CO2 cycloaddition to epoxides promoted by bis-thioether-phenolate Fe(II) and Fe(III) complexes

The synthesis and characterization of three new bis-thioether-phenolate monoanionic tridentate [SOS]-type ligands, and the synthesis of the corresponding Fe(R) and Fe(III) complexes are reported. Their application, in combination with tetrabutylammonium bromide (TBAB), as catalysts for the cycloaddition of CO2 to a number of monosubstituted epoxides and cyclohexene oxide was investigated. For example, epichlorohydrin is converted in the corresponding cyclic carbonate, with a turnover frequency of 155 h(-1) (0.1 mol% of iron, 0.5 mol% of TBAB, 10 bar of CO2). With the aim to obtain further information about the catalyst structure-activity relationship and the reaction mechanism, the reaction between the catalyst precursors and TBAB was investigated, suggesting the formation of an anionic metallate species in the case of the Fe(III) complexes that is supposed to be true catalyst. A mechanistic scenario explaining the different behaviour of the Fe(II) and Fe(III) complexes is proposed.