What Distinguishes the Strength and the Effect of a Lewis Acid: Analysis of the Gutmann–Beckett Method

Abstract

IUPAC defines Lewis acidity as the thermodynamic tendency for Lewis pair formation. This strength property was recently specified as global Lewis acidity (gLA), and is gauged for example by the fluoride ion affinity. Experimentally, Lewis acidity is usually evaluated by the effect on a bound molecule, such as the induced 31P NMR shift of triethylphosphine oxide in the Gutmann–Beckett (GB) method. This type of scaling was called effective Lewis acidity (eLA). Unfortunately, gLA and eLA often correlate poorly, but a reason for this is unknown. Hence, the strength and the effect of a Lewis acid are two distinct properties, but they are often granted interchangeably. The present work analyzes thermodynamic, NMR specific, and London dispersion effects on GB numbers for 130 Lewis acids by theory and experiment. The deformation energy of a Lewis acid is identified as the prime cause for the critical deviation between gLA and eLA but its correction allows a unification for the first time.

Ligand‐Enabled Disproportionation of 1,2‐Diphenylhydrazine at a PV‐Center**

“…In contrast, the isodesmically computed fluoride ion affinity (FIA) and hydride ion affinity (HIA) for 19 are higher than that reported for 21 or 22 , and only slightly surpassed by that of octa‐chlorinated 23 . Note that the FIA and HIA values include the reagent deformation penalty to form the corresponding fluoride and hydride adducts [29] . This energy is not negligible for 22 and 23 because their geometries substantially change from tetrahedral to trigonal bipyramidal to form the corresponding adduct, while 19 only requires a minimum displacement of the central P‐atom outside of the ONNO plane.…”

Ligand enabled disproportionation of 1,2-diphenylhydrazine at a P(V)-center

“…Note that the FIA and HIA values include the reagent deformation penalty to form the corresponding fluoride and hydride adducts. [29] This energy is not negligible for 22 and 23 because their geometries substantially change from tetrahedral to trigonal bipyramidal to form the corresponding adduct, while 19 only requires a minimum displacement of the central P-atom outside of the ONNO plane. For comparison purposes we also calculated the FIA and HIA values for quasi tetrahedral 20.…”

“…In an effort to shed light on the mechanism governing this transformation, electronic structure calculations were carried out. The reaction intermediates and selected transition states were optimized at the PBE-D3(BJ)/def2-SVP level of theory, [29] making use of the D3 Grimme dispersion correction with Becke-Johnson damping. [30] The nature of the stationary points was confirmed by harmonic frequency calculations, which were also used for the thermodynamical corrections to the free energy.…”

Formation of a Strong Heterogeneous Aluminum Lewis Acid on Silica

“…The 31 P NMR chemical shift of Lewis adducts containing O=PEt 3 are also diagnostic of the strength of a Lewis acids in solution [21] or on solid surfaces [22] . The reaction of 1 with 1.1 equiv O=PEt 3 forms 2 [Eq.…”