Multielectron Redox Afforded by a Pincer Ligand Promoting Kumada Cross-Coupling Reactions

Area of Science:

• Organometallic Chemistry
• Catalysis
• Organic Synthesis

Background:

• Pincer ligands are crucial in catalysis due to their robust coordination.
• Redox-active ligands offer unique electronic properties for catalytic transformations.
• Nickel complexes are versatile catalysts for cross-coupling reactions.

Purpose of the Study:

• To explore the catalytic activity of a redox-active NNN nickel pincer complex in C-C cross-coupling.
• To elucidate the mechanism, particularly the role of the ligand's radical character.
• To demonstrate the efficiency of the catalyst under mild conditions.

Main Methods:

• Synthesis and structural characterization of the nickel pincer complex.
• Kumada cross-coupling reactions using aryl halides and Grignard reagents.
• Mechanistic studies including TEMPO quenching, radical-clock experiments, and DFT calculations.

Main Results:

• The nickel complex, featuring a Ni2N2 diamond core, exists as a dimer with the ligand in the iminosemiquinonate form.
• Efficient Kumada cross-coupling of aryl halides (I, Br, Cl) with aryl Grignard reagents using 3 mol% catalyst loading.
• Evidence supporting a radical pathway involving the redox-active ligand, confirmed by radical quenching and clock experiments.

Conclusions:

• The redox-active pincer ligand plays a key role in promoting C-C cross-coupling via a radical mechanism.
• The cooperative action between the ligand and nickel center drives the catalytic cycle.
• This system offers a mild and efficient method for synthesizing biaryl compounds.