DFT Studies on the Methane Elimination Reaction of a Trinuclear Rare-Earth Polymethyl Complex: σ‑Bond Metathesis Assisted by Cooperation of Multimetal Sites

DFT studies have been performed for the process of methane elimination via an intramolecular C–H bond activation of the polymethyl trinuclear rare-earth-metal complex [(η⁵-C₅Me₄SiMe₃)­Tm­(μ₂-CH₃)₂]₃. It has been found that intermetallic cooperation plays an important role in achieving such an intramolecular C–H bond activation process, and the cooperation of trimetallic centers makes the C–H bond activation kinetically easier in comparison with that of bimetallic centers. The methane elimination reaction occurs through rearrangement of Tm–CH₃ connections and subsequent C–H bond activation. The C–H bond activation is the rate-determining step of the whole process, and the corresponding transition state is characterized by a σ-bond metathesis assisted by cooperation of multimetal sites. Such a C–H bond activation step was accomplished by a change in the coordination manner (from μ₂ to μ₃ fashion) of a methyl group and a subsequent hydrogen transfer, and the two events are asynchronous concerted processes. The computational results also suggest that the reactivity of the C–H bond of the metal-connected CH₃ group follows the order μ₃-C­(H₂)–H > μ₂-C­(H₂)–H > μ₁-C­(H₂)–H. This is unlike the case of a metal–CH₃ bond, where the terminal methyl (monometal-connected μ₁-CH₃) is generally more reactive than multimetal-connected methyls. In addition, the effect of 4f electrons of such trinuclear organolanthanide compounds on the geometry and energy profile studied here is also discussed