Structures and properties of gallium-MOFs with MIL-53-topology based on aliphatic linker molecules

Abstract

Employing gallium nitrate and one of the aliphatic dicarboxylic acids trans-1,4-cyclohexanedicarboxylic acid (H2C8H10O4) or (+)-camphoric acid (H2C10H14O4), respectively, two new metal–organic frameworks exhibiting the MIL-53-topology were synthesised in N,N-dimethylformamide. All structural characterisations were carried out based on Powder-XRD data using a combination of structural relationship, force-field calculations and Rietveld refinement. The compound based on camphoric acid [GaOH(C10H14O4)] or Ga-MIL-53-Cam (1) crystallises in the polar orthorhombic space group P212121 (a=14.0453(3), b=12.0124(3), c=6.7658(2)Å) . Ga-MIL-53-Cam exhibits an unporous dense framework which is stable up to 280°C in air. The second compound [GaOH(C8H10O4)]·0.5DMF (2) or Ga-CAU-13-lp (lp stands for large pore) crystallises in the triclinic space group P-1 (a=6.7114(2), b=10.3019(3), c=9.6265(2)Å, α=110.743(2), β=106.883(3), γ=94.679(3)°). While this compound shows no porosity towards nitrogen at 77K, water molecules can be adsorbed into the pores upon exposure to air after thermal removal of the DMF molecules. The thus obtained [GaOH(C8H10O4)]·H2O (3) or Ga-CAU-13-np (np stand for narrow pore) exhibits the same space group symmetry as Ga-CAU-13-lp but with changed cell parameters (P-1, a=6.7200(2), b=9.2179(3), c=9.5082(3)Å, α=107.359(2), β=102.691(3), γ=97.913(2)°) and thus a breathing of the framework is observed, which is shown to be reversible. Moreover, large molecules like p-xylene can be adsorbed into the pores, underlining the compound’s porosity. The particular nature of the breathing effect is substantially different compared with that known for Ga-MIL-53 based on the aromatic terephthalic acid.