Catalytic Dehydrogenation of Alkanes by PCP–Pincer Iridium Complexes Using Proton and Electron Acceptors

Dehydrogenation
to give olefins offers the most broadly applicable
route to the chemical transformation of alkanes. Transition-metal-based
catalysts can selectively dehydrogenate alkanes using either olefinic
sacrificial acceptors or a purge mechanism to remove H2; both of these approaches have significant practical limitations.
Here, we report the use of pincer-ligated iridium complexes to achieve
alkane dehydrogenation by proton-coupled electron transfer, using
pairs of oxidants and bases as proton and electron acceptors. Up to
97% yield was achieved with respect to oxidant and base, and up to
15 catalytic turnovers with respect to iridium, using t-butoxide as base coupled with various oxidants, including oxidants
with very low reduction potentials. Mechanistic studies indicate that
(pincer)­IrH2 complexes react with oxidants and base to
give the corresponding cationic (pincer)­IrH+ complex, which
is subsequently deprotonated by a second equivalent of base; this
affords (pincer)Ir which is known to dehydrogenate alkanes and thereby
regenerates (pincer)­IrH2.