Noninteracting, Vicinal Frustrated P/B-Lewis Pair at the Norbornane Framework: Synthesis, Characterization, and Reactions

Synthesis and Photochemical Uncaging of Alkene-Protected, Polymer-Bound Vicinal Frustrated Lewis Pairs

Polymeric materials bearing Frustrated Lewis Pair (FLP) functionality are promising candidates for use as heterogeneous catalysts and adaptive materials, but synthetic access to FLP-functional polymers remains limited due to the incompatibility of FLPs with standard polymerization chemistries. Herein, we describe a synthetic approach that "cages" highly reactive vicinal phosphine-borane FLPs as covalent alkene adducts, which are stable to Ni-mediated vinyl addition polymerization. We discovered that the caged FLP adducts can be photochemically activated to liberate vicinal FLPs, enabling spatiotemporally controlled release of FLPs from polymeric precursors.

Nitrogen monoxide and calix[4]pyrrolato aluminate: structural constraint enabled NO dimerization

The dimerization of nitrogen monoxide (NO) is highly relevant in homo- and heterogeneous biochemical and environmental redox processes, but a broader understanding is challenged by the endergonic nature of this equilibrium. The present work describes NO-dimerization leveraged by structurally constrained aluminum and metal-ligand cooperativity at the anionic calix[4]pyrrolato aluminate(III). Quantum chemical calculations reveal the driving force for N-N bond formation, while reactivity tests shed light on subsequent redox chemistry and NO decomposition at metal surfaces. Inhibiting the dimerization pathway by saturating NO's unpaired electron with a phenyl group (nitrosobenzene) allows trapping the 1,2-adduct as a key intermediate. Elevated temperatures result in an unprecedented and high-yielding rearrangement of the calix[4]pyrrolato ligand scaffold. Kinetic and theoretical studies provide a comprehensive picture of the rearrangement mechanism and delineate systematics for ring modification of the prominent calix[4]pyrrole macrocycle.

CeO2-Based Frustrated Lewis Pairs via Defective Engineering: Formation Theory, Site Characterization, and Small Molecule Activation

Activation of small molecules is considered to be a central concern in the theoretical investigation of environment- and energy-related catalytic conversions. Sub-nanostructured frustrated Lewis pairs (FLPs) have been an emerging research hotspot in recent years due to their advantages in small molecule activation. Although the progress of catalytic applications of FLPs is increasingly reported, the fundamental theories related to the structural formation, site regulation, and catalytic mechanism of FLPs have not yet been fully developed. Given this, it is attempted to demonstrate the underlying theory of FLPs formation, corresponding regulation methods, and its activation mechanism on small molecules using CeO2 as the representative metal oxide. Specifically, this paper presents three fundamental principles for constructing FLPs on CeO2 surfaces, and feasible engineering methods for the regulation of FLPs sites are presented. Furthermore, cases where typical small molecules (e.g., hydrogen, carbon dioxide, methane oxygen, etc.) are activated over FLPs are analyzed. Meanwhile, corresponding future challenges for the development of FLPs-centered theory are presented. The insights presented in this paper may contribute to the theories of FLPs, which can potentially provide inspiration for the development of broader environment- and energy-related catalysis involving small molecule activation.

Mechanistic Insight Into the Reactivity of Frustrated Lewis Pairs: Liquid-State NMR Studies

Frustrated Lewis pairs (FLPs) have been widely investigated as promising catalysts due to their metal-free feature and ability to activate small molecules. Over the last few years, the structure, dynamics and interactions between the Lewis centers and their effects on the reactivity with different substrates have been studied. Nuclear magnetic resonance (NMR) is a powerful tool in studying the reaction intermediates, kinetics and mechanism of frustrated Lewis pairs (FLPs). Various NMR experiments have been applied to precisely determine the association or cooperativity of FLPs and one or two-dimensional spectra were obtained. Herein, insights coming from NMR spectroscopy for FLPs are presented, the structure and reactivity of FLPs in solution are described, and their effects on the kinetics and mechanism of different substrates are also illustrated in this review.

Systematic Assessment of the Catalytic Reactivity of Frustrated Lewis Pairs in C-H Bond Activation

Unreactive C-H bond activation is a new horizon for frustrated Lewis pair (FLP) chemistry. This study provides a systematic assessment of the catalytic reactivity of recently reported intra-molecular FLPs on the activation of typical inert C-H bonds, including 1-methylpyrrole, methane, benzyl, propylene, and benzene, in terms of density functional theory (DFT) calculations. The reactivity of FLPs is evaluated according to the calculated reaction thermodynamic and energy barriers of C-H bond activation processes in the framework of concerted C-H activation mechanisms. As for 1-methylpyrrole, 14 types of N-B-based and 15 types of P-B-based FLPs are proposed to be active. Although none of the evaluated FLPs are able to catalyze the C-H activation of methane, benzyl, or propylene, four types of N-B-based FLPs are suggested to be capable of catalyzing the activation of benzene. Moreover, the influence of the strength of Lewis acid (LA) and Lewis base (LB), and the differences between the influences of LA and LB on the catalytic reactivity of FLPs, are also discussed briefly. This systematic assessment of the catalytic activity of FLPs should provide valuable guidelines to aid the development of efficient FLP-based metal-free catalysts for C-H bond activation.

Frustrated Lewis Pair Chelation in the p-Block

Frustrated Lewis pairs (FLPs) have been the subject of considerable study since the field's inception. While much of the research into FLPs has centered around small molecule activation for diverse stoichiometric and catalytic transformations, intramolecular FLPs also show promise as chelating ligands. The cooperative action of Lewis basic and acidic moieties enables intramolecular FLPs to stabilize low oxidation state centers and (consequently) reactive molecular fragments through a donor-acceptor approach, making them an attractive ligand class in main group element chemistry. This review outlines the state of FLP chelation to date throughout the p-block, encompassing primarily groups 13-16.

Insights into Single-Electron-Transfer Processes in Frustrated Lewis Pair Chemistry and Related Donor-Acceptor Systems in Main Group Chemistry

The activation and utilization of substrates mediated by Frustrated Lewis Pairs (FLPs) was initially believed to occur solely via a two-electron, cooperative mechanism. More recently, the occurrence of a single-electron transfer (SET) from the Lewis base to the Lewis acid was observed, indicating that mechanisms that proceed via one-electron-transfer processes are also feasible. As such, SET in FLP systems leads to the formation of radical ion pairs, which have recently been more frequently observed. In this review, we aim to discuss the seminal findings regarding the recently established insights into the SET processes in FLP chemistry as well as highlight examples of this radical formation process. In addition, applications of reported main group radicals will also be reviewed and discussed in the context of the understanding of SET processes in FLP systems.

Selective 3,3-Rearrangement of Azobenzenes upon Complexation by a Frustrated Lewis Pair

The reaction of the oxygen-bridged frustrated Lewis pairs (FLPs) tBu₂ P-O-Si(C₂ F₅ )₃ (1) and tBu₂ P-O-AlBis₂ (2) with azobenzene, promoted by UV irradiation, led to a selective complexation of the cis-isomer. The addition product of 2 is stable, while the adduct of 1 isomerizes in solution in an ortho-benzidine-like [3,3]-rearrangement by cleavage of the N-N bond, saturation of the nitrogen atoms with hydrogen atoms and formation of a new bond between two phenyl ortho-carbon atoms. Similar rearrangements take place with different para-substituted azobenzenes (R=Me, OMe, Cl) and di(2-naphthyl)diazene, while ortho-methylated azo compounds do not form adducts with 1. All adducts were characterized by multinuclear NMR spectroscopy and elemental analyses and the mechanism of the rearrangement was explored by quantum-chemical calculations.

Intramolecular frustrated Lewis pair mediated approach to the CO bond activation and cleavage of carbon dioxide

A thermally stable FLP-CO₂ adduct of pronounced nucleophilic properties that forms a range of Lewis acid-base adducts with strong Lewis acids is reported. Upon addition of Tf₂O, it generates a cationic triflate, which undergoes C-O bond cleavage to give the formal FLP adduct of the elusive dication C₂O₃²⁺.

Controlling the Lewis Acidity and Polymerizing Effectively Prevent Frustrated Lewis Pairs from Deactivation in the Hydrogenation of Terminal Alkynes

Two strategies were reported to prevent the deactivation of Frustrated Lewis pairs (FLPs) in the hydrogenation of terminal alkynes: reducing the Lewis acidity and polymerizing the Lewis acid. A polymeric Lewis acid (P-BPh₃) with high stability was designed and synthesized. Excellent conversion (up to 99%) and selectivity can be achieved in the hydrogenation of terminal alkynes catalyzed by P-BPh₃. This catalytic system works quite well for different substrates. In addition, the P-BPh₃ can be easily recycled.

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.

A BH Borenium-Derived Thioxoborane, Its Persulfide, and Their Li+-Induced Reactions with Alkynes and with Carbon Dioxide

Insertion of sulfur into the B-H bond of the BH borenium salt [IMes(C₆F₅)BH]⁺ followed by deprotonation gave the thioxoborane IMes(C₆F₅)B═S. Subsequent treatment with additional sulfur gave the corresponding boron persulfide, a NHC-stabilized boradithiirane. The B═S compound reacted with carbon dioxide in the presence of the lithium salt Li[B(C₆F₅)₄] by formal [2+2] cycloaddition to give a boron thiocarbonate-type product. The boron persulfide formally inserted phenyl acetylene into the B-S bond in the presence of Li[B(C₆F₅)₄] to give the respective five-membered heterocycle.

Solid-State NMR Techniques for the Structural Characterization of Cyclic Aggregates Based on Borane-Phosphane Frustrated Lewis Pairs

Modern solid-state NMR techniques offer a wide range of opportunities for the structural characterization of frustrated Lewis pairs (FLPs), their aggregates, and the products of cooperative addition reactions at their two Lewis centers. This information is extremely valuable for materials that elude structural characterization by X-ray diffraction because of their nanocrystalline or amorphous character, (pseudo-)polymorphism, or other types of disordering phenomena inherent in the solid state. Aside from simple chemical shift measurements using single-pulse or cross-polarization/magic-angle spinning NMR detection techniques, the availability of advanced multidimensional and double-resonance NMR methods greatly deepened the informational content of these experiments. In particular, methods quantifying the magnetic dipole-dipole interaction strengths and indirect spin-spin interactions prove useful for the measurement of intermolecular association, connectivity, assessment of FLP-ligand distributions, and the stereochemistry of adducts. The present review illustrates several important solid-state NMR methods with some insightful applications to open questions in FLP chemistry, with a particular focus on supramolecular associates.

Twenty-five years of bis-pentafluorophenyl borane: a versatile reagent for catalyst and materials synthesis

Highlights of the extensive chemistry and applications of bis-pentafluorophenyl borane (“Piers’ borane”) from the 25 years since its first appearance are featured.

Dihydrogen Splitting by Intramolecular Borane-Phosphane Frustrated Lewis Pairs: A Comprehensive Characterization Strategy Using Solid State NMR and DFT Calculations

Four hydrogenated intramolecular phosphane-borane frustrated Lewis pair (B/P FLP) compounds bearing unsaturated cyclic or aromatic carbon backbones have been synthesized and structurally characterized using ¹¹ B, ³¹ P, ¹ H and ² H solid-state NMR spectroscopy. A comparison of the spectra with those of the corresponding free B/P FLPs shows that both ¹¹ B isotropic chemical shifts as well as nuclear electric quadrupolar coupling constants decrease significantly upon FLP hydrogenation, revealing the breakage of the partial B-P bond present in the starting materials. Likewise, the ³¹ P isotropic chemical shift, the chemical shift anisotropy, and the asymmetry parameter decrease significantly upon FLP hydrogenation, reflecting the formation of a more symmetric, C_3v -like local environment. ¹¹ B{³¹ P} rotational echo double resonance (REDOR) experiments can be used to measure the B-P internuclear distance (about 3.2 Å) of these compounds. Observation of the hydrogen atoms bound to the Lewis centers is best accomplished via ³¹ P{¹ H} and ¹¹ B{¹ H} cross-polarization-heteronuclear correlation experiments or by direct observation of the ² H MAS NMR signals on especially prepared FLP-D₂ adducts. For accurately measuring the phosphorus-deuterium distance via ³¹ P{² H} rotational echo adiabatic passage double resonance (REAPDOR), it is essential to take the secondary dipolar coupling of ³¹ P with the boron-bonded ² H nuclei explicitly into consideration, by simulating a ² H_P -³¹ P-² H_B three-spin system based on structural input. All of the experimental NMR interaction parameters are found in excellent agreement with values calculated by DFT methods, using the geometries obtained either by energy optimization or from single-crystal structures.

Radicals derived from Lewis acid/base pairs

While conventional approaches to stabilizing main group radicals have involved the use of Lewis acids or bases, this tutorial review focuses on new avenues to main group radicals derived from combinations of donor and acceptor molecules.

Homolytic cleavage of peroxide bonds via a single electron transfer of a frustrated Lewis pair

Single electron transfer (SET) reactions are effected by the combination of a Mes₃P/B(C₆F₅)₃ frustrated Lewis pair with benzoyl peroxide derivatives. The resulting homolytic cleavage of the peroxide bonds affords the radical salts [Mes₃P˙]⁺[RCOOB(C₆F₅)₃]^- (R = Ph 1, p-BrC₆H₄2, p-MeC₆H₄3). These species react with Ph₃SnH to give [Mes₃PH]⁺[RCOOB(C₆F₅)₃]^- (R = Ph 10, p-BrC₆H₄11, p-MeC₆H₄12) and (Ph₃Sn)₂. The mechanism of these SET reactions is discussed in light of control experiments and DFT calculations.

Formation of macrocyclic ring systems by carbonylation of trifunctional P/B/B frustrated Lewis pairs

The trifunctional P/B/B frustrated Lewis pairs 11a-c featuring bulky aryl groups at phosphorus [Dmesp (a), Tipp (b), Mes* (c)] react with H2 by heterolytic hydrogen splitting followed by cleavage of HB(C6F5)2 to give the zwitterionic six-membered heterocyclic PH phosphonium/borate products 14a-c. Compounds 11a,b react with carbon monoxide by means of a Lewis acid induced CO insertion/rearrangement sequence that eventually results in the formation of the macrocyclic dimers 17a,b. The respective carbonylation reaction of the Mes*P/B/B FLP gives the macrocyclic trimer 18c. The new products were characterized spectroscopically and by X-ray diffraction and the reaction mechanism was analyzed by DFT calculations.

Interception of intermediates in phosphine oxidation by mesityl nitrile-N-oxide using frustrated Lewis pairs

While phosphine oxidation by MesCNO is rapid, FLPs can be used to intercept 1,3-addition intermediates. These species react with tBuOK or [Bu₄N]F permitting the oxidation to proceed.

(Dicyclohexyl(2-(dimesitylboryl)phenyl)phosphine: en route to stable frustrated Lewis pairs-hydrogen adducts in water

The new ansa-phosphinoborane (dicyclohexyl(2-(dimesitylboryl)phenyl)phosphine was synthesized via an one-pot protocol in 67% yield. The compound has been characterized by 1H, 13C, 11B and 31P NMR, and its solid-state structure determined by a single crystal X-ray diffraction analysis. The ansa-phosphinoborane does not react with molecular hydrogen or water at room or elevated temperature. According to performed DFT studies, heterolytic splitting of water or hydrogen by the phosphinoborane are both endergonic but close in thermodynamics. In polar solvents, such as in methanol or acetonitrile, addition of hydrogen is energetically more favorable than of water.

CO/CO and NO/NO coupling at a hidden frustrated Lewis pair template

N-Allyltetramethylpiperidine is readily isomerized to the corresponding enamine by treatment with catalytic amounts of B(C6F5)3. It adds HB(C6F5)2 at the nucleophilic enamine carbon atom to form a C/B Lewis adduct. This reacts with two molar equivalents of carbon monoxide by selective head to tail coupling to give a five-membered C2O2B heterocycle. In contrast the enamine/HB(C6F5)2 Lewis pair reacts with two molar equiv. of nitric oxide by head to head coupling. This reaction probably proceeds via equilibrium with the corresponding vicinal N/B Lewis pair. Most products were characterized by X-ray diffraction.

The Chemistry of a Non-Interacting Vicinal Frustrated Phosphane/Borane Lewis Pair

The dimesitylphosphinocyclopentene/HB(C₆ F₅ )₂ -derived vicinal trans-1,2-P/B frustrated Lewis pair (FLP) 4 shows no direct phosphane-borane interaction. Toward some reagents it behaves similar to an intermolecular FLP; it cleaves dihydrogen, deprotonates terminal alkynes, and adds to organic carbonyl compounds including CO₂ . It shows typical intramolecular FLP reaction modes (cooperative 1,1-additions) to mesityl azide, to carbon monoxide, and to NO. The latter reaction yields a persistent P/B FLPNO nitroxide radical, which undergoes H-atom abstraction reactions. The FLP 4 serves as a template for the CO reduction by [HB(C₆ F₅ )₂ ] to generate a FLP-η² -formylborane. The formylborane moiety is removed from the FLP template by reaction with pyridine to yield a genuine pyridine stabilized formylborane that undergoes characteristic borane carbaldehyde reactions (Wittig olefination, imine formation). Most new products were characterized by X-ray diffraction.

An Ethylene-Bridged Phosphane/Borane Frustrated Lewis Pair Featuring the -B(Fxyl)2 Lewis Acid Component

Hydroboration of dimesitylvinylphosphane with bis[3,5-bis(trifluoromethyl)phenyl]borane [HB(Fxyl)2 ] gave the intramolecular ethylene-bridged P/B frustrated Lewis pair (FLP) Mes2 PCH2 CH2 B(Fxyl)2 . The new compound underwent a variety of typical FLP reactions such as P/B-addition to the carbonyl group of p-chloro-benzaldehyde. Cooperative N,N-addition to nitric oxide gave the respective persistent P/B FLPNO(.) radical, which readily reacted with 1,4-cyclohexadiene by H-atom abstraction to yield the corresponding P/B FLPNOH product. The B(Fxyl)2 -containing FLP reacted as a template for the HB(C6 F5 )2 reduction of carbon monoxide to the formyl stage to give the respective FLP(η(2) -formylborane) product. Most products were characterized by single-crystal X-ray crystal structure analysis.

Aminoxyl Radicals of B/P Frustrated Lewis Pairs: Refinement of the Spin-Hamiltonian Parameters by Field- and Temperature-Dependent Pulsed EPR Spectroscopy

Q-band and X-band pulsed electron paramagnetic resonance spectroscopic methods (EPR) in the solid state were employed to refine the parameters characterizing the anisotropic interactions present in six nitroxide radicals prepared by N,N addition of NO to various borane-phosphane frustrated Lewis pairs (FLPs). The EPR spectra are characterized by the g-anisotropy as well as by nuclear hyperfine coupling between the unpaired electron and the 11B/10B, 14N and 31P nuclear magnetic moments. It was previously shown that continuous-wave spectra measured at X-band frequency (9.5 GHz) are dominated by the magnetic hyperfine coupling to 14N and 31P, whereas the g-tensor values and the 11B hyperfine coupling parameters cannot be refined with high precision from lineshape fitting. On the other hand, the X-band electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation (HYSCORE) spectra are completely dominated by the nuclear hyperfine coupling to the 11B nuclei, allowing a selective determination of their interaction parameters. In the present work this analysis has been further validated by temperature dependent ESEEM measurements. In addition, pulsed EPR data measured in the Q-band (34 GHz) are reported, which present an entirely different situation: the g-tensor components can be measured with much higher precision, and the ESEEM and HYSCORE spectra contain information about all of the 10B, 11B, 14N and 31P hyperfine interaction parameters. Based on these new results, we report here high-accuracy and precision data of the EPR spin Hamiltonian parameters measured on six FLP-NO radical species embedded in their corresponding hydroxylamine host structures. While the ESEEM spectra at Q-band frequency turn out to be very complex (due to the multinuclear contribution to the overall signal) in the HYSCORE experiment the extension over two dimensions renders a better discrimination between the different nuclear species, and the signals arising from hyperfine coupling to 10B, 11B, 14N and 31P nuclei can be individually analyzed.

A Highly Reactive Geminal P/B Frustrated Lewis Pair: Expanding the Scope to C-X (X=Cl, Br) Bond Activation

The geminal frustrated Lewis pair tBu2 PCH2 B(Fxyl)2 (1; Fxyl=3,5-(CF3 )2 C6 H3 ) is accessible in 65 % yield from tBu2 PCH2 Li and (Fxyl)2 BF. According to NMR spectroscopy and X-ray crystallography, 1 is monomeric both in solution and in the solid state. The intramolecular P⋅⋅⋅B distance of 2.900(5) Å and the full planarity of the borane site exclude any significant P/B interaction. Compound 1 readily activates a broad variety of substrates including H2 , EtMe2 SiH, CO2 /CS2 , Ph2 CO, and H3 CCN. Terminal alkynes react with heterolysis of the C-H bond. Haloboranes give cyclic adducts with strong P-BX3 and weak R3 B-X bonds. Unprecedented transformations leading to zwitterionic XP/BCX3 adducts occur on treatment of 1 with CCl4 or CBr4 in Et2 O. In less polar solvents (C6 H6 , n-pentane), XP/BCX3 adduct formation is accompanied by the generation of significant amounts of XP/BX adducts. FLP 1 catalyzes the hydrogenation of PhCH=NtBu and the hydrosilylation of Ph2 CO with EtMe2 SiH.

Cyclopropane formation under frustrated Lewis pair conditions

Reaction of the –CH₂OSiMe₃ substituted allyldimesitylphosphane with HB(C₆F₅)₂ resulted in a hydroboration/(C₆F₅)₂BOSiMe₃ elimination sequence to give the phosphinomethyl substituted cyclopropane derivative, probably via a phosphiranium type intermediate.

Indirect synthesis of a pair of formal methane activation products at a phosphane/borane frustrated Lewis pair

The endergonic formal FLP methane activation products were obtained by an indirect stepwise synthesis and shown to be kinetically stable.

Reactivity of Amine/E(C6F5)3 (E = B, Al) Lewis Pairs toward Linear and Cyclic Acrylic Monomers: Hydrogenation vs. Polymerization

This work reveals the contrasting reactivity of amine/E(C6F5)3 (E = B, Al) Lewis pairs toward linear and cyclic acrylic monomers, methyl methacrylate (MMA) and biorenewable γ-methyl-α-methylene-γ-butyrolactone (γMMBL). While mixing of 2,2,6,6-tetramethylpiperidine (TMP) and B(C6F5)3 leads to a frustrated Lewis pair (FLP), Et3N reacts with B(C6F5)3 to form disproportionation products, ammonium hydridoborate ionic pair and iminium zwitterion. On the other hand, the stoichiometric reaction of either TMP or Et3N with Al(C6F5)3 leads to clean formation of a classic Lewis adduct (CLA). Neither TMP nor Et3N, when paired with E(C6F5)3, polymerizes MMA, but the Et3N/2B(C6F5)3 pair promotes transfer hydrogenation of MMA to form methyl isobutyrate. In contrast, the amine/E(C6F5)3 pairs promote rapid polymerization of γMMBL carrying the more reactive exocyclic methylene moiety, achieving full conversion in less than 3 min even at a low catalyst loading of 0.0625 mol %. TMP is more effective than Et3N for the polymerization when paired with either the borane or the alane, while the alane exhibits higher polymerization activity than the borane when paired with Et3N. Overall, the TMP/Al(C6F5)3 system exhibits the highest polymerization activity, achieving a maximum turn-over frequency of 96,000 h-1 at 0.125 mol % of catalyst loading, producing high molecular weight PγMMBL with Mn = 1.29 × 105 g∙mol-1.