Identification of Cyclohexadienyl Hydrides as Intermediates in Molybdenum-Catalyzed Arene Hydrogenation

Treatment of phosphino(imino)pyridine (PIP) molybdenum cyclooctadiene (COD) complexes [(PIP)Mo(COD)] with dihydrogen in the presence of benzene selectively furnished the molybdenum cyclohexadienyl hydrides [(PIP)MoH(η⁵-C₆H₇)], which are precatalysts for the hydrogenation of benzene to cyclohexane. [(PIP)MoH(η⁵-C₆H₇)] arises from a rarely observed insertion of benzene into a molybdenum-hydride bond, a key step in the molybdenum-catalyzed homogeneous hydrogenation of arenes. The reaction with toluene afforded a single isomer of the corresponding molybdenum cyclohexadienyl hydride while para-xylene predominantly formed the molybdenum η⁶-arene complex with the insertion product being a minor component. Addition of carbon monoxide to a cyclohexane-d₁₂ solution of [(PIP)MoH(η⁵-C₆H₇)] liberated cyclohexadiene, providing experimental support for a higher kinetic barrier for the subsequent steps en route to cycloalkanes.