Unveiling the hydroesterification of 1,3-butadiene with carbonyl cobalt ionic liquids: Facilitation of cationic protonic hydrogen for active species formation

[Display omitted] •Hydroesterification of 1,3-butadiene under lower pressure catalyzed by carbonyl cobalt ionic liquid.•Formation of the active species HCo(CO)4 through proton exchange between anions and cations in ionic liquids.•Qualitative kinetics for hydroesterification process was established. The 1,3-butadiene hydroesterification products, such as methyl pentenoate and dimethyl adipate, play a significant role in the production of nylon, plasticizers, and pharmaceutical intermediates. However, due to its unique π-π conjugated system, slow reaction rate, and difficulties in controlling regioselectivity, butadiene presents itself as an incredibly challenging substrate. The development of efficient and low-cost catalysts has gained substantial attention in both theoretical studies and industrial applications due to the high cost or extremely high pressure requirements. In this study, carbonyl cobalt ionic liquids were innovatively employed as catalysts to facilitate the synthesis of monoesters from hydroesterification of butadiene under mild conditions. A series of carbonyl cobalt ionic liquids with different cations were prepared and characterized for their structural properties using IR, ESI, IC, and DFT calculations. The yield of methyl pentenoate was comparable to that of Co2CO8 at 400 bar, and the catalysts exhibited high selectivity to methyl 3-pentenoates (>95 %), establishing a stable, efficient, and low-cost catalytic system. The experiments revealed that the type of the cation in ionic liquids remarkably influence their catalytic performance. Electrostatic potential calculations further confirmed that this performance is tightly related to the dissociation and migration behaviors of protons, leading to the proposal of a screening method for ionic liquids. In this paper, we further calculated the qualitative reaction kinetics and proposed a reaction mechanism in conjunction with previous studies. The reaction is hypothesized to begin with the formation of the active species HCo(CO)4 by proton exchange of [HX]+[Co(CO)4]−, in line with our experimental findings and theoretical calculations. This study presents an innovative approach to overcome the challenges associated with butadiene hydroesterification and paves the way for the development of more efficient and cost-effective catalytic systems.