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

Illuminating the multiple Lewis acidity of triaryl-boranes via atropisomeric dative adducts

Using the principle that constrained conformational spaces can generate novel and hidden molecular properties, we challenged the commonly held perception that a single-centered Lewis acid reacting with a single-centered Lewis base always forms a single Lewis adduct. Accordingly, the emergence of single-centered but multiple Lewis acidity among sterically hindered and non-symmetric triaryl-boranes is reported. These Lewis acids feature several diastereotopic faces providing multiple binding sites at the same Lewis acid center in the interaction with Lewis bases giving rise to adducts with diastereomeric structures. We demonstrate that with a proper choice of the base, atropisomeric adduct species can be formed that interconvert via the dissociative mechanism rather than conformational isomerism. The existence of this exotic and peculiar molecular phenomenon was experimentally confirmed by the formation of atropisomeric piperidine-borane adducts using state-of-the-art NMR techniques in combination with computational methods.

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

Group 4 Lewis acids are well-known catalysts and precursors for (non-aqueous) sol-gel chemistry. Titanium, zirconium and hafnium halides, and alkoxy halides are precursors for the controlled synthesis of nanocrystals, often in the presence of Lewis base. Here, we investigate the interaction of Lewis bases with the tetrahalides (MX4, X = Cl, Br) and metal alkoxy halides (MXx(OR)4-x, x = 1-3, R = OiPr, OtBu). The tetrahalides yield the expected Lewis acid-base adducts MX4L2 (L = tetrahydrofuran or phosphine oxide). The mixed alkoxy halides react with Lewis bases in a more complex way. 31P NMR spectroscopy reveals that excess of phosphine oxide yields predominantly the complexation product, while a (sub)stoichiometric amount of phosphine oxide causes disproportionation of the MXx(OR)4-x species into MXx+1(OR)3-x and MXx-1(OR)5-x. The combination of complexation and disproportionation yields an atypical Job plot. In the case of zirconium isopropoxy chlorides, we fitted the concentration of all observed species and extracted thermodynamic descriptors from the Job plot. The complexation equilibrium constant decreases in the series: ZrCl3(OiPr) > ZrCl2(OiPr)2 ≫ ZrCl(OiPr)3, while the disproportionation equilibrium constant follows the opposite trend. Using calculations at the DFT level of theory, we show that disproportionation is driven by the more energetically favorable Lewis acid-base complex formed with the more acidic species. We also gain more insight into the isomerism of the complexes. The disproportionation reaction turns out to be a general phenomenon, for titanium, zirconium and hafnium, for chlorides and bromides, and for isopropoxides and tert-butoxides.

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

We present herein the synthesis of a nearly square-pyramidal chlorophosphorane supported by the tetradentate bis(amidophenolate) ligand, N,N'-bis(3,5-di-tert-butyl-2-phenoxy)-1,2-phenylenediamide. After chloride abstraction the resulting phosphonium cation efficiently promotes the disproportionation of 1,2-diphenylhydrazine to aniline and azobenzene. Mechanistic studies, spectroscopic analyses and theoretical calculations suggest that this unprecedented reactivity mode for PV -centres is induced by the high electrophilicity at the cationic PV -center, which originates from the geometry constraints imposed by the rigid pincer ligand, combined with the ability of the o-amidophenolate moieties to act as electron reservoir. This study illustrates the promising role of cooperativity between redox-active ligands and phosphorus for the design of organocatalysts able to promote redox processes.

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

The most active alkyne metathesis catalysts rely on well-defined Mo alkylidynes, X₃Mo≡CR (X = OR), in particular the recently developed canopy catalyst family bearing silanolate ligand sets. Recent efforts to understand catalyst reactivity patterns have shown that NMR chemical shifts are powerful descriptors, though previous studies have mostly focused on ligand-based NMR descriptors. Here, we show in the context of alkyne metathesis that ⁹⁵Mo chemical shift tensors encode detailed information on the electronic structure of these catalysts. Analysis by first-principles calculations of ⁹⁵Mo chemical shift tensors extracted from solid-state ⁹⁵Mo NMR spectra shows a direct link of chemical shift values with the energies of the HOMO and LUMO, two molecular orbitals involved in the key [2 + 2]-cycloaddition step, thus linking ⁹⁵Mo chemical shifts to reactivity. In particular, the ⁹⁵Mo chemical shifts are driven by ligand electronegativity (σ-donation) and electron delocalization through Mo-O π interactions, thus explaining the reactivity patterns of the silanolate canopy catalysts. These results further motivate exploration of transition metal NMR signatures and their relationships to electronic structure and reactivity.

Lewis Acid-Base Adducts of Al(N(C6F5)2)3 and Ga(N(C6F5)2)3 – Structural Features and Adduct Formation Enthalpies

Herein we present the molecular structures of six neutral Lewis acid-base adducts of the Lewis superacid Al(N(C6F5)2)3 and its higher homolog Ga(N(C6F5)2)3 with the electron pair donors MeCN, CNtBu, THF...