3-Center-3-Electron σ-Adduct Enables Silyl Radical Transfer below the Minimum Barrier for Silyl Radical Formation

Transition-metal-catalyzed cleavage of the Si-H bond in silanes to yield silyl radicals requires substantial amounts of energy, which are commonly supplied by photons. For Rh(II) porphyrins, efficient hydrosilylation catalysis becomes accessible only upon site isolation in a metal-organic framework (MOF), and the formation of free silyl radicals likewise requires irradiation. Within the MOF, however, an uncommonly facile direct silyl radical transfer to olefin substrates is also possible, which makes thermal olefin hydrosilylation accessible at room temperature. The ability of MOF-supported Rh(II) metalloradicals to furnish an unprecedented 3-center-3-electron (3c-3e) Rh(II)-silane σ-adduct enables the assembly of a tricomponent transition state that is comprised of Rh(II), silane, and ethylene. The tricomponent transition state bypasses the high-energy silyl radical species and enables silyl radical transfer with an activation free energy ∼15 kcal·mol^-1 below the minimum energy barrier for silyl radical formation. We report direct observation of the 3c-3e silane σ-adduct, which is a stable species in the absence of light and olefins. Furthermore, a combination of experiments and quantum chemical calculations shows that direct silyl radical transfer to ethylene is promoted by the temporary oxidation of the transition structure by a proximal Rh(II) center. Thus, the crucial role of the MOF matrix is to fix the inter-Rh separation in our catalyst at a value large enough for 3c-3e silane adduct formation but short enough for facile electron transfer.