Metal-organic frameworks (MOFs) based on 4-connected nodes are of special interest because of their zeolite-like topologies and open architectures.[1–2] A fundamental structural feature of zeolite topology is tetrahedral nodes (e.g., Si4+) crosslinked by a bicoordinated bridge (e.g., O2− as in SiO2). This bonding feature can be emulated by metal cations (or clusters) and organic crosslinking ligands. Until now, several metallic elements (e.g., Zn, Cd, and In) have been found to generate zeolite-like MOFs including zeolitic imidazolate frameworks (ZIFs) and indium-based ZMOFs.[3–6] The replacement of these transition (or post-transition) metals with lightweight main group metals (e.g., Li, Mg, and Al) has potential to produce a lower framework density, which is desirable for enhancing gravimetric energy storage capacity of gas storage materials. In the past several years, the synthesis of MOFs based on lightweight metals has attracted much attention with the greatest successes being achieved in Mg- and Al-MOFs.[7–8] Nevertheless, to date, the framework based on only 4-connected lithium nodes remains unknown, even though Li-containing boron imidazolate frameworks and frameworks containing Li-O clusters or chains are known.[9]
The use of Li as the framework 4-connected node is very attractive because lithium is the lightest metal. However, the design and synthesis of Li-MOFs are not without significant challenges. Compared to common MOFs based on di- and trivalent metal ions, one unique feature of Li+ is its lowest possible positive charge. For the charge balance requirement, each tetrahedral Li+ node only needs one mononegatively charged ligand such as imidazolate (i.e., LiL) or one-half of a dicarboxylate ligand (i.e., LiL0.5). On the other hand, like SiO2, 4-connected MOFs require a metal:ligand ratio of 1:2, which means if imidazolate or dicarboxylate ligands are used as crosslinking ligands, the resulting 4-connected LiL2 framework would be excessively negative (−1 per Li site for imidazolate framework or −3 for dicarboxylate framework).
In this work, we demonstrate a versatile synthetic method capable of generating a large family of Li-MOF materials. This method is based on the use of mixed charge-complementary ligands (L− and L0, L = ligand) specifically chosen to mimic SiO2 composition and to create Li+L−L0-type Li-ZIFs. With this strategy, a total of twelve Li-ZIFs with various structures including 2D and 3D frameworks have been made (Table 1). As expected, two types of 3D framework materials reported here possess the general SiO2-type framework composition of LiL0L−. In comparison, 2D structures contain both L− and L0 within the polymeric layer (i.e., LiL−1L00.5), together with a pendant neutral ligand occupying the fourth Li+ coordination site, leading to a general framework stoichiometry of LiL−1L00.5L0 (as in layered silicates). The 3-connected lithium site with a pendant molecule in 2D structures can be desirable because of its potential to serve as an open Li+ site upon solvent removal. Furthermore, as shown here, pendant solvent molecules on Li sites can also serve as pillars between neutral Li/L−1/L0 layers to generate porosity.
Table 1.
Summary of crystal data and refinement results.[a]
| Code | Formula | Space group | a [Ǻ] | b [Ǻ] | c [Ǻ] | α[°]/γ[°] | β[°] | R[F] | Net[b] |
|---|---|---|---|---|---|---|---|---|---|
| 1 | [Li(bim)(4,4′-bpy)] | Pna21 | 11.2007(3) | 17.3036(6) | 7.9293(3) | 90 | 90 | 0.0412 | New {65.8} |
| 2 | [Li(im)(4,4′-bpy)] | Cc | 7.8645(6) | 16.6945(13) | 9.1195(6) | 90 | 97.941(2) | 0.0296 | dia |
| 3 | Li(5,6-DMBim)(5,6-HDMBim)(4,4′-bpy)2 | P-1 | 10.5566(7) | 18.0187(11) | 18.5348(12) | 83.260(5)/86.453(4) | 74.662(4) | 0.1152 | 0D |
| 4 | [Li(bim)(bpe)1/2·(bpe)] | P21/n | 9.4954(6) | 10.1626(6) | 22.5395(11) | 90 | 95.911(4) | 0.0566 | hcb (63) |
| 5 | [Li(bim)(pyrazine)1/2·(CH3CN)] | P21/n | 7.7706(2) | 8.8075(2) | 16.1226(4) | 90 | 98.294(1) | 0.0362 | hcb (63) |
| 6 | Li(bim)(CH3CN)2 | P21 | 7.6318(3) | 9.5156(3) | 8.3864(3) | 90 | 110.474(2) | 0.0333 | 1D |
| 7 | [Li2(bim)2(DABCO)·(Hbim) ·(CH3CN)] ·DABCO·pyrazine | P-1 | 10.0374(5) | 10.0674(5) | 21.5782(9) | 86.561(3)/61.901(3) | 84.426(3) | 0.0782 | hcb (63) |
| 8 | [Li(bim)(DABCO)1/2·(H2O)]2·2.5DABCO·pyrazine·CH3CN | Pnnm | 26.2411(6) | 10.4790(3) | 16.2609(4) | 90 | 90 | 0.0775 | hcb (63) |
| 9 | [Li(bim)(DABCO)1/2·(CH3CN)]2·pyrazine | C2/c | 17.8720(2) | 10.1645(11) | 17.8991(2) | 90 | 104.399(8) | 0.1136 | hcb (63) |
| 10 | [Li(bim)(pyrazine)1/2·(4,4′-bpy)]2·CH3CN | P21/n | 17.0212(3) | 10.2313(2) | 17.8680(3) | 90 | 113.784(1) | 0.0445 | hcb (63) |
| 11 | [Li(bim)(DABCO)1/2·(4,4′-bpy)] | Fdd2 | 78.333(9) | 10.3201(12) | 18.1154(18) | 90 | 90 | 0.0561 | hcb (63) |
| 12 | [Li(5,6-dmbim)(DABCO)] ·CH3CN | Pbca | 12.1648(2) | 11.8199(2) | 23.5880(3) | 90 | 90 | 0.0434 | sq1(44.62) |
im = imidazolate; bim = benzimidazolate; 5,6-dmbim = 5,6-Dimethylbenzimidazole; 4,4′-bpy = 4,4′-bipyridine; DABCO = 1,4-Diazabicyclo[2.2.2]octane; bpe = trans-1,2-Bis(4-pyridyl)-ethylene.
For definitions of three-letter abbreviations, see Reticular Chemistry Structure Resource (http://rcsr.anu.edu.au/).
Compound 1 represents the first 3D 4-connected Li-ZIF. As shown in Figure 1a, each Li+ ion in 1 is coordinated by four N atoms from two negative benzimidazolate (bim) ligands and two neutral 4,4′-bpy ligands to form a 4-connected node. In turn, each organic ligand (bim and 4,4′-bpy) bridges between two Li nodes, leading to an overall 3D 4-connected framework containing 1D hexagonal channels along c-axis direction (Figure 1b). The channel wall is made up of Li6(bim)4(4,4′-bpy)2 6-membered rings through edge-sharing, while each channel is constructed from five infinite [Li6(bim)2(4,4′-bpy)4]∞ spiral chains joined together by additional bim bridges (Figure 1c, S1). It is noteworthy that the 3D framework of 1 exhibits a previously unknown 4-connected uninodal topology with a vertex symbol of 6.6.6.6.62.1012 and a Schläfli symbol of (65.8).
Figure 1.
a) Coordination mode of Li+ in 1. b) View of 3D structure of 1 based on Li+ as nodes and bim/4,4′-bpy as linker. c) View of 1D channel structure of 1.
The use of charge-complementary ligands here creates an extra freedom to tune the framework composition and topology, because either L0 ligand or L− ligand can be varied. The replacement of bim ligand with imidazolate (im) or 5,6-dimethylbenzimidazolate (5,6-dmbim, Figure 2a) led to the synthesis of 2 and 3, respectively. It is clear that substituents on the imidazolate ring have a dramatic effect on the framework structure. With the smaller im ligand, a new 3D 4-connected Li-ZIF (2) was obtained. In the absence of any substituents on the im ring, the 3D framework of 2 exhibits an open diamond network (Figure 2b,c). However, the open pore space of the dia net is used up by two additional dia networks, leading to an overall 3-fold interpenetrated structure in 2 (Figure S2). In comparison, the use of larger 5,6-dmbim ligand led to the formation of a discrete 4-coordinated lithium complex 3, (Figure 2d).
Figure 2.
a) View of different negative ligands. b) and d) Coordination mode of Li+ ions in 2 and 3, respectively. c) View of dia structure of 2. LiN4 tetrahedron: grey.
In addition to the use of different L− ligands, we have also explored different neutral L0 ligands (4,4′-bpy as shown above in 1–3, bpe, pyrazine, and DABCO described below). Compared to L− ligands which can be crosslinking (as in 1–2) or dangling (in the form of HL and L− as in 3), L0 ligands exhibit three structural roles: crosslinking, dangling, and pore-filling. In this work, each L0 ligand is employed individually (4,4′-bpy in 1–3, bpe in 4, pyrazine in 5, and DABCO in 12) or in combination with another L0 ligand, as is the case for 7–11 in which two kinds of L0 ligands (pyrazine/DABCO in 7–9, pyrazine/4,4′-bpy in 10, 4,4′-bpy/DABCO in 11) are incorporated into structures, some as extra-framework species occupying the interlayer space (Table 1). As demonstrated below, the use of two L0 ligands together also allows us to probe the relative bonding affinity of L0 ligands to Li sites, which is relevant to the design of the framework materials and the creation of open Li sites. It is worth noting that the role of L0 ligand and the crystallization product can be altered by solvent, because solvent molecules compete for coordination to Li sites and can also be included as extra-framework species. The incorporation of solvent molecules is often desirable because it can contribute to the generation of porosity upon solvent removal.
In 4, a longer ligand (bpe) is used, as compared to 4,4′-bpy in 1–3. This results in the formation of a 2D Li-ZIF. Each tetrahedral Li centre, defined by four N atoms from two L− bim and two L0 bpe ligands, is linked to three adjacent Li sites by two bim and one bpe ligands to form the graphite-type 2D net consisting of Li6(bim)4(bpe)2 six-rings (Figures 3, 4a). The second bpe ligand on each Li site just serves as a pendant ligand (L0-terminating) that points towards the centre of the six-ring of the adjacent layer. Such bpe ligands can be considered as a special type of pillars between two adjacent layers (Figure S3).
Figure 3.
View of different neutral ligands and their corresponding Li6 six-ring structures. LiN4 tetrahedron: grey.
Figure 4.
a)–c) View of 2D structures of 4, 5, and 7, respectively. d) View of the 3D porous stacking structure of 7. LiN4 tetrahedron: grey.
In compounds 1–4, L0 ligands (4,4′-bpy and bpe) are significantly longer than L− ligands (imidazolate-based) in terms of the distance between two N-donor sites. By using L0 ligands (pyrazine and DABCO) with size more comparable to that of imidazolate, additional new phases have been made. When only pyrazine is used, a new 2D Li-ZIF 5, also with a graphite-type net was prepared. The layer is made of Li6(bim)4(pyrazine)2 six-rings (Figure 3, 4b) with the fourth Li coordination site occupied by one pendant CH3CN (solvent-terminating). It is of interest to note that when the concentration of pyrazine is reduced by half, pyrazine can no longer be retained as the crosslinking ligand, resulting in a 1D Li-bim chain (6) in which there are two pendant CH3CN ligands at each Li site (Figure S4). These results suggest that pyrazine is not competitive for bonding with Li and even solvent CH3CN molecules can prevent pyrazine from bonding with Li sites.
The use of two mixed and size-similar L0 ligands, DABCO and pyrazine (the short-short combination), led to three porous 2D Li-ZIFs (7–9) (Figure S5), by adjusting the amount of DABCO (or pyrazine) relative to other chemicals. As shown in Figure 4c and Figure S6, the 2D layer structure of 7 consists of Li6(bim)4-(DABCO)2 six-rings with two kinds of pendant ligands, CH3CN and Hbim alternating in the same layer (simultaneous solvent-terminating and HL-terminating). Compared with pyrazine, DABCO is more bulky. Thus, the ligands inside each Li6(bim)4(DABCO)2 six-ring can be separated further due to increased steric hindrance. As a result, the Li6(bim)4(DABCO)2 six-rings possess hexagonal porous rings with a diameter of ~1.1 nm, while Li6(bim)4(pyrazine)2 six-rings in 5 are rectangular rings with the interior space occupied by bim ligands (Figure 3). The layers in 7 lie parallel to the ab plane and the porous Li6(bim)4(DABCO)2 six-rings from different layers are stacked together to generate 1D infinite channels where the uncoordinated DABCO and pyrazine are located (dual L0-pore-filling) (Figure 4d). Compounds 8–9 have the same polymeric Li-bim-DABCO layers as 7, their differences lie in the types of the terminal ligands on lithium sites and extra-framework solvents molecules (pyrazine/DABCO in 7, pyrazine/DABCO/CH3CN in 8, pyrazine only in 9, Table 1). A comparison of 7–9 suggests that the roles of L0 and solvent ligands and their bonding to Li sites are strongly affected by the relative concentrations of these species (in addition to their chemical structures). All these variations provide multiple ways to pillar the 2D layers and to fill the interlayer pore space for porosity control.
In addition to DABCO/pyrazine used for 7–9, the use of mixed size-complementary DABCO/4,4′-bpy or pyrazine/4,4′-bpy (the long-short combination), led to 2D Li-ZIFs 10 and 11. The common feature in 10 and 11 is that short neutral ligands (DABCO or pyrazine) serve as crosslinkers in the construction of hexagonal Li6(bim)4(DABCO)2 or Li6(bim)4(pyrazine)2 six-rings, respectively, while long neutral ligands 4,4′-bpy serve as pendant ligands (Figure S7). Combining all above results, it is proposed here that the tendency of L0 ligands to serve as crosslinking ligands follows the order: DABCO > pyrazine > 4,4′-bpy. The tendency to serve as dangling ligands is 4,4′-bpy > CH3CN > pyrazine > DABCO (no pendant DABCO observed so far). Furthermore, with the exception of long 4,4′-bpy and bpe ligands, short DABCO, pyrazine, and CH3CN have also been found to serve as the pore filling molecules.
The formation of the graphite-type topology in compounds 4–5 and 7–11 highlights a unique ability to create “pillared” Li-ZIF structures with pores controlled by L0 and solvent molecules. This is in part due to the different bonding strength between Li-L− and Li-L0. Furthermore, the relative strength between Li-Ligand bond and Ligand-Ligand steric repulsion also plays a role. In Zn-based ZIFs, the ligand-ligand steric repulsion is not strong enough to prevent the formation of im-Zn-im linkages, even though it is strong enough to change the 3D topological types. In comparison, in the Li-ZIF system, the ligand-ligand repulsion also contributes to the formation of low-dimensional structures because of relatively weaker Li-L0 bond. On the basis of this work, it can be suggested that in the Li-ZIF system longer crosslinking ligands (e.g., 4,4′-bpy) may have a stronger tendency to form 3D structures through reduced ligand-ligand repulsion while shorter crosslinking ligand (e.g., DABCO) may adopt lower-dimensional structures to better relieve the steric repulsion. Thus, both 3D and 2D frameworks are accessible through the ligand choice. One interesting aspect of this work is that even with the same 2D net topology, there are great variations in these structures as evidenced by the differences in (a) the crosslinking L0 ligand, (b) pendant pillaring ligands, and (c) the pore-filling L0 ligands or solvent molecules. These differences can significantly alter their porous properties.
All aforementioned DABCO-containing Li-ZIFs were made with bim. Because it was observed here (see below) that DABCO-containing compounds tend to give materials with a large solvent accessible volume, we further studied the effect of replacing bim with larger 5,6-dmbim in the DABCO system, which led to a new 2D Li-ZIF 12 built from square-shaped four-rings Li4(5,6-dmbim)2(DABCO)2 with chessboard-like pattern (Figure 5). The dimension of the Li4 four-ring is 0.59 × 0.71 nm, which is smaller than that of Li6 six-ring. The layers are parallel to the ab plane and stacked together to form 1D channels along the c-axis direction in which CH3CN molecules are located (Figure S8).
Figure 5.
Coordination mode of Li+ ion in 12. b) View of the 2D structure of 12. LiN4 tetrahedron: grey.
The total potential solvent accessible volumes calculated with PLATON,[10] are 8.5% (for 1), 33.2% (for 7), 60.4% (for 8), 27.1% (for 9) and 20.0 % (for 12), which correlate well with the ratio (0, 1, 2.25, 0.5, and 1, respectively) between the number of uncoordinated solvent molecules and the number of Li sites. One anomaly from this trend is compound 12, which can be explained by its pore-blocking two methyl groups on the crosslinking ligand (5,6-dmmim) and smaller pore-filling solvent molecules (CH3CN). To determine porous properties of such materials, products 1, 7 and 8 were selected for gas adsorption measurements performed on a Micromeritics ASAP 2010 surface area and pore size analyzer. These samples were degassed at room temperature for 24 hours under vacuum prior to the measurement. The maximum CO2 adsorptions of 1, 7 and 8 at 273 K and 1 atm are 9.1, 27.8 and 48.8 cm3g−1, respectively (Figure 6), which agree well with the trend in the calculated solvent accessible volumes in these phases (8.5%, 33.2%, and 60.4%, respectively). The CO2 uptake in 8 is very high and is in fact higher than the CO2 uptake (35.6 cm3/g) by BIF-9-Li which is a porous Li-B imidazolate framework with zeolite RHO type topology.9d Additionally, the CO2 storage capacities of some highly porous ZIFs are in the range of 19 – 55 cm3g−1 under the same condition.[1b] However, likely due to the limitation of pore aperture, 1,7, and 8 do not show appreciable N2 adsorption.
Figure 6.
CO2 adsorption isotherms of 1 (●), 7 (■) and 8 (▲).
In summary, a family of lithium imidazolate frameworks has been synthesized by using the charge-complementary ligand strategy, which is designed to control the local charge density surrounding the lithium sites and to allow the formation of SiO2-like structure that have so far been rather rare for Li-ZIFs. These lithium imidazolate frameworks, mediated with neutral ligands, not only exhibit various types of structures, but also tunable porous features and gas sorption properties. Such results reported here demonstrate that the charge-complementary-ligand synthetic stratege is an effective synthetic method for generating interesting Li-based porous materials with potential applications in gas sorption.
Experimental Section
Typical synthesis of 1
Li2S (0.092 g, 2 mmol), bim (0.110 g, 1 mmol), 4,4′-bpy (0.156 g, 1 mmol), and CH3CN (3.0 g) were placed in a 20 ml vial. The sample was heated at 90 °C for 5 days, and then cooled to room-temperature. After washed by acetone, the colorless crystals were obtained.
Synthesis of 2–12
2 and 3 were similar with that of 1, except for the replacement of bim by im (0.062 g, 1 mmol) and 5,6-dmbim (0.069 g, 0.5 mmol), respectively. 4, 6, 7, 8, 9, 10, and 11 were similar with that of 1, except for the replacement of 4,4′-bpy by bpe (0.182 g, 1.0 mmol), pyrazine (0.080 g, 1 mmol), DABCO·6H2O/pyrazine (0.110 g, 0.5 mmol/0.156 g, 2.0 mmol), DABCO·6H2O/pyrazine (0.220 g, 1.0 mmol/0.156 g, 2.0 mmol), DABCO·6H2O/pyrazine (0.110 g, 0.5 mmol/0.078 g, 1.0 mmol), pyrazine/4,4′-bpy (0.156 g, 1.43 mmol/0.156 g, 1.0 mmol), and 4,4′-bpy/DABCO·6H2O (0.156 g, 1.0 mmol/0.110 g, 0.5 mmol), respectively. 5 was similar with that of 6, except for the replacement of bim/pyrazine (0.110 g, 1.0 mmol/0.08 g, 1.0 mmol) by bim/pyrazine (0.055 g, 0.50 mmol/0.156 g, 1.0 mmol). 12 was similar with that of 3, except for the replacement of 4,4′-bpy by DABCO·6H2O (0.165 g, 0.75 mmol).
Crystal structure determination
Each crystal was glued to a glass fiber with epoxy resin and mounted on a Bruker APEX II diffractometer equipped with a fine focus, 2.0 kW sealed tube X-ray source (MoK radiation, λ = 0.71073 Ǻ) operating at 50 kV and 30 mA. The empirical absorption correction was based on equivalent reflections. Each structure was solved by direct methods 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. CCDC 777431–777442 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Other physical measurements
Gas adsorption measurements (H2, CO2, and N2) were performed on a Micromeritics ASAP 2020 surface-area and pore-size analyzer. Thermal analyses were performed in a dynamic nitrogen atmosphere with a heating rate of 10 °C/min, using a SDTQ600 thermal analyzer. Powder XRD patterns were obtained using a Bruker D8 Advance X-ray powder diffractometer with CuKα radiation (λ = 1.54056 Å).
Supplementary Material
Acknowledgments
We thank the support of this work by NSF (X. B. DMR-0846958, P. F. CHEM-0809335), Research Corporation (X.B. CC6593), and DOE (P. F. DE-SC0002235), and. X. B is a Henry Dreyfus Teacher Scholar.
Footnotes
Supporting information for this article is available on the WWW under http://www.chemeurj.org/or from the author.
Contributor Information
Prof. Dr. Pingyun Feng, Email: pingyun.feng@ucr.edu.
Prof. Dr. Xianhui Bu, Email: xbu@csulb.edu.
References
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