A bulky 1,4,7-triazacyclononane and acetonitrile, a Goldilocks system for probing the role of Ni^III and Ni^I centers in cross-coupling catalysis

In nickel-catalyzed cross-coupling reactions involving alkyl substrates, catalysts supported by N-donor ligands are proposed to involve paramagnetic Ni(I) and Ni(III) species as key intermediates. Herein, we report the use of a bulky ligand, 1,4,7-triisopropyl-1,4,7-triazacyclononane (iPr₃TACN), that facilitated the detection or isolation of uncommon organometallic Ni(I) and Ni(III) complexes involved in well-defined oxidative addition, transmetalation, and reductive elimination steps. Moreover, (iPr₃TACN)Ni(II) complexes were shown to be efficient catalysts for alkyl-alkyl Kumada cross-coupling. The presence of acetonitrile and other alkyl nitriles led to an increased yield of cross-coupled products, and the multifaceted beneficial role of the nitrile additive during catalysis was thoroughly investigated. Overall, these studies provide unambiguous evidence for the involvement of both Ni(III) and Ni(I) organometallic species in Ni-catalyzed alkyl-alkyl cross-coupling reactions and suggest the presence of a coordinating solvent and a weak π-acceptor, such as acetonitrile, could have a beneficial effect for a range of Ni-mediated organometallic transformations.