Photocatalyzed Enantioselective Functionalization of C(sp3)-H Bonds.

Owing to its diverse activation processes including single-electron transfer (SET) and hydrogen-atom transfer (HAT), visible-light photocatalysis has emerged as a sustainable and efficient platform for organic synthesis. These processes provide a powerful avenue for the direct functionalization of C(sp3)-H bonds under mild conditions. Over the past decade, there have been remarkable advances in the enantioselective functionalization of the C(sp3)-H bond via photocatalysis combined with conventional asymmetric catalysis. Herein, we summarize the advances in asymmetric C(sp3)-H functionalization involving visible-light photocatalysis and discuss two main pathways in this emerging field: (a) SET-driven carbocation intermediates are followed by stereospecific nucleophile attacks; and (b) photodriven alkyl radical intermediates are further enantioselectively captured by (i) chiral π-SOMOphile reagents, (ii) stereoselective transition-metal complexes, and (iii) another distinct stereoscopic radical species. We aim to summarize key advances in reaction design, catalyst development, and mechanistic understanding, to provide new insights into this rapidly evolving area of research.

Organic photoredox catalyzed C(sp3)-H functionalization of saturated aza-heterocycles via a cross-dehydrogenative coupling reaction.

Herein we present a novel protocol to access α-functionalized saturated aza-heterocycles, and a variety of nucleophilic groups, such as indole, naphthol, phenol, pyrrole, furyl, nitromethyl, and cyano, could be easily installed into saturated aza-heterocycles. Furthermore, a range of biologically valuable 3,3'-diindolylmethane derivatives could also be readily accessed under mild photocatalytic conditions.

Zr(OH)4 -Catalyzed Controllable Selective Oxidation of Anilines to Azoxybenzenes, Azobenzenes and Nitrosobenzenes.

The selective oxidation of aniline to metastable and valuable azoxybenzene, azobenzene or nitrosobenzene has important practical significance in organic synthesis. However, uncontrollable selectivity and laborious synthesis of the expensive required catalysts severely hinders the uptake of these reactions in industrial settings. Herein, we have pioneered the discovery of Zr(OH)4 as an efficient heterogeneous catalyst capable of the selective oxidation of aniline, using either peroxide or O2 as oxidant, to selectively obtain various azoxybenzenes, symmetric/unsymmetric azobenzenes, as well as nitrosobenzenes, by simply regulating the reaction solvent, without the need for additives. Mechanistic experiments and DFT calculations demonstrate that the activation of H2 O2 and O2 is primarily achieved by the bridging hydroxyl and terminal hydroxyl groups of Zr(OH)4 , respectively. The present work provides an economical and environmentally friendly strategy for the selective oxidation of aniline in industrial applications.

Grain Boundary Engineering with Self-Assembled Porphyrin Supramolecules for Highly Efficient Large-Area Perovskite Photovoltaics.

Grain boundary management is critical to the performance and stability of polycrystalline perovskite solar cells (PSCs), especially large-area devices. However, typical passivators are insulating in nature and limit carrier transport. Here, we design a supramolecular binder for grain boundaries to simultaneously passivate defects and promote hole transport across perovskite grain boundaries. By doping the monoamine porphyrins (MPs, M = Co, Ni, Cu, Zn, or H) into perovskite films, MPs self-assemble into supramolecules at grain boundaries. Organic cations in perovskites protonate MPs in supramolecules to form ammonium porphyrins bound on the perovskite grain surface, to passivate defects and extract holes from the perovskite lattice. Periodic polarons in supramolecules (especially NiP-supramolecule) promote the transport of extracted holes across boundaries, reducing nonradiative carrier recombination. The NiP-doped PSCs reveal a certified efficiency of 22.1% for an active area of 1.0 cm2 with the remarkably improved open-circuit voltage and fill factor. The unencapsulated device retained over 80% initial performance under AM 1.5G solar light continuous illumination or heating at 85 °C over 3000 h.

Non-Interpenetrated Single-Crystal Covalent Organic Frameworks.

Growth of covalent organic frameworks (COFs) as single crystals is extremely challenging. Inaccessibility of open-structured single-crystal COFs prevents the exploration of structure-oriented applications. Herein we report for the first time a non-interpenetrated single-crystal COF, LZU-306, which possesses the open structure constructed exclusively via covalent assembly. With a high void volume of 80 %, LZU-306 was applied to investigate the intrinsic dynamics of reticulated tetraphenylethylene (TPE) as the individual aggregation-induced-emission moiety. Solid-state 2 H NMR investigation has determined that the rotation of benzene rings in TPE, being the freest among the reported cases, is as fast as 1.0×104  Hz at 203 K to 1.5×107  Hz at 293 K. This research not only explores a new paradigm for single-crystal growth of open frameworks, but also provides a unique matrix-isolation platform to reticulate functional moieties into a well-defined and isolated state.

Exploring Applications of Covalent Organic Frameworks: Homogeneous Reticulation of Radicals for Dynamic Nuclear Polarization.

Rapid progress has been witnessed in the past decade in the fields of covalent organic frameworks (COFs) and dynamic nuclear polarization (DNP). In this contribution, we bridge these two fields by constructing radical-embedded COFs as promising DNP agents. Via polarization transfer from unpaired electrons to nuclei, DNP realizes significant enhancement of NMR signal intensities. One of the crucial issues in DNP is to screen for suitable radicals to act as efficient polarizing agents, the basic criteria for which are homogeneous distribution and fixed orientation of unpaired electrons. We therefore envisioned that the crystalline and porous structures of COFs, if evenly embedded with radicals, may work as a new "crystalline sponge" for DNP experiments. As a proof of concept, we constructed a series of proxyl-radical-embedded COFs (denoted as PR( x)-COFs) and successfully applied them to achieve substantial DNP enhancement. Benefiting from the bottom-up and multivariate synthetic strategies, proxyl radicals have been covalently reticulated, homogeneously distributed, and rigidly embedded into the crystalline and mesoporous frameworks with adjustable concentration ( x%). Excellent performance of PR( x)-COFs has been observed for DNP 1H, 13C, and 15N solid-state NMR enhancements. This contribution not only realizes the direct construction of radical COFs from radical monomers, but also explores the new application of COFs as DNP polarizing agents. Given that many radical COFs can therefore be rationally designed and facilely constructed with well-defined composition, distribution, and pore size, we expect that our effort will pave the way for utilizing radical COFs as standard polarizing agents in DNP NMR experiments.

Mechanistic insight into the formation of acetic acid from the direct conversion of methane and carbon dioxide on zinc-modified H-ZSM-5 zeolite.

Methane and carbon dioxide are known greenhouse gases, and the conversion of these two C1-building blocks into useful fuels and chemicals is a subject of great importance. By solid-state NMR spectroscopy, we found that methane and carbon dioxide can be co-converted on a zinc-modified H-ZSM-5 zeolite (denoted as Zn/H-ZSM-5) to form acetic acid at a low temperature range of 523-773 K. Solid-state (13)C and (1)H MAS NMR investigation indicates that the unique nature of the bifunctional Zn/H-ZSM-5 catalyst is responsible for this highly selective transformation. The zinc sites efficiently activate CH4 to form zinc methyl species (-Zn-CH3), the Zn-C bond of which is further subject to the CO2 insertion to produce surface acetate species (-Zn-OOCCH3). Moreover, the Brønsted acid sites play an important role for the final formation of acetic acid by the proton transfer to the surface acetate species. The results disclosed herein may offer the new possibility for the efficient activation and selective transformation of methane at low temperatures through the co-conversion strategy. Also, the mechanistic understanding of this process will help to the rational design of robust catalytic systems for the practical conversion of greenhouse gases into useful chemicals.

4-(N,N-dimethylamino)pyridine-embedded nanoporous conjugated polymer as a highly active heterogeneous organocatalyst.

We report herein for the first time the incorporation of a versatile organocatalyst, 4-(N,N-dimethylamino)pyridine (DMAP), into the network of a nanoporous conjugated polymer (NCP) by the "bottom-up" approach. The resulting DMAP-NCP material possesses highly concentrated and homogeneously distributed DMAP catalytic sites (2.02 mmol g(-1)). DMAP-NCP also exhibits enhanced stability and permanent porosity due to the strong covalent linkage and the rigidity of the "bottom-up" monomers. As a result, DMAP-NCP shows excellent catalytic activity in the acylation of alcohols with yields of 92-99 %. The DMAP-NCP catalyst could be easily recovered from the reaction mixture and reused in at least 14 consecutive cycles without measurable loss of activity. Moreover, the catalytic acylation reaction could be performed under neat and continuous-flow conditions for at least 536 h of continuous work with the same catalyst activity.

Ultrafine PdCo bimetallic nanoclusters confined in N-doped porous carbon for the efficient semi-hydrogenation of alkynes.

Semi-hydrogenation of alkynes to prepare alkenes is an important reaction in the petrochemical and fine chemical industries. The use of conventional Pd nanoparticle-based catalysts is limited by alkyne over-hydrogenation and low Pd utilization. In this study, a nitrogen-doped mesoporous carbon material (m-NC), which was rich in defect sites after Zn volatilization, was fabricated by the carbonization of ZIF-8. Ultrafine PdCo bimetallic nanoclusters with Co atom-modified Pd active site electronic and compositional structure were highly dispersed and confined in m-NC. As-obtained Pd0.43Co1/m-NC was used for the semi-hydrogenation of alkynes and it exhibited high selectivity with high conversion under mild reaction conditions. Pd0.43Co1/m-NC also exhibited excellent stability in leaching tests and maintained its catalytic activity for at least nine reaction cycles. The highly dispersed active sites in Pd0.43Co1/m-NC served as the active sites for the catalytic semi-hydrogenation of alkynes; as a regulator, the second metal Co effectively improved selectivity, and m-NC endowed the catalyst with excellent stability. The research work presented here may provide a foundation for the design of highly active, selective, and stable Pd-based bimetallic catalysts for selective hydrogenation.

Robotic soft swim bladder using liquid-vapor phase transition.

The swim bladder is crucial to underwater robots to enhance their overall performance and to expand their range of motion. However, previous attempts to incorporate this function have failed or have adopted mechanical swim bladders with high-disturbances. This study presents an entirely soft swim bladder capable of controlling buoyancy selectively and noiselessly, making it applicable to sensitive underwater environments. The soft swim bladder, which consists of an elastic cover layer, flexible heating elements, and three expandable pouches filled with low boiling point fluid, can express four modes of motion by varying buoyancy: sinking, suspending, rising, and fast-rising. The varying buoyancy is achieved through liquid-vapor phase transition of the fluid in the selected pouches when Joule heated above its boiling temperature. Moreover, the swim bladder is integrated with a shape memory alloy-based fishtail to form a soft fish robot. The synergy between the bladder and the tail allows the robot to explore a total of ten disparate modes of maneuvers, and their dynamic performance has been evaluated. The results of this study present the potential for the soft swim bladder to be utilized in any underwater robotic applications to enhance their swimming performance.

A TEMPO-Functionalized Ordered Mesoporous Polymer as a Highly Active and Reusable Organocatalyst.

The properties of high stability, periodic porosity, and tunable nature of ordered mesoporous polymers make these materials ideal catalytic nanoreactors. However, their application in organocatalysis has been rarely explored. We report herein for the first time the incorporation of a versatile organocatalyst, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), into the pores of an FDU-type mesoporous polymer via a pore surface engineering strategy. The resulting FDU-15-TEMPO possesses a highly ordered mesoporous organic framework and enhanced stability, and shows excellent catalytic activity in the selective oxidation of alcohols and aerobic oxidative synthesis of 2-substituted benzoxazoles, benzimidazoles and benzothiazoles. Moreover, the catalyst can be easily recovered and reused for up to 7 consecutive cycles.

Reactivity of C1 surface species formed in methane activation on Zn-modified H-ZSM-5 zeolite.

Solid-state (13)C magic angle spinning (MAS) NMR spectroscopy investigations identified zinc methyl species, formate species, and methoxy species as C(1) surface species formed in methane activation on the zeolite Zn/H-ZSM-5 catalyst at T≤573 K. These C(1) surface species, which are possible intermediates in further transformations of methane, were prepared separately by adsorption of (13)C-enriched methane, carbon monoxide, and methanol onto zinc-containing catalysts, respectively. Successful isolation of each surface species allowed convenient investigations into their chemical nature on the working catalyst by solid-state (13)C MAS NMR spectroscopy. The reactivity of zinc methyl species with diverse probe molecules (i.e., water, methanol, hydrochloride, oxygen, or carbon dioxide) is correlated with that of organozinc compounds in organometallic chemistry. Moreover, surface formate and surface methoxy species possess distinct reactivity towards water, hydrochloride, ammonia, or hydrogen as probe molecules. To explain these and other observations, we propose that the C(1) surface species interconvert on zeolite Zn/H-ZSM-5. As implied by the reactivity information, potential applications of methane co-conversion on zinc-containing zeolites might, therefore, be possible by further transformation of these C(1) surface species with rationally designed co-reactants (i.e., probe molecules) under optimized reaction conditions.

Palladium Nanoclusters Confined in MOF@COP as a Novel Nanoreactor for Catalytic Hydrogenation.

Metal-nanocluster-doped porous materials are attracting considerable research attention due to their specific catalytic performance. In this study, core-shell metal-organic frameworks@covalent organic polymer (MOF@COP) nanocomposites were formed by the covalent linking of chemically stable COP on the surface of size-selective UiO-66-NH2. Pd nanoclusters with an average diameter of ∼0.8 nm were successfully confined in UiO-66-NH2@COP, and the obtained nanoreactor, referred to as UiO-66-NH2@COP@Pd, exhibited abundant porosity, high stability, and large surface area. Notably, the UiO-66-NH2@COP@Pd nanoreactor exhibited superior catalytic activity and stability for the catalytic reduction of 4-nitrophenol and hydrogenation of other nitroarenes, demonstrating the potential of Pd-cluster-doped MOF@COP hybrid materials as candidates for efficient catalytic hydrogenation. This study may provide new avenues for the construction of MOF@COP-hybrid-material-based heterogeneous catalysts for efficient catalytic applications.

Ru nanoclusters confined in porous organic cages for catalytic hydrolysis of ammonia borane and tandem hydrogenation reaction.

The fabrication of narrow-sized metal nanoclusters for heterogeneous catalysis has attracted widespread research attention. Nevertheless, it is still a significant challenge to fabricate highly dispersed metal-nanocluster-based catalysts with high activity and stability. In this study, 1,3,5-benzenetricarboxylate and 1,2-diaminocyclohexane were used as precursors to fabricate porous organic cages (POCs), CC3-R. CC3-R exhibited a high specific surface area and a microporous-mesoporous structure. In addition, ultrafine Ru nanoclusters were successfully encapsulated in CC3-R with high dispersion via impregnation and subsequent reduction, affording Ru nanoclusters with a precisely controlled size of ∼0.65 nm. As-obtained Ru(1.45%)@CC3-R exhibited significantly enhanced catalytic activities toward the hydrolysis of ammonia borane (AB) and exhibited high conversion and selectivity for the tandem hydrogenation of nitroarenes and hydrogenation of quinoline in water under mild conditions. In addition, the Ru(1.45%)@CC3-R catalyst exhibited high stability and good recyclability. This study should provide a novel strategy for fabricating highly dispersed ultrafine nanocluster-based catalysts for various catalysis applications.

Ultrafine palladium nanoparticles confined in core-shell magnetic porous organic polymer nanospheres as highly efficient hydrogenation catalyst.

The development of porous organic polymer (POP)-based materials with controllable structures is highly desirable for catalysis, drug delivery, and chemical adsorption. In this work, we prepared unique porous magnetic core-shell POP nanospheres (Fe3O4@PDA@POP) through a facile strategy. These nanospheres contained a core of magnetic Fe3O4 nanoparticles (NPs), a hydrophilic intermediate layer of dopamine and a POP outer layer. The Fe3O4@PDA@POP showed high porosity, making it an ideal supporting material for fabricating ultrafine and highly dispersed noble-metal NPs (NMNPs). Thus, highly dispersed ultrafine Pd NPs (1.5-2.1 nm) were confined and stabilized within the pores of Fe3O4@PDA@POP via a reverse double-solvent approach (RDSA) to obtain Fe3O4@PDA@POP@Pd catalyst. The Fe3O4@PDA@POP@Pd-2.5% catalyst showed excellent catalytic performance and recyclability towards the hydrogenation of nitrobenzene, alkenes, and alkynes. Hence, this work can pave the way for the development and application of functionalized POP materials to construct efficient catalytic systems.

PdCo nanoparticles supported on carbon fibers derived from cotton: Maximum utilization of Pd atoms for efficient reduction of nitroarenes.

In the present work, a facile and environment-friendly route is illustrated for the efficient fabrication of highly dispersed PdCo nanoparticles (NPs) by modified cotton-derived carbon fibers (PdCo/CCF). Firstly, commercial cotton was impregnated with CoCl2, followed by pyrolysis under high calcination temperature to obtain the Co NPs modified CCF sample (Co/CCF). Secondly, Co/CCF was treated with Pd(AcO)2 aqueous solution, wherein, through a spontaneous replacement reaction process, Pd2+ is reduced to metallic Pd and mostly covered on the surface of the Co NPs. Thus, the PdCo/CCF catalyst was obtained avoiding the use of toxic reductants like NaBH4, NH2NH2 and HCHO. The PdCo/CCF catalyst exhibits excellent catalytic activity and recyclability for the reduction of 4-nitrophenol and other nitroarenes compared with Pd/CCF, PdCo NPs and many other noble metals based catalysts. The reasons could be attributed to the uniformly dispersed and accessible PdCo NPs on the surface of the CCF, and the Pd atoms deposited on the Co NPs surface that makes the Pd active sites available for optimum use. The PdCo/CCF catalyst also exhibits potential application for catalytic reduction of nitroarenes in a fixed bed reactor under mild reaction conditions. Furthermore, the PdCo/CCF catalyst can be magnetically recycled and reused for at least ten cycles without either losing catalytic activity or leaching of Pd active sites, thereby confirming its superior stability.

Immobilization of Pt nanoparticles in hollow mesoporous silica nanocapsules: An aggregation- and leaching-resistant catalyst.

In this study, hollow mesoporous silica nanocapsules (h-mNSiO2) with uniformly dispersed Pt nanoparticles (Pt NPs) in their hollow core (Pt@h-mNSiO2) were successfully fabricated. The as-synthesized Pt@h-mNSiO2 was core@shell like structure with silica shells and Pt-rich cores. The catalyst was synthesised in an oil-water biphasic stratification system, then the self-assembly of reactants occurred in the oil-water interface for one-pot sustaining interfacial growth. The as-prepared Pt@h-mNSiO2 catalyst exhibited superior activity for hydrogen generation from the hydrolysis of ammonia borane, with a turnover frequency of 371.7 molH2 mol-1Pt min-1 at ambient temperature, probably owing to the abundant mesopores and high surface area, leading to a considerable increase in the accessible active sites. Besides, almost no Pt NP aggregation was observed during reusability tests. Notably, Pt leaching was not observed during the reaction, possibly related to the protective effect of the mesoporous silica shell. Thus, this study provides a facile route to synthesise aggregation- and leaching-resistant catalysts with superior activity, accessibility, and recyclability for the environment and energy chemistry.

Single-crystal x-ray diffraction structures of covalent organic frameworks.

The crystallization problem is an outstanding challenge in the chemistry of porous covalent organic frameworks (COFs). Their structural characterization has been limited to modeling and solutions based on powder x-ray or electron diffraction data. Single crystals of COFs amenable to x-ray diffraction characterization have not been reported. Here, we developed a general procedure to grow large single crystals of three-dimensional imine-based COFs (COF-300, hydrated form of COF-300, COF-303, LZU-79, and LZU-111). The high quality of the crystals allowed collection of single-crystal x-ray diffraction data of up to 0.83-angstrom resolution, leading to unambiguous solution and precise anisotropic refinement. Characteristics such as degree of interpenetration, arrangement of water guests, the reversed imine connectivity, linker disorder, and uncommon topology were deciphered with atomic precision-aspects impossible to determine without single crystals.

Wobbling and Hopping: Studying Dynamics of CO2 Adsorbed in Metal-Organic Frameworks via (17)O Solid-State NMR.

Knowledge of adsorbed gas dynamics within microporous solids is crucial for the design of more efficient gas capture materials. We demonstrate that (17)O solid-state NMR (SSNMR) experiments allow one to obtain accurate information on CO2 dynamics within metal-organic frameworks (MOFs), using CPO-27-M (M = Mg, Zn) as examples. Variable-temperature (VT) (17)O SSNMR spectra acquired from 150 to 403 K yield key parameters defining the CO2 motions. VT (17)O SSNMR spectra of CPO-27-Zn indicate relatively weaker metal-oxygen binding and increased CO2 dynamics. (17)O SSNMR is a sensitive probe of CO2 dynamics due to the presence of both the quadrupolar and chemical shielding interactions, and holds potential for the investigation of motions within a variety of microporous materials.