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Ni-catalyzed cross-electrophile coupling (XEC) reactions have gained prominence for the construction of C–C bonds. Prior studies of XEC routes to biaryls have invoked several different mechanisms for the formation of key Ni(Ar)2 intermediates. Here, we provide evidence for a previously unrecognized pathway involving reductively induced transmetalation between NiI(Ar) and NiII(Ar)X species. Chemical and electrochemical reduction of (tBubpy)NiII(2-tolyl)Br (tBubpy = 4,4’-di-tert-butyl-2,2’-bipyridine) to (tBubpy)NiI(2-tolyl) is shown to initiate rapid transmetalation of the 2-tolyl ligand to a second equivalent of (tBubpy)NiII(2-tolyl)Br, affording (tBubpy)NiII(2-tolyl)2 and (tBubpy)NiIBr as well defined products. Experimental and computational data show that the NiI-to-NiII transmetalation mechanism is much more favorable than NiII-to-NiII transmetalation. Oxidation of (tBubpy)NiII(2-tolyl)Br results in rapid reductive elimination of 2-tolyl–Br, rather than promoting the analogous oxidatively induced NiII/NiIII transmetalation. The NiII(2-tolyl)2 product of NiI-to-NiII transmetalation is stable at room temperature, while sterically less encumbered NiII(Ar)2 species undergo rapid reductive elimination to afford biaryl and a Ni0 byproduct. The latter species can serve as a source of electrons to promote further transmetalation and biaryl formation. The unhindered complex (tBubpy)NiII(4-CF3-phenyl)Br undergoes biaryl formation in the absence of added reductant; however, kinetic analysis reveals an induction period and autocatalytic time course. Addition of catalytic quantities of a cobaltocene-based reductant eliminates the induction period and accelerates biaryl formation, consistent with the NiI-to-NiII transmetalation pathway. The results of this study provide a new rationale for previously reported results in the literature and introduce an alternative pathway to consider in the development of Ni-catalyzed biaryl coupling reactions. 

Transition metal oxo species are key intermediates for the activation of strong C–H bonds. As such, there has been interest in understanding which structural or electronic parameters of metal oxo complexes determine their reactivity. Factors such as ground state thermodynamics, spin state, steric environment, oxygen radical character, and asynchronicity have all been cited as key contributors, yet there is no consensus on when each of these parameters is significant or the relative magnitude of their effects. Herein, we present a thorough statistical analysis of parameters that have been proposed to influence transition metal oxo mediated C–H activation. We used density functional theory (DFT) to compute parameters for transition metal oxo complexes and analyzed their ability to explain and predict an extensive data set of experimentally determined reaction barriers. We found that, in general, only thermodynamic parameters play a statistically significant role. Notably, however, there are independent and significant contributions from the oxidation potential and basicity of the oxo complexes which suggest a more complicated thermodynamic picture than what has been shown previously.