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

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.


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 H 2 storage 3 and CO 2 capture 4 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) H 2 or CO 2 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 Specically, decoration of the pores with basic nitrogen-containing groups (such as -NH 2 , 6 triazole, 7 and tetrazole 8 ) are effective methods to improve CO 2 adsorption uptakes in MOFs, as the heteroatoms can afford large dipolequadrupole interactions with CO 2 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][12][13][14] These ubt-type 13,15 frameworks are constructed from {Cu 2 (RCOO) 4 } paddlewheels connected by C 3 -symmetric hexacarboxylate linkers, where three isophthalates are connected through a central core in a planar fashion. All these frameworks consist of the same [Cu 24 (isophthalate) 24 ] cuboctahedron which is formed by 24 linkers and 12 {Cu 2 } units. The ubt-type net can be viewed as the face-centered cubic (fcc) packing of cuboctahedra in 3D space. These materials show high H 2 uptakes at both low and high pressures attributed to the unique structural feature of the ubttype network showing close packing of cuboctahedral cages containing 24 exposed Cu(II) sites and extra-high surface area. The {Cu 24 } cage has strong affinity to gas molecules, which has been conrmed experimentally by neutron powder diffraction studies. 12,16 Previous studies revealed that the low-pressure H 2 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 smallsized hexacarboxylate struts. However, there is a limitation to the size of linker that can be used to form (3,24)-connected fccpacked 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 C 3 -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 We report the introduction of ve-membered heterocycles into the C 3 -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 CO 2 molecules. 4b The 1,2,3-triazole rings have been incorporated previously into MOFs, including a exible framework MIL-112, but no porosity was observed for this system. 19 Most recently, the triazole group has been incorporated into porous organic polymers 20 using "click" chemistry. 21 Herein, we report the use of the 1,2,3-triazolecontaining linker 5,5 0 ,5 00 -(4,4 0 ,4 00 -(benzene-1,3,5-triyl)tris(1H-1,2,3-triazole-4,1-diyl))triisophthalic acid (H 6 BTTI) 22 to construct a highly porous (3,24)-connected polyhedral framework (denoted NOTT-122) incorporating {Cu 2 } paddlewheel units and having a high density of accessible nitrogen-donor sites, 23 and achieving high adsorption capacities for both H 2 and CO 2 .

Results and discussion
H 6 BTTI 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 H 6 BTTI with Cu(NO 3 ) 2 $2.5H 2 O in DMF (DMF ¼ N,N-dimethylformamide) in the presence of HCl afforded truncated octahedron-shaped bluegreen crystals of NOTT-122 in high yield. The complex Single crystal X-ray diffraction conrms ‡ that NOTT-122 is a (3,24)-connected network in which a cuboctahedron (Cage A) is constructed by 12 {Cu 2 (COO) 4 } paddlewheels and 24 isophthalates from 24 different (BTTI) 6À units, and each linker acts as a 3-connected node connected to 3 cuboctahedra. The face of the (BTTI) 6À 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 t within these cavities have diameters of 12.2Å and 19.3Å 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 triazolerings. In contrast, the triazole groups in most other triazolatebridged 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.
The total accessible volume of NOTT-122 aer removal of the guest and coordinated water molecules is estimated to be 74% Fig. 1 (a) Hexacarboxylate linker with co-planar linearly connected isophthalates. (b) Two close packed {Cu 24 } 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 {Cu 24 } 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.
Scheme 1 Synthesis of H 6 BTTI. using the PLATON/VOID routine, 26 and the calculated density of the desolvated framework is 0.589 g cm À3 . Thermogravimetric analysis (TGA) conrmed that the framework is thermally stable up to 270 C. The phase purity of the bulk sample of NOTT-122 was conrmed by powder X-ray diffraction (PXRD). The acetoneexchanged sample was degassed under dynamic vacuum at 110 C for 12 hours to afford the activated sample of NOTT-122a. The PXRD proles of the activated NOTT-122a indicate that the framework retains its crystallinity, reecting a highly robust structure attributed to the (3,24)-connected net (Fig. S9 †).
N 2 sorption at 77 K in NOTT-122a (Fig. S12 †) exhibits a reversible type-I sorption prole with small changes in slope in the pressure region P/P 0 0.03-0.1 indicating the sequential lling and desorption of gas within micro-and meso-porous cavities. The apparent Brunauer-Emmett-Teller (BET) surface area is 3286 m 2 g À1 , and the calculated pore size, derived from the N 2 isotherm and based on the nonlocal density functional theory (NLDFT) model, is distributed between 10-15Å, and 15-30Å (Fig. S14 †), consistent with the single crystal structure analysis. NOTT-122a has a total pore volume of 1.41 cm 3 g À1 calculated from the N 2 isotherm data.
The gravimetric H 2 sorption isotherms for NOTT-122a were recorded at 77 and 87 K up to 20 bar ( Fig. 3a and S15 †). Signicantly, NOTT-122a can adsorb 2.61 wt% of H 2 at 1 bar and 77 K [wt% ¼ 100(weight of adsorbed H 2 )/(weight of host material)]. To date, there are only a handful of MOFs with H 2 uptakes exceeding 2.6 wt% under the same conditions. 3 The H 2 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 H 2 uptakes in the range of 1.44-2.42 wt% at 1 bar and 77 K. The isosteric heat of adsorption (Q st ) for H 2 has an estimated value of 6.0 kJ mol À1 at zero coverage (see ESI †), consistent with H 2 binding to the open Cu(II) sites. The remarkable low-pressure H 2 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 H 2 molecules. 27 Due to the large BET surface area and pore volume, NOTT-122a has a total H 2 adsorption capacity of 7.0 wt% (70 mg g À1 ) at 77 K and 20 bar, among the highest reported to date for MOFs under the  The open Cu(II) sites and high porosity also enable high methane storage in NOTT-122a (Fig. 3b). At 20 bar, the total CH 4 uptakes of NOTT-122a reach 21.5 wt% [176 cm 3 (STP) cm À3 , STP: standard temperature and pressure] and 16.1 wt% (132.1 cm 3 cm À3 ) at 273 and 298 K, respectively. The volumetric CH 4 uptake values for NOTT-122a are comparable to those of other porous MOFs with high CH 4 storage such as UTSA-20 28 (total 195 cm 3 cm À3 at 300 K and 35 bar) and PCN-11 29 (excess 171 cm 3 cm À3 , 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 CO 2 , and indeed at 1 bar, NOTT-122a exhibits remarkable CO 2 adsorption capacities of 39.7 wt% (9.02 mmol g À1 , 202.1 cm 3 g À1 , STP) at 273 K and 20.4 wt% (4.63 mmol g À1 , 103.8 cm 3 g À1 , STP) at 298 K (Fig. 3), exceeding the CO 2 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-TDPAT 17 (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) CO 2 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 CO 2 uptake of 25.6 mmol g À1 at 20 bar and 298 K, higher than most other highly porous MOFs such as Cu-TPBTM (23.5 mmol g À1 ), PCN-61 (21.4 mmol g À1 ) and PCN-68 (22.1 mmol g À1 ) 10c,d under the same conditions. To evaluate the selective CO 2 capture in NOTT-122a, CH 4 and N 2 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 CO 2 /N 2 adsorption selectivity factors for NOTT-122a are 23.4 : 1 at 273 K and 14.3 : 1 at 298 K. The CO 2 /N 2 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 CO 2 adsorption. NOTT-122a also shows respectable CO 2 /CH 4 selectivity of 9.2 : 1 at 273 K and 6.4 : 1 at 298 K. The analysis of isosteric heats of CO 2 adsorption using the virial method reveals a moderate adsorption enthalpy of 24.5 kJ mol À1 at zero coverage, reecting the expected van der Waals host-guest interactions in this large pore system and consistent with the shallow gradient observed for the CO 2 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 H 2 , CH 4 and CO 2 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 CO 2 adsorption at different pressures (10, 50 and 100 kPa) at 273 K have been employed to dene potential interactions between CO 2 molecules and the framework. Simulations were performed based on a model where the CO 2 -framework interactions were described by electrostatic and van der Waals potentials. There are three observed preferential binding sites for CO 2 in NOTT-122a identied at 10 kPa ( Fig. 4a and b): the open Cu 2+ site (I), denoted CuA, inside the cuboctahedral cage (Cage A) (another type of Cu 2+ ion sited outside of a cuboctahedral cage is denoted CuB); the triangular-shaped window site (II) with the CO 2 sites at the center of the cage opening formed by three isophthalate units and three Cu 2 (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 BTTI 6À units, where the CO 2 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 CO 2 binding in this particular framework. When the pressure is increased to 50 kPa, a signicant amount of CO 2 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 BTTI 6À units connecting the B and C cages begins (Fig. 4c and d). At 100 kPa, aer occupying the cuboctahedral cage, the narrow triangular pocket and the cavity connecting the Cage B and C, CO 2 molecules continue to diffuse into the remaining empty space of the porous structure, revealing a sequential pore lling mechanism of CO 2 in this hierarchically assembled structure.
GCMC simulations identied the triazole groups of NOTT-122a as binding sites for CO 2 molecules, and a number of congurations occurred regularly in the simulations (Fig. 5). At pressures of 50 kPa and 100 kPa, a chain of CO 2 molecules can be formed via CO 2 -Cu(II), CO 2 -triazole, and CO 2 -CO 2 interactions, with the triazole ring a particular "hot spot" for binding (Fig. 6).

Conclusions
In conclusion, we present here the construction of a highly porous (3,24)-connected framework NOTT-122a by using a C 3symmetric 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 {Cu 24 } cuboctahedra, coupled with the high density of polarizing triazole groups, is responsible for its high H 2 , CH 4 and CO 2 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 CO 2 . The GCMC simulation study conrms the combination of effects from open metal sites and accessible triazole rings that affords high CO 2 uptakes in NOTT-122a.