Stoichiometric Reduction of CO2 to CO by Phosphine/AlX3‑Based Frustrated Lewis Pairs

The reactions of the bulky phosphines Mes3P or (otol)3P with AlX3 (X = I, Br, Cl) and CO2 are probed and shown to give complexes of the form Mes3PC­(OAlX3)2 (X = I (3), Br (4), Cl (5)) and (otol)3PC­(OAlI3)2 (6). The former compounds under CO2 are further transformed to Mes3PC­(OAlX2)2OAlX3 (X = I (8), Br (10)) and [Mes3PX]­[AlX4] (X = I (9), Br (11)). These latter reactions are thought to proceed via dissociation of alane, as evidenced by the generation of (otol)3PC­(OAl­(C6F5)3) from (otol)3PC­(OAl­(C6F5)3)2 (7) and the isolation of (otol)3PC­(O)­OAl­(OC­(CF3)3)3 (15). Subsequent insertion of CO2 into the Al–X bond is evidenced by the characterization of [(C6F5)­C­(O)­OAl­(C6F5)2]2 (14) from the reaction of Al­(C6F5)3 and CO2. The isolation of Al­(C6F5)3·2­(C6H5Br) (16) also suggests dissociation of Al­(C6F5)3 from 7 may be facilitated by interactions with solvent. Kinetics of the formation of 8/9 from 3 show the reaction is first order in 3 and CO2 and the rate-determining step involves an associative process. A mechanism involving dissociation of alane and subsequent insertion of CO2 into the Al–X bond is proposed.