Dehydrogenation of Formic Acid Promoted by a Trihydride-Hydroxo-Osmium(IV) Complex: Kinetics and Mechanism

Article abstract of DOI:10.1021/acscatal.8b02370

The preparation of the hydroxo-osmium(IV) complex OsH₃(OH){xant(PⁱPr₂)₂} (xant(PⁱPr₂)₂ = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) and its catalytic efficiency for the dehydrogenation of formic acid to H₂ and CO₂ are reported. The mechanism of the dehydrogenation has been unambiguously stablished by combining the kinetic analysis of the catalysis, the isolation of the intermediates and the kinetic analysis of their decomposition, and density functional theory (DFT) calculations on the rate-determining step. Under catalytic conditions, the trihydride-hydroxo complex reacts with formic acid to afford OsH₃{κ¹-O-(HCO₂)}{xant(PⁱPr₂)₂}, which isomerizes into OsH₃{κ¹-H-(HCO₂)}{xant(PⁱPr₂)₂} by means of the slippage of the metal center through a formate O-C-H path. The κ¹-H-formate intermediate releases CO₂ to give the previously reported tetrahydride OsH₄{xant(PⁱPr₂)₂}, which undergoes protonation with formic acid. The resulting OsH₅ cation exists as an equilibrium mixture of the tautomers trihydride-compressed dihydride [OsH₃(H···H){xant(PⁱPr₂)₂}]⁺ and hydride-compressed dihydride-dihydrogen [OsH(H···H)(η²-H₂){xant(PⁱPr₂)₂}]⁺. The dissociation of H₂ from the latter leads to [OsH₃{xant(PⁱPr₂)₂}]⁺, which coordinates HCO₂^- to regenerate the trihydride-(κ¹-O-formate) complex and close the cycle. The release of CO₂ from the κ¹-H-formate intermediate is the rate-determining step of the catalysis.