Photolytic Activation of Ni(II)X2(dtbbpy) Explains How Ni-Mediated Cross Coupling Begins

We show that Ni-mediated carbon-carbon cross coupling can be initiated by trans- formation of Ni(II)X2(dtbbpy) to Ni(I)X(dtbbpy) by light. Photolysis of the Ni(II)- X bond (X= Cl, Br) either via direct excitation or triplet energy transfer produces Ni(I)X(dtbbpy) and a halogen radical, X•. Hydrogen atom abstraction, often from the solvent, subsequently produces a C(sp3) radical, R•, that recombines with Ni(I) to form novel organonickel(II) complexes, Ni(II)XR(dtbbpy). These solvent-derived organonickel(II) species can be subsequently photolyzed back to Ni(I), thereby serving as a reservoir state that protects the system from Ni dimer formation and deactivation. A combination of x-ray absorption (XAS), nuclear-magnetic resonance (NMR), elec- tronic absorption (UV-Vis), and electron paramagnetic resonance (EPR) spectroscopies confirm the identity of the final photo-products, whilst the R• addition to Ni(I) was in- dependently observed via pulse radiolysis and found to form the same NiXR(dtbbpy) complex on the μs timescale. Finally, subsequent irradiation of the NiXR(dtbbpy) reservoir state converts it to known NiX(Caryl)(dtbbpy) compounds in the presence of an aryl bromide. These results explain how Ni(II) pre-catalysts convert to Ni(I) and ultimately drive oxidative addition of aryl halides in many pivotal methodologies for C(sp2)-C(sp3) bond formation, for example in cross electrophile couplings and C-H activations.