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

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The preparation of the hydroxo-osmium(IV) complex OsH 3 (OH){xant(P i Pr 2 ) 2 } (xant(P i Pr 2 ) 2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) and its catalytic efficiency for the dehydrogenation of formic acid to H 2 and CO 2 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 OsH 3 {κ 1 -O-(HCO 2 )}{xant(P i Pr 2 ) 2 }, which isomerizes into OsH 3 {κ 1 -H-(HCO 2 )}{xant(P i Pr 2 ) 2 } by means of the slippage of the metal center through a formate O–C–H path. The κ 1 -H-formate intermediate releases CO 2 to give the previously reported tetrahydride OsH 4 {xant(P i Pr 2 ) 2 }, which undergoes protonation with formic acid. The resulting OsH 5 cation exists as an equilibrium mixture of the tautomers trihydride-compressed dihydride [OsH 3 (H···H){xant(P i Pr 2 ) 2 }] + and hydride-compressed dihydride–dihydrogen [OsH(H···H)(η 2 -H 2 ){xant(P i Pr 2 ) 2 }] + . The dissociation of H 2 from the latter leads to [OsH 3 {xant(P i Pr 2 ) 2 }] +, which coordinates HCO 2 – to regenerate the trihydride-(κ 1 -O-formate) complex and close the cycle. The release of CO 2 from the κ 1 -H-formate intermediate is the rate-determining step of the catalysis.