Evidence for a Single Electron Shift in a Lewis Acid–Base Reaction

Light-Induced Polymeric Frustrated Radical Pairs as Building Blocks for Materials and Photocatalysts

Polymeric frustrated Lewis pairs, or poly(FLP)s, have served to bridge the gap between functional polymer science and main group catalysis, pairing the uniqueness of sterically frustrated Lewis acids and bases with a polymer scaffold to create self-healing gels and recyclable catalysts. However, their utilization in radical chemistry is unprecedented. In this paper, we disclose the synthesis of polymeric frustrated radical pairs, or poly(FRP)s, by in situ photoinduction of FLP moieties, where their Lewis acidic and basic centers are tuned to promote single electron transfer (SET). Through systematic manipulation of the chemical structure, we demonstrate that inclusion of ortho-methyl groups on phosphine monomers is crucial to enable SET. The generation of radicals is evidenced by monitoring the stable polymeric phosphine radical cations via UV/vis and EPR spectroscopy. These new poly(FRP)s enable both catalytic hydrogenation and radical-mediated photocatalytic perfluoroalkylations. These polymeric radical systems open new avenues to design novel functional polymers for catalysis and photoelectrical chemistry.

Frustrated Radical Pairs in Organic Synthesis

Frustrated radical pairs (FRPs) describe the phenomenon that two distinct radicals─which would otherwise annihilate each other to form a closed-shell covalent adduct─can coexist in solution, owing to steric repulsion or weak bonding association. FRPs are typically formed via spontaneous single-electron transfer between two sterically encumbered precursors─an oxidant and a reductant─under ambient conditions. The two components of a FRP exhibit orthogonal chemical properties and can often act in cooperativity to achieve interesting radical reactivities. Initially observed in the study of traditional frustrated Lewis pairs, FRPs have recently been shown to be capable of homolytically activating various chemical bonds. In this Perspective, we will discuss the discovery of FRPs, their fundamental reactivity in chemical bond activation, and recent developments of their use in synthetic organic chemistry, including in C-H bond functionalization. We anticipate that FRPs will provide new reaction strategies for solving challenging problems in modern organic synthesis.

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.

Cooperative Bond Activation and Catalytic CO2 Functionalization with a Geometrically Constrained Bis(silylene)-Stabilized Borylene

Metal-ligand cooperativity has emerged as an important strategy to tune the reactivity of transition-metal complexes for the catalysis and activation of small molecules. Studies of main-group compounds, however, are scarce. Here, we report the synthesis, structural characterization, and reactivity of a geometrically constrained bis(silylene)-stabilized borylene. The one-pot reaction of [(SiNSi)Li(OEt₂)] (SiNSi = 4,5-bis(silylene)-2,7,9,9-tetramethyl-9H-acridin-10-ide) with 1 equiv of [BBr₃(SMe₂)] in toluene at room temperature followed by reduction with 2 equiv of potassium graphite (KC₈) leads to borylene [(SiNSi)B] (1), isolated as blue crystals in 45% yield. X-ray crystallography shows that borylene (1) has a tricoordinate boron center with a distorted T-shaped geometry. Computational studies reveal that the HOMO of 1 represents the lone pair orbital on the boron center and is delocalized over the Si-B-Si unit, while the geometric perturbation significantly increases its energy. Borylene (1) shows single electron transfer reactivity toward tris(pentafluorophenyl)borane (B(C₆F₅)₃), forming a frustrated radical pair [(SiNSi)B]^•+[B(C₆F₅)₃]^•-, which can be trapped by its reaction with PhSSPh, affording an ion pair [(SiNSi)BSPh][PhSB(C₆F₅)₃] (3). Remarkably, the cooperation between borylene and silylene allows the facile cleavage of the N-H bond of aniline, the P-P bond in white phosphorus, and the C═O bond in ketones and carbon dioxide, thus representing a new type of main-group element-ligand cooperativity for the activation of small molecules. In addition, 1 is a strikingly effective catalyst for carbon dioxide reduction. Computational studies reveal that the cooperation between borylene and silylene plays a key role in the catalytic chemical bond activation process.

Visible light-controlled living cationic polymerization of methoxystyrene

Photo-controlled living polymerization has received great attention in recent years. However, despite the great success therein, the report on photo-controlled living cationic polymerization has been greatly limited. We demonstrate here a novel decolorable, metal-free and visible light-controlled living cationic polymerization system by using tris(2,4-dimethoxyphenyl)methylium tetrafluoroborate as the photocatalyst and phosphate as the chain transfer agent (CTA) for polymerization of 4-methoxystyrene. This polymerization reaction under green LED light irradiation shows clear living characteristics including predictable molar mass, low molar-mass dispersity (Đ = 1.25), and sequential polymerization capability. In addition, the photocatalytic system exits excellent "on-off" photo switchability and shows the longest "off period" of 36 h up to now for photo-controlled cationic polymerization. Furthermore, the residual photo-catalyst is easily deactivated and decolored with addition of a base after the polymerization. The present study has extended the photo-controlled living cationic polymerization systems with new organic photocatalysts, phosphate CTA and polymerizable monomer as well as the new properties of excellent photostability and in-situ decolored capacity.

The Varied Frustrated Lewis Pair Reactivity of the Germylene Phosphaketene (CH{(CMe)(2,6‐ i Pr 2 C 6 H 3 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.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Site-Fixed Hydroboration of Terminal and Internal Alkenes using BX3 /i Pr2 NEt*

An unprecedented and general hydroboration of alkenes with BX₃ (X=Br, Cl) as the boration reagent in the presence of ⁱ Pr₂ NEt is reported. The addition of ⁱ Pr₂ NEt not only suppresses alkene polymerization and haloboration side reactions but also provides an "H" source for hydroboration. More importantly, the site-fixed installation of a boryl group at the original position of the internal double bond is readily achieved in contrast to conventional transition-metal-catalyzed hydroboration processes. Further application to the synthesis of 1,n-diborylalkanes (n=3-10) is also demonstrated. Preliminary mechanistic studies reveal a major reaction pathway that involves radical species and operates through a frustrated Lewis pair type single-electron-transfer mechanism.

B(C6F5)3-Catalyzed Electron Donor-Acceptor Complex-Mediated Aerobic Sulfenylation of Indoles under Visible-Light Conditions

An efficient B(C₆F₅)₃-catalyzed aerobic oxidative C-S cross-coupling reaction of thiophenol with indoles was developed, affording a wide range of diaryl sulfides in good yields. An electron donor-acceptor complex between B(C₆F₅)₃ and indoles was formed, facilitating the photoinduced single-electron transfer (SET) from indole substrates to the B(C₆F₅)₃ catalyst. This protocol demonstrates a new reaction model using B(C₆F₅)₃ as a single-electron oxidant.

Boranyl-Functionalized [Ge9 ] Clusters: Providing the Idea of Intramolecular Ge/B Frustrated Lewis Pairs

The unique three-dimensional structure of spherical, homoatomic nine-atom germanium clusters opens various possibilities for the spatial arrangement of functional groups. Ligands comprising lone pairs have recently been introduced in the cluster sphere, and we now report the addition of a boranyl group to the cluster featuring a Ge-B exo-cluster bond. The reaction of the twofold-silylated cluster [Ge9 {Si(TMS)3 }2 ]2- (TMS=trimethylsilyl) with 2-chloro-1,3,2-diazaborolidines DABR -Cl leads to the first boranyl-functionalized [Ge9 ] clusters [Ge9 {Si(TMS)3 }2 DABR ]- (R=methyl (1 a), iso-propyl (2 a), ortho-tolyl (3 a)). The anions 2 a and 3 a were structurally characterized as [NHCDipp Cu]+ complexes (NHCDipp =1,3-di(2,6-diisopropylphenyl)imidazolylidine) through single crystal X-ray structure determination. Quantum-chemical calculations manifest the frustrated Lewis pair (FLP) character of the boranyl-functionalized cluster [Ge9 {Si(TMS)3 }2 BCy2 ]- (4 a).

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.

B-H and O-H bonds activation via a single electron transfer of frustrated radical pairs

The rare examples of B-H bond activation in a frustrated radical pair regime have been observed by treatment of TEMPO radicals with Piers' borane HB(C6F5)2 or bis-borane, respectively. The resulting concomitant formation of zwitterionic products and geminal N/B frustrated Lewis pairs implied a one electron process. In addition, the reaction of a TEMPO/B(C6F5)3 pair with H2O·B(C6F5)3 was assumed to involve one-electron reduction of water. Our results provide insights into chemical bond (e.g. B-H and O-H) activation via a single electron transfer.

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(C6 F5 )3 or not. The reactions of H2 and Ph3 SnH 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.

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.

Trialkylphosphines Having a Bulky Phosphacyclopentane Backbone: Structural and Redox Properties Depending on the Exocyclic Alkyl Groups and EPR Observation of a Persistent Trialkylphosphine Radical Cation

Bulky phosphines and their redox properties have received increased attention in the view of useful auxiliary ligands for transition metal catalysts and Lewis-base components of frustrated Lewis pairs for chemical transformations. Herein we report the synthesis, structure, and properties of a series of trialkylphosphines 2R (R = methyl, ethyl, isopropyl, tert-butyl, 1-adamantyl) that possess the bulky 2,2,5,5-tetrakis(trimethylsilyl)-1-phosphacyclopentane as a structural backbone. Among these phosphines, 2Ad, which contains an adamantyl moiety, has a very large buried volume (%V_bur) for a trialkylphosphine (62.0) and shows a quasi-reversible oxidative wave at a lower oxidation potential (-0.12 V in CH₂Cl₂, vs ferrocene/ferrocenium couple) by cyclic voltammetry. The reaction of 2Ad with AgPF₆ afforded a cationic silver aquo complex [Ag(2Ad)(H₂O)]⁺[PF₆]^-, whereas the reaction with NOSbF₆ gave a persistent phosphine radical cation [2Ad]^•+. Based on the EPR spectra and DFT studies, the spin and positive charge of [2Ad]^•+ are localized on the phosphorus atom.

Main-Group Metal Complexes in Selective Bond Formations Through Radical Pathways

Recent years have witnessed remarkable advances in radical reactions involving main-group metal complexes. This includes the isolation and detailed characterization of main-group metal radical compounds, but also the generation of highly reactive persistent or transient radical species. A rich arsenal of methods has been established that allows control over and exploitation of their unusual reactivity patterns. Thus, main-group metal compounds have entered the field of selective bond formations in controlled radical reactions. Transformations that used to be the domain of late transition-metal compounds have been realized, and unusual selectivities, high activities, as well as remarkable functional-group tolerances have been reported. Recent findings demonstrate the potential of main-group metal compounds to become standard tools of synthetic chemistry, catalysis, and materials science, when operating through radical pathways.

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(C6 F5 )3 and PtBu3 /B(C6 F5 )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.

Redox-Controlled Reactivity at Boron: Parallels to Frustrated Lewis/Radical Pair Chemistry

We report the synthesis of new Lewis-acidic boranes tethered to redox-active vanadium centers, (Ph₂N)₃V(μ-N)B(C₆F₅)₂ (1a) and (N(CH₂CH₂N(C₆F₅))₃)V(μ-N)B(C₆F₅)₂ (1b). Redox control of the V^IV/V couple resulted in switchable borane versus "hidden" boron radical reactivity, mimicking frustrated Lewis versus frustrated radical pair (FLP/FRP) chemistry, respectively. Whereas heterolytic FLP-type addition reactions were observed with the V^V complex (1b) in the presence of a bulky phosphine, homolytic peroxide, or Sn-hydride bond cleavage reactions were observed with the V^IV complex, [CoCp₂^*][(N(CH₂CH₂N(C₆F₅))₃)V(μ-N)B(C₆F₅)₂] (3b), indicative of boron radical anion character. The extent of radical character was probed by spectroscopic and computational means. Together, these results demonstrate that control of the V^IV/V oxidation states allows these compounds to access reactivity observed in both FLP and FRP chemistry.

Isolable Anionic, Neutral and Cationic Radicals from Reactions of N,N'-Dimesityldiamidocarbene and Lewis Acids

B(C₆ F₅ )₃ undergoes nucleophilic attack by N,N'-dimesityldiamidocarbene (DAC) with fluoride transfer to the boron center, resulting in a new zwitterion (1). This B-F fluoride can be replaced or abstracted to give the corresponding hydride (2) or triflate (3) derivatives or the corresponding cation (4). These species are reduced with KC₈ or Cp₂ Co to give isolable anionic and neutral radicals (5-8). Similarly, the [Ph₃ C] cation undergoes nucleophilic attack by DAC resulting in the spontaneous formation of the radical cation (9).

SET processes in Lewis acid-base reactions: the tritylation of N-heterocyclic carbenes

Reactions between N-heterocyclic carbenes (NHCs) and the trityl cation, [Ph₃C]⁺, give covalent adducts of type [NHC-CPh₃]⁺ and/or [NHC-C₆H₅-CPh₂]⁺. EPR spectroscopy, UV-Vis analyses, and trapping experiments imply that adduct formation involves carbene radical cations and the trityl radical. The results demonstrate that single electron transfer (SET) processes should be considered for reaction of NHCs with oxidizing Lewis acids.

Exploiting single-electron transfer in Lewis pairs for catalytic bond-forming reactions

A single-electron transfer (SET) between tris(pentafluorophenyl)borane (B(C₆F₅)₃) and N,N-dialkylanilines is reported, which is operative via the formation of an electron donor–acceptor (EDA) complex involving π-orbital interactions as a key intermediate under dark conditions or visible-light irradiation depending on the structure of the aniline derivatives. This inherent SET in the Lewis pairs initiates the generation of the corresponding α-aminoalkyl radicals and their additions to electron-deficient olefins, revealing the ability of B(C₆F₅)₃ to act as an effective one-electron redox catalyst.

Radical–ion pair generation from common Lewis pairs and its application to catalytic carbon–carbon bond formation.

Trialkylsilyl-Substituted Silole and Germole Dianions as Precursors for Unusual Silicon and Germanium Compounds

Group 14 element heteroles are the heavier analogues of cyclopentadienes in which a heavier group 14 element atom replaces the sp³ carbon atom. In particular siloles and, to a somewhat smaller degree, germoles attracted considerable attention since the early 1990s due to their favorable photophysical properties which allowed the construction of OLEDs using group 14 element heteroles as emissive or electron-transport layers. Anions and in particular dianions derived from group 14 element heteroles have been of substantial interest due to the possible occurrence of Hückel aromaticity involving the heavier main group atom. Aromaticity is not the only notable electronic feature of silole and germole dianions; the spatial and energetic alignment of their frontier orbitals is equally remarkable. With a high lying lone pair at the heteroatom, which is orthogonal to a delocalized π-system, their frontier orbital sequence closely resembles that of N-heterocyclic carbene analogues. Despite these intriguing parallels between carbene analogues and silole and germole dianions, disappointingly little is known about their reactivity. The installation of trialkylsilyl substituents in the 2,5-positions of the heterocyclopentadiene ring as in K₂[I] has a remarkable effect on the stability of silole and germole dianions and allows us to study their reactivity and to evaluate their synthetic potential in detail. Simple double salt metathesis reactions with different dihalides provided heterofulvenes. These were detected either as intermediates or, in the case of carbon dihalides, isolated in the form of their ylidic isomers II. In other cases, the heterofulvenes were the starting point for complex reaction sequences leading to novel binuclear complexes of titanium and zirconium III or for simple isomerization reactions that lead to bicyclohexene-type tetrylenes (BCH-tetrylenes) IV, a novel class of heavier carbenes. These bicyclic carbene analogues are significantly stabilized by homoconjugation between the electron deficient tetrel atom and the remote C═C double bond. Compound IV with E'R₂═SiR₂ and E = Si is a valence isomer of disilabenzene and is a stable derivative of the global minimum of the Si₂C₄H₆ potential energy surface. With group 13 dihalides, as for example with boron dichlorides, topological closely related compounds V were isolated. These Ge(II) complexes of borole dianions are isolobal to half-sandwich complexes of main group elements such as aluminum(I) cyclopentadienide or can be viewed as nido-type clusters. These analogies already open a broad field for future investigations of their reactivity. Trialkylsilyl-substituted heterole dianions I provide a facile synthetic approach to several novel intriguing compound classes with the tetrel element in unusual coordination states. The reactivity and the synthetic potential of these new compounds is however widely unexplored and calls for future systematic studies. Gratifyingly, the periodic table of the elements stills holds a lot of potential for future research on the reactivity of silole and germole dianions.

Ge=B π-Bonding: Synthesis and Reversible [2+2] Cycloaddition of Germaborenes

Phosphine-stabilized germaborenes featuring an unprecedented Ge=B double bond with short B⋅⋅⋅Ge contacts of 1.886(2) (4) and 1.895(3) Å (5) were synthesized starting from an intramolecular germylene-phosphine Lewis pair (1). After oxidative addition of boron trihalides BX3 (X=Cl, Br), the addition products were reduced with magnesium and catalytic amounts of anthracene to give the borylene derivatives in yields of 78 % (4) and 57 % (5). These halide-substituted germaborenes were characterized by single-crystal structure analysis, and the electronic structures were studied by quantum-chemical calculations. According to an NBO NRT analysis, the dominating Lewis structure contains a Ge=B double bond. The germaborenes undergo a reversible, photochemically initiated [2+2] cycloaddition with the phenyl moiety of a terphenyl substituent at room temperature, forming a complex heterocyclic structure with GeIV in a strongly distorted coordination environment.

Control of Nanomorphology in Fullerene-Free Organic Solar Cells by Lewis Acid Doping with Enhanced Photovoltaic Efficiency

Generating desired efficiency enhancements in organic solar cells (OSCs) by charge-transfer doping requires to obtain modified optoelectronic properties while retaining the favorable nanomorphology. We report a thermally assisted doping based on Lewis acid tris(pentafluorophenyl)-borane (BCF) as a p-dopant for two groups of OSCs comprising the PBDB-TF and PBDB-T donors and a nonfullerene acceptor IT-4F. We found that the face-on molecular packing in the PBDB-TF:IT-4F blend or neat PBDB-TF donor films is favorably modified with the formation of frustrated Lewis pairs (FLPs) in the donor, which is in contrast to the hampered π-π stacking in the doped PBDB-T film. The different impacts of BCF dopants on the morphology lead to contrasting photovoltaic behaviors where the PBDB-TF-based devices receive enhanced power conversion efficiencies (PCEs) in the presence of BCF, while reduction of efficiencies is observed in the PBDB-T device. In the best doping conditions with the proposed hot-film deposition, we achieve a boosted PCE of 14.1% in PBDB-TF:IT-4F solar cells at low BCF concentrations. Based on the same fluorinated donor, the described BCF doping also applies to NF-solar cells based on the NF-acceptor Y6, leading to an increase in the PCE to 16.0%. Our results suggest that controlling the degree of FLP formation in the donor component with the addition of BCF is key to obtaining desired improvements on nanomorphology and relevant photophysical properties in OSCs.

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.

A Germacalicene: Synthesis, Structure, and Reactivity

The synthesis of the germacalicene 7 from the reaction of the dipotassium germole dianion K₂ [6] with 1,2-bis-diisopropylamino-3-chlorocyclopropenyl perchlorate is reported. Based on the crystal structure analysis and the results of DFT calculations, the germacalicene 7 can be viewed as a cyclopropenium germacyclopentadienide ylide that is isoelectronic to α-cationic phosphanes. First reactivity studies revealed its nucleophilic character and resulted in the isolation of the air- and moisture-stable carbonyl iron complex 15 and the cationic silver complex 20. One-electron oxidation of the germacalicene 7 was achieved by its reaction with [Ph₃ C][B(C₆ F₅ )₄ ] and the bis-cationic Ge-Ge-bonded dimer 22 was isolated.