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 ₂C=) for emphasis). The ligand precursor P ₂CH ₂ can be prepared in good yield in two steps from pyrrole, formaldehyde, and chlorodiisopropylphosphine. The reaction between P ₂CH ₂ and [(p-cymene)RuCl ₂] ₂ in the presence of Et ₃N cleanly furnishes (P ₂C=)RuHCl. The latter possesses a Y-shaped five-coordinate geometry about Ru, as determined in an X-ray diffraction study. Reactions of (P ₂C=)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 ₂C=)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 ₂C=)Ru(-CPh=CH ₂)-Cl. (P ₂C=)RuHCl can be converted to (P ₂C=)RuCl ₂ by action of an iminium chloride reagent. (P ₂C=)RuCl ₂ adds one molecule of CO with retention of the carbene moiety, even at elevated temperatures. These results demonstrate the potential of (P ₂C=) as a robust and electronically distinctive ancillary ligand.