Unexpected Acidity Enhancement Triggered by AlH₃ Association to Phosphines

The complexes formed by the interaction between a series of phosphines R–PH₂ (R = H, CH₃, c-C₃H₅, C₆H₅) and AlH₃ have been investigated through the use of high-level G4 ab initio calculations. These very stable complexes behave as much stronger acids than the isolated phosphines. This dramatic acidity enhancement, which can be as high as 174 kJ mol^–1, results from a much greater stabilization of the anionic deprotonated species with respect to the neutral one, upon AlH₃ association. This effect depends quantitatively on the nature of the substituent R and is smaller for R = C₆H₅ because of the conjugation of the P lone pair with the aromatic system. More unexpectedly, however, the phosphine–alane complexes, RPH₂:AlH₃, are more acidic than the corresponding phosphine–borane RPH₂:BH₃ analogues. This unexpected result is due to the enhanced stability of the anionic deprotonated species for complexes involving AlH₃, because the delocalization of the newly created P lone pair with the P–Al bonding density is more favorable when the Lewis acid is aluminum trihydride than when it is borane.