Reduction of CO to Methanol with Recyclable Organic Hydrides

The reaction steps for the selective conversion of a
transition
metal carbonyl complex to a hydroxymethyl complex that releases methanol
upon irradiation with visible light have been successfully quantified
in acetonitrile solution with dihydrobenzimidazole organic hydride
reductants. Dihydrobenzimidazole reductants have been shown to be
inactive toward H2 generation in the presence of a wide
range of proton sources and have been regenerated electrochemically
or photochemically. Specifically, the reaction of cis-[Ru(bpy)2(CO)2]2+ (bpy = 2,2′-bipyridine)
with one equivalent of a dihydrobenzimidazole quantitatively yields
a formyl complex, cis-[Ru(bpy)2(CO)(CHO)]+, and the corresponding benzimidazolium on a seconds time
scale. Kinetic experiments revealed a first-order dependence on the
benzimidazole hydride concentration and an unusually large kinetic
isotope effect, inconsistent with direct hydride transfer and more
likely to occur by an electron transfer-proton-coupled electron transfer
(EΤ−PCET) or related mechanism. Further reduction/protonation
of cis-[Ru(bpy)2(CO)(CHO)]+ with two equivalents of the organic hydride yields the hydroxymethyl
complex cis-[Ru(bpy)2(CO)(CH2OH)]+. Visible light excitation of cis-[Ru(bpy)2(CO)(CH2OH)]+ in the presence
of excess organic hydride was shown to yield free methanol. Identification
and quantification of methanol as the sole CO reduction product was
confirmed by 1H NMR spectroscopy and gas chromatography.
The high selectivity and mild reaction conditions suggest a viable
approach for methanol production from CO, and from CO2 through
cascade catalysis, with renewable organic hydrides that bear similarities
to Nature’s NADPH/NADP+.