CO2 Activation with Bulky Neutral and Cationic Phenoxyalanes

The sterically crowded aluminum diphenolate (2,6-Mes2C6H3O)2AlEt (2; Mes =2,4,6-Me3C6H2−) was converted into the ionic species [(2,6-Mes2C6H3O)2Al]+[CHB11Cl11]− (6) by ethide abstraction with the silylium salt [Et3Si]­[CHB11Cl11] or by a combination of β-hydride abstraction and concomitant ethylene elimination with the trityl salt [Ph3C]­[CHB11Cl11]. Compound 6 consists of solvent-separated ions, and the cation features very short Al···C contacts involving the flanking mesityl groups, leading to an overall distorted-tetrahedral coordination geometry around the aluminum center. Analogous reactions with the diphenolate (2,6-tBu2-4-MeC6H2O)2AlEt (1) led to tert-butyl transfer to the benzene solvent to afford tert-butylbenzene as the main product. A 1:1 mixture of the triphenolate (2,6-Ph2C6H3O)3Al and tri-tert-butylphosphine forms a frustrated Lewis pair, which reacts with CO2 at room temperature to give the compound (2,6-Ph2C6H3O)3AlOC­(O)­Pt-Bu3 (8). The high Lewis acidity of 6 catalyzes the scrambling of the subsituents of Et3SiH at room temperature to give Et4Si, Et2SiH2, and EtSiH3 and also the reduction of CO2 to d5-toluene (C6D5CH3) and CH4 at 82 °C in d6-benzene solution.