Cobalt-Catalyzed Additive-Free Dehydrogenation of Neat Formic Acid

Abstract

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 ( iPr PP R P)CoH(PMe 3 ) ( iPr PP R P = (o-i Pr 2 PC 6 H 4 ) 2 PR; R = H or Me) are shown to catalyze the additive-free dehydrogenation of neat formic acid to carbon dioxide. The iPr PP Me P-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 ( iPr PP Me P)CoH(PMe 3 ) reveals that the hydride complex is initially converted to [( iPr PP Me P)CoH 2 (PMe 3 )] + and then to "( iPr PP Me P)Co-(OCHO)" as the key intermediates for releasing H 2 and CO 2 , respectively. As the catalytic reaction proceeds, decarbonylation of formic acid produces CO, which transforms the intermediates to [( iPr PP Me P)Co(CO)(PMe 3 )] + and ( iPr PP Me P)Co-(CO)H as the less active forms of the catalyst. Further degradation to [( iPr PP Me P)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 PMe 3 from the coordination sphere (via phosphine protonation) to generate a more reactive intermediate.