Covalent Organic Frameworks Based Photoenzymatic Nano-reactor for Asymmetric Dynamic Kinetic Resolution of Secondary Amines.

Bio-catalysis represents a highly efficient and stereoselective method for the synthesis of valuable chiral compounds, however, the poor stability and limited reaction types of free enzymes restrict their wide application in industrial production. In this work, to overcome these problems, a multifunctional photoenzymatic nanoreactor CALB@COF-Ir was developed through the encapsulation of Candida antarctica lipase B (CALB) in a photosensitive covalent organic framework COF-Ir. This bio-nanocluster serves as efficient catalysts in asymmetric dynamic kinetic resolution (DKR) of secondary amines to give a series of chiral amines in high yields (up to 99 %) and enantioselectivities (up to 99 % ee). The well-designed COF-Ir not only acts as safety cover to prevent CALB from deactivation but promotes racemization of secondary amines via photo-induced hydrogen atom transfer (HAT) process. Photoelectric characterization and TDDFT calculation revealed that (ppy)2Ir units in COF-Ir play crucial role in this photocatalytic system which enhance its photo-redox properties through facilitating the separation between photoelectrons (e-) and holes (h+). Furthermore, the heterogeneous photoenzymatic nanoreactor could be recycled for five rounds with slight decline of catalytic reactivity.

Enhanced protonation ability of covalent organic frameworks via N,O-bidentate chelation for photocatalytic H2 evolution.

Inspired by the bidentate coordination chemistry of metal ions, we incorporated hydroxyl (OH) and methoxy (OMe) groups into the skeleton of imine-linked COFs to improve their protonation ability via intramolecular hydrogen bonds (O-H⋯NC). In comparison with the pristine COFs possessing monodentate nitrogen coordination sites, OH and OMe functionalized COFs with (N,O)-bidentate chelating sites exhibited up to 13.8 times faster photocatalytic hydrogen evolution rates (HERs).

Embedding Hydrogen Atom Transfer Moieties in Covalent Organic Frameworks for Efficient Photocatalytic C-H Functionalization.

Covalent organic frameworks (COFs) have emerged as efficient heterogeneous photocatalysts for a wide range of relatively simple organic reactions, whereas their application in complex organic transformations, like site-selective functionalization of unactivated C-H bonds, is underexplored, which can be mainly attributed to the lack of highly active organophotocatalytic cores. Herein through bonding oxygen atoms at the N-terminus of quinolines in nonsubstituted quinoline-linked COFs (NQ-COFs), we successfully realized the embedding of active hydrogen atom transfer (HAT) moieties into the skeleton of COFs. This novel designed COF (NQ-COFE5 -O), serving as both an excellent photosensitizer and HAT catalyst, exhibited much higher efficiency in C-H functionalization than the corresponding NQ-COFE5 . Specially, we evaluated the photocatalytic performance of NQ-COFE5 -O on ten different substrates, including quinolines, benzothiazole, and benzoxazole, all of which were transferred to desired products in moderate to high yields (up to 93 %). Furthermore, the as-synthesized NQ-COFE5 -O displayed excellent photostability and could be reused with negligible loss of activity for five catalytic cycles.

One-pot cascade construction of nonsubstituted quinoline-bridged covalent organic frameworks.

Irreversible locking of imine linkages into stable linkages represents a promising strategy to improve the robustness and functionality of covalent organic frameworks (COFs). We report, for the first time, a multi-component one-pot reaction (OPR) for imine annulation to construct highly stable nonsubstituted quinoline-bridged COFs (NQ-COFs), and that equilibrium regulation of reversible/irreversible cascade reactions by addition of MgSO4 desiccant is crucial to achieve high conversion efficiency and crystallinity. The higher long-range order and surface area of NQ-COFs synthesized by this OPR than those of the reported two-step post-synthetic modification (PSM) facilitate charge carrier transfer and photogeneration ability of superoxide radicals (O2˙-), which makes these NQ-COFs more efficient photocatalysts for O2˙- mediated synthesis of 2-benzimidazole derivatives. The general applicability of this synthetic strategy is demonstrated by fabricating 12 other crystalline NQ-COFs with a diversity of topologies and functional groups.

Construction of Ultrastable Nonsubstituted Quinoline-Bridged Covalent Organic Frameworks via Rhodium-Catalyzed Dehydrogenative Annulation.

Exploring new routes to lock the dynamic C=N bonds in imine-linked covalent organic frameworks (COFs) is highly desired for enhancing their stability and functionality. Herein, a novel C=N bridge locking strategy via rhodium-catalyzed [4+2] annulation is developed to construct nonsubstituted quinoline-linked COFs (NQ-COFs). The notable feature of this strategy includes high C=N conversion efficiency, oxidant-free, and generality for synthesis of a variety of NQ-COFs with high chemical stability. Particularly, after post-synthetic modification, the crystallinity, topology, and porosity of pristine imine-linked COFs are well retained. When used as photocatalysts, NQ-COFs display better visible light absorption and carriers' separation efficiency due to enhanced in-plane π conjugation ability, as well as more facile generation of superoxide anion radicals than their counterparts, thus leading to efficient synthesis of 2,4,6-tris(aryl)pyridines, benzimidazole, and sulfoxide derivatives.

Preparation of ultrathin carbon-coated CdS nanobelts for advanced Li and Na storage.

In this paper, we report a simple hydrothermal method for preparation of ultrathin carbon-coated CdS (CdS@C) nanobelts. The CdS@C nanobelts show superior electrochemical properties as an anode material for Li-ion batteries. The optimized CdS@C composites deliver a reversible capacity around 910 mAhg-1 and 48 mAhg-1 at 0.1 Ag-1 and 30.0 Ag-1, respectively. Moreover, the optimized nanobelts are also potential materials for Na storage. A stable capacity around 240 mAhg-1 is obtained at 0.1 Ag-1, even after 100 cycles.

[Observational characteristics of the concentrations of NOx, O3, SO2 and CO over Bohai Sea in summer].

By using the observational data obtained over the Bohai sea in Aug.-Sep., 2000, the variation characteristics of NOx, O3, SO2 and CO concentrations were firstly shown. The effects of solar radiation and weather conditions on the trace concentrations were analyzed. SO2 concentration was very stable, its' average value was about 0.006 mg.m-3. O3 concentration was mainly controlled by solar radiation. The concentration of CO and NOx was around 2.5-3.5 mg.m-3 and 0.1 mg.m-3, respectively, in weak weathers. Typhoon weather may made their concentrations increasing abruptly in a short time. The distinct differences of the pollutant concentrations between land and sea were also revealed, and the air quality over the Bohai sea in summer season was assessed based on the measurements.