Metal–Organic Frameworks with Hexakis(4-carboxyphenyl)benzene: Extensions to Reticular Chemistry and Introducing Foldable Nets

Nine metal–organic frameworks have been prepared with the hexagon-shaped linker 1,2,3,4,5,6-hexakis(4-carboxyphenyl)benzene (H6cpb) by solvothermal reactions in dimethylformamide (dmf) or dimethylacetamide (dmac) with acetic acid or formic acid as modulators: [Bi2(cpb)(acetato)2(dmf)2]·2dmf CTH-6 forms a rtl-net; 2(H2NMe2)[Cu2(cpb)] CTH-7 forms a kgd-net; [Fe4(cpb)(acetato)2(dmf)4] CTH-8 and [Co4(cpb)(acetato)2(dmf)4] CTH-9 are isostructural and form yav-nets; 2(HNEt3)[Fe2(cpb)] CTH-10 and the two polymorphs of 2(H2NMe2)[Zn2(cpb)]·1.5dmac, Zn-MOF-888 and CTH-11, show kgd-nets; [Cu2(cpb)(acetato)2(dmf)2]·2dmf, CTH-12, forms a mixed coordination and hydrogen-bonded sql-net; and 2(H2NMe2)[Zn2(cpb)] CTH-13, a similarly mixed yav-net. Surface area values (Brunauer–Emmett–Teller, BET) range from 34 m2 g–1 for CTH-12 to 303 m2 g–1 for CTH-9 for samples activated at 120 °C in dynamic vacuum. All compounds show normal (10-fold higher) molar CO2 versus N2 uptake at 298 K, except the 19-fold CO2 uptake for CTH-12 containing Cu(II) dinuclear paddle-wheels. We also show how perfect hexagons and triangles can combine to a new 3D topology laf, a model of which gave us the idea of foldable network topologies, as the laf-net can fold into a 2D form while retaining the local geometry around each vertex. Other foldable nets identified are cds, cds-a, ths, sqc163, clh, jem, and tfc covering the basic polygons and their combinations. The impact of this concept on “breathing” MOFs is discussed. I2 sorption, both from gas phase and from MeOH solution, into CTH-7 were studied by time of flight secondary ion mass spectrometry (ToF-SIMS) on dried crystals. I2 was shown to have penetrated the crystals, as layers were consecutively peeled off by the ion beam. We suggest ToF-SIMS to be a method for studying sorption depth profiles of MOFs.


Synthesis of the linker
Hexakis(4-carboxyphenyl)benzene 5 (H 6 CPB) was prepared according to a literature-known procedure. 1 The synthesis route of H 6 CPB is shown in Figure S1. Methyl-4-iodobenzoate reacts in a first reaction step via Sonogashira cross coupling with trimethylsilylacetylene 2 to 1,4-bis(pcarbomethoxybenzene)-1,2-acetylene 3. Intermediate 3 reacted via cyclotrimerization reaction to ester 4, which was in a final step hydrolyzed to product 5. mL vial. This was followed by the addition of 1 mL of glacial acetic acid. The mixture was heated and stirred for few minutes. The solution was then transferred to a 10 mL Pyrex tube and heated at 120 ⁰C for 3 days to produce colorless rectangular crystals. All preparations of CTH-6 were contaminated with a small amount of concomitantly occurring black phase interpreted as Bi(s) and we were not able to get high yields of CTH-6. PXRD also indicated the presence of another minor phase. TGA shows solvent loss between 80-200°C that is difficult to quantify due to the impurities.
Both TGA and elemental analysis is consistent with a substantial amount of Bi formed.

Single Crystal X-ray Diffraction Analysis.
Single crystal x-ray diffraction data were collected on an Oxford Diffraction Xcalibur 3 system using ω-scans and Mo Kα (λ = 0.71073 Å) for CTH-6. Intensity data for CTH-7 and CTH-13 were collected with a Rigaku Synergy, Dualflex, AtlasS2 diffractometer using Cu Kα radiation (λ = 1.54184 Å) and the CrysAlis PRO Data reduction was conducted using the SAINT-Plus 4 software, and the absorption correction of the collection intensities were performed using the SADABS program. 5-6 Direct methods were S 7 used for all structures, the refinements were established by full-matrix least squares with SHELX-2018/3. 7 using the X-seed 8 platform as a graphical interface. All non-hydrogen atoms in all structures were found in the difference electron density map and refined anisotropically except in case of disordered molecules and the structures having abnormal anisotropic thermal parameters (CTH-9 and CTH-10). All hydrogens except the COOH, and the hydrogen attached to the amine nitrogen were placed with geometric constraints and refined isotropically.

Transmission Electron Microscopy Images
Scanning electron microscopy images of Zn-MOF-888 and CTH-11 were collected on a JEOL JEM-2100 TEM.    Figure S4. Two adjacent layers of CTH-11 (shown for two different data sets). The viewing is set to be perpendicular to the individual layers.

Thermal Analysis
Thermogravimetric analysis (TGA) data were collected on a Mettler Toledo TGA/DSC 3 + under a purge gas of air flow at a scan rate of 10 ⁰C min -1 . TGA of all samples were performed between 30 ⁰C and 800 ⁰C. CTH-6 TGA trace is shown in Figure S5 and the first step illustrates a weight loss of 2.62% which is associated with half an acetic acid molecule. A further 12.87% weight loss, corresponding to half acetic acid and two dmf molecules (calc; 2dmf + acetic acid = 16.14 %, experimental; 2dmf + acetic acid = 15.49 %). CTH-6 crystals were coated with dark grey moieties which correspond to Bi 2 O 3 and were difficult to separate to perform thermogravimetric analysis, so the Bi 2 O 3 mass was included when calculating the percentage weight loss of the solvents (dmf and acetic acid) in the framework. In CTH-7 ( Figure S6), there is a small % mass loss ranging from 100% to 90.77% (9.23%) corresponding to the loss of one dma molecule (calc; 8.91%) and a 78.42 % mass loss due to the linker (calc; 78.29 %). The CTH-8 TGA curve is illustrated in Figure S7 and the TGA trace shows a one step weight loss, (dmf and acetic acid solvents are coordinated to Fe atoms in the framework) corresponding to four dmf, two acetic acid and the linker. According to the thermogravimetric analysis of CTH-9 in Figure S8, the trace demonstrates a weight loss of 8.04 % corresponding to two acetic acid molecules (calc; 8.35 %). A further 20.56 % weight loss is observed, which is attributed to four dmf molecules (calc; 20.32 %). The rest of the mass loss in the curve is the % weight of the linker. The mass loss of 13.9 % (calc. 18,3 %) in S 12 CTH-10 ( Figure S9) is the loss of triethylamine (tea) molecules and then followed by the decomposition of the linker. The Zn-MOF-888 and CTH-11 trace is showed in Figure S10 with three weight loss steps: the first step is at 5.52%, the second at 6.32 % and the third at 7.46 % corresponding to 1.5 dmac molecules (calc; 22.35 %), all of these steps are within the 2% requirement between the calculated and experimental % weight loss. Three steps are demonstrated in the CTH-12 TGA curve ( Figure S11). The first step at 22.21 % is the release of two dmf molecules (calc; 21.9 0 %). The second step at 10.55 % is attributed to the acetic acid (calc; 9.0 %) and the last step to the decomposition of the linker. Figure S12 shows the TGA trace of CTH-13, where the first weight loss is 4.83% (calc; 3.43 %) corresponding to the coordinated water molecule, the second weight loss at 6.78% (calc; 8.58%) corresponding to the protonated dimethylamine in the framework then 75.36% (calc; 75.20%) which is the loss of the linker.

Gas Adsorption Measurements
Gas adsorption isotherms were obtained using a Micromeritics ASAP2020 surface area analyzer (Atlanta, Georgia, USA). Prior to the analysis, the samples were pre-treated by heating to 120 °C for 6 hours under dynamic vacuum (1 × 10-4 Pa) using a Micromeritics SmartVacPrep 067 instrument (Atlanta, Georgia, USA). The BET (SBET) and Langmuir surface area (SLang) values were calculated using the N 2 adsorption isotherms obtained at -196 °C (with data within a relative pressure between 0.05 and 0.15). CO 2 , and N 2 isotherms were also obtained at 20 °C using the same instrument.

Secondary Ion Mass Spectrometer
ToF-SIMS. ToF-SIMS analysis was performed using a TOFSIMS 5 instrument (ION-TOF GmbH, Münster, Germany). A 25 keV Bi 3+ cluster ion gun was used as the primary ion source and a 10 keV C60 + gun was used for etching and depth profiling. The samples were analyzed in the delayed extraction mode using a pulsed primary ion beam (Bi 3 2+ 0.2 pA at 50 keV) with a focus of approximately 400 nm. The mass resolution was approximately M/ΔM = 3000 fwhm at m/z 100.
Depth profile analysis was performed using a C 60 2+ beam at 20keV with a current of 0.18 nA in the non-interlaced mode with 1s of analysis, 1s of sputtering and a pause of 1s for each sputter cycle.
The maximum ion dose density of Bi 3 2+ was kept between 1×1012 and 5×1012 cm −2 over the whole depth profiling experiment, while the ion dose for C602+ ranged from 1x1014 to 6x1014 ions cm −2 . Low energy electrons were used for charge compensation during analysis. All spectra and images were analyzed using the Surface Lab software (version 6.