Ligand and base additive effects on the reversibility of nucleophilic addition in palladium-catalyzed allylic aminations monitored by nucleophile crossover

A nucleophile crossover experiment was used to monitor the reversibility of nucleophilic addition of benzylamine to π-allylpalladium complexes. Dppe, dppp, dppb, and PHOX showed more crossover than PPh₃ and dppm in both DMF and dichloromethane. Crossover was inhibited by the addition of DBU or Cs₂CO₃, but much less elimination to diene side products was observed with Cs₂CO₃. Analysis of percent crossover vs. percent reaction completion using the PHOX ligand revealed that with added DBU or Cs₂CO₃ crossover only began occurring after 100% completion had been reached.     

ESI activity of Br−, BF4−, ClO4− and BPh4− anions in the presence of Li+ and NBu4+ counter‐ions

To improve our understanding of the electrospray ionization (ESI) process, we have subjected equimolar mixtures of salts A⁺X⁻ (A⁺ = Li⁺, NBu₄⁺; X⁻ = Br⁻, ClO₄⁻, BF₄⁻, BPh₄⁻) in different solvents (CH₃CN, tetrahydrofuran, CH₃OH, H₂O) to negative‐ion mode ESI and analyzed the relative ESI activity of the different anionic model analytes. The ESI activity of the large and hydrophobic BPh₄⁻ ion greatly exceeds that of the smaller and more hydrophilic anions Br⁻, ClO₄⁻ and BF₄⁻, which we ascribe to its higher surface activity. Moreover, the ESI activity of the anions is modulated by the action of the counter‐ions and their different tendency toward ion pairing. The tendency toward ion pairing can be reduced by the addition of the chelating ligands 12‐crown‐4 and 2.2.1 cryptand and is, although to a smaller degree, further influenced by the variation of the solvent. Complementary electrical conductivity measurements afford additional information on the interactions of the ionic constituents of the sample solutions. Copyright © 2017 John Wiley & Sons, Ltd.     

Reliable and efficient reaction path and transition state finding for surface reactions with the growing string method

The computational challenge of fast and reliable transition state and reaction path optimization requires new methodological strategies to maintain low cost, high accuracy, and systematic searching capabilities. The growing string method using internal coordinates has proven to be highly effective for the study of molecular, gas phase reactions, but difficulties in choosing a suitable coordinate system for periodic systems has prevented its use for surface chemistry. New developments are therefore needed, and presented herein, to handle surface reactions which include atoms with large coordination numbers that cannot be treated using standard internal coordinates. The double‐ended and single‐ended growing string methods are implemented using a hybrid coordinate system, then benchmarked for a test set of 43 elementary reactions occurring on surfaces. These results show that the growing string method is at least 45% faster than the widely used climbing image‐nudged elastic band method, which also fails to converge in several of the test cases. Additionally, the surface growing string method has a unique single‐ended search method which can move outward from an initial structure to find the intermediates, transition states, and reaction paths simultaneously. This powerful explorative feature of single ended‐growing string method is demonstrated to uncover, for the first time, the mechanism for atomic layer deposition of TiN on Cu(111) surface. This reaction is found to proceed through multiple hydrogen‐transfer and ligand‐exchange events, while formation of H‐bonds stabilizes intermediates of the reaction. Purging gaseous products out of the reaction environment is the driving force for these reactions. © 2017 Wiley Periodicals, Inc.     

How computational methods and relativistic effects influence the study of chemical reactions involving Ru‐NO complexes?

Two treatments of relativistic effects, namely effective core potentials (ECP) and all‐electron scalar relativistic effects (DKH2), are used to obtain geometries and chemical reaction energies for a series of ruthenium complexes in B3LYP/def2‐TZVP calculations. Specifically, the reaction energies of reduction ( A ‐ F ), isomerization ( G‐I ), and Cl⁻ negative trans influence in relation to NH₃ ( J ‐ L ) are considered. The ECP and DKH2 approaches provided geometric parameters close to experimental data and the same ordering for energy changes of reactions A ‐ L . From geometries optimized with ECP, the electronic energies are also determined by means of the same ECP and basis set combined with the computational methods: MP2, M06, BP86, and its derivatives, so as B2PLYP, LC‐wPBE, and CCSD(T) (reference method). For reactions A ‐ I , B2PLYP provides the best agreement with CCSD(T) results. Additionally, B3LYP gave the smallest error for the energies of reactions J ‐ L . © 2017 Wiley Periodicals, Inc.     

The Quest for Mononuclear Gold(II) and Its Potential Role in Photocatalysis and Drug Action

The chemistry of gold strongly focuses on the ubiquitous oxidation states +I and +III. The intermediate oxidation state +II is generally avoided in mononuclear gold species. In recent years, gold(II) has been increasingly suggested as a key intermediate in artificial photosynthesis systems, with gold(III) moieties acting as electron acceptors, as well as in gold‐catalyzed photoredox catalysis and radical chemistry. This Minireview provides a concise summary of confirmed and characterized mononuclear open‐shell gold(II) complexes. Recent findings on structural motifs and reactivity patterns will be discussed. Exciting developments in the fields of photosynthesis, photocatalysis, and potential roles in medicinal chemistry will be outlined.     

CsPbBr3 QD/AlOx Inorganic Nanocomposites with Exceptional Stability in Water,Light, and Heat

Herein, the assembly of CsPbBr₃ QD/AlO_x inorganic nanocomposites, by using atomic layer deposition (ALD) for the growth of the amorphous alumina matrix (AlO_x ), is described as a novel protection scheme for such QDs. The nucleation and growth of AlO_x on the QD surface was thoroughly investigated by miscellaneous techniques, which highlighted the importance of the interaction between the ALD precursors and the QD surface to uniformly coat the QDs while preserving the optoelectronic properties. These nanocomposites show exceptional stability towards exposure to air (for at least 45 days), irradiation under simulated solar spectrum conditions (for at least 8 h), and heat (up to 200 °C in air), and finally upon immersion in water. This method was extended to the assembly of CsPbBr_x I_3−x QD/AlO_x and CsPbI₃ QD/AlO_x nanocomposites, which were more stable than the pristine QD films.     

Effects of Shape of Crowders on Dynamics of a Polymer Chain Closure

Using 3D Langevin dynamics simulations,we investigate the effects of the shape of crowders on the dynamics of a polymer chain closure.The chain closure in spherical crowders is dominated by the increased medium viscosity so that it gets slower with the increasing volume fraction of crowders.By contrast,the dynamics of chain closure becomes very complicated with increasing volume fraction of crowders in spherocylindrical crowders.Notably,the mean closure time is found to have a dramatic decrease at a range of volume fraction of crowders 0.36-0.44.We then elucidate that an isotropic to nematic transition of spherocylindrical crowders at this range of volume fraction of crowders is responsible for the unexpected dramatic decrease in the mean closure time.     

FeOx Coating on Pd/C Catalyst by Atomic Layer Deposition Enhances the Catalytic Activity in Dehydrogenation of Formic Acid

Hydrogen generation from formic acid (FA) has received significant attention.The challenge is to obtain a highly active catalyst under mild conditions for practical applications.Here atomic layer deposition (ALD) of FeO_x was performed to deposit an ultrathin oxide coating layer to a Pd/C catalyst,therein the FeO_x coverage was precisely controlled by ALD cycles.Transmission electron microscopy and powder X-ray diffraction measurements suggest that the FeO_x coating layer improved the thermal stability of Pd nanoparticles (NPs).X-ray photoelectron spectroscopy measurement showed that deposition of FeO_x on the Pd NPs caused a positive shift of Pd3d binding energy.In the FA dehydrogenation reaction,the ultrathin FeO_x layer on the Pd/C could considerably improve the catalytic activity,and Pd/C coated with 8 cycles of FeO_x showed an optimized activity with turnover frequency being about 2 times higher than the uncoated one.The improved activities were in a volcanoshape as a function of the number of FeO_x ALD cycles,indicating the coverage of FeO_x is critical for the optimized activity.In summary,simultaneous improvements of activity and thermal stability of Pd/C catalyst by ultra-thin FeO_x overlayer suggest to be an effective way to design active catalysts for the FA dehydrogenation reaction.