An Intermediate Cobalt(IV) Nitrido Complex and its N‑Migratory Insertion Product

Low-temperature photolysis experiments
(T = 10
K) on the tripodal azido complex [(BIMPNMes,Ad,Me)­CoII(N3)] (1) were monitored by EPR spectroscopy
and support the formation of an exceedingly reactive, high-valent
Co nitrido species [(BIMPNMes,Ad,Me)­CoIV(N)]
(2). Density functional theory calculations suggest a
low-spin d5, S = 1/2, electronic configuration
of the central cobalt ion in 2 and, thus, are in line
with the formulation of complex 2 as a genuine, low-spin
Co­(IV) nitride species. Although the reactivity of this species precludes
handling above 50 K or isolation in the solid state, the N-migratory
insertion product [(NH-BIMPNMes,Ad,Me)­CoII]­(BPh4) (3) is isolable and was reproducibly synthesized
as well as fully characterized, including CHN elemental analysis,
paramagnetic 1H NMR, IR, UV–vis, and EPR spectroscopy
as well as SQUID magnetization and single-crystal X-ray crystallography
studies. A computational analysis of the reaction pathway 2 → 3 indicates that the reaction readily occurs
via N-migratory insertion into the Co–C bond (activation barrier
of 2.2 kcal mol–1). In addition to the unusual reactivity
of the nitride 2, the resulting divalent cobalt complex 3 is a rare example of a trigonal pyramidal complex with four
different donor ligands of a tetradentate chelatean N-heterocyclic
carbene, a phenolate, an imine, and an aminebinding to a high-spin
Co­(II) ion. This renders complex 3 chiral-at-metal.