Cooperative photocatalysis of hafnium-based metal–organic framework and TEMPO for selective oxidation of sulfides

Metal–organic frameworks (MOFs) have been exploited through their metal–oxo clusters and organic ligands toward promising photocatalysis. In this work, a hafnium (Hf)-based MOF, Hf-NU-1000, consisted of Hf6(μ3-OH)8(-OH)8 and 1,3,6,8-tetrakis(p-benzoic acid)pyrene is assembled with biphenyl-4-carboxylic acid as a modulator. The metal–oxo cluster Hf6(μ3-OH)8(-OH)8 becomes the nexus for cooperative photocatalysis by anchoring a redox mediator, 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl (HOOC-TEMPO) to enhance the hole transfer over Hf-NU-1000. Adding 1 mol% HOOC-TEMPO could dramatically augment the selective oxidation of sulfides under the irradiation of blue light, resulting in conversions above 3 times that of pristine Hf-NU-1000. For the selective oxidation of various sulfides to sulfoxides, cooperative photocatalysis exhibits excellent durability and admirable applicability at ambient conditions. This work presents a novel perspective that MOFs can be an ideal platform to anchor a redox mediator for improving photocatalytic selective oxidations.