Holli

Composites of mesoporous polymeric carbon nitride and tungsten(VI) oxide show very high photocatalytic activity for the evolution of hydrogen from water under visible light and in the presence of sacrificial electron donors. Already addition of very small amounts of WO₃ yields up to a twofold increase in the efficiency when compared to bulk carbon nitrides and their composites and more notably even to the best reported mesoporous carbon nitride-based photocatalytic materials. The higher activity can be attributed to the high surface area and synergetic effect of the carbon nitrides and the WO₃ resulting in improved charge separation through a photocatalytic solid-state Z-scheme mechanism.

CN/WO₃ composites for enhanced H₂ evolution: Mesoporous CN/WO₃ composites prepared by a simple dispersion method are efficient photocatalysts for enhanced hydrogen production from water. Compared to other nonporous composites based on carbon nitrides, a twofold increase in activity is observed, which is attributed to the high surface area and efficient solid-state Z-scheme-induced charge separation in these composites.

The first stable copper borohydride complex [(CAAC)CuBH₄] [CAAC=cyclic(alkyl)(amino)carbene] bearing a single monodentate ligand was prepared by addition of NaBH₄ or BH₃NH₃ to the corresponding [(CAAC)CuCl] complex. Both complexes are air-stable and promote the catalytic hydrolytic dehydrogenation of ammonia borane. The amount of hydrogen released reaches 2.8 H₂/BH₃NH₃ with a turnover frequency of 8400 mol mol_cat⁻¹ h⁻¹ at 25 °C. In a fifteen-cycle experiment, the catalyst was reused without any loss of efficiency.

Copper works! [(CAAC)CuCl] reacts with both NaBH₄ and BH₃NH₃ to afford [(CAAC)CuBH₄] as a thermally and air-stable complex. Both complexes efficiently promote the hydrolytic dehydrogenation of ammonia borane at room temperature, with a turnover frequency of up to 8400 mol mol_cat⁻¹ h⁻¹. In addition, these air-stable catalysts are readily recyclable. CAAC=cyclic (alkyl)(amino)carbene.

Wiley-VCH, Weinheim 2015. 264 pp., € 126.00.—ISBN 978-3527335534

Characteristics of zeolite formation, such as being kinetically slow and thermodynamically metastable, are the main bottlenecks that obstruct a fast zeolite synthesis. We present an ultrafast route, the first of its kind, to synthesize high-silica zeolite SSZ-13 in 10 min, instead of the several days usually required. Fast heating in a tubular reactor helps avoid thermal lag, and the synergistic effect of addition of a SSZ-13 seed, choice of the proper aluminum source, and employment of high temperature prompted the crystallization. Thanks to the ultra-short period of synthesis, we established a continuous-flow preparation of SSZ-13. The fast-synthesized SSZ-13, after copper-ion exchange, exhibits outstanding performance in the ammonia selective catalytic reduction (NH₃-SCR) of nitrogen oxides (NO_x), showing it to be a superior catalyst for NO_x removal. Our results indicate that the formation of high-silica zeolites can be extremely fast if bottlenecks are effectively widened.

Bottle opener: SSZ-13 is a zeolite commercialized as a catalyst to remove nitrogen oxides (NO_x). However, its long synthesis time is one of the biggest barriers to large-scale production. An ultrafast synthesis route now allows SSZ-13 to be synthesized in 10 min and thus facilitates continuous preparation of SSZ-13. The fast-synthesized SSZ-13 exhibits outstanding performance for the NO_x removal.

Solid bases, such as SBA-15-oxynitrides, have attracted considerable interest for potential applications as catalysts in important industrial processes. Reported herein is that by simply tuning the temperature of nitridation (ammonolysis), the catalytic activity of these solid bases can be enhanced. Solid-state NMR spectroscopy and XPS studies provided the reasoning behind this change in activity.

Oxynitride solid: Solid bases, such as SBA-15-oxynitrides, have attracted considerable interest for potential applications as catalysts in important industrial processes. Reported here is that tuning the temperature of nitridation (ammonolysis) leads to enhanced catalytic activity of these solid bases. Detailed studies are discussed to explain this change in activity.

A series of dithienylethene-containing phosphole derivatives has been designed, synthesized and characterized. One of the compounds has been characterized by X-ray crystallography. Upon photoexcitation, the compounds exhibit drastic color changes, ascribed to the reversible photochromic behavior. Their photochromic, photophysical and electrochemical properties have been studied. They show photochromic reactivities with high photocyclization quantum yields. Their photophysical and photochromic properties are found to be facilely tuned in this system by substitution at the phosphole backbone, as well as variation on the extent of π-conjugation of the phosphole backbone. Some selected compounds have been demonstrated to exhibit photochromic properties in polymethylmethacrylate (PMMA) films.

Robust photoswitch: A new series of robust dithienylethene-containing phosphole derivatives has been synthesized (see figure); their photophysical and photochromic properties are found to be readily tuned by simple modifications at the phosphorus center, substitution at the phosphole backbone, as well as variation on the extent of π-conjugation of the phosphole backbone. Both the open and closed forms of the phosphole compounds have good thermal stability in benzene solution and in PMMA film.

Transfer hydrogenation Transfer hydrogenation is a simple safe way for reduction reactions and using water as a solvent has the additional benefit of being “green”. In their Full paper on page 5370 ff., J. Xiao et al. report that a cyclometalated iridium complex catalyzes the transfer hydrogenation of various nitrogen heterocycles in an aqueous solution of formate under mild conditions. The catalyst shows excellent functional-group compatibility and high turnover number, with loadings as low as 0.01 mol % being feasible. A mechanistic study of quinoline reduction suggests that the reaction proceeds via both 1,2- and 1,4-addition pathways, with the catalytic turnover limited by hydride transfer.

Dual role for catalysts: Novel routes for the generation of asymmetric stereocenters using photoredox catalysis were recently developed. Different chiral catalytic systems allowed new CC bonds to form in good yields and enantioselectivities using a mild methodology in which light is used as the energy source.

Killing two birds with one stoneIn the presence of a bis(η⁵,η¹-pentafulvene) titanium complex, N-methylanilines undergo two bond activation reactions to directly give titanaaziridines. Based on this transformation, S. Doye, R. Beckhaus, et al. report in their Communication on page 4383 ff. the isolation and investigation of all key intermediates of the titanium-catalyzed hydroaminoalkylation of alkenes. The results represent the first experimental proof of the suggested reaction mechanism of this industrially promising reaction.

What is an oxidation state? Oxidation state has defining algorithms but lacks a comprehensive definition. Results of an IUPAC project to find such a definition have recently been published in an extensive Technical Report. A summary in this Essay is illustrated with applications on Lewis, bond-graph, and summary formulas of molecules, ions, or solids, together with the most recent information regarding tricky cases.

Eight [Ir(bpy)Cp*Cl]⁺-type complexes (bpy= bipyridine, Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) containing differently substituted bipyridine ligands were synthesized and characterized. Cyclic voltammetry (CV) of the complexes in Ar-saturated acetonitrile solutions showed that the redox behavior of the complexes could be fine tuned by the electronic properties of the substituted bipyridine ligands. Further CV in CO₂-saturated MeCN/H₂O (9:1, v/v) solutions showed catalytic currents for CO₂ reduction. In controlled potential electrolysis experiments (MeCN/MeOH (1:1, v/v), E_app=−1.80 V vs Ag/AgCl), all of the complexes showed moderate activity in the electrocatalytic reduction of CO₂ with good stability over at least 15 hours. This electrocatalytic process was selective toward formic acid, with only traces of dihydrogen or carbon monoxide and occasionally formaldehyde as byproducts. However, the turnover frequencies and current efficiencies were quite low. No direct correlation between the redox potentials of the complexes and their catalytic activity was observed.

Promising option: Selective electrocatalytic CO₂ reduction was achieved at room temperature under atmospheric CO₂ pressure with [Ir(bpy)Cp*Cl]⁺-type complexes (see picture) in solvent mixtures containing protic solvents. Catalytic activity subtly depended on the electron density of the Ir center, which was influenced by the bipyridine substitution pattern. Turnover frequencies are moderate, but the catalysts show stability over at least 15 hours.