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

Abstract

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 <b>1-THF</b> and the five-coordinate THF-free complex <b>1</b> are in a high-spin <i>S</i> = 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 <b>1</b> or <b>1-THF</b> 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