A Tris(triphenylphosphine)aluminum Ambiphilic Precatalyst for the Reduction of Carbon Dioxide with Catecholborane

The ambiphilic species Al­(C<sub>6</sub>H<sub>4</sub>(<i>o</i>-PPh<sub>2</sub>))<sub>3</sub> (<b>2</b>) was synthesized and fully characterized, notably using X-ray diffraction. Species <b>2</b> exhibits pseudo-bipyramidal-trigonal geometry caused by the two Al–P interactions. <b>2</b> reacts with CO<sub>2</sub> to generate a CO<sub>2</sub> adduct commonly observed in the activation of CO<sub>2</sub> using frustrated Lewis pairs (FLPs). This ambiphilic species serves as a precatalyst for the reduction of CO<sub>2</sub> in the presence of catecholborane (HBcat) to generate CH<sub>3</sub>OBcat, which can be readily hydrolyzed in methanol. The reaction mixture confirms that, in the presence of HBcat, <b>2</b> generates the known CO<sub>2</sub> reduction catalyst 1-Bcat-2-PPh<sub>2</sub>-C<sub>6</sub>H<sub>4</sub> (<b>1</b>) and intractable catecholate aluminum species. It was, however, possible to isolate a single crystal of Al­(κ<sup>2</sup><i>O</i>,<i>O</i>-(MeO)<sub>2</sub>Bcat)<sub>3</sub> (<b>5</b>) supporting this hypothesis. Also, a borane-protected analogue of <b>2</b>, Al­(C<sub>6</sub>H<sub>4</sub>(<i>o</i>-PPh<sub>2</sub>·BH<sub>3</sub>))<sub>3</sub> (<b>4</b>), does not react with catecholborane, suggesting the influence of the pendant phosphines in the transformation of <b>2</b> into <b>1</b>.