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

The reactions of RhH(CO)L 3 (L=P(3,5-(CF 3 ) 2 C 6 H 3 ) 3 ) with CO, H 2 , C 2 H 4 and mixtures of these in supercritical carbon dioxide (scCO 2 ) were investigated using high-pressure FTIR spectroscopy. The results were compared to the behavior of the conventional catalyst, RhH(CO)(PPh 3 ) 3 , in organic solvents. RhH(CO)L 3 does not dissociate in scCO 2 and it is converted to RhH(CO) 2 L 2 and to [Rh(CO) 2 L 2 ] 2 in the presence of CO and mainly to RhH(CO)L 2 in the presence of an equimolar mixture of CO and H 2 . In the presence of CO and C 2 H 4 , 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)L 2 (COEt), Rh(CO) 2 L 2 (COEt), and Rh(CO) 3 L(COEt). Similar species were also observed during the hydroformylation reaction. The first ever detection of the presence of Rh(CO)L 2 (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 CF 3 groups.