Cuibo Liu

Synthesis of Hollow CdxZn1−xSe Nanoframes through the Selective Cation Exchange of Inorganic–Organic Hybrid ZnSe–Amine Nanoflakes with Cadmium Ions

Novel applications of nanostructures in catalysis, electronics, photonics, and bionanotechnology are driving the exploration of synthetic approaches to control and manipulate the morphology and chemical composition of nanostructures. Among these nanomaterials, hollow skeleton-like structures involving nanorings, nanoskeletons, and nanoframes have attracted increasing interest owing to their unique structures, interesting properties, which are different from their solid counterparts, and promising applications in different fields. In recent years, hollow skeleton-like or framelike Au, Co, Fe, Cu2O, Fe2O3, ZnO nanostructures have been successfully fabricated by using many advanced methods including self-assembly or oriented attachment of nanocrystals, template-assisted chemical etching, and self-coiling of nanobelts. However, with present methods the synthesis of frame-shaped semiconductors of controlled composition is a great challenge. Thus, it should be of great interest to develop a facile approach to generate frame-like hollow materials with adjustable composition, because the properties of these materials are strongly dependent on their composition. Since Alivisatos and coworkers discovered the cationexchange reaction in nanocrystals, the cation-exchange reaction has received growing attention as an effective way to modulate the composition, the structure, and the properties of nanomaterials. For instance, a partial cation-exchange reaction was adopted to create semiconductor superlattice structures in nanorods. Chalcogenide gels and metal phosphides and sulfides were successfully generated by the ionexchange reaction of corresponding nanostructures by Brock s group. Dorn et al. developed an in situ electric measuring method to monitor the exchange process of nanowires. Our recent study demonstrated that the vaporphase cation-exchange reaction of CdS with organic zinc could generate one-dimensional nanostructures with adjustable composition, morphology, and optical properties. Interestingly, the selectivity of facet reactivity was also observed in the case of the cation-exchange reactions of CdS nanorods and octapod-shaped CdS nanocrystals. At present, research efforts mainly focused on modulating the composition, structure, and properties of solid inorganic nanocrystals or one-dimensional nanostructures, while preserving their size and shape. Recently, we adopted the inorganic–organic hybrid sheets as starting materials and described a facile cation-exchange strategy to fabricate singlecrystal-like porous nanosheets. Although these advances have been achieved, the development of selective cationexchange reactions to synthesize frame-like nanostructures, especially with adjustable composition, is still in its infancy. Herein, we report a facile and effective protocol to prepare CdxZn1 xSe nanoframes through the selective cationexchange reaction of unpurified inorganic–organic ZnSe– amine nanosheets with Cd ions (see Figure 1 a). We show that CdxZn1 xSe nanoframes with adjustable composition are accessible by varying the relative ratio of hybrid precursors to cations in this approach. Moreover, the selective cationexchange strategy of hybrid nanosheets is suitable for fabricating other frame-like nanostructures. Furthermore, we demonstrate that the as-prepared Cd0.33Zn0.67Se nanoframes are highly active for photocatalytic H2 evolution from water splitting. The composition of hollow CdxZn1 xSe nanoframes has exerted an important influence on their photocatalytic activities. The unpurified ZnSe–DETA nanosheets (DETA = diethylenetriamine) were first prepared using a modified amineassisted solvothermal method, and were then reacted with different concentrations of Cd ions to obtain CdxZn1 xSe nanoframes (see the Supporting Information). The ZnSe– DETA nanosheets were firstly examined using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). SEM images (Figure 1b,c) clearly demonstrate that ZnSe–DETA nanosheets were successfully synthesized in high yields. The typical X-ray diffraction (XRD) pattern of the as-prepared hybrid precursors shown in Figure 1 f is identical to that of the reported ZnSe(DETA)0.5 nanobelts, [15] thus suggesting that the composition of the as-prepared hybrid nanosheets is ZnSe(DETA)0.5 (named ZnSe–DETA here). When ZnSe–DETA nanosheets exchange with an appropriate amount of Cd ions at elevated temperature, the solid sheets become hollow frame-like ones with a skeleton size of 30–40 nm (Figure 1d, e). The FTIR spectra shown in Figure 1 g also confirm that the hybrid precursors can be successfully transformed into inorganic materials. The composition of hollow CdxZn1 xSe nanoframes [*] X. Wu, Y. Yu, Y. Liu, Y. Xu, C. Liu, Prof. Dr. B. Zhang Department of Chemistry, School of Science, Tianjin University Tianjin 300072 (P. R. China) E-mail: bzhang@tju.edu.cn

A Facile Parallel Synthesis of Trifluoroethyl-Substituted Alkynes†

The physicochemical and biological properties of an organic compound are profoundly modified by the presence of fluorine functional groups, which alter the steric, electronic, lipophilic, and metabolic characteristics of the compound. The incorporation of fluorinated moieties into organic molecules has captured the attention of synthetic chemists over the past decades. As a reactive intermediate, 2,2,2trifluorodiazoethane is an attractive C2 synthon for the construction of fluorine-containing building blocks. The earlier studies involving this reagent primarily focused on noncatalytic reactions. These methods usually suffer from relatively harsh reaction conditions and limited substrate scope. To overcome these problems, recent research has paid more attention to metal-catalyzed transformations of 2,2,2trifluorodiazoethane. For instance, the Simonneaux and Komarov groups reported metal-catalyzed cyclopropanation reactions of gaseous F3CCHN2 with various olefins. [3] Carreira and co-workers demonstrated the catalytic generation of 2,2,2-trifluorodiazoethane in situ from CF3CH2NH2·HCl. [4] Several metal catalysts have been found to be compatible with the diazotization reaction, allowing a tandem transformation to take place in aqueous media. However, the generation of Csp3 CH2CF3 or Csp2 CH2CF3 bonds has been the focus of nearly all such studies. In sharp contrast, the formation of Csp CH2CF3 bonds through simple trifluoroethylation of terminal alkynes has not been reported to date, and still remains an interesting challenge. A survey of the literature reveals that the use and preparation of trifluoroethyl-substituted alkynes is extremely rare, which likely correlates with the absence of practical general methods for their synthesis. Only one recent report by Shibata and co-workers has addressed the trifluoromethylation of 3-(4’-nitrophenyl)propargyl bromide with [CuCF3] species, which were generated in situ from an electrophilic trifluoromethylating reagent and a stoichiometric amount of copper, but the desired product was obtained in only 36% yield (Scheme 1a). Herein, we report our efforts in developing a direct catalytic trifluoroethylation of terminal alkynes by using 2,2,2-trifluorodiazoethane (Scheme 1 b). This crosscoupling reaction can be conducted under mild conditions without the need for additional base or ligands. Furthermore, the ease of preparation and workup allows for the quick and efficient parallel synthesis of a broad variety of trifluoroethylated alkynes. These products, which bear a CH2CF3 group in the propargyl position, are versatile precursors for the synthesis of other types of fluorinated molecules. Additionally, both experimental and theoretical analyses indicate that this trifluoroethylation could proceed by a concerted Csp H insertion process. In general, reactions of terminal alkynes with diazo compounds lead to cyclopropenation or Csp H insertion. The reaction course strongly depends on the nature of the metal and the catalyst structure. For example, the groups of P rez and Doyle described cyclopropenation reactions of alkynes with diazoesters catalyzed by Cu or Rh complexes, whereas the groups of Fu, Fox, and Wang reported the cross-coupling of alkynes with diazo compounds catalyzed by Cu or Pd to afford the corresponding products. Very recently, Morandi and Carreira disclosed a rhodium-catalyzed cyclopropenation of alkynes by CF3CHN2 generated in situ from CF3CH2NH2·HCl in aqueous media. [4b] Based on these important precedents, we examined the ability of various Cu and Cu salts to catalyze the trifluoroethylation of phenylacetylene 1a using gaseous CF3CHN2. A preliminary result was obtained using CuI as a catalyst under mild conditions to provide the corresponding trifluoroethylated product 2 a in 63% yield without the use of extra base or ligands. However, a disadvantage of this method is that it requires the use of a large excess of CF3CH2NH2·HCl (5–8 equiv). In order to use gaseous CF3CHN2 more efficiently, we needed to set up a recycling system. After the reactor, a storage balloon was placed to trap and reuse the escaping gaseous CF3CHN2. Gratifyingly, examination of the same test reaction with the recycling system in place revealed that the yield can be significantly improved to 85 % and that the amount of CF3CH2NH2·HCl can be lowered to three equivalents. Furthermore, a multireactor setup has been designed to provide an integrated system for simultaneously running Scheme 1. Synthesis of trifluoroethyl-substituted alkynes. NMP = N-methyl-2-pyrrolidone, OTf = trifluoromethanesulfonate.

Conversion of CuO nanoplates into porous hybrid Cu2O/polypyrrole nanoflakes through a pyrrole-induced reductive transformation reaction.

Porous hybrid Cu2O/polypyrrole nanoflakes have been synthesized from solid CuO nanoplate templates through the pyrrole-induced reductive transformation reaction at elevated temperature. The conversion mechanism involves the reductive transformation of CuO to Cu2O and the in situ oxidative polymerization of pyrrole to polypyrrole. In addition, the morphology of the as-converted nanohybrids depends on the shape of the CuO precursors. The strategy enables us to transform single-crystalline CuO nanosheets into hollow hybrid Cu2O/polypyrrole nanoframes. The ability to transform CuO and an organic monomer into porous hybrid materials of conducting polymer and Cu2O with macrosized morphological retention opens up interesting possibilities to create novel nanostructures. Electrochemical examinations show that these porous hybrid Cu2O/polypyrrole nanostructures exhibit efficient catalytic activity towards oxygen reduction reaction (ORR), excellent methanol tolerance ability, and catalytic stability in alkaline solution, thus making them promising nonprecious-metal-based catalysts for ORR in alkaline fuel cells and metal-air batteries.

Light triggered addition/annulation of 2-isocyanobiphenyls toward 6-trifluoromethyl-phenanthridines under photocatalyst-free conditions

A photocatalyst-free, light promoted sequential radical addition/annulation of 2-isocyanobiphenyls to 6-trifluoromethyl phenanthridines is presented. Wide substrate scopes and scale-up experiment demonstrate the promising efficiency and utility of this strategy.

Base-promoted direct and highly selective alkynylation of electron-deficient octafluorotoluene

A strategy for direct alkynylation of electron-deficient octafluorotoluene via selective C–F bond cleavage is exhibited. The synthesized products are super intermediates for further transformations. Additionally, we give a preliminary explanation of the mechanism for the reaction of terminal alkynes with octafluorotoluene using density functional theory (DFT).

Facile Access to Fluoroaromatic Molecules by Transition-Metal-Free C-F Bond Cleavage of Polyfluoroarenes: An Efficient, Green, and Sustainable Protocol.

The creation of new bonds via C-F bond cleavage of polyfluoroarenes has proven to be an important and powerful tool in synthetic chemistry. Using such a strategy, a myriad of valuable partially fluoroaromatic molecules and building blocks can be obtained. The transition-metal-free nucleophilic aromatic substitution (SN Ar) strategy has aroused the continuing interest of researchers due to its simple, mild, economical, and environmentally benign characteristics, which have been successfully applied to C-F bond functionalizations. In this account, we present a summary of the recent investigations of polyfluoroarenes involving SN Ar reactions and discuss some of our recent endeavors in the construction of partially fluoroaromatic molecules. Through this strategy, many new bonds including C-C, C-N, C-O, C-S, and C-H bonds can be created. Additionally, brief discussions on the transformation mechanisms are also provided. Finally, we discuss the existing limitations of the SN Ar reactions of polyfluoroarenes as well as our perspective on the future development of this chemistry.

A water-soluble glucose-functionalized cobalt(III) complex as an efficient electrocatalyst for hydrogen evolution under neutral conditions.

A water-soluble glucose-functionalized cobalt(III) complex [Co(III)(dmgH)2(py-glucose)Cl] is active and stable for electrocatalytic hydrogen production in neutral aqueous solution.