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

The ambiphilic species Al(C 6 H 4 ( o -PPh 2 )) 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 CO 2 to generate a CO 2 adduct commonly observed in the activation of CO 2 using frustrated Lewis pairs (FLPs). This ambiphilic species serves as a precatalyst for the reduction of CO 2 in the presence of catecholborane (HBcat) to generate CH 3 OBcat, which can be readily hydrolyzed in methanol. The reaction mixture confirms that, in the presence of HBcat, 2 generates the known CO 2 reduction catalyst 1-Bcat-2-PPh 2 -C 6 H 4 ( 1 ) and intractable catecholate aluminum species. It was, however, possible to isolate a single crystal of Al(κ 2 O, O -(MeO) 2 Bcat) 3 ( 5 ) supporting this hypothesis. Also, a borane-protected analogue of 2, Al(C 6 H 4 ( o -PPh 2 ·BH 3 )) 3 ( 4 ), does not react with catecholborane, suggesting the influence of the pendant phosphines in the transformation of 2 into 1 .