The iron member of the CPO-27 coordination polymer series: Synthesis, characterization, and intriguing redox properties

The microporous coordination polymer CPO-27-Fe was synthesized from iron salts and 2,5-dihydroxyterephthalic acid by microwave assisted solvothermal synthesis. The crystal structures of the as-synthesized compounds were determined by Rietveld refinement from powder X-ray diffraction data using synchrotron radiation, revealing a honeycomb-type framework, isostructural to the other compounds in the CPO-27-M series. Exposure to oxygen was found to have pronounced effects on the material, like change of color, band gap, and structural details which we associate with oxidation of the iron(II) in the M2(dhtp) framework to iron(III). XPS measurements confirm the presence of iron in these oxidation states in the respective compounds. The desolvation process of CPO-27-Fe was investigated using variable temperature powder X-ray diffraction and mass spectrometry. CPO-27-Fe passes through several phase transitions when heated up during which it reversibly changes between oxidation states +2 and +3, remaining in divalent state in the empty framework structure Fe2(dhtp) in the last crystalline phase. These measurements also indicate that methanol contained in the pore after synthesis is transformed into formaldehyde during the heating process, potentially making CPO-27-Fe a viable catalyst in redox processes. The effect of the extraordinary high concentration of accessible open metal sites in the desolvated CPO-27-Fe was investigated by gas adsorption experiments using hydrogen, carbon dioxide and oxygen. Oxygen adsorption was reversible at low temperatures, but exposure to oxygen at room temperature led to blocking of the open metal site and partial deconstruction of the framework. Significantly larger amounts of oxygen than nitrogen are adsorbed at room temperature.

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► The coordination polymer CPO-27-Fe contains a high concentration of open metal sites.
► The coordinatively unsatured metal sites affect the adsorption properties.
► The material might be useful in nitrogen–oxygen separation.
► The iron(II) in the framework is redox-active.
► Repeated changes of oxidation state are observed upon heating.