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

摘要

Metal-oxygen complexes, such as metal-oxo [M-(O~(2-))], -hydroxo [M(OH~-)], -peroxo [M(O_2~(2-))], -hydro-peroxo [M(OOH~-)], and -superoxo [M(O_2·~-)] species, are capable of conducting oxygen atom transfer (OAT) reactions with organic substrates, such as thioanisole (PhSMe) and triphenyl-phosphine (Ph_3P). However, OAT of metal-aqua complexes, [M(OH_2)]~(n+), has yet to be reported. We report herein OAT of a mononuclear non-heme Mn(Ⅲ)-aqua complex, [(dpaq)-Mn~Ⅲ(OH_2)]~(2+) (1, dpaq = 2-[bis(pyridin-2-ylmethyl)]arnino-N-quinolin-8-yl-acetamidate), to PhSMe and Ph_3P derivatives for the first time; it is noted that no OAT occurs from the corresponding Mn(Ⅲ)-hydroxo complex, [(dpaq)Mn~(Ⅲ)(OH)]~+ (2), to the substrates. Mechanistic studies reveal that OAT reaction of 1 occurs via electron transfer from 4-methoxythioanisole to 1 to produce the 4-methoxythioanisole radical cation and [(dpaq)Mn~Ⅱ(OH_2)]~+, followed by nucleophilic attack of H_2O in [(dpaq)Mn~Ⅱ(OH_2)]~+ to the 4-methoxythioanisole radical cation to produce an OH adduct radical, 2,4-(MeO)_2C_6H_3S·(OH)Me, which disproportionates or undergoes electron transfer to 1 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 1 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 1 to Ph_3P occurs via electron transfer from Ph_3P to 1, and the proton effect on the reaction rate has been discussed. The rate constants of electron transfer from electron donors, including PhSMe and Ph_3P derivatives, to 1 are fitted well by the electron transfer driving force dependence of the rate constants predicted by the Marcus theory of outer-sphere electron transfer.