A New Application Area for Ag-NHCs: CO2 Fixation Catalyst

Abstract

The first example of the silver-NHC catalyzed cycloaddition of CO2 to terminal epoxides is reported. The choice of NHC substituents, halides and the nature of additives were determined to be important. Ionic [(Cnmim)2Ag]2[Ag2Br4] (Cnmim=1-(CnH2n+1)-3-methylimidazol-2-ylidene; n=418) catalysts exhibited a high yield of styrene carbonate (7792?%) in the presence of 4-dimethylaminopyridine (DMAP) as a cocatalyst without needing any solvent. The catalytic activity improved as the length of the alkyl groups (n) increased, although substituents longer than C14 did not produce any further beneficial effect. For the protocol performed at 1.5 MPa pressure and 100 degrees C the TOF was calculated to be 440 h-1. A prolonged reaction time and increased pressure of CO2 increased the yield. Used without any additive, compound C14 was able to fix CO2 with PO (propylene oxide) quantitatively at 120 degrees C, 2.0 MPa and could be used at least seven times without loss of activity and selectivity. The nucleophilic nature of the anion is decisive for the formation of the product.