Unexpected Gas-Phase Reactivity of the CH3OH Adduct of Michler's Hydrol Blue: Proton-Shuttle Catalysis and Stepwise Radical Expulsions

Abstract

Electrospray ionization of a methanolic solution of Michler's hydrol blue, bis(4-(N,N-dimethylamino)phenyl)methyl tetrafluoroborate, 1+BF(4)-, produces the formal methanol adduct [1+.CH(3)OH], which shows an unusual gas-phase chemistry. Tandem mass-spectrometry experiments and complementary theoretical studies indicate that this adduct corresponds to the methyl ether of Michler's hydrol protonated at one of the dimethylamino groups, cation 3+. Collision-induced dissociation of mass-selected 3+ leads to two sequential expulsions of open-shell species, resulting in a formal loss of [C(2)H(6)O], whereas no expulsion of methanol is observed. In contrast, interaction of gaseous 3+ with a single molecule of a suitable base triggers an exoergic loss of methanol via proton-shuttle catalysis within the collision complex. The occurrence of this exothermic proton transfer also prevents the application of the otherwise successful kinetic method for the determination of the gas-phase proton affinity of the methyl ether of Michler's hydrol.