Syntheses, structures, and spectroscopic properties of cobalt(III) complexes containing 3- or 4-pyridyl-substituted nitronyl and imino nitroxide ligands

Several new cobalt(III) complexes incorporating 4,4,5,5-tetramethyl-2-(3- or 4-pyridyl)imidazolin-1-oxyl 3-oxide (NITnpy) (n = 3 or 4) or 4,4,5,5-tetramethyl-2-(3- or 4-pyridyl)-imidazolin-1-oxyl (IMnpy) have been prepared. The X-ray crystallographic analyses and the spectroscopic investigations have revealed that the nitronyl or imino nitroxides with the radical character are bound through the pyridyl-N atom to a Co^III center. There are some influences from the radical unpaired electron toward the Co^III complex moiety and those from the coordination of the Co^III ion toward the nature of these radicals which can be detected by ⁵⁹Co NMR and UV/Vis spectroscopy. Such observed changes coincide with the proposed spin density on the N atom of the pyridyl substituent. Furthermore, it was found that trans-[Co-(acac)₂(PPh₃)(NIT4py)]⁺ in methanol was gradually converted into the corresponding IM4py complex, while free NIT4py is stable in methanol. The Co^II species formed on the partial decomposition of the NIT4py complex in methanol seems to catalyze the deoxygenation of NIT4py to IM4py.