IrII(ethene):: Metal or carbon radical?: Part II:: Oxygenation via iridium or direct oxygenation at ethene?

Treatment of [(Me(3)tpa)Ir-II(ethene)](2+) (Me(3)tpa = N,N,N-tri(6-methyl-2-pyridylmethyl)amine) (1(2+)) with dioxygen in weakly coordinating solvents results in formation of [(Me(3)tpa)Ir-III (ethene)(superoxo)](2+) (4a(2+)). In the presence of DMPO (DMPO = 5,5-dimethyl-2-pyrrolidine-1-oxide) DMPO is substituted for ethene, and subsequently oxidized to DMPOX by the superoxo fragment to give [(Me(3)tpa)Ir-III (DMPOX)](2+) (7(2+); DMPOX = 5,5-dimethyl-2-pyrrolidone-1-oxide). In acetonitrile, in the absence of DMPO, oxygenation of 1(2+) to [(Me(3)tpa)Ir-III (formylmethyl) (MeCN)](2+) (2(2+)) is observed. In the presence of DMPO the formation of 2(2+) and 7(2+) is competing. Oxygenation of 1(2+) to 2(2+) may proceed via 4a(2+), involving an insertion mechanism at the metal. However, a mechanism based on olefin ligand non-innocence seems a reasonable alternative. This involves formation of acetonitrile adduct [(Me(3)tpa)Ir-(ethene)(MeCN)](2+) (3(2+)), which has a significant metalla-ethyl radical (Ir-III-CH2CH2.) character, allowing attack of O-3(2) directly at the ethene ligand. Both pathways are discussed on the basis of experimental observations and DFT geometry optimizations.