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.