In Situ High-Pressure NMR Studies of Co2(CO)6[P(p-CF3C6H4)3]2 in Supercritical Carbon Dioxide: Ligand Substitution, Hydrogenation, and Hydroformylation Reactions

Abstract

The dimeric cobalt complex Co2(CO)6[P(p-CF3C6H4)3]2 (1) reacts reversibly with hydrogen to produce HCo(CO)3[P(p-CF3C6H4)3] (4). The carbonyl and the phosphine ligands of both 1 and 4 are very labile. Compound 1 reacts with CO to give Co2(CO)7[P(p-CF3C6H4)3] (2), and compound 4 reacts with CO and P(p-CF3C6H4)3 (L) to give HCo(CO)4 (5) and HCo(CO)2[P(p-CF3C6H4)3]2 (6), respectively. The 31P NMR studies show that, in the presence of 1, the line width of the 31P resonance of L is temperature dependent, and at constant temperature, its broadening is proportional to the square root of the concentration of 1. This broadening is attributed to its exchange reaction with the mononuclear cobalt radical (CO)3LCo• (3), which is generated by the homolysis of 1. Compound 1 catalyzes the hydroformylation of olefins in supercritical carbon dioxide. In contrast to the unsubstituted Co2(CO)8, the phosphine-modified catalyst system is stable under low CO pressures and the hydroformylation reactions can be carried out at low pressures. In situ monitoring of 31P and 59Co NMR spectra of the solution shows that the phosphine-containing hydrido cobalt complexes 4 and 6 are the only hydrido cobalt complexes present in detectable concentrations in 1-catalyzed hydroformylation reactions; nevertheless, the possibility that the observed activity for 1 comes primarily from the more active HCo(CO)4, present in concentrations below detectable limits, has not been rigorously excluded.