Pd(II)-Mediated Si-H Activation of Ortho-Silyl Arylphosphines: Complexes with Pd-Si Bonds and their Utility in Carbon-Carbon Coupling Reactions.

Ortho-silyl arylphosphines bearing Si-R groups (R = H, CH3, Ph) exhibit distinct reactivity profiles when they are bound to transition metals. In this study, a series of Pd(II) square planar complexes featuring silyl phosphine species has been prepared and established their molecular structures. Reactions of phosphines containing Si-H with [Pd(η3-allyl)X2] (X = Cl, Br, and I) afforded square planar Pd(II) complexes of the type [{P,Si}PdX]2 (X = Cl, Br, and I), featuring Pd-Si bonds by Si-H bond cleavage. This set of Si-H bond activation pathways was further analyzed by experimental and computational analysis. Detailed DFT studies revealed two competing mechanistic pathways: a stepwise route involving Si-H oxidative addition and propene elimination, and a ligand-assisted low-barrier pathway (ΔG‡ ≈ 4 kcal/mol), forming a key mononuclear intermediate. The latter route becomes favorable in the presence of excess ligand and rationalizes the experimentally observed complex formation. Substituting Si-H with Si-CH3 alters the reactivity by inhibiting key migratory steps, in agreement with the experimental results. Furthermore, the catalytic performance of one of these palladium complexes was also explored as a molecular catalyst in selective hetero carbon-carbon coupling reactions over Glaser coupling products and prepared a series of alkynes in good to excellent yields.