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

2006-10-23T00:00:00Z (GMT) by Wei Weng Sean Parkin Oleg V. Ozerov
Ru complexes of a new neutral PCP pincer ligand containing a central carbene ligand have been prepared (referred to as (P<sub>2</sub>C) for emphasis). The ligand precursor P<sub>2</sub>CH<sub>2</sub> can be prepared in good yield in two steps from pyrrole, formaldehyde, and chlorodiisopropylphosphine. The reaction between P<sub>2</sub>CH<sub>2</sub> and [(<i>p</i>-cymene)RuCl<sub>2</sub>]<sub>2</sub> in the presence of Et<sub>3</sub>N cleanly furnishes (P<sub>2</sub>C)RuHCl. The latter possesses a Y-shaped five-coordinate geometry about Ru, as determined in an X-ray diffraction study. Reactions of (P<sub>2</sub>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<sub>3</sub> gives rise to both types of products. (P<sub>2</sub>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<sub>2</sub>C)Ru(−CPhCH<sub>2</sub>)Cl. (P<sub>2</sub>C)RuHCl can be converted to (P<sub>2</sub>C)RuCl<sub>2</sub> by action of an iminium chloride reagent. (P<sub>2</sub>C)RuCl<sub>2</sub> adds one molecule of CO with retention of the carbene moiety, even at elevated temperatures. These results demonstrate the potential of (P<sub>2</sub>C) as a robust and electronically distinctive ancillary ligand.