Hydride Ligands Make the Difference: Density Functional Study of the Mechanism of the Murai Reaction Catalyzed by [Ru(H)2(H2)2(PR3)2] (R=cyclohexyl)
Résumé
The catalytic cycle for the Murai reaction at room temperature between ethylene and acetophenone catalyzed by [Ru(H)2(H2)2(PMe3)2] has been studied computationally at the B3PW91 level. The active species is the ruthenium dihydride complex [Ru(H)2(PMe3)2]. Coordination of the ketone group to Ru induces very easy C[BOND]H bond cleavage. Coordination of ethylene after ketone de-coordination, followed by ethylene insertion into a Ru[BOND]H bond, creates the Ru[BOND]ethyl bond. Isomerization of the complex to a RuIV intermediate creates the geometry adapted to C[BOND]C bond formation. Re-coordination of the ketone before the C[BOND]C coupling lowers the energy of the corresponding TS. The highest point on the potential energy surface (PES) is the TS for the isomerization to the RuIV intermediate, which prepares the catalyst geometry for the C[BOND]C coupling step. Inclusion of dispersion corrections significantly lowers the height of the overall activation barrier. The actual bond cleavage and bond forming processes are associated to low activation barriers because of the presence of hydrogen atoms around the Ru center. They act as redox buffers through formation and breaking of H[BOND]H bonds in the coordination sphere. This flexibility allows optimal repartition of the various ligands according to the change in stereoelectronic demands along the catalytic cycle.