Manganese(III) complexes with tetradentate O^C^C^O ligands: Synthesis, characterization and catalytic studies on the CO2 cycloaddition with epoxides

A novel class of manganese(III) complexes bearing bis(NHC)-bis(phenolate) (O^C^C^O) type ligands was successfully synthesized. Three differently substituted imidazolium salts (with 2-hydroxyphenyl, (H4L1)Br2, 5‑tert‑butyl‑2-hydroxyphenyl, (H4L2)Br2, and 3,5-di‑tert‑butyl‑2-hydroxyphenyl, (H4L3)Br2, groups) were prepared as precursors of the (O^C^C^O) ligands and a convenient high-yield complexation reaction using manganese(III) acetate was developed. Electrospray ionization mass spectrometry (ESI-MS) and single-crystal X-ray diffraction (SC-XRD) data confirm the formation of the complexes of general formula [MnBrL1–3] and clarify their coordination geometry. The complexes were studied as homogeneous catalysts in the cycloaddition of CO2 to benzyl glycidyl ether (BGE) to form the corresponding cyclic carbonate, using tetrabutylammonium bromide (TBAB) or bis(triphenylphosphine)iminium bromide (PPNBr) as co-catalysts. The complex [MnBrL3] shows the highest activity, and kinetic investigations revealed a pseudo-first order dependence with respect to BGE under neat conditions. The temperature effect was also investigated using the Eyring and Arrhenius equations and the activation parameters for the neat reaction using [MnBrL3] and TBAB were experimentally determined (ΔH‡ = 11.2 kcal·mol-1 and ΔS‡ = -50 cal·mol-1·K-1). On the basis of the performed mechanistic studies and DFT investigations, a catalytic cycle which involves the CO2 1,2-insertion as the rate determining step is proposed.