Ruthenium(II) supported by phosphine-functionalized N-heterocyclic carbene ligands as catalysts for the transfer hydrogenation of ketones

We have prepared and characterized two ruthenium(II) complexes supported by phosphine-functionalized N-heterocyclic (NHC) ligands. One of the complexes (2a) underwent ortho-metalation of the N-phenyl moiety giving rise to a tridentate PCNHCC− coordinating ligand whereas 2b bears an N-mesityl group in order to prevent C–H activation of the aryl ring thereby enforcing a PCNHC bidentate binding mode. Both 2a and 2b were shown to catalyze transfer hydrogenation of ketones at 82 °C in 2-propanol in the presence of KOtBu albeit with vastly different catalytic activities. Catalytic transfer hydrogenation by 2b was shown to proceed at room temperature and in air using unpurified 2-propanol as solvent and hydrogen donor. Time studies revealed unique kinetic profiles for the two precatalysts; this may shed light on the difference in their catalytic activities.