Control over Selectivity in Alkene Hydrosilylation Catalyzed by Cobalt(III) Hydride Complexes

Area of Science:

• Organometallic Chemistry
• Catalysis
• Coordination Chemistry

Background:

• Pincer ligands, particularly those based on phosphorus ([PCP]) and silicon ([SiCSi]), are crucial in stabilizing metal centers for catalytic applications.
• Cobalt complexes are increasingly explored as cost-effective alternatives to precious metals in homogeneous catalysis, including hydrosilylation.
• Understanding ligand effects on catalyst activity and selectivity is key to designing efficient catalytic systems.

Purpose of the Study:

• To synthesize and characterize novel bisphosphine [PCP] and bissilylene [SiCSi] pincer cobalt(III) hydrides.
• To evaluate the catalytic performance of these new cobalt complexes in alkene hydrosilylation reactions.
• To investigate the influence of ligand structure and substrate type on the regioselectivity of the hydrosilylation process.

Main Methods:

• Synthesis of three new cobalt(III) hydrides: two [PCP] bisphosphine complexes and one [SiCSi] bissilylene complex, via reaction with CoCl(PMe3)3.
• Catalytic testing of the synthesized cobalt complexes in alkene hydrosilylation across a scope of 28 substrates.
• Mechanistic studies, including the synthesis and evaluation of a cobalt(I) complex, to elucidate reaction pathways.

Main Results:

• The [PCP] pincer cobalt(III) hydrides demonstrated higher catalytic activity than the [SiCSi] complex, with both affording anti-Markovnikov selectivity for various alkenes.
• The [SiCSi] bissilylene complex showed comparable activity to a phosphine complex but exhibited substrate-dependent regioselectivity, yielding anti-Markovnikov products for aliphatic alkenes and Markovnikov products for aromatic alkenes.
• Mechanistic insights were gained through the study of a cobalt(I) complex, revealing variations dependent on reaction conditions and substrates.

Conclusions:

• The electronic and steric properties of [PCP] and [SiCSi] pincer ligands significantly impact the catalytic behavior of cobalt(III) hydrides in alkene hydrosilylation.
• Ligand design and substrate choice are critical factors in controlling regioselectivity, offering pathways to selectively synthesize different isomers.
• The study provides a foundation for developing tailored cobalt catalysts for specific hydrosilylation applications by understanding mechanistic nuances.