2019, Rosenthal, Uwe

Many examples exist for the chromium catalyzed selective ethylene oligomerization in which the influence of ligands is essential for the formation of products. Regarding the tri- and tetramerization to 1-hexene or 1-octene mostly PNP ligands are responsible for the tetra- and some of such modified ligands for the trimerization. A very special case in these reactions are PNPN−H ligands, showing in most cases highly selective trimerization of ethylene to 1-hexene. In this review all existing published information about these PNPN−H ligands is accumulated and compared to some other related PNP, PNPN and NPNPN ligands in the chromium catalyzed selective ethylene oligomerization with respect to the switch from tetra- to trimerization and back by different substituent pattern of PNP ligand. Mechanistic information and arguments are collected to explain the switch from tetra- to trimerization and back by substitution of functional groups in classical PNP to PNPN−H ligands as a result of mono- and dinuclear catalytic species. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

2016, Steele, J.A., Lewis, R.A., Horvat, J., Nancarrow, M.J.B., Henini, M., Fan, D., Mazur, Y.I., Schmidbauer, M., Ware, M.E., Yu, S.-Q., Salamo, G.J.

Herein we investigate a (001)-oriented GaAs1−xBix/GaAs structure possessing Bi surface droplets capable of catalysing the formation of nanostructures during Bi-rich growth, through the vapour-liquid-solid mechanism. Specifically, self-aligned “nanotracks” are found to exist trailing the Bi droplets on the sample surface. Through cross-sectional high-resolution transmission electron microscopy the nanotracks are revealed to in fact be elevated above surface by the formation of a subsurface planar nanowire, a structure initiated mid-way through the molecular-beam-epitaxy growth and embedded into the epilayer, via epitaxial overgrowth. Electron microscopy studies also yield the morphological, structural and chemical properties of the nanostructures. Through a combination of Bi determination methods the compositional profile of the film is shown to be graded and inhomogeneous. Furthermore, the coherent and pure zincblende phase property of the film is detailed. Optical characterisation of features on the sample surface is carried out using polarised micro-Raman and micro-photoluminescence spectroscopies. The important light producing properties of the surface nanostructures are investigated through pump intensity-dependent micro-PL measurements, whereby relatively large local inhomogeneities are revealed to exist on the epitaxial surface for important optical parameters. We conclude that such surface effects must be considered when designing and fabricating optical devices based on GaAsBi alloys.

2020, Rosenthal, Uwe

For linear and cyclic coupling reactions of CO, among other products, the formation of the hexapotassium salt of hexahydroxybenzene is of particular interesting. The interaction of metallic potassium and CO offers, via the assumed K[OC≡CO]K as the result of several carbon monoxide coupling reactions, the formation of C6(OK)6 among other products. To date, only speculations exist about the reaction pathway for these products, which were first described by Liebig in 1834. A novel concept is suggested here, which consists of the single steps (i) reductive coupling of CO, (ii) formation of dihetero-metallacyclopentynes (cis-2,5-diheterobutatriene as formal ethylenedione O=C=C=O complexes), (iii) formation of its dinuclear 1-metalla-2,5-dioxo-cyclopentyne complexes by external coordination of the triple bond, (iv) insertion of CO into the M−C bond of the formed metallacyclopropene, and (v) the reductive elimination of C6(OK)6. The novel aspect of this concept is the formation of dihetero-metallacyclopentynes (in analogy to the well characterized all-C-metallacyclopentynes), which have not been considered in the mechanism of reductive CO coupling reactions. It is expected that the presence of transition-metal impurities would promote the reaction. © 2020 The Authors. Published by Wiley-VCH GmbH

2021, Hu, Yuya, Wei, Zhihong, Frey, Anna, Kubis, Christoph, Ren, Chang-Yue, Spannenberg, Anke, Jiao, Haijun, Werner, Thomas

A series of hydroxy-functionalized phosphonium salts were studied as bifunctional catalysts for the conversion of CO2 with epoxides under mild and solvent-free conditions. The reaction in the presence of a phenol-based phosphonium iodide proceeded via a first order rection kinetic with respect to the substrate. Notably, in contrast to the aliphatic analogue, the phenol-based catalyst showed no product inhibition. The temperature dependence of the reaction rate was investigated, and the activation energy for the model reaction was determined from an Arrhenius-plot (Ea =39.6 kJ mol-1 ). The substrate scope was also evaluated. Under the optimized reaction conditions, 20 terminal epoxides were converted at room temperature to the corresponding cyclic carbonates, which were isolated in yields up to 99 %. The reaction is easily scalable and was performed on a scale up to 50 g substrate. Moreover, this method was applied in the synthesis of the antitussive agent dropropizine starting from epichlorohydrin and phenylpiperazine. Furthermore, DFT calculations were performed to rationalize the mechanism and the high efficiency of the phenol-based phosphonium iodide catalyst. The calculation confirmed the activation of the epoxide via hydrogen bonding for the iodide salt, which facilitates the ring-opening step. Notably, the effective Gibbs energy barrier regarding this step is 97 kJ mol-1 for the bromide and 72 kJ mol-1 for the iodide salt, which explains the difference in activity.