In Situ Determination of the Active Catalyst in Hydrosilylation Reactions Using Highly Reactive Pt(0) Catalyst Precursors

Abstract

The mechanism of hydrosilylation using the highly active precatalyst Karstedt's precatalyst (Pt x -(M vinyl M vinyl ) y , M vinyl M vinyl ) divinyltetramethyldisiloxane) was investigated using extended X-ray absorption fine structure (EXAFS), small-angle X-ray scattering (SAXS), and ultraviolet-visible (UV-vis) spectroscopy. Contrary to previous reports suggesting colloidal catalysts, we find that regardless of the stoichiometric ratio of hydrosilane to olefin, the catalyst is a monomeric platinum compound with silicon and carbon in the first coordination sphere. The platinum end product, however, is a function of the stoichiometry of the reactants. At excess olefin concentration, the platinum end product contains only platinum-carbon bonds, whereas at high hydrosilane concentration, the platinum end product is multinuclear and also contains platinum-silicon bonds. The two end products can interconvert by adding additional aliquots of the stoichiometrically deficient reagent. An explanation of the "oxygen" effect is also given. In the absence of oxygen, hydrosilylation of certain olefins does not occur. Oxygen serves to disrupt multinuclear platinum species that are formed when poorly stabilizing olefins are employed. Finally, we discuss the olefin isomerization reaction that may accompany hydrosilylation of terminal olefins. When the hydrosilylation reaction proceeds slowly due to poorly reactive olefins, the olefin isomerization products become significant. EXAFS analysis of solutions after olefin isomerization has occurred shows the presence of platinum-platinum bonded compounds.

Pd‐Catalyzed Z‐Selective Semihydrogenation of Alkynes: Determining the Type of Active Species

“…The opposite, in which anticipated NP or nanocluster materials can be precursors for molecular catalysts, has been demonstrated as well. [11,[23][24][25][26] It is important to know the type of active catalyst, especially for the development of molecular catalysts. These catalysts are developed based on mechanistic understanding.…”

Evidence of colloidal rhodium formation during the biphasic hydroformylation of 1-octene with thermoregulated phase-transfer phosphine rhodium(I) catalyst

“…Because the colouration in homogeneous catalysis generally arises from colloidal metal formation, 14,34 in the present study the aqueous layers after reaction were examined by TEM and UV-Vis spectroscopy. As shown in Fig.…”

Kinetics and mechanism of the reaction of allyl chloride with trichlorosilane catalyzed by carbon‐supported platinum

“…The mechanistic scheme presents conventional oxidative addition±reductive elimination steps to explain how the hydrosilylation occurs. The oxidative addition of trisubstituted silanes to a metal±alkene complex (usually with d 8 or d 10 configuration) is followed by migratory insertion of alkene into an M-H bond and the resulting metal±(silyl)(alkyl) complex undergoes reductive elimination by Si-C bond formation and regeneration of metal±alkene complex in excess of alkene. Since a facile reductive elimination of silylalkene from alkyl-(M)-SiR 3 species was not well established in a stoichiometric reaction, a modified Chalk± Harrod mechanism was proposed that involves alkene insertion into the metal±silyl bond followed by C-H reductive elimination 4±6,20 (Scheme 1).…”

“…Extensive studies, carried out, particularly by Lewis and Stein and their co-workers from General Electric, allowed one to conclude that, regardless of the stoichiometric ratio of hydrosilane to olefin, catalytic hydrosilylation is a molecular process proceeding via metal clusters and that formation of colloids is associated with deactivation of the system. 7,8 Platinum supported on active carbon (usually with a metal loading of 1±5 wt%) is the most frequently used and efficient metal catalyst for commercial hydrosilylation of carbon± carbon multiple bonds, and particularly for the addition of trichlorosilane (TCS) to allyl chloride (AC). 2±4,9,10 It was reported recently that a heterogeneous catalyst, such as platinum supported on carbon, 11 obtained by treatment of carbon with mesitylene-solvated platinum atoms 12 showed good efficiency in the hydrosilylation of terminal and internal alkynes, 11,12 and bimetallic catalysts (e.g.…”