What Factors Control the Reactivity of Cobalt-Imido Complexes in C-H Bond Activation via Hydrogen Abstraction?

Metal-imido complexes are critical intermediates in transition metal-catalyzed C-H amination reactions. Discerning the factors that control their reactivity, however, remains largely open for exploration, particularly for the territory of cobalt-imido's. Herein we describe a systematic computational exploration of this new frontier via the C-H activation mechanisms of typical well-defined cobalt-imido complexes, whose formal oxidation states cover an extremely wide range from Co(II) to Co(V). Hydrogen atom abstraction (HAA) is found to be the rate-limiting step in all these systems, with the open-shell electronic states of radical character consistently bearing kinetic advantage over the closed-shell ones. Surprisingly, there is no correlation found between the cobalt oxidation state and the HAA reactivity. To render a more accessible HAA channel, the dichotomous EER/anti-EER electron-shift scenarios for the open-shell electronic structure are dependent on the cobalt oxidation states [Co(III), different from others], implying a paradigm shift from an EER to an anti-EER scenario in the periodic table from Fe to Co. In contrast to the kinetic factor that determines the HAA reactivity, the reaction outcomes of C-H activation (amination or cyclometalation product) in cobalt-imido complexes are found to be controlled by the thermodynamic stabilities of the products. Our results for the cobalt-imido complexes imply that, in addition to HAA chemistry of metal-oxo's, the HAA promoted by metal-imido species could also be subject to the radical-facilitated reactivity. From this work, it is predictable that the stabilization of the less reactive closed-shell singlet state relative to other more reactive open-shell states is generally not beneficial to the HAA reactivity of cobalt-imido species.