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, monometallic (Zn-dtc₂) and bimetallic ((Zn-bdtc)₂), were synthesized and characterized using ¹H/¹³C NMR and ex situ ATR-FTIR spectroscopy. Both complexes served as efficient catalysts for cyclic carbonate 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 turnover frequency values ranging from 1000 to 2000 h⁻¹ for the synthesis of 4-chloromethyl-2-oxo-1,3-dioxolane. Notably, Zn-dtc₂ displayed high activity toward the rigid and sterically hindered internal cyclohexene oxide with conversions up to 96% in DMSO. Furthermore, density functional theory 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.