Cooperative Lewis Pairs Based on Late Transition Metals: Activation of Small Molecules by Platinum(0) and B(C6F5)3

A Lewis basic platinum(0)–CO complex supported by a diphosphine ligand and B(C₆F₅)₃ act cooperatively, in a manner reminiscent of a frustrated Lewis pair, to activate small molecules such as hydrogen, CO₂, and ethene. This cooperative Lewis pair facilitates the coupling of CO and ethene in a new way.     

Frustrated Radical Pairs: Insights from EPR Spectroscopy

Progress in frustrated Lewis pair (FLP) chemistry has revealed the importance of the main group elements in catalysis, opening new avenues in synthetic chemistry. Recently, new reactivities of frustrated Lewis pairs have been uncovered that disclose that certain combinations of Lewis acids and bases undergo single‐electron transfer (SET) processes. Here an electron can be transferred from the Lewis basic donor to a Lewis acidic acceptor to generate a reactive frustrated radical pair (FRP). This minireview aims to showcase the recent advancements in this emerging field covering the synthesis and reactivities of frustrated radical pairs, with extensive highlights of the results from Electron Paramagnetic Resonance (EPR) spectroscopy to explain the nature and stability of the different radical species observed.     

Aggregation Behavior of a Six‐Membered Cyclic Frustrated Phosphane/Borane Lewis Pair: Formation of a Supramolecular Cyclooctameric Macrocyclic Ring System

A new six‐membered cyclic frustrated phosphane/borane Lewis pair was liberated from its HB(C₆F₅)₂ adduct by treatment with vinylcyclohexane. The system is an active frustrated Lewis pair that undergoes cycloaddition reactions with suitable π reagents and it splits dihydrogen. At room temperature in solution the new compound is a monomer, however, in the crystal and in solution at low temperature it aggregates to a thermodynamically favoured supramolecular macrocyclic cyclooctamer.     

Structure and dynamic features of an intramolecular frustrated Lewis pair

Di(mesityl)cyclohexenylphosphine undergoes hydroboration with Piers' borane [HB(C₆F₅)₂] to yield the cyclohexylene‐anellated frustrated Lewis pair 5 . This P/B pair splits H₂ with the formation of the product 4 and adds to the C?O double bond of phenyl isocyanate to yield 6 . In the crystal, compound 5 features a puckered four‐membered heterocyclic core structure with a long P? B bond (av. 2.197(5) Å). The activation energy of the P? B cleavage of the frustrated Lewis pair 5 was determined by dynamic ¹⁹F NMR spectroscopy at ΔG^≠(298 K)=12.1±0.3 kcal mol^?1.     

Bis-boranes in the frustrated Lewis pair activation of carbon dioxide

Using a frustrated Lewis pair approach, the 1,1-bis-(C(6)F(5))(2)BOB(C(6)F(5))(2) is shown to bind CO(2) in a monodentate fashion, while the bis-boranes (X(2)B)(2)C=CMe(2) (X = Cl, C(6)F(5)) give bidentate chelation of CO(2) affording unique heterocycles.     

Chemistry of a geminal frustrated Lewis pair featuring electron withdrawing C6F5 substituents at both phosphorus and boron

Hydroboration of the electron poor phosphine (1-propenyl)P(C(6)F(5))(2) with Piers' borane [HB(C(6)F(5))(2)] gave the geminal frustrated Lewis pair (C(6)F(5))(2)P-CH(Et)-B(C(6)F(5))(2). It undergoes 1,2-addition reactions to an alkene and an alkyne and to the C=N bond of an isocyanate. With mesityl azide it undergoes a 1,3-addition reaction.     

Intramolecular heterolytic dihydrogen cleavage by a bifunctional frustrated pyrazolylborane Lewis pair

The reaction of bis(pentafluorophenyl)borane, HB(C(6)F(5))(2), with 3,5-di-tert-butyl-1H-pyrazole (3) affords the zwitterionic pyrazolium-borate trans-5 and, after dehydrogenation by use of the frustrated carbene-borane Lewis pair 1/B(C(6)F(5))(3), the bifunctional pyrazolylborane 6, which is able to cleave dihydrogen heterolytically with the formation of a mixture of cis-5 and trans-5.     

Concerted attack of frustrated Lewis acid-base pairs on olefinic double bonds: a theoretical study

A computational approach reveals cooperative action of the preorganized acidic and basic centers of the frustrated P(t-Bu)(3)/B(C(6)F(5))(3) Lewis pair on olefinic bonds as the key to the observed regioselective addition reaction.     

Heterolytic outer-sphere cleavage of h(2) for the reduction of n(2) in the coordination sphere of transition metals-a DFT study

Cleaved by frustration, an H(2) molecule inserts into the void between the components of the frustrated Lewis pair 1, which consists of a dicationic ruthenium pincer complex as the Lewis acid and (tBu)(3) P as the Lewis base. The end-on-coordinated N(2) in 1 accepts the hydride, and this is the initial step, which could, in principle, lead to the complete reduction of N(2) to NH(3) .     

Coupling of Carbon Monoxide with Nitrogen Monoxide at a Frustrated Lewis Pair Template

Coupling of carbon monoxide with nitrogen monoxide was achieved at a frustrated Lewis pair template. This unique reaction uses hydride as an auxiliary, which reductively activates carbon monoxide at the frustrated Lewis pair. The CO/NO coupling reaction then takes place through a pathway involving a radical reaction in which the hydrogen atom auxiliary is eventually removed again.     

Group 6 Transition‐Metal/Boron Frustrated Lewis Pair Templates Activate N2 and Allow its Facile Borylation and Silylation

The reaction of trans ‐[M(N₂)₂(dppe)₂] (M=Mo, 1_Mo , M=W, 1_W ) with B(C₆F₅)₃ ( 2 ) provides the adducts [(dppe)₂M=N=N‐B(C₆F₅)₃] ( 3 ) which can be regarded as M/B transition‐metal frustrated Lewis pair (TMFLP) templates activating dinitrogen. Easy borylation and silylation of the activated dinitrogen ligands in complexes 3 with a hydroborane and hydrosilane occur by splitting of the B−H and Si−H bonds between the N₂ moiety and the perfluoroaryl borane. This reactivity of 3 is reminiscent of conventional frustrated Lewis pair chemistry and constitutes an unprecedented approach for the functionalization of dinitrogen.     

Mechanistic aspects of bond activation with perfluoroarylboranes

In the mid-1990s, it was discovered that tris(pentafluorophenyl)borane, B(C(6)F(5))(3), was an effective catalyst for hydrosilylation of a variety of carbonyl and imine functions. Mechanistic studies revealed a counterintuitive path in which the function of the borane was to activate the silane rather than the organic substrate. This was the first example of what has come to be known as "frustrated Lewis pair" chemistry utilizing this remarkable class of electrophilic boranes. Subsequent discoveries by the groups of Stephan and Erker showed that this could be extended to the activation of dihydrogen, initiating an intense period of activity in this area in the past 5 years. This article describes the early hydrosilylation chemistry and its subsequent applications to a variety of transformations of importance to organic and inorganic chemists, drawing parallels with the more recent hydrogen activation chemistry. Here, we emphasize the current understanding of the mechanism of this process rather than focusing on the many and emerging applications of hydrogen activation by fluoroarylborane-based frustrated Lewis pair systems.     

N,P‐Heterocyclic Germylene/B(C6F5)3 Adducts: A Lewis Pair with Multi‐reactive Sites

An N,P‐heterocyclic germylene/B(C₆F₅)₃ Lewis adduct 2 presenting multi‐reactive sites (P/B Lewis pair, germylene, Ge=P π‐bond) is reported. In contrast to classical frustrated Lewis pairs or divalent Group 14 element species, 2 is able to activate two small molecules simultaneously. Of particular interest, 2 reacts with silanes leading to the formation of original cationic germylenes 3 , and can be used as a metal‐free catalyst for selective CO₂‐hydrosilylation to H₂C(OSiEt₃)₂.     

Electrophilic Fluorophosphonium Cations in Frustrated Lewis Pair Hydrogen Activation and Catalytic Hydrogenation of Olefins

The combination of phosphorus(V)‐based Lewis acids with diaryl amines and diaryl silylamines promotes reversible activation of dihydrogen and can be further exploited in metal‐free catalytic olefin hydrogenation. Combined experimental and density functional theory (DFT) studies suggest a frustrated Lewis pair type activation mechanism.     

Formation of Frustrated Lewis Pairs in Ptx‐Loaded Zeolite NaY

The formation of a frustrated Lewis pair consisting of sodium hydride (Na⁺H^?) and a framework‐bound hydroxy proton O(H⁺) is reported upon H₂ treatment of zeolite NaY loaded with Pt nanoparticles (Pt_x/NaY). Frustrated Lewis pair formation was confirmed using in situ neutron diffraction and spectroscopic measurements. The activity of the intrazeolite NaH as a size‐selective catalyst was verified by the efficient esterification of acetaldehyde (a small aldehyde) to form the corresponding ester ethyl acetate, whereas esterification of the larger molecule benzaldehyde was unsuccessful. The frustrated Lewis pair (consisting of Na⁺H^? and O(H⁺)) generated within zeolite NaY may be a useful catalyst for various catalytic reactions which require both H^? and H⁺ ions, such as catalytic hydrogenation or dehydrogenation of organic compounds and activation of small molecules.     

α‐Hydroxymethylation of Pyridines at a Frustrated Lewis Pair Template

The “η²‐formylborane” moiety formed by CO reduction with HB(C₆F₅)₂ at a P/B frustrated Lewis pair template undergoes a hydroxymethylation reaction at the α‐position to nitrogen in pyridine or isoquinoline. The analogous reaction with pyrimidine revealed a mechanism related to the Tschitschibabin reaction.     

Rapid Dihydrogen Cleavage by Persistent Nitroxide Radicals under Frustrated Lewis Pair Conditions

Persistent radicals undergo hydrogen atom abstraction reactions with a great variety of substrates, but not with dihydrogen. It has now been found that the TEMPO radical splits dihydrogen under mild conditions in the presence of the strong bulky B(C₆F₅)₃ boron Lewis acid. The reaction is thought to proceed by a typical frustrated Lewis pair mechanism with the TEMPO radical acting as the active Lewis base. The reaction was analyzed by DFT, which indicates that no significant spin density on the hydrogen atoms is accumulated along the H₂ splitting reaction path.     

Organometallic frustrated Lewis pair chemistry

Frustrated Lewis pairs are playing an increasingly important role in organometallic chemistry. Examples are presented and discussed where organometallic systems themselves serve as the Lewis base or Lewis acid components in frustrated Lewis pair chemistry, mostly through their attached functional groups. Activation of dihydrogen takes place easily in many of these systems. This may lead to the generation of novel catalyst systems but also in many cases to the occurrence of specific reactions at the periphery of the organometallic frameworks. Increasingly, FLP reactions are used to carry out functional group conversions in organometallic systems under mild reaction conditions. The limits of typical FLP reactivity are explored with selected organometallic examples, a discussion that points toward new developments, such as the discovery of facile new 1,1-carboboration reactions. Learning more and more about the broad spectrum of frustrated Lewis pair chemistry helps us to find novel reactions and applications.     

CO2‐Cross‐Linked Frustrated Lewis Networks as Gas‐Regulated Dynamic Covalent Materials

The design of structurally dynamic molecular networks can offer strategies for fabricating stimuli‐responsive adaptive materials. Herein we first report a gas‐responsive dynamic gel system based on frustrated Lewis pair (FLP) chemistry. Two trefoil‐like molecules with bulky triphenylborane and triphenylphosphine groups are synthesized as complementary Lewis acid and base with trivalent sites. They can together bind CO₂ gas molecules and further form a cross‐linked network via the bonding interactions between FLPs and CO₂. Such CO₂‐bridged dative linkages are shown to be dynamic covalent bonds, which endow the frustrated Lewis network with adaptable behaviors and unprecedented gas‐regulated viscoelastic, mechanical, and self‐healing performance. This study is an initial attempt to apply the FLP concept in materials chemistry, but we believe that this strategy will open a promising future for gas‐sensitive smart materials.     

Chiral Frustrated Lewis Pair@Metal-Organic Framework as a New Platform for Heterogeneous Asymmetric Hydrogenation

Asymmetric hydrogenation, a seminal strategy for the synthesis of chiral molecules, remains largely unmet in terms of activation by non-metal sites of heterogeneous catalysts. Herein, as demonstrated by combined computational and experimental studies, we present a general strategy for integrating rationally designed molecular chiral frustrated Lewis pair (CFLP) with porous metal–organic framework (MOF) to construct the catalyst CFLP@MOF that can efficiently promote the asymmetric hydrogenation in a heterogeneous manner, which for the first time extends the concept of chiral frustrated Lewis pair from homogeneous system to heterogeneous catalysis. Significantly, the developed CFLP@MOF, inherits the merits of both homogeneous and heterogeneous catalysts, with high activity/enantio-selectivity and excellent recyclability/regenerability. Our work not only advances CFLP@MOF as a new platform for heterogeneous asymmetric hydrogenation, but also opens a new avenue for the design and preparation of advanced catalysts for asymmetric catalysis.