Catalytic hydrofunctionalization and small molecule reactivity at a low-valent cobalt center supported by a bis(carbene) CCC pincer ligand

The ubiquity of noble metal catalysts in both industrial and academic settings is a testament to their impressive reactivity, versatility, and modularity. Recent advances in cross-coupling catalyses have principally been achieved with this privileged class of compounds, in large part due to the ability of noble metals to engage in well-defined two-electron redox events that are amenable to analysis and prediction. In contrast, the reactivity of first-row transition metals is mostly characterized by one-electron redox changes that can be deleterious and has precluded the systemic incorporation of these more earth-abundant and affordable metals in certain settings. Considerations of the intrinsic electronic structure of these base metals provides insight into their apparently orthogonal redox reactivity to second and third-row transition metals. For many first-row transition metal compounds, the presence of a weak ligand field engenders a high-spin electron configuration and one-electron events at the expense of two-electron elementary steps. Accordingly, our lab and many others have endeavored to utilize a strong ligand field approach whereby a highly-donating ligand set is installed at the first-row transition metal center to impart a strong ligand field and thereby disfavor one-electron redox changes. Early work focused on the synthesis and characterization of cobalt derivatives of the (DIPPCCC) pincer ligand, (DIPPCCC = bis(diisopropylphenyl-benzimidazol-2-ylidene)phenyl), a platform which provides highly-donating carbenes and an aryl carbon linkage to the coordinated metal. Multinuclear NMR spectroscopies, cyclic voltammetry, X-ray crystallography, and EPR spectroscopies have established the successful isolation of Co(I), Co(II), and Co(III) derivatives with this ligand platform. The diamagnetic natures of the Co(I) and Co(III) compounds, (DIPPCCC)Co(N2) and (DIPPCCC)CoCl2py, respectively, confirmed the ability of the CCC ligand framework to support low-spin electron configurations at the first-row transition metal center and motivated additional studies into the reactivity of the Co(I) dinitrogen compound. Initial studies explored the catalytic competency of the (DIPPCCC)CoN2 species, specifically towards hydrosilylation and hydroboration, two processes typically mediated by precious metal catalysts. Hydrosilylation of terminal alkene substrates proceeded in good yields with secondary and tertiary silanes, an important feat for earth-abundant catalysts for which examples of hydrosilylation with tertiary silanes and hydrosiloxanes are relatively uncommon. Interestingly, the catalyst exhibited high chemoselectivity and 1,2-regioselectivity even towards challenging substrates bearing unprotected hydroxyl, amino, nitrile, formyl, and conjugated diene functionality, a consequence of both steric and electronic factors. A variety of experiments, including 29Si NMR and two-dimensional NMR spectroscopy experiments, confirmed oxidative addition of silane and established that this catalysis likely proceeds through a Chalk-Harrod-type mechanism, thereby establishing that a Co(I)/Co(III) redox couple is operative for this process. Extension of this catalysis towards hydroboration was successful. In addition to exhibiting a chemoselectivity and regioselectivity similar to that of the hydrosilylation protocol, the (DIPPCCC)Co(N2) catalyst was also found to be competent towards nitrile reduction, a challenging process for earth-abundant metal catalysts. Mechanistic studies establish the intermediacy of a cobalt hydride, as well as the negotiation of insertion, isomerization, and -hydride elimination processes. These results allowed us to propose a mechanism, similar to that proposed for the hydrosilylation protocol, whereby the cobalt catalyst accesses a Co(I)/Co(III) redox couple, highlighting the ability of this bis(carbene) CCC pincer platform to impart noble-metal-type reactivity onto base metals. Finally, the stoichiometric reactivity of the CoI complex towards potential two-electron oxidants was probed in order to assess the viability of Co(I)/Co(III) redox couple with additional substrates and advance our understanding of the reactivity of these interesting compounds