Robust pyrrole-Schiff base Zinc complexes as novel catalysts for the selective cycloaddition of CO2 to epoxides

A series of Zn (II) complexes with nitrogen Schiff base ligands (NSB) has been synthesized from pyrrole carboxaldehyde. The prepared complexes were fully characterized. Single crystal X-ray diffraction analysis showed various coordination modes, as monomers or as helical dimers according to the number of available nitrogen atoms surrounding the zinc cation. The complex structure optimizations were carried out using the density functional theory (DFT) with ADF program and the dimerization energies and the temperature effect were examined. The Zn complexes were used as Lewis acids (LA) combined with tetrabutylammonium halides (N4BuX) as Lewis bases (LB), for the catalytic valorization of CO2 into cyclic carbonates. A new momomeric Zn-NSB was chosen to study the influence of different reaction parameters (temperature, CO2 pressure, LA/LB molar ratio, catalyst amount). Thus, optimized conditions afforded the solvent-free selective formation of styrene cyclic carbonate in good yields (up to 98 %) and allowed the reuse of this Zn-NSB/N4BuX catalytic system without any loss of activity.