Double C−H Activation Results in Ruthenium Complexes of a Neutral PCP Ligand with a Central Carbene Moiety

Ru complexes of a new neutral PCP pincer ligand containing a central carbene ligand have been prepared (referred to as (P 2 C ) for emphasis). The ligand precursor P 2 CH 2 can be prepared in good yield in two steps from pyrrole, formaldehyde, and chlorodiisopropylphosphine. The reaction between P 2 CH 2 and [( p -cymene)RuCl 2 ] 2 in the presence of Et 3 N cleanly furnishes (P 2 C )RuHCl. The latter possesses a Y-shaped five-coordinate geometry about Ru, as determined in an X-ray diffraction study. Reactions of (P 2 C )RuHCl with a series of potential ligands have been examined. Addition of pyridine, MeCN, or ethylene leads to a simple adduct formation and addition of CO leads to coordination of two CO ligands to Ru with concomitant hydride-to-carbene migration, while addition of PMe 3 gives rise to both types of products. (P 2 C )RuHCl is a moderately active precatalyst for transfer hydrogenation of ketones. Addition of phenylacetylene leads to the insertion of the triple bond into Ru−H to give (P 2 C )Ru(−CPh CH 2 )Cl. (P 2 C )RuHCl can be converted to (P 2 C )RuCl 2 by action of an iminium chloride reagent. (P 2 C )RuCl 2 adds one molecule of CO with retention of the carbene moiety, even at elevated temperatures. These results demonstrate the potential of (P 2 C ) as a robust and electronically distinctive ancillary ligand.