Showing papers in "Chemical Science in 2012"

Rethinking the term “pi-stacking”

Abstract: It has become common to reference “pi-stacking” forces or “pi–pi interactions” when describing the interactions between neighbouring aromatic rings. Here, we review experimental and theoretical literature across several fields and conclude that the terms “pi-stacking” and “pi–pi interactions” do not accurately describe the forces that drive association between aromatic molecules of the types most commonly studied in chemistry or biology laboratories. We therefore propose that these terms are misleading and should no longer be used. Even without these terms, electrostatic considerations relating to polarized pi systems, as described by Hunter and Sanders, have provided a good qualitative starting place for predicting and understanding the interactions between aromatics for almost two decades. More recent work, however, is revealing that direct electrostatic interactions between polarized atoms of substituents as well as solvation/desolvation effects in strongly interacting solvents must also be considered and even dominate in many circumstances.

Synergistic catalysis: A powerful synthetic strategy for new reaction development

Abstract: Synergistic catalysis is a synthetic strategy wherein both the nucleophile and the electrophile are simultaneously activated by two separate and distinct catalysts to afford a single chemical transformation. This powerful catalysis strategy leads to several benefits, specifically synergistic catalysis can (i) introduce new, previously unattainable chemical transformations, (ii) improve the efficiency of existing transformations, and (iii) create or improve catalytic enantioselectivity where stereocontrol was previously absent or challenging. This perspective aims to highlight these benefits using many of the successful examples of synergistic catalysis found in the literature.

Fe, Co, and Ni ions promote the catalytic activity of amorphous molybdenum sulfide films for hydrogen evolution

Abstract: Molybdenum sulfide materials have been shown as promising non-precious catalysts for hydrogen evolution. This paper describes the study of the promotional effects of certain transition metal ions on the activity of amorphous MoS3 films. Ternary metal sulfide films, M–MoS3 (M = Mn, Fe, Co, Ni, Cu, Zn), have been prepared by cyclic voltammetry of aqueous solutions containing MCl2 and (NH4)2[MoS4]. Whereas the Mn–, Cu–, and Zn–MoS3 films show similar or only slightly higher catalytic activity as the MoS3 film, the Fe–, Co–, and Ni–MoS3 films are significantly more active. The promotional effects of Fe, Co, and Ni ions exist under both acidic and neutral conditions, but the effects are more pronounced under neutral conditions. Up to a 12-fold increase in exchange current density and a 10-fold increase in the current density at an overpotential of 150 mV are observed at pH = 7. It is shown that Fe, Co, and Ni ions promote the growth of the MoS3 films, resulting a high surface area and a higher catalyst loading. These changes are the main contributors to the enhanced activity at pH = 0. However, at pH = 7, Fe, Co, and Ni ions appear to also increase the intrinsic activity of the MoS3 film.

Graphene-based electronic sensors

Abstract: Graphene, the archetypal two-dimensional material, is attracting increasing attention due to its unique and superior properties. The atomic thickness of the graphene sheet is extremely sensitive towards the change of local environment, making it an ideal channel material in field-effect transistors used as electronic sensors. In this minireview, we review the graphene-based electronic sensors for detection of various chemicals and biomolecules. We first introduce the different kinds of graphene materials used in the electronic sensors and how they affect the device sensing performance. Then we focus on the use of the reduced graphene oxide for the fabrication of cost-efficient, high-yield and highly reproducible sensing devices.

Carboxylates as sources of carbon nucleophiles and electrophiles: comparison of decarboxylative and decarbonylative pathways

Abstract: This tutorial review provides a comparison between the concepts of catalytic decarboxylative and decarbonylative couplings for the ipso-substitution of carboxylate groups, and illustrates their potential benefits over alternative C–C bond-forming reactions. Redox-neutral decarboxylative reactions allow generating organometallic species with nucleophilic reactivity via the extrusion of carbon dioxide from metal carboxylates. Such C–C bond activating processes provide a way of employing carboxylate salts as substitutes for the traditional sources of carbon nucleophiles, i.e. stoichiometric organometallic reagents. If the decarboxylation of carboxylic acids is performed under oxidative conditions, organometallic species with electrophilic reactivity are obtained instead. These can alternatively be accessed via the extrusion of carbon monoxide from acyl–metal species generated via the oxidative addition of activated carboxylic acid derivatives (e.g. acid chlorides, anhydrides or esters) to metal complexes. In the latter two reaction types, carboxylic acids thus become substitutes for organohalides. The complementary redox-neutral and oxidative decarboxylative and decarbonylative reaction modes allow the broad use of carboxylic acids as substrates in C–C bond-forming reactions. Their applicability, scope and limitations are discussed using the examples of Heck reactions, cross-couplings and direct arylations.

Fullerene crystallisation as a key driver of charge separation in polymer/fullerene bulk heterojunction solar cells

Abstract: Solution processed polymer/fullerene blend films are receiving extensive attention as the photoactive layer of organic solar cells. In this paper we report a range of photophysical, electrochemical, physicochemical and structural data which provide evidence that formation of a relatively pure, molecularly ordered phase of the fullerene component, phenyl-C61-butyric acid methyl ester (PCBM), may be the key factor driving the spatial separation of photogenerated electrons and holes in many of these devices. PCBM crystallisation is shown to result in an increase in its electron affinity, providing an energetic driving force for spatial separation of electrons and holes. Based upon our observations, we propose a functional model applicable to many organic bulk heterojunction devices based upon charge generation in a finely intermixed polymer/fullerene phase followed by spatial separation of electrons and holes at the interface of this mixed phase with crystalline PCBM domains. This model has significant implications for the design of alternative acceptor materials to PCBM for organic solar cells.

A highly selective ratiometric near-infrared fluorescent cyanine sensor for cysteine with remarkable shift and its application in bioimaging

Abstract: We developed a highly selective ratiometric near-infrared cyanine-based probe CyAC for cysteine (Cys) over homocysteine (Hcy) and glutathione (GSH). Upon the addition of Cys to the solution of CyAC, remarkable shifts in the spectra of CyAC can be monitored (from 770 nm to 515 nm in absorption spectra and from 780 nm to 570 nm in emission spectra). For the first time, the novel strategy that reversibly modulates the polymethine π-electron system by conjugation and removal of the specific trigger moiety was implemented for the generation of a ratiometric cyanine-based sensor. Hydroxy cyanine CyAE was chosen as the flurophore scaffold because the tautomerism (CyAE and CyAK or CyAD) can cause the reversible change in the π-conjugation system of the dyes with large shifts in the spectra. An acrylate group containing a α, β-unsaturated ketone as a functional trigger moiety was incorporated with CyAK to form the sensor CyAC. This specific response for Cys was based on the differences of the kinetics of intramolecular adduct/cyclizations. Moreover, CyAC was successfully applied for bioimaging Cys in living cancer cells. This paradigm by modulation of the polymethine π-electron system in the cyanine dye provides a promising methodology for the design of ratiometric cyanine-based sensors.

Changing and challenging times for service crystallography

Abstract: Crystallography is no longer solely the preserve of the specialist, a situation that has implications for the operation of crystallographic service facilities. This mini-review provides an overview of the challenges in operating a crystallographic facility from the perspective and experience of the UK National Crystallography Service – a modern mid-range facility. Examples of chemical research generating the greatest challenges for the modern crystallography service and the state-of-the-art tools, hardware, facilities and expertise that are required to address them are highlighted. An overview of current research trends in single crystal diffraction research, which will ensure the future development of the technique, is presented. The remit of the service crystallographer is examined, together with proposed examples of best practice.

Cooperative Lewis acid/N-heterocyclic carbene catalysis

Abstract: Lewis acid activation with N-heterocyclic carbene (NHC) catalysis has presented new opportunities for enantioselective reaction development. Recent findings illustrate that Lewis acids can play an important role in homoenolate annulations by: enhancement of the reactivity, reversal of the diastereo- or regioselectivity, and activation of previously inactive electrophiles. Additionally, the incorporation of a Lewis acid into Bronsted base-catalyzed conjugate addition allowed for an increase in yields.

Postsynthetic ligand exchange as a route to functionalization of ‘inert’ metal–organic frameworks

Abstract: Herein, we report that the exchange of ligands from an intact metal–organic framework (MOF) can be exploited as a means to introduce functionalized ligands into MOFs under mild conditions. It is shown that ligand exchange can occur with ‘inert’ Zr(IV)-based UiO-66 MOFs in a solvent dependent manner. We call this process postsynthetic exchange (PSE) and show that it provides access to MOFs that are not readily prepared in high quality by solvothermal methods. It was found that ligand exchange can occur between UiO-66 MOFs as monitored by aerosol time-of-flight mass spectrometry (ATOFMS). ATOFMS was used to analyze the chemical composition of microcrystalline MOFs on the single particle level, providing information not available through bulk analysis. PSE is an important postsynthetic approach to the modification of MOFs, and the ligand exchange revealed by ATOFMS requires a re-evaluation of the assumed ‘stability’ of even the most robust MOFs.

Recent progress in enantioselective synthesis of C3-functionalized oxindoles: rare earth metals take action

Abstract: The enantioselective synthesis of 3-functionalized oxindole derivatives has experienced an explosive development. This minireview introduces the recent application of rare earth (RE) metal complex catalysts in the synthesis of targeted frameworks. The direct addition reactions of 3-substituted oxindoles or isatins are described, together with a discussion of the catalytic mechanism and related transformations to pharmaceuticals.

Mesoporous g-C3N4 nanorods as multifunctional supports of ultrafine metal nanoparticles: hydrogen generation from water and reduction of nitrophenol with tandem catalysis in one step

Abstract: Flexible mesoporous g-C3N4 nanorods with open channels, synthesized via nanocasting, can act as catalyst support, solid stabilizer and photosensitizer simultaneously for anchoring uniform metal nanoparticles with high compatibility with the composition, the synthetic method and the particle size. The final hybrid nanorods exhibit high stability and catalytic/photocatalytic activity.

Stimuli-responsive Pd2L4metallosupramolecular cages: towards targeted cisplatin drug delivery

Abstract: Metallosupramolecular cages are an emerging, but as of yet relativity unexplored, drug delivery vector. Herein we show that discrete dipalladium(II) molecular cages of the formula [Pd2L4](X)4 can be quantatively self-assembled from a simple tripyridyl ligand (2,6-bis(pyridin-3-ylethynyl)pyridine) and [Pd(CH3CN)4](X)2 (X = BF4− or SbF6−). The cages have been fully characterised using 1H, 13C and DOSY NMR spectroscopy, elemental analysis, IR spectroscopy, and high resolution electrospray mass spectrometry (HR-ESMS). Additionally, the molecular structure of the [Pd2L4](SbF6)4 cage was confirmed unequivocally using X-ray diffraction. These [Pd2L4](X)4 cages are stimuli-responsive and can be reversibly disassembled/reassembled upon the addition/removal of suitable competing ligands. The central cavities of the [Pd2L4](X)4 cages are lined with four hydrogen bond accepting pyridine units which enable the encapsulation of two cisplatin molecules within the metallosupramolecular architecture through hydrogen bonding interactions between the cage and the amine ligands of the cisplatin guest. The structure of the [Pd2L4⊃(cisplatin)2](BF4)4 host–guest adduct has been confirmed by 1H NMR spectroscopy, HR-ESMS and X-ray crystallography. Additionally we have demonstrated that the cage–cisplatin host–guest adduct can be quantatively disassembled upon the addition of a competing ligand, releasing the cisplatin guest. This is the first crystallographically characterised example of a discrete metallosupramolecular cage encapsulating an FDA-approved inorganic drug molecule. This host–guest chemistry could open the way to relatively unexplored methods of drug delivery, which circumvent the malicious side effects and drug resistance associated with cisplatin and other anticancer therapeutics.

What makes efficient circularly polarised luminescence in the condensed phase: aggregation-induced circular dichroism and light emission

Abstract: In this contribution, we conceptually present a new avenue to construction of molecular functional materials with high performance of circularly polarised luminescence (CPL) in the condensed phase. A molecule (1) containing luminogenic silole and chiral sugar moieties was synthesized and thoroughly characterized. In a solution of 1, no circular dichroism (CD) and fluorescence emission are observed, but upon molecular aggregation, both the CD and fluorescence are simultaneously turned on, showing aggregation-induced CD (AICD) and emission (AIE) effects. The AICD effect is supported by the fact that the molecules readily assemble into right-handed helical nanoribbons and superhelical ropes when aggregated. The AIE effect boosts the fluorescence quantum efficiency (ΦF) by 136 fold (ΦF, ∼0.6% in the solution versus ∼81.3% in the solid state), which surmounts the serious limitations of aggregation-caused quenching effect encountered by conventional luminescent materials. Time-resolved fluorescence study and theoretical calculation from first principles conclude that restriction of the low-frequency intramolecular motions is responsible for the AIE effect. The helical assemblies of 1 prefer to emit right-handed circularly polarised light and display large CPL dissymmetry factors (gem), whose absolute values are in the range of 0.08–0.32 and are two orders of magnitude higher than those of commonly reported organic materials. We demonstrate for the first time the use of a Teflon-based microfluidic technique for fabrication of the fluorescent pattern. This shows the highest gem of −0.32 possibly due to the enhanced assembling order in the confined microchannel environment. The CPL performance was preserved after more than half year storage under ambient conditions, revealing the excellent spectral stability. Computational simulation was performed to interpret how the molecules in the aggregates interact with each other at the molecular level. Our designed molecule represents the desired molecular functional material for generating efficient CPL in the solid state, and the current study shows the best results among the reported organic conjugated molecular systems in terms of emission efficiency, dissymmetry factor, and spectral stability.

pH-induced mechanistic changeover from hydroxyl radicals to iron(IV) in the Fenton reaction

Abstract: A major pathway in the reaction between Fe(II) and H2O2 at pH 6–7 in non-coordinating buffers exhibits inverse kinetic dependence on [H+] and leads to oxidation of dimethyl sulfoxide (DMSO) to dimethyl sulfone (DMSO2). This step regenerates Fe(II) and makes the oxidation of DMSO catalytic, a finding that strongly supports Fe(IV) as a Fenton intermediate at near-neutral pH. This Fe(IV) is a less efficient oxidant for DMSO at pH 6–7 than is (H2O)5FeO2+, generated by ozone oxidation of Fe(H2O)62+, in acidic solutions. Large concentrations of DMSO are needed to achieve significant turnover numbers at pH ≥ 6 owing to the rapid competing reaction between Fe(II) and Fe(IV) that leads to irreversible loss of the catalyst. At pH 6 and ≤0.02 mM Fe(II), the ratio of apparent rate constants for the reactions of Fe(IV) with DMSO and with Fe(II) is ∼104. The results at pH 6–7 stand in stark contrast with those reported previously in acidic solutions where the Fenton reaction generates hydroxyl radicals. Under those conditions, DMSO is oxidized stoichiometrically to methylsulfinic acid and ethane. This path still plays a role (1–10%) at pH 6–7.

N-rich zeolite-like metal–organic framework with sodalite topology: high CO2 uptake, selective gas adsorption and efficient drug delivery

Abstract: A novel zeolite-like metal–organic framework (ZMOF) with sodalite topology, [Zn(HL)]·DMA (IFMC-1, L = 4,5-di(1H-tetrazol-5-yl)-2H-1,2,3-triazole and IFMC = Institute of Functional Material Chemistry), was solvothermally synthesized based on an N-rich aromatic ligand without a NH2 group. It exhibits high CO2 uptake and selective CO2/N2 adsorption capacity. For the first time, we investigated the influence of a large number of uncoordinated nitrogen atoms from aromatic rings for CO2 adsorption in ZMOFs. This result reveals that the high percentage of open N-donor sites leads to the high uptake capacity for CO2, even in the absence of any NH2 groups and open metal sites. In addition, it also exhibits efficient drug delivery capacity.

Gram-scale synthesis and crystal structures of [8]- and [10]CPP, and the solid-state structure of C60@[10]CPP

Abstract: A cost-effective gram-scale synthesis of [8]- and [10]cycloparaphenylenes (CPPs) has been developed for the first time. Both [8]- and [10]cycloparaphenylene organized into herringbone geometries in the crystalline state with well-defined cylindrical cavities of 1.1 and 1.4 nm, respectively. With large amounts of material available, the highly efficient convex–concave π–π interactions between [10]CPP and C60 in the solid-state was validated by X-ray diffraction analysis.

Layered manganese oxides for water-oxidation: alkaline earth cations influence catalytic activity in a photosystem II-like fashion

Abstract: In reaction sequences for light driven water-splitting into H2 and O2, water-oxidation is a crucial reaction step. In vivo, the process is catalysed within a photoenzyme called photosystem II (PSII) by a μ-oxido CaMn4 cluster, the oxygen-evolving complex (OEC). The OEC is known to be virtually inactive if Ca2+ is removed from its structure. Activity can be restored not only by the addition of Ca2+ but also Sr2+ ions. We have recently introduced layered calcium manganese oxides of the birnessite mineral family as functional synthetic model compounds for the OEC. Here, we present the syntheses of layered manganese oxides where we varied the interlayer cations, preparing a series of K-, Ca-, Sr- and Mg-containing birnessites. Structural motifs within these materials were determined using X-ray absorption spectroscopy (XAS) showing that all materials have similar atomic structures despite their different elemental compositions. Water-oxidation experiments were carried out to elucidate structure-reactivity relations. These experiments demonstrated that the oxides—like the OEC—require the presence of calcium in their structures to reach maximum catalytic activity. As another similarity to the OEC, Sr2+ is the “second best choice” for the secondary cation. The results thus support mechanistic proposals which involve an important catalytic role for Ca2+ in biological water-oxidation. Additionally, they offer valuable hints for the development of synthetic, manganese-based water-oxidation catalysts for artificial photosynthesis.

A solvent-driven molecular spring

Abstract: A solvent-driven doubly threaded rotaxane dimer based on an amino-modified copillar[5]arene was prepared using bis(trifluoromethyl)phenyl isocyanate as stoppers. By comparison of proton NMR spectra of the rotaxane dimer and the control compound, the inclusion-induced shielding effects of the decyl protons of the dumbbell compound were estimated. From the crystal structures of previously reported analogous pillar[5]arene/alkane pseudorotaxanes, we know that four methylenes can be totally encapsulated in the pillar[5]arene cavity. When a pillar[5]arene is swaying along a guest with a long linear alkyl chain (more than four methylenes), its cavity statistically locates on the four methylenes whose protons showed relatively larger upfield shifts. Based on this, the length of the rotaxane dimer can be estimated. In CDCl3, it was in a contracted state with a length of 31 A. In DMSO-d6, it was in a extended state with a length of 37 A. Moreover, as the polarity of the solvent is changing, the length of the rotaxane dimer can change continuously as the contraction/stretching systems work in living organisms. Therefore, we can control the length of this molecular spring as needed.

A highly selective red-emitting FRET fluorescent molecular probe derived from BODIPY for the detection of cysteine and homocysteine: an experimental and theoretical study

Abstract: A red-emitting BODIPY-based fluorescent-resonance-energy-transfer (FRET) molecular probe 1 for selective detection of cysteine and homocysteine was designed. The fluorescence OFF–ON switch is triggered by cleavage of the 2,4-dinitrobenzensulfonyl (DNBS) unit from the fluorophore by thiols. The FRET energy donor (λabs = 498 nm, λem = 511 nm) is a parent BODIPY moiety and the energy acceptor is based on 4-hydroxylstyryl BODIPY moiety (λabs = 568 nm, λem = 586 nm). The unique C–C linker between the energy donor and acceptor was established using a Suzuki cross-coupling reaction. A polyether chain was also introduced into the probe to improve solubility in aqueous solution. While probe 1 itself is non-fluorescent, in the presence of cysteine or homocysteine a red emission at 590 nm is switched on (excitation at 505 nm), producing a pseudo-Stokes shift of up to 77 nm, which is in stark contrast to the small Stokes shift (ca. 10 nm) observed for typical BODIPY dyes. Excitation of the energy donor leads to the red emission from the acceptor of the probe, and demonstrates a high energy transfer efficiency. The probe was used for in vivo fluorescent imaging of cellular thiols. The fluorescence sensing mechanism of the probe and the photophysical properties of the fluorescent intermediates were fully rationalized by DFT calculations. The lack of fluorescence of probe 1 is attributed to the dark excited state S1 (oscillator strength f = 0.0007 for S0 → S1, based on the optimized S1 state geometry), which is due to the electron sink effect of the DNBS moiety. Cleavage of the DNBS moiety from the fluorophore by thiols re-establishes the emissive S1 state of the fluorophore (f = 1.4317 for S0 → S1), thus the red emission can be observed in the presence of thiols (fluorescence is turned on). The FRET effect of the probe was rationalized by DFT calculations which indicated that upon excitation into the S4 excited state (localized on the energy donor unit), the S1 state (localized on the energy acceptor, i.e. styryl-BODIPY) is populated via internal conversion (IC), thus red emission from the styryl-BODIPY energy acceptor is observed (Kasha's rule).

Cu2O|NiOx nanocomposite as an inexpensive photocathode in photoelectrochemical water splitting

Abstract: A photoelectrochemical (PEC) cell for overall water splitting made of a Cu2O nanowire photocathode modified with a thin film of NiOx coupled to a WO3 nanosheet photoanode is presented. The photocathode was prepared by thermal annealing of Cu(OH)2 nanowires on a Cu foil under N2, followed by the deposition of a 10 nm NiOx film on the Cu2O nanowires (aspect ratio > 40). XPS spectra revealed that the surface species of NiOx is a mixture of NiO and Ni(OH)2, which enhances charge separation in photoexcited Cu2O, as confirmed by electrochemical impedance spectroscopy. The optimized NiOx modified Cu2O electrode shows a photocurrent density up to −4.98 mA cm−2 at −0.33 V and −0.56 mA cm−2 at 0.1 V vs. the normal hydrogen electrode (NHE) under white-light irradiation (26 mW cm−2) in an aqueous electrolyte solution at pH 6 and 25 °C. The formation of H2 gas was only observed by gas chromatography for NiOx-modified Cu2O and was not detectable for unmodified Cu2O electrodes during prolonged irradiation. The nanocomposite structure also resulted in a three-fold increase in photostability of Cu2O; 72 ± 3% of the initial photocurrent density remained for the NiOx modified Cu2O electrode after 20 min irradiation at 0.1 V vs. NHE. The optimized photocathode was subsequently used in a two-electrode PEC cell with an n-type WO3 nanosheet photoanode for overall water splitting. The different band gap of Cu2O (2 eV) and WO3 (2.6 eV) permits for efficient and complementary light absorption and sunlight-driven water splitting. The p/n heterojunction PEC cell operates with a small output of electricity even in the absence of an external bias. We demonstrate that a Cu2O-based electrode for H2 evolution can be prepared free of noble metals and we show its utilization in a PEC water splitting cell made solely from earth abundant elements.

Thermodynamic analysis of Xe/Kr selectivity in over 137000 hypothetical metal–organic frameworks

Abstract: Metal–organic frameworks (MOFs) are porous crystals with the potential to improve many industrial gas separation processes Because there is a practically unlimited number of different MOFs, which vary in their pore geometry and chemical composition, it is challenging to find the best MOF for a given application Here, we applied high-throughput computational methods to rapidly explore thousands of possible MOFs, given a library of starting materials, in the context of Xe/Kr separation We generated over 137 000 structurally diverse hypothetical MOFs from a library of chemical building blocks and screened them for Xe/Kr separation For each MOF, we calculated geometric properties via Delaunay tessellation and predicted thermodynamic Xe/Kr adsorption behavior via multicomponent grand canonical Monte Carlo simulations Specifically, we calculated the pore limiting diameter, largest cavity diameter, accessible void volume, as well as xenon and krypton adsorption at 10, 50 and 10 bar at 273 K From these data we show that MOFs with pores just large enough to fit a single xenon atom, and having morphologies resembling tubes of uniform width, are ideal for Xe/Kr separation Finally, we compare our generated MOFs to several known structures (IRMOF-1, HKUST-1, ZIF-8, Pd-MOF, & MOF-505) and conclude that significantly improved materials remain to be synthesized All crystal structure files are freely available for download and browsing in an online database

Photocatalytic oxidation of water by polymeric carbon nitride nanohybrids made of sustainable elements

Abstract: The search for earth-abundant semiconductors and co-catalysts for water oxidation is an important step towards conversion of solar energy into fuel chemicals in a sustainable manner. Here, we report the integration of Co3O4 nanoparticles within graphitic carbon nitride to construct a nanohybrid photocatalyst made of abundant elements for the photocatalytic generation of oxygen from water, achieving an apparent quantum efficiency of 1.1% at 420 nm for water splitting to oxygen. The present system shows great promise in polymer photocatalysis for applications in future energy cycles based on sustainable materials.

Transforming CdS into an efficient visible light photocatalyst for selective oxidation of saturated primary C–H bonds under ambient conditions

Abstract: Selective activation of saturated sp3 C–H bonds to high-value-added chemicals remains a significant but challenging task for the sustainable exploitation of available feedstocks. However, the selective oxidation of C–H bonds with environmentally benign oxygen is often very difficult to control. Research works available in thermal heterogeneous catalysis often involve the use of transition metal particles together with harsh reaction conditions, e.g., high temperature and high pressure, which results in the difficulty in controlling the selectivity. Here, we report a very simple room temperature method to prepare a cubic phase, sheet structured semiconductor CdS sample. The as-prepared CdS is able to be used as a visible-light-driven photocatalyst for the selective oxidation of saturated primary C–H bonds in alkyl aromatics with high activity and selectivity using molecular oxygen as a benign oxidant and benzotrifluoride as the solvent under ambient conditions, i.e., room temperature and atmospheric pressure. The superior photocatalytic performance of CdS can be attributed to its unique assembly of sheet structure with cubic phase, high surface area and efficient separation of photogenerated charge carriers. The possible reaction mechanism for the photocatalytic selective oxidation of such C–H bonds over the CdS semiconductor has also been proposed.

Iron imidazolate framework as precursor for electrocatalysts in polymer electrolyte membrane fuel cells

Abstract: The extensive use of Pt and platinum group metals (PGM) as electrocatalysts poses a significant cost barrier for the commercialization of polymer electrolyte membrane fuel cells (PEMFC). Replacing Pt with non-PGM electrocatalysts is a long-term pursuit of the scientific community. In this study, non-PGM cathode electrocatalysts for PEMFC were prepared by pyrolyzing an iron imidazolate framework. The new catalyst demonstrated excellent activity towards oxygen reduction reaction in the acidic medium. The catalytic activity was further improved by mixing with a zinc imidazolate framework, ZIF-8. The membrane electrode assembly made of such catalyst as the cathode demonstrated an onset potential of 0.977 V and measured volumetric current density of 12 A cm−3 at 0.8 V in a single cell test.

Alternating layer addition approach to CdSe/CdS core/shell quantum dots with near-unity quantum yield and high on-time fractions

Abstract: We report single-particle photoluminescence (PL) intermittency (blinking) with high on-time fractions in colloidal CdSe quantum dots (QD) with conformal CdS shells of 1.4 nm thickness, equivalent to approximately 4 CdS monolayers. All QDs observed displayed on-time fractions >60% with the majority >80%. The high on-time fraction blinking is accompanied by fluorescence quantum yields (QY) close to unity (up to 98% in an absolute QY measurement) when dispersed in organic solvents and a monoexponential ensemble photoluminescence (PL) decay lifetime. The CdS shell is formed in high synthetic yield using a modified selective ion layer adsorption and reaction (SILAR) technique that employs a silylated sulfur precursor. The CdS shell provides sufficient chemical and electronic passivation of the QD excited state to permit water solubilization with greater than 60% QY via ligand exchange with an imidazole-bearing hydrophilic polymer.

Copper-catalyzed olefinic trifluoromethylation of enamides at room temperature

Abstract: Copper-catalyzed olefinic trifluoromethylation and oxytrifluoromethylation of enamides is reported. The direct olefinic C–H trifluoromethylation constitutes an efficient method for the stereoselective synthesis of β-trifluoromethyl substituted enamides.

Selective liquid phase oxidation with supported metal nanoparticles

Abstract: Over the past twenty years there has been intense interest in the design and understanding of catalysis by gold. More recently, it has been observed that alloying gold with a second metal greatly enhances the catalytic efficacy. These supported nanoparticles offer great potential as catalysts for the synthesis of fine chemicals and they are now at the stage where they can contribute to the sustainable development of chemical processes, in particular selective oxidation. A number of factors have contributed to this increased interest; namely, environmental issues promoting the need for more atom efficient processes and the new advances in the synthesis of nanoparticles as well as the characterisation methods available for their study. New catalytic materials obtained by careful control of the morphology of the metal nanoparticles and their use under solvent-free conditions all contribute to the latest developments. Most importantly their use can obviate the need to use stoichiometric oxidants. In this perspective we demonstrate the recent advances in these materials for selective oxidation reactions.

Catalytic intermolecular hydroacylation of C–C π-bonds in the absence of chelation assistance

Abstract: Intermolecular rhodium catalyzed hydroacylation in the absence of chelation assistance remains a largely unmet challenge due to competing decarbonylation of the acylrhodium intermediates to form catalytically inactive carbonyl complexes. Here, catalytic systems for intermolecular hydroacylation in the absence of chelation assistance are reviewed, with an emphasis on recently described processes that operate through mechanistic pathways beyond aldehyde C–H oxidative addition.

Copper-catalyzed decarboxylative alkenylation of sp3 C–H bonds with cinnamic acids via a radical process

Abstract: A copper-catalyzed decarboxylative coupling of vinylic carboxylic acids with simple alcohols, ethers, and hydrocarbons was achieved. In the past decades, most of the sp3 α-C–H activation/C–C bond formation reactions proceeded via an addition of a α-hydroxy carbon-centered radical to heterocycles, alkenes, and alkynes. The present system exhibits a novel pathway for the functionalization of various sp3 C–H bonds via radical addition–elimination of aryl-substituted vinyl carboxylic acids. This strategy allows for rapid and selective access to a variety of (E)-alkenes such as allylic alcohols, allylic ethers, and substituted styrenes. In addition, this procedure could be scaled up to gram level, which would be useful to prepare natural products and pharmaceuticals that contain chromene and its derivatives.