RESUMEN
Cleavage of strong C-O bonds without breaking C-C/C-H bonds is a key step for catalytic conversion of renewable biomass to hydrocarbon feedstocks. Herein we report multistep sequential engineering of orthogonal Lewis acid and palladium nanoparticle (NP) catalysts in a metal-organic framework (MOF) built from (Al-OH)n secondary building units and a mixture of 2,2'-bipyridine-5,5'-dicarboxylate (dcbpy) and 1,4-benzenediacrylate (pdac) ligands (1) for tandem C-O bond cleavage. Ozonolysis of 1 selectively removed pdac ligands to generate Al2(OH)(OH2) sites, which were subsequently triflated with trimethylsilyl triflate to afford strongly Lewis acidic sites for dehydroalkoxylation. Coordination of Pd(MeCN)2Cl2 to dcbpy ligands followed by in situ reduction produced orthogonal Pd NP sites in 1-OTf-PdNP as the hydrogenation catalyst. The selective and precise transformation of 1 into 1-OTf-PdNP was characterized step by step using powder X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, inductively coupled plasma mass spectrometry, infrared spectroscopy, and X-ray absorption spectroscopy. The hierarchical incorporation of orthogonal Lewis acid and Pd NP active sites endowed 1-OTf-PdNP with outstanding catalytic performance in apparent hydrogenolysis of etheric, alcoholic, and esteric C-O bonds to generate saturated alkanes via a tandem dehydroalkoxylation-hydrogenation process under relatively mild conditions. The reactivity of C-O bonds followed the trend of tertiary carbon > secondary carbon > primary carbon. Control experiments demonstrated the heterogeneous nature and recyclability of 1-OTf-PdNP and its superior catalytic activity over the homogeneous counterparts. Sequential engineering of multiple catalytic sites in MOFs thus presents a unique opportunity to address outstanding challenges in sustainable catalysis.
RESUMEN
We report here the design of two multifunctional metal-organic frameworks (MOFs), mPT-Cu/Co and mPT-Cu/Re, comprising cuprous photosensitizers (Cu-PSs) and molecular Co or Re catalysts for photocatalytic hydrogen evolution (HER) and CO2 reduction (CO2RR), respectively. Hierarchical organization of Cu-PSs and Co/Re catalysts in these MOFs facilitates multielectron transfer to drive HER and CO2RR under visible light with an HER turnover number (TON) of 18â¯700 for mPT-Cu/Co and a CO2RR TON of 1328 for mPT-Cu/Re, which represent a 95-fold enhancement over their homogeneous controls. Photophysical and electrochemical investigations revealed the reductive quenching pathway in HER and CO2RR catalytic cycles and attributed the significantly improved performances of MOFs over their homogeneous counterparts to enhanced electron transfer due to close proximity between Cu-PSs and active catalysts and stabilization of Cu-PSs and molecular catalysts by the MOF framework.
RESUMEN
The reduction of alkenes to their respective alkanes is one of the most important transformations in organic chemistry, given the abundance of natural and commercial olefins. Metal-catalyzed hydrogenation is the most common way to reduce alkenes; however, the use of H2 gas in combination with the precious metals required for these conditions can be impractical, dangerous, and expensive. More complex substrates often require extremely high pressures of H2, further emphasizing the safety concerns associated with these hydrogenation reactions. Here we report a safe, cheap, and practical photochemical alkene reduction using a readily available organophotocatalyst, catalytic thiol, and formate. These conditions reduce a variety of di-, tri-, and tetra-substituted alkenes in good yield as well as dearomatize pharmaceutically relevant heterocycles to generate sp3-rich isosteres of benzofurans and indoles. These formal-hydrogenation conditions tolerate a broad range of functionalities that would otherwise be sensitive to typical hydrogenations and are likely to be important for industry applications.