Cross-linking Zr-based metal–organic polyhedra via postsynthetic polymerization†

Dongsik Nam ^a, Jihyun Huh ^a, Jiyoung Lee ^a, Ja Hun Kwak ^b, Hu Young Jeong ^c, Kyungmin Choi ^d and Wonyoung Choe *^a
aDepartment of Chemistry, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan 44919, Republic of Korea. E-mail: choe@unist.ac.kr
^bDepartment of Chemical Engineering, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan 44919, Republic of Korea
^cUNIST Central Research Facilities, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan 44919, Republic of Korea
^dDepartment of Chemical and Biological Engineering, Sookmyung Women’s University, 100 Cheongpa-ro 47-gil, Seoul 04310, Korea

First published on 25th September 2017

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Introduction

The phenomenal success of metal–organic frameworks (MOFs) achieved over the past two decades stems from several factors, one of which is certainly the rich choice of building blocks, namely metal nodes and organic linkers, leading to the generation of over 70000 MOFs with a vast array of advanced functions.^1–3 The underlying synthetic protocol for these MOFs, however, is rather simple, involving self-assembly of the metal nodes and organic building blocks.^4–6 Instead of this simple protocol, sequential self-assembly utilizes nano-sized building blocks such as metal–organic polyhedra (MOPs) to build higher-dimensional metal–organic materials.^7–9 Some MOPs are intrinsically porous and are therefore attractive building blocks for the cage-based construction of porous materials.^10–13 The distinct advantage of such a modular, multi-step synthesis protocol is a delicate control of the topology and functions by utilizing pre-designed building blocks with high connectivity and symmetry.^13,14

Such elaborate conceptual design attracted synthetic chemists to utilize MOPs and MOFs as building blocks for porous materials.^8–11 A few examples include the formation of 3D MOFs, by connecting MOPs with pyridyl pillars to build 3D MOFs, as reported by Zhou, Su, Wang, and others.^15–22 In these cases, rigid organic linkers connect discrete MOP cages via coordination-driven self-assembly. However, connecting MOPs with flexible organic linkers is highly desirable because both moieties can bring the features of polymers and MOFs, thus combining the advantages of two different classes of material to enhance chemical stability and processability.^23,24

A similar synthetic approach, cross-linking with flexible organic moieties in MOFs, has been well-established, as pioneered by Lee²⁵ in 2000, and recently demonstrated by Sada^26,27 and Wang.²⁸ MOF–polymer hybrid materials often lack molecular-level homogeneity due to the size of MOF particles being a few hundred nanometers (Fig. 1a).²⁸ On the other hand, MOP counterparts, built from metal–organic nanosized cages (1–3 nm) with intrinsic pores, can be potentially useful in energy applications such as gas separation.²⁹ Cohen,³⁰ Nitschke,³¹ and Johnson^32,33 have used polymeric ligands together with cage moieties for the synthesis of 3D polymeric frameworks (Fig. 1b). Recently, Kitagawa and Furukawa developed MOPs attached with flexible molecules.^34,35 None of these MOP–polymer hybrid materials, however, have demonstrated intrinsic porosity originating from the MOP cages, possibly due to the fragile nature of the MOPs.^36,37 Furthermore, cross-linking MOPs with covalent bonds, i.e. postsynthetic polymerization^28,38 of MOPs, has not been demonstrated to our knowledge.

Herein, we demonstrate cross-linking MOPs via a condensation reaction between MOP cages and flexible organic molecules (Fig. 1c and 2). First, we synthesized a robust amine-functionalized Zr-based MOP (UMOP-1-NH₂, UMOP for UNIST metal–organic polyhedra), [Cp₃Zr₃O(OH)₃]₄[BDC-NH₂]₆[(C₂H₅)₂NH₂]₂Cl₆, with a tetrahedral cage and connectivity similar to that found in the iconic MOF, UiO-66.³⁹ Utilizing this robust MOP as a starting material, we carefully selected a simple and effective condensation reaction between the amine group of the Zr-MOP cage and the acyl chloride group of the linker to avoid the use of catalysts or high temperature. A series of highly crystalline and porous cross-linked MOPs (hereafter CLMOPs) was thus synthesized. These results demonstrate that MOPs can be used in the organic cross-linking reaction, providing easy access to a new class of crystalline porous material for advanced applications.

Results and discussion

A Zr-based MOP (UMOP-1-NH₂) was synthesized from Cp₂ZrCl₂ (Cp = η⁵-C₅H₅) and 2-aminoterephthalic acid (H₂BDC-NH₂). Single crystal X-ray diffraction (SCXRD) showed that UMOP-1-NH₂ crystallized in the Fmm space group with a cell parameter of 36.75 Å (Table S1†). The structure consists of tetrahedral cages with trinuclear Zr clusters ([Cp₃Zr₃O(OH)₃(RCO₂)₃]⁺) at the vertexes. The Zr clusters are formed by hydrolysis of Cp₂ZrCl₂.^40,41 The BDC-NH₂ ligands are coordinated to the Zr clusters, forming the edge of the cage. Surprisingly, the crystal structure of UMOP-1-NH₂ is quite similar to that of UiO-66 (Fig. 3). Both systems have structurally analogous tetrahedral cages. The cages in UMOP-1-NH₂ are packed with rhombicuboctahedral and cubic voids (Fig. S1†), interacting via hydrogen bonding with Cl⁻ ions. In the cubic voids, eight [Cp₃Zr₃O(OH)₃(RCO₂)₃]⁺ clusters are interconnected via twelve Cl⁻ ions forming hydrogen bonds with μ₂-OH in the Zr clusters (Fig. S2 and S3†). A neutral charge is satisfied by four (C₂H₅)₂NH₂⁺ cations, produced from decomposition of N,N-diethylformamide (DEF), as evidenced by the ¹H-NMR spectra of UMOP-1-NH₂ (Fig. S4†). Although a similar tetrahedral MOP (ZrT-1) was reported by Yuan and co-workers, the packing pattern of the tetrahedral MOP cage is quite different.⁴⁰

The permanent porosity of UMOP-1-NH₂ was confirmed by N₂ sorption analysis at 77 K (Fig. 4a) with notable BET and Langmuir surface areas of 782 m² g⁻¹ and 900 m² g⁻¹, respectively. Although UMOP-1-NH₂ lost its crystallinity in activation (Fig. S5†), the pore size distribution (PSD) data (Fig. 4b and S6†) obtained from the N₂ adsorption isotherm clearly indicate a pore width of 6 Å, corresponding to the pore of the tetrahedral cage. The pore sizes of the cube and rhombicuboctahedron (10 Å and 18 Å, respectively) were also observed in the PSD data, supporting that the pore structure is partly maintained despite amorphization in the activation. The structural similarity between UMOP-1-NH₂ and UiO-66-NH₂ is further supported by the PSD data, representing a tetrahedral pore of 6 Å in both structures (Fig. 4b). The CO₂ isotherms of UMOP-1-NH₂ acquired at 273 K and 293 K indicated a moderate CO₂ uptake of 2.3 mmol g⁻¹ and 1.4 mmol g⁻¹ at 1 bar, respectively (Fig. S7†). The isosteric heat of adsorption (Q_st) calculated by fitting the CO₂ isotherms to the dual-site Langmuir–Freundlich equation indicated an initial Q_st value of 26.5 kJ mol⁻¹ (Fig. S8 and S9†), comparable to the values for UiO-66 (26.5 kJ mol⁻¹)⁴² and UiO-66-NH₂ (28.6 kJ mol⁻¹).⁴³ Interestingly, the powder X-ray diffraction (PXRD) pattern of activated UMOP-1-NH₂ was restored to the original when the sample was soaked in N,N-diethylformamide (DEF) (Fig. S12†), suggesting that the arrangement of tetrahedral cages was not totally disordered. We further studied the stability of the discrete UMOP-1-NH₂ cage by dissolving it in water. An electrospray ionization (ESI) mass spectrum of the UMOP-1-NH₂ solution in water was obtained after 1 week and revealed a mass to charge ratio (m/z) consistent with a fully intact cage, [U]⁴⁺ (U: a cage without Cl⁻ ions), with additional peaks for [U–H]³⁺ and [U–2H]²⁺ (Fig. 4c). The observed m/z values of the intact cage represent the remarkable stability of the UMOP-1-NH₂ cage in water, while typical MOPs are not stable in water.⁴⁴ The stability in MeOH was also identified with an ESI mass spectrum of a UMOP-1-NH₂ solution in MeOH (Fig. S13†). The discrete UMOP-1-NH₂ cage with a size of 1.5 nm was confirmed with a high-angle annular dark-field (HAADF) scanning transmission electron microscopy image (Fig. 4d). Elemental mapping images showing the distribution of Zr supported that the observed particle was UMOP-1-NH₂ (Fig. S14†).

The CLMOPs were designed to exploit the condensation reaction between UMOP-1-NH₂ and the acyl chloride linkers. The target reaction is the condensation between the amine and acyl chloride groups, resulting in the formation of a secondary amide and HCl. Three acyl chloride linkers with different lengths were used for cross-linking, i.e., suberoyl chloride (La) for CLMOP-1a, sebacoyl chloride (Lb) for CLMOP-1b, and dodecanedioyl dichloride (Lc) for CLMOP-1c, as shown in Fig. 2b.

To directly confirm successful condensation between the amine and acyl chloride groups forming the secondary amide, FT-IR spectra of UMOP-1-NH₂ and the CLMOPs were obtained. The blue region in Fig. 5a shows ν(C–H) bands, derived from the alkyl chain in the CLMOPs. The existence of the secondary amide can be confirmed by the appearance of the ν(C–N) + δ(CNH) and δ(NH) + δ(OCN) bands, distinguished from the peaks of the aromatic amine.⁴⁵ In the spectra of the CLMOPs, the peaks in the orange region in Fig. 5a (observed with shoulders) correspond to ν(C–N) + δ(CNH). In the green region in Fig. 5a, peaks for δ(NH) + δ(OCN) were observed around 1305 cm⁻¹ for the CLMOPs. It is notable that a peak around 1340 cm⁻¹ for the primary amine in UMOP-1-NH₂ was not observed in the spectra of the CLMOPs. This result indicates that the secondary amide is a major group in the CLMOPs. The peak changes observed in the transformation of UMOP-1-NH₂ to the CLMOPs were similar to those observed in postsynthetic modification of UiO-66-NH₂.^45,46

As another convincing point of evidence supporting successful condensation, the amide dimer was directly detected through ¹H-NMR measurement after digestion of the CLMOPs (Fig. 5b and S15–S17†). CLMOP-1b was analyzed as a representative of the CLMOPs (orange graph; Fig. 5b). A mixture of UMOP-1-NH₂ and Lb (black graph) was also analyzed to prove that condensation between UMOP-1-NH₂ and Lb did not occur under the digestion conditions. Peaks of the aromatic protons of the amide dimer were detected around 8.7 ppm, 7.8 ppm, and 7.4 ppm (a, b and c in Fig. 5b, respectively), whereas peaks of the protons of H₂BDC-NH₂ were observed around 7.5 ppm, 7 ppm, and 6.9 ppm (a′, b′ and c′ in Fig. 5b). This proves that almost all the amines reacted with the acyl chlorides during cross-linking, while UMOP-1-NH₂ did not undergo condensation with Lb under the digestion conditions. Peaks of the alkyl chain in the amide dimer were observed at three different chemical shifts in the region of 1.1–2.4 ppm (d, e and f in Fig. 4b). We also detected the amide monomer, indicating that cross-linking was not perfectly achieved. The peaks for the protons labeled d′ and e′ in the amide monomer (in Fig. 5b) were observed upfield, as distinguished from the d and e peaks, due to their proximity to the carboxyl group produced by the reaction of acyl chloride with water. The proportion of interconnected ligands was calculated by comparing the integrated value of the b, d, and d′ peaks. 85% of the total amount of BDC-NH₂ was interconnected by Lb. 13% of the total amount reacted with Lb, but terminated with a carboxyl group. The remaining 2% was from H₂BDC-NH₂, produced by hydrolysis of the amide group during digestion. The data show that a meaningful portion of the cages were cross-linked in the CLMOPs. We also confirmed that 88% of the total amount of the ligand was interconnected in CLMOP-1c (Table S3†), but CLMOP-1a was unable to be analyzed due to peak overlap.

After cross-linking, a drastic solubility change was observed (Fig. S18†). UMOP-1-NH₂ dissolved rapidly in MeOH, but the CLMOPs did not dissolve in MeOH and maintained a crystal shape even after being shaken for 2 days in MeOH. Considering that the UMOP-1-NH₂ cages remained fully intact in MeOH, the observed solubility change suggests successful cross-linking of the cages, thereby preventing the crystals from dissolving in MeOH.

The CLMOPs were further characterized via PXRD, thermogravimetric analysis (TGA), and gas sorption analysis. The PXRD patterns of the as-synthesized CLMOPs were identical to those of UMOP-1-NH₂ (Fig. 6a). The data show the robustness of the tetrahedral cages and their packing even under HCl produced from the condensation. In the TGA analysis, the CLMOPs indicated a larger weight loss at high temperature than UMOP-1-NH₂ (Fig. S19†). The greater weight loss in the CLMOPs corresponds to decomposition of a larger amount of the organic fragment, caused by the addition of the alkyl chains. The permanent porosity of the CLMOPs was analyzed by acquisition of N₂ isotherms (Fig. 6b). CLMOP-1a, CLMOP-1b, and CLMOP-1c had respective total N₂ uptakes of 140 cm³ g⁻¹, 130 cm³ g⁻¹, and 75 cm³ g⁻¹. The BET surface areas were 502 m² g⁻¹, 469 m² g⁻¹, and 277 m² g⁻¹, respectively. The decreased N₂ uptake and BET surface area of the CLMOPs, compared to UMOP-1-NH₂, can be ascribed to the reduced space in the pores due to incorporation of the alkyl chains. The pore size distribution of the CLMOPs indicated a pore width of 6 Å, consistent with the pore width of the tetrahedral cage (Fig. S20†). The excellent agreement between the pore size of UMOP-1-NH₂ and the CLMOPs suggests that the tetrahedral cages remain fully intact even after condensation. CO₂ sorption studies of CLMOP-1a, -1b, and -1c showed respective CO₂ uptakes of 1.9 mmol g⁻¹, 1.8 mmol g⁻¹, and 1.6 mmol g⁻¹ at 1 bar at 273 K (Fig. S21†). The initial Q_st values for the CLMOPs were compared with the values for UMOP-1-NH₂ (Fig. 6c). The values for the CLMOPs were significantly increased (32.2 kJ mol⁻¹ for CLMOP-1a, 30.7 kJ mol⁻¹ for CLMOP-1b, and 36.2 kJ mol⁻¹ for CLMOP-1c). These values were even higher than the values for UiO-66 (26.5 kJ mol⁻¹)⁴² and UiO-66-NH₂ (28.6 kJ mol⁻¹).⁴³ The increased enthalpy is ascribed to the van der Waals interaction between the alkyl groups and CO₂ molecules.⁴⁷ As observed in UMOP-1-NH₂, the PXRD pattern of activated CLMOP-1c revealed an amorphous structure, but the crystallinity was recovered when the sample was soaked in DEF (Fig. S25†), implying that the cage structure was stable during activation.

Structural models of the CLMOPs are suggested by considering the connection by the acyl chloride linkers between the tetrahedral cages, based on the structure of UMOP-1-NH₂. Three possible connections are shown in Fig. 7a with distances of 8.1 Å, 14.2 Å, and 18.4 Å in UMOP-1-NH₂. When the amide dimers are fully stretched, the distances between the centers of the phenyl rings are approximately 17 Å, 19 Å, and 22 Å for the linkers La, Lb, and Lc, respectively (Fig. 7b). La and Lb are possibly long enough to connect units with the distance of 8.1 Å, but not long enough to create links over the distances of 14.2 Å and 18.4 Å (Fig. S26†). In contrast, Lc is sufficiently long to connect even the longest distances. In Fig. 7c, we show a structure model for CLMOP-1c with only one of three connections over the distance of 18.4 Å (orange), representatively.

Surprisingly, we obtained SCXRD data for CLMOP-1c with acceptable quality (R = 7.9%; Table S6†), while the CLMOP-1a and CLMOP-1b crystals used for the SCXRD analysis represented poor single crystallinity based on the diffraction images (Fig. S27†). The SCXRD image of CLMOP-1c showed the single crystal nature (Fig. 7d). The longest connection by Lc possibly keeps the tetrahedral cages intact, otherwise rotational rearrangement of the cages might be involved. The structure of CLMOP-1c was solved in the Rm space group, while UMOP-1-NH₂ crystallized in the Fmm space group. Although the positions of the tetrahedral cages were clearly assignable for CLMOP-1c, we were unable to model the alkyl chains. This is a common phenomenon for MOFs with alkyl chains, as observed for modified IRMOF-3.⁴⁸ The packing pattern of CLMOP-1c is essentially the same as that of UMOP-1-NH₂, except for distortion corresponding to compression along the 3-fold axis of rotational symmetry (Fig. 7e). Such structural difference is also observed in the PXRD pattern of CLMOP-1c, supporting the Rm space group (Fig. S28†).

Conclusions

In this work, we synthesized a Zr-based MOP, UMOP-1-NH₂, with a structure similar to an iconic MOF, UiO-66. UMOP-1-NH₂ showed permanent porosity and stability in water unlike typical fragile MOPs. Exploiting such a robust MOP, we demonstrated cross-linking with flexible organic molecules. Reactive condensation between the amines in UMOP-1-NH₂ and acyl chlorides was employed. Even after cross-linking accompanied by the production of HCl, the porosity and crystallinity of the CLMOPs were maintained, providing evidence that the tetrahedral cages remained fully intact. It is remarkable that the CLMOPs are microporous while permanent porosity of other MOPs combined with flexible organic moieties has not been demonstrated. We confirmed that the Zr-based MOP, which was sufficiently stable, can be utilized in the condensation reaction while preserving the pore structure. To our knowledge, the CLMOPs are the first examples of the covalent cross-linking of MOPs, highlighting the potential of MOPs as building blocks in polymerization. Using this approach, a combination of MOPs with various organic linkers is now possible. For example, amine-rich organic linkers will provide a strong interaction with CO₂ molecules and multitopic linkers will form denser cross-linked systems with highly tunable physical properties. We expect that this cross-linking strategy can be applied to advanced applications, contributing to the bridging of two classes of material, namely metal–organic materials and polymers, both of which are extensively studied to target high selectivity and processability in molecular separation. Specifically, we expect that such materials might provide an important clue to the synthesis of polymeric MOP membranes.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by National Research Foundation (NRF) of Korea (NRF-2015R1D1A1A02061738) and the Ulsan National Institute of Science and Technology (UNIST). The authors acknowledge the Pohang Accelerator Laboratory (PAL) for 2D beamline use (2014-3rd-2D-024) and 6D beamline use (2016-2nd-6D-A011).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental methods, instrumentation, and details for the characterization of UMOP-1-NH2 and CLMOPs. CCDC 1520037 and 1520038. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7sc03847j

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