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 (P2C=) for emphasis). The ligand precursor P2CH2 can be prepared in good yield in two steps from pyrrole, formaldehyde, and chlorodiisopropylphosphine. The reaction between P2CH2 and [(p-cymene)RuCl2](2) in the presence of Et3N cleanly furnishes (P2C=) RuHCl. The latter possesses a Y-shaped five-coordinate geometry about Ru, as determined in an X-ray diffraction study. Reactions of (P2C=) 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 PMe3 gives rise to both types of products. (P2C=) 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 (P2C=)Ru(-CPh=CH2)Cl. (P2C=) RuHCl can be converted to (P2C=) RuCl2 by action of an iminium chloride reagent. (P2C=) RuCl2 adds one molecule of CO with retention of the carbene moiety, even at elevated temperatures. These results demonstrate the potential of (P2C=) as a robust and electronically distinctive ancillary ligand.