Investigation of rhodium catalyzed hydroformylation of ethylene in supercritical carbon dioxide by in situ FTIR spectroscopy

The reactions of RhH(CO)L3 (L=P(3,5-(CF3)2C6H3)3) with CO, H2, C2H4 and mixtures of these in supercritical carbon dioxide (scCO2) were investigated using high-pressure FTIR spectroscopy. The results were compared to the behavior of the conventional catalyst, RhH(CO)(PPh3)3, in organic solvents. RhH(CO)L3 does not dissociate in scCO2 and it is converted to RhH(CO)2L2 and to [Rh(CO)2L2]2 in the presence of CO and mainly to RhH(CO)L2 in the presence of an equimolar mixture of CO and H2. In the presence of CO and C2H4, the peaks observed in the acyl region and the terminal metal carbonyl region indicate the formation of three different acylrhodium complexes which are Rh(CO)L2(COEt), Rh(CO)2L2(COEt), and Rh(CO)3L(COEt). Similar species were also observed during the hydroformylation reaction. The first ever detection of the presence of Rh(CO)L2(COEt) under hydroformylation conditions provides direct evidence for the mechanism originally proposed by Wilkinson and co-workers. The carbonyl stretching frequencies of all of the rhodium–carbonyl species are shifted to higher wavenumbers due to a reduction of electron density at the metal center by the CF3 groups.