Reactivity Studies of Bipyridine-Ligated Nickel(I) and Nickel(0) Complexes Inform the Mechanism in Modern Cross-Coupling Reactions
T. Judah Raab, Abigail G. Doyle
Abstract
Bipyridine-ligated nickel(I) and nickel(0) intermediates are widely proposed in Ni-catalyzed cross-coupling reactions. However, few isolable Ni I and Ni 0 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 -Bu bpy)Ni I and ( t -Bu bpy)Ni 0 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 -Bu bpy)NiBr, measured rates for the activation of aryl halides and alkyl halides by ( t -Bu bpy)Ni I and ( t -Bu bpy)Ni 0, and found that the reduction of ( t -Bu bpy)NiBr to Ni 0 is inefficient with common heterogeneous metal reductants. Taken together, these studies enable us to propose a general mechanism for cross-electrophile coupling reactions.