MECHANISTIC ASPECTS OF THE RHODIUM-CATALYZED HYDROGENATION OF CO2 TO FORMIC-ACID - A THEORETICAL AND KINETIC-STUDY

The mechanism of the rhodium-catalyzed hydrogenation of CO2 to formic acid was investigated by initial rate measurements using the complex [ (dppp)Rh(hfacac)] (A) (dppp = Ph2P(CH2)(3)PPh2, hfacac = hexafluoroace tyl-acetonate) as a catalyst precursor in DMSO/NEt3 and by ab initio c alculations using cis-[(H3P)(2)Rh] as a model fragment for the catalyt ically active site. The kinetic data are consistent with a mechanism t hat involves rate limiting product formation by liberation of formic a cid from an intermediate that is formed via two reversible reactions o f the actual catalytically active species first with CO2 and then with H-2. The calculations provide for the first time a theoretical analys is of the full catalytic cycle of CO2 hydrogenation. They give detaile d insight into the structure of possible intermediates and their trans formations during the individual steps. The results suggest sigma-bond metathesis as an alternative low energy pathway to a classical oxidat ive addition/reductive elimination sequence for the reaction of the fo rmate intermediate with dihydrogen.