As a new class of inorganic-organic hybrid materials, metalorganic frameworks (MOFs) have attracted great interests not only for their potential applications in gas storage and separation ^{[1, 2, 3, 4]}, sensor ^{[5, 6, 7]}, electrochemistry ^{[8, 9]} and catalysis ^{[10, 11, 12]} but also for fascinating structures as well as abundant topologies ^{[13, 14]}. MOFs are crystalline solids with extended network structures, which are comprised of metal-containing nodes and organic ligand linkages linked via coordination bonds ^[15]. Metalcontaining nodes (known as secondary building units, SBU) play important roles in the design and synthesis of MOFs with various structures and topologies ^{[16, 17, 18]}. As a result, many efforts have been devoted to designing and constructing particular SBUs for the synthesis of MOFs with desired structures. For examples, a series of reticular MOFs with homogeneous periodic pores have been obtained based on Cu₂(H₂O)₂(COO)₄ SBU ^{[19, 20]}, and highly stable Zr-based MOFs with various, unique properties have been reported based on Zr₆O₄(OH)₄ SBU ^{[21, 22]}.

In addition to SBUs, the exploration on organic linkers is also very critical in the design and synthesis of MOFs. Among the diverse organic linkers, carboxylate ligands have been widely used in the construction ofMOFs with specific ormultifunctional properties due to their diverse configurations and coordination modes ^[23]. As we know, most reported MOFs are based on highly symmetric ligands. However, only rare efforts have been focused on the ligands with lowered symmetry. In thiswork, we designed and synthesized a new desymmetric 4-connected ligand, 5-(2, 6-bis(4-carboxyphenyl)pyridin- 4-yl)isophthalic acid (H₄BCPIA). The assembly of BCPIA^4- with predesigned 4-connected paddle-wheel plane quadrilateral Cu₂(H₂O)₂(COO)₄ or tetrahedral In(COO)₄ nodes yielded two threedimensional (3D) MOFs, [Cu₂(H₂O)₂(BCPIA)] (BUT-20, where BUT = Beijing University of Technology) and (Me₂NH₂)[In(BCPIA)] (BUT-21), respectively. Their structures were determined by singlecrystal X-ray diffractions. In BUT-20, each twelve Cu₂(H₂O)₂(COO)₄ SBUs are connected together by six BCPIA^4- to form an octahedral cage. Each face of the cage unit is shared by other octahedral cages to form a 3D framework with an Nbo topology. In BUT-21, In(COO)₄ nodes are linked by the ligands to form again a 3D framework with two types of open channels along the a axis, and a Unc topology. Obviously, the formation of different 4-connected metal-containing nodes plays a key role in the topological network diversity of the resulting MOFs.

All general reagents and solvents (AR grade) were commercially available and used as received without further purification. The ¹H NMR data were collected on a Mercury 400 spectrometer. Fourier transform infrared (FT-IR) spectra were recorded on an IRAffinity-1 instrument. The powder X-ray diffraction patterns (PXRD) were recorded on a BRUKER D8-Focus Bragg-Brentano X-ray Powder Diffractometer equipped with a Cu sealed tube (λ = 1.54178) at room temperature (r.t.). Simulation of the PXRD spectrum was carried out by the single-crystal data and diffraction-crystal module of theMercury programavailable free of charge via internet at http://www.ccdc.cam.ac.uk/mercury/. Thermogravimetric analysis (TGA) data were obtained on a TGA-50 (SHIMADZU) thermogravimetric analyzer with a heating rate of 10 ℃min^-1 under N₂ atmosphere. Elemental microanalyses (EA) were performed by VarioMacro cube elementar.

H₄BCPIA was synthesized following a procedure as shown in Scheme 1.

Scheme 1

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Scheme 1.Synthesis procedure for H4BCPIA.

4-(3, 5-Dimethylphenyl)-2, 6-di-p-tolylpyridine (3): 3, 5-Dimethyl benzaldehyde (2.0 g, 15mmol) and 4-methyl acetophenone (4.6 g, 34mmol) andNaOH(9.6 g, 240 mmol)were crushedtogether with amortar and pestle for 2 h manually tomake the two reactants reactwell. Then, the obtained yellowpowderwas added to a stirred solution of ammonium acetate (15 g, excess) in ethanol (300 mL). The resulted mixture was heated at reflux for 48 h. Upon cooling to r.t., the precipitatewas filtered, washedwithwater three times, and dried to afford the crude product. It was further purified by flash column chromatography on silica. Elution with n-hexane/ethyl acetate (1:1) gave a white solid of 4-(3, 5-dimethylphenyl)-2, 6-di-ptolylpyridine (yield: 3.4 g, 63.0%.). ¹H NMR (CDCl₃, 400 MHz): δ 8.14 (d, 4H), 7.85 (s, 2H), 7.35 (t, 6H), 7.14 (s, 1H), and 2.46 (s, 12H).

5-(2, 6-Bis(4-carboxyphenyl)pyridin-4-yl)isophthalic acid (4, H₄BCPIA): 0.5 g of 4-(3, 5-dimethylphenyl)-2, 6-di-p-tolylpyridine, 2 mL concentrated HNO₃, and 15 mL H₂O were added to a 20 mL Teflon-lined stainless steel autoclaves and heated at 180 ℃ for 24 h. After reaction, the product was filtered andwashedwithwater three times. The obtained orange solid of 5-(2, 6-bis(4-carboxyphenyl)pyridin- 4-yl)isophthalic acid (H₄BCPIA) was dried at 80 ℃ for 12h (yield: 0.55 g, 83.3%.). ¹H NMR (400MHz, DMSO-d6): δ 8.69 (s, 2H), 8.59 (s, 1H), 8.50 (d, 4H), 8.41 (s, 2H), and 8.11 (d, 4H). FT-IR (KBr pellets): 3014 (m), 1695 (s), 1596 (s), 1280 (m), 1238 (m), 858 (w), 759 (w), and 742 (w) (Fig. S3 in Supporting information).

[Cu₂(H₂O)₂(BCPIA)]·nS (BUT-20, S represents non-assignable solvent molecules): A solution of Cu(NO₃)₂·2.5H₂O (20 mg, 0.120 mmol) and H₄BCPIA (12 mg, 0.025 mmol) in 3 mL of N, Ndimethylformamide (DMF) was sealed in a 5 mL glass vial and heated at 80 ℃ for 24 h, after cooling down spontaneously, green block crystals were collected (yield: 10 mg). Elemental analysis (EA) for BUT-20, C₂₇H₁₃Cu₂NO₁₀ (638): Calcd. C 50.78, H 2.04, N 2.19. Found. C 49.82, H 2.23, N 2.08. FT-IR (KBr pellets): 3367 (m), 1608 (s), 1552 (s), 1402 (s), 862 (w), 788 (w), and 777 (w) (Fig. S3).

(Me₂NH₂)[In(BCPIA)]·nS (BUT-21): A solution of In(NO₃)₃·5H₂O (10 mg, 0.034 mmol), H₄BCPIA (8 mg, 0.017 mmol), and 4 drops of HBF₄ (48% water solution) in 1.5 mL of N, N-dimethylacetamide (DMA) was sealed in a 5 mL glass vial and heated at 100 ℃ for 5 days, after cooling down spontaneously, colorless block crystals were obtained (yield: 7 mg). Elemental analysis (EA) for BUT-21, C₂₉H₁₉InN₂O₈ (637.82): Calcd. C 54.56, H 2.98, N 4.39. Found. C 53.63, H 2.43, N 4.16. FT-IR (KBr pellets): 3430 (m), 1618 (s), 1384 (s), 1083 (s), 863 (w), 790 (w), and 779 (w) (Fig. S3).

Single-crystal X-ray diffraction data of BUT-20 and -21 were collected on a Bruker-AXS APEX-Ⅱ CCD X-ray diffractometer equipped with a fine-focus sealed-tube X-ray source (graphite monochromaticMo-Karadiation, λ = 0.71073Å , ω-scanswitha 0.5°step). Raw data collection and reduction were performed using APEX2 software ^[24]. Absorption correctionswere applied using the SADABS routine ^[25]. The structures were solved by direct methods and refined by full-matrix least-squares on F² with anisotropic displacement using the SHELXTL software package ^[26]. All non-H atoms were refined anisotropically during the final cycles. The hydrogen atoms of ligands were calculated at idealized positions with a riding model and refined isotropically. In addition, the hydrogen atomsof coordinatedwatermolecules in BUT-20 were not considered in its structural refinement. There are large solvent accessible pore volumes in the crystals of BUT-20 and -21, which are occupied by highly disordered solvent molecules, or counter-ions (in BUT-21). No satisfactory disorder model for these guest molecules could be achieved, and therefore the SQUEEZE program implemented in PLATON was used to remove the electron densities of these disordered species ^[27]. Thus, all of the electron densities from free guest molecules have been “squeezed” out. The details of structural refinement can be found in Table S1 in Supporting information, while the selected bond lengths and angles are summarized in Tables S2 and S3 in Supporting information.

The new ligand, H4BCPIA, was synthesized by following a previously reported procedure with modifications (Scheme 1) ^[28]. The point group of the ligand, BCPIA^4- in the structure of BUT-20 and -21 is C_2v. Solvothermal reactions of H₄BCPIA with Cu(NO₃)₂·2.5H₂O, or In(NO₃)₃·5H₂O in DMF or DMA yielded crystals of BUT-20 and -21, respectively. Their phase purity was checked by PXRD. As shown in Fig. S1 in Supporting information, the experimental PXRD patterns match well with ones simulated from the single-crystal data, indicating the pure phase of them. The TGA plots of the prepared samples of the two MOFs are shown in Fig. S2 in Supporting information, confirming that BUT-20 and -21 are stable up to ca. 260 and 520 ℃, respectively. In addition, as shown in Fig. S3, slight blue-shifts of the characteristic bands of the carbonyl group in the two MOFs compared with that of H₄BCPIA could be observed, illustrating the metal coordination of carboxylate groups in the ligand.

Single-crystal X-ray diffraction analysis reveals that BUT-20 crystallizes in a non-centrosymmetric hexagonal space group R- 3m and possesses an extended 3D framework structure. In BUT-20, both Cu^Ⅱ atoms lie on a twofold axis and each is coordinated by one water molecule and four oxygen atoms from carboxyl groups of four different BCPIA^4- ligands. Two Cu^Ⅱ atoms are linked by four carboxylate groups from different ligands into a classical Cu₂(H₂O)₂(COO)₄ paddle-wheel SBU with a Cu-Cu distance of 2.6599(5) Å (Fig. 1a). As shown in Fig. 1b, four carboxylate groups in the BCPIA^4- ligand only exhibit a bis-monodentate coordination mode. It should be pointed out that the prominent structural feature of BUT-20 is the presence of an interesting, cavity-like substructures, where twelve dimeric Cu₂(COO)₄ SBUs are connected together by six BCPIA^4- ligands to form an octahedral cage, which can be filled by a ball with the diameter of about 7.1Å (Fig. 1c and d). Moreover, each octahedral cage is surrounded by eight equal ones (Fig. 1e). Each face of the octahedral cage is shared by other octahedral cages to extend to a 3D framework (Fig. 2). After removal of free solvent molecules, the total solventaccessible volume of BUT-20 is estimated to be 73.1% by PLATON ^[27]. From the topological viewpoint, each Cu₂(H₂O)₂(COO)₄ SBU and BCPIA^4- ligand can be viewed as 4-connected nodes (Fig. 1a and b), the overall 3D structure of BUT-20 can thus be simplified as a 4-connected net with the point symbol of (6⁴·8²), corresponding to Nbo-type topology (Fig. 1f). The reported Nbo-type MOFs in the literature such as NOTT-105 ^[29], PCN-14 ^[30], MOF-505 ^[31], UTSA-76 ^[32] and UTSA-80 ^[33] are well-known and they all crystallize in space group R-3m. In our case, BUT-20 crystallizes in a lower space group R-3m with the loss of an inversion center.

Fig. 1

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Fig. 1.(a) Viewof the [Cu2(H2O)2(CO2)4] paddle-wheel SBU; (b) coordinationmode of BCPIA4- ligand; (c) the cage-type substructure; (d) octahedron cavity in BUT-20; (e) natural tiling of BUT-20 showing the packing of the octahedron cavities in BUT-20; and (f) topological representation of BUT-20 (H atoms on the ligands are omitted for clarity).

Fig. 2

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Fig. 2.(a) The 3D framework structure of BUT-20 with highlighted cages and (b) the natural tiling of BUT-20 viewed along the [111] direction (H atoms on the ligands are omitted for clarity).

BUT-21 crystallizes in an orthorhombic chiral space group of P2₁2₁2₁, which suggests the presence of 2₁ helical chains along the a, b and c axes. In the structure of BUT-21, the In^Ⅲ ion adopts a tetrahedral linkage geometry through coordinating to eight oxygen atomsfromfour carboxylate groups of four different BCPIA^4- ligands (Fig. 3a). The In-O distances range from 2.2617(1)Å to 2.2905(1)Å , which are comparable to those of reported indium complexes ^[34]. As shown in Fig. 3b, the carboxylate groups in BCPIA^4- ligand exhibit a chelating coordination mode, where each one coordinates to one In^Ⅲ ion. Furthermore, in the structure there exist two different open channels along the a axis with rectangular windows having dimensions of 8.9 × 17.5Å and 11.8 × 18.6Å (atomto atom distance), respectively (Fig. 3c) and along the a axis, there exists a right-handed helical chain (Fig. 3c and d). Two carboxylate groups in position 1 and position 2 of the BCPIA^4- ligand (illustrated in Fig. 3b) connect two adjacent In(COO)₄ entities to form 2₁ righthanded helical chains along a axis (the helical pitch is 14.49Å , given by one full rotation around the 2₁ helical axis). Furthermore, each large channel is surrounded by four small channels, while the small channel is surrounded by four large channels to extend to a 3D framework (Fig. 3c). After removal of free solvent molecules, the total solvent-accessible volume of BUT-21 is estimated to be 75.8% by PLATON ^[27]. From the topological viewpoint, each In(COO)4 node and BCPIA^4- ligand can be viewed as 4-connected nodes, and the 3D structure of BUT-21 can thus be simplified as a 4-connected net with the point symbol of (6⁶), corresponding to a Unc-type topology (Fig. 3d). It should also be pointed out that in the pore of BUT-21, there should exist counter-cations balancing the charge of the framework. However, these counter-cations are highly disordered, and could not be identified by single-crystal X-ray diffraction analysis. EA analysis was then carried out, which confirmed extra N content inside the pore. The result implies the presence of (CH₃)₂NH₂⁺, which is a common by-product in solvothermal reactions with DMF or DMA as solvent ^[35]. Furthermore, the TGA curve showed a 7.1% weight loss between 200 ℃ and 340 ℃, which corresponds well to the presence of one (CH₃)₂NH₂⁺ ion per formula.

Fig. 3

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Fig. 3.(a) View of the [In(CO2)4] node; (b) coordination mode of the BCPIA4- ligand; (c) 3D framework structure; and (d) topological representation of BUT-21 (H atoms on the ligands are omitted for clarity).

As described above, the reaction between BCPIA^4- ligand and Cu^Ⅱ or In^Ⅲ under solvothermal conditions gives two MOFs, each with distinct network structures. The generation of these different networks could be mainly attributed to the distinct metal-containingnode represented in the structures. In BUT-20, the BCPIA^4- ligand connects with the plane quadrilateral Cu₂(H₂O)₂(COO)₄ SBU, while in BUT-21, the BCPIA^4- ligand connects with the tetrahedral In(COO)₄ node.

In summary, we have designed and synthesized a new desymmetric 4-connected ligand, H₄BCPIA, and was used to construct novel MOFs with two predesigned 4-connected metalcontaining nodes, plane quadrilateral Cu₂(H₂O)₂(COO)₄ and tetrahedral In(COO)₄. As a result, two new MOFs with fascinating structures of Nbo and Unc topologies were created. Clearly, the linkage configurations of the metal-containing nodes play important roles in determining the framework structures of the resulting MOFs. Currently, investigations involving desymmetric ligands in the design and construction of new MOFs are in progress in our laboratory.

ThisworkwasfinanciallysupportedbytheNSFC(Nos.21322601, 21271015, 21406006, andU1407119) and ProgramforNewCentury Excellent Talents in University (No. NCET-13-0647).

Supplementary data associated with this article can be found, in the online version, at: http://dx.doi.org/10.1016/j.cclet.2015.12.034.