TEMPO-Appended Metal-Organic Frameworks as Highly Active, Selective, and Reusable Catalysts for Mild Aerobic Oxidation of Alcohols

Metal-organic frameworks (MOFs) decorated with stable organic radicals are highly promising materials for redox catalysis. Unfortunately however, the synthesis of chemically robust MOFs typically requires harsh solvothermal conditions, which are not compatible with organic radicals. Here, we describe the synthesis of two isoreticular families of stable, mixed component, zirconium MOFs with UiO-66 and UiO-67 structures and controlled amounts of covalently attached TEMPO radicals. The materials were obtained using a relatively low-temperature, HCl-modulated de novo method developed by Hupp and Farha and shown to contain large amounts of missing cluster defects, forming nanodomains of the reo phase with 8-connected clusters. In the extreme case of homoleptic UiO-67-TEMPO(100%), the material exists as an almost pure reo phase. Large voids due to missing clusters and linkers allowed these materials to accommodate up to 2 times more of bulky TEMPO substituents than theoretically predicted for the idealized structures and proved to be beneficial for catalytic activity. The TEMPO-appended MOFs were shown to be highly active and recyclable catalysts for selective aerobic oxidation of a broad range of primary and secondary alcohols under exceptionally mild conditions (room temperature, atmospheric pressure of air). The influence of various parameters, including the pore size and TEMPO content, on the catalytic activity was also comprehensively investigated.