Mechanistic Insights into the Methylenation of Ketone by a Trinuclear Rare-Earth-Metal Methylidene Complex

Trinuclear rare-earth-metal methylidene (CH₂^2–) complexes are an emerging class of compounds that serve as methylidene transfer agents for methylenation of carbonyl compounds. Herein, the reaction of a trinuclear scandium methylidene complex with acetophenone was used as a model reaction of the multimetallic-cooperating methylidene transfer case, and its detailed mechanism has been investigated by the DFT approach. The analyses of Wiberg bond index, electron occupation, the frontier molecular orbital, and natural charge provide us a clear and comprehensive understanding of the CH₂^2–/O^2– group interchange process assisted by cooperating multimetal sites. The mechanism presented here is markedly different from conventional Wittig and transition-metal-mediated Wittig-type reactions. In addition, the behavior of μ₃-CH₂ in a multinuclear complex system is also demonstrated. This study not only enriches the chemistry of metal Wittig-type reactions but also sheds light on the intermetallic cooperation for methylidene transfer.