Fe/Ga-CFA-6 – metal organic frameworks featuring trivalent metal centers and the 4,4′-bipyrazolyl ligand

The synthesis and crystal structures of the new porous coordination polymers M-CFA-6 (M = Ga, Fe) are described. The structure motif of the M-CFA-6 framework (termed Coordination Framework Augsburg University-6, CFA-6) is closely related to that of the MIL-53 due to its octahedrally coordinated metal centers, bridging hydroxyl groups and the bifunctional 4,4′-bipyrazolyl ligand. Structural properties of the compounds were obtained via XRPD and single-crystal diffraction methods. Ga-CFA-6 and Fe-CFA-6 are isomorphous and crystallize in the orthorhombic crystal system within the space group Imma (no. 74), with the following unit cell parameters: Ga-CFA-6, a = 14.8281(16) A, b = 6.4872(5) A, c = 11.3503(12) A, V = 1091.82(19) A3; Fe-CFA-6·0.6DMAc, a = 14.8424(19) A, b = 6.6238(9) A, c = 11.7467(18) A, V = 1154.9(3) A3. Coordination polymers M-CFA-6 were characterized by elemental and thermogravimetric analyses. Variable temperature powder X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy and BET measurements confirmed the stability of the frameworks up to 250 and 300 °C (Ga-CFA-6 and Fe-CFA-6, respectively) and the porous characters of these compounds. The connectivity of the framework and symmetry of the space group were confirmed by MAS-NMR spectroscopy of Ga-CFA-6. Mossbauer spectroscopy and magnetic measurements were applied to determine the oxidation state of the iron centers in Fe-CFA-6.