Reductive Cleavage of Dioxygen Mediated by a Lewis Superacidic Bis(borane)

The bis(borane) o-{(C6F5)2B}2C6F4 (1) reacts with dioxygen
in the presence of decamethylferrocene (FeCp*2, Cp* = pentamethylcyclopentadienyl)
to deliver the salt [(μ–OH)-o-{B(C6F5)2}2C6F4][FeCp*2] (2) featuring
a hydroxide sequestered by the two adjacent boron atoms. Mechanistic
investigations of this formal 4-electron reduction of O2 suggest that it goes through the formation of a superoxide adduct
[1-O2]−, which evolves through
disproportionation into O2 and a peroxo-adduct [1-O2]2–. Upon coordination of another
equivalent of 1, the thus-generated 4-fold boron-sequestered
peroxide {[1]2-O2}2– undergoes homolytic O–O bond cleavage to yield a pair of
oxyl radicals [1-O•]−. These highly reactive intermediates subsequently perform hydrogen
atom abstraction to lead to the hydroxide salt 2. Our
observations point to FeCp*2 being the H atom purveyor
in this final step.