Challenging the Kinetic Likelihood of Bimolecular Bipyridine-NickelIII Aryl Dihalide Reactions

<jats:p>Nickel-catalyzed cross-coupling reactions are frequently proposed to proceed through NiI/NiIII catalytic cycles in which high-valent NiIII intermediates participate in bimolecular elementary steps, including NiI/NiIII comproportionation and ligand exchange with nucleophiles. Despite their central role in many mechanistic proposals, the kinetic viability of these processes has not been experimentally established. Here, we examine whether bipyridine-ligated NiIII aryl dihalide complexes possess sufficient lifetimes to engage in such chemistry. Cyclic and square-wave voltammetry studies demonstrate that reductive elimination of aryl halides from NiIII occurs sufficiently rapidly to outcompete NiI/NiIII comproportionation, even under conditions that maximize the local concentrations of both odd-electron Ni intermediates at the electrode surface. Additionally, independent stoichiometric oxidation studies further show that NiIII aryl dihalide complexes do not undergo productive ligand exchange with methanol prior to reductive elimination. These findings indicate that bipyridine-supported NiIII aryl dihalide complexes are too short-lived to participate in many bimolecular reactions commonly invoked in catalytic mechanisms. Instead, the data supports a reaction landscape dominated by unimolecular decay of NiIII and bimolecular processes involving NiI and NiII species. These results challenge the widespread assumption that NiIII intermediates routinely engage in bimolecular reactions during every catalytic turnover and instead favor mechanisms in which more persistent NiI complexes serve as the catalytically relevant reactive intermediates. The data presented here enable us to revise previously accepted mechanistic proposals for both Ni-photochemical C(sp2)–C(sp3) and C(sp2)–heteroatom cross-coupling reactions.</jats:p>