An Intermediate
Cobalt(IV) Nitrido Complex and its
N‑Migratory Insertion Product
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Abstract
Low-temperature photolysis experiments
(<i>T</i> = 10
K) on the tripodal azido complex [(BIMPN<sup>Mes,Ad,Me</sup>)Co<sup>II</sup>(N<sub>3</sub>)] (<b>1</b>) were monitored by EPR spectroscopy
and support the formation of an exceedingly reactive, high-valent
Co nitrido species [(BIMPN<sup>Mes,Ad,Me</sup>)Co<sup>IV</sup>(N)]
(<b>2</b>). Density functional theory calculations suggest a
low-spin d<sup>5</sup>, <i>S</i> = 1/2, electronic configuration
of the central cobalt ion in <b>2</b> and, thus, are in line
with the formulation of complex <b>2</b> 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-BIMPN<sup>Mes,Ad,Me</sup>)Co<sup>II</sup>](BPh<sub>4</sub>) (<b>3</b>) is isolable and was reproducibly synthesized
as well as fully characterized, including CHN elemental analysis,
paramagnetic <sup>1</sup>H 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 <b>2</b> → <b>3</b> indicates that the reaction readily occurs
via N-migratory insertion into the Co–C bond (activation barrier
of 2.2 kcal mol<sup>–1</sup>). In addition to the unusual reactivity
of the nitride <b>2</b>, the resulting divalent cobalt complex <b>3</b> is a rare example of a trigonal pyramidal complex with four
different donor ligands of a tetradentate chelatean N-heterocyclic
carbene, a phenolate, an imine, and an aminebinding to a high-spin
Co(II) ion. This renders complex <b>3</b> chiral-at-metal