Single-Crystal to Single-Crystal Transformation of a Nonporous Fe(II) Metal–Organic Framework into a Porous Metal–Organic Framework via a Solid-State Reaction

We report the synthesis of an air-stable nonporous coordination compound based on iron(II) centers, formate anions, and a 4,4′-bipyrazole (H2BPZ) ligand. Upon thermal treatment, a porous metal–organic framework(MOF) formed due to decomposition of the incorporated formate anions. This decomposition step and the following structural changes constituted a single-crystal to single-crystal transformation. The resulting [Fe(BPZ)] framework contained tetrahedrally coordinated iron(II) metal centers. The framework was sensitive toward oxidation by molecular oxygen even at temperatures of 183 K, as followed by oxygen sorption measurements and a color change from colorless to metallic black. The semiconductor properties of the oxidized material were studied by diffuse reflectanceUV/vis/NIR spectroscopy and dielectric spectroscopy.