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Olefin Hydrosilylation Catalyzed by a Bis-N-Heterocyclic Carbene Rhodium Complex. A Density Functional Theory Study

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journal contribution
posted on 22.04.2013, 00:00 by Yin Wu, Virve A. Karttunen, Shane Parker, Alexander Genest, Notker Rösch
Using a density functional theory method, we explored four reaction mechanisms of hydrosilylation of silane and ethylene as model substrates, catalyzed by a Rh­(I) complex with a bidendate ethylene-bridged bis-N-heterocyclic carbene ligand. We examined in detail the energy profiles of the Glaser–Tilley, Chalk–Harrod, and modified Chalk–Harrod mechanisms, as well as of σ-bond metathesis. The Chalk–Harrod mechanism and σ-bond metathesis were determined most favorable, with the calculated highest relative activation enthalpies of 9.3 and 8.6 kcal mol–1, respectively. We also studied a potential side reaction in the σ-bond metathesis that leads to the formation of ethane; its rate-limiting activation enthalpy is sufficiently high, 20.9 kcal mol–1 (14.6 kcal mol–1 higher than the competing barrier on the main pathway), not to be competitive. The feasibility of crucial reaction steps, C–H and C–Si bond formation, was found to correlate with the ease of conformational changes of the bis-N-heterocyclic carbene ligand, thus providing a hint at optimum ligand design.