Ethanol Upgrading with N,N,N-Pincer-Based Ru Catalysts: Delineating Key Factors Governing Catalyst Evolution and Stability

The 5-CH3-bpi (bpi = 1,3-bis(2′-pyridylimino)isoindolate) ruthenium catalyst is among the most active catalysts to generate higher order alcohols from ethanol, via the Guerbet reaction, yet the reason(s) for this beneficial CH3-subsitution position (5- relative to 4- and 6-) were not known. To understand the influence of this unique positional activity, we report a mechanistic investigation of the N,N,N-pincer Ru(bpi)(PPh3)2Cl catalyst. In situ IR and ex situ NMR/ESI-MS analyses of catalytic reactions identified a series of Ru carbonyl species generated during catalysis. We find that the primary decomposition pathway is via hydrogenolysis of the bpi ligands and that incorporating a methyl group at the 5-position disfavors this deactivation pathway.