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(η(2):η(2)-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][BAr(F)4] (Ar(F) = 3,5-di(trifluoromethyl)phenyl) to give the cobalt(V) imido species [(IMes)Co(NDipp)2][BAr(F)4] (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.