Dehydrogenation of Formic Acid Catalyzed by an Osmium-Polyhydride: Relevance of Acid Assistance in the CO2 Formation Stage

Complex OsH₄{κ¹-P,η²-GeH-[ⁱPr₂PCH(Me)CH₂GeEt₂H]}(PⁱPr₃) (1) breaks down formic acid into H₂ and CO₂. The decomposition is catalytic with complex 1 being the main metallic species detected spectroscopically during the process. The kinetic analysis of the catalysis reveals that the decomposition rate is first order in the catalyst and independent of the concentration of formic acid, with the calculated activation parameters being: ΔH^⧧ = 23 ± 2 kcal mol^–1, ΔS^⧧ = −1 ± 5 cal mol^–1 K^–1, and ²⁹⁸ΔG^⧧ = 23 ± 3 kcal mol^–1. Complex 1 also shows stoichiometric reactivity with benzoic and acetic acids. The reactions lead to OsH₂{κ²-O,O-[O₂CR]}{κ²-P,Ge-[ⁱPr₂PCH(Me)CH₂GeEt₂]}(PⁱPr₃) (R = Ph (9), Me (10)). On the basis of these findings and DFT calculations, the following mechanism for the decomposition is proposed: complex 1 releases one molecule of H₂ to produce an osmium(IV)-trihydride unsaturated intermediate, which promotes heterolytic activation of the O–H bond of formic acid. The metal fragment of the resulting osmium(IV)-(κ¹-O-formate)-saturated derivative slides along the formate group, following the O–C–H pathway. The displacement is assisted externally by a molecule of formic acid and generates an osmium(IV)-(κ¹-H-formate) species, which releases CO₂ to regenerate 1 and close a cycle. The dissociation of H₂ from the latter is the rate-determining step of catalysis.