Ba-MOFs with tetrazole-based acetic acids: unusual configuration, novel topology and high proton conductivity

Metal-organic frameworks (MOFs) as proton-conductive materials have attracted increasing attention due to their applications in proton-exchange membrane fuel cells. While the majority of the MOFs are based on transition metals, group II metals with unusual configurations have received relatively less attention. In this work, we selected three tetrazole-based acetic acids, N,N-di(2-carboxymethyltetrazol-5-yl)amine (H₂L¹), 1,3,5-tri(2-carboxymethyltetrazol-5-yl)benzene (H₃L²), and 4,5-di(tetrazol-5-yl)imidazolylacetic acid (H₃L³), as ligands and prepared three MOFs with barium(ii) ions: [Ba₂(L¹)₂(H₂O)₅]·2H₂O (1), (Me₂NH₂)[Ba(L²)(H₂O)]·3H₂O (2), and [Ba₃(μ₂-H₂O)(L³)₂(H₂O)₃]·2H₂O (3). Ba-MOFs have been unambiguously characterized by elemental, FT-IR spectroscopy and single-crystal X-ray diffraction analyses. In the solid state, 2D MOF 1 with two nine-coordinated Ba(ii) centers shows a rare hula hoop-like geometry at the Ba1 atom and a distorted tricapped trigonal prismatic geometry around the Ba2 one. Unusual sphenocorona geometry is also found in MOF 2 with a ten-coordinated Ba(ii) ion. In MOF 3, three different Ba(ii) ions coexist with two nine-coordinated Ba1 and Ba2 displaying a muffin-like configuration and a distorted tricapped trigonal prism geometry, respectively, and an eight-coordinated Ba3 having a distorted biaugmented trigonal prism geometry. In addition, MOF 2 exhibits an unprecedented trinodal 3,7,7-connected 3D network with the Schläfli symbol (3⁷·4⁶·5²·6²·7⁴)(3⁷·4⁶·5²·6³·7³)₂(6³) and MOF 3 displays a novel trinodal 4,5,9-connected 3D framework with the Schläfli symbol (4²¹·6¹⁵)(4⁵·6)(4⁸·6²). Due to the presence of extensive hydrogen bonded networks consisting of dimethyl ammonium and water molecules in the 1D channels, MOF 2 shows a high proton conductivity of 4.47 × 10^-3 S cm^-1 at 85 °C and 98% relative humidity (RH).