Mechanistic Aspects of Samarium-Mediated σ-Bond Activations of Arene C−H and Arylsilane Si−C Bonds

To investigate the potential role of Sm−Ph species as intermediates in the samarium-catalyzed redistribution of PhSiH3 to Ph2SiH2 and SiH4, the samarium phenyl complex [Cp*2SmPh]2 (1) was prepared by oxidation of Cp*2Sm (2) with HgPh2. Compound 1 thermally decomposes to yield benzene and the phenylene-bridged disamarium complex Cp*2Sm(μ-1,4-C6H4)SmCp*2 (3). This decomposition reaction appears to proceed through dissociation of 1 into monomeric Cp*2SmPh species which then react via unimolecular and bimolecular pathways, involving rate-limiting Cp* metalation and direct C−H activation, respectively. The observed rate law for this process is of the form: rate = k 1[1] + k 2[1]2. Complex 1 efficiently transfers its phenyl group to PhSiH3, with formation of Ph2SiH2 and [Cp*2Sm(μ-H)]2 (4). Quantitative Si−C bond cleavage of C6F5SiH3 is effected by the samarium hydride complex 4, yielding silane and [Cp*2Sm(μ-C6F5)]2 (5). In contrast, Si−H activation takes place upon reaction of 4 with o-MeOC6H4SiH3, affording the samarium silyl species (7). Complex 7 rapidly decomposes to [Cp*2Sm(μ-o-MeOC6H4)]2 (6) and other samarium-containing products. Compounds 5 and 6 were prepared independently by oxidation of 2 with Hg(C6F5)2 and Hg(o-MeOC6H4)2, respectively. The mechanism of samarium-mediated redistribution at silicon, and chemoselectivity in σ-bond metathesis reactions, are discussed.