Cross-coupling catalysts are prone to unproductive side reactivity that can limit their practical use in synthetic chemistry. A detailed understanding of these pathways and the conditions that enable them is important for reaction optimization and rational catalyst design. In this work, we report the off-cycle reactivity of a monoligated, CyJohnPhos-bound Ni0 complex following product-forming reductive elimination. In the absence of substrate, free phosphine ligand, or π-accepting additives, dimerization of (CyJohnPhos)Ni0 occurs, followed by C-P bond activation of the ligand to form a phosphido-bridged Ni0/NiII dimer; both the Ni0/Ni0 and Ni0/NiII dimers were structurally characterized. Monomeric (CyJohnPhos)Ni0 must be intercepted by substrate or free ligand to prevent irreversible dimerization and catalyst deactivation.

Nickels +1 oxidation state has received much interest due to its varied and often enigmatic behavior in increasingly popular catalytic methods. In part, the lack of understanding about NiI results from common synthetic strategies limiting the breadth of complexes that are accessible for mechanistic study and catalyst design. We report an oxidative approach using tribromide salts that allows for the generation of a well-defined precursor, [NiI(COD)Br]2, as well as several new NiI complexes. Included among them are complexes bearing bulky monophosphines, for which structure-speciation relationships are established and catalytic reactivity in a Suzuki-Miyaura coupling (SMC) is investigated. Notably, these routes also allow for the synthesis of well-defined monomeric t-Bubpy-bound NiI complexes, which has not previously been achieved. These complexes, which react with aryl halides, can enable previously challenging mechanistic investigations and present new opportunities for catalysis and synthesis.