Activation and deactivation of a carbene containing non-classical ruthenium hydride complex in catalytic processes involving C–H bond cleavage

The carbene-containing non-classical ruthenium hydride complex [(PCy3)Ru(H)2(H2)2(IMes)] 1 (IMes=1,3-Bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene) is an active catalyst for H/D exchange in aromatic ketones. It is also capable of combining sp2 C–H bond activation with C–C bond formation. Comparing the chemo- and regio-selectivities of the H/D exchange process and the C–C bond formation clearly indicates that different intermediates are involved in the two processes. High pressure NMR studies provide strong evidence that the key intermediate for the C–C coupling reaction is analogous to that for other ruthenium catalysts reported previously. Catalytic turnover is limited by the instability of this intermediate in the presence of the olefinic coupling partner.