Photo-Induced Reaction of Cp*Ru(mu-H)4RuCp*with Arenes Resulting in Irreversible Formation of mu-eta 2:eta 2-Cyclohexadiene Complexes

In contrast to the thermal reaction of [Cp*Ru(mu- H)4RuCp*] (1: Cp* = eta 5-C5Me5) with benzene at 70 degrees C yielding an equilibrium mixture of 1 and [{Cp*Ru(mu-H)}2(mu-eta 2:eta 2-C6H8)] (2), the UV irradiation of 1 at 25 degrees C in benzene at 365 nm exclusively produced the dinuclear cyclohexadiene complex 2. The photo-induced reaction of 1 featured remarkably high tolerance toward substituted benzene, including toluene and p-xylene. The photo-induced reaction of 1 with toluene afforded a regio-isomeric mixture of bridging methylcyclohexadiene complexes 5a and 5b in high yields, although the thermal reaction of 1 with toluene did not proceed. Dearomatization via insertion into the Ru-H bond did not occur in the reaction with the fused aromatic system due to the preservation of the additional aromatic system. The eta 2:eta 2-5,6-benzocyclohexadiene complex, [{Cp*Ru(mu-H)}2(mu-eta 2:eta 2-C10H8)] (7), was selectively obtained in the photo-induced reaction of 1 with naphthalene. Photochemically induced transient species with an expanded Ru2 skeleton were proposed based on the thermal reaction of 2,6-dimethylcyclohexa-1,3-diene complex, [{Cp*Ru(mu-H)}2(mu-eta 2:eta 2-Me2C6H6)] (6), with toluene, yielding the mixture of 5a and 5b. Time-dependent density functional theory calculations of 1 provided insight into the mechanism.