Synthesis and Reversible H2 Activation by Coordinatively Unsaturated Rhodium NHC Complexes

Abstract We report the synthesis of coordinatively unsaturated cationic rhodium complexes bearing the sterically encumbered electron‐rich NHC ligand IPr* OMe . The COD (1,5‐cyclooctadiene) complex Rh(IPr* OMe )(COD)BF 4 adopts a tilted, pseudo‐square planar coordination geometry, where bonding to the ipso ‐carbon of the NHC aryl substituent was observed in the solid state. Hydrogenation of this complex afforded a metastable dihydride complex Rh(IPr* OMe )(H) 2 BF 4 with an unusual internal coordination to an arene of the ligand. In the absence of a hydrogen atmosphere, spontaneous reductive elimination of H 2 afforded a rhodium complex Rh(IPr* OMe )BF 4 with a single chelating ligand that stabilizes the highly unsaturated metal by two‐fold π‐face donation as suggested by NMR spectroscopy and computational studies. This unusual complex might serve as a versatile precatalyst for a variety of transformations.