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

The ambiphilic species Al­(C₆H₄(o-PPh₂))₃ (2) was synthesized and fully characterized, notably using X-ray diffraction. Species 2 exhibits pseudo-bipyramidal-trigonal geometry caused by the two Al–P interactions. 2 reacts with CO₂ to generate a CO₂ adduct commonly observed in the activation of CO₂ using frustrated Lewis pairs (FLPs). This ambiphilic species serves as a precatalyst for the reduction of CO₂ in the presence of catecholborane (HBcat) to generate CH₃OBcat, which can be readily hydrolyzed in methanol. The reaction mixture confirms that, in the presence of HBcat, 2 generates the known CO₂ reduction catalyst 1-Bcat-2-PPh₂-C₆H₄ (1) and intractable catecholate aluminum species. It was, however, possible to isolate a single crystal of Al­(κ²O,O-(MeO)₂Bcat)₃ (5) supporting this hypothesis. Also, a borane-protected analogue of 2, Al­(C₆H₄(o-PPh₂·BH₃))₃ (4), does not react with catecholborane, suggesting the influence of the pendant phosphines in the transformation of 2 into 1.