Mechanistic Insights into Hydroformylation Catalyzed by Cationic Cobalt(II) Complexes: In Silico Modification of the Catalyst System

The hydroformylation reaction is used on a large industrial scale to convert olefins and synthesis gas (CO + H-2) into aldehydes. Researchers have recently discovered that a class of cationic Co(II) complexes of the formula [Co-II(PP)(acac)](+) (PP = diphosphine, acac = acetylacetonate) can catalyze hydroformylation with activity approaching that of the widely used rhodium catalysts (Hood, D. M. et al. Science 2020, 367, 542-548). This density functional theory (DFT) study reveals the detailed workings of the cationic Co(II) catalyst system. The precatalyst [Co-II(PP)(acac)](+) is initiated by reacting with H-2 and CO to generate active species [HCoII(CO)(2)(PP)](+). In comparison with the 18-electron neutral Co(I) catalytic species HCoI(CO)(3)(PR3), these cationic Co(II) species, with their unique 17-electron and square pyramidal structure, invoke a lower-energy pathway through different elementary steps such as associative alkene uptake and heterolytic H-2 cleavage. The regioselectivity for linear aldehyde products is due to a combination of electronic and steric effects that favor the anti-Markovnikov insertion of a terminal alkene into the Co-H bond. DFT calculations predict that addition of PMe3 would facilitate the precatalyst initiation, thereby decreasing the reaction temperature or shortening the induction period. The insights gained by this theoretical study can be useful for the further development of hydroformylation catalysts.