Mono- versus Binuclear Zn-Carbamodithioate Complexes as Efficient Homogeneous Catalysts for the Cycloaddition of Epoxides with CO2

Abstract

(A)cyclic Zn²⁺ dithiocarbamate complexes containing imidazolium moieties, mono-metallic (Zn-dtc₂) and bi-metallic ((Znbdtc)₂), were synthesized and fully characterized using ¹H/¹³C NMR and ex situ ATR-FTIR spectroscopy. Both complexes served as efficient catalysts for CCs synthesis via CO₂ coupling with a variety of terminal and internal epoxides under mild reaction conditions (1 bar CO₂, 80 °C, 4 h and 1 mol% catalytic loading). Interestingly, both catalytic systems achieved TOF values up to 2000 h^-1 for 4-chloromethyl-2-oxo-1,3-dioxolane synthesis. Notably, Zn-dtc₂ displayed high activity toward the rigid and sterically hindered internal cyclohexene oxide with conversions up to 96% in DMSO. Furthermore, DFT calculations were employed to elucidate the structures of the Zn complexes and to validate the proposed cycloaddition mechanism, identifying the ring-closure step as the rate-determining stage.