Photo-Induced Reaction of Cp*Ru(μ-H)₄RuCp* with Arenes Resulting in Irreversible Formation of μ-η²:η²-Cyclohexadiene Complexes

In contrast to the thermal reaction of [Cp*Ru(μ-H)4RuCp*] (1: Cp* = η5-C5Me5) with benzene at 70 °C yielding an equilibrium mixture of 1 and [{Cp*Ru(μ-H)}2(μ-η2:η2-C6H8)] (2), the UV irradiation of 1 at 25 °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 η2:η2-5,6-benzocyclohexadiene complex, [{Cp*Ru(μ-H)}2(μ-η2:η2-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(μ-H)}2(μ-η2:η2-Me2C6H6)] (6), with toluene, yielding the mixture of 5a and 5b. Time-dependent density functional theory calculations of 1 provided insight into the mechanism.