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.

Complexation and disproportionation of group 4 metal (alkoxy) halides with phosphine oxides

“…Using the calculated energies, we computed the enthalpy changes for the (gas-phase) complexation reactions ( Table 3 ), which represent the global Lewis acidity since the latter is defined as the thermodynamic tendency to form Lewis acid–base pairs. 55 All reactions are exothermic and the complexation with TEPO is favored over complexation with THF, in agreement with experimental results. The complexation reaction becomes less exothermic in the series: ZrCl 4 > ZrCl 3 (O i Pr) > ZrCl 2 (O i Pr) 2 > ZrCl(O i Pr) 3 .…”

“…which represent the global Lewis acidity since the latter is defined as the thermodynamic tendency to form Lewis acidbase pairs. 55 All reactions are exothermic and the complexation with TEPO is favored over complexation with THF, in agreement with experimental results. The complexation reaction becomes less exothermic in the series: ZrCl Lewis acidity agrees with the trend in effective Lewis acidity, given by the 31 P NMR shifts.…”

Complexation and Disproportionation of Group 4 Metal (Alkoxy) Halides with Phosphine Oxides

“…Using the calculated energies, we computed the enthalpy changes for the (gas-phase) complexation reactions (Table 3), which represent the global Lewis acidity since the latter is defined as the thermodynamic tendency to form Lewis acid-base pairs. 69 All reactions are exothermic and the complexation with TEPO is favored over complexation with THF, in agreement with experimental results. The complexation reaction becomes less exothermic in the series:…”

“…Furthermore, the uncoordinated Lewis acid is a tetrahedral monomer in the calculations, and thus dimerization (or polymerization in case of ZrCl 4 ) may account for discrepancies with experimental results. 69 As a final disclaimer, the calculations were performed in the gas phase and no solvent effects were taken into account.…”

Classifying and Understanding the Reactivities of Mo-Based Alkyne Metathesis Catalysts from 95Mo NMR Chemical Shift Descriptors

“…In this context, nuclear magnetic resonance (NMR) spectroscopy has emerged as a powerful tool for establishing the molecular origins of reactivity. − Natural localized molecular orbital (NLMO) analysis of NMR parameters such as chemical shift or electric field gradient (efg) tensors can provide a link between experimentally accessible observables and reactivity. This link arises from the direct relation of these tensors to frontier molecular orbitals and electron distributions. − Orbital analyses of NMR parameters have elucidated reactivity patterns of organometallic and coordination compounds in stoichiometric and catalytic reactions by providing detailed information about metal–ligand binding, focusing mostly on NMR signatures of nuclei in ligands bound to metal sites. − A relevant example is the analysis of Mo alkylidynes X 3 MoCR with X = alkoxide or silanolate ligands, where the 13 C chemical shielding of the alkylidyne carbon can be related to the energy of the vacant π*­(MC) orbital as modulated by the monoanionic X ligands. , Direct analysis of catalytic centers through NMR of metal nuclei such as 95 Mo is an appealing and potentially more powerful alternative. The high potential resolution of this approach is manifested by the extraordinarily large 95 Mo chemical shift range, known to span >5500 ppm , compared to ∼300 ppm for typical 13 C chemical shifts.…”