Mechanistic insights into CO2 cycloaddition of styrene oxide on paddle-wheel metal clusters: a theoretical study

Abstract

The CO₂ cycloaddition of styrene oxide (SO) catalyzed by different paddlewheel-type di-metal clusters has been investigated by means of density functional calculations. Without a co-catalyst, the reaction proceeds in a single step involving the breaking of a C–O bond of adsorbed epoxide and the simultaneous formation of two new C–O bonds between the adsorbed epoxide and a carbon dioxide. The reaction starts with the adsorption of a styrene oxide molecule on the catalytic site of an M–BTC catalyst via its oxygen atom with adsorption free energies of −12.3, −10.2, −8.0, −3.9, and −13.9 kcal mol⁻¹ for SO adsorption on Fe–, Co–, Ni–, Cu–, and Zn–BTC systems, respectively. From NBO analysis, it was found that the interaction between SO and M–BTC causes a higher polarization of C–O bonds of the epoxide. The more polarized C–O bond of the adsorbed SO allows the nucleophile to attack more easily. The calculated results unambiguously highlight the important role of unsaturated cationic sites in the paddlewheel unit in promoting the catalytic cycloaddition of CO₂ with SO. Based on the activation energy, the trend of catalytic activity is in the order Fe–BTC > Zn–BTC > Co–BTC > Ni–BTC > Cu–BTC. From DFT calculations, the Fe–BTC is theoretically proved to have the potential to be a desirable catalyst for the CO₂ cycloaddition of styrene oxide in the absence of a co-catalyst.