A Mononuclear Non-Heme Manganese(III)-Aqua Complex in Oxygen Atom Transfer Reactions via Electron Transfer.

Metal-oxygen complexes, such as metal-oxo [M(O)], -hydroxo [M(OH)], -peroxo [M(O)], -hydroperoxo [M(OOH)], and -superoxo [M(O)] species, are capable of conducting oxygen atom transfer (OAT) reactions with organic substrates, such as thioanisole (PhSMe) and triphenylphosphine (PhP). However, OAT of metal-aqua complexes, [M(OH)], has yet to be reported. We report herein OAT of a mononuclear non-heme Mn(III)-aqua complex, [(dpaq)Mn(OH)] (, dpaq = 2-[bis(pyridin-2-ylmethyl)]amino--quinolin-8-yl-acetamidate), to PhSMe and PhP derivatives for the first time; it is noted that no OAT occurs from the corresponding Mn(III)-hydroxo complex, [(dpaq)Mn(OH)] (), to the substrates. Mechanistic studies reveal that OAT reaction of occurs via electron transfer from 4-methoxythioanisole to to produce the 4-methoxythioanisole radical cation and [(dpaq)Mn(OH)], followed by nucleophilic attack of HO in [(dpaq)Mn(OH)] to the 4-methoxythioanisole radical cation to produce an OH adduct radical, 2,4-(MeO)CHS(OH)Me, which disproportionates or undergoes electron transfer to to yield methyl 4-methoxyphenyl sulfoxide. Formation of the thioanisole radical cation derivatives is detected by the stopped-flow transient absorption measurements in OAT from to 2,4-dimethoxythioanisole and 3,4-dimethoxythioanisole, being compared with that in the photoinduced electron transfer oxidation of PhSMe derivatives, which are detected by laser-induced transient absorption measurements. Similarly, OAT from to PhP occurs via electron transfer from PhP to , and the proton effect on the reaction rate has been discussed. The rate constants of electron transfer from electron donors, including PhSMe and PhP derivatives, to are fitted well by the electron transfer driving force dependence of the rate constants predicted by the Marcus theory of outer-sphere electron transfer.