Three-Coordinate Cobalt(IV) and Cobalt(V) Imido Complexes with N‑Heterocyclic Carbene Ligation: Synthesis, Structure, and Their Distinct Reactivity in C–H Bond Amination

The reaction of the
cobalt(0) alkene complex [(IMes)­Co­(η2:η2-dvtms)] (1) (IMes = 1,3-bis­(1′,3′,5′-trimethylphenyl)­imidazol-2-ylidene,
dvtms = divinyltetramethyldisiloxane) with 2 equiv of DippN3 (Dipp = 2,6-diisopropylphenyl) afforded the cobalt­(IV) imido complex
[(IMes)­Co­(NDipp)2] (2), which could be oxidized
by [Cp2Fe]­[BArF4] (ArF = 3,5-di­(trifluoromethyl)­phenyl) to give the cobalt­(V) imido species
[(IMes)­Co­(NDipp)2]­[BArF4] (3). The molecular structures of all these complexes were established
by single-crystal X-ray diffraction studies. Characterization data
and theoretical calculations suggest ground spin states of S = 1/2 and S = 0
for the cobalt­(IV) and cobalt­(V) species, respectively. When heated,
the cobalt­(IV) imido species was converted to a cobalt­(II) diamido
complex via an intramolecular C–H bond amination reaction,
but the cobalt­(V) species was stable under similar conditions. The
different outcomes suggest that a high oxidation state does not guarantee
C–H bond activation reactivity of late-transition-metal imido
species.