Cobalt-Catalyzed\nAdditive-Free Dehydrogenation of\nNeat Formic Acid

Dehydrogenation of formic acid without\nusing additives\nand solvents\nis a challenging research problem in base metal catalysis. In this\nstudy, cobalt complexes of the type (<sup><i>i</i>Pr</sup>PP<sup>R</sup>P)CoH(PMe<sub>3</sub>) (<sup><i>i</i>Pr</sup>PP<sup>R</sup>P = (<i>o</i>-<sup><i>i</i></sup>Pr<sub>2</sub>PC<sub>6</sub>H<sub>4</sub>)<sub>2</sub>PR; R = H or\nMe) are shown to catalyze the additive-free dehydrogenation of neat\nformic acid to carbon dioxide. The <sup><i>i</i>Pr</sup>PP<sup>Me</sup>P-ligated cobalt hydride is particularly effective,\ngiving catalytic turnover numbers of up to 7122 with a single load\nof formic acid and 10,338 with a continuous addition of formic acid.\nMechanistic investigation focusing on (<sup><i>i</i>Pr</sup>PP<sup>Me</sup>P)CoH(PMe<sub>3</sub>) reveals that the hydride complex\nis initially converted to [(<sup><i>i</i>Pr</sup>PP<sup>Me</sup>P)CoH<sub>2</sub>(PMe<sub>3</sub>)]<sup>+</sup> and then\nto “(<sup><i>i</i>Pr</sup>PP<sup>Me</sup>P)Co(OCHO)”\nas the key intermediates for releasing H<sub>2</sub> and CO<sub>2</sub>, respectively. As the catalytic reaction proceeds, decarbonylation\nof formic acid produces CO, which transforms the intermediates to\n[(<sup><i>i</i>Pr</sup>PP<sup>Me</sup>P)Co(CO)(PMe<sub>3</sub>)]<sup>+</sup> and (<sup><i>i</i>Pr</sup>PP<sup>Me</sup>P)Co(CO)H as the less active forms of the catalyst. Further degradation\nto [(<sup><i>i</i>Pr</sup>PP<sup>Me</sup>P)Co(CO)<sub>2</sub>]<sup>+</sup>, protonated phosphine ligands, and cobalt formate ends\nthe catalyst’s life. Contrary to many other catalytic systems,\nthe cobalt catalysts described here are more active in neat formic\nacid than in formic acid solutions, which can be attributed to the\nremoval of PMe<sub>3</sub> from the coordination sphere (via phosphine\nprotonation) to generate a more reactive intermediate.