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.