Sb(V) in strong Brønsted acid
solvents is traditionally assumed
to react with light alkanes through superacid protonolysis, which
results in carbocation intermediates, H2, and carbon oligomerization.
In contrast to this general assumption, our density functional theory
(DFT) calculations revealed an accessible barrier for C–H activation
between methane and Sb(V) in sulfuric acid that could potentially
outcompete superacid protonolysis. This prompted us to experimentally
examine this reaction in sulfuric acid with oleum, which has never
been reported because of presumed superacid reactivity. Reaction of
methane at 180 °C for 3 h resulted in very high yields of methyl
bisulfate without significant overoxidation. Our DFT calculations
show that a C–H activation and Sb-Me bond functionalization
mechanism to give methyl bisulfate outcompetes methane protonolysis
and many other possible reaction mechanisms, such as electron transfer,
proton-coupled electron transfer, and hydride abstraction. Our DFT
calculations also explain experimental hydrogen–deuterium exchange
studies and the absence of methane carbo-functionalization/oligomerization
products. Overall, this work demonstrates that in very strong Brønsted
acid solvent, Sb(V) can induce innersphere reaction mechanisms akin
to transition metals and outcompete superacid reactivity.