An Intermediate Cobalt(IV) Nitrido Complex and its N-Migratory Insertion Product
Résumé
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.