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