Rhodium Complexes Bearing 1,10-Phenanthroline Analogue Bis-NHC Ligands Are Active Catalysts for Transfer Hydrogenation of Ketones

Synthesis of a new example of the pyridazine annelated bis­(N-heterocyclic carbene) ligand 3b (vegi) bearing benzyl substituents is reported, as well as the synthesis of its cationic Rh­(cod) complex 5b. The mechanism of formation of the [Rh­(cod)­(vegi)]+ complexes was investigated, showing a stepwise deprotonation of the imidazolium moieties via the mono-carbene imidazolium Rh­(cod)Cl species 4. The [Rh­(cod)­(vegi)]+ complexes 5a,b show catalytic activity in the transfer hydrogenation of even sterically hindered ketones.