The discovery and development of novel porous materials is important for burgeoning applications such as gas separation, since adsorptive separation is more energy efficient than traditional separations based on cryogenic distillation or amine scrubbing.1 Recent years have witnessed a surge in the number of metal‐organic frameworks (MOFs),2 with several showing a superior gas separation performance than zeolites for the separation of some important industrial gases such as C2H2/C2H4,3 C2H4/C2H6,4 C2H2/CO2,5 CO2/CH4,6C3H4/C3H6,7 and C3H6/C3H8.8 Compared with MOFs, which are self‐assembled by coordination bonding, supramolecular organic frameworks (SOFs) or hydrogen‐onded organic frameworks (HOFs) are constructed from a range of weaker interactions (e.g. hydrogen bonds, π‐π stacking, CH⋅⋅⋅π, and van der Waals interactions), and have also recently emerged as potential porous materials.9 However, as a consequence of the weakness of supramolecular interactions, few SOF materials show permanent porosity with reversible gas sorption properties, which limits the practical application.10

Current research on MOFs and SOFs usually focuses on the use of purely organic ligands. Inorganic anion hybrid MOFs, a unique subclass of MOFs that were named as hybrid ultramicroporous materials (HUMs), have been of particular interest for gas adsorption/separation because of their high recognition ability for gas molecules.11 The large variety of inorganic anions, organic linkers, and metal ions enable exquisite control over the pore dimensions and the electrostatics of the pore surface. So far, there are four families of HUMs that can be distinguished on the basis of the types of anions: MFSIX networks pillared by hexafluorometallate anions (e.g. SiF62−, GeF62−, TiF62−) in a linear fashion,11a–11b MOxFy pillared networks with anions containing octahedral metal centers bonded through both O and F atoms (e.g. NbOF52−, VOF52−),11c mmo topology networks with tetrahedral MO42− oxyanions (e.g. CrO42−, MoO42−),11d and DICRO coordination networks that are maintained with Cr2O72− dianions.11e The incorporation of metal ions and inorganic anions into HOFs or SOFs can provide facile access to unique pore structures, dimensions, topologies, functionalities, and enhanced stability, as demonstrated by MPM‐1‐TIFSIX and HOF‐21.10c, 10d However, porous supramolecular frameworks containing metal complexes have rarely been reported, and neither have those constructed from nonclassic interactions such as B−H⋅⋅⋅M interactions and proton‐hydride (Hδ−⋅⋅⋅Hδ+) dihydrogen interactions.12

The icosahedral closo‐dodecaborate dianion [B12H12]2− is a highly stable cluster molecule consisting of 12 identical B‐H vertices and is considered a 3D analogue of benzene (Scheme 1 a).13 The cage size defined by the distance between opposite hydrogen atoms is about 5.6 Å. The salt [Et3NH]2[B12H12] can be easily prepared on a 25 g scale by a one‐pot synthesis from commercially available reagents. Neutral carboranes C2B10H12,14 as relatives of [B12H12]2−, have been utilized as unique building blocks for MOFs after linking them through pyridyl or carboxy groups, which serve as the coordinating moieties. These MOFs have unique physical and chemical properties that are distinct from those of normal MOFs with purely organic ligands.15 However, as a consequence of the limited number of methods to selectively functionalize [B12H12]2− as well as the potential disadvantage of its weakly coordinating nature, the construction and applications of MOFs using [B12H12]2− have not yet been reported.

Herein, we report the first microporous MOFs supramolecularly assembled from 1,2‐is(4‐pyridyl)acetylene (bpa) and copper(II) dodecaborate. The dodecaborate dianion inside not only serves as a counterion but also maintains the 3D framework as a spherical pillar. The resulting material [CuB12H12(bpa)2] (termed BSF‐1 for boron cage based supramolecular metal‐organic framework) resembles the structures of HUMs and can be considered as the fifth family of HUMs that utilize cluster dodecaborates as the pillars. On the other hand, it can be regarded as a unique SOF interconnected by nonclassical hydrogen interactions such as B−H⋅⋅⋅M interactions and B−Hδ−⋅⋅⋅Hδ+−C interactions. Furthermore, the potential use of BSF‐1 for gas separation was investigated, which revealed a highly selective gas separation performance for C3H8/CH4 and C2H6/CH4 at room temperature.

BSF‐1 was readily prepared by stirring a mixture of Na2[B12H12], CuNO3⋅3 H2O, and bpa in MeOH/H2O for 24 h at room temperature (Scheme 1 b). Single crystals of BSF‐1 were obtained by layering a MeOH solution of bpa onto an aqueous solution of Na2[B12H12] and CuNO3⋅3 H2O. X‐ ay structural analysis of BSF‐1 revealed that it crystallizes in a three‐dimensional (3D) framework in the monoclinic space group C2/c. Each hexacoordinate Cu center serves as a six‐connected node: four pyridyl units from different bpa ligands comprise the equatorial plane, which extends into a square‐lattice (sql) 2D layer, the axial positions of the Cu nodes coordinate to hydrogen atoms from [B12H12]2−, which bridge the planar sql nets to generate an infinite 3D primitive cubic (pcu) array (Schemes 1 c,d). Crystallographic details and selected bond lengths and angles are summarized in Tables S1, S2, and S3 as well as Figure S2. Notably, an unprecedented linear B‐H‐metal coordination was observed in BSF‐1, which has not been reported for metal dodecaborate complexes to the best of our knowledge. As a result of the electron deficiency and charge delocalization in borane clusters, dodecaborate anions usually coordinate with two or three B‐H moieties to metal centers through a three‐center two‐electron (3c‐2e) bond.16 However, in the present case, the multiple weak B−H⋅⋅⋅H−C dihydrogen bonding interactions between [B12H12]2− and the pyridine units results in the cage showing a clear preference for direct B‐H orientation towards the Cu center, thereby probably reducing the steric hindrance and maximizing the interactions of the [B12H12]2− pillars within the sql networks (Scheme 1 c). The B−H⋅⋅⋅H−C dihydrogen interactions could also be observed by IR spectroscopy (Figure S15): some C(sp2)−H stretching peaks were shifted to lower frequencies (2850–2950 cm−1) with enhanced strength. In addition, at least two different B‐H vibrating signals can be distinguished (2467 and 2383 cm−1), consistent with the X‐ ay structure, which shows B−H bonds are located in several different chemical environments. The inferred distances of B−H⋅⋅⋅Cu are slightly different (2.110 and 2.259 Å, Figure S2), possibly because of Jahn–Teller distortion. The pcu networks feature rhombohedral cavities with dimensions of about 10.0×13.6×13.6 Å3 (defined by the Cu⋅⋅⋅Cu distances) that enable twofold interpenetration, whereby the node of the second net is offset from the center of the cavity of the first net (Scheme 1 d,e). Such a mode of interpenetration is commonly encountered in twofold‐interpenetrated pcu networks such as DICRO‐ased HUMs,11e and can be explained by the interaction of dodecaborates with adjacent pyridyl groups. Notably, the offset interpenetration results in three crystallographically distinct channels parallel to the c‐axis with a void space of 29.3 % calculated by PLATON with a probe radius of 1.2 Å. These channels are present in a ratio of 2:1:1 and are marked by yellow, pink, and green in Scheme 1 e. However, only the pink channel is large enough to accommodate guest molecules (Scheme 1 f).

The porous nature of BSF‐1 revealed by its crystal structure promoted us to investigate its permanent porosity using gas adsorption experiments. First, the as‐synthesized BSF‐1 was exchanged with methanol twice every 24 h for 2 days and then evacuated under vacuum at 353 K for activation. Comparison of the powder X‐ ay diffraction (PXRD) patterns showed the activated sample had the same structure as BSF‐1. An N2 adsorption measurement was conducted at 77 K to study the pore chemistry. This clearly indicates its microporous nature, with a BET surface area of 535 m2 g−1 (Figures S11 and S12) and a pore volume of 0.25 cm3 g−1, close to the calculated value of 0.27 cm3 g−1 from single‐crystal data.

The establishment of permanent microporosity in BSF‐1 encouraged us to examine its utility as an adsorbent for industrially important gas separations such as C2H2/C2H4 and C2H2/CO2. The C2H2, C2H4, and CO2 sorption isotherms were collected at 273, 298, and 313 K, and all reveal Langmuir (type I) isotherms. At 298 K and 1 bar, the CO2 and C2H4 uptakes were 39.7 cm3 g−1 and 36.6 cm3 g−1 respectively, whereas the C2H2 uptake was 52.6 cm3 g−1 (Figure 1 a). More importantly, the adsorption isotherm of C2H2 exhibits a steeper slope than those of CO2 and C2H4 under low pressures, which indicates a higher affinity of BSF‐1 for C2H2, which is a basis for separating C2H2 from C2H4 and CO2. The isotherms were fitted using the dual‐site Langmuir–Freundlich Equation (Tables S4–S6), and the isosteric enthalpies of adsorption (Qst) were calculated using the Clausius–Clapeyron Equation. The Qst values for C2H2, C2H4, and CO2 at a low loading in BSF‐1 were calculated to be −31, −26, and −22 kJ mol−1, respectively, which were consistent with the sorption isotherms showing that BSF‐1 accommodated C2H2 more favorably than C2H4 and CO2 (Figure 1 b).

The growing developments based on the exploitation of natural gas, which consists primarily of methane (CH4) and various amounts of ethane (C2H6) and propane (C3H8), has resulted in the separation of light alkanes from each other becoming increasingly important. Traditional methods such as cryogenic distillation and solvent absorption are energy‐intensive and environmentally unfriendly and, therefore, the potential separation performance of C1–C3 alkanes by BSF‐1 was investigated. To our delight, BSF‐1 exhibits quite different sorption behavior for CH4, C2H6, and C3H8. The CH4, C2H6, and C3H8 sorption isotherms were collected at 273, 298, and 313 K. At 298 K and 1 bar, the C3H8 and C2H6 uptakes were 43.5 and 35.2 cm3 g−1, respectively, whereas the CH4 uptake was only 10.5 cm3 g−1 (Figure 1 c). In addition, the C3H8 adsorption isotherm is extraordinarily steep and the uptakes of C3H8 and C2H6 were 26.9 cm3 g−1 and 9.4 cm3 g−1 at 0.045 bar. In contrast, the uptake of CH4 was a negligible 0.55 cm3 g−1, thus indicating a good separation performance for C3H8/CH4 and C2H6/CH4 (Figure 1 d). The IAST selectivity for C3H8/CH4 (1:1) and C2H6/CH4 (1:1) gas mixtures at 298 K were calculated using ideal adsorbed solution theory (IAST) to be as high as 353 and 23 at 1 atm after fitting isotherms to the dual‐site Langmuir–Freundlich Equation (Figure 1 e). These values are much larger than those of many reported MOFs such as JUC‐100 (80:11), JUC‐103 (55:8), JUC‐106 (75:13),17 UTSA‐35a (80:20),18 flexible MFM‐202a (87:10),19 HKUST‐1‐like tbo‐MOF‐1 (32:5) and tbo‐MOF‐2 (60:14) with large pores,20 JLU‐Liu22 (272:14) with open metal sites,21 and comparable with that of anionic MOFs FJI‐C1 (471:22),22 and FJI‐C4 (293:40)23 (Figure 1 f). Other porous materials including carbons, zeolites, and MOFs that have been studied for alkane separation are listed in Table S7; BSF‐1 is still one of the benchmark materials for alkane separation. Such high C3H8/CH4 selectivity is ascribed to the appropriate pore size and slightly electronegative pore surface arising from [B12H12]2−enhancing the interactions between BSF‐1 and C3H8. The selectivity of BSF‐1 for other gas mixtures that need separating in industry such as C2H2/C2H4, C2H2/CO2, C2H2/CH4, and CO2/CH4 were also calculated and the values are 2.3, 3.3, 46.9, and 7.5 at 298 K and 1 bar (Figure 1 d).

To understand the gas adsorption behavior within BSF‐1, modeling studies using first‐principles DFT‐D (dispersion‐corrected density functional theory) calculations were conducted. Notably, a strong B−H⋅⋅⋅H−C dihydrogen bond (Figure 2 a, 1.99 Å) is observed between BSF‐1 and C2H2, with a binding energy of −35.7 kJ mol−1. The binding energy of CO2within BSF‐1 is only −23.3 kJ mol−1, thus indicating a good selectivity for C2H2/CO2 (Figure S26, Table S8). For C3H8, with its larger molecular size, there are multiple weak interactions between the gas and [B12H12]2− (C−H⋅⋅⋅H‐B 2.48 Å, 3.21 Å) as well as the organic linkers (C−H⋅⋅⋅π 2.89 Å, 3.18 Å and C−H⋅⋅⋅C≡C 2.91 Å), which lead to a higher binding energy of −47.0 kJ mol−1 (Figure 2 b) and trap C3H8 tightly in the pore. The optimized adsorption configurations of C2H6 and CH4 molecules in the BSF‐1 channels were also calculated to have binding energies of −32.9 kJ mol−1 and −25.5 kJ mol−1, respectively, in agreement with the Qst values. (Figures S24 and S25, Table S8). The striking difference in the binding energies also accounts for the high selectivity of C3H8/CH4 and C2H6/CH4. The adsorption behavior of C2H2 and C3H8 was further revealed by a grand canonical Monte Carlo (GCMC) simulation, which suggests a distinct synergistic effect during the adsorption (Figures S27 and S28).

To further evaluate the dynamic separation of C3H8/CH4 and C2H6/CH4 gas mixtures, breakthrough experiments were conducted in which equimolar C3H8/CH4 and C2H6/CH4mixtures were flowed over a packed bed of BSF‐1 with a total flow of 1 mL min−1 at 298 K. Figures 3 a,b show the breakthrough curves of BSF‐1 for C3H8/CH4 and C2H6/CH4. Compared with C3H8 and C2H6, CH4 always eluted first, which can be explained by the favorable affinity of BSF‐1 for C3H8 and C2H6, in agreement with the single‐component adsorption isotherms in Figure 1 c. The typical roll‐up of the CH4 curves also revealed the weak affinity of BSF‐1 for CH4. To simulate the practical conditions of natural gas, a three‐component gas mixture of C3H8/C2H6/CH4 (5:10:85) was tested under different flow rates (1 mL min−1, 10 mL min−1, and 13.5 mL min−1) as well as under saturated moisture conditions (Figures S17–19), with all the studies suggesting a good separation performance. Notably, the excellent breakthrough results were retentive with saturated moisture, which indicates the high tolerance of BSF‐1 for water (Figure S17). These breakthrough tests suggested that BSF‐1 primarily adsorbed C3H8 followed by C2H6 under mixture flow, consistent with the IAST results. Thus, it is possible to achieve the efficient separation of C3H8/CH4, C2H6/CH4, and C3H8/C2H6/CH4mixtures through the packed bed of BSF‐1 material to recover each component in nearly pure form.

In view of the importance of the stability of porous materials for practical applications, the cyclability as well as the water and thermal stability of BSF‐1 were investigated. There was no noticeable loss in the CO2 adsorption capacity after five cycles of adsorption/desorption experiments. Long‐ ime soaking of BSF‐1 in water did not change the porous structure of BSF‐1, as demonstrated by the PXRD patterns as well as the CO2 adsorption isotherms (Figure 3 c,d). Thermogravimetric analysis (TGA) indicated that BSF‐1 did not lose weight until about 240 °C and a weight loss of only about 9 % was observed after heating to 800 °C, thus showing its potential as a unique carbon precursor for carbonization (Figure S22). The thermal stability was further confirmed by variable‐ emperature PXRD patterns, which remained unchanged up to 220 °C (Figure S21). Such unexpected stability can be attributed to the contracted/interpenetrated structure with multiple interactions between the [B12H12]2− dianions, Cu2+ ions, and bpa linkers.

In conclusion, a supramolecularly assembled porous dodecaborate complex framework BSF‐1 is reported, which represents the first porous material comprising the icosahedral [B12H12]2− anion. The gas sorption behavior of BSF‐1 for N2, C2H2, C2H4, CO2, C3H8, C2H6, and CH4 was fully studied experimentally and theoretically. The studies indicated excellent separation selectivities for C3H8/CH4, C2H6/CH4, and C2H2/CH4 as well as moderately high separation selectivities for C2H2/C2H4, C2H2/CO2, and CO2/CH4. Moreover, the practical separation performance of C3H8/C2H6/CH4 was confirmed by dynamic breakthrough experiments. The good cyclability and high water/thermal stability render it suitable for real industrial applications. This study not only illustrates a novel porous material for efficient gas separation but also opens a new avenue for the synthesis and application of MOFs comprised purely of boron‐ased cages. Considering the variety of substituted dodecaborate derivatives as well as their unique properties compared to those of organic compounds, we believe that a wealth of dodecaborate‐containing porous materials will emerge in the near future for various applications inspired by our pioneering work.