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.