Interplay between Solvent and Counteranion Stabilization of Highly Unsaturated Rhodium( III ) Complexes: Facile Unsaturation‐Induced Dearomatization

Abstraction of the chloride ligand from the PCN‐based chloromethylrhodium complex 2 by AgX (X=BF 4 − , CF 3 SO 3 − ) or a direct C–C cleavage reaction of the PCN ligand 1 with [(coe) 2 Rh(solv) n ] + X − (coe=cyclooctene) lead to the formation of the coordinatively unsaturated rhodium( III ) complexes 3 . Compound 3 a (X = BF 4 − ) exhibits a unique medium effect; the metal center is stabilized by reversible coordination of the bulky counteranion or solvent as a function of temperature. Reaction of [(PCN)Rh(CH 3 )(Cl)] with AgBAr f in diethyl ether leads to an apparent rhodium( III ) 14‐electron complex 4 , which is stabilized by reversible, weak coordination of a solvent molecule. This complex coordinates donors as weak as diethyl ether and dichloromethane. Upon substitution of the BF 4 − ion in [(PCN)Rh(CH 3 )]BF 4 by the noncoordinating BAr f − ion in a noncoordinating medium, the resulting highly unsaturated intermediate undergoes a 1,2‐metal‐to‐carbon methyl shift, followed by β ‐hydrogen elimination, leading to the Rh‐stabilized methylene arenium complex 5 . This process represents a unique mild, dearomatization of the aromatic system induced by unsaturation.