Stepwise Iodide-Free Methanol Carbonylation via Methyl Acetate Activation by Pincer Iridium Complexes

Iodide is an essential promoter in
the industrial production of
acetic acid via methanol carbonylation, but it also contributes to
reactor corrosion and catalyst deactivation. Here we report that iridium
pincer complexes mediate the individual steps of methanol carbonylation
to methyl acetate in the absence of methyl iodide or iodide salts.
Iodide-free methylation is achieved under mild conditions by an aminophenyl­phosphinite
pincer iridium­(I) dinitrogen complex through net C–O oxidative
addition of methyl acetate to produce an isolable methyliridium­(III)
acetate complex. Experimental and computational studies provide evidence
for methylation via initial C–H bond activation followed by
acetate migration, facilitated by amine hemilability. Subsequent CO
insertion and reductive elimination in methanol solution produced
methyl acetate and acetic acid. The net reaction is methanol carbonylation
to acetic acid using methyl acetate as a promoter alongside conversion
of an iridium dinitrogen complex to an iridium carbonyl complex. Kinetic
studies of migratory insertion and reductive elimination reveal essential
roles of the solvent methanol and distinct features of acetate and
iodide anions that are relevant to the design of future catalysts
for iodide-free carbonylation.