Reaction of an “Invisible” Frustrated N/B Lewis Pair with Dihydrogen

D ‐(+)‐Camphor forms the enamine 2 with piperidine. Compound 2 adds HB(C₆F₅)₂ at the enamine carbon atom C3 to form a Lewis acid/Lewis base adduct (exo‐/endo‐isomers of 3 ). Exposure of 3 to dihydrogen (2.5 bar, room temperature) leads to heterolytic splitting of H₂ to form the H⁺/H^? addition products ( 4 , two diastereoisomers) of the “invisible” frustrated Lewis pairs ( 5 , two diastereoisomers) that were apparently generated in situ by enamine hydroboration under equilibrium conditions.     

A Frustrated and Confused Lewis Pair

We report a new class of frustrated Lewis pairs (FLPs) by the hydroboration of bulky isocyanates ^iPr2ArNCO (^iPr2Ar=2,6‐iPr₂C₆H₃) and ^Ph2tBuArNCO (^Ph2tBuAr=2,6‐Ph₂‐4‐tBuC₆H₂) with Piers’ borane (HB(C₆F₅)₂). While hydroboration of smaller isocyanates such as ^iPr2ArNCO leads to isocyanate—N/B FLP adducts, hydroboration of the bulkier ^Ph2tBuArNCO allows isolation of the substrate‐free aminoborane with a short, covalent N?B bond. This confused FLP reversibly binds unsaturated substrates such as isocyanates and isocyanides, suggesting the intermediacy of a “normal” FLP along the reaction pathway, supported by high‐level DFT studies and variable‐temperature NMR spectroscopy. These results underscore the possibility of FLP behavior in systems that possess no obvious frustrated Lewis acid–base interaction.     

Ab Initio Molecular Dynamics with Explicit Solvent Reveals a Two‐Step Pathway in the Frustrated Lewis Pair Reaction

The role solvent plays in reactions involving frustrated Lewis pairs (FLPs)—for example, the stoichiometric mixture of a bulky Lewis acid and a bulky Lewis base—still remains largely unexplored at the molecular level. For a reaction of the phosphorus/boron FLP and dissolved CO₂ gas, first principles (Born–Oppenheimer) molecular dynamics with explicit solvent reveals a hitherto unknown two‐step reaction pathway—one that complements the concerted (one‐step) mechanism known from the minimum‐energy‐path calculations. The rationalization of the discovered reaction pathway—that is, the stepwise formation of P?C and O?B bonds—is that the environment (typical organic solvents) stabilizes an intermediate which results from nucleophilic attack of the phosphorus Lewis base on CO₂. This finding is significant because presently the concerted reaction‐path paradigm predominates in the rationalization of FLP reactivity. Herein we point out how to attain experimental proof of our results.     

Functionalization of Intramolecular Frustrated Lewis Pairs by 1,1‐Carboboration with Conjugated Enynes

The vicinal P/B frustrated Lewis pair (FLP) Mes₂PCH₂CH₂B(C₆F₅)₂ undergoes 1,1‐carboboration reactions with the Me₃Si‐substituted enynes to give ring‐enlarged functionalized C₃‐bridged P/B FLPs. These serve as active FLPs in the activation of dihydrogen to give the respective zwitterionic [P]H⁺/[B]H^? products. One such product shows activity as a metal‐free catalyst for the hydrogenation of enamines or a bulky imine. The ring‐enlarged FLPs contain dienylborane functionalities that undergo “bora‐Nazarov”‐type ring‐closing rearrangements upon photolysis. A DFT study had shown that the dienylborane cyclization of such systems itself is endothermic, but a subsequent C₆F₅ migration is very favorable. Furthermore, substituted 2,5‐dihydroborole products are derived from cyclization and C₆F₅ migration from the photolysis reaction. In the case of the six‐membered annulation product, a subsequent stereoisomerization reaction takes place and the resultant compound undergoes a P/B FLP 1,2‐addition reaction with a terminal alkyne with rearrangement.     

Metal‐Free Hydrogenation Catalyzed by an Air‐Stable Borane: Use of Solvent as a Frustrated Lewis Base

In recent years ‘frustrated Lewis pairs’ (FLPs) have been shown to be effective metal‐free catalysts for the hydrogenation of many unsaturated substrates. Even so, limited functional‐group tolerance restricts the range of solvents in which FLP‐mediated reactions can be performed, with all FLP‐mediated hydrogenations reported to date carried out in non‐donor hydrocarbon or chlorinated solvents. Herein we report that the bulky Lewis acids B(C₆Cl₅)_x(C₆F₅)_3?x (x=0–3) are capable of heterolytic H₂ activation in the strong‐donor solvent THF, in the absence of any additional Lewis base. This allows metal‐free catalytic hydrogenations to be performed in donor solvent media under mild conditions; these systems are particularly effective for the hydrogenation of weakly basic substrates, including the first examples of metal‐free catalytic hydrogenation of furan heterocycles. The air‐stability of the most effective borane, B(C₆Cl₅)(C₆F₅)₂, makes this a practically simple reaction method.     

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.     

Photoinduced and Thermal Single-Electron Transfer to Generate Radicals from Frustrated Lewis Pairs

Archetypal phosphine/borane frustrated Lewis pairs (FLPs) are famed for their ability to activate small molecules. The mechanism is generally believed to involve two-electron processes. However, the detection of radical intermediates indicates that single-electron transfer (SET) generating frustrated radical pairs could also play an important role. These highly reactive radical species typically have significantly higher energy than the FLP, which prompted this investigation into their formation. Herein, we provide evidence that the classical phosphine/borane combinations PMes₃/B(C₆F₅)₃ and PtBu₃/B(C₆F₅)₃ both form an electron donor–acceptor (charge-transfer) complex that undergoes visible-light-induced SET to form the corresponding highly reactive radical-ion pairs. Subsequently, we show that by tuning the properties of the Lewis acid/base pair, the energy required for SET can be reduced to become thermally accessible.     

Ru–η6‐Arene Cations [{(Ph2PC6H4)2B(η6‐Ph)}RuX]+ (X=Cl,H) as Lewis Acids

The reactivity of [{(Ph₂PC₆H₄)₂B(η⁶‐Ph)}RuCl][B(C₆F₅)₄] ( 1 ) as a Lewis acid was investigated. Treatment of 1 with mono and multidentate phosphorus Lewis bases afforded the Lewis acid–base adducts with the ortho‐carbon atom of the coordinated arene ring. Similar reactivity was observed upon treatment with N‐heterocyclic carbenes; however, adduct formation occurred at both ortho‐ and para‐carbon atoms of the bound arene with the para‐position being favoured by increased steric demands. Interestingly treatment with isocyanides resulted in adduct formation with the B‐centre of the ligand framework. The hydride‐cation [{(Ph₂PC₆H₄)₂B(η⁶‐Ph)}RuH] [B(C₆F₅)₄] was prepared via reaction of 1 with silane. This species in the presence of a bulky phosphine behaves as a frustrated Lewis pair (FLP) to activate H₂ between the phosphorus centre and the ortho‐carbon atom of the η⁶‐arene ring.     

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.     

Metal‐Free Dihydrogen Oxidation by a Borenium Cation: A Combined Electrochemical/Frustrated Lewis Pair Approach

In order to use H₂ as a clean source of electricity, prohibitively rare and expensive precious metal electrocatalysts, such as Pt, are often used to overcome the large oxidative voltage required to convert H₂ into 2 H⁺ and 2 e^?. Herein, we report a metal‐free approach to catalyze the oxidation of H₂ by combining the ability of frustrated Lewis pairs (FLPs) to heterolytically cleave H₂ with the in situ electrochemical oxidation of the resulting borohydride. The use of the NHC‐stabilized borenium cation [(IiPr₂)(BC₈H₁₄)]⁺ (IiPr₂=C₃H₂(NiPr)₂, NHC=N‐heterocyclic carbene) as the Lewis acidic component of the FLP is shown to decrease the voltage required for H₂ oxidation by 910 mV at inexpensive carbon electrodes, a significant energy saving equivalent to 175.6 kJ mol^?1. The NHC–borenium Lewis acid also offers improved catalyst recyclability and chemical stability compared to B(C₆F₅)₃, the paradigm Lewis acid originally used to pioneer our combined electrochemical/frustrated Lewis pair approach.     

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.     

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.     

Phosphino-Phosphination Reactions: Frustrated Lewis Pair Reactivity of Phosphino-Phosphonium Cations with Alkynes

The phosphino-phosphonium cations of the form [R₃PPR′₂]⁺ are labile and provide access to the constituent Lewis acidic and Lewis basic fragments. This permits frustrated Lewis pair-type addition reactions to alkynes, affording unprecedented phosphino-phosphination reactions and giving cations of the form [cis-R₃PCHC(R′′)PR′₂]⁺. This reactivity is further adapted to prepare several examples of a rare class of dissymmetric cis-olefin-linked bidentate phosphines.     

Atypical “Masked” Frustrated Lewis Pair Character in a Geminal-Linked Phosphine-Borane

We report the synthesis and reactivity of the geminal-linked fluorene-derived P/B frustrated Lewis pair (FLP) ⁱPr₂P(C₁₃H₈)BCy₂ ( 1 ). Compound 1 displays anomalous behavior towards small molecules, acting as a “masked” FLP. This behavior stems from significant polarization of the B−C(fluorene) bond in 1 , as identified by density functional theory (DFT) computations. We exploit this B−C bond polarity through reactions with PhPCl₂, H₃N ⋅ BH₃, and ⁱPrNH₂, demonstrating P−Cl and N−H bond activation, respectively.     

Formation of Active Cyclic Five-membered Frustrated Phosphane/Borane Lewis Pairs and their Cycloaddition Reactions

The cyclic five-membered frustrated phosphane/borane Lewis pairs 11 a , b featuring the bulky octaethylhydrindacenyl- (Eind) substituent or its mono-bromo derivative (^BrEind) at phosphorus are monomeric at room temperature. The reactive frustrated Lewis pairs (FLPs) cleave dihydrogen. The cyclic FLP 11 b (^BrEind) undergoes 1,2-P/B addition to ethylene to give the zwitterionic heteronorbornane derivative 14 b . It reacts similarly with the carbon–carbon double bond of norbornene. With a variety of organic π-reagents, the cyclic FLP 11 b often undergoes reaction sequences reminiscent of the Alder–Rickert reaction: the cycloaddition reaction is followed by rapid cycloreversion to form new five-membered heterocyclic FLP products with extrusion of ethene. Reactions of 11 b with benzaldehyde or with acetylenes follow this reaction pattern.     

N‐Methylacridinium Salts: Carbon Lewis Acids in Frustrated Lewis Pairs for σ‐Bond Activation and Catalytic Reductions

N‐methylacridinium salts are Lewis acids with high hydride ion affinity but low oxophilicity. The cation forms a Lewis adduct with 4‐(N,N‐dimethylamino)pyridine but a frustrated Lewis pair (FLP) with the weaker base 2,6‐lutidine which activates H₂, even in the presence of H₂O. Anion effects dominate reactivity, with both solubility and rate of H₂ cleavage showing marked anion dependency. With the optimal anion, a N‐methylacridinium salt catalyzes the reductive transfer hydrogenation and hydrosilylation of aldimines through amine–boranes and silanes, respectively. Furthermore, the same salt is active for the catalytic dehydrosilylation of alcohols (primary, secondary, tertiary, and ArOH) by silanes with no observable over‐reduction to the alkanes.     

A Combined “Electrochemical–Frustrated Lewis Pair” Approach to Hydrogen Activation: Surface Catalytic Effects at Platinum Electrodes

Herein, we extend our “combined electrochemical–frustrated Lewis pair” approach to include Pt electrode surfaces for the first time. We found that the voltammetric response of an electrochemical–frustrated Lewis pair (FLP) system involving the B(C₆F₅)₃/[HB(C₆F₅)₃]^? redox couple exhibits a strong surface electrocatalytic effect at Pt electrodes. Using a combination of kinetic competition studies in the presence of a H atom scavenger, 6‐bromohexene, and by changing the steric bulk of the Lewis acid borane catalyst from B(C₆F₅)₃ to B(C₆Cl₅)₃, the mechanism of electrochemical–FLP reactions on Pt surfaces was shown to be dominated by hydrogen‐atom transfer (HAT) between Pt, [Pt?H] adatoms and transient [HB(C₆F₅)₃] ^? electrooxidation intermediates. These findings provide further insight into this new area of combining electrochemical and FLP reactions, and proffers additional avenues for exploration beyond energy generation, such as in electrosynthesis.     

Formylborane Formation with Frustrated Lewis Pair Templates

Boranes R₂BH react with carbon monoxide by forming the respective borane carbonyl compounds R₂BH(CO). The formation of (C₆F₅)₂BH(CO) derived from the Piers borane, HB(C₆F₅)₂, is a typical example. Subsequent CO‐hydroboration does not take place, since the formation of the formylborane is usually endothermic. However, an “η²‐formylborane” was formed by CO‐hydroboration with the Piers borane at vicinal phosphane/borane frustrated Lewis pair (FLP) templates. Subsequent treatment with pyridine liberated the intact formylborane from the FLP framework, and (pyridine)(C₆F₅)₂B? CHO was then isolated as a stable compound. This product underwent typical reactions of carbonyl compounds, such as Wittig olefination.     

Reactions of Rare‐Earth Metal Based Lewis Pairs with Azides†

The reactivity of homoleptic rare‐earth metal aryloxide based Lewis pairs toward organic azide substrates has been investigated herein. Treatment of RE(OAr)₃ (RE = La, Sm, Y, and Sc, Ar = 2,6‐^tBu₂‐C₆H₃), PEt₃ and Me₃SiN₃ in 2 : 1 : 1 molar ratio resulted in formation of separated ion pair complexes [Me₃Si‐PEt₃]⁺[(ArO)₃RE‐N=N=N‐RE(OAr)₃]^– under mild conditions. Replacement of phosphine with the nitrogen‐containing Lewis base 1,4‐diazabicyclo[2.2.2]octane (DABCO) produced analogous rare‐earth azide complexes with [Me₃Si‐DABCO]⁺ counterions. In contrast, reaction of the La(OAr)₃/PEt₃ Lewis pair with 1‐adamantyl azide (AdN₃) afforded the typical frustrated Lewis pair‐type 1,1‐addition product. A tetrahydrofuran ring‐opening reaction was also observed for the resulting rare‐earth azide complex with the [Me₃Si‐PEt₃]⁺ cation, with cleavage of the C—O bond by Si/P cooperation.     

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.