Expanding the Reticular Chemistry Building Block Library toward Highly Connected Nets: Ultraporous MOFs Based on 18-Connected Ternary, Trigonal Prismatic Superpolyhedra

The chemistry of metal–organic frameworks (MOFs) continues to expand rapidly, providing materials with diverse structures and properties. The reticular chemistry approach, where well-defined structural building blocks are combined together to form crystalline open framework solids, has greatly accelerated the discovery of new and important materials. However, its full potential toward the rational design of MOFs relies on the availability of highly connected building blocks because these greatly reduce the number of possible structures. Toward this, building blocks with connectivity greater than 12 are highly desirable but extremely rare. We report here the discovery of novel 18-connected, trigonal prismatic, ternary building blocks (tbb's) and their assembly into unique MOFs, denoted as Fe-tbb-MOF-x (x: 1, 2, 3), with hierarchical micro- and mesoporosity. The remarkable tbb is an 18-c supertrigonal prism, with three points of extension at each corner, consisting of triangular (3-c) and rectangular (4-c) carboxylate-based organic linkers and trigonal prismatic [Fe3(μ3-Ο)(−COO)6]+ clusters. The tbb’s are linked together by an 18-c cluster made of 4-c ligands and a crystallographically distinct Fe3(μ3-Ο) trimer, forming overall a 3-D (3,4,4,6,6)-c five nodal net. The hierarchical, highly porous nature of Fe-tbb-MOF-x (x: 1, 2, 3) was confirmed by recording detailed sorption isotherms of Ar, CH4, and CO2 at 87, 112, and 195 K, respectively, revealing an ultrahigh BET area (4263–4847 m2 g–1) and pore volume (1.95–2.29 cm3 g–1). Because of the observed ultrahigh porosities, the H2 and CH4 storage properties of Fe-tbb-MOF-x were investigated, revealing well-balanced high gravimetric and volumetric deliverable capacities for cryoadsorptive H2 storage (11.6 wt %/41.4 g L–1, 77 K/100 bar–160 K/5 bar), as well as CH4 storage at near ambient temperatures (367 mg g–1/160 cm3 STP cm–3, 5–100 bar at 298 K), placing these materials among the top performing MOFs. The present work opens new directions to apply reticular chemistry for the construction of novel MOFs with tunable porosities based on contracted or expanded tbb analogues.

Powder X-Ray Diffraction Patterns were collected using a Panalytical X'pert Pro MPD System Cu Kα (λ=1.5418Å) radiation operated at 45 kV and 40 mA.A typical scan rate was 3 sec/step with a step size of 0.02 deg.

Single Crystal X-Ray Diffraction
In house single crystal X-ray diffraction data were collected on a Bruker D8 Venture diffractometer equipped with a Cu Incoatec microfocus IμS 3.0 source, a Photon II detector operating in shutterless mode and a cryostrem 800 system (Oxford Cryosystems) for temperature regulation.
Synchrotron Single Crystal X-ray Diffraction was performed on the PROXIMA 2A micro-focused beamline in SOLEIL synchrotron (λ = 0.729319 Å) using an EIGER X9M 2D hybrid photon counting detector.Data were collected at 200 K. Subsequent data integration and reduction were undertaken with Xia2 (Winter, G., xia2: an expert system for macromolecular crystallography data reduction.J. Appl.Crystallogr.2009,  43, 186-190).No corrections for solvent were applied.The structure was solved using the direct method and refined by full-matrix least-squares on F2 by the SHELXTL-2014 software package.The disorder was modeled using standard crystallographic methods, including constraints, restraints and rigid bodies where necessary.All carbon-bound hydrogen atoms were added in idealized positions and refined using a riding model.Several restraints were used to obtain reasonable parameters.H-atoms were refined isotropically, while the other atoms were refined anisotropically.All the phenyl rings are constrained to the ideal six-membered ring.The solvent molecules were highly disordered, and attempts to locate and refine the solvent peaks were unsuccessful.Contributions from the solvent molecules were removed using the SQUEEZE routine of PLATON, structures were then refined again using the data generated under Olex2-1.5 (Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. J. Appl.Crystallogr.2009, 42, 339-341).The contents of the solvent region were not represented in the unit cell contents in the crystal data.Crystal data and details of the data collection are given in Table S1.
Scanning electron microscopy (SEM) images were collected on a field emission JSM-IT700HR instrument.

1
H NMR spectra were recorded on a 500MHz Bruker spectrometer.MOF samples were prepared by digesting a small portion (~2mg) of the acetonitrile exchanged solids with a drop of concentrated HCl acid (37%) in a DMSO-d6 solution.
Gas sorption measurements at low pressures.Low Pressure nitrogen, argon, carbon dioxide and methane gas sorption measurements were carried at different temperatures up to 1 bar using Autosorb-iQ2 instrument from Quantachrome equipped with a cryocooler system capable of temperature control from 20 to 320 K. Prior to analysis the as made samples were washed with warm N,Ndimethylformamide five times per day for 2 days to remove any unreacted starting materials from the pores.Then the samples were soaked in warm acetonitrile (40 o C) over a period of 5 days, replenishing the acetonitrile 4 times per day.Finally, the wet samples were transferred to 6 mm sample cells using a pipette and activated under dynamic vacuum at 80 o C for 12 hours until the outgas rate was less than 2 mTorr/min.After evacuation, the samples were weighed to obtain their precise mass and the cells were transferred to the analysis port of the gas adsorption instrument.
Thermogravimetric analyses (TGA), were performed using a TA Instrument TGA 5500.An amount of approximately 10 mg of activated Fe-tbb-MOF-x was placed inside a quartz cap and heated up to at least 700 o C under N2 flow with a heating rate of 5 o C/min.
Compound 2. To a 20ml Teflon-lined stainless steel autoclave, compound 1 (0.20 gr, 0.34 mmol) was mixed with 65% HNO3 (1 ml) and H2O (6 ml).The autoclave was sealed and heated to 180 o C for 24h.Then, the reaction mixture was cooled down to room temperature and the resulting orange powder was collected through filtration, washed thoroughly with water and dried under vacuum.Yield: 45%. 1   The tritopic carboxylate ligand H3TATB (4,4',4''-s-triazine-2,4,6-triyltribenzoic acid) was synthesized according to previous literature reports with slight modifications. 2 The precise synthetic procedure is described below (Scheme 2).Compound 3. To a dried under Ar double neck round bottom flask, AlCl3 (10.0 gr, 75.0 mmol) and dry toluene (25 ml) were added.The mixture was gently heated to 60 o C and then cyanuric chloride (4.15 gr, 22.5 mmol) was added in 12 portions over the course of an hour.The resulting mixture was stirred overnight.After that period, the reaction was quenched with ice and stirred for an additional 30 minutes.The resulting mixture was extracted with CHCl3 (2 x 25 ml).The combined organic layers were concentrated in vacuo.The crude product was recrystallized from toluene to afford a white needle-like crystalline solid.Yield: 58%. 1 H NMR (500 MHz, CDCl3): 8.66 (d, 6H), 7.37 (d, 6H), 2.48 (s, 9H).Compound 4. To a dried under Ar double neck round bottom flask, compound 3 (0.20 gr, 0.57 mmol) was dissolved in acetic acid (12 ml) and H2SO4 (0.5 ml).The solution was cooled using an ice bath and chromium oxide (0.45 gr) and acetic anhydride (0.7 ml) were slowly added to the flask.The resulting dark-green reaction mixture was warmed to room temperature and stirred overnight.After that period, the reaction was quenched with ice, stirred for an additional 60 minutes and finally filtered.The offwhite solid was washed thoroughly with water and then dissolved in an aqueous solution of NaOH (~10 ml, 2 M).Residues from unreacted starting material were removed by filtration and the resulting solution was acidified with an aqueous solution of HCl (until pH<2) to produce a white precipitate which was filtered, washed with water and dried under vacuum.Yield: 84%. 1   The tritopic carboxylate ligand H3PTB (4,4',4''-(Pyridine-2,4,6-triyl)tribenzoic acid) was synthesized according to previous literature reports with slight modifications. 3The precise synthetic procedure is described below (Scheme 3).Compound 5. To a 50 ml round bottom flask, 4-methylacetophenone (0.57 gr, 4.25 mmol), ptolualdehyde (0.50 gr, 4.16 mmol) and an aqueous solution of NaOH 0.5 N (15 ml) were added.The mixture was vigorously stirred at room temperature for 60 minutes and then heated to 60 o C overnight.After that period, the mixture was cooled down to room temperature and the yellow precipitate was filtered, washed several times with water and dried under vacuum.Yield: 87%. 1   Compound 6.Using a mortar and a pestle, compound 5 (1.45 gr, 6.14 mmol), 4-methylacetophenone (0.830 gr, 6.19 mmol) and powder NaOH (0.970 gr, 24.3 mmol) were ground together for 2 hours.The resulting yellow tacky solid was transferred to a 250-ml round bottom flask, containing a solution of ammonium acetate (7.00 gr, 90.8 mmol) in ethanol (80 ml).The reaction mixture was heated to reflux and stirred overnight.After that period, the mixture was cooled down to room temperature, filtered and washed several times with EtOH and the filtrate was concentrated in vacuo.The crude residue was purified by flash column chromatography (silica gel, petroleum ether : EtOAc = 90:1).Yield: 35%. 1   Compound 7. To a 20ml Teflon-lined stainless steel autoclave, compound 6 (0.10 gr, 0.29 mmol) was mixed with 65% HNO3 (0.5 ml) and H2O (3 ml).The autoclave was sealed and heated to 180 o C for 24h.Then, the reaction mixture was cooled down to room temperature and the resulting orange powder was collected through filtration, washed thoroughly with water and dried under vacuum.Yield: 65%. 1   The tritopic carboxylate ligand H3BTB (4,4',4''-benzene-1,3,5-triyl-tribenzoate) was synthesized according to previous literature reports with slight modifications. 4The precise synthetic procedure is described below (Scheme 4).Compound 8. To a dried under Ar double neck round bottom flask, acetyl chloride (25 ml, 0.35 mmol) and AlCl3 (5 gr, 37.5 mmol) were added.The content of the flask was cooled and retained at 0 o C for 15min.After that period, a solution of 1,3,5 triphenyl benzene (1.5 gr, 4.9 mmol) dissolved in 20 ml CH2Cl2 was added to the flask.The mixture was stirred under argon at 0 o C for 15min and then at 25 o C for 90min.During that time, the mixture's color turned into deep red.The mixture was transferred in a flask which contained ~ 100ml of ice.The solution, which became yellow, was extracted with CH2Cl2 (x3).The organic layers were combined and washed with saturated aqueous solution of NaHCO3 and then dried over MgSO4, filtered and concentrated under reduced pressure.Yield: 87%. 1   Compound 9. Compound 8 (0.5 gr, 1.16 mmol) was dissolved in 1,4-dioxane (25 ml).The solution was placed in a 100ml round bottom flask.At the same time, NaOH (1.8 gr, 45 mmol) was dissolved in H2O (12 ml) in a 50 ml beaker.The solution in the beaker was cooled at 0 o C using an ice bath and then Br2 (0.9 ml, 17.6 mmol) was added.The new solution was stirred at 0 o C for 15min.Then, the solution was added gently to the round bottom flask and the mixture was heated at 65 o C for 2.5h.After completion of the reaction, the mixture was cooled at room temperature and Na2S2O3•5H2O (0.3 gr) and c.HCl (4 ml) were added.The resulting solid was filtered, washed with H2O and dried under vacuum.Yield: 75%.
Subsequently the derived virial coefficients αi and bi were used in order to determine heat of adsorption for zero coverage (Qst0) as well as the heat of adsorption as a function of the total adsorbed amount according to the following two equations:     Pressure, bar           Uptake, mg g

Figure S13 .
Figure S13.Representative SEM images of the hexagonal crystals of Al-tbb-MOF-1 obtained as a mixed phase with Al-soc-MOF as revealed by PXRD.

Figure S15 .
Figure S15.The structure of Fe-ttb-MOF-1 looking down the b-axis (left) and slightly off the ac-plane (right).

Figure S16 .
Figure S16.a) Regular soc-type cuboidal cages observed in Fe-pbpta (see ref. #36 in the manuscript) and b) distorted cuboidal cages in Fe-tbb-MOF-1.Both cages are made from eight Fe3(μ3-Ο)(-COO)6 clusters occupying the corners and six PBPTA 4-ligands.As described in the manuscript, the origin of the distortion in the case of Fe-tbb-MOF-1, originates from the relative arrangement of PBPTA ligands in neighboring cuboidal faces.Yellow spheres denote the empty space.c) The structure of Fe-pbpta made of edge-shared regular soc-type, cuboidal cages.

Figure S19 .
Figure S19.Hypothetical acs type MOF constructed from corner-shared tbb's using suitable angular dicarboxylate bridges.

-Figure S21 .
Figure S21.Topological analysis of Fe-tbb-MOF-1 where the 4-connected PBPTA linker is considered as two interconnected 3-c building units (a-c).d) The derived non-augmented tft and e) the augmented tft-a (3,3,3,3,6,6)-c 6-nodal net.The augmented tbb, soc-type and mesopore cages are shown in (f), (g) and (i), respectively.The simplified non-augmented tft net (d) is obtained by reducing the 6-c Fe3O(CO2)6-a and Fe3O(CO2)6-b clusters to 6-c orange and purple nodes respectively, the 3-c PTB ligand to a 3-c yellow node, the 4-c PBPTA-a to two blue interconnected 3-c triangular nodes (b), while the 4-c PBPTA-b to two interconnected distinct 3-c nodes, green and dark red, shown in (c).It is noted that the reduced 4-c PBPTA-a linker bridges four different orange nodes within the same tbb unit (b), while the reduced 4-c PBPTA-b linker bridges three orange 6-c nodes from three different tbb units and one purple 6-c node (c).For this reason, the topological analysis considering the 4-c PBPTA linkers as interconnected 3-c units, results in three different 3-c nodes, one blue, one green and one dark red.Adding to this the triangular 3-c yellow node from the reduction of the 3-c PTB ligand and the two 6-c orange and purle nodes from the reduction of the 6-c Fe3O(CO2)6 clusters, the resulting 6-nodal tft net has a connectivity of (3,3,3,3,6,6)-c.

Figure S24 . 1 H
Figure S24.(Top) Experimental PXRD pattern of as-made Al-tbb-MOF-1 obtained by a post-synthetic metal exchange reaction from Fe-tbb-MOF-1, along with the calculated pattern from the single crystal structure of the latter.(Middle) Representative Al-tbb-MOF-1 crystals obtained after the post-synthetic metal exchange reaction.(Bottom) The complete metal exchange reaction was confirmed by SEM/EDS analysis, showing the absence of Fe.Details will be published elsewhere.

Figure
Figure S29.a) BETSI analysis for Fe-tbb-MOF-2 and b) the corresponding regression diagnostics.

Figure S37 :
Figure S37: H2 isosteric heat of adsorption in Fe-tbb-MOF-1 as a function of coverage

Figure S45 :
Figure S45: H2 isosteric heat of adsorption in Fe-tbb-MOF-2 as a function of coverage

Figure S50 .
Figure S50.Gravimetric and volumetric CH4 deliverable capacity under pressure swing conditions for Fe-tbb-MOF-2 and representative best performing MOFs.

Table S2 .
Total pore volume of Fe-tbb-MOF-x (x: 1, 2, 3) at 0.99 p/p0 obtained from the corresponding gas adsorption isotherm recorded at the respective boiling point.