Pincer–Ruthenium-Catalyzed Reforming of MethanolSelective High-Yield Production of Formic Acid and Hydrogen

A series of NNN pincer–ruthenium
complexes of the type (R2NNN)RuCl2(CH3CN) based on bis(imino)pyridine ligands were synthesized
and characterized. These
pincer ruthenium acetonitrile complexes, along with their phosphine
and carbonyl counterparts, were tested for the reforming of methanol
in water in the presence of a base. The catalyst (Cy2NNN)RuCl2(PPh3) was found to be the most efficient in comparison
to other considered catalysts. Among the bases screened, KOtBu (1.5 equiv with respect to water) was found to
give the best results at temperatures as low as 100 °C. Under
these conditions, while (Cy2NNN)RuCl2(PPh3) (0.2 mol %) in a mixture of methanol and water in a 2:1
ratio gave a yield of up to 81% each of hydrogen and formic acid (FA)
at 100% selectivity, the corresponding reaction with 2 mol % (Cy2NNN)RuCl2(PPh3) gave up to 90% of
hydrogen and 73% of FA at 80% selectivity. On the other hand, the
(Cy2NNN)RuCl2(PPh3) (0.8 mol %) catalyzed
reforming of a 3:1 methanol/water mixture gave good yields (84%) of
hydrogen with 81% FA at 95% selectivity. The yield of hydrogen was
cross-verified by using it to reduce unsaturated compounds and determining
the corresponding yield of the reduced product, which was found to
be consistent. Isotope-labeling studies suggest the involvement of
C–H activation as a part of the catalytic cycle and not as
a part of the rate-determining step (RDS) with an average secondary
KIE of 1.96. The reaction was observed to have a first-order dependence
of rate on the concentration of both (Cy2NNN)RuCl2(PPh3) and methanol. DFT studies are in agreement with
this, and the σ-bond metathesis leading to the elimination of
the first molecule of hydrogen is computed to be the RDS either for
the cycle leading to FA and 2 moles of hydrogen or for the cycle that
results in carbon dioxide and 3 moles of hydrogen. The Ru–H
species (Cy2NNN)RuCl(H) plays a decisive role in the unprecedented
selectivity toward FA. In its choice to undergo a σ-bond metathesis
either with the O–H of methanol (that completes the FA cycle)
or with the O–H of FA that leads to carbon dioxide, it chooses
the former as it is kinetically more favored by 4.58 kcal/mol. The
current catalytic system comprising of NNN pincer-ruthenium phosphines
based on bis(imino)pyridine ligands that gives high
yields of H2 and FA at unprecedented selectivity at low
operating temperature offers immense promise in the transformation
of methanol to clean-burning hydrogen and high-value FA.