Cobalt-Catalyzed Additive-Free Dehydrogenation of Neat Formic Acid

Dehydrogenation of formic acid without
using additives
and solvents
is a challenging research problem in base metal catalysis. In this
study, cobalt complexes of the type (iPrPPRP)CoH(PMe3) (iPrPPRP = (o-iPr2PC6H4)2PR; R = H or
Me) are shown to catalyze the additive-free dehydrogenation of neat
formic acid to carbon dioxide. The iPrPPMeP-ligated cobalt hydride is particularly effective,
giving catalytic turnover numbers of up to 7122 with a single load
of formic acid and 10,338 with a continuous addition of formic acid.
Mechanistic investigation focusing on (iPrPPMeP)CoH(PMe3) reveals that the hydride complex
is initially converted to [(iPrPPMeP)CoH2(PMe3)]+ and then
to “(iPrPPMeP)Co(OCHO)”
as the key intermediates for releasing H2 and CO2, respectively. As the catalytic reaction proceeds, decarbonylation
of formic acid produces CO, which transforms the intermediates to
[(iPrPPMeP)Co(CO)(PMe3)]+ and (iPrPPMeP)Co(CO)H as the less active forms of the catalyst. Further degradation
to [(iPrPPMeP)Co(CO)2]+, protonated phosphine ligands, and cobalt formate ends
the catalyst’s life. Contrary to many other catalytic systems,
the cobalt catalysts described here are more active in neat formic
acid than in formic acid solutions, which can be attributed to the
removal of PMe3 from the coordination sphere (via phosphine
protonation) to generate a more reactive intermediate.