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

Abstract

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.

Heterobimetallic Trihydrido Complex of Ruthenium and Rhodium Supported by Cyclopentadienyl Groups with Different Steric Demands

“…This excitation should lengthen the Ru–Rh distance, and the expanded dinuclear site allows the facile uptake of benzene at the Ru center. Similar δ* → π* transitions were also suggested for the excitation of [Cp*Ru­(μ-H) 4 RuCp*] …”

“…Similar δ* → π* transitions were also suggested for the excitation of [Cp*Ru(μ-H) 4 RuCp*]. 29 As shown in Figure S48, almost the same results were obtained from TD-DFT calculations for 3c. Therefore, the reason why a μ-phenyl complex was not formed in the photoreaction of 3c can be attributed to a steric reason, namely the bulky Cp ‡ ligand on the Rh center, which suppresses the migration of a phenyl group from the Ru center to the Rh center.…”

“…In fact, three-ring critical points among each Ru–H–Rh moiety were found. Similarly, a bond path was also found between the two Ru atoms in the related diruthenium tetrahydrido complex, [Cp*Ru­(μ-H) 4 RuCp*], by QTAIM analysis despite the lack of a direct Ru–Ru interaction …”

“…Similarly, a bond path was also found between the two Ru atoms in the related diruthenium tetrahydrido complex, [Cp*Ru(μ-H) 4 RuCp*], by QTAIM analysis despite the lack of a direct Ru−Ru interaction. 29 More importantly, the bond paths between the Ru atom and bridging hydrides were considerably bent inward, while those between Rh and H were straight (Figure 4). This indicates that the three-center interaction is composed of the η 2 -coordination of the Rh−H σ-bond to the Ru atom.…”

“…Formation of the μ-phenyl complex is in marked contrast to the photoreaction of the related diruthenium tetrahydrido complex, [Cp*Ru(μ-H) 4 RuCp*], with benzene, yielding a μη 2 :η 2 -cyclohexadiene complex, [{Cp*Ru(μ-H)} 2 (μ-η 2 :η 2 -C 6 H 8 )], via the migration of hydrides to the aromatic ring. 29 The molecular structure of 11b was determined by XRD, as represented in Figure 8. As seen in other heterobimetallic complexes possessing a bridging phenyl ligand, 38−43 2.0753(19) Å).…”