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

Treatment of [(Me3tpa)IrII(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 [(Me3tpa)IrIII(ethene)(superoxo)]2+ (4a2+). 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 [(Me3tpa)IrIII(DMPOX)]2+ (7(2+); DMPOX = 5,5-dimethyl-2-pyrrolidone-1-oxide). In acetonitrile, in the absence of DMPO, oxygenation of 1(2+) to [(Me3tpa)IrIII(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 [(Me3tpa)Ir(ethene)(MeCN)]2+ (3(2+)), which has a significant metalla-ethyl radical (IrIII-CH2CH2*) character, allowing attack of 3O2 directly at the ethene ligand. Both pathways are discussed on the basis of experimental observations and DFT geometry optimizations.