Z‐Selective (Cross‐)Dimerization of Terminal Alkynes Catalyzed by an Iron Complex

Efficient iron‐catalyzed homocoupling of terminal alkynes and cross‐dimerization of aryl acetylenes with trimethylsilylacetylene is reported. The complex [Fe(H)(BH₄)(iPr‐PNP)] ( 1 ) catalyzed the (cross‐)dimerization of alkynes at room temperature, with no need for a base or other additives, to give the corresponding dimerized products with Z selectivity in excellent yields (79–99 %).     

Hydroboration of Nitriles,Esters, and Carbonates Catalyzed by Simple Earth-Abundant Metal Triflate Salts

During the past decade earth-abundant metals have become increasingly important in homogeneous catalysis. One of the reactions in which earth-abundant metals have found important applications is the hydroboration of unsaturated C−C and C−X bonds (X=O or N). Within these set of transformations, the hydroboration of challenging substrates such as nitriles, carbonates and esters still remain difficult and often relies on elaborate ligand designs and highly reactive catalysts (e. g., metal alkyls/hydrides). Here we report an effective methodology for the hydroboration of challenging C≡N and C=O bonds that is simple and applicable to a wide set of substrates. The methodology is based on using a manganese(II) triflate salt that, in combination with commercially available potassium tert-butoxide and pinacolborane, catalyzes the hydroboration of nitriles, carbonates, and esters at room temperature and with near quantitative yields in less than three hours. Additional studies demonstrated that other earth-abundant metal triflate salts can facilitate this reaction as well, which is further discussed in this report.     

P2‐NaCo0.5Mn0.5O2 as a Positive Electrode Material for Sodium‐Ion Batteries

As a promising positive electrode material for sodium‐ion batteries (SIBs), layered sodium oxides have attracted considerable attention in recent years. In this work, stoichiometric P2‐phase NaCo_0.5Mn_0.5O₂ was prepared through the conventional solid‐state reaction, and its structural and physical properties were studied in terms of XRD, XPS, and magnetic susceptibility. Furthermore, the P2‐NaCo_0.5Mn_0.5O₂ electrode delivered a discharge capacity of 124.3 mA h g^?1 and almost 100 % initial coulombic efficiency over the potential window of 1.5–4.15 V. It also showed good cycle stability, with a reversible capacity and capacity retention reaching approximately 85 mA h g^?1 and 99 %, respectively, at the 5 C rate after 100 cycles. Additionally, cyclic voltammetry and ex situ XRD were employed to explain the electrochemical behavior at the different electrochemical stages. Owing to the applicable performances, P2‐NaCo_0.5Mn_0.5O₂ can be considered as a potential positive electrode material for SIBs.     

Tunable Bidirectional Electroosmotic Flow for Diffusion-Based Separations

We present a new concept for on-chip separation that leverages bidirectional flow, to tune the dispersion regime of molecules and particles. The system can be configured so that low diffusivity species experience a ballistic transport regime and are advected through the chamber, whereas high diffusivity species experience a diffusion dominated regime with zero average velocity and are retained in the chamber. We detail the means of achieving bidirectional electroosmotic flow using an array of alternating current (AC) field-effect electrodes, experimentally demonstrate the separation of particles and antibodies from dyes, and present a theoretical analysis of the system, providing engineering guidelines for its design and operation.     

Low-pressure hydrogenation of carbon dioxide catalyzed by an iron pincer complex exhibiting noble metal activity

A highly active iron catalyst for the hydrogenation of carbon dioxide and bicarbonates works under remarkably low pressures and achieves activities similar to some of the best noble metal catalysts. A mechanism is proposed involving the direct attack of an iron trans-dihydride on carbon dioxide, followed by ligand exchange and dihydrogen coordination.     

Dynamic Nuclear Polarization in the solid state: a transition between the cross effect and the solid effect

Proton Dynamic Nuclear Polarization (DNP) experiments were conducted on a 3.4 T homebuilt hybrid pulsed-EPR-NMR spectrometer, on static samples containing 10 mM or 40 mM TEMPOL in frozen glassy solutions of DMSO/water. During DNP experiments proton-NMR signals are enhanced with the help of microwave (MW) irradiation on or close to the Electron Paramagnetic Resonance (EPR) spectrum of the free radicals in the sample, transferring polarization from the free electrons to the nuclei. In the solid state a distinction is made between three DNP enhancement mechanisms: the Solid Effect (SE), the Cross Effect (CE) and Thermal Mixing (TM). In an effort to determine the dominant DNP mechanisms responsible for the enhancement of the nuclear signals, electron and nuclear spin-lattice relaxation rates, enhancement buildup times and microwave (MW) swept DNP spectra were measured as a function of temperature and MW irradiation strength. We observed lineshape variations of the DNP spectra that indicated changes in the relative contributions of SE-DNP and CE-DNP with temperature and MW power. Using a theoretical model describing the SE-DNP and CE-DNP the DNP spectra could be analyzed without involving the TM-DNP mechanism and the relative SE-DNP and CE-DNP contributions to the nuclear enhancement could be determined. From this analysis it follows that lowering the temperature beyond 20 K increases the SE-DNP and decreases the CE-DNP contributions. Possible explanations for this behavior are suggested.     

Slow Unfolded-State Structuring in Acyl-CoA Binding Protein Folding Revealed by Simulation and Experiment

Protein folding is a fundamental process in biology, key to understanding many human diseases. Experimentally, proteins often appear to fold via simple two- or three-state mechanisms involving mainly native-state interactions, yet recent network models built from atomistic simulations of small proteins suggest the existence of many possible metastable states and folding pathways. We reconcile these two pictures in a combined experimental and simulation study of acyl-coenzyme A binding protein (ACBP), a two-state folder (folding time ～10 ms) exhibiting residual unfolded-state structure, and a putative early folding intermediate. Using single-molecule FRET in conjunction with side-chain mutagenesis, we first demonstrate that the denatured state of ACBP at near-zero denaturant is unusually compact and enriched in long-range structure that can be perturbed by discrete hydrophobic core mutations. We then employ ultrafast laminar-flow mixing experiments to study the folding kinetics of ACBP on the microsecond time scale. These studies, along with Trp-Cys quenching measurements of unfolded-state dynamics, suggest that unfolded-state structure forms on a surprisingly slow (～100 μs) time scale, and that sequence mutations strikingly perturb both time-resolved and equilibrium smFRET measurements in a similar way. A Markov state model (MSM) of the ACBP folding reaction, constructed from over 30 ms of molecular dynamics trajectory data, predicts a complex network of metastable stables, residual unfolded-state structure, and kinetics consistent with experiment but no well-defined intermediate preceding the main folding barrier. Taken together, these experimental and simulation results suggest that the previously characterized fast kinetic phase is not due to formation of a barrier-limited intermediate but rather to a more heterogeneous and slow acquisition of unfolded-state structure.     

Reactivity of long conjugated systems: selectivity of Diels-Alder cycloaddition in oligofurans

Taking advantage of the synthetic availability and solubility of long oligofurans, their reactivity toward dienophiles was studied as a model for the rarely investigated reactivity of long conjugated systems. Unlike oligoacenes, the reactivity of oligofurans decreases or remains constant with increasing chain length. Terminal ring cycloadducts of oligofurans are kinetically and thermodynamically favored, whereas central ring cycloadducts are preferred in oligoacenes, because of the different driving forces in the two reactions: π-conjugation in oligofurans and aromatization/dearomatization in oligoacenes.     

Optical characteristics and plasma parameters of a blue-green exciplex lamp

The optical characteristics and plasma parameters of an exciplex lamp radiating in the blue-green spectral range are studied. A plasma is generated by an atmospheric-pressure barrier discharge initiated in a quaternary mixture including mercury dibromide, sulfur hexafluoride, nitrogen, and helium. It is shown that the exciplex lamp can radiate at an elevated repetition rate of pump pulses (1–12 kHz) under the conditions of mixture self-heating. A tradeoff between the radiation power and nitrogen partial pressure is found. The mean specific radiation power in the blue-green range at a level of 480 mW/cm³ is achieved at partial vapor pressures of mercury dibromide, sulfur hexafluoride, nitrogen, and helium of 0.70, 0.07, 4.00, and 117.20 kPa, respectively. The plasma parameters, namely, the electron energy distribution function; concentration, temperature, and mean energy of electrons; transport properties; and rate constants of elastic and inelastic electron scattering by the working mixture components are determined as functions of ratio E/N (where E is the electric field strength and N is the total concentration of mercury dibromide, sulfur hexafluoride, and nitrogen molecules and helium atoms). It is found that mercury monobromide molecules and also excited and higher energy states take part in the population of exciplex molecules HgBr* (B²Σ₁^2/+ states) in the course of quenching these exciplexes by sulfur hexafluoride and nitrogen molecules.