Hydroformylation of olefins under mild conditions Part I : the Co4-nRhn(CO)12+xL (n = 0, 2, 4; x = 0-9) system and preformed Rh4(CO)12-xLx clusters (x = 1-4).

The hydroformylation of cyclohexene, 1-pentene and styrene under mild conditions (25-50 °C, 1 atm equimolar mixture of CO and H2) has been investigated using as catalyst precursor either the Co4-nRhn(CO)12 + x L (n = 0, 2, 4; x = 0 - 9) system or preformed Rh4CO)12-xLx (x = 1 - 4) substituted clusters, where L is a trisubstituted phosphine or phosphite. The activity of these systems increases as a function of x, and reaches a maximum for a L/Co4-nRhn(CO)12 (n = 2, 4) molar ratio of ca. 5 - 6. A further increase in this ratio corresponds to a smooth decrease in the activity. This ratio has apparently a negligible effect on the regioselectivity in the hydroformylation of both 1-pentene and styrene. In contrast, both the activity and the regioselectivity are significantly affected by the nature of the ligand employed as cocatalyst. When working with Rh4(CO)12 as well as Rh6(CO)16, and trisubstituted phosphites as ligands, infrared spectroscopy and 31P NMR invariably show the presence of Rh4(CO)9L3 as the most substituted rhodium carbonyl species present in solution, and there is no evidence of fragmentation of the tetranuclear cluster during the catalytic process. In contrast, when using phosphine ligands such as PPh3, evidence of fragmentation to Rh2(CO)6(PPh3)2 or to Rh2(CO)4(PPh3)4 species has been obtained at the higher PPh3/Rh4(CO)12 molar ratios. Degradation of the ligand employed as cocatalyst, particularly the arylsubstituted phosphines, is observed, and this is probably at the origin of the loss of catalytic activity of some of these systems with time.