Porous hexacyanocobaltates(III): Role of the metal on the framework properties

The extended porous framework of divalent transition metal hexacyanocobaltates(III) was studied from the refined crystal structures and adsorption isotherms of H₂O, CO₂ and N₂. From the obtained adsorption data the pore accessibility, pore volume, adsorption potentials and nature of the guest-host interactions were evaluated. The properties of the porous framework are modulated by the metal used to form the 3D framework from the elemental building block, the hexacyanocobaltate(III) ion. From that fact, this family of microporous compounds can be considered as tunable zeolites, with a pore volume and a system of pore windows appropriate for separation and storage of small molecules. The adsorption isotherms also reveal that the electric field gradient at the pore surface and the pore accessibility are determined by the metal linked at the N end of the CN groups. These compounds are usually obtained as hydrates. The dehydration process and the thermal stability were studied from thermo-gravimetry combined with X-ray diffraction. The crystal water is lost below 100 °C and then the anhydrous structure remains stable, preserving its porous features, up to 250 °C. Upon water removal a progressive cell contraction which amounts 4% of cell volume reduction was observed.