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

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.