Reactivity Studies of Bipyridine-Ligated Nickel(I) and Nickel(0) Complexes Inform the Mechanism in Modern Cross-Coupling Reactions

Bipyridine-ligated nickel(I) and nickel(0) intermediates are widely proposed in Ni-catalyzed cross-coupling reactions. However, few isolable NiI and Ni0 complexes with catalytically relevant bipyridine ligands are known, limiting our understanding of these complexes' speciation and reactivity. In this work, we identify and investigate well-defined, isolable (t-Bubpy)NiI and (t-Bubpy)Ni0 complexes to characterize their behavior in catalytic systems. Employing spectroscopic and stoichiometric studies, we identified a solvent dependence on rates of the irreversible dimerization of (t-Bubpy)NiBr, measured rates for the activation of aryl halides and alkyl halides by (t-Bubpy)NiI and (t-Bubpy)Ni0, and found that the reduction of (t-Bubpy)NiBr to Ni0 is inefficient with common heterogeneous metal reductants. Taken together, these studies enable us to propose a general mechanism for cross-electrophile coupling reactions.