[Al2(OH)2(TCPB)] – An Al-MOF based on a tetratopic linker molecule

• An isoreticular Al-MOF was obtained using an extended linker molecule.
• The permanently porous Al-MOF is thermally stable at least up to 480 °C in air.
• Each carboxylate group of the tetratopic linker coordinates Al octahedra bidentately.
• Distribution of 27Al quadrupolar coupling constants reflects framework flexibility.

High-throughput methods were employed in the discovery and synthesis optimization of the new microcrystalline metal-organic framework (MOF) [Al2(OH)2(TCPB)] · 2 DMF (denoted CAU-9as), which contains the tetratopic linker ion 1,2,4,5-tetrakis-(4-carboxylatophenyl)-benzene (TCPB4−). The compound is obtained under solvothermal reaction conditions and activation at 150 °C leads to the removal of the guest molecules (CAU-9act). Detailed characterization was carried out by FT-IR and NMR spectroscopy, thermogravimetric analysis, gas sorption measurements and X-ray powder diffraction. CAU-9act crystallizes in an orthorhombic unit cell (space group Cmmm) with the cell parameters: a = 31.562(4), b = 6.642(7), c = 10.6612(7) Å. Modeling by force field calculations in combination with Rietveld refinement was used to determine the structure of CAU-9. The content of the asymmetric unit was corroborated based on NMR crystallographic strategies evaluating high-resolution 1H, 13C and 27Al MAS NMR spectra. The structure of CAU-9 is isoreticular to the one of MIL-118B [Al2(OH)2(BTEC)] (BTEC = 1,2,4,5-benzenetetracarboxylate) and Al-PMOF [Al2(OH)2(TCPP)] (TCPP = meso-tetra(4-carboxylato-phenyl)porphyrin). According to the temperature dependent PXRD measurements and the thermogravimetric analysis, it is stable up to 480 °C in air. The nitrogen sorption measurement at 77 K has given a Brunauer–Emmett–Teller (BET) surface area of 1118 m2 g−1 and a micropore volume of 0.45 cm3 g−1. In contrast to MIL-118 no rearrangement of the carboxylate groups from a mono-to bidentate coordination mode was observed upon activation.

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