Modulating the packing of [Cu₂₄(isophthalate)₂₄] cuboctahedra in a triazole-containing metal–organic polyhedral framework

Abstract

The highly porous (3,24)-connected framework NOTT-122 incorporates a C₃-symmetric angularly connected isophthalate linker containing 1,2,3-triazole rings and shows body-centered tetragonal packing of [Cu₂₄(isophthalate)₂₄] cuboctahedra. This unique packing, coupled with the high density of free N-donor sites, is responsible for the simultaneous high H₂, CH₄ and CO₂ adsorption capacities in desolvated NOTT-122a.

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Introduction

Metal–organic frameworks (MOFs) are an emerging class of crystalline materials constructed from metal centers (nodes) bridged by organic linkers (spacers) with a high level of structural modularity and diversity.¹ The concept of supramolecular building blocks to engineer 3-dimensional crystalline architectures allows the design and prediction of novel structures.² Porous MOFs show great promise for potential applications in H₂ storage³ and CO₂ capture⁴ owing to their chemical and thermal stability, permanent porosity, and high surface area.⁵ It is now well established that the high-pressure (>20 bar) H₂ or CO₂ uptake in MOFs is correlated with the surface area and/or the pore volume of the host, consistent with the physisorption process, while the low-pressure (under 1 bar) adsorption performance is strongly correlated with the strength of interaction between the framework and gas molecules.³ Current synthetic efforts to enhance gas–framework interactions include introduction of open metal sites,^3d and adjustment of pore shape and size to enhance overlapping energy potential of the pore walls.^3d,e,4b Specifically, decoration of the pores with basic nitrogen-containing groups (such as –NH₂,⁶ triazole,⁷ and tetrazole⁸) are effective methods to improve CO₂ adsorption uptakes in MOFs, as the heteroatoms can afford large dipole–quadrupole interactions with CO₂ molecules.^4b

Our synthetic strategy focuses on the design and synthesis of metal–organic polyhedral cage materials based upon (3,24)-connected frameworks^5b,9,10 possessing high structural stability and high surface area and pore volume.^11–14 These ubt-type^13,15 frameworks are constructed from {Cu₂(RCOO)₄} paddlewheels connected by C₃-symmetric hexacarboxylate linkers, where three isophthalates are connected through a central core in a planar fashion. All these frameworks consist of the same [Cu₂₄(isophthalate)₂₄] cuboctahedron which is formed by 24 linkers and 12 {Cu₂} units. The ubt-type net can be viewed as the face-centered cubic (fcc) packing of cuboctahedra in 3D space. These materials show high H₂ uptakes at both low and high pressures attributed to the unique structural feature of the ubt-type network showing close packing of cuboctahedral cages containing 24 exposed Cu(II) sites and extra-high surface area. The {Cu₂₄} cage has strong affinity to gas molecules, which has been confirmed experimentally by neutron powder diffraction studies.^12,16 Previous studies revealed that the low-pressure H₂ adsorption uptake of these ubt-type frameworks can be enhanced by reducing the distance between the two closest cuboctahedra,^10c,12 which can be achieved by employing small-sized hexacarboxylate struts. However, there is a limitation to the size of linker that can be used to form (3,24)-connected fcc-packed cuboctahedra as this type of packing will be forbidden by steric hindrance between two closest cuboctahedra.¹⁷ By modifying the topology of the hexacarboxylate linker to give more angular connectivity through the C₃-symmetric central core, we argued that tight packing of the cuboctahedra might be modulated to give more densely packed cuboctahedra compared to more common fcc-structures (Fig. 1).^17,18

Fig. 1 (a) Hexacarboxylate linker with co-planar linearly connected isophthalates. (b) Two close packed {Cu₂₄} cuboctahedra within a ubt-type network aligned along the same axis. There has to be a minimum distance between these two polyhedra due to the steric hindrance from the two axial water molecules. (c) Hexacarboxylate linker with co-planar angularly connected isophthalate units of lower symmetry; (d) two near neighbour {Cu₂₄} cuboctahedra packed in a distorted manner due to the angular hexacarboxylate linker. The framework constructed from the angular hexacarboxylate linker has the tendency to form a more close packed bct-ubt net compared to the more common fcc-ubt network.

We report the introduction of five-membered heterocycles into the C₃-symmetric hexacarboxylate linker to introduce an angular character to the external arms between the terminal isophthalates and the central core. Additionally, heterocycles can potentially provide enhanced binding sites for CO₂ molecules.^4b The 1,2,3-triazole rings have been incorporated previously into MOFs, including a flexible framework MIL-112, but no porosity was observed for this system.¹⁹ Most recently, the triazole group has been incorporated into porous organic polymers²⁰ using “click” chemistry.²¹ Herein, we report the use of the 1,2,3-triazole-containing linker 5,5′,5′′-(4,4′,4′′-(benzene-1,3,5-triyl)tris(1H-1,2,3-triazole-4,1-diyl))triisophthalic acid (H₆BTTI)²² to construct a highly porous (3,24)-connected polyhedral framework (denoted NOTT-122) incorporating {Cu₂} paddlewheel units and having a high density of accessible nitrogen-donor sites,²³ and achieving high adsorption capacities for both H₂ and CO₂.

Results and discussion

H₆BTTI was obtained in good yield via Cu(I)-catalyzed 1,3-cycloaddition of 1,3,5-triethynylbenzene with 5-azidoisophthalic acid (Scheme 1; see ESI†). Solvothermal reaction of H₆BTTI with Cu(NO₃)₂·2.5H₂O in DMF (DMF = N,N-dimethylformamide) in the presence of HCl afforded truncated octahedron-shaped blue-green crystals of NOTT-122 in high yield. The complex [Cu₃(BTTI)(H₂O)₃]·10DMF·10H₂O crystallizes in tetragonal space group I4/m with large unit cell [a = b = 30.926(5) Å, c = 45.103(7) Å]. Single crystal X-ray diffraction confirms‡ that NOTT-122 is a (3,24)-connected network in which a cuboctahedron (Cage A) is constructed by 12 {Cu₂(COO)₄} paddlewheels and 24 isophthalates from 24 different (BTTI)⁶⁻ units, and each linker acts as a 3-connected node connected to 3 cuboctahedra. The face of the (BTTI)⁶⁻ unit has a distorted hexagonal geometry and the overall structure is based upon body-centered tetragonal (bct) packing of cuboctahedra in 3D space (Fig. S7†). This is of lower symmetry compared to the more usual fcc-packing in a ubt net. Two other types of polyhedral cages, a distorted truncated tetrahedron (Cage B) and a distorted truncated octahedron (Cage C) are observed in this structure, and the three types of cages tessellate in the same way as in the ubt-type network. One Cage B is connected to 4 cuboctahedra and one Cage C is connected to 6 cuboctahedra, by sharing the triangular and square windows of Cage A, respectively. The ratio of the different cages (Cage A : B : C) is 1 : 2 : 1. Taking into account the van der Waals surface of the cage interior, the largest spheres that can fit within these cavities have diameters of 12.2 Å and 19.3 Å for Cage B and Cage C, respectively. The length of the largest cage in the framework can reach ca. 3.2 nm and lies within the mesoporous regime. This structural analysis reveals that the bct-packing is another type of efficient tiling of cuboctahedra for creating hierarchically assembled cavities. It is worth noting that both Cage B and C have a remarkably high density of nitrogen-donor sites on the pore walls (Fig. 2), and the apertures which access the empty space in these cavities are densely decorated with uncoordinated triazole-rings. In contrast, the triazole groups in most other triazolate-bridged frameworks are coordinated to metal ions by donating one or two nitrogen atoms of the heterocycle,^24,25 thus drastically reducing the number of accessible nitrogen-donor sites within the pores.

Scheme 1 Synthesis of H₆BTTI.

Fig. 2 The crystal structure of NOTT-122 showing a (3,24)-connected network. The framework is decorated with a high density of N-donor sites.

The total accessible volume of NOTT-122 after removal of the guest and coordinated water molecules is estimated to be 74% using the PLATON/VOID routine,²⁶ and the calculated density of the desolvated framework is 0.589 g cm⁻³. Thermogravimetric analysis (TGA) confirmed that the framework is thermally stable up to 270 °C. The phase purity of the bulk sample of NOTT-122 was confirmed by powder X-ray diffraction (PXRD). The acetone-exchanged sample was degassed under dynamic vacuum at 110 °C for 12 hours to afford the activated sample of NOTT-122a. The PXRD profiles of the activated NOTT-122a indicate that the framework retains its crystallinity, reflecting a highly robust structure attributed to the (3,24)-connected net (Fig. S9†).

N₂ sorption at 77 K in NOTT-122a (Fig. S12†) exhibits a reversible type-I sorption profile with small changes in slope in the pressure region P/P₀ 0.03–0.1 indicating the sequential filling and desorption of gas within micro- and meso-porous cavities. The apparent Brunauer–Emmett–Teller (BET) surface area is 3286 m² g⁻¹, and the calculated pore size, derived from the N₂ isotherm and based on the nonlocal density functional theory (NLDFT) model, is distributed between 10–15 Å, and 15–30 Å (Fig. S14†), consistent with the single crystal structure analysis. NOTT-122a has a total pore volume of 1.41 cm³ g⁻¹ calculated from the N₂ isotherm data.

The gravimetric H₂ sorption isotherms for NOTT-122a were recorded at 77 and 87 K up to 20 bar (Fig. 3a and S15†). Significantly, NOTT-122a can adsorb 2.61 wt% of H₂ at 1 bar and 77 K [wt% = 100(weight of adsorbed H₂)/(weight of host material)]. To date, there are only a handful of MOFs with H₂ uptakes exceeding 2.6 wt% under the same conditions.³ The H₂ capacity of NOTT-122a is higher than all the (3,24)-connected ubt-type frameworks NOTT-112 to NOTT-116,^12–14 NOTT-119,¹¹ PCN-61,¹⁰ and NU-100^5b which show H₂ uptakes in the range of 1.44–2.42 wt% at 1 bar and 77 K. The isosteric heat of adsorption (Q_st) for H₂ has an estimated value of 6.0 kJ mol⁻¹ at zero coverage (see ESI†), consistent with H₂ binding to the open Cu(II) sites. The remarkable low-pressure H₂ adsorption performance is attributed to the combined effects of the close bct-packing of the cuboctahedra with exposed Cu(II) sites and the high concentration of nitrogen-donating sites in the structure. Theoretical calculations predict that N-containing aromatic rings can enhance the affinity to H₂ molecules..²⁷ Due to the large BET surface area and pore volume, NOTT-122a has a total H₂ adsorption capacity of 7.0 wt% (70 mg g⁻¹) at 77 K and 20 bar, among the highest reported to date for MOFs under the same conditions. Thus, NOTT-122a is a new member of the family of the (3,24)-connected Cu-based polyhedral structures showing high H₂ sorption capacities at both low and high pressures.

Fig. 3 (a) H₂ isotherms for NOTT-122a at 77 K in the pressure range 0 to 20 bar. The inset shows the experimental and simulated H₂ isotherms up to 1 bar; (b) experimental and simulated CH₄ isotherms for NOTT-122a at both 273 and 298 K; (c) experimental CO₂, CH₄ and N₂ sorption isotherms for NOTT-122a at both 273 and 298 K up to 1 bar; (d) experimental and simulated CO₂ isotherms for NOTT-122a in the pressure range 0–1 bar. Adsorption and desorption branches are shown with closed and open symbols, respectively.

The open Cu(II) sites and high porosity also enable high methane storage in NOTT-122a (Fig. 3b). At 20 bar, the total CH₄ uptakes of NOTT-122a reach 21.5 wt% [176 cm³(STP) cm⁻³, STP: standard temperature and pressure] and 16.1 wt% (132.1 cm³ cm⁻³) at 273 and 298 K, respectively. The volumetric CH₄ uptake values for NOTT-122a are comparable to those of other porous MOFs with high CH₄ storage such as UTSA-20²⁸ (total 195 cm³ cm⁻³ at 300 K and 35 bar) and PCN-11²⁹ (excess 171 cm³ cm⁻³, at 298 K and 35 bar).

The incorporation of accessible polarised 1,2,3-triazole rings in the framework is expected to enhance the affinity to CO₂, and indeed at 1 bar, NOTT-122a exhibits remarkable CO₂ adsorption capacities of 39.7 wt% (9.02 mmol g⁻¹, 202.1 cm³ g⁻¹, STP) at 273 K and 20.4 wt% (4.63 mmol g⁻¹, 103.8 cm³ g⁻¹, STP) at 298 K (Fig. 3), exceeding the CO₂ capacities of most MOFs containing exposed metal sites.⁴ These values are comparable to the highest values for MOFs such as Mg/DOBDC (35.2 wt%, 296 K),³⁰ Cu–TDPAT¹⁷ (44.5 wt%, 273 K; 25.8 wt%, 298 K), and Cu–TPBTM^10d (42.6 wt%, 273 K; 23.3 wt%, 298 K). Possessing a similar pore size to another (3,24)-connected framework PCN-61^10c,d with ubt-topology, NOTT-122a shows a higher low-pressure (1 bar) CO₂ uptake capacity (PCN-61: 25.2 wt%, 273 K; 13.9 wt%, 298 K), which is ascribed to the high nitrogen-content and the different packing mode of the cuboctahedra. NOTT-122a also shows a high CO₂ uptake of 25.6 mmol g⁻¹ at 20 bar and 298 K, higher than most other highly porous MOFs such as Cu–TPBTM (23.5 mmol g⁻¹), PCN-61 (21.4 mmol g⁻¹) and PCN-68 (22.1 mmol g⁻¹)^10c,d under the same conditions. To evaluate the selective CO₂ capture in NOTT-122a, CH₄ and N₂ sorption isotherms were also measured at ambient temperatures (Fig. 3c). By determining the ratios of the Henry's Law constants from single component isotherms,³¹ the CO₂/N₂ adsorption selectivity factors for NOTT-122a are 23.4 : 1 at 273 K and 14.3 : 1 at 298 K. The CO₂/N₂ selectivity in NOTT-122a is higher than both the ubt-type frameworks Cu–TPBTM (∼22 : 1) and PCN-61 (∼15 : 1) at 273 K, further suggesting that the polar triazole functionalities have a positive effect on CO₂ adsorption. NOTT-122a also shows respectable CO₂/CH₄ selectivity of 9.2 : 1 at 273 K and 6.4 : 1 at 298 K. The analysis of isosteric heats of CO₂ adsorption using the virial method reveals a moderate adsorption enthalpy of 24.5 kJ mol⁻¹ at zero coverage, reflecting the expected van der Waals host–guest interactions in this large pore system and consistent with the shallow gradient observed for the CO₂ isotherm. Also, no hysteresis is observed in the measured gas isotherms. This is to be expected given the high porosity and high pore volume in NOTT-122.

To analyse further the gas adsorption properties of NOTT-122a, grand canonical Monte Carlo (GCMC) simulations for H₂, CH₄ and CO₂ adsorption were performed (see ESI†). The simulated isotherms for these three gases are in good agreement with the experimental data (Fig. 3). GCMC simulations for CO₂ adsorption at different pressures (10, 50 and 100 kPa) at 273 K have been employed to define potential interactions between CO₂ molecules and the framework. Simulations were performed based on a model where the CO₂–framework interactions were described by electrostatic and van der Waals potentials. There are three observed preferential binding sites for CO₂ in NOTT-122a identified at 10 kPa (Fig. 4a and b): the open Cu²⁺ site (I), denoted CuA, inside the cuboctahedral cage (Cage A) (another type of Cu²⁺ ion sited outside of a cuboctahedral cage is denoted CuB); the triangular-shaped window site (II) with the CO₂ sites at the center of the cage opening formed by three isophthalate units and three Cu₂(COO)₄ paddlewheels;^32,33 and site (III), located in the corner of the narrow channel in Cage B formed by three peripheral arms from three BTTI⁶⁻ units, where the CO₂ is close to the N1 atom of one triazole ring from the channel wall. These simulations further suggest that the open metal sites and the polar pore environment created by the triazole groups contribute synergically to the CO₂ binding in this particular framework. When the pressure is increased to 50 kPa, a significant amount of CO₂ was found in the cuboctahedral cage and the narrow channel of the truncated tetrahedral cage, and then population of the small cavity surrounded by four BTTI⁶⁻ units connecting the B and C cages begins (Fig. 4c and d). At 100 kPa, after occupying the cuboctahedral cage, the narrow triangular pocket and the cavity connecting the Cage B and C, CO₂ molecules continue to diffuse into the remaining empty space of the porous structure, revealing a sequential pore filling mechanism of CO₂ in this hierarchically assembled structure.

Fig. 4 GCMC simulations of CO₂ adsorption in NOTT-122a at 10 (a and b) and 50 kPa (c and d). Three likely CO₂ adsorption sites in NOTT-122a derived from GCMC at 10 kPa are indicated: site I (CuA center) is shown in (a) and sites (II) and (III) are shown in (b). At 50 kPa, CO₂ molecules were found in the narrow channel of Cage B shown in (c) and in the small cavity created by four BTTI⁶⁻ units connecting Cages B and C (d).

GCMC simulations identified the triazole groups of NOTT-122a as binding sites for CO₂ molecules, and a number of configurations occurred regularly in the simulations (Fig. 5). At pressures of 50 kPa and 100 kPa, a chain of CO₂ molecules can be formed via CO₂–Cu(II), CO₂–triazole, and CO₂–CO₂ interactions, with the triazole ring a particular “hot spot” for binding (Fig. 6).

Fig. 5 Possible orientations of CO₂ molecules binding at the triazole ring: (a) the CO₂ molecule is parallel to the triazole ring, (b) the CO₂ molecule interacts with the N-center of the triazole ring and the nearest hydrogen, (c) the CO₂ molecule is located above the triazole ring, (d) two CO₂ molecules interact simultaneously with the triazole ring.

Fig. 6 Snapshot of the GCMC simulations showing preferential binding sites for CO₂ molecules in NOTT-122a: (a) at 50 kPa, (b) at 100 kPa.

Conclusions

In conclusion, we present here the construction of a highly porous (3,24)-connected framework NOTT-122a by using a C₃-symmetric angularly connected isophthalate strut bearing high content of free N-donor sites. The bct-packing of the cuboctahedra in NOTT-122a is another type of efficient 3D tessellation of polyhedra, having the advantage of allowing a large number of heterocycles to be sited within distorted hexagonal faces without the necessity of increasing the size of the linker. The bct-packing of {Cu₂₄} cuboctahedra, coupled with the high density of polarizing triazole groups, is responsible for its high H₂, CH₄ and CO₂ uptake, making NOTT-122a one of the rare examples of MOFs showing simultaneous high adsorption performance for these gases. This study also demonstrates that 1,2,3-triazole functionalities can be effectively integrated into porous robust MOFs using “click” chemistry, leading to rigid structures decorated with Lewis basic sites for enhanced interactions with CO₂. The GCMC simulation study confirms the combination of effects from open metal sites and accessible triazole rings that affords high CO₂ uptakes in NOTT-122a.

Acknowledgements

We thank the EPSRC (U.K. Sustainable Hydrogen Energy Consortium, http://www.uk-shec.org.uk/) and the University of Nottingham for funding. We are grateful to the EPSRC-funded National Crystallography Service and Diamond Light Source for data collection on Diamond Beamline I19. M.S. gratefully acknowledges receipt of an ERC Advanced Grant, an EPSRC Programme Grant and a Royal Society Wolfson Merit award.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: CCDC number for structure of Experimental details, crystallographic parameters, adsorption isotherms, and GCMC calculations. CCDC 906058. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3sc21769h

‡ Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication CCDC 906058.

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