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

Area of Science:

• Organometallic Chemistry
• Catalysis
• Materials Science

Background:

• Transition-metal-catalyzed hydrosilylation typically requires high energy input, often from photons, to cleave Si-H bonds and form silyl radicals.
• Site isolation of Rh(II) porphyrins within metal-organic frameworks (MOFs) is known to facilitate hydrosilylation catalysis.
• Free silyl radical formation usually necessitates photoirradiation.

Purpose of the Study:

• To investigate a novel catalytic pathway for thermal olefin hydrosilylation at room temperature.
• To explore the formation and reactivity of a unique 3-center-3-electron (3c-3e) Rh(II)-silane σ-adduct within a MOF.
• To elucidate the mechanism of direct silyl radical transfer to olefin substrates facilitated by MOF-supported catalysts.

Main Methods:

• Synthesis and characterization of Rh(II) metalloradicals immobilized within a metal-organic framework (MOF).
• Spectroscopic and computational studies to observe and analyze the 3-center-3-electron (3c-3e) silane σ-adduct.
• Kinetic and mechanistic investigations using a combination of experimental techniques and quantum chemical calculations.

Main Results:

• MOF-supported Rh(II) catalysts enable facile thermal olefin hydrosilylation at room temperature, bypassing the need for photoirradiation.
• Direct observation of a stable 3-center-3-electron (3c-3e) Rh(II)-silane σ-adduct, a key intermediate.
• The tricomponent transition state involving Rh(II), silane, and ethylene lowers the activation barrier for silyl radical transfer by approximately 15 kcal·mol^-1.
• Quantum chemical calculations and experiments confirm that proximal Rh(II) centers in the MOF promote silyl radical transfer via temporary oxidation.

Conclusions:

• The MOF matrix plays a critical role by controlling the inter-Rh separation, enabling both the formation of the 3c-3e silane adduct and efficient electron transfer.
• This work presents a new paradigm for thermal hydrosilylation catalysis by circumventing high-energy silyl radical intermediates.
• The findings open avenues for developing more energy-efficient catalytic processes in organosilicon chemistry.