Article pubs.acs.org/IC
Exploration of Gate-Opening and Breathing Phenomena in a Tailored Flexible Metal−Organic Framework Sung-min Hyun,†,# Jae Hwa Lee,† Gwan Yeong Jung,‡ Yun Kyeong Kim,† Tae Kyung Kim,† Sungeun Jeoung,† Sang Kyu Kwak,*,‡ Dohyun Moon,*,§ and Hoi Ri Moon*,† †
Department of Chemistry, and ‡School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea § Beamline Division, Pohang Accelerator Laboratory, 80 Jigokro-127-beongil, Nam-gu Pohang, Gyungbuk 37673, Republic of Korea S Supporting Information *
ABSTRACT: Flexible metal−organic frameworks (MOFs) show the structural transition phenomena, gate opening and breathing, upon the input of external stimuli. These phenomena have significant implications in their adsorptive applications. In this work, we demonstrate the direct capture of these gate-opening and breathing phenomena, triggered by CO2 molecules, in a well-designed flexible MOF composed of rotational sites and molecular gates. Combining Xray single crystallographic data of a flexible MOF during gate opening/closing and breathing with in situ X-ray powder diffraction results uncovered the origin of this flexibility. Furthermore, computational studies revealed the specific sites required to open these gates by interaction with CO2 molecules.
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INTRODUCTION The tunability and modularity of metal−organic frameworks (MOFs) with a variety of metal building blocks and organic linkers have generated a myriad of MOFs that allow for their applications in a wide range of areas.1,2 Recently, flexible MOFs have attracted great attention because they show distinctive properties that cannot be achieved with rigid MOFs.3,4 For example, flexible MOFs can selectively respond to a specific adsorbate in a mixture of small gas molecules with similar kinetic diameters and small differences in physical properties. The flexible nature of MOFs originates from the structural transformability of molecular components upon external stimuli, which includes the environmental change of secondary building units (SBUs) as well as the regional movements of organic linkers such as twisting, rotating, bending, or tilting.5−8 In addition, for the case of interpenetrating networks, host− host and host−guest interactions can also afford the dynamic behavior of the frameworks.9,10 The responsive dynamic structural transformations in flexible MOFs have been widely investigated upon the application of various external stimuli such as light,11 temperature,12−14 and guest molecules.15−17 The two prominent dynamic structural changes of MOFs upon gas sorption are “gate-opening/closing”18−21 and “breathing” behaviors.22−25 Gate-opening/closing is the transition from a closed and nonporous to an open and porous phase, upon the application of external stimuli. The breathing phenomenon refers to structural transitions such as an abrupt expansion/compression of the unit cell, induced by atomic displacement caused by interactive guest molecules.25−27 The utility and importance of these two phenomena were emphasized by Kitagawa and Férey and have subsequently emerged as a major area of research interest. Kitagawa et al. © XXXX American Chemical Society
reported a pillared-layer coordination polymer exhibiting a locking/unlocking behavior before and after guest adsorption.19 This was accompanied by the rotation of molecular gates as well as the expansion of cell dimensions. On the other hand, a typical example of the breathing phenomenon was found in the MIL-53 series by Férey et al.22,24,28 In their works, the dried MIL-53-ht structure, prepared by heating MIL-53 at high temperature and cooling it in air, could transform into the MIL53-lt structure with reabsorbed water molecules. This transition involved the reversible change of its cell volume. To date, these two phenomena have been studied with flexible MOFs by using various instrumentation techniques.22,24 Nevertheless, tailoring the dynamic properties in flexible MOFs is still challenging, and further detailed in situ characterizations and computational studies are expected to deserve new insights into the flexibility design of MOFs.4 Recently, structural studies with model MOF systems, mostly based on X-ray diffraction analyses,29,30 have been carried out to obtain more compelling evidence to understand these interesting phenomena. Theoretical calculations, including the formulation of thermodynamic isotherms and ab initio molecular dynamics simulations,27,31 were also conducted to understand these intriguing phenomena in flexible MOFs. However, the substantiation of the origin of the flexibility is still challenging because both phenomena are inevitably accompanied by a high degree of atomic displacement. The MOFs showing these structural changes are mostly responsive by CO2 over other gases. Nevertheless, the reason for this selectivity only has been explained as the strong interaction of the CO2 Received: December 11, 2015
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DOI: 10.1021/acs.inorgchem.5b02874 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry molecules with hosts due to its quadrupole moment,32−34 without providing the definite evidence. Herein, we report a well-designed flexible MOF at the synthesis stage, which shows both the gate-opening and breathing effects upon CO2 adsorption. The structural information before and after structural arrangements are successfully provided by the X-ray powder diffraction (XRPD) and single-crystal diffraction (SCD) results, which enable the capture of the snapshots of the gate-opening and breathing phenomena. In addition, during the CO2 adsorption, the specific sites in the flexible MOF that interact with CO2 molecules to open the gate are verified by computational studies of multiscale simulations that are a combination of the density functional theory (DFT), molecular dynamics (MD), and grand canonical Monte Carlo (GCMC) simulations. This is the first study which elucidates the framework flexibility in the entire responsive process upon CO2 molecules, before and after as well as during adsorption.
them to rotate more freely than other metal clusters that are extended toward multidirections. It also has two bulky and flexible propyl pendant arms that can act as a molecular gate that is dependent on their orientation. Meanwhile, H4BPTC has many rotational sites (such as the bond between the two phenyl rings and that between the phenyl ring and a carboxylate group) endowing high flexibility to the MOF. The self-assembly of these two rotational moieties, [NiLpropyl](ClO4)2 and H4BPTC, yielded {[(NiLpropyl)2(BPTC)]·4DMF·4H2O} (asf lexMOF; DMF = N,N-dimethylformamide) (see also experimental details in the Supporting Information).36 The asymmetric unit of as-f lexMOF contains two kinds of Ni(II) macrocyclic complexes, Ni1A and Ni2B, with half occupancy for each type of complex and the other half for the BPTC4− ligand. As described in Figure 1b, the linear connection between [NiLpropyl]2+ and BPTC4− is formed via a coordinate bond between Ni(II) and the carboxylate, and a hydrogen bond between the oxygen atoms of the carboxylate and the secondary amines in [NiLpropyl]2+. This resulted in six-coordinate octahedral Ni(II) centers. It is noteworthy that the macrocyclic complex of Ni1A involves four hydrogen bonds (Figure S1), namely all the secondary amines form hydrogen bonds with the oxygen atoms of the carboxylates. On the other hand, the Ni2B macrocycle has only two hydrogen bonds. The two phenyl rings of BPTC4− are twisted at an angle of 45.98(5)° and bridged by four macrocycles to construct a three-dimensional (3D) network (Figure 1c). Thus, as-f lexMOF has 3D interconnected pores that are formed by intersecting 1D channels in [100] and [010] directions. The PLATON calculation indicates that as-f lexMOF contains 40% void space (2880 Å3 per unit cell volume).37 Consequently, the potentially flexible MOF, as-f lexMOF, that embodies a revolving module at the Ni(II) macrocyclic complex was successfully synthesized. Herein the importance of hydrogen bonds needs to be emphasized. These are relatively labile intermolecular interactions that enable the dynamic transformations of the MOF. It is revealed that the number of hydrogen bonds has a decisive effect on its flexible behavior upon CO2 gas uptake, as discussed later. Framework Stability of f lexMOF. To activate asf lexMOF, its guest molecules were exchanged with the more volatile solvent acetonitrile (ex-f lexMOF) and dried by heating under vacuum (d-flexMOF) (see experimental details in the Supporting Information). A structural transformation was observed during their activation as evidenced from the comparison of XRPD patterns (Figure S2). The pattern of ex-f lexMOF slightly shifted from that of as-f lexMOF, whereas d-f lexMOF showed significant changes in the XRPD patterns. The change of cell parameters in single-crystal diffraction (SCD) data of as-flexMOF and ex-f lexMOF also support these results (Table S1).36 Unfortunately, SCD analysis for df lexMOF could not be performed as it lost its single crystallinity during evacuation. Thus, the space group and cell parameters of d-flexMOF were extracted from the refinement based on XRPD patterns (Figure S3) and revealed a drastic decrease in cell volume (5096 Å3) of 29% when compared to that of as-flexMOF (7168 Å3). From these results, we deduced that the present MOF is clearly flexible with regard to the types and presence of guest molecules, while the atomic displacement occurs in a significant range. Gas Sorption Properties of d-f lexMOF. Gas sorption results for N 2, CH 4, and CO2 also reflect the same phenomenon. The N2 sorption isotherm of d-f lexMOF at 77
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RESULTS AND DISCUSSION Synthesis and Crystal Structure of as-flexMOF. To elucidate the gate-opening and breathing phenomena of a flexible MOF, we designed a flexible MOF that possesses rotatable molecular gates for opening/closing of the pores and robust but flexible connectivity for easy atomic displacement (Figure 1).35 To construct the flexible MOF (designated as flexMOF), we employed a square planar Ni(II) macrocyclic complex, [Ni(C14H34N6)]2+ ([NiLpropyl]2+), as a metal building block and 2,2′,5,5′-biphenyltetracarboxylic acid (H4BPTC) as an organic linker (Figure 1a). [NiLpropyl]2+ is used as a linear linker with only two accessible coordination sites, allowing
Figure 1. (a) Metal and organic building blocks of as-f lexMOF and (b) the linear connectivity via coordinate and hydrogen bonds. (c) SCD structure of as-f lexMOF, shown in the ac plane. Color scheme: Ni, yellow; C, black; O, red; N, blue; H, white. B
DOI: 10.1021/acs.inorgchem.5b02874 Inorg. Chem. XXXX, XXX, XXX−XXX
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result comparable to that of Zn-MOF-74.41 Upon increasing the sorption temperatures from 195 to 273 to 298 K, the CO2 pressures corresponding to a vertical uptake increased from 0.18 to 7.8 to 15.0 bar, respectively. At high temperatures, the kinetic energies of the gas molecules become large enough to surpass the intermolecular potential energy (i.e., the potential energy between the gas molecules and framework). To allow the gas molecules to adsorb onto the host framework, the density should be increased at higher temperatures. Based on thermodynamics, this directly leads to a higher pressure. In Situ XRPD Experiments upon CO2 Sorption. In order to directly observe the structural changes of d-flexMOF upon CO2 sorption, we conducted in situ XRPD measurements under the same conditions as the CO2 sorption measurements at 298 K. These exhibited a stepwise adsorption/desorption isotherm (Figure 3, and also see experimental detail in the Supporting Information). During these measurements, to allow us to reach the stabilized structure at each pressure, the sample was exposed to CO2 gas at the target pressure for 5 min. The CO2 adsorption in the first stage, up to 15 bar (5.36 wt %), mainly occurred in the void volume of d-flexMOF (909 Å3). Meanwhile its XRPD patterns remained identical to those of df lexMOF. In the pressure range for vertical uptake (>15 bar), the typical adsorption profile of the gate-opening phenomenon was observed. In the vertical region, the XRPD pattern completely changed from that of d-f lexMOF and was almost coincident with that of as-f lexMOF (Figure S7). Calculation of its cell volume by XRPD indexing indicated a 146% cell expansion from 5096 to 7454 Å3. This structure was retained during desorption down to a pressure of 12 bar. As the pressure decreased below 9 bar, the XRPD pattern greatly changed again, and its calculated cell volumes were 6590 Å3 and reached 5628 Å3 below 5 bar. This is definitely “breathing behavior” that is associated with reversible movement between expansion and contraction. Interestingly, even under very low pressure, the adsorbed CO2 molecules were not removed completely from the flexMOF, showing a large hysteresis that does not merge,
K showed typical type II sorption behavior with a Brunauer− Emmett−Teller (BET) surface area of 7 m2/g and a total pore volume of 0.04 cm3/g, indicating a nonporous nature (Figure S4). Even at high pressures of up to 30 bar at 298 K, dflexMOF adsorbed only negligible amounts of N2 and CH4, 0.52 and 0.98 wt %, respectively (Figure S5). In contrast, as shown in Figure 2, d-f lexMOF showed interesting adsorption
Figure 2. CO2 adsorption isotherms of flexMOF at 195, 273, and 298 K. Desorption isotherms were omitted for clarity, and the full curves are provided in Figure S6.
behavior upon CO2 adsorption at the various temperatures. At 195 K, d-f lexMOF adsorbed 4.96 wt % of CO2 up to a pressure of 0.18 bar, where it exhibited the next abrupt adsorption of CO2 over 30 wt % (6.8 mmol of CO2/g of host). The small CO2 uptake observed below 0.18 bar might be associated with the void of d-f lexMOF which exists even before the significant structural changes but has selectivity for CO2 over N2 and CH4.38−40 It finally adsorbed up to 45.6 wt % (10.4 mmol of CO2/g of host) at 1 bar. Adsorption isotherms of CO2 at 273 and 298 K also traced similar two-step adsorption profiles that reached ca. 24 wt % (5.4 mmol of CO2/g of host) at 30 bar, a
Figure 3. (a) CO2 adsorption/desorption isotherm at 298 K; (b) in situ XRPD experimental results with calculated cell volumes based on the XRPD patterns. C
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Figure 4. Comparison of SCD structures of p-f lexMOF and as-f lexMOF representing (a) the gate-opening and (b) breathing phenomena upon CO2 adsorption.
breakage and reformation of the H-bond between the carboxylate oxygen of the ligand and the secondary amine hydrogen in the macrocycle, from blue to pink hydrogen, as clearly evidenced by SCD results (Figure 4a). Out of four crystallographically independent Ni(II) macrocyclic complexes in the p-f lexMOF structure, only two macrocycles participated in such gate-opening motions; one macrocycle did not rotate during the gate opening, another two macrocycles (highlighted in Figure 4a) rotated and opened the “gate” by changing the position of the secondary amine that formed the hydrogen bond, and the last one, in spite of its rotation, was not involved in the pore “opening/blocking” behavior. It should be noted that the nonrotational macrocycle involved the four H-bonds, whereas the rotating macrocycles had only two H-bonds. These differences consequently exerted a significant influence on the rotational behavior upon guest molecule removal and gas uptake. Simultaneously, SCD also verified that the present MOF underwent breathing (Figure 4b), showing good agreement with the results of the in situ XRPD experiment. As shown in Figure 4b, the rectangular dimension in f lexMOF increased from 9.19 × 20.78 Å2 to 12.00 × 29.79 Å2 upon CO2 uptake (for p-flexMOF and as-flexMOF, respectively). This expansion is mainly related to the dihedral angles between the two phenyl rings as well as the carboxylate planes and benzene rings of the BPTC4− ligand (Figure S10) upon CO2 adsorption, which were introduced as rotational sites at the synthetic stage. In the present study, the breathing shows the same pattern to human inhalation/exhalation, in which expansion is associated with an input and contraction with an output, while the reverse behavior was observed in MIL-53 that is a representative of breathing MOFs. Theoretical Calculation of Heat of Adsorption. To investigate the driving force that leads to the flexible behavior selectively triggered by CO2 molecules over N2 and CH4, multiscale computational analyses were conducted at 298 K (see the simulation details in the Supporting Information for model systems and methods). From the MD, the heat of
and heating was always needed to reactivate this MOF. The result of the second measurement revealed that the flexible behavior of the MOF is reversibly proceeded (Figure S8). The XRPD pattern of f lexMOF obtained after the CO2 sorption experiment was similar but shifted to lower angles when compared with that of d-f lexMOF. These results revealed that the crystal structures are similar, but their cell volumes are different (5628 vs 5096 Å3). Although the gate-opening and breathing phenomena occur simultaneously in our system and have been commonly mingled to explain anomalous sorption isotherm patterns in MOFs, in a strict sense as mentioned previously, they clearly display different behavior and have to be designated separately. To uncover the origin that leads to this distinguished behavior, in situ XRPD analyses were insufficient, and more discernible information about the structural transformation was needed. Comparison of SCD Structures of p-f lexMOF and asf lexMOF. The SCD experiments for this MOF are suitable to clearly elucidate these transitions as well as local structural changes. Since d-flexMOF lost its single crystallinity during activation, we mildly evacuated ex-f lexMOF as an alternative solution to the limit where its single crystallinity was maintained and successfully obtained an SCD structure of the partially dried crystal (p-flexMOF) (Table S6).36 Fortunately, the simulated XRPD pattern of p-flexMOF was the same as that of the flexMOF obtained after finishing the desorption experiment (Figures 3b and S9). Thus, this information can be used to explain the representative structure which has a small cell volume. Interestingly, as shown in Figure 4a (left), pflexMOF has a closed structure in which the pendant arms of the macrocycles block the pore apertures. However, as CO2 molecules strike the closed structure with enough pressure to open the pores, the macrocycles rotate upon the axis generated by coordination between the carboxylate and Ni(II) center. This regional rotational motion of the macrocycles leads to gate opening, allowing a large amount of CO2 molecules to access the pores (Figure 4a, right). It is inevitably accompanied by the D
DOI: 10.1021/acs.inorgchem.5b02874 Inorg. Chem. XXXX, XXX, XXX−XXX
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rotation of the macrocyle. The adsorption of CO2 on the framework provides the energy to rotate the propyl pendant group in the direction of the gate opening. Subsequently, the hydrogen bond (O4C···H2B in Ni2B) in the inner side of the complex is broken, not from chemical interaction by CO2, which is inaccessible to interior part, but from mechanical energy. As a result, the carboxylate oxygen forms a new hydrogen bond with the adjacent secondary amine (O4C··· H1B), as described in the SCD results. This theoretical calculation provided a clear explanation for the selectivity of CO2 induced by vdW interactions, as well as quadrupole moments, and confirmed the specific interaction sites needed to initiate this flexible behavior. In conclusion, a flexible MOF showing gate-opening properties as well as breathing effects has been designed and synthesized by the introduction of rotational and interactive moieties with CO2 molecules. It is noticeable that the implementation of flexible MOFs through the direct rotation of metal components is quite rare. In this work, introducing a square planar Ni(II) macrocyclic complex containing the dangling side chains led to the MOF by acting as a rotating door. The in-depth elucidation of the structural change, which is only triggered by CO2 sorption, was feasible through direct structural comparison based on in situ XRPD and SCD data. The computational simulation clearly revealed the site and the kind of the interaction occurring with the CO2 molecules, namely, a very weak van der Waals interaction that finally led to significant structural changes such as gate opening and breathing. Therefore, the design insight of this MOF and the collective elucidation of its flexibility can successfully provide the rational tool for the construction of flexible MOFs with new dynamic properties as well as the idea of designing the responsive MOFs for applications in green and renewable energy.
adsorption was estimated during the adsorption process where the energy of gas molecules such as CO2, N2, and CH4 was transferred to the host framework. The heat of adsorption (ΔEads) is determined by the sum of electrostatic (ΔECoulomb) and van der Waals energy (ΔEvdW) components on the basis of a single Ni2B macrocycle (Table 1 and Figure S11). As Table 1. Heats of Adsorption and Its Energy Components for Each Gas Molecule Obtained by MD Simulation Performed at 298 K and 14.99 bar gases
ΔECoulomba (kJ/mol)
ΔEvdWb (kJ/mol)
ΔEadsc (kJ/mol)
CO2 N2 CH4
−8.44 0.00 −0.20
−29.81 −13.13 −17.17
−38.25 −13.13 −17.37
a ΔECoulomb is the Coulombic energy component. bΔEvdW is the van der Waals energy component. cΔEads is the heat of adsorption per single Ni2B macrocycle, which is the sum of Coulombic and van der Waals energy components (see the simulation details in Supporting Information).
expected, ΔECoulomb for CO2 adsorption only has a considerably large value of −8.44 kJ/mol. This is attributed to its quadrupole moment. On the other hand, although all the gas molecules show substantial ΔEvdW values of −29.81, −13.13, and −17.17 kJ/mol for CO2, N2, and CH4, respectively, ΔEvdW for CO2 is twice as large as that of the others. The simulated adsorption isotherms of CO2 obtained from GCMC simulations agreed well with experimental results (Figure S12). In particular, when the GCMC simulation was performed at the pressure needed to start the structural changes upon CO2 adsorption, most of the CO2 molecules were found near the propyl pendant group in the macrocycle complex (Figures 5 and S13). The distances
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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.5b02874. Crystallographic information files for as-f lexMOF (CIF) Crystallographic information files for ex-f lexMOF(CIF) Crystallographic information files for p-f lexMOF (CIF) Experimental details; crystallographic data; additional structural figures; XRPD patterns; gas sorption isotherms; multiscale simulation results (PDF)
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Figure 5. GCMC snapshot for binding configuration of CO2 molecules at the gate-opening condition.
AUTHOR INFORMATION
Corresponding Authors
between the oxygen atoms of the CO2 molecules and the hydrogen atoms of the propyl group are in the range of 2.772− 3.480 Å. This implies that a weak but meaningful interaction exists. The van der Waals (vdW) interaction, which has been explained by Torrisi et al., with alkyl functionalized benzene,42 reflects the high ΔEvdW for CO2 adsorption. As a result, ΔEads at 298 K was determined as −38.25, −13.13, and −17.37 kJ/mol for CO2, N2, and CH4, respectively, by the sum of ΔECoulomb and ΔEvdW. Consequently, as the rotation energy barrier of flexMOF obtained by the DFT calculation was estimated to be ∼26.44 kJ/mol (Figure S14), CO2 gas was the only adsorbate to produce the energy surpassing the energy threshold of the
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# Department of Chemistry, Texas A&M University, 401 Joe Routt Blvd., College Station, TX 77843, United States (S.m.H.).
Notes
The authors declare no competing financial interest. E
DOI: 10.1021/acs.inorgchem.5b02874 Inorg. Chem. XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea (NRF-2013K1A3A1A04076417; NRF2014R1A5A1009799; NRF-2014R1A1A2058815). The SCD and in situ XRPD experiments at PLS-II BL2D-SMC beamline were supported in part by MSIP and POSTECH (2015-second2D-009). The computational resources come from UNISTHPC and KISTI-PLSI. J.H.L. acknowledges the Global Ph.D. Fellowship (NRF-2013H1A2A1033501).
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DOI: 10.1021/acs.inorgchem.5b02874 Inorg. Chem. XXXX, XXX, XXX−XXX
