RHODIUM-CATALYZED HYDROGENATION OF CARBON-DIOXIDE TO FORMIC-ACID

The complex [Rh(NBD)(PMe2Ph)3]BF4 (2; NBD = norbornadiene) has been found to serve as a precatalyst for the hydrogenation of carbon dioxide to formic acid at moderate temperatures in THF solution, with turnover numbers of 10-60/day. Water accelerates formic acid production, whereas PMe2Ph is an inhibitor. Kinetic studies show that the rate of formic acid appearance is first order each in [2], p(H-2), and p(CO2) in the range 50-300 psi (following prehydrogenation). In situ high-pressure IR and NMR experiments reveal that the addition of H-2 to [Rh(NBD)(PMe2Ph)3]BF4 (2) produces rhodium dihydride complexes [H2Rh(PMe2Ph)3(S)]BF4 (4, 5; S = H2O, THF) and [H2Rh(PMe2Ph)4]BF4 (3). IR and NMR studies of the reaction of 3-5 with CO2 indicate that 3 is unreactive toward CO2 but that 4 and 5 insert CO2 to give species 6 and 7, formulated as formato complexes [HRh(S)(PMe2Ph)2(eta-2-OCHO)]BF4 and [HRh(S)1,2(PMe2Ph)3,2(eta-1-OCHO)]BF4, respectively; complexes 6 and 7 are also detected under catalytic conditions by IR spectroscopy. Aquo dihydride complex 4 has been found to insert CO2 more rapidly than the THF complex 5. [H2Rh(PMe2Ph)3(S)]BF4(4,5) also catalyze the decomposition of formic acid to CO2 and H-2. Combined kinetic and spectroscopic results suggest that reductive elimination of formic acid from the intermediate formato complexes is the rate-limiting step in the catalytic cycle.