Nitrogen‐Doped Carbon Dots: A Facile and General Preparation Method,Photoluminescence Investigation,and Imaging Applications

Carbon dots (Cdots) are an important probe for imaging and sensing applications because of their fluorescence property, good biocompatibility, and low toxicity. However, complex procedures and strong acid treatment are often required and Cdots suffer from low photoluminescence (PL) emission. Herein, a facile and general strategy using carbonization of precursors and then extraction with solvents is proposed for the preparation of nitrogen‐doped Cdots (N‐Cdots) with 3‐(3,4‐dihydroxyphenyl)‐L ‐alanine (L ‐DOPA), L ‐histidine, and L ‐arginine as precursor models. After they are heated, the precursors become carbonized. Nitrogen‐doped Cdots are subsequently extracted into N,N′‐dimethylformamide (DMF) from the carbogenic solid. A core–shell structure of Cdots with a carbon core and the oxygen‐containing shell was observed. Nitrogen has different forms in N‐Cdots and oxidized N‐Cdots. The doped nitrogen and low oxidation level in N‐Cdots improve their emission significantly. The N‐Cdots show an emission with a nitrogen‐content‐dependent intensity and Cdot‐size‐dependent emission‐peak wavelength. Imaging of HeLa cells, a human cervical cancer cell line, and HepG2 cells, a human hepatocellular liver carcinoma line, was observed with high resolution using N‐Cdots as a probe and validates their use in imaging applications and their multicolor property in the living cell system.     

Formation and Properties of Self‐Assembly‐Driven Fluorescent Nanoparticle Sensors

Fluorescent nanoparticles (FNPs) are obtained in water by self‐assembly from a polymeric ionic liquid, fluorescent carboxylate moiety, and a surfactant through two main supramolecular interactions, that is, ionic bonds and hydrophobic/hydrophilic interactions. The hydrophobicity of the surfactant is tunable and a highly hydrophobic surfactant increases the fluorescence intensity and stability of the FNPs. The fluorescence of the FNPs is sensitive to a quenching effect by various ions with high selectivity, and consequently, they may be used as sensors. The self‐assembly approach used to generate the FNPs is considerably simpler than other methods based on more challenging synthetic methods and the flexibility of the approach should allow a wide and diverse range of FNPs to be prepared with specific sensor applications.     

Supramolecular Approaches towards Ordered Polymer Materials

Controlled organization of polymer chains into ordered structures is highly important to tune or enhance the properties of the polymeric materials. A supramolecular approach using host–guest chemistry has allowed rational design of chain assemblies with many functional properties. Nanoporous materials with ordered channel structures are particularly useful for attaining precise assemblies of polymer chains through nanoconfinement.     

Silver‐Catalyzed Cross‐Coupling of Propargylic Alcohols with Isocyanides: An Atom‐Economical Synthesis of 2,3‐Allenamides

Cross‐coupling reactions between propargylic alcohols and isocyanides, by means of silver catalysis, have been described. This new reaction is both atom and step efficient and is applicable to a broad scope of substrates, allowing the synthesis of a range of synthetically valuable 2,3‐allenamides in moderate to excellent yields.     

On the Reaction Mechanism of the Complete Intermolecular O2 Transfer between Mononuclear Nickel and Manganese Complexes with Macrocyclic Ligands

The recently described intermolecular O₂ transfer between the side‐on Ni‐O₂ complex [(12‐TMC)Ni‐O₂]⁺ and the manganese complex [(14‐TMC)Mn]²⁺, where 12‐TMC and 14‐TMC are 12‐ and 14‐membered macrocyclic ligands, 12‐TMC=1,4,7,10‐tetramethyl‐1,4,7,10‐tetraazacyclododecane and 14‐TMC=1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane, is studied by means of DFT methods. B3LYP calculations including long‐range corrections and solvent effects are performed to elucidate the mechanism. The potential energy surfaces (PESs) compatible with different electronic states of the reactants have been analyzed. The calculations confirm a two‐step reaction, with a first rate‐determining bimolecular step and predict the exothermic character of the global process. The relative stability of the products and the reverse barrier are in line with the fact that no reverse reaction is experimentally observed. An intermediate with a μ‐η¹:η¹‐O₂ coordination and two transition states are identified on the triplet PES, slightly below the corresponding stationary points of the quintet PES, suggesting an intersystem crossing before the first transition state. The calculated activation parameters and the relative energies of the two transition sates and the products are in very good agreement with the experimental data. The calculations suggest that a superoxide anion is transferred during the reaction.     

Versatile Iridicycle Catalysts for Highly Efficient and Chemoselective Transfer Hydrogenation of Carbonyl Compounds in Water

Cyclometalated iridium complexes are shown to be highly efficient and chemoselective catalysts for the transfer hydrogenation of a wide range of carbonyl groups with formic acid in water. Examples include α‐substituted ketones (α‐ether, α‐halo, α‐hydroxy, α‐amino, α‐nitrile or α‐ester), α‐keto esters, β‐keto esters and α,β‐unsaturated aldehydes. The reduction was carried out at substrate/catalyst ratios of up to 50 000 at pH 4.5 and required no organic solvent. The protocol provides a practical, easy and efficient way for the synthesis of β‐functionalised secondary alcohols, such as β‐hydroxyethers, β‐hydroxyamines and β‐hydroxyhalo compounds, which are valuable intermediates in pharmaceutical, fine chemical, perfume and agrochemical synthesis.     

High-Pressure Synthesis of Fully Occupied Tetragonal and Cubic Tungsten Bronze Oxides

A high-pressure reaction yielded the fully occupied tetragonal tungsten bronze K₃W₅O₁₅ (K_0.6WO₃). The terminal phase shows an unusual transport property featuring slightly negative temperature-dependence in resistivity (dρ/dT<0) and a large Wilson ratio of R_W=3.2. Such anomalous metallic behavior possibly arises from the low-dimensional electronic structure with a van Hove singularity at the Fermi level and/or from enhanced magnetic fluctuations by geometrical frustration of the tungsten sublattice. The asymmetric nature of the tetragonal tungsten bronze K_xWO₃-K_0.6−yBa_yWO₃ phase diagram implies that superconductivity for x≤0.45 originates from the lattice instability because of potassium deficiency. A cubic perovskite KWO₃ phase was also identified as a line phase—in marked contrast to Na_xWO₃ and Li_xWO₃ with varying quantities of x (<1). This study presents a versatile method by which the solubility limit of tungsten bronze oxides can be extended.     

Enantioselective Synthesis of (+)-Mitomycin K by a Palladium-Catalyzed Oxidative Tandem Cyclization

The mitomycins, a family of bioactive natural products, feature a compact 6/5/5-fused polycyclic ring structure densely decorated with highly reactive and/or fragile quinone, amino ketal, and aziridine as well as carbamate moieties. It is this striking feature that has defeated numerous synthetic attempts towards these apparently small molecules, rendering them one of the most formidable targets for total synthesis. We herein report the first enantioselective synthesis of (+)-mitomycin K, a representative of G series mitomycins. The key step of this synthesis is an enantioselective oxidative cyclization catalyzed by a palladium/(+)-sparteine system that had previously been developed by our group. The robustness of this method bodes well for further applications in the asymmetric total synthesis of natural products, particularly those with characteristic 6/5/5-fused pyrroloindole skeletons.     

The Interstitial Carbon of the Nitrogenase FeMo Cofactor is Far Better Stabilized than Previously Assumed

The first quantum-mechanical calculations of all relevant potential constants in both the iron-molybdenum cofactor and the iron-vanadium cofactor of nitrogenase suggest that the carbide is bound to the center of the enzyme much more strongly than hitherto assumed. Previous studies seemed to indicate a dummy function of the interstitial carbon, with a weak force constant (ca. 0.32 N cm⁻¹). Our new investigations confirm a different picture: the central carbon atom binds the iron-sulfur cluster through six covalent C−Fe bonds. With a potential constant of more than 1.3 N cm⁻¹, the interstitial carbon also appears to be dynamically persistent. According to our investigations, the values for the elasticity within the iron-sulfur cluster have to be corrected too. These new details on the mechano-chemical properties of the FeMo cofactor will be important for elucidating the catalytic cycle of nitrogen fixation. By implementing our new algorithm in the freely available COMPLIANCE program, the dependence on the coordinates during the calculation of Hesse matrices is eliminated completely.     

Activation and Photoinduced Release of Alkynes on a Biomimetic Tungsten Center: The Photochemical Behavior of the W−S-Phoz System

The synthesis and structural determination of four tungsten alkyne complexes coordinated by the bio-inspired S,N-donor ligand 2-(4′,4′-dimethyloxazoline-2′-yl)thiophenolate (S-Phoz) is presented. A previously established protocol that involved the reaction of the respective alkyne with the bis-carbonyl precursor [W(CO)₂(S-Phoz)₂] was used for the complexes [W(CO)(C₂R₂)(S-Phoz)₂] (R=H, 1 a ; Me, 1 b ; Ph, 1 c ). Oxidation with pyridine-N-oxide gave the corresponding W-oxo species [WO(C₂R₂)(S-Phoz)₂] (R=H, 2 a ; Me, 2 b ; Ph, 2 c ). All W-oxo-alkyne complexes ( 2 a , b , c ) were found to be capable of alkyne release upon light irradiation to afford five-coordinate [WO(S-Phoz)₂] ( 3 ). The photoinduced release of the alkyne ligand was studied in detail by in situ ¹H NMR measurements, which revealed correlation of the photodissociation rate constant ( 2 b>2 a>2 c ) with the elongation of the alkyne C≡C bond in the molecular structures. Oxidation of [WO(S-Phoz)₂] ( 3 ) with pyridine-N-oxide yielded [WO₂(S-Phoz)₂] ( 4 ), which shows highly fluxional behavior in solution. Variable-temperature ¹H NMR spectroscopy revealed three isomeric forms with respect to the ligand arrangement versus each other. Furthermore, compound 4 rearranges to tetranuclear oxo compound [W₄O₄(μ-O)₆(S-Phoz)₄] ( 5 ) and dinuclear [{WO(μ-O)(S-Phoz)}₂] ( 6 ) over time. The latter two were identified by single-crystal X-ray diffraction analyses.