Fe(0)-Mediated Reductive Disproportionation of CO2

The zero-valent iron phosphine [Fe­(N2)­(dmpe)2] (2) reacts with CO2 to form the κ2C,O-FeC2O4 5-membered metalacycle cis-[Fe­(C­(═O)­OC­(═O)­O-κ2C,O)­(dmpe)2] (3) through the reductive coupling of two molecules of carbon dioxide. The formation of metalacycle 3 proceeds through a stepwise reaction with carbon dioxide to form, initially, the isolable η2(C,O)-CO2 complex cis-[Fe­(CO2-η2C,O)­(dmpe)2] (1), which reacts with a second molecule of CO2 to form the 5-membered metalacycle. This sequence represents the first experimental evidence for the formation of a κ2C,O-FeC2O4 metalacycle via an η2(C,O)-CO2 precursor. Isotopic labeling was used to demonstrate that the initial coordinated CO2 molecule forms the metal-bound carbonyl moiety in the metalacycle, while the second molecule of CO2 forms the carbonyl β to the iron center. The metalo-2,4-dioxolane-3,5-dione cis-[Fe­(C­(═O)­OC­(═O)­O-κ2C,O)­(dmpe)2] (3) breaks down with loss of CO to yield the iron carbonate cis-[Fe­(CO3-κ2O,O)­(dmpe)2] (5). In the presence of a CO scavenger such as [Fe­(N2)­(dmpe)2], CO is trapped to give [Fe­(CO)­(dmpe)2] (4). This is the first direct experimental evidence for a κ2C,O-C2O4 metalacycle being a key intermediate in the reductive disproportionation of CO2 to CO and CO32–.