Co2 and Co3 Mixed Cluster Secondary Building Unit Approach toward a Three-Dimensional Metal-Organic Framework with Permanent Porosity

Large and permanent porosity is the primary concern when designing metal-organic frameworks (MOFs) for specific applications, such as catalysis and drug delivery. In this article, we report a MOF Co11(BTB)6(NO3)4(DEF)2(H2O)14 (1, H3BTB = 1,3,5-tris(4-carboxyphenyl)benzene; DEF = N,N-diethylformamide) via a mixed cluster secondary building unit (SBU) approach. MOF 1 is sustained by a rare combination of a linear trinuclear Co3 and two types of dinuclear Co2 SBUs in a 1:2:2 ratio. These SBUs are bridged by BTB ligands to yield a three-dimensional (3D) non-interpenetrated MOF as a result of the less effective packing due to the geometrically contrasting SBUs. The guest-free framework of 1 has an estimated density of 0.469 g cm−3 and exhibits a potential solvent accessible void of 69.6% of the total cell volume. The activated sample of 1 exhibits an estimated Brunauer-Emmett-Teller (BET) surface area of 155 m2 g−1 and is capable of CO2 uptake of 58.61 cm3 g−1 (2.63 mmol g−1, 11.6 wt % at standard temperature and pressure) in a reversible manner at 195 K, showcasing its permanent porosity.

From the ligand perspective, designing an organic ligand alone by increasing its bulkiness to prevent interpenetration and to obtain highly porous MOFs is challenging, and may instead lead to the formation of MOFs with reduced porosity.To this end, two methods are widely practiced to obtain highly porous MOFs.The first method involves the in situ replacement of an existing ligand from the as-synthesized MOF with a similar (usually longer) one, taking advantage of the kinetic lability of the metal-ligand association in solvents [44][45][46][47][48][49][50].Hupp et al. termed this process as solvent-assisted linker exchange (SALE) [48].It is notable that even the most robust MOFs, such as UiO-66, MIL-53, and ZIF (e.g., CdIF-4 with RHO topology), are susceptible to SALE in the solution state because of the small energy difference generated by different linker derivatives results in a dynamic situation [49,51,52].The other more direct approach concerns the use of mixed ligands with different length/angularity but with similar coordination preference to reduce the effective packing of the molecule to achieve non-interpenetrated MOFs with high porosity [53,54].
The limitations of SALE and mix-ligand approach are also obvious.As SALE involves competitive binding of one ligand for the other, usually driven by the difference of the pKa values of the two ligands, the parent MOF should be porous enough to allow the incoming ligands to diffuse into the pores, yet labile enough to allow the competitive binding, and ultimately robust enough to maintain the framework connectivity throughout the exchange process so that the MOFs are not undergoing decomposition or dissolution-recrystallization to yield the undesired complexes [55][56][57].As for the mix-ligand process, there is a potential risk that each linker forms their own preferred MOFs to achieve the energy minimum of the reaction system [54,58].
By analogy to the mix-ligand approach, the mix-SBU approach also serves to prevent the structure interpenetration to yield stable MOFs with high porosity.An existing and reliable mix-SBU approach is the bottom-up MOF engineering from pre-formed cluster units.For example, Zaworotko et al. [59] adapted the pinwheel [M 3 (µ 3 -O)(O 2 CR) 6 ] cluster as the SBU to further construct MOFs for targeted applications.Li et al. [60] demonstrated the compatibility of a triangular Cu 3 -pyrazolate SBU with paddle-wheel Zn 2 and tetrahedral Zn 4 SBUs to yield a class of mixed-cluster MOFs.These MOFs feature a notable reversible redox switch between Cu I 3 (PyC) 3 and Cu II 3 (µ-OH)(PyC) 3 (OH) 3 (H 2 PyC = 4-pyrazolecarboxylic acid) with the retention of the SBU geometry.The Cu 3 cluster SBU thus functions as an electron reservoir during the catalytic oxidation of CO/aromatic alcohols, and the decomposition of H 2 O 2 .
To this end, the formation of mix SBUs in one MOF remains scarce, presumably because of the usually less compatible geometries of two or more high-connecting SBUs in one MOF.Zhao et al. [61] reported highly porous MOFs respectively sustained by [Cu 12 I 12 ] + [Gd 3 ] and [Cu 3 I 2 ] + [Gd 4 ] cluster SBUs for the carboxylation reactions of CO 2 with terminal alkynes under mild conditions.In this paper, we explore the synthesis of a mix-SBU MOF by using Co(NO 3 ) 2 •6H 2 O as the metal source and H 3 BTB as the ligand in DEF (H 3 BTB = 1,3,5-tris(4-carboxyphenyl)benzene; DEF = N,N-diethylformamide).We chose Co II because the tetrahedral and octahedral Co 2+ were demonstrated to coexist in equilibrium via the stabilization of oxygen donor ligands [62].It is therefore not uncommon that the tetrahedral and octahedral Co 2+ coexist in one SBU of the MOFs [63,64].We select a large and triangular H 3 BTB as the ligand because it typically forms non-interpenetrated MOFs.DEF is further selected because it is less volatile compared with DMF (N,N-dimethylformamide) and will guarantee the stability of the MOF for subsequent material characterizations.Thus the one-pot solvothermal reaction of Co(NO 3 ) 2 •6H 2 O and H 3 BTB in DEF yielded the 3D MOF of Co 11 (BTB) 6 (NO 3 ) 4 (DEF) 2 (H 2 O) 14 (1) in a high yield of 87%.The guest-free framework of 1 has an estimated density of 0.469 g cm −3 and features a potentially 69.6% solvent accessible volume.The gas adsorption studies of 1 indicated that it reversibly uptakes CO 2 over N 2 and demonstrated permanent porosity.

Synthesis and Material Characterization of MOF 1
MOF 1, characterized to have a general formula of Co 11 (BTB) 6 (NO 3 ) 4 (DEF) 2 (H 2 O) 14 , has been prepared as block purple crystals from a facile one-pot reaction of Co(NO 3 ) 2 •6H 2 O and H 3 BTB in DEF under mild reaction conditions (Figure S1).The isolation yield of MOF 1 reaches as high as 87% (based on Co).MOF 1 is stable in MeOH, EtOH, and DMF but experiences a rapid decomposition when immersed in H 2 O.The FT-IR spectrum of 1 contains a sharp peak at 1385 cm −1 , diagnostic of the presence of NO 3 − [65].The FT-IR spectrum further features peaks at 2975 cm −1 , 2930 cm −1 , and 1643 cm −1 , corresponding to the presence of -CH 3 , -CH 2 -, and -C=O functionalities and hence the DEF solvate [66].These observations corroborate the structure deduced from the X-ray crystallographic analysis.The thermogravimetric analysis (TGA) indicates a continuous mass loss of 1 before ca.500 • C (Figure 1).The first weight loss before 200 • C is likely due to the solvate evaporation, while the second weight loss between 200-500 • C is due to the framework decomposition.Given the large pore percentage (69.6%) of the material and the complexity of solvates included (both coordinated and free DEF and H 2 O), it is not possible to assign the correct ratio of solvents within the pore.
The powder X-ray diffraction (PXRD) patterns of MOF 1 indicate its crystalline nature with a primary match between the experimental and that of the simulated (Figure 2).However, the intensities of these peaks do not match well.This is typically due to either the different orientation of the crystallites in the bulk material or the pore filling effect [67,68].The pore filling effect describes that the diffraction intensity deviation (either enhanced or reduced) exists between the solvent-free (simulated) and solvated (experimental) samples [57,68].
When looking along the [010] direction (Figure 3e) and the crystallographic a direction (Figure 3f), small parallelogram and large oval-shaped pores are feasible.These pore volumes sum up to 19,398.6 Å 3 per unit cell, which occupied 69.6% of the total cell volume (27,861.0Å 3 ) as calculated by the Platon program [74].As a result, a guest-free scaffold of MOF 1 features a low density of 0.469 g cm −3 (Table 2).
where n is the number of reflections, and p is total number of parameters refined.

Gas Adsorption Behavior of MOF 1
The pore surfaces of MOF 1 are decorated with charged/polar NO 3 − /H 2 O/DEF, an ideal trait of an absorbent for N 2 and CO 2 with quadruple moments of 1.52 × 10 −26 esu cm 2 and 4.30 × 10 −26 esu cm 2 [75,76].The sample activation was guided by the TGA results (Figure 1).To maintain the framework integrity of MOF 1 for subsequent porosity measurement, the sample activation temperature and pressure were set to 323 K and 10 −2 kPa.The solvents in the pores were quickly exchanged with CHCl 3 twice before transferring to the instrument for further activation.We observed no obvious absorption of N 2 at 77 K (Figure 4) but notable uptake of CO 2 at 195 K for MOF 1.This presumably results from the larger kinetic size of N 2 (3.64 Å) than CO 2 (3.30Å) [75,77].
In addition, the surface crack of the crystals is also a potential limiting factor for the differentiation of gas uptake, as demonstrated by Matzger et al. [78,79] using the positron annihilation lifetime spectroscopy.Furthermore, CO 2 exhibits a significant quadrupole moment than N 2 , which makes it more susceptible to induce interactions with MOFs, such as association with open metal sites, involving in hydrogen bonding, etc. [80].In our case, we found that the TGA of the activated sample exhibits similar scaffold decomposition temperature as that of the as-synthesized (Figure 1), but its PXRD pattern becomes amorphous (Figure S2), suggesting some collapse of the crystal under the activation condition and may ultimately lead to the reduction of the pore size to exclude the N 2 uptake.MOF 1 exhibited a type I CO 2 adsorption profile with steep uptake in the low-pressure region, typical of a microporous material with open channels in the degassed phase [45,[81][82][83].The CO 2 adsorption isotherm for MOF 1 exhibits a negligible hysteresis upon desorption, an indication of the scaffold rigidity.While being fully aware that N 2 is the most utilized probe molecule to estimate the BET surface areas, other probes, such as Ar and CO 2 , may also serve as useful alternative to determine the BET surface areas, particularly for those MOFs featuring ultramicropores [84,85].The Brunauer-Emmett-Teller (BET) surface area of MOF 1 can be roughly estimated based on the CO 2 adsorption data in the linear range (P/P 0 < 0.07) to be 155 m 2 g −1 .MOF 1 also adsorbs up to 58.61 cm 3 g −1 CO 2 (2.63 mmol g −1 , 11.6 wt % at standard temperature and pressure, STP), comparable with the respective data found for the 3D MOFs of [Zn(iso-hmn)] (1.63 mmol g −1 ; 7.23 wt % at STP; iso-hmn = 2-(hydroxymethyl)isonicotinate) [81], [Cd 3 (BTB) 2 (bpe)(H 2 O) 2 ] (3.94 mmol g −1 ; 17.3 wt % at STP; bpe = trans-1,2-bis(4-pyridyl)ethylene) [45], and Mn(2,6-ndc) (3.0 mmol g −1 ; 13.0 wt % at STP; ndc = 2,6-naphthalenedicarboxylate) [86].It is also notable that the activated samples of MOF 1 exhibit nearly no CO 2 uptake at 273 K and 298 K (Figure 4).

General
Co(NO 3 ) 2 •6H 2 O (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China, ≥98.5%), benzene-1,3,5-tribenzoic acid (H 3 BTB, >98.0%,TCI), N,N-diethylformamide (DEF, >99.0%,TCI) were commercially available and used without further purification.IR spectrum was measured on a Varian 1000 FT-IR spectrometer (Varian, Inc., Palo Alto, CA, USA) as KBr disks (400-4000 cm −1 ).Elemental analyses for C, H, and N were carried out on a Carlo-Erba CHNO-S microanalyzer (Carlo Erba, Waltham, MA, USA).The Thermogravimetric Analyzer was used at a heating rate of 5 • C min −1 under a nitrogen gas flow in an Al 2 O 3 pan.Powder X-ray diffraction (PXRD) patterns were recorded with a Bruker D8 GADDS (General Area Detector Diffraction System) micro-diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) equipped with a VANTEC-2000 area detector (Bruker AXS GmbH, Germany) with Φ rotation method.The gas adsorption-desaorption isotherms were recorded using BELSORP-max (MicrotracBEL Corp., Osaka, Japan).The sample was air dried in the fume hood and exchanged with CHCl 3 twice before transferring to the instrument for activation.The sample was heated to proper temperature under a vacuum of 10 −2 kPa for 36 h.The evacuated sample tube was weighed again, and the sample mass was determined by subtracting the mass of the previously.The N 2 and CO 2 isotherms were measured using a liquid nitrogen bath (77 K), dry ice/acetone (195 K), and cooling machines EtOH/H 2 O (v/v = 1:1) as coolant (273 K and 298 K), respectively.

X-Ray Crystallography for MOF 1
The single crystal of MOF 1 was analyzed on a Bruker D8 Quest CCD X-ray diffractometer with graphite monochromated Mo Kα (λ = 0.71073 Å) radiation.Refinement and reduction of the collected data were achieved by the program Bruker SAINT with absorption correction (multi-scan) applied [87].The structure was solved by direct method and refined on F 2 by full-matrix least-squares techniques with SHELXTL-2013 [88].In 1, each BTB ligand contains one disordered phenyl group with relative ratios of 0.56/0.in the lattice was found but acceptable refinement results could not be obtained for this electron density.The solvent contribution was then modeled using SQUEEZE in the Platon program suite [89].Crystallographic data for MOF 1 has been deposited in the Cambridge Crystallographic Data Center (CCDC) as supplementary publication number of 1,826,274.These data can be obtained free of charge either from the CCDC via www.ccdc.cam.ac.uk/data_request/cif (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk) or from the Supporting Information.A summary of the key crystallographic data is listed in Table 1, and the selected bond lengths are listed in Table 2.

Conclusions
In summary, we have prepared and characterized a porous 3D MOF (MOF 1) sustained by a Co 3 and two types of Co 2 mixed cluster SBUs.The activated sample of MOF 1 exhibits CO 2 gas uptake at 195 K and permanent porosity.In contrast, MOF 1 showed negligible N 2 uptake at 77 K, as well as CO 2 at 273 K and 295 K, indicating the collapse of the MOF scaffold, probably due to the improper activation.In our future work, we will employ a more effective activation method such as supercritical CO 2 , to obtain MOFs with high surface areas.The ionic nature of MOF 1 with solvent-like or weakly associated NO 3 − in its pores coupled with its wide pore opening may further allow solution-based anion exchange for the removal of environmentally relevant anions, such as TcO induced by better size match between these anions and the host framework.We are currently exploring this anion exchange possibility of MOF 1.

Figure 1 .
Figure 1.The thermogravimetric analysis (TGA) curves of the as-synthesized and activated samples of metal−organic framework (MOF) 1 showing the continuous weight loss before ca.500 • C.

Figure 2 .
Figure 2. The powder X-ray diffraction (PXRD) of MOF 1 showing the simulated (black) and the experimental (red) patterns.

Figure 3 .
Figure 3.The structure of MOF 1 showing the Co 3 secondary building unit (SBU) supported by Co1 and Co2 viewing from two directions (a,b), the Co 2 SBU supported by Co3 and Co4 (c), the Co 2 SBU supported by Co5 and Co6 (d), the 3D network viewing along the [010] direction (e), and the 3D network viewing from the crystallographic a direction (f).All H atoms are omitted.For (a-e), only one of the disordered portion of the BTB ligand is depicted (H 3 BTB = 1,3,5-tris(4-carboxyphenyl)benzene), and for (c,d), only one disordered portion of the coordinated anion/solvent is depicted.Color legends: Co (magenta), O (red), N (blue), C (black).

Figure 4 .
Figure 4. N 2 (77 K) and CO 2 (195 K, 273 K, and 298 K) sorption isotherms for MOF 1 with pressures of up to 1 bar.The black square and red circle represent CO 2 absorption and desorption at 195 K, the blue and pink triangles represent CO 2 absorption and desorption at 273 K, the green square and the dark blue triangle represent CO 2 absorption and desorption at 298 K, while the purple triangle and dark purple hexagon represent N 2 absorption and desorption.P 0 is the saturated vapor pressure of the adsorbates at the measurement temperatures.

Table 1 .
Crystallographic data and refinement parameters for MOF 1.

Table 2 .
Selected bond distances of MOF 1 involving the Co centers.
Symmetry transformations used to generate equivalent atoms: # 44, 0.56/0.44,and 0.60/0.40refined for the two components.The Co3 coordination site is associated with 0.25 NO 3 Similar to that for Co3, each of the coordination site for Co5 and Co6 is associated with 0.33 NO 3 − (contains N4)/0.67H 2 O.In addition, each coordination site for both Co5 and Co6 is also associated with 0.50 DEF /0.50 H 2 O.A large amount of spatially delocalized electron density (1408 electrons)