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 H₂ afforded a rhodium complex Rh(IPr*^OMe)]BF₄ 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.