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.