Enriching Transmetalation Routes to Stable and Highly Porous Crystals of Chromium-Based Metal-Organic Frameworks

Among the rich chemical and structural diversity of metal-organic frameworks (MOFs), those based on trivalent chromium (Cr(III)-MOFs) have attracted significant interest due to their typically high stability. However, their synthesis as highly crystalline materials remains challenging due to chromium's relative inertness, ability to adopt multiple oxidation states, and intricate electronic interactions with ligands. This study presents an innovative transmetalation approach from cobalt- and indium-based MOFs, expanding beyond the previously reported routes, mostly limited to iron-based MOF precursors. Moreover, the transmetalation can be performed on cations with varied accessibility in clusters with diverse connectivity (6 to 9). By expanding the library of possible metal precursors, we additionally overcome some limitations encountered with the traditional iron-based MOFs while increasing the structural diversity of highly porous Cr-MOFs. Specifically, Co-sod-ZMOF-307 and In-soc-MOF-1, when transformed to their chromium analogs, exhibit significantly enhanced stability and porosity compared to their initial as-synthesized forms. Cr-soc-MOF-1 (from In), for instance, shows a substantial increase in porosity, reaching 1.9 cm³/g, with a BET of 5668 m²/g, while Cr-sod-ZMOF-307 (from In) stands as one of the most porous Cr-MOFs reported, with pore volume reaching 2.4 cm³/g.