Exploring the potential of group III salen complexes for the conversion of CO2 under ambient conditions

Abstract

Monometallic and bimetallic scandium and yttrium-salen complexes were synthesized and applied for the first time as homogeneous Lewis acids for the cycloaddition of CO2 to epoxides in the presence of quaternary ammonium salts. The developed rare-earth-based complexes showed the ability to catalyze the coupling of CO2 and industrially attractive epoxides such as epichlorohydrin and propylene oxide into cyclic carbonates under ambient conditions using low metal loading and leading to turnover numbers (TON) over 100. Additionally, functionalized cyclic carbonates with potential for application in polymer synthesis were obtained under mild conditions (T = 40−60 °C) under atmospheric CO2 pressure. DFT calculations were carried out to gain insight into the reaction mechanism justifying the observed difference of activity between the different complexes and indicating milder reaction barriers for the yttrium and scandium-based salen species compared to a homologous chromium complex. Moreover, advanced buried volume calculations were performed to assess the accessibility of the catalytic pockets in monometallic salen complexes versus bimetallic salen complexes.