Frustrated Lewis Pair Reactivity Research Articles

Constructing frustrated Lewis pairs on Mo-doped Fe2O3 catalyst to enhance oxidative desulfurization efficiency

The emission of sulfur oxides will not only cause a series of environmental pollution but also threaten human health. At present, oxidative desulfurization is considered one of the technologies to achieve high-efficiency conversion of sulfur-containing compounds. Herein, the special frustrated Lewis pairs (FLPs) for efficient oxidative desulfurization are created by regulating surface doping in a redox iron-based oxide. According to experimental results, the Lewis acidic sites can be considered as the iron atoms adjacent to the molybdenum dopant, while the doped metal proximal to the oxygen vacancies serves as Lewis basic sites to create FLPs in redox iron-based oxide. This reactivity of FLPs is further adapted to the chemical activation of molecular oxygen and aromatic sulfur-containing compounds. The Fe2O3-FLP catalyst can achieve 100 % sulfur removal at 3 h and maintains 96.9 % after seven cycles, higher than other previously reported iron-based oxidative desulfurization catalysts. In addition, the design of FLPs offers a promising approach for developing high-efficiency catalysts in the oxidative desulfurization process.

Preferred Electric Field Mechanism for Frustrated Lewis Pair Reactivity.

This study employs computational methods to investigate the mechanism of H2 activation by frustrated Lewis pair (FLP) species, including both intermolecular and intramolecular nitrothane/borane FLP systems. Previous studies have proposed two qualitative reactivity mechanism models to explain the facile cleavage of H2 by FLPs. The findings of this study support the electric field mechanism as the favorable pathway for H2 cleavage. Utilizing frontier molecular orbital theory and energy decomposition analysis, the study explores the electronic structure and nature of the reactions under an external electric field (EEF). Analysis using the activation strain model highlights the significant influence of geometrical deformation energies of FLPs on the activation barriers of H2 activation reactions. Computational results suggest that H2 activation by FLP molecules follows the electric field mechanism, indicating the potential of the FLP/EEF combination as an effective activator for inert molecules.

Aluminum in Frustrated Lewis Pair Chemistry.

This review article describes the development of the use of aluminum compounds in the chemistry of frustrated Lewis pairs (FLPs) over the last 14 years. It also discusses the synthesis, reactivity and catalytic applications of intermolecular, intramolecular and so-called hidden FLPs with phosphorus, nitrogen and carbon Lewis bases. The intrinsically higher acidity of aluminum compounds compared to their boron analogs opens up different reaction pathways. The results are presented in a more or less chronological order. It is shown that Al FLPs react with a variety of polar and non-polar substrates and form both stable adducts and reversibly activate bonds. Consequently, some catalytic applications of the title compounds were presented such as dimerization of alkynes, hydrogenation of tert-butyl ethylene and imines, C-F bond activation, reduction of CO2, dehydrogenation of amine borane and transfer of ammonia. In addition, various Al FLPs were used as initiators in polymerization reactions.

Coordination chemistry and FLP reactivity of 1,1- and 1,2-bis-boranes.

Coordination chemistry and frustrated Lewis pair (FLP) chemistry have been most commonly studied using monodentate Lewis acids. In this paper, we examine the corresponding reactions employing the 1,1- and 1,2-bis-boranes, PhCH2CH(B(C6F5)2)21 and Me3SiCH(B(C6F5)2)CH2B(C6F5)22, respectively. Coordination of isocyanide to these species results in the formation of the products RCH(B(C6F5)2CNtBu)CH2(B(C6F5)2CNtBu) (R = Ph 3, Me3Si 4). The rearrangement of 1 to give the 1,2-bis-borane adduct 3 was probed and attributed to a donor-induced retrohydroboration and subsequent hydroboration. The analogous reaction of 1 is evident in efforts to use the Gutman-Beckett method to assess its Lewis acidity. However, in combination with tBu3P, bis-boranes 1 and 2 form FLPs and react with H2 to give [tBu3PH][PhCH2CH(B(C6F5)2)2(μ-H)] 5a and [tBu3PH][Me3SiCH(B(C6F5)2)CH2(B(C6F5)2)(μ-H)] 6, respectively. Reactions of 1 and 2 with various donors and PhCCH were shown to give deprotonation and addition products, depending on the nature of the base. However, in the case of 1, products resulting from retrohydroboration, and subsequent hydroboration are evident. Several of these alkyne products are crystallographically characterized.

A Facile Preparation of N-Heterocyclic Olefins: Ring-Opening Polymerization of β-Butyrolactone and Frustrated Lewis Pair Reactivity.

A direct synthesis of N-heterocyclic olefins (NHOs) and their mesoionic congeners (mNHOs) from N-heterocyclic carbenes and N-aziridinylimines is reported. The reaction provided diverse functionalized (m)NHOs and π-extended analogues. The prepared NHOs initiated the ring-opening polymerization of β-butyrolactone, and insertion of aldehyde and nitrile into an NHO-B(C6 F5 )3 adduct was demonstrated.

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

The phosphino-phosphonium cations of the form [R3 PPR'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-R3 PCHC(R'')PR'2 ]+ . This reactivity is further adapted to prepare several examples of a rare class of dissymmetric cis-olefin-linked bidentate phosphines.

Phosphino‐phosphinations: 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.

Frustrated Lewis Pair Catalyzed Reactions

AbstractIn recent years, frustrated Lewis pairs have been widely used for the activation of small molecules and in catalytic transformations. This graphical review aims to provide a fundamental understanding of frustrated Lewis pair reactivity and the exploitation thereof in catalytic reactions.

Metal‐Free Catalytic Functionalization of Second −Csp2−H Bond of 1‐Methyl Pyrrole Using Bishomocubane‐Derived Aminoborane Frustrated Lewis Pairs: A Computational Study

AbstractBishomocubane, as a molecular scaffold, has been designed to functionalize the −Csp2−H bond of 1‐methyl pyrrole. The newly designed molecular scaffold showed noteworthy improvements in the reaction energetics than the phenylene scaffold. The DFT (ωB97XD/6‐31+G(d,p)) calculations revealed that the dimethyl amine (A) and piperidine (B) analogues of bishomocubane frustrated Lewis pairs (FLP) can catalyze the first −Csp2−H functionalization much more easily than that of the phenylene FLP. The previous results showed that the second −Csp2−H functionalization was found to be energetically difficult. The DFT calculations suggest that the piperidine analogue of bishomocubane, 1‐Pip‐2‐BH2‐C10H10 (B), can lead the second −Csp2−H functionalization facile than the reported FLPs. The rate‐determining step of the second −Csp2−H functionalization is significantly lower with the catalyst (B) by ∼9.0 kcal/mol compared to the phenylene catalyst (II). The improved reactivity of bishomocubane FLPs has been examined with molecular electrostatic potential (MESP) analysis and the conceptual density functional theory (CDFT) calculations. The hyperconjugative stabilization interaction (n→σ*) and the strain relief between the Lewis pairs play a crucial role in the stability of rate‐determining transition states. This study reveals that saturated hydrocarbon scaffolds are promising candidates as FLP catalysts in such reactions.

An Isolable Phosphaborene Stabilized by an Intramolecular Lewis Base

AbstractPhosphaborenes are P/B analogs of alkynes. Herein, we describe the isolation of a phosphaborene stabilized by an intramolecular Lewis base. This species is achieved via the installation of the phosphaborene with a sterically encumbering phosphino substituent. Although this species features an aromatic P2B three‐membered ring, the ring strain allows the hemi‐labile behavior of the P−B donor‐acceptor bond, thereby exhibiting frustrated Lewis pair reactivity. Our finding paves the way for further exploration of the reactivity patterns of monomeric phosphaborenes.

The Varied Frustrated Lewis Pair Reactivity of the Germylene Phosphaketene (CH{(CMe)(2,6-i Pr2 C6 H3 N)}2 )GePCO.

The germylene species (CH{(CMe)(2,6‐iPr2C6H3N)}2)GePCO 1 is shown to react with the Lewis acids (E(C6F5)3 E=B, Al). Nonetheless, 1 participates in FLP chemistry with electron deficient alkynes or olefins, acting as an intramolecular FLP. In contrast, in the presence of B(C6F5)3 and an electron rich alkyne, 1 behaves as Ge‐based nucleophile to effect intermolecular FLP addition to the alkyne. This reactivity demonstrates that the reaction pathway is controlled by the nature of the electrophile and nucleophile generated in solution, as revealed by extensive DFT calculations.

Polymeric frustrated Lewis pairs in CO2/cyclic ether coupling catalysis.

Frustrated Lewis pairs (FLPs) are now ubiquitous as metal-free catalysts in an array of different chemical transformations. In this paper we show that this reactivity can be transferred to a polymeric system, offering advantageous opportunities at the interface between catalysis and stimuli-responsive materials. Formation of cyclic carbonates from cyclic ethers using CO2 as a C1 feedstock continues to be dominated by metal-based systems. When paired with a suitable nucleophile, discrete aryl or alkyl boranes have shown significant promise as metal-free Lewis acidic alternatives, although catalyst reuse remains illusive. Herein, we leverage the reactivity of FLPs in a polymeric system to promote CO2/cyclic ether coupling catalysis that can be tuned for the desired epoxide or oxetane substrate. Moreover, these macromolecular FLPs can be reused across multiple reaction cycles, further increasing their appeal over analogous small molecule systems.

Can Superhalogen Ligand Make More Reactive Frustrated Lewis Pairs?

AbstractNew Frustrated Lewis Pairs (FLPs) are designed and are attempted to activate molecular hydrogen. The new FLPs are modeled by changing the ligands in the Lewis acid (LA) part of the TPFPB (Tris(pentafluorophenyl)borane) molecule. The modifications are done by replacing the phenyl group in TPFPB with three different aromatic heterocyclic ligands. The stability and reactivity of FLPs: [(C2BNSHF2)]3B−P(tBu)3, [(C2BNSF3)]3B−P(tBu)3 and [C2BNO(CN)3]3B−P(tBu)3 is analysed in terms of the dual descriptors, NBO/Hirshfeld charges, VDE, hardness (η) and electrophilicity (ω). Although all three modeled FLPs show their capability towards activating molecular hydrogen, the [C2BNO(CN)3]3B−P(tBu)3 shows better performance than the conventionally used FLP: (C6F5)3B−P(tBu)3.

Synergistic Catalysis by Brønsted Acid/Carbodicarbene Mimicking Frustrated Lewis Pair‐Like Reactivity

AbstractCarbodicarbene (CDC), unique carbenic entities bearing two lone pairs of electrons are well‐known for their strong Lewis basicity. We demonstrate herein, upon introducing a weak Brønsted acid benzyl alcohol (BnOH) as a co‐modulator, CDC is remolded into a Frustrated Lewis Pair (FLP)‐like reactivity. DFT calculation and experimental evidence show BnOH loosely interacting with the binding pocket of CDC via H‐bonding and π‐π stacking. Four distinct reactions in nature were deployed to demonstrate the viability of proof‐of‐concept as synergistic FLP/Modulator (CDC/BnOH), demonstrating enhanced catalytic reactivity in cyclotrimerization of isocyanate, polymerization process for L‐lactide (LA), methyl methacrylate (MMA) and dehydrosilylation of alcohols. Importantly, the catalytic reactivity of carbodicarbene is uniquely distinct from conventional NHC which relies on only single chemical feature of nucleophilicity. This finding also provides a new spin in diversifying FLP reactivity with co‐modulator or co‐catalyst.

Cationic Aluminium Complexes as Catalysts for Imine Hydrogenation

Strongly Lewis acidic cationic aluminium complexes, stabilized by β–diketiminate (BDI) ligands and free of Lewis bases, have been prepared as their B(C6F5)4 − salts and were investigated for catalytic activity in imine hydrogenation. The backbone (R1) and N (R2) substituents on the R1,R2BDI ligand (R1,R2BDI=HC[C(R1)N(R2)]2) influence sterics and Lewis acidity. Ligand bulk increases along the row Me,DIPPBDI<Me,DIPePBDI≈tBu,DIPPBDI<tBu,DIPePBDI; DIPP=2,6‐C(H)Me2‐phenyl, DIPeP=2,6‐C(H)Et2‐phenyl. The Gutmann‐Beckett test showed acceptor numbers of: (tBu,DIPPBDI)AlMe+ 85.6, (tBu,DIPePBDI)AlMe+ 85.9, (Me,DIPPBDI)AlMe+ 89.7, (Me,DIPePBDI)AlMe+ 90.8, (Me,DIPPBDI)AlH+ 95.3. Steric and electronic factors need to be balanced for catalytic activity in imine hydrogenation. Open, highly Lewis acidic, cations strongly coordinate imine rendering it inactive as a Frustrated Lewis Pair (FLP). The bulkiest cations do not coordinate imine but its combination is also not an active catalyst. The cation (tBu,DIPPBDI)AlMe+ shows the best catalytic activity for various imines and is also an active catalyst for the Tishchenko reaction of benzaldehyde to benzylbenzoate. DFT calculations on the mechanism of imine hydrogenation catalysed by cationic Al complexes reveal two interconnected catalytic cycles operating in concert. Hydrogen is activated either by FLP reactivity of an Al⋅⋅⋅imine couple or, after formation of significant quantities of amine, by reaction with an Al⋅⋅⋅amine couple. The latter autocatalytic Al⋅⋅⋅amine cycle is energetically favoured.

N‐Heterocyclic Silylene Main Group Element Chemistry: Adduct Formation, Insertion into E−X Bonds and Cyclization of Organoazides

AbstractInvestigations concerning the reactivity of the N‐heterocyclic silylene Dipp2NHSi (1, 1,3‐bis(2,6‐diisopropylphenyl)‐1,3‐diaza‐2‐silacyclopent‐4‐en‐2‐ylidene) towards selected alanes and boranes, elemental halides X2 (X=Br, I), selected halide containing substrates such as tin chlorides and halocarbons, as well as organoazides are presented. The NHSi adducts Dipp2NHSi⋅AlI3 (2), Dipp2NHSi⋅Al(C6F5)3 (3), and Dipp2NHSi⋅B(C6F5)3 (4) were formed by the reaction of Dipp2NHSi with the corresponding Lewis acids AlI3, Al(C6F5)3 and B(C6F5)3. Adducts 3 and 4 were tested with respect to their ability to activate small organic molecules, but no frustrated Lewis pair reactivity was observed. Reactions of Dipp2NHSi with Br2, I2, Ph2SnCl2 and Me3SnCl led to formation of Dipp2NHSiBr2 (5), Dipp2NHSiI2 (6), Dipp2NHSiCl2 (7) and {(Me3Sn)N(Dipp)CH}2 (8), respectively. The reaction with the halocarbons methyl iodide, benzyl chloride, and benzyl bromide afforded the insertion products Dipp2NHSi(I)(CH3) (9), Dipp2NHSi(Cl)(CH2Ph) (10) and Dipp2NHSi(Br)(CH2Ph) (11). Reaction of Dipp2NHSi with the organoazides Ad‐N3 (Ad=adamantyl) and TMS‐N3 (TMS=trimethylsilyl) led to the formation of 1‐Dipp2NHSi‐2,5‐bis(adamantyl)‐tetrazoline (12) and bis(trimethylsilyl)amido azido silane (13), respectively. For 2,6‐(diphenyl)phenyl‐N3 C−H activation occurs and a cyclosilamine 14 was isolated.

Formation of Nucleophilic Allylboranes from Molecular Hydrogen and Allenes Catalyzed by a Pyridonate Borane that Displays Frustrated Lewis Pair Reactivity.

Here we report the in situ generation of nucleophilic allylboranes from H2 and allenes mediated by a pyridonate borane that displays frustrated‐Lewis‐pair reactivity. Experimental and computational mechanistic investigations reveal that upon H2 activation, the covalently bound pyridonate substituent becomes a datively bound pyridone ligand. Dissociation of the formed pyridone borane complex liberates Piers borane and enables a hydroboration of the allene. The allylboranes generated in this way are reactive towards nitriles. A catalytic protocol for the formation of allylboranes from H2 and allenes and the allylation of nitriles has been devised. This catalytic reaction is a conceptually new way to use molecular H2 in organic synthesis.

The activity and reversibility of intramolecular B/N Frustrated Lewis Pairs in the hydrogenation

Frustrated Lewis Pairs (FLPs) as the zwitter compound is not only an effective catalyst in the hydrogenation process, but also a potential excellent hydrogen-storage material. Therefore, the activity and the reversibility of the ab-/desorbing hydrogen process of FLPs are generally determined by the acidity of the Lewis acid (LA) and the alkalinity of the Lewis base (LB). Herein, we learn three classic B/N intramolecular FLPs composed by Pier's acid and N-heterocyclic groups to evaluate the relationship between the energy of deformation of LA/LB and the reactivity of FLPs in the hydrogenation. The acidity is determined by the energy of structural transformation in activating hydrogen, while the alkalinity of LB is determined by the electron density of connected groups. Different from the traditional point that the strong LA and LB form a perfect FLPs, the excellent FLPs appears reversibility and fast reaction rate in the hydrogenation when it composed by a strong LA (72.97 = GA ≤ 120.15 kcal/mol) and a relative weak LB (GB ≥ 155.28 kcal/mol). The results help us to establish a quantitative relationship between the deformation energy of FLPs and the reversibility/activity in the Ab-/Desorbing hydrogen process of the intramolecular B/N FLPs.

Semihydrogenation of Alkynes Catalyzed by a Pyridone Borane Complex: Frustrated Lewis Pair Reactivity and Boron–Ligand Cooperation in Concert

The metal‐free cis selective hydrogenation of alkynes catalyzed by a boroxypyridine is reported. A variety of internal alkynes are hydrogenated at 80 °C under 5 bar H2 with good yields and stereoselectivity. Furthermore, the catalyst described herein enables the first metal‐free semihydrogenation of terminal alkynes. Mechanistic investigations, substantiated by DFT computations, reveal that the mode of action by which the boroxypyridine activates H2 is reminiscent of the reactivity of an intramolecular frustrated Lewis pair. However, it is the change in the coordination mode of the boroxypyridine upon H2 activation that allows the dissociation of the formed pyridone borane complex and subsequent hydroboration of an alkyne. This change in the coordination mode upon bond activation is described by the term boron‐ligand cooperation.

Synthesis and Reactivity of (NHC)AuI–Mercaptopyridine Complexes

Reaction of (NHC)Au–X (X = halide) complexes with 2- or 4-mercaptopyridine under basic conditions allows for the preparation of a series of (NHC)AuI–mercaptopyridine complexes 1–4 in high yields. All complexes have been fully characterized by means of 1H and 13C NMR spectroscopy, melting point, and elemental analysis. Single crystals of complexes 1–3 were obtained, and the solid-state structures display a linear Au(I) center with coordination to the NHC ligand and the sulfur atom of the mercaptopyridine moiety. Interestingly, weak γ-anagostic C–H···Au interactions are found in complexes 1–3 regardless of the relative N or S position in the mercaptopyridine. Taking advantage of the free pyridine nitrogen, reaction of complexes 1 and 2 with palladium allyl chloride dimer permits the isolation of heteronuclear Au/Pd complexes 5 and 8 featuring a bridging ambidentate mercaptopyridine ligand. Further reactivity of complexes 1–3 toward B(C6F5)3 produces complexes 6, 7, and 9, which display frustrated Lewis pair reactivity with the activation of a H–F bond.