Synthesis and Reversible H2 Activation by Coordinatively Unsaturated Rhodium NHC Complexes

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)]BF4 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)]BF4 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))]BF4 with a single chelating ligand that stabilizes the highly unsaturated metal by two-fold pi-face donation as suggested by NMR spectroscopy and computational studies. This unusual complex might serve as a versatile precatalyst for a variety of transformations.