A Pair of Cobalt(III/IV) Terminal Imido Complexes

The reaction of the cobalt(I) complex [(TIMMNmes)Co-I](BPh4) (2) (TIMMNmes=tris-[2-(3-mesitylimidazolin-2-ylidene)methyl]amine) with 1-adamantylazide yields the cobalt(III) imido complex [(TIMMNmes)Co-III(NAd)](BPh4) (3) with concomitant release of dinitrogen. The N-anchor in diamagnetic 3 features an unusual, planar tertiary amine, which results from repulsive electrostatic interaction with the filled d(z(2))-orbital of the cobalt ion and negative hyperconjugation with the neighboring methylene groups. One-electron oxidation of 3 with [FeCp2](OTf) provides access to the rare, high-valent cobalt(IV) imido complex [(TIMMNmes)Co-IV(NAd)](OTf)(2) (4). Despite a half-life of less than 1 h at room temperature, 4 could be isolated at low temperatures in analytically pure form. Single-crystal X-ray diffractometry and EPR spectroscopy corroborate the molecular structure and the d(5) low-spin, S=1/2 , electron configuration. A computational analysis of 4 suggests high covalency within the Co-IV=NAd bond with non-negligible spin density located at the imido moiety, which translates into substantial triplet nitrene character.