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

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