Reversible Co(II)-Co(III) Transformation in a Family of Metal-Dipyrazolate Frameworks

Transformation between oxidation states is widespread in transition metal coordination chemistry and biochemistry, typically occurring in solution. However, air-induced oxidation in porous crystalline solids with retention of crystallinity is rare due to the dearth of materials with high structural stability that are inherently redox active. Herein, we report a new family of such materials, four isostructural cobalt-pyrazolate frameworks of face-centered cubic, fcu, topology, fcu-L-Co, that are sustained by Co₈ molecular building blocks (MBBs) and dipyrazolate ligands, L. fcu-L-Co were observed to spontaneously transform from Co(II)₈ to Co(III)₈ MBBs in air with retention of crystallinity, marking the first such instance in metal-organic frameworks (MOFs). This transformation can also be achieved through water vapor sorption cycling, heating, or chemical oxidation. The reverse reactions were conducted by exposure of fcu-L-Co(III) to aqueous hydrazine. fcu-L-Co(II) exhibited high gravimetric water vapor uptakes of 0.55-0.68 g g^-1 at 30% relative humidity (RH), while in fcu-L-Co(III) the inflection point shifted to lower RH and framework stability improved. Insight into the transformation between fcu-L-Co(II) and fcu-L-Co(III) was gained from single crystal X-ray diffraction and in situ spectroscopy. Overall, the crystal engineering approach we adopted has afforded a new family of MOFs that exhibit cobalt redox chemistry in a confined space coupled with high porosity.