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

•Unexpected ortho-metalation led to a ruthenium(II)–PCNHCC− complex.•ortho-Metalated catalyst exhibits slow initiation kinetics.•Catalyst derived from ruthenium(II)–PCNHC complex is active at room temperature and in air using unpurified solvent.

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.

Graphical abstractTwo ruthenium(II) complexes supported by phosphine-functionalized N-heterocyclic carbene ligands have been prepared, characterized and evaluated as catalysts for transfer hydrogenation of ketones.Figure optionsDownload full-size imageDownload as PowerPoint slide