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 access'ble 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.