Zwitterionic bifunctional stable zinc complexes as single catalysts for the valorization of CO2 into cyclic carbonates

Bifunctional pyrrole containing nitrogen Schiff base zinc complexes were developed as single catalysts for the solvent free CO2 cycloaddition into epoxides. Four zwitterionic complexes (Zn-N-SB+/X-) were prepared by quaternarization of their neutral zinc precursor (Zn-N-SB) with different alkyl halides. The experimental compound characterization (NMR, IR, HRMS) was completed using the density functional theory (DFT) to obtain optimized structures. All the bifunctional zinc complexes efficiently promoted the solvent free cycloaddition of CO2 into styrene oxide, acting as both, Lewis acid and Lewis base catalyst, to form styrene cyclic carbonate in very good yields (up to 90 %) reaching TOF values up to 257 h-1. Best catalytic activity was noted for the iodine containing zwitterionic species (Zn-N-SB+/I-) that was used in very small loading (0.2 mol%) to enlarge the reaction scope to other epoxides in solvent free conditions, allowing high cyclic carbonate yields in the case of terminal epoxides. The comparison of the catalytic system formed by the neutral zinc complex (Zn-N-SB) combined to an iodine ammonium salt and its derived zwitterionic iodine containing complex (Zn-N-SB+/I-), showed that the bifunctional catalyst is more active. In order to get mechanistic insights, for both catalytic systems, the reaction order was calculated by VTNA (variable time normalization analysis) and the epoxide opening regioselectivity was determined using aniline as nucleophile.