Reactive Nickel Species Relevant to Cross-Coupling

Abstract

Nickel catalysis has emerged as a powerful means of accomplishing diverse cross-coupling reactions. Although nickel catalysis has traditionally relied on ground state reactivity in the Ni(0) and Ni(II) oxidation states, recent progress has unlocked many novel transformations by exploiting the reactivity of excited or odd-electron nickel species. However, the transient and unstable nature of these species renders them difficult to study, a statement that holds true for catalytic systems bearing bipyridine-type ligands that are employed in many synthetic methodologies. Direct studies of catalytically relevant nickel complexes in their excited states or odd-valent forms would significantly advance mechanistic understanding. This dissertation details the study of two classes of reactive nickel species relevant to cross coupling: excited state Ni(II) and ground state Ni(I).Complexes of the type (bipyridine)NiII(aryl)(halide) are commonly proposed catalytic intermediates, the photoexcitation of which can be responsible for productive cross coupling. We experimentally clarified the photophysics of excited states for these complexes, establishing that they initially access a metal-to-ligand charge transfer state but rapidly relax to a low-energy 3d-d state with weakened metal–ligand bonds. Organometallic studies revealed that these complexes can undergo photoinduced homolysis of the Ni(II)–aryl bond, providing a mechanism for accessing Ni(I) species. Since Ni(I)-bipyridine species are believed to be responsible for catalytic activity in a variety of cases, photolysis can thus serve to accomplish catalyst activation. This is now believed to be operative in several synthetic methodologies. To investigate the ground state reactivity of Ni(I)-bipyridine species, we synthesized the well-defined complex [(CO2Etbpy)NiICl]4 (CO2Etbpy = diethyl 2,2′-bipyridine-4,4′-dicarboxylate). The speciation of this complex in solution is characterized by dissociation into its monomeric form, and a redox equilibrium with Ni(0) and Ni(II) species. This complex readily reacts with aryl halides, undergoing a formal bimetallic oxidative addition. Kinetic and stoichiometric studies supported that oxidative addition occurs from a Ni(I) monomer to initially generate a Ni(III) species. This occurs through either a concerted mechanism or a halogen atom abstraction followed by in-cage recombination with the same Ni center. Subsequently, the Ni(III) complex undergoes comproportionation with a second equivalent of Ni(I) to form Ni(II) products. We hope that the understanding gained from this work may inspire advances in nickel catalysis more broadly.