Reactions of Neutral Cobalt(II) Complexes of a Dianionic Tetrapodal Pentadentate Ligand: Cobalt(III) Amides from Imido Radicals
Résumé
Neutral cobalt(II) complexes of the dianionic tetrapodal pentadentate ligand B(2)Pz(4)Py, in which borate linkers supply the dianionic [GRAPHIC] charges, are repotted. 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-cydohexadiene 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 tare, examples of octahedral CO amid compounds; structural determinations reveal significant pytarrndalization of the amido nitrogens due to pi-pi repulsion wherein the amido is primarily a sigma 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 B(2)Pz(4)Py ligand framework render the imido ligand formed highly reactive.