Energetics of Homogeneous Intermolecular Vinyl and Allyl Carbon−Hydrogen Bond Activation by the 16-Electron Coordinatively Unsaturated Organometallic Fragment [Tp‘Rh(CNCH₂CMe₃)]

Reaction of the complex Tp‘Rh(CNneo)(CHCH₂)Cl (neo = CH₂CMe₃, Tp‘ = hydridotris(3,5-dimethylpyrazolyl)borate) with Cp₂ZrH₂ leads to the formation of Tp‘Rh(CNneo)(CHCH₂)H. This complex is also formed upon photolysis of a solution of Tp‘Rh(CNneo)(PhNCNneo) containing ethylene or by thermal reaction of Tp‘Rh(CNneo)(c-hexyl)H with ethylene. The vinyl hydride complex rearranges to the more stable η²-ethylene complex with a half-life of 8 h at 22 °C. Photolysis of a solution of Tp‘Rh(CNneo)(PhNCNneo) in liquid propylene produces the allylic activation product Tp‘Rh(CNneo)(CH₂CHCH₂)H, which rearranges (t_1/2 = 3 days at 22 °C) to the η²-propylene complex. Allylic activation is also seen with isobutylene, but loss of olefin is observed at 22 °C in benzene solution to generate Tp‘Rh(CNneo)(Ph)H (t_1/2 = 16.6 h). Photolysis of a tert-butylethylene solution of Tp‘Rh(CNneo)(PhNCNneo) produces the trans vinyl hydride complex, which loses tert-butylethylene to generate Tp‘Rh(CNneo)(Ph)H (t_1/2 = 113 days at 22 °C). A combination of kinetic selectivity and reductive elimination experiments have allowed for calculation of relative Rh−C bond strengths for both the rhodium allyl and vinyl hydride complexes and for the inclusion of these new data in an analysis of bond strength correlations. The results show that the trend for relative Rh−C bond strengths parallels the trend of hydrocarbon C−H bond strengths, i.e., Rh−Ph > Rh−vinyl > Rh−methyl > Rh−alkyl (1°) > Rh−cycloalkyl (2°) > Rh−benzyl > Rh−allyl, but that differences in M−C bond strengths typically exceed the differences in C−H bond strengths.