Frustrated Lewis Pair System Research Articles

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.

Dinitrogen Activation within Frustrated Lewis Pairs Is Promoted by Adding External Electric Fields.

This study uses computational means to explore the feasibility of N2 cleavage by frustrated Lewis pair (FLPs) species. The employed FLP systems are phosphane/borane (1) and carbene/borane (2). Previous studies show that 1 and 2 react with H2 and CO2 but do not activate N2. The present study demonstrates that N2 is indeed inert, and its activation requires augmentation of the FLPs by an external tool. As we demonstrate here, FLP-mediated N2 activation can be achieved by an external electric field oriented along the reaction axis of the FLP. Additionally, the study demonstrates that FLP -N2 activation generates useful nitrogen compound, e.g., hydrazine (H2N-NH2). In summary, we conclude that FLP effectively activates N2 in tandem with oriented external electric fields (OEEFs), which play a crucial role. This FLP/OEEF combination may serve as a general activator of inert molecules.

Single electron reduction of NHC-CO2-borane compounds.

The carbon dioxide radical anion [CO2˙-] is a highly reactive species of fundamental and synthetic interest. However, the direct one-electron reduction of CO2 to generate [CO2˙-] occurs at very negative reduction potentials, which is often a limiting factor for applications. Here, we show that NHC-CO2-BR3 species - generated from the Frustrated Lewis Pair (FLP)-type activation of CO2 by N-heterocyclic carbenes (NHCs) and boranes (BR3) - undergo single electron reduction at a less negative potential than free CO2. A net gain of more than one volt was notably measured with a CAAC-CO2-B(C6F5)3 adduct, which was chemically reduced to afford [CAAC-CO2-B(C6F5)3˙-]. This room temperature stable radical anion was characterized by EPR spectroscopy and by single-crystal X-ray diffraction analysis. Of particular interest, DFT calculations showed that, thanks to the electron withdrawing properties of the Lewis acid, significant unpaired spin density is localised on the carbon atom of the CO2 moiety. Finally, these species were shown to exhibit analogous reactivity to the carbon dioxide radical anion [CO2˙-] toward DMPO. This work demonstrates the advantage provided by FLP systems in the generation and stabilization of [CO2˙-]-like species.

Evidence for a kinetic FLP reaction pathway in the activation of benzyl chlorides by alkali metal-phosphine pairs.

Kinetic frustrated Lewis pairs (FLP) allow facile cleavage of a number of E-H bonds (E = H, Si, C, B) where both the Lewis base and Lewis acid are involved in the bond activation transition state. More recently, kinetic FLP systems have been extended to the cleavage of C-X (X = F, Cl, Br) bonds. We report on the role of sodium tetrakis(pentafluorophenyl)borate in the benzylation of triarylphosphines, where the sodium cation and phosphine support a kinetic FLP type transition state.

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.

Structure-Reactivity Relationships in Borane-Based FLP-Catalyzed Hydrogenations, Dehydrogenations, and Cycloisomerizations.

ConspectusThe activation of molecular hydrogen by main-group element catalysts is an extremely important approach to metal-free hydrogenations. These so-called frustrated Lewis pairs advanced within a short period of time to become an alternative to transition metal catalysis. However, deep understanding of the structure-reactivity relationship is far less developed compared to that of transition metal complexes, although it is paramount for advancing frustrated Lewis pair chemistry.In this Account, we provide detailed insight into how Lewis acidity and Lewis basicity correlate to reactivity. The reactivity of frustrated Lewis pairs will be systematically discussed in context with selected reactions. The influence of major electronic modifications of the Lewis pairs is correlated with the ability to activate molecular hydrogen, to channel reaction kinetics and reaction pathways, or to achieve C(sp3)-H activations.First, we will describe how we entered this emerging field of research after quickly realizing that information was lacking on how the reactivity changes with modification of the frustrated Lewis pair. This led us to the development of a qualitative and quantitative structure-reactivity relationship in metal-free imine hydrogenations. The imine hydrogenation was utilized as the model reaction to experimentally determine the activation parameters of the FLP-mediated hydrogen activation for the first time. This kinetic study revealed autoinduced catalytic profiles when Lewis acids weaker than tris(pentafluorophenyl)borane were applied, opening up to study the Lewis base dependency within one system. With this knowledge of the interplay between Lewis acid strength and Lewis basicity, we developed methods for the hydrogenation of densely functionalized nitroolefins, acrylates, and malonates. Here, the reduced Lewis acidity needed to be counterbalanced by a suitable Lewis base to ensure efficient hydrogen activation. The opposite measure was necessary for the hydrogenation of unactivated olefins. For these, comparably less electron-releasing phosphanes were required to generate strong Brønsted acids by hydrogen activation. These systems displayed highly reversible hydrogen activation even at temperatures as low as -60 °C. A systematic study of these systems enabled the development of acceptorless dehydrocouplings of amines with silanes and dehydrogenations of aza-heterocycles by C(sp3)-H activations. Furthermore, the C(sp3)-H and π-activation was utilized to achieve cycloisomerizations by carbon-carbon and carbon-nitrogen bond formations. Lastly, new frustrated Lewis pair systems featuring weak Lewis bases as active components in the hydrogen activation were developed for the reductive deoxygenation of phosphane oxides and carboxylic acid amides.

Exploring the metal-free catalytic reduction of CO2 to methanol with saturated adamantane scaffolds of phosphine-borane frustrated Lewis pair: A DFT study

Frustrated Lewis pair (FLP) chemistry is an alternative strategy in the area of metal-free organic catalysis. This study reports adamantane as FLP molecular scaffold used to design new FLP (ambiphilic molecule) for the metal-free catalytic reduction of CO2 to methoxy borane (CH3O[B]) via hydroboration process. The DFT [B97D/6-31G(d,p), (SMD,benzene)//B3PW91/6-31G(d,p),SDD] calculations have been employed to examine the reaction. The saturated hydrocarbon showed a marked improvement in the energetics of the reduction process compared to the unsaturated phenylene FLPs employed for such reactions. The saturated scaffold improves the Lewis acid and base characters in adamantyl-derived FLPs. The gauche arrangement of Lewis pairs in the adamantyl FLP system is critical to lower the energy barriers in the potential energy surfaces compared to the unsaturated phenylene FLPs. The adamantyl scaffold FLP lowered the activation barriers by ∼6.0 kcal/mol in the key steps of the reduction of CO2 with the reducing agent compared to the phenylene FLPs. These calculated results corroborate the activation of the reducing agent and carbon dioxide to function as efficient catalysis in all steps. The enhancement in the Lewis characters of FLPs with adamantane scaffold was elucidated with conceptual density functional theory (CDFT) calculations.

Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen.

The articulation of the notion of "frustrated Lewis pairs" (FLPs) emerged from the discovery that H2 can be reversibly activated by combinations of sterically encumbered main group Lewis acids and bases. This has prompted numerous studies focused on various perturbations of the Lewis acid/base combinations and the applications to organic reductions. This Perspective focuses on the new directions and developments that are emerging from this FLP chemistry involving hydrogen. Three areas are discussed including new applications and approaches to FLP reductions, the reductions of small molecules, and the advances in heterogeneous FLP systems. These foci serve to illustrate that despite having its roots in main group chemistry, this simple concept of FLPs is being applied across the discipline.

Effect of non-aqueous solvents on kinetics of carbon dioxide absorption by tBu3P/B(C6F5)3 frustrated Lewis pairs

Frustrated Lewis pairs (FLPs), combinations of sterically hindered Lewis acids and bases, are known for their ability to capture CO2. Although there have been several theoretical studies on the mechanisms of the reactions between CO2 and some FLP systems, experimental studies on the reaction kinetics have been inconclusive. In this study, the mechanism and kinetics of CO2 absorption by an FLP system consisting of tri-tert-butylphosphine (tBu3P) and tris(pentafluorophenyl)borane (B(C6F5)3) in bromobenzene, cyclopentyl methyl ether (CPME), and tert-butyl methyl ether (MTBE) were investigated using the stopped-flow method. The pseudo-first-order reaction rate constants, ko (s−1), were measured for a concentration range of 0.02–0.035 M and over a temperature range of 298–323 K. The experimental data were fitted according to modified termolecular mechanisms with average absolute relative deviations of 4.34%, 4.63%, and 3.51% for the CO2–FLP:bromobenzene, CO2–FLP:CPME, and CO2–FLP:MTBE systems, respectively. The forward reaction rate constants, k (m3 kmol−1 s−1), were calculated based on the proposed reaction mechanism. The forward reaction rate constants were higher than those for various aqueous tertiary amine systems but lower than those for aqueous monoethanolamine and piperazine systems. Moreover, the activation energies were estimated from Arrhenius plots. They were calculated to be 22.0, 19.7, and 21.8 kJ mol−1 for the CO2–FLP:bromobenzene, CO2–FLP:CPME, and CO2–FLP:MTBE systems, respectively. This study promotes the development of novel efficient solvent formulations for CO2 capture.

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.

Optimizing the Energetics of FLP-Type H2 Activation by Modulating the Electronic and Structural Properties of the Lewis Acids: A DFT Study.

The great potential of frustrated Lewis pairs (FLPs) as metal-free catalysts for activation of molecular hydrogen has attracted increasing interest as an alternative to transition-metal catalysts. However, the complexity of FLP systems, involving the simultaneous interaction of three molecules, impedes a detailed understanding of the activation mechanism and the individual roles of the Lewis acid (LA) and Lewis base (LB). In the present work, using density functional theory (DFT) calculations, we examine the reactivity of 75 FLPs for the H2 splitting reaction, including a series of experimentally investigated LAs combined with conventional phosphine-based (tBu3P) and oxygen-based (i.e., ethereal solvent) Lewis bases. We find that the catalytic activity of the FLP is the result of a delicate balance of the LA and LB strengths and their bulkiness. The H2 splitting reaction can be changed from endergonic to exergonic by tuning the electrophilicity of the LA. Also, a more nucleophilic LB results in a more stable ion pair product and a lower barrier for the hydrogen splitting. The bulkiness of the LB leads to an early transition state to reduce steric hindrance and lower the barrier height. The bulkiness of the fragments determines the cavity size in the FLP complex, and a large cavity allows for a larger charge separation in the ion pair configuration. A shorter proton–hydride distance in this product complex correlates with a stronger attraction between the fragments, which forms more reactive ion pairs and facilitates the proton and hydride donations in the subsequent hydrogenation process. These insights may help with rationalizing the experimentally observed reactivities of FLPs and with designing better FLP systems for hydrogenation catalysis and hydrogen storage.

Tuning Activity and Selectivity during Alkyne Activation by Gold(I)/Platinum(0) Frustrated Lewis Pairs.

Introducing transition metals into frustrated Lewis pair systems has attracted considerable attention in recent years. Here we report a selection of three metal-only frustrated systems based on Au(I)/Pt(0) combinations and their reactivity toward alkynes. We have inspected the activation of acetylene and phenylacetylene. The gold(I) fragments are stabilized by three bulky phosphines bearing terphenyl groups. We have observed that subtle modifications on the substituents of these ligands proved critical in controlling the regioselectivity of acetylene activation and the product distribution resulting from C(sp)–H cleavage of phenylacetylene. A mechanistic picture based on experimental observations and computational analysis is provided. As a result of the cooperative action of the two metals of the frustrated pairs, several uncommon heterobimetallic structures have been characterized.

Triple Frustrated Lewis Pair-Type Reactivity on a Single Rare-Earth Metal Center.

Rare-earth metal cations have been used rarely as Lewis-acidic components in the chemistry of frustrated Lewis pairs (FLPs). Herein, we report the first cerium/phosphorus system (2) employing a heptadentate N4 P3 ligand, which exhibits triple FLP-type reactivity towards a series of organic substrates, including isocyanates, isothiocyanates, diazomethane, and azides on a single rare-earth Lewis acidic Ce center. This result shows that the Ce center and three P atoms in 2 could simultaneously activate three equivalents of small molecules under mild conditions. This study broadens the diversity of FLPs and demonstrates that rare earth based FLP exhibit unique properties compared with other FLP systems.

A Radical FLP Approach to C–C Coupling

In the February issue of Cell Reports Physical Science, Melen and co-workers report the reactivity of diaryl-substituted esters with R3P/B(C6F5)3 frustrated Lewis pairs (FLPs). Although ester C–O activation occurs for all FLP systems, the use of Mes3P induces single-electron transfer to generate a [Mes3P]⋅+/[C(H)Ar2]⋅ radical ion pair. In the presence of olefins, this reactivity is harnessed in an sp2-sp3 C–C heterocoupling reaction to generate α,β-substituted olefins.

Bis(pentafluorophenyl)phenothiazylborane - an intramolecular frustrated Lewis pair catalyst for stannane dehydrocoupling.

We synthesized a novel Lewis acidic aminoborane containing a phenothiazyl substituent and demonstrated its potential to catalytically promote the dehydrocoupling of tin hydrides. The observed reactivity would imply a homolytic frustrated Lewis pair type mechanism, however computational analysis suggests a heterolytic mechanism for this reaction. This result represents one of the first frustrated Lewis pair systems to dehydrocouple stannanes in a heterolytic fashion.

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. Reactions involving the use of both classical Lewis acid-base adducts and frustrated Lewis pair systems are discussed and the subsequent reactivity is considered.

Recent Advances in Water-Tolerance in Frustrated Lewis Pair Chemistry

A water-tolerant frustrated Lewis pair (FLP) combines a sterically encumbered Lewis acid and Lewis base that in synergy are able to activate small molecules even in the presence of water. The main challenge introduced by water comes from its reversible coordination to the Lewis acid which causes a marked increase in the Brønsted acidity of water. Indeed, the oxophilic Lewis acids typically used in FLP chemistry form water adducts whose acidity can be comparable to that of strong Brønsted acids such as HCl, thus they can protonate the Lewis base component of the FLP. Irreversible proton transfer quenches the reactivity of both the Lewis acid and the Lewis base, precluding small molecule activation. This short review discusses the efforts to overcome water-intolerance in FLP systems, a topic that in less than five years has seen significant progress.1 Introduction2 Water-Tolerance (or Alcohol-Tolerance) in Carbonyl Reductions3 Water-Tolerance with Stronger Bases4 Water-Tolerant Non-Boron-Based Lewis Acids in FLP Chemistry5 Conclusions

Living Polymerization of Conjugated Polar Alkenes Catalyzed by N-Heterocyclic Olefin-Based Frustrated Lewis Pairs

The living polymerization of conjugated polar alkenes such as methacrylates by a noninteracting, authentic frustrated Lewis pair (FLP) has remained elusive ever since the report on FLP-promoted polymerization in 2010. Here we report that the polymerization of alkyl methacrylates by a FLP system based on a strongly nucleophilic N-heterocyclic olefin (NHO) Lewis base and sterically encumbered but modestly strong Lewis acid MeAl(4-Me-2,6-tBu2-C6H2O)2 is not only rapid but also living. This living polymerization was indicated by the formation of a linear, living chain, capped with NHO/H chain ends, without backbiting-derived cyclic chain ends. The true livingness of this FLP-promoted polymerization has been unequivocally verified by five lines of evidence, including the predicted polymer number-average molecular weight (Mn, up to 351 kg·mol–1) coupled with low dispersity (Đ = 1.05) values; obtained high to quantitative initiation efficiencies; an observed linear increase of polymer Mn vs monomer conversion an...

Silyl Ketene Acetals/B(C₆F₅)₃ Lewis Pair-Catalyzed Living Group Transfer Polymerization of Renewable Cyclic Acrylic Monomers.

This work reveals the silyl ketene acetal (SKA)/B(C6F5)3 Lewis pair-catalyzed room-temperature group transfer polymerization (GTP) of polar acrylic monomers, including methyl linear methacrylate (MMA), and the biorenewable cyclic monomers γ-methyl-α-methylene-γ-butyrolactone (MMBL) and α-methylene-γ-butyrolactone (MBL) as well. The in situ NMR monitored reaction of SKA with B(C6F5)3 indicated the formation of Frustrated Lewis Pairs (FLPs), although it is sluggish for MMA polymerization, such a FLP system exhibits highly activity and living GTP of MMBL and MBL. Detailed investigations, including the characterization of key reaction intermediates, polymerization kinetics and polymer structures have led to a polymerization mechanism, in which the polymerization is initiated with an intermolecular Michael addition of the ester enolate group of SKA to the vinyl group of B(C6F5)3-activated monomer, while the silyl group is transferred to the carbonyl group of the B(C6F5)3-activated monomer to generate the single-monomer-addition species or the active propagating species; the coordinated B(C6F5)3 is released to the incoming monomer, followed by repeated intermolecular Michael additions in the subsequent propagation cycle. Such neutral SKA analogues are the real active species for the polymerization and are retained in the whole process as confirmed by experimental data and the chain-end analysis by matrix-assisted laser desorption/ionization time of flight mass spectroscopy (MALDI-TOF MS). Moreover, using this method, we have successfully synthesized well-defined PMMBL-b-PMBL, PMMBL-b-PMBL-b-PMMBL and random copolymers with the predicated molecular weights (Mn) and narrow molecular weight distribution (MWD).

A Free Energy Landscape of CO2 Capture by Frustrated Lewis Pairs

Frustrated Lewis pairs (FLPs) are known for their ability to capture CO2. Although many FLPs have been reported experimentally and several theoretical studies have been carried out to address the reaction mechanism, the individual roles of the Lewis acid and base of FLPs in the capture of CO2 are still unclear. In this study, we employed density functional theory (DFT) based metadynamics simulations to investigate the complete reaction path for the capture of CO2 by the tBu3P/B(C6F5)3 pair and to understand the roles of the Lewis acid and base. Interestingly, we find that the Lewis acid plays a more important role than the Lewis base. Specifically, the Lewis acid is crucial for the catalytic properties and is responsible for both kinetic and thermodynamics control. The Lewis base, however, has less of an effect on the catalytic performance and is mainly responsible for the formation of a FLP system. On the basis of these findings, we propose a rule of thumb for the future synthesis of FLP-based catalysts ...