Abstract
Palladium catalyst turnover by reoxidation of a low-valent Pd species dominates the proceeding of an efficient oxidative transformation, but the state-of-the-art catalysis approaches still have great challenges from the perspectives of high efficiency, atom-economy and environmental-friendliness. Herein, we report a new strategy for addressing Pd reoxidation problem by the fabrication of spatially proximate IrIII photocatalyst and PdII catalyst into metal-organic framework (MOF), affording MOFs based Pd/photoredox catalysts UiO-67-Ir-PdX2 (X = OAc, TFA), which are systematically evaluated using three representative Pd-catalyzed oxidation reactions. Owing to the stabilization of single-site Pd and Ir catalysts by MOFs framework as well as the proximity of them favoring fast electron transfer, UiO-67-Ir-PdX2, under visible light, exhibits up to 25 times of Pd catalyst turnover number than the existing catalysis systems. Mechanism investigations theoretically corroborate the capability of MOFs based Pd/photoredox catalysis to regulate the competitive processes of Pd0 aggregation and reoxidation in Pd-catalyzed oxidation reactions.
Highlights
Palladium catalyst turnover by reoxidation of a low-valent Pd species dominates the proceeding of an efficient oxidative transformation, but the state-of-the-art catalysis approaches still have great challenges from the perspectives of high efficiency, atom-economy and environmental-friendliness
The merger of photocatalyst and transition metal catalysts into metalorganic framework (MOF) have been reported recently, and this elegant methodology has demonstrated their successful applications in photocatalytic water splitting[27,28,29], CO2 reduction[30,31,32], and organic transformations[33,34,35,36] by promoting electron transfer and stabilizing active intermediates
The poly(pyridine)-IrIII complexes have won profound reputation as photosensitizers (PS) because they allow the visible lightmediated charge separation with long-lived excited states, and can be facilely modulated through the ligand design to improve their photophysical properties, which made them extensively studied in varieties of photocatalytic transformations[37]
Summary
Palladium catalyst turnover by reoxidation of a low-valent Pd species dominates the proceeding of an efficient oxidative transformation, but the state-of-the-art catalysis approaches still have great challenges from the perspectives of high efficiency, atom-economy and environmental-friendliness. Compared with stoichiometric oxidants, the catalytic amounts of ETMs or photocatalyst in solution reduce the chance to interact with Pd0 species, and the overall Pd catalyst turnover efficiency is unsatisfactory For these reactions, the Pd reoxidation still remains problematic in current Pd-catalyzed oxidation reactions, and high Pd loadings (e.g., 10 mol%) are inevitably involved in most catalytic systems. Low Pd catalyst consumption is sufficient to support an efficient oxidative transformation when the reoxidation rate of Pd0 species exceeds its aggregation rate, and this requires an effective strategy to both accelerate the reoxidation and restrain the Pd0 aggregation processes in the catalytic cycle In this context, metal-organic frameworks (MOFs) as highly porous and tunable platform would be a judicious selection[21,22,23,24,25,26]. The welldefined structures of MOFs can provide facile opportunity to reveal the stepwise electron transfer process between Pd, photocatalyst, and O2, which further gives insight for the elucidation of the Pd reoxidation pathway
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