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

Abstract

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.