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

Abstract

The dimeric cobalt complex Co 2 (CO) 6 [P( p -CF 3 C 6 H 4 ) 3 ] 2 ( 1 ) reacts reversibly with hydrogen to produce HCo(CO) 3 [P( p -CF 3 C 6 H 4 ) 3 ] ( 4 ). The carbonyl and the phosphine ligands of both 1 and 4 are very labile. Compound 1 reacts with CO to give Co 2 (CO) 7 [P( p -CF 3 C 6 H 4 ) 3 ] ( 2 ), and compound 4 reacts with CO and P( p -CF 3 C 6 H 4 ) 3 ( L ) to give HCo(CO) 4 ( 5 ) and HCo(CO) 2 [P( p -CF 3 C 6 H 4 ) 3 ] 2 ( 6 ), respectively. The 31 P NMR studies show that, in the presence of 1, the line width of the 31 P 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) 3 LCo • ( 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 Co 2 (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 31 P and 59 Co 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.