Cobalt complexes with α-amino acid ligands catalyze the incorporation of CO2 into cyclic carbonates

Arguably one of the largest research areas involving carbon dioxide (CO₂) fixation is the coupling of CO₂ to epoxides to form cyclic carbonates and polycarbonates. In this sense, there is an ever-increasing demand for the development of higher-performing catalytic systems that could counterbalance sustainability and energy efficiency in the production of cyclic carbonates. The use of abundant first-row transition metals combined with naturally occurring amino acids may be an ideal catalytic platform to fulfill this demand. Nevertheless, detailed information on the interactions between metal centers and natural products as catalysts in this transformation is lacking. Here a series of Co(III) amino acid catalysts operating in a binary system showed outstanding performance for the coupling reaction of epoxides and CO₂. Nine new complexes of the type trans(N)-[Co(aa)₂(bipy)]Cl (aa: ala, asp, lys, met, phe, pro, ser, tyr, and val) were used to explore the structure-activity relationship influenced by the complex outer coordination sphere, and its effect on the catalytic activity in the coupling reaction of CO₂ and epoxides.

[OSSO]-Type Chromium(III) Complexes for the Reaction of CO2 with Epoxides

In this work, four new mononuclear Cr(III) complexes (2-5) bearing bis-thioether-diphenolate, [OSSO]-type ligands, were synthesized and characterized. These complexes in combination with bis(triphenylphosphine)iminium chloride (PPNCl) promoted the coupling of CO2 with epoxides. Depending on the type of substrate and the conditions, the reaction results in the selective formation of either polycarbonate or cyclic carbonate. For example, the reactions in the presence of complex 2 led to the exclusive formation of poly(cyclohexene carbonate, PCHC) from cyclohexene oxide (CHO) (TOF up to 39 h-1 , at T=45-100 °C, time=24 h, pCO2 =20 bar, epoxide/2 (mol/mol)=1000, and PPNCl/2=0.5-2.0 mol %). Under the same conditions and PPNCl/2=0.5-5.0 mol %, the reactions of CO2 with styrene oxide (SO), epichlorohydrin (ECH), 1,2 epoxydodecane (EDD), and allyl glycidyl ether (AGE) have shown selective conversion to the corresponding cyclic carbonates (TOF up to 41 h-1 ).