On the Lewis Basicity of Phosphoramides: A Critical Examination of Their Donor Number through Comparison of Enthalpies of Adduct Formation with SbCl5 and BF3

The Lewis basicity of a series of phosphoryl compounds was examined using DFT and ab initio methods, including solvation effects. The enthalpies of adduct formation with two archetypal Lewis acids, antimony pentachloride and boron trifluoride, used to define the donor number DN and the BF3 affinity (BF(3)A) respectively, were examined. The BF3 adducts allow the use of the high-accuracy G4 approach, whereas for SbCl5 adducts, three different DFT formalisms, including empirical dispersion corrections, were used because the G4 formalism is not available for third-row elements. For a comparison with experimental data, solvation effects were taken into account by using the polarizable continuum model. The experimental BF3 affinities were well reproduced by G4 calculations when including PCM solvation. Conversely, comparisons of our calculated values and experimental results reported in the literature show that SbCl5 enthalpies for phosphoramides are in error. In particular the DN for HMPA should be revised.