Square-planar imido complexes of cobalt: synthesis, reactivity and computational study

Abstract

Treatment of [Co(N₂)(^tBuPNP)] (^tBuPNP = anion of 2,5-bis(di-tert-butylphosphinomethyl)pyrrole) with one equivalent of an aryl azide generates the four-coordinate imido complexes [Co(NAr)(^tBuPNP)] (Ar = mesityl, phenyl, or 4-^tBu-phenyl). X-ray crystallographic analysis of the compounds shows an unusual square-planar geometry about cobalt with nearly linear imido units. In the presence of the hydrogen atom donor, TEMPOH, [Co(NPh)(^tBuPNP)] undergoes addition of the H atom to the imido nitrogen to generate the corresponding amido complex, [Co(NHPh)(^tBuPNP)], whose structure and composition were verified by independent synthesis. Despite the observation of H atom transfer reactivity with TEMPOH, the imido complexes do not show catalytic activity for C–H amination or aziridination for several substrates examined. In the case of [Co(NPh)(^tBuPNP)], addition of excess azide produced the tetrazido complex, [Co(N₄Ph₂)(^tBuPNP)], whose bond metrics were most consistent with an anionic Ph₂N₄ ligand. Density Functional Theory (DFT) investigations of the imido and tetrazido species suggest that they adopt a ground state best described as possessing a low-spin cobalt(II) ion ferromagnetically coupled to an iminyl radical.