Phosphine-Catalyzed Activation of Phenylsilane for Benzaldehyde Reduction.

Hydrosilylation reactions are commonly used for the reduction of carbonyl bonds in fine chemistry, catalyzed by transition metal complexes. The current challenge is to expand the scope of metal-free alternative catalysts, including in particular organocatalysts. This work describes the organocatalyzed hydrosilylation of benzaldehyde with a phosphine, introduced at 10 mol%, and phenylsilane at room temperature. The activation of phenylsilane was highly dependent on the physical properties of the solvent such as the polarity, and the highest conversions were obtained in acetonitrile and propylene carbonate with yields of 46 % and 97 %, respectively. The best results of the screening over 13 phosphines and phosphites were obtained with linear trialkylphoshines (PMe3 , Pn Bu3 , POct3 ), indicating the importance of their nucleophilicity, with yields of 88 %, 46 % and 56 %, respectively. With the help of heteronuclear 1 H-29 Si NMR spectroscopy, the products of the hydrosilylation (PhSiH3-n (OBn)n ) were identified, allowing a monitoring of the concentration in the different species, and thereby of their reactivity. The reaction displayed an induction period of ca. 60 min, followed by the sequential hydrosilylations presenting various reaction rates. In agreement with the formation of partial charges in the intermediate state, we propose a mechanism based on a hypervalent silicon center via the Lewis base activation of the silicon Lewis acid.

The delicate balance of phase speciation in bimetallic nickel cobalt nanoparticles.

Bimetallic nickel-cobalt nanoparticles are highly sought for their potential as catalytic and magnetic nanoparticles. These are typically prepared in organic solvents in the presence of strong stabilizing ligands such as tri-n-octylphosphine (TOP). Due to the variety of cobalt crystallographic phases and to the strong interaction of the ligands with the metallic surfaces, forming fcc nanoparticles rather than a phase mixture is a challenging endeavor. Here, using a two-step synthesis strategy that aims at a core-shell nickel-cobalt morphology, we demonstrated that many parameters have to be adjusted: concentration of the metal precursors, stoichiometry of TOP, and heating program from room temperature to 180 °C. We found optimized conditions to form size-controlled fcc NiCo nanoparticles from preformed Ni nanoparticles, and the phase attribution was confirmed with a combination of X-Ray diffraction on powder and X-Ray absorption spectroscopy at the Co K edge. We then investigated the early stages of Co nucleation on the nickel using a lower stoichiometry of Co, down to 0.05 equiv. vs. Ni. Using X-ray photoelectron spectroscopy and scanning transmission electron microscopy coupled to energy-dispersive X-Ray spectroscopy and electron energy loss spectroscopy, we showed that cobalt reacts first on the nickel nanoparticles but easily forms cobalt-rich larger aggregates in the further steps of the reaction.