Magnesium Nickelate Complexes and Their Implications in Ni-Catalyzed Cross-Couplings of Aryl Fluorides and Aryl Ethers with Grignard Reagents

The nickel-catalyzed Kumada-Tamao-Corriu cross-coupling reaction is widely used to form C-C bonds and receives continued interest due to the unique ability of nickel to activate challenging organic electrophiles containing C-F and C-O bonds. Recent studies on the nickel-catalyzed cross-coupling of Ar-F and Ar-OMe electrophiles with organolithium nucleophiles have unveiled the key involvement of highly reactive anionic nickelates, in which Li and Ni cooperatively promote the activation of these substrates. However, the possible formation of related heterobimetallic intermediates when employing organomagnesium nucleophiles as cross-coupling partners still remains widely underexplored. Filling this gap in the knowledge, we use air-stable Ni-tris(olefin), Ni(4-Me-stb)3 (where stb = stilbene), as a Ni(0) precursor to systematically investigate its reactivity toward several organomagnesium and organolithium reagents, which has allowed for the structural and spectroscopic characterization of a new family of nickelate complexes. Their possible implications and the influence of their constitution and speciation on the outcome of Kumada-Tamao-Corriu cross-couplings are also investigated through a series of stoichiometric and catalytic reactions, which have uncovered a dramatic solvent effect, hinting at the formation of contacted ion pair species as key to maximizing Mg (or Li)/Ni(0) cooperativity.