breathing mechanism difficult to identify. We herein describe the rational design and synthesis of a new flexible paddle-wheel-based pillared-layered MOF, [Zn2(tdc)2(pvq)]· 1.5DMF (H2tdc = 2,5-thiophenedicarboxylic acid, pvq = 5-(2(pyridin-4-yl)vinyl)quinoline, DMF = N,N-dimethylformamide) with square grid (sql) topology, hereinafter denoted as MOF-1as. Ethanol-exchanged MOF-1as can be converted into a fully desolvated one, MOF-1des, in a single-crystal-tosingle-crystal (SCSC) manner by heating at 120 °C in vacuo. MOF-1as exhibits a switchable stimuli-driven “breathing” behavior between lp and np phases in a SCSC transformation. Such a “breathing” behavior is mainly caused by the rotation motion of the quinoline group, the bending of Zn-carboxylate bonds and subnetwork displacement of two-interpenetrated
INTRODUCTION Metal−organic frameworks (MOFs) are a class of porous crystalline materials with a large surface area, high degree of structural tenability, and functional diversity arising from a vast scope of potential metal cluster secondary building units (SBUs) and organic linkers that have been widely demonstrated to be capable of gas storage and separation,1−4 chemical sensing,5−8 drug delivery,9−11 solar energy harvesting,12−14 heterogeneous catalysis, and others.15−18 Compared with the robust porous crystalline materials (such as zeolites, rigid MOFs), as a subclass of MOFs, flexible MOFs have emerged and attracted great interest for their dynamically structural transformability upon various external stimuli, such as pressure, temperature, light, electric fields, and guest molecules.19−26 The most prominent phenomena associated with a structural transformation between different crystalline states are often termed “breathing” and “gating” behavior.27−31 Significantly, the switchable “breathing” or “gating” behavior of flexible MOFs can result in an dramatic expansion or contraction of the unit cell volume while changing the pore environment, which may be a matter of great importance for molecular recognition,32,33 sensors,34,35 gas separation,36−38 selectivity catalysis,39,40 and energy savings.41,42 Despite intensive efforts dedicated to the investigation of the flexible MOFs, their rational assembly still remains challenging owing to the uncertainty in the crystallization process and intrinsic structural rigidity.43−47 Indeed, the flexible nature of these MOFs derives from the dynamics of structural transformations of the flexible organic linkers and coordination geometry of SBUs, such as rotation, twisting, bending, and © XXXX American Chemical Society
Received: May 22, 2018
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DOI: 10.1021/acs.inorgchem.8b01408 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry frameworks, which may be due to the structural flexibility of MOF-1as. Moreover, a highly effective separation of toluene from aromatic analogues (benzene and p-xylene) has been achieved in MOF-1des. Described below are the syntheses and structural characterization of MOF-1as and MOF-1des, their performances in the sorption of N2 and CO2, and their adsorption selectivity toward toluene.
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(w), 1807 (w), 1645 (s), 1644 (s), 1532 (m), 1400 (ms), 1108 (m), 1035 (ms), 802 (s), 766 (s), 671 (ms), 508 (ms). X-ray Crystallography. Single crystals of MOF-1as and its desolvated phase MOF-1des were mounted on a Bruker APEX-II single-crystal X-ray diffractometer at 120 K equipped with a CCD area detector and operated at 45 kV, 0.64 mA to generate Mo Kα radiation (λ = 0.71073 Å). Data were processed with the Bruker APEX2 software package,59 integrated using SAINT v8.34A and corrected for the absorption by SADABS 2014/5 routines (no correction was made for extinction or decay).60 The structures were solved by intrinsic phasing (SHELXT) and refined by full-matrix least-squares on F2 (SHELXL-2013).61 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were geometrically calculated and refined as riding atoms unless otherwise noted. Diffused electron densities resulting from these solvent molecules were removed from the data set using the SQUEEZE routine of PLATON and refined further using the data generated.62 The final formula of these compounds were determined according to the SQUEEZE results, combined with elemental analysis and thermogravimetric analysis data. The selected crystallographic data for the three compounds are summarized in Tables S1 and S2. CCDC 1840340−1840341 contain crystallographic data for this paper. Adsorption Measurements and Isosteric Heat of Adsorption (Qst). The adsorption experiments were executed on the BELSORP-max (BEL, Japan) surface area and pore size analyzer. Typical samples of ca. 100 mg, preactivated at 120 °C to remove all residual solvents, were transferred in an Ar-filled glovebox to a preweighed analysis tube. The tube was capped with a BELSORP-max Tran Seal, brought out of the glovebox, and transferred to the analysis port of the gas sorption analyzer. The N2 adsorption/desorption isotherms were proceeded at 77 K under a liquid Nitrogen bath. The CO2 adsorption/desorption isotherms were collected at 273 and 298 K in an ice−water and water bath, respectively. The vapor adsorption/desorption isotherms were collected at 298 K in a water bath. The isosteric heat of adsorption (Qst) is calculated using the Clausius−Clapeyron equation (see the Supporting Information).
EXPERIMENTAL SECTION
Materials and Methods. All chemicals and solvents used in the synthesis were of reagent grade and used without further purification. Elemental analyses (EA), Fourier transform infrared (FT-IR) spectra, and powder X-ray diffraction (PXRD) measurements were reported in our previous work.58 1H NMR spectra were recorded at ambient temperature on an Agilent 400 MHz NMR spectrometer. 13C NMR spectra were recorded at ambient temperature on a Varian UNITY plus-600 spectrometer. Chemical shifts were calculated using the solvent resonances as internal standards (1H: 7.26 ppm for CDCl3, 13 C: 77.00 ppm for CDCl3). Thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) were performed by heating the crystalline sample on a Mettler Toledo Star System under a nitrogen atmosphere at a heating rate of 10 °C min−1. Scanning electron microscope (SEM) images and energy dispersive X-ray spectroscopy (EDS) were taken with a HITACHI S-4700 cold fieldemission scanning electron microscope operated at 15 kV. X-ray photoelectron spectra (XPS) were collected with an SSI S-Probe XPS Spectrometer. Synthesis of 5-(2-(Pyridin-4-yl)vinyl)quinoline (pvq). 5Bromoquinoline (1.035 g, 5 mmol), 4-vinylpyridine (0.656 g, 6.15 mmol), K2CO3 (1.38 g, 10 mmol), and Pd(PPh3)2Cl2 (70 mg, 0.1 mmol) in anhydrous DMF (20 mL) was degassed for 10 min. Then, the suspension was stirred for 24 h at 120 °C under nitrogen atmosphere. After cooling down to room temperature, the mixture was concentrated via vacuum filtration and extracted with DCM. The brown solution was washed with brine, dried over Na2SO4, and then evaporated to dryness under reduced pressure. The crude product was purified by silica gel column chromatography with petroleum ether/ ethyl acetate (8:1 v/v) as the eluent, affording desired product pvq as a yellow solid. Yield: (0.73 g, 64%). FT-IR (KBr, cm−1): 3443 (m), 2953 (w), 2866 (w), 1722 (s), 1609 (m), 1439 (m), 1416 (w). 1H NMR (600 MHz, DMSO): δ 8.94 (dd, J = 9.1, 5.0 Hz, 2H), 8.59 (dd, J = 4.6, 1.4 Hz, 2H), 8.35 (d, J = 16.2 Hz, 1H), 8.03 (t, J = 8.4 Hz, 2H), 7.84−7.78 (m, 1H), 7.75 (dd, J = 4.7, 1.3 Hz, 2H), 7.61 (dd, J = 8.5, 4.1 Hz, 1H), 7.37 (d, J = 16.1 Hz, 1H). 13C NMR (151 MHz, DMSO): δ 151.04, 150.48, 148.40, 144.54, 134.45, 132.82, 130.09, 129.69, 128.77, 126.46, 124.33, 121.98, 121.76. Synthesis of [Zn2(tdc)2(pvq)]·1.5DMF (MOF-1as). A mixture of pvq (0.046 g, 0.2 mmol), ZnSO4·7H2O (0.143 g, 0.5 mmol), and H2tdc (0.034 g, 0.2 mmol) was dissolved in 10 mL of anhydrous DMF in a 25 mL vial with a screw cap and sonicated until the solid was completely dissolved. Then, the solution was heated in an isothermal oven at 120 °C for 72 h. Yellow cubic-like crystals were collected, washed with DMF and dried in air. Yield: 82.7 mg (51% based on pvq). Anal. Calcd (%) for C32.5H26.5O9.5N3.5S2Zn2: C, 48.06; H, 3.26; N, 6.04%. Found: C, 48.11; H, 3.35; N, 5.91%. FT-IR (KBr disk, cm−1): 3430 (w), 3101 (w), 2926 (w), 2597 (w), 1950 (w), 1825 (w), 1678 (s), 1644 (s), 1531 (m), 1386 (ms), 1226 (w), 1089 (m), 802 (s), 770 (s), 672 (ms), 506 (s). Synthesis of [Zn2(tdc)2(pvq)] (MOF-1des). As-synthesized MOF-1as (∼150 mg) samples were washed with DMF three times and EtOH three times, followed by soaking in EtOH for 5 days to allow solvent exchange. During the solvent exchange process, EtOH was decanted and replaced with fresh anhydrous EtOH three times per day. After that, the ethanol-exchanged crystals were isolated by centrifugation. Then, each sample was activated under vacuum for 12 h and then degassed under vacuum at 120 °C for 12 h to form yellow crystals of MOF-1des. Anal. Calcd (%) for C28H16O8N2S2Zn2: C, 47.82; H, 2.29; N, 3.98%. Found: C, 47.91; H, 2.16; N, 3.83%. FT-IR (KBr disk, cm−1): 3410 (w), 3080 (m), 2588 (w), 2079 (w), 1948
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RESULTS AND DISCUSSION Syntheses and Characterization. Solvothermal reaction of Zn(NO3)2·7H2O with H2tdc and pvq in DMF gave yellow crystals of MOF-1as in 51% yield (Scheme S1). The single crystal X-ray diffraction analysis reveals that it crystallizes in the monoclinic space group P21/c. Four carboxylates of tdc2− linkers bridge Zn2 paddle-wheel SBUs (Figure S1) to form a distorted two-dimensional (2D) square grid with 10.9 Å × 9.8 Å in one unit length. The 2D square grids are axially coordinated by pvq pillar ligands along with b axis to form a three-dimensional (3D) framework (Figure 1a). The dihedral angle between the pyridine and quinoline rings in the “trans” conformer of pvq is ca. 29.1°. Two identical 3D frameworks are unsymmetrically interpenetrated by π−π interactions between parallel quinoline groups (ca. 3.9 Å), giving rise to one-dimensional (1D) channels with a cross section of approximately 7 Å × 11 Å in the a-direction, which are occupied by highly disordered DMF guest molecules (Figures1c, S2, and S3). The accessible volume of the fully desolvated MOF-1as is 32.0%, as estimated using the PLATON program.62 The chemical formula of MOF-1as was determined as [Zn2(tdc)2(pvq)]·1.5DMF on the basis of single crystal X-ray analysis and elemental analysis The phase purity of the bulk MOF-1as material was independently confirmed by powder X-ray diffraction (PXRD) and the simulated data from SCXRD (Figure S4). The thermogravimetric analysis (TGA) revealed that MOF-1as can lose DMF guest molecules at 120 °C but is thermally stable up to 300 °C (Figure S5). In addition, MOF-1as can remain intact without B
DOI: 10.1021/acs.inorgchem.8b01408 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
orthogonal directions, resulting in a collapsed, nonporous framework (Figures 1b,d and S9). In particular, phase transition is accommodated by folding up of “wine-rack” type array while the tetrahedral coordination geometry of Zn2+ centers and the framework topology remains invariant (Figures S10 and S11).64−66 During the phase transition, the pore contraction associated with rotational motion of the pvq linkers (12.8° compared to 29.1°) is accompanied by an elongation of the square grids owing to the “kneecap” rotational O−O axis of the tdc2+ carboxylate groups which are located on the 2-fold rotational axis (Figures S12 and S13).56 More intriguingly, the effect of this phase transition associated with a symmetry-breaking is evident in the “breathing” behavior upon DMF molecule removal, which is also consistent with a remarkable 27.2% contraction of the c lattice parameter (Figures 3 and S14). Figure 1. Perspective view of the switchable SCSC structural transformation between MOF-1as (a) and MOF-1des (b) upon desolvation/resolvation. (c and d) Space-filling representations of the structures of the MOF-1as and MOF-1des along the a-axis, respectively. The two interpenetrating frameworks are shown in rose and aqua. The disordered DMF molecules are omitted for clarity.
loss of its crystallinity after 3 days of stability testing (Figure S6). Switchable Breathing Behavior. To demonstrate the switchable “breathing” behavior, the ethanol exchanged MOF1as is further activated under high vacuum at 120 °C for 12 h to give the fully desolvated frameworks MOF-1des in a SCSC transformation (Figure S7). For comparison, the diffraction patterns for MOF-1as and MOF-1des are quite different, which confirmed that the phase transformation resulting from desolvation is complete (Figure S8).63 Notably, both the positions and intensities of the Bragg diffraction peaks assignable to the [100], [011], and [111] were substantially changed (Figure 2).
Figure 3. Ball−stick representation of the structure of MOF-1as (a) and MOF-1des (b). The H atoms in MOF-1as and MOF-1des are omitted for clarity.
Remarkably, resolvation of the MOF-1des phase in DMF at 60 °C for 3 days leads to recovery of the initial MOF-1as phase via SCSC transformation, as confirmed by SCXRD and PXRD (Figure S15). Energy dispersive X-ray spectrometry (EDS) mapping analysis of MOF-1as and MOF-1des unambiguously reveals the extremely homogeneous distribution of Zn, S, O, and N elements (Figures S16 and S17). The survey X-ray photoemission spectroscopy (XPS) spectra of MOF-1as and MOF-1des further confirm the chemical states of Zn, C, S, and N elements (Figures S18−S20). To assess the porosity changes associated with the reversible phase transformation, the N2 adsorption isotherm at 77 K of MOF-1des was measured and its profile reveals a remarkable adsorption behavior which is different from all six previous reports of isotherm types with a Brunauer−Emmett−Teller (BET) surface area of 437 m2 g−1 within the relative pressure range of 0.05 < P/P0 < 0.3 (Figures 4a and S21−S23). During the adsorption, almost negligible adsorption of N2 uptake (1.01 cm3 g−1) at low relative pressures (0.15) can be observed. However, the relative pressure rapidly shifts to relatively low pressure at 8.19 × 10−4 following a sharp step in the adsorption isotherm, which indicates the “gate-opening” phenomenon with the saturation uptake of 145.6 cm3 g−1 (Figure 4b).67 The significant hysteresis of the desorption isotherm is markedly different from the adsorption branch. The stepwise adsorption isotherm coupled with an uncommon hysteresis loop can be explained by a dynamic structural phase transformation between np phase and lp phase. Also noteworthy is the fact
Figure 2. Comparison of the powder X-ray diffraction patterns of MOF-1as (green) and MOF-1des (red).
Furthermore, high-quality single crystals of MOF-1des suitable for crystal structure determination were obtained. The structural characterization of MOF-1des, based on X-ray crystallography, shows that the removal of DMF molecules results in a phase transition from monoclinic space group P21/c in MOF-1as to orthorhombic space group Pcc2 in MOF-1des with a pronounced volume overall shrinkage of about 17.9% in C
DOI: 10.1021/acs.inorgchem.8b01408 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 5. Carbon dioxide adsorption isotherms of MOF-1des extrudated at 273 and 298 K. Filled and open symbols represent adsorption and desorption curves, respectively.
significant quadrupole moment (13.4 × 10−4 C m2) and the higher polarizability (2.51 Å3) diffusion into the narrow pores of the framework is hindered at low relative pressure while the lower polarizability of N2 (1.71 Å3) favors its adsorption on the polar micropore walls decorated with the thiophene groups exposed toward the channels.72 To quantitatively evaluate the binding strength of the polar channels toward CO2, the isosteric heat of adsorption (Qst) of MOF-1des was calculated through the Clausius−Clapeyron equation based on the CO2 adsorption isotherms at 273 and 298 K. The corresponding Qst at low coverage is 21.3 kJ·mol−1 and then gradually increases to 29.4 kJ·mol−1 at high CO2 loading (Figure S24). The slightly lower value of Qst indicates that the weaker interaction of CO2 with the polar channels results in significantly small amount of CO2 absorbed. The recyclability of MOF-1des for selective adsorption is an important aspect for practical application. Vapor Adsorption Properties. Considering the nature of the “gate-opening” flexibility in MOF-1des, we further explored guest molecules recognition of flexible MOFs, using benzene, toluene, and p-xylene. The separation of toluene from benzene, p-xylene with promising performance under ambient conditions is challenging due to their markedly similar physicochemical properties.73 Various types of porous solid materials have been investigated thoroughly as selective adsorbents for separation, such as zeolites, and rigid MOFs.74−76 However, flexible frameworks may be an alternative way to overcome the aforementioned limitations owing to their drastic structural changes upon external stimuli.77−79 Consistent with our hypothesis, the significantly different adsorption capacity for toluene, benzene and p-xylene was observed (Figure 6). The toluene sorption features a reversible type-I isotherm, and reaches a saturated adsorption at P/P0 = 0.05. The steep slope reveals that toluene can easily diffuse into the channels, indicating the stronger host−guest interactions (Figure S25).80 A lower adsorbed amount of benzene is observed, even though the adsorption isotherms of toluene and benzene are similar (Figure S26). The toluene uptake is nearly 1.5 times larger than that of benzene, amounting to 38 cm3 g−1. For comparison, however, p-xylene sorption features a multistep isotherm with a steep relative pressure at 0.05. The saturated uptake amount of p-xylene reaches 2.53 cm3 g−1 at P/P0 = 0.95, indicating that the adsorption processes are restricted because of the weaker host−guest interactions (Figure S27). The noticeable differ-
Figure 4. (a) Nitrogen adsorption isotherms of MOF-1des at 77 K. Inserted plot shows the pore size distribution of MOF-1des. (b) Magnified view of the relative pressure region below 0.15. (c) Breathing behaviors of MOF-1des. N2 sorption isotherms of MOF1des for four adsorption−desorption cycles. Filled and open symbols represent adsorption and desorption curves, respectively.
that the dynamic structural transformations with gas-induced gate-opening behavior are fully reproducible over at least four adsorption−desorption cycles (Figure 4c). Similarly, consistent with the first, second, third, and fourth adsorption−desorption cycles, a phase transformation also occurs. However, the follwing adsorption step gradually shifts to higher relative pressures, and the maximum uptake of N2 reduces to 116.2, 113.6, and 111.3 cm3 g−1, respectively (Figure 4c, insert). Indeed, the switchable gas-induced gate-opening behavior reveals that the transformation from the collapsed phase to open pore phase can lead to a sudden expansion of the overall pore volume at a certain threshold pressure of nitrogen and thus result in a negative pressure.68,69 Furthermore, the metastable state of the open pore phase releases some heat (ΔH < 0) to the environment to offset the endothermic desorption of N2 (ΔH > 0), and returns back to the original collapsed phase upon releasing the absorbed gas.70 Thus, the adsorption/desorption isotherm can be cycled many times. To obtain a deeper insight into the guest-induced “breathing” effect, the CO2 adsorption isotherm at 273 K was also measured. Compared to N2 adsorption profile, no appreciable CO2 adsorption and gate-opening phenomenon with stepwise sorption could be observed (Figure 5). The much lower uptake of CO2 at 273 K (13.3 cm3 g−1) prompted us to further study the significant selectivity adsorption of N2 over CO2. Despite the smaller kinetic diameter of CO2 (3.3 Å) compared with that of N2 (3.64 Å),71 the difference of molecular dipole moment may be responsible for the selective adsorption property. It is plausible that for CO2 molecules with a D
DOI: 10.1021/acs.inorgchem.8b01408 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Tel.: +86-512-65882865. Fax: +86-512-65880328. *E-mail:
[email protected]. ORCID
Wen-Hua Zhang: 0000-0001-9047-8881 Jian-Ping Lang: 0000-0003-2942-7385 Notes
The authors declare no competing financial interest. Figure 6. Adsorption isotherms of benzene (olive), toluene (blue), and p-xylene (red) vapor on the MOF-1des samples at 298 K.
ACKNOWLEDGMENTS
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REFERENCES
We thank the National Natural Science Foundation of China (Grant Nos. 21531006, 21671143, and 21773163), the State Key Laboratory of Organometallic Chemistry of Shanghai Institute of Organic Chemistry (2018kf-05), the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Project of Scientific, and Technologic Infrastructure of Suzhou (SZS201708) for financial support. S.Y.X. thanks the Innovative Research Program for Postgraduates in Universities of Jiangsu Province.
ences in vapor adsorption properties of MOF-1des can probably be attributed to the electron affinity and geometrical features of vapor molecules toward the polar micropore walls decorated with thiophene groups associated with self-structural adjustment.81 We have also tried to obtain the guest-loaded MOF-1des but failed to gain insights in this case. Overall, this result proves the potential application of MOF-1des for the effective separation of toluene over benzene and p-xylene.
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CONCLUSION We presented the reversible phase transformation with a breathing effect of the flexible pillared-layered MOF, MOF1as, and its desolvated species MOF-1des as a guest-response adsorbent for selective adsorption and separation. Interestingly, the gate-opening behavior of MOF-1des has been observed only in N2 adsorption isotherm with hysteresis loops associated with a pronounced negative gas adsorption pressure, which can be attributed to the dynamic structural transformation between np phase and lp phase induced by guest molecules. The structure can return back to its original state, even after four adsorption/desorption cycles. Furthermore, the introduction of thiophene groups in MOF was shown to lead to more efficient absorption of toluene than benzene and pxylene. The pores vary not only in polarity but also in their flexibility, which results in a remarkable diversity of guest−host interactions. The results presented in this work should not only provide a better fundamental understanding of the nature of flexible MOF triggered by external stimuli but also illustrate the potential application of flexible MOFs in molecular recognition and separation.
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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.inorgchem.8b01408. Full details for sample preparation, characterization results, and the isosteric heats of adsorption (PDF) Accession Codes
CCDC 1840340−1840341 contain 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 E
DOI: 10.1021/acs.inorgchem.8b01408 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.8b01408 Inorg. Chem. XXXX, XXX, XXX−XXX
