Effect of Phosphines on the Thermodynamics of the Cobalt-Catalyzed Hydroformylation System

Thermodynamic parameters relevant to the phosphine-modified cobalt hydroformylation reaction are reported. Equilibrium constants for the hydrogenation of Co-2(CO)(6)L-2 to yield HCo(CO)(3)L were determined using in situ H-1 and P-31 NMR spectroscopy between 75 and 175 degrees C for various solvents and phosphine ligands. Special emphasis was placed on n-Bu3P, as this ligand is prototypical of the Shell hydroformylation process. The resultant van't Hoff plots yield the enthalpy and entropy change (Delta H = 7.0 +/- 0.4 kcal/mol and Delta S = 2 +/- 1 cal/mol center dot K) for the case of L = n-Bu3P in benzene solvent. These parameters were found to be relatively insensitive to changes in the solvent, suggesting that the hydride product is not very polar. Even for isobutyl alcohol solvent, the resultant enthalpy and entropy changes (Delta H = 5.8 +/- 0.4 kcal/mol and Delta S = -2 +/- 1 cal/mol center dot K) were found to be similar to those obtained in benzene and dioxane. Analysis of the P-31 NMR line widths allows rigorous lower limits to be established for the catalytically relevant Co-Co and Co-H bond energies in the case of L = n-Bu3P (Co-Co >= 23 kcal/mol and Co-H >= 60 kcal/mol) relative to the previously reported values for the case of L = CO (Co-Co = 19 +/- 2 kcal/mol and Co-H = 59 +/- 1 kcal/mol).