Infrared Spectroscopy Evidence of Weak Interactions in Frustrated Lewis Pairs Formed by Tris(pentafluorophenyl)borane

Frustrated Lewis pairs (FLPs) have been widely investigated as promising catalysts due to their metal-free feature and ability to activate small molecules. Since their discovery, many works have been investigating how these Lewis pairs (intermolecular pairs) are held together in an encounter complex. This prompted several studies based on theoretical investigations, but experimental ones are limited yet. In this communication we show evidence of weak intermolecular interactions between Lewis acids and Lewis bases, distinguishing the Lewis adduct from FLPs, by probing fluorine-carbon vibrational modes using infrared spectroscopy. The main evidence is based on the band shifts occurring in FLPs due to weak hydrogen bonds between the hydrogen atoms of the Lewis base and the fluorine atoms of Lewis acid.     

Asymmetric Hydrogenation of Ketones and Enones with Chiral Lewis Base Derived Frustrated Lewis Pairs

The concept of frustrated Lewis pairs (FLPs) has been widely applied in various research areas, and metal‐free hydrogenation undoubtedly belongs to the most significant and successful ones. In the past decade, great efforts have been devoted to the synthesis of chiral boron Lewis acids. In a sharp contrast, chiral Lewis base derived FLPs have rarely been disclosed for the asymmetric hydrogenation. In this work, a novel type of chiral FLP was developed by simple combination of chiral oxazoline Lewis bases with achiral boron Lewis acids, thus providing a promising new direction for the development of chiral FLPs in the future. These chiral FLPs proved to be highly effective for the asymmetric hydrogenation of ketones, enones, and chromones, giving the corresponding products in high yields with up to 95 % ee. Mechanistic studies suggest that the hydrogen transfer to simple ketones likely proceeds in a concerted manner.     

New insights into frustrated Lewis pairs: structural investigations of intramolecular phosphane-borane adducts by using modern solid-state NMR techniques and DFT calculations

Covalent bonding interactions between the Lewis acid and Lewis base functionalities have been probed in a series of "frustrated Lewis pairs" (FLPs) (mainly substituted vinylene linked intramolecular phosphane-borane adducts), using solid-state nuclear magnetic resonance techniques and accompanying DFT calculations. Both the (11)B NMR isotropic chemical shifts and nuclear electric quadrupolar coupling parameters turn out to be extremely sensitive experimental probes for such interactions, revealing linear correlations with boron-phosphorus internuclear distances. The principal component V(zz) of the (11)B electric field gradient tensor is tilted slightly away (~20°) from the boron-phosphorus internuclear vector, leading to an improved understanding of the remarkable reactivity of the FLPs. Complementary (31)P{(1)H}-CPMAS experiments reveal significant (31)P-(11)B scalar spin-spin interactions ((1)J ≈ 50 Hz), evidencing covalent bonding interactions between the reaction centers. Finally, (11)B{(31)P} rotational echo double resonance (REDOR) experiments show systematic deviations from calculated curves based on the internuclear distances from X-ray crystallography. These deviations suggest non-zero contributions from anisotropic indirect spin-spin (J anisotropy) interactions, thereby offering additional evidence for covalent bonding.     

On the Mechanism of Hydrogen Activation by Frustrated Lewis Pairs

We report herein a comprehensive theoretical study of the thermodynamics and kinetics of molecular hydrogen activation by frustrated Lewis pairs (FLPs). A series of intermolecularly combined boranes (Lewis acids) and phosphines (Lewis bases), with experimentally established different reactivities towards H₂, have been subjected to DFT and (SCS‐)MP2 calculations, and analyzed in terms of their structural properties, the energetics of association of the FLPs, and the kinetics of their interactions with H₂ and hydrogenation to the ion‐pair products. The analysis included the following steps: 1) assessment of the ability/inability of the Lewis species to preorganize into FLPs with an optimum arrangement of the acid and base sites for preconditioning the reaction with H₂, 2) comprehension of the different thermodynamics of hydrogenation of the selected FLPs by comparing the Gibbs energies of the overall reactions, and 3) estimation of the mechanism of the activation of H₂ by identifying the reaction steps and the associated kinetic barriers. The results of our studies correlate well with experimental findings and have clarified the reasons for the observed different reactivities of the investigated systems, ranging from reversible or nonreversible activation to no reaction with H₂. The derived predictions could assist the future design of Lewis acid–base systems with desired properties and applicability as metal‐free hydrogenation catalysts.     

Extended weak bonding interactions in DNA: pi-stacking (base-base), base-backbone, and backbone-backbone interactions

We report on several weak interactions in nucleic acids, which, collectively, can make a nonnegligible contribution to the structure and stability of these molecules. Fragments of DNA were obtained from previously determined accurate experimental geometries and their electron density distributions calculated using density functional theory (DFT). The electron densities were analyzed topologically according to the quantum theory of atoms in molecules (AIM). A web of closed-shell bonding interactions is shown to connect neighboring base pairs in base-pair duplexes and in dinuleotide steps. This bonding underlies the well-known pi-stacking interaction between adjacent nucleic acid bases and is characterized topologically for the first time. Two less widely appreciated modes of weak closed-shell interactions in nucleic acids are also described: (i) interactions between atoms in the bases and atoms belonging to the backbone (base-backbone) and (ii) interactions among atoms within the backbone itself (backbone-backbone). These interactions include hydrogen bonding, dihydrogen bonding, hydrogen-hydrogen bonding, and several other weak closed-shell X-Y interactions (X, Y = O, N, C). While each individual interaction is very weak and typically accompanied by perhaps 0.5-3 kcal/mol, the sum total of these interactions is postulated to play a role in stabilizing the structure of nucleic acids. The Watson-and-Crick hydrogen bonding is also characterized in detail at the experimental geometries as a prelude to the discussion of the modes of interactions listed in the title.     

受阻路易斯酸碱对在高分子聚合上的应用

受阻路易斯酸碱对(frustrated Lewis pairs,FLPs)是大位阻的路易斯酸和大位阻的路易斯碱在溶液中受空间位阻因素影响而不能形成配位键所得到的组合。 在这种特殊的组合中,路易斯酸和路易斯碱未能被中和淬灭,依旧保持着的反应活性。 而当H₂等小分子靠近时,FLPs可以将H₂的化学键异裂,进而得到一个阳离子和一个阴离子。 这种独特的反应特性使得FLPs在催化加氢、小分子气体活化、烯烃聚合和开环聚合等方面展现出了一些具有新特性的研究思想和方法。 尤其是在烯烃聚合和开环聚合中,FLPs具有很强的催化活性。 本文简要介绍了FLPs的发展历史及其在小分子活化中的应用,并重点介绍了其在高分子催化领域中的应用。     

Insight into the relative reactivity of "frustrated Lewis pairs" and stable carbenes in activating H2 and CH4: a comparative computational study

Computational study has been conducted to gain insight into the relative reactivity of stable carbenes (1 and 2) and typical frustrated Lewis pairs (FLPs, 3-6) in activating H(2) and CH(4). For the FLP H(2) activations, despite the quite different basicities of the Lewis base components, they have comparable reactivities. The unexpected relative reactivity can be attributed to the following two factors: (i) the vacant carbene C: p(π) orbital, which is important when carbene works alone but does not participate in the FLP activation; and (ii) the electrostatic interaction between the Lewis base center and the approaching H atom which plays an important role and can either favor or disfavor a reaction. These explanations are also applicable to methane activations. The study brings two messages to the experimentalists for constructing FLPs: (i) it is recommended to use P- and N-centered Lewis bases to construct FLPs for H(2) activation because using more reactive components does not benefit the activation; and (ii) the FLPs are less reactive in activating CH(4) than H(2). In addition, using more reactive carbenes as Lewis bases in FLPs does not necessarily benefit the methane activation.     

In Situ Formation of Frustrated Lewis Pairs in a Water‐Tolerant Metal‐Organic Framework for the Transformation of CO2

Frustrated Lewis pairs (FLPs) consist of sterically hindered Lewis acids and Lewis bases, which provide high catalytic activity towards non‐metal‐mediated activation of “inert” small molecules, including CO₂ among others. One critical issue of homogeneous FLPs, however, is their instability upon recycling, leading to catalytic deactivation. Herein, we provide a solution to this issue by incorporating a bulky Lewis acid‐functionalized ligand into a water‐tolerant metal‐organic framework (MOF), named SION‐105 , and employing Lewis basic diamine substrates for the in situ formation of FLPs within the MOF. Using CO₂ as a C1‐feedstock, this combination allows for the efficient transformation of a variety of diamine substrates into benzimidazoles. SION‐105 can be easily recycled by washing with MeOH and reused multiple times without losing its identity and catalytic activity, highlighting the advantage of the MOF approach in FLP chemistry.     

Frustrated Lewis Pair Chemistry: Development and Perspectives

Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases in solution that are deterred from strong adduct formation by steric and/or electronic factors. This opens pathways to novel cooperative reactions with added substrates. Small‐molecule binding and activation by FLPs has led to the discovery of a variety of new reactions through unprecedented pathways. Hydrogen activation and subsequent manipulation in metal‐free catalytic hydrogenations is a frequently observed feature of many FLPs. The current state of this young but rapidly expanding field is outlined in this Review and the future directions for its broadening sphere of impact are considered.     

“Anti-electrostatic” Halogen Bonding between Ions of Like Charge

Halogen bonding occurs between molecules featuring Lewis acidic halogen substituents and Lewis bases. It is often rationalized as a predominantly electrostatic interaction and thus interactions between ions of like charge (e. g., of anionic halogen bond donors with halides) seem counter-intuitive. Herein, we provide an overview on such complexes. First, theoretical studies are described and their findings are compared. Next, experimental evidences are presented in the form of crystal structure database analyses, recent examples of strong “anti-electrostatic” halogen bonding in crystals, and the observation of such interactions also in solution. We then compare these complexes to select examples of “counter-intuitive” adducts formed by other interactions, like hydrogen bonding. Finally, we comment on key differences between charge-transfer and electrostatic polarization.     

Borane/silane frustrated Lewis pairs for polymerization of β-substituted Michael acceptors

Frustrated Lewis pairs (FLPs) of Lewis acid (LA) B(C₆F₅)₃ and Lewis base hydrosilane [SiH] have been utilized to promote controlled polymerization of a challenging β-substituted Michael acceptor, methyl crotonate (MC), devoid of chain transfer side reactions. Mechanistic studies show that chain initiation involves LA-catalyzed 1,4-hydrosilylation of MC with [SiH] via FLP-type activation, generating a silyl ketene acetal nucleophile that participates in chain propagation via classic LA activation of monomer and a bimolecular conjugate addition mechanism. The role of the LA is conflicting in the two different catalytic cycles: the FLP activation in chain initiation requires LA-substrate (monomer) dissociation (or weak interaction) while the classic LA activation in chain propagation demands LA-monomer association (or strong interaction).     

Single-Electron Transfer in Frustrated Lewis Pair Chemistry

Frustrated Lewis pairs (FLPs) are well known for their ability to activate small molecules. Recent reports of radical formation within such systems indicate single-electron transfer (SET) could play an important role in their chemistry. Herein, we investigate radical formation upon reacting FLP systems with dihydrogen, triphenyltin hydride, or tetrachloro-1,4-benzoquinone (TCQ) both experimentally and computationally to determine the nature of the single-electron transfer (SET) events; that is, being direct SET to B(C₆F₅)₃ or not. The reactions of H₂ and Ph₃SnH with archetypal P/B FLP systems do not proceed via a radical mechanism. In contrast, reaction with TCQ proceeds via SET, which is only feasible by Lewis acid coordination to the substrate. Furthermore, SET from the Lewis base to the Lewis acid–substrate adduct may be prevalent in other reported examples of radical FLP chemistry, which provides important design principles for radical main-group chemistry.     

Frustrated Lewis Pairs: Metal‐free Hydrogen Activation and More

Sterically encumbered Lewis acid and Lewis base combinations do not undergo the ubiquitous neutralization reaction to form “classical” Lewis acid/Lewis base adducts. Rather, both the unquenched Lewis acidity and basicity of such sterically “frustrated Lewis pairs (FLPs)” is available to carry out unusual reactions. Typical examples of frustrated Lewis pairs are inter‐ or intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C₆F₅)₂ components. Many examples of such frustrated Lewis pairs are able to cleave dihydrogen heterolytically. The resulting H⁺/H^? pairs (stabilized for example, in the form of the respective phosphonium cation/hydridoborate anion salts) serve as active metal‐free catalysts for the hydrogenation of, for example, bulky imines, enamines, or enol ethers. Frustrated Lewis pairs also react with alkenes, aldehydes, and a variety of other small molecules, including carbon dioxide, in cooperative three‐component reactions, offering new strategies for synthetic chemistry.     

Understanding the C−F Bond Activation Mediated by Frustrated Lewis Pairs: Crucial Role of Non-covalent Interactions

The activation of a single C−F bond in di- and trifluoromethyl groups by frustrated Lewis pairs (FLPs) has been computationally explored by means of Density Functional Theory calculations. It is found that in this activation reaction the FLP partners exhibit a peculiar cooperative action, which is markedly different from related FLP-mediated processes, and where non-covalent interactions established between the Lewis base and the substrate play a decisive role. In addition, the process proceeds through the intermediacy of a hypervalent species featuring a pentacoordinate carbon atom, which is rare in the chemistry of FLPs. The physical factors controlling this process as well as the bonding situation of these hypervalent intermediates have been quantitatively analyzed in detail by using state-of-the-art computational methods to not only rationalize the mechanism of the transformation but also to guide experimentalists towards the realization of these so far elusive hypervalent systems.     

Dihydrogen Activation with a Neutral,Intermolecular Silicon(IV)-Amine Frustrated Lewis Pair

The heterolytic cleavage of dihydrogen constitutes the hallmark reaction of frustrated Lewis pairs (FLP). While being well-established for planar Lewis acids, such as boranes or silylium ions, the observation of the primary H₂ splitting products with non-planar Lewis acid FLPs remained elusive. In the present work, we report bis(perfluoro-N-phenyl-ortho-amidophenolato)silane and its application in dihydrogen activation to a fully characterized hydridosilicate. The strict design of the Lewis acid, the limited selection of the Lewis base, and the distinct reaction conditions emphasize the narrow tolerance to achieve this fascinating process with a tetrahedral Lewis acid.     

“受阻Lewis酸碱对”化学的研究进展

受阻Lewis酸碱对(Frustrated Lewis Pairs,FLPs)是一类具有特殊反应活性的Lewis酸碱对。自发现以来,FLPs受到了广泛关注并在许多领域崭露头角。本文对FLPs在不对称氢化、高分子聚合、CO_2催化还原等应用领域取得的突破进行了介绍;同时对过渡金属FLPs和FLPs配位的过渡金属催化体系进行了综述;最后对FLPs领域未来的发展前景进行了展望。     

Rational design of FLP catalysts for reversible H2 activation: A DFT study of the geometric and electronic effects

The thermodynamics of reversible H₂ activation could be controlled by adjusting substituents of LA group and using different polar solvents, which forges a guide to design potential FLPs catalysts for reversible H₂ activation.     

A New Mode of Chemical Reactivity for Metal‐Free Hydrogen Activation by Lewis Acidic Boranes

We herein explore whether tris(aryl)borane Lewis acids are capable of cleaving H₂ outside of the usual Lewis acid/base chemistry described by the concept of frustrated Lewis pairs (FLPs). Instead of a Lewis base we use a chemical reductant to generate stable radical anions of two highly hindered boranes: tris(3,5‐dinitromesityl)borane and tris(mesityl)borane. NMR spectroscopic characterization reveals that the corresponding borane radical anions activate (cleave) dihydrogen, whilst EPR spectroscopic characterization, supported by computational analysis, reveals the intermediates along the hydrogen activation pathway. This radical‐based, redox pathway involves the homolytic cleavage of H₂, in contrast to conventional models of FLP chemistry, which invoke a heterolytic cleavage pathway. This represents a new mode of chemical reactivity for hydrogen activation by borane Lewis acids.