Isolated Organometallic Nickel(III) and Nickel(IV) Complexes Relevant to Carbon-Carbon Bond Formation Reactions.

Nickel-catalyzed cross-coupling reactions are experiencing a dramatic resurgence in recent years given their ability to employ a wider range of electrophiles as well as promote stereospecific or stereoselective transformations. In contrast to the extensively studied Pd catalysts that generally employ diamagnetic intermediates, Ni systems can more easily access various oxidation states including odd-electron configurations. For example, organometallic NiIII intermediates with aryl and/or alkyl ligands are commonly proposed as the active intermediates in cross-coupling reactions. Herein, we report the first isolated NiIII-dialkyl complex and show that this species is involved in stoichiometric and catalytic C-C bond formation reactions. Interestingly, the rate of C-C bond formation from a NiIII center is enhanced in the presence of an oxidant, suggesting the involvement of transient NiIV species. Indeed, such a NiIV species was observed and characterized spectroscopically for a nickelacycle system. Overall, these studies suggest that both NiIII and NiIV species could play an important role in a range of Ni-catalyzed cross-coupling reactions, especially those involving alkyl substrates.