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, <b>fcu</b>, topology, <b>fcu-L-Co</b>, that are sustained by Co₈ molecular building blocks (MBBs) and dipyrazolate ligands, <b>L</b>. <b>fcu-L-Co</b> 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 <b>fcu-L-Co(III)</b> to aqueous hydrazine. <b>fcu-L-Co(II)</b> exhibited high gravimetric water vapor uptakes of 0.55-0.68 g g^-1 at 30% relative humidity (RH), while in <b>fcu-L-Co(III)</b> the inflection point shifted to lower RH and framework stability improved. Insight into the transformation between <b>fcu-L-Co(II)</b> and <b>fcu-L-Co(III)</b> was gained from single crystal X-ray diffraction and <i>in situ</i> 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.