Organization of Lithium Cubane Clusters into Three-Dimensional

Article pubs.acs.org/crystal

Organization of Lithium Cubane Clusters into Three-Dimensional Porous Frameworks by Self-Penetration and Self-Polymerization Xitong Chen,† Xianhui Bu,*,‡ Qipu Lin,† Quan-Guo Zhai,† Xiang Zhao,† Yuan Wang,† and Pingyun Feng*,† †

Department of Chemistry, University of California, Riverside, California 92521, United States Department of Chemistry and Biochemistry, California State University Long Beach, 1250 Bellflower Boulevard, Long Beach, California 90840, United States

‡

S Supporting Information *

ABSTRACT: Metal−organic frameworks based on lithium inherit the unique chemical and structural features of the metal ion itself. While the monomeric lithium node, usually 4connected, is very desirable for designing zeolite-type networks, the resulting lithium MOF usually has limited stability, especially for 3-connected nets due to the solvent termination. The conventional design strategy based on lithium aryloxide clusters makes use of phenol-type ligands for cluster formation and a separate bifunctional ligand for cross-linking, which also leads to 4-connected nets. By integrating the roles of cluster-formation and frameworkformation into a single ligand, 4-hydroxypyridine was previously shown to give a highly stable 8-connected framework. Still, its shortness and rigidity limit both the porosity and the type of framework topologies. In this work, we demonstrate the new chemical and structural features of lithium cubane clusters with an elongated ligand, which results in two high-connected 3-D framework materials characterized by self-penetration and self-polymerization, respectively, unlike the commonly observed interpenetration. Such a method provides a feasible path to tune both stability and porosity in lithium-based MOFs.

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INTRODUCTION

In the exploration of metal cations or clusters for building MOFs, the lightweight main group metal ions and clusters can exhibit novel chemical and structural features, as well as desirable properties beyond those simply derived from the gravimetric advantage.40−50 Unlike transition metal ions which often exhibit some similarities, each lightweight element (e.g., Li, B, Mg, Al) has distinct chemistry unique to itself and thus brings rich new opportunities. On the other hand, it also presents synthetic challenges, because each requires different strategies for their synthetic development, instead of benefiting from established methods derived from studies of transition metal ions. To develop Li-MOFs, we have created several strategies, such as using charge-complementary inorganic building blocks (+1 and +3)49−51 or ligands (−1 and 0).52−54 These strategies are based on the consideration of charge-density matching at a local coordination level surrounding individual ions as well as at a global level between host framework and extra-framework guest species. While highly successful in developing new LiMOFs, especially those with zeolite-type framework topologies, these methods make use of individual monomeric lithium sites with a maximum connectivity of 4. As such, the thermal stability

Metal−organic frameworks (MOFs), which belong to the family of crystalline porous materials (CPMs),1−13 have emerged as arguably the most versatile type of porous materials in the past two decades. MOFs, characterized by high surface area, tunable pore size, and well-defined crystal structure, have exhibited promising properties in gas sorption and separation,14−23 chemical sensing,24−27 biomedicine,28 and so on. MOFs are built of metal cations or clusters and organic linkers. Functionalization or length modification of the organic linkers can not only broaden the family of MOFs but also endow MOFs with specific features.29−31 Another component of MOFs, inorganic building blocks,32−35 such as “square paddlewheel” dimer [Cu2(CO2)4], trigonal prismatic trimer [M3O(CO2)6], octahedral tetramer [Zn4O(CO2)], and dodecahedral hexamer [Zr6O4(CO2)12], have been well studied and these clusters are commonly called secondary building units (SBUs). Such SBUs with diverse composition and tunable connectivity can be a determining factor in the construction of MOF structures. For example, in PCN-521,36 the 12-connected Oh Zr6O4 cluster can be reduced to 8-connected D4h symmetry in order to match the symmetry of the ligand, forming fluorite type framework. In addition, a lot of efforts have been made to develop heterometallic clusters for fabricating MOFs recently.37−39 © XXXX American Chemical Society

Received: August 18, 2016 Revised: September 29, 2016

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DOI: 10.1021/acs.cgd.6b01229 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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of such materials can be limited, compared with MOFs made from higher-valent metal ions with higher connectivity. A modification of synthetic strategies has allowed us to synthesize new forms of lithium clusters (dimer as well as tetramer), leading to the development of new porous Li-MOFs.55 Such lithium clusters are novel and were not known even in molecular forms prior to our study. While developing entirely new forms of lithium clusters is certainly desirable, new opportunities for developing Li-MOFs could also be found by extending well-known molecular chemistry to frameworks, in a manner similar to those done for transition metal ions. For lithium cations, upon combination with aryloxide, different types of metal clusters such as double four ring (D4R) and double six ring (D6R) are known to exist.56−58 Among them, D4R, Li4(OAr)4 cubane cluster have been well researched. However, there are few MOFs based on lithium aryloxide clusters.40,41,59,60 One interesting example is a three-dimensional (3D) structure built from the cubane cluster.59 In this structure, Li4O4 cubane cluster is made from lithium cations and oxygen atoms of naphthol. Neutral 1,4dioxane ligand is used to connect the cubane clusters, forming 3-D diamondoid network. The O source of lithium cubane cluster comes from naphthol, a dangling group that blocks the pore access. The connectivity of lithium cubane here is thus only 4. For lithium cubane clusters, if both cationic and anionic sites can be used for intercluster connectivity, the number of the connectivity can be up to 8, resulting in a much more stable framework. However, prior to our study, no synthetic strategies were known to realize this goal. We developed a method that efficiently eliminates the blocking effect and increases the surface area.40 In the earlier literature method, naphthol can only play the cluster-forming role and the bulk of the ligand remains dangling and pore-blocking instead of serving to link clusters into the framework. We replaced naphthol with 4pyridinol (HOPy). As seen in Figure 1, OPy can provide O atoms for cluster formation. At the other side, the nitrogen

atoms from neutral pyridyl groups can bond to lithium cations, thus linking the cluster together. This method makes very efficient use of the backbone of the organic ligand, turning it from pendant pore-blocking into cross-linking with one end embedded within the cluster, thereby enhancing both porosity and stability. Based on these considerations, we synthesized a new framework, Li4(OPy)4 with the ACO zeotype and 8-ring channels in all three directions. It is well worth noting that this framework has an impressive high thermal stability and is stable up to 520 °C. The success with the synthesis of the ACO-type framework with its high-connectivity lithium cubane cluster ushered in a new direction in the development of stable and porous LiMOFs. Still, so far, only one such material (i.e., Li4(OPy)4) is known. Despite its high stability, the ligand length and rigidity place a limit on both the porosity as well as the types of framework topologies that could be realized. In this work, we seek to further explore the chemical and structural properties of the lithium aryloxide system by systematically tuning synthetic conditions and also employing an elongated ligand, 4-[(E)-2(4-pyridinyl)ethenyl]phenol (PyEP). By studying the lithiumPyEP assembly process under different metal-to-ligand ratios and different solvents, two different types of frameworks have been obtained. These two structures unveil new patterns of organization by cubane clusters that are responsive to the variation in the synthetic conditions. They also demonstrate that while the functional groups (−OH and Py) continue to dictate the cluster formation and local coordination geometry, the geometrical features of the ligand, especially the length, dictate the framework topology. Of particular interest, instead of adopting the same orientation of all clusters, which could lead to ACO-like interpenetrated structure, the cubane clusters in the first compound, [Li4(PyEP)4] (denoted as compound 1), adopt two different orientations, leading to a self-penetrating structure with a previously unknown 8-connected framework topology. Also of interest is the finding of self-polymerization of cubane clusters into 1-D chains in [Li4(PyEP)3(OH)] (denoted as compound 2), which is likely a response to the insufficient amount of ligand concentration needed for capping all 8 corners of the cluster. These new findings reveal a potentially rich synthetic and structural chemistry of lithium aryloxide clusters.

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EXPERIMENTAL SECTION

Chemicals. All materials were used as purchased without further purification. Ligand 4-[(E)-2-(4-pyridinyl)ethenyl]phenol was synthesized according to the literature.61 Synthesis of Compound 1. To a mixture of acetonitrile (4 mL) and 4-[(E)-2-(4-pyridinyl)ethenyl]phenol (40 mg, 0.2 mmol) in 20 mL glass vial was injected 0.2 mL tert-BuOLi (1.0 M solution in hexane). After stirring for 25 min, the vial was placed in 80 °C for 2 days. Pale yellow block-shaped crystals were obtained after cooling to room temperature. The yield is ∼60% based on the metal. Synthesis of Compound 2. To a mixture of tert-butanol (5 mL) and 4-[(E)-2-(4-pyridinyl)ethenyl]phenol (20 mg, 0.1 mmol) in 20 mL glass vial was injected 0.2 mL tert-BuOLi (1.0 M solution in hexane). After stirring for 25 min, the vial was placed in 80 °C for 2 days. Yellow hexagonally shaped crystals were obtained after cooling to room temperature. The yield is ∼85% based on the ligand. Single-Crystal X-ray Crystallography Studies. Single-crystal Xray analysis was performed on a Bruker APEX II CCD area diffractometer with nitrogen-flow temperature controller using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å), operating in the ω and φ scan mode. The SADABS program was used for absorption correction. The structure was solved by direct methods

Figure 1. Comparison of two synthetic strategies involving lithium cubane clusters: (a) Li4O4 cubane cluster, (b,d) 3-connected lithium cubane cluster, (c,e) 8-connected lithium cubane cluster. (lime: Li; red: O; blue: N; gray: C) B



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followed by successive difference Fourier methods. All non-hydrogen atoms were refined anisotropically. Computations were performed using SHELXTL and final full-matrix refinements were against F2.62 See Table 1 for Crystal Data and Structural Refinement Results.

1 atm of gas pressure by the static volumetric method. All gases used were of 99.99% purity, and the impurity trace water was removed by passing the gases through the molecular sieve column equipped in the gas line. The gas sorption isotherms for CO2 and CH4 were measured at 273 or 298 K. The gas sorption isotherms for H2 were measured at 77 K. Prior to the measurement, compound 2 was soaked in acetonitrile for 2 days with solvent refreshed twice a day. Then the samples were filtered, dried, and transferred to the sample tube. The sample was degassed at 120 °C overnight before measurement. The isosteric heat of adsorption for CO2 was estimated from the CO2 sorption data measured at 273 and 298 K by using a virial-type expression.63

Table 1. Crystal Data and Structural Refinement Results formulaa FW crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z R1, wR2b GOF

1

2

Li4(PyEP)4 812.68 Orthorhombic Pcc2 11.252(2) 20.934(4) 21.890(4) 90 90 90 5156.2(18) 4 0.0824, 0.2081 0.966

[Li4(PyEP)3(OH)] 633.46 Trigonal R3 26.419(2) 26.419(2) 16.2344(16) 90 90 120 9813.2(15) 9 0.0567, 0.1630 1.003

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RESULTS AND DISCUSSION One interesting finding is that the pattern of organization by lithium cubane clusters can be dramatically impacted by the synthetic conditions such as the metal/ligand ratio and the type of solvents, even when the same framework building units are used (Li+ and PyEP). This has allowed the synthesis of two distinct new phases. In addition to being joined by ligands into different spatial arrangements, we found for the first time that these clusters can also self-aggregate into infinite 1-D chains which are then cross-linked by organic ligands in other directions. This finding demonstrates a new level of possibilities in the structural chemistry of lithium cubane clusters. In addition, the chain formation has the potential to enhance the stability of cubane-cluster-based Li-MOFs, as compared to the cross-linking of discrete cubane clusters. Another interesting finding is that despite dramatically different crystal structures between two structures reported here, both have noncentrosymmetric symmetry. This is likely related to our synthetic strategy of using 4-pyridinol-type ligands that are intrinsically noncentrosymmetric. However, noncentrosymmetric ligands can often form centrosymmetric crystals by simply flipping their orientations between adjacent locations. In our system with cubane clusters, the orientation of

PyEP = 4-[(E)-2-(4-pyridinyl)ethenyl]phenol. bR1 = ∑||F0|−|Fc||/∑| F0| and wR2 = {∑w(F02−Fc2)2/∑w(F02)2}1/2.

a

Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) data were performed a Bruker D8 Advance powder diffraction meter with Cu Kα radiation (40 kV, 40 mA, λ = 1.5418 Å). The simulated powder pattern was calculated using single-crystal X-ray diffraction data and processed by the Mercury 2.3 program provided by the Cambridge Crystallographic Data Centre. Thermogravimetric Measurement. Thermogravimetric analysis (TGA) was performed on a TA Instruments TGA Q500 in the temperature range 30−1000 °C under nitrogen flow with a heating rate of 5 °C/min. Gas Sorption. Gas sorption isotherms were measured on a Micromeritics ASAP 2020 M surface-area and pore-size analyzer up to

Figure 2. Structure of 1. (a) 8-connected lithium cubane cluster. (b) Octahedral cage formed by lithium cubane cluster and organic linker. (c) Three-dimensional structure. (lime: Li; red: O; blue: N; gray: C. The long red, blue, and gray sticks represent the organic linker.) C



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is observed. The chain formation occurs when two opposite cube corners occupied by Li and O, respectively, connect with two other clusters directly through Li−O bonds, along the c axis (Figure 3b). To the best of our knowledge, this is the first

ligands is not dictated by the close-packing requirement, and instead its orientation is dictated by the requirement of cluster formation and subsequent cross-linking. Such specific requirements seem to promote the formation of noncentrosymmetric materials. It is likely that the controlled organization of organic cubane clusters by 4-pyridinol-type ligands may be a fertile area for developing noncentrosymmetric or polar materials. When acetonitrile is used as solvent and the metal−ligand molar ratio in the starting mixture is 1, solvothermal reaction of a mixture of PyEP and t-BuOLi (1.0 M in hexane) results in cubane clusters cross-linked by ligands from all corners of the cluster. Single crystal X-ray analysis reveals that 1 crystallizes in orthorhombic space group Pcc2. Each lithium is tetrahedrally coordinated and bonds to three oxygen atoms and one nitrogen atom, for the purpose of cluster formation and intercluster connection. The Li−O bond lengths are 1.8868−2.0414 Å while Li−N bond lengths fall within the range 2.0040−2.0838 Å. The O−Li−O angles vary from 92.541° to 95.566° while N−Li−O angles are between 116.596° and 127.268°, reflecting the fact that oxygens perform both cluster-forming and framework-forming roles while nitrogen serves only the framework-forming role. As shown in Figure 2a, the lithium cubanes in 1 can be treated as eight-connected nodes, each bonding to eight nearest neighbors through eight ligands. In this structure, six lithium cubanes and eight organic linkers form an octahedral cage (Figure 2b). At the center of each octahedral cage resides another lithium cubane cluster which also serves as a corner of another octahedral cage. In other words, the octahedral cages are interconnected with each other to form a self-penetrated framework. It is worth noting that 1 represents a previously unknown 8-connected topology with the point symbol of {416.612}, if each lithium cubane cluster is treated as 8connected node and PyEP is treated as ditopic linear linker. It is of particular interest to note that the framework topology of 1 is totally different from that of Li-OPy-ACO reported earlier, despite the fact that at a local coordination level they both contain Li cubane clusters with the 8-connected mode. Solvothermal reaction of HOPy and t-BuOLi gave the ACO zeotype structure. However, under similar conditions, the longer ligand PyEP results in the self-penetrated structure. From the structural view, the topological difference can be attributed to the orientations of lithium cubane clusters (Figure 2b and Figure S3). The orientations of lithium cubanes in the ACO type framework are all the same, in contrast with those in 1 that have two different orientations, as illustrated by two adjacent octahedral cages of 1. The self-penetration rather than expansion of the ACO-type framework when the ligand changes from 4-pyridinol to PyEP demonstrates a new and alternative route for the spatial organization of cubane clusters. A more commonly observed phenomenon with the ligand elongation is interpenetration, in contrast with the selfpenetration observed here. When the solvent is changed from acetonitrile to tert-butanol and the metal−ligand ratio is increased from 1 to 2, compound 2 with the metal−ligand ratio of 4/3 is obtained. It is of interest to note that an increase in the metal−ligand ratio during the synthesis leads to an increase of metal−ligand ratio in the resulting crystals, even though the two ratios are not exactly the same. Mechanistically, likely due to the lack of sufficient amount of the ligands in the synthesis mixture to terminate all 8 corners of the cubane cluster, an unusual structural feature, the homopolymerization of cubane clusters into infinite 1-D chains,

Figure 3. Structure of 2. (a) 8-connected lithium cubane cluster. (b) One-dimensional lithium cubane chain. (c) Triangular channel along c axis. (d) Triangular channel viewed along b axis. (e) Threedimensional structure along c axis. (lime: Li; red: O; blue: N; gray: C)

example containing 1-D lithium cubane chains. The Li−O bond lengths in the cubane vary from 1.8576 to 2.0915 Å and the length of Li−O bond connecting adjacent cubanes is 1.9153 Å. Each chain is surrounded by six other chains and is connected to these chains via 6 PyEP ligands. Overall, the large hexagonal channels are partitioned into six triangular channels (Figure 3c). The free volume accessible to guest species is 34.9%, as calculated with the PLATON program.64 For compound 2 with open architecture, powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), and gas sorption properties were carried out. PXRD adds evidence to the purity of the bulk sample. TGA of as-synthesized 2 under N2 atmosphere revealed a sharp weight loss of 16% under 160 °C, attributed to the removal of the solvent molecules. No steep weight loss was observed between 160 and 430 °C. Isotherms of N2 (77 K), H2 (77 K), CH4 (273 K), and CO2 (273 and 298 K) were measured. While 2 was found to be D



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nonporous with respect to N2, it exhibits significant H2 and CO2 uptake (Figure 4), especially when compared to other Li-

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Award No. DE-SC0010596.

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Figure 4. Reversible H2 (77 K), CO2 (273, 298 K), and CH4 (273 K) isotherms for 2.

MOFs. At 1 atm and 77 K, the hydrogen uptake is 108.7 cm3 g−1, which is among the highest in lithium based MOFs. At 1 bar, 2 exhibits the CO2 adsorption of 43.2 and 27.3 cm3 g−1 at 273 and 298 K, respectively. The isosteric heat of adsorption (Qst) for CO2 is 23.3 kJ mol−1 at low loading (Figure S5). IAST calculations based on experimental CO2 and CH4 uptake suggest CO2/CH4 (50/50) selectivity of 6.0 at 273 K and 18.3 at 298 K, respectively.

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CONCLUSION In conclusion, two MOFs based on lightweight lithium clusters have been successfully synthesized. As shown here, lithium cubane clusters can serve as a promising platform for building novel frameworks with tunable structures and properties. The responsiveness in the organization of clusters to synthetic parameters is particularly worth noting as it suggests the potential rich structural variety. Instead of the commonly observed interpenetration effect based on the same cluster orientation and connectivity, the lithium-ligand coassembly process responds to the elongation of ditopic bifunctional organic linker by maintaining the same cluster type and connectivity and yet adopting two complementary orientations, leading to a self-penetrating framework. On the other hand, in response to the relative concentrations of metal ions and ligands, cubane clusters self-polymerize into an unprecedented 1D lithium cubane chain which contributes to thermal stability. These features contribute to relatively high uptakes of H2 and CO2 observed here compared to other known Li-MOFs.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01229. Experimental details, TGA and PXRD patterns, additional figures (PDF) Accession Codes

CCDC 1497079−1497080 contains 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



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