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Various Lewis acid–Lewis base interactions are discussed as initiating chemical reactions and processes. For example, the hydrogen bond is often a preliminary stage of the proton transfer process or the tetrel and pnicogen bonds lead sometimes to the SN2 reactions. There are numerous characteristics of interactions being first stages of reactions; one can observe a meaningful electron charge transfer from the Lewis base unit to the Lewis acid; such interactions possess at least partly covalent character, one can mention other features. The results of different methods and approaches that are applied in numerous studies to describe the character of interactions are presented here. These are, for example, the results of the Quantum Theory of Atoms in Molecules, of the decomposition of the energy of interaction or of the structure-correlation method.

In this concept article, we consider the notion of ‘frustrated Lewis pairs’ (FLPs). While the original use of the term referred to steric inhibition of dative bond formation in a Lewis pair, work in the intervening decade demonstrates the limitation of this simplistic view. Analogies to known transition metal chemistry and the applications in other areas of chemistry are considered. In the light of these findings, we present reflections on the criteria for a definition of the term ‘frustrated Lewis pair’. Segregation of the Lewis acid and base and the kinetic nature of FLP reactivity are discussed. We are led to the conclusion that, while an all-inclusive definition of FLP is challenging, the notion of ‘FLP chemistry’ is more readily recognized. This article is part of the themed issue ‘Frustrated Lewis pair chemistry’.

The growing demands for ammonia in agriculture and transportation fuel stimulate researchers to develop sustainable electrochemical methods to synthesize ammonia ambiently, to get past the energy-intensive Haber-Bosch process. However, the conventionally used aqueous electrolytes limit N₂ solubility, leading to insufficient reactant molecules in the vicinity of the catalyst during electrochemical nitrogen reduction reaction (NRR). This hampers the yield and production rate of ammonia, irrespective of how efficient the catalyst is. Herein, we introduce an aqueous electrolyte (NaBF₄), which not only acts as an N₂-carrier in the medium but also works as a full-fledged “co-catalyst” along with our active material MnN₄ to deliver a high yield of NH₃ (328.59 μg h−1 mgcat −1) at 0.0 V versus reversible hydrogen electrode. BF₃-induced charge polarization shifts the metal d-band center of the MnN₄ unit close to the Fermi level, inviting N₂ adsorption facilely. The Lewis acidity of the free BF₃ molecules further propagates their importance in polarizing the N≡N bond of the adsorbed N₂ and its first protonation. This push-pull kind of electronic interaction has been confirmed from the change in d-band center values of the MnN₄ site as well as charge density distribution over our active model units, which turned out to be effective enough to lower the energy barrier of the potential determining steps of NRR. Consequently, a high production rate of NH₃ (2.45 × 10−9 mol s−1 cm−2) was achieved, approaching the industrial scale where the source of NH₃ was thoroughly studied and confirmed to be chiefly from the electrochemical reduction of the purged N₂ gas.

Prototypical Lewis acids, such as boranes, derive their reactivity from electronic unsaturation. Here, we report the Lewis acidity and catalytic application of electronically saturated phosphorus-centered electrophilic acceptors. Organofluorophosphonium salts of the formula [(C₆F₅) 3-x Ph x PF][B(C₆F₅)₄] (x = 0 or 1; Ph, phenyl) are shown to form adducts with neutral Lewis bases and to react rapidly with fluoroalkanes to produce difluorophosphoranes. In the presence of hydrosilane, the cation [(C₆F₅)₃PF]⁺ is shown to catalyze the hydrodefluorination of fluoroalkanes, affording alkanes and fluorosilane. The mechanism demonstrates the impressive fluoride ion affinity of this highly electron-deficient phosphonium center.

In enzymatic catalysis, the redox site and Lewis acid are the two main roles played by metal to assist amino acids. However, the reported enzyme mimics only focus on the redox-active metal as redox site, while the redox-inert metal as Lewis acid has, to the best of our knowledge, not been studied, presenting a bottleneck of enzyme mimics construction. Based on this, a series of highly efficient M x V 2 O 5 ·nH 2 O peroxidase mimics with vanadium as redox site and alkaline-earth metal ion (M 2+ ) as Lewis acid are reported. Experimental results and theoretical calculations indicate the peroxidase-mimicking activity of M x V 2 O 5 ·nH 2 O show a periodic change with the Lewis acidity (ion potential) of M 2+ , revealing the mechanism of redox-inert M 2+ regulating electron transfer of V-O through non-covalent polarization and thus promoting H 2 O 2 adsorbate dissociation. The biomimetic synergetic effect of redox site and Lewis acid is expected to provide an inspiration for design of enzyme mimics. The so far reported enzyme mimics focus on the redox-active metal as a redox site, while the redox-inert metal as Lewis acid has so far not been reported. Here, the authors report efficient M x V 2 O 5 ·nH 2 O peroxidase mimics with vanadium as redox site and alkaline-earth metal ion (M 2+ ) as Lewis acid, where the peroxidase-mimicking activity shows a periodic change with the Lewis acidity of the metal.

In contrast to the wealth of catalytic systems that are available to control the stereochemistry of thermally promoted cycloadditions, few similarly effective methods exist for the stereocontrol of photochemical cycloadditions. A major unsolved challenge in the design of enantioselective catalytic photocycloaddition reactions has been the difficulty of controlling racemic background reactions that occur by direct photoexcitation of substrates while unbound to catalyst. Here, we describe a strategy for eliminating the racemic background reaction in asymmetric [2 + 2] photocycloadditions of α,β-unsaturated ketones to the corresponding cyclobutanes by using a dual-catalyst system consisting of a visible light–absorbing transition-metal photocatalyst and a stereocontrolling Lewis acid cocatalyst. The independence of these two catalysts enables broader scope, greater stereochemical flexibility, and better efficiency than previously reported methods for enantioselective photochemical cycloadditions.

Surface Lewis acid-base sites in crystal structure may influence the physicochemical properties and the catalytic performances in nanozymes. Understanding the synergistic effect mechanism of Co 3 O 4 nanozymes towards substances (3,3′,5,5′-tetramethylbenzidine (TMB) and hydrogen peroxide (H 2 O 2 )) induced by surface Lewis acid-base sites is important to enhance the efficiency for peroxidase-like reaction. Herein, ultrathin porous Co 3 O 4 nanosheets with abundant Lewis acid-base sites were prepared by sodium borohydride (NaBH 4 ) reduction treatment, which exhibited high-efficiency peroxidase-like activity compared with original Co 3 O 4 nanosheets. The Lewis acid-base sites for ultrathin porous Co 3 O 4 nanosheets nanozyme were owing to the coordination unsaturation of Co ions and the formation of defect structure. Ultrathin porous Co 3 O 4 nanosheets had 18.26-fold higher catalytic efficiency (1.27 × 10 −2 s −1 ·mM −1 ) than that of original Co 3 O 4 (6.95 × 10 −4 s −1 ·mM −1 ) in oxidizing TMB substrate. The synergistic effect of surface acid and base sites can enhance the interfacial electron transfer process of Co 3 O 4 nanosheets, which can be a favor of absorption substrates and the generation of reactive intermediates such as radicals. Furthermore, the limit of detection of hydroquinol was 0.58 µM for ultrathin porous Co 3 O 4 nanosheets, 965-fold lower than original Co 3 O 4 (560 µM). Besides, the linear range of ultrathin porous Co 3 O 4 nanosheets was widely with the concentration of 5.0–1,000 µM. Colorimetric detection of hydroquinol by agarose-based hydrogel membrane was provided based on excellent peroxidase-like properties. This study provided insights into designing high-performance nanozymes for peroxidase-like catalysis via a strategy of solid surface acid-base sites engineering.

Highlights NiCo–OH has a relatively high Brønsted acid sites (BASs) content (89.6%), which can promote the adsorption of OH − and inhibit the co-adsorption of OH − and alcohols, resulting in poor alcohol oxidation reaction (AOR) activity but higher oxygen evolution reaction activity. NiCo–OH-derived NiCo 2 O 4 solid-acid electrocatalysts with balanced BASs (46.9%) and Lewis acid sites (53.1%) facilitates co-adsorption of alcohols molecules and OH − , thereby favoring the AOR. In the AOR on NiCo 2 O 4 , as the number of hydroxyl groups in C 1 -C 6 saturated alcohols increases, the activity shows an increasing trend: C 1

Journal Article

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