Anthraquinone-based covalent organic framework as a recyclable direct hydrogen atom transfer photocatalyst for C-H functionalization.

Photocatalytic direct hydrogen atom transfer (d-HAT) is a synthetically important strategy to convert C-H bonds to useful C-X bonds. Herein we report the synthesis of an anthraquinone-based two-dimensional covalent organic framework, DAAQ-COF, as a recyclable d-HAT photocatalyst for C-H functionalization. Powder X-ray diffraction, N2 sorption isotherms, solid-state NMR spectra, infrared spectra, and thermogravimetric analysis characterized DAAQ-COF as a crystalline, porous COF with a stable ketoenamine linkage and strong absorption in the visible region. Under visible light irradiation, DAAQ-COF is photo-excited to cleave C(sp3)-H or C(sp2)-H bonds via HAT to generate reactive carbon radicals, which add to different radical acceptors to achieve C-N or C-C coupling reactions. DAAQ-COF is easily recovered from the reaction mixture via centrifugation or filtration and used in six consecutive reaction runs without any decrease in catalytic efficiency. The ease of catalyst separation allows sequential conversion of the C-N coupling intermediate to synthetically useful amide, ester, or thioester products. Photophysical and isotope labelling experiments support the d-HAT mechanism of DAAQ-COF-catalyzed C-H bond functionalization.

Self-adaptive Metal-Organic Framework Assembles Di-iron Active Sites to Mimic Monooxygenases.

Despite significant efforts, it remains a challenge to design artificial enzymes that can mimic both structures and functions of natural enzymes. Here, we report the post-synthetic construction of binuclear iron catalysts in MOF-253 to mimic natural di-iron monooxygenases. The adjacent bipyridyl (bpy) linkers in MOF-253 can freely rotate to form the [(bpy)FeIII(µ2-OH)]2 active site in a self-adaptive fashion. The composition and structure of the [(bpy)FeIII(µ2-OH)]2 active sites in MOF-253 were characterized by a combination of inductively coupled plasma-mass spectrometry, thermogravimetric analysis, X-ray absorption spectrometry, and Fourier-transform infrared spectroscopy. The MOF-based artificial monooxygenase effectively catalyzed oxidative transformations of organic compounds, including C-H oxidation and alkene epoxidation reactions, using O2 as the only oxidant, which indicates the successful recapitulation of the structure and functions of natural monooxygenases using readily accessible MOFs. The di-iron system exhibited at least 27 times higher catalytic activity than the corresponding mononuclear control. DFT calculations showed that the binuclear system had a 14.2 kcal/mol lower energy barrier than the mononuclear system in the rate-determining C-H activation process, suggesting the importance of cooperativity of the iron centers in the [(bpy)FeIII(µ2-OH)]2 active site in the rate-determining step. The stability and recyclability of the MOF-based artificial monooxygenase were also demonstrated.