Energetics of intermediates and reaction steps involved in the hydroformylation reaction catalyzed by HCo(CO)4. A theoretical study based on density functional theory

Abstract

A theoretical study based on density functional theory has been carried out on the CO dissociation of HCo(CO)/sub 4/ and the migratory insertion reaction RCo(CO)/sub 4/ /yields/ RC(O)Co(CO)/sub 3/ for R = H and R = CH/sub 3/, respectively. RCo(CO)/sub 4/ was shown to form two stable isomers of trigonal-bipyramidal geometry. The configuration with R in axial position was lower in energy (63 kJ mol/sup /minus/1/ for R = H and 42 kJ mol/sup /minus/1/ for R = CH/sub 3/) than the corresponding isomer with R along the basal axis. The Co-CO dissociation energy of HCo(CO)/sub 4/ was found to be 186 kJ mol/sup /minus/1/. The resulting intermediate HCo(CO)/sub 3/ had as its most stable conformation a butterfly structure with the hydride in apical position. The migratory insertion of CO into the Co-CH/sub 3/ bond affords a coordinatively unsaturated acyl intermediate CH/sub 3/C(O)Co(CO)/sub 3/ that was found to form several stable isomers, with the ones that have the acyl oxygen facing the vacant site lowest in energy due to the formation of stable /eta//sup 2/ interactions. The reaction profile of the migration process was investigated by a linear transit procedure. The methyl 1,2-shift reaction to a cis-carbonyl group was foundmore » to have a low activation barrier and a reaction enthalpy of not more than 70 kJ mol/sup /minus/1/, whereas the direct insertion of a CO ligand into the Co-CH/sub 3/ bond was disfavored by an activation barrier of /approx/ 200 kJ mol/sup /minus/1/. The corresponding hydride migration to a neighboring CO group was considerably more endothermic and did not produce stable formyl intermediates on the Hartree-Fock-Slater energy surface. The alternative isomers with the formyl oxygen pointing toward the empty site were found to be stable but high in energy.« less