ACTIVATION OF CARBON-DIOXIDE .4. RHODIUM-CATALYZES HYDROGENATION OF CARBON-DIOXIDE TO FORMIC-ACID

Rhodium catalysts formed in situ from suitable precursors and the bidentate phosphane Ph2P-(CH2)4-PPh2 (1) catalyse the reversible formation of formic acid from carbon dioxide and dihydrogen very efficiently. Up to 2200 moles of formic acid may be formed per mole of rhodium with turnover frequencies as high as 375 h-1 using [(Rh(cod)(mu-H))4] (3) (cod = 1,5-cyclooctadiene) as precursor. In agreement with thermodynamical considerations, the highest yields of formic acid are achieved in polar aprotic solvents in the presence of secondary or tertiary amines. Experiments with various catalyst precursors together with kinetic measurements in a specially designed reactor suggest that neutral rhodium(I) hydride complexes are the actual active species in the catalytic cycle. A mechanism is proposed on the basis of these results that allows explanation of all experimental data including the induction period and the activation of the catalyst by treatment with H-2 or HCO2H prior to the reaction observed with the precursor [(Rb(cod)(mu-Cl)}2] (2).