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 ((PP)-P-iPr (R) P)CoH(PMe3) ((PP)-P-iPr (R) P = (o-(Pr2PC6H4)-Pr-i)(2)PR; R = H or Me) are shown to catalyze the additive-free dehydrogenation of neat formic acid to carbon dioxide. The (PPP)-P-iPr-P-Me-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 ((PPP)-P-iPr-P-Me)CoH(PMe3) reveals that the hydride complex is initially converted to [((PPP)-P-iPr-P-Me)CoH2(PMe3)](+) and then to "((PPP)-P-iPr-P-Me)Co(OCHO)" as the key intermediates for releasing H-2 and CO2, respectively. As the catalytic reaction proceeds, decarbonylation of formic acid produces CO, which transforms the intermediates to [((PPP)-P-iPr-P-Me)Co(CO)(PMe3)](+) and ((PPP)-P-iPr-P-Me)Co(CO)H as the less active forms of the catalyst. Further degradation to [((PPP)-P-iPr-P-Me)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.