Highly stable and porous porphyrin-based zirconium and hafnium phosphonates-electron crystallography as an important tool for structure elucidation

The Ni-metallated porphyrin-based tetraphosphonic acid (Ni-tetra(4-phosphonophenyl)porphyrin, Ni-H₈TPPP) was used for the synthesis of highly porous metal phosphonates containing the tetravalent cations Zr⁴⁺ and Hf⁴⁺. The compounds were thoroughly characterized regarding their sorption properties towards N₂ and H₂O as well as thermal and chemical stability. During the synthesis optimization the reaction time could be substantially decreased under stirring from 24 to 3 h in glass vials. M-CAU-30, [M₂(Ni-H₂TPPP)(OH/F)₂]·H₂O (M = Zr, Hf) shows exceptionally high specific surface areas for metal phosphonates of a_BET = 1070 and 1030 m² g^-1 for Zr- and Hf-CAU-30, respectively, which are very close/correspond to the theoretical values of 1180 and 1030 m² g^-1. CAU-30 is always obtained as mixtures with one mol ZrO₂/HfO₂ per formula unit as proven by TEM, electron diffraction, TG and elemental analysis. Hence experimentally derived specific surface areas are 970 and 910 m² g^-1, respectively. M-CAU-30 is chemically stable in the pH range 0 to 12 in HCl/NaOH and thermally up to 420 °C in air as determined by variable-temperature powder X-ray diffraction (VT-PXRD). The crystal structure of M-CAU-30 was determined by combining electron diffraction tomography for structure solution and powder X-ray diffraction data for the structure refinement. The crystal structure consists of chains of corner sharing MO₆ octahedra interconnected by the partly deprotonated linker molecules Ni-H₂TPPP^6-. Thus 1D channels with pore diameters of 1.3 × 2.0 nm are formed. The redox activity of Zr-CAU-30 was investigated by cyclic voltammetry resulting in a reversible redox process at a half-wave potential of E_1/2 = -0.649 V.