Reactions of Neutral Cobalt(II) Complexes of a Dianionic Tetrapodal Pentadentate Ligand: Cobalt(III) Amides from Imido Radicals

Neutral cobalt­(II)
complexes of the dianionic tetrapodal pentadentate ligand B2Pz4Py, in which borate linkers supply the anionic charges,
are reported. Both the six-coordinate THF adduct 1-THF and the five-coordinate THF-free complex 1 are in a
high-spin S = 3/2 configuration in the ground state
and have been structurally characterized by X-ray crystallography.
These two Co­(II) starting materials react rapidly with aryl azides
of moderate steric bulk. The thermodynamic products of these reactions
are low-spin, diamagnetic, Co­(III) amido complexes that are either
monomeric, when an external hydrogen atom source such as 1,4-cyclohexadiene
is present, or dimeric products formed via C–C coupling of
the azide aryl group and internal transfer of H• to the nitrogen. These products are fully characterized and are
rare examples of octahedral Co amido compounds; structural determinations
reveal significant pyramidalization of the amido nitrogens due to
π–π repulsion wherein the amido ligand is primarily
a σ donor. The amido products arise from highly reactive Co­(III)
imido radical intermediates that are the kinetic products of the reactions
of 1 or 1-THF with the azide reagents. The
imido radicals can be detected by X-band EPR spectroscopy and have
been probed by density functional theory computations, which indicate
that this doublet species is characterized by a high degree of spin
localization on the imido ligand, accounting for the reactivity with
hydrogen atom sources and dimerization chemistry observed. The high
coordination number and the electron-rich nature of the dianionic
B2Pz4Py ligand framework render the imido ligand
formed highly reactive.