CO₂ to Cyclic Carbonate: A Mechanistic Insight of a Benign Route Using Zinc(II) Salophen Complexes

Abstract

Zinc(II) Schiff base complexes with different substituents at 5, 5’ positions have been synthesized to study the effect of the electronic environment of the metal towards the cycloaddition reaction between CO2 and epoxide. The complexes have been characterized by FT‐IR, XPS, NMR, electronic spectroscopy, LC‐MS, and TGA analysis. We have used density functional theory to study the electronic structure of the Zn(II) complexes and modelled the electronic spectra and the mechanism of catalysis. Results from DFT and LC‐MS data show the existence of all the complexes as dimers except ZnL4. The monomeric ZnL4 has the strongest electron withdrawing group, i.e., ‐NO2 attached to it along two labile water molecules. The dimeric complexes exhibit good to moderate yield for cycloaddition reaction to styrene carbonate under solvent‐free conditions and a relatively low reaction temperature of 80 °C, with CO2 pressure of ~1 atm. The best yield has been achieved by ZnL4. Different rate‐determining steps are captured by the DFT studies for dimeric and monomeric complexes. Without taking any preventing measure of dimerization by attaching the bulky tBu groups, relatively lesser catalytic amounts of all the complexes have shown yields of cyclic carbonate between 53‐74%, depending on the nature of the substituent present.