The Atmosphere Matters: The Effect of “Inert” Gas on the Catalytic Outcomes in Cobalt-Mediated Alkyne and Olefin Hydroboration

Abstract

We present a highly efficient cobalt-mediated hydroboration reaction of alkynes and alkenes enabled by a π-acidic and redox-active pyrimidinediimine (PPymDI) core. The entry point in the catalytic cycle is on a cobalt dinitrogen complex rather than cobalt hydride species, which are commonly postulated in hydroelementation reactions. Stoichiometric studies have demonstrated that both cobalt hydrides and dinitrogen complexes can be generated from the same precursors and under the same reaction conditions, with the sole difference being the reaction atmosphere (argon vs N2). Nevertheless, while the PPymDI-based cobalt dinitrogen complex is highly active (TOF = 1100 h–1 at t1/2, RT), the hydride analogue displays only modest conversions at slow reaction rates. Under the optimized conditions, a wide range of vinyl and alkyl organoboron derivatives can be obtained with high catalytic efficiency. Mechanistic studies suggest that, due to the increased π-acidity of the PPymDI core, initial formation of Co-π-complexes is preferred, followed by the oxidative addition of borane, which is the turnover-limiting step. This sequence of catalytic steps is supported by a Hammett analysis, which shows that the hydroboration reaction proceeds more rapidly for electron-rich substrates. The electronic structure of all relevant species was investigated in detail by computational, crystallographic, and spectroscopic means, revealing ligand involvement in the redox processes.