A 9-Borylated Acridinyl Radical

Halogen Atom Transfer-Induced Homolysis of C-F Bonds by the Excited-State Boryl Radical

A novel reactivity toward C-F bond functionalization has been developed, which could be designated as fluorine atom transfer (FAT). A photoexcited state of an N-heterocyclic carbene-ligated boryl radical exhibits a transcendent reactivity, capable of activating chemically inert carbon-fluorine bonds through homolysis. Combined experimental and computational studies suggest that the ligated boryl radical species directly abstracts a fluorine atom from the organofluoride substrates to provide valuable carbon-centered radicals.

Reactivity of a Hexaaryldiboron(6) Dianion as Boryl Radical Anions

Our study unveils a novel approach to accessing boryl radicals through the spontaneous homolytic cleavage of B-B bonds. We synthesized a hexaaryl-substituted diboron(6) dianion, 1, via the reductive B-B coupling of 9-borafluorene. Intriguingly, compound 1 exhibits the ability to undergo homolytic B-B bond cleavage, leading to the formation of boryl radical anions, as confirmed by EPR studies, in the presence of the 2.2.2-cryptand at room temperature. Moreover, it directly reacts with diphenylacetylene, producing an unprecedented 1,6-diborylated allene species, where the phenyl ring is dearomatized. Density functional theory computational studies suggest that homolytic B-B bond cleavage is favored in the reaction path, and the formation of the boryl radical anion is crucial for dearomatization. Additionally, it achieves the dearomative diborylation of anthracene and the activation of elemental sulfur/selenium under mild conditions. The borylation products have been successfully characterized by NMR spectra, HRMS, and X-ray single-crystal diffraction.

Synthesis and Reactivity of a Dialane-Bridged Diradical

Radicals of the lightest group 13 element, boron, are well established and observed in numerous forms. In contrast to boron, radical chemistry involving the heavier group 13 elements (aluminum, gallium, indium, and thallium) remains largely underexplored, primarily attributed to the formidable synthetic challenges associated with these elements. Herein, we report the synthesis and isolation of planar and twisted conformers of a doubly CAAC (cyclic alkyl(amino)carbene)-radical-substituted dialane. Extensive characterization through spectroscopic analyses and X-ray crystallography confirms their identity, while quantum chemical calculations support their open-shell nature and provide further insights into their electronic structures. The dialane-connected diradicals exhibit high susceptibility to oxidation, as evidenced by electrochemical measurements and reactions with o-chloranil and a variety of organic azides. This study opens a previously uncharted class of dialuminum systems to study, broadening the scope of diradical chemistry and its potential applications.

Screening seven-electron boron-centered radicals for dinitrogen activation

The activation of dinitrogen is significant as nitrogen-containing compounds play an important role in industries. However, the inert NN triple bond caused by its large HOMO-LUMO gap (10.8 eV) and high bond dissociation energy (945 kJ mol-1 ) renders its activation under mild conditions particularly challenging. Recent progress shows that a few main group species can mimic transition metal complexes to activate dinitrogen. Here, we demonstrate that a series of seven-electron (7e) boron-centered radical can be used to activate N2 via density functional theory calculations. It is found that boron-centered radicals containing amine ligand perform best on the thermodynamics of dinitrogen activation. In addition, when electron-donating groups are introduced at the boron atom, these radicals can be used to activate N2 with low reaction barriers. Further analysis suggests that the electron transfer from the boron atom to the π* orbitals of dinitrogen is essential for its activation. Our findings suggest great potential of 7e boron radicals in the field of dinitrogen activation.

Dianionic and Neutral Diboron-Centered Classical Diradicaloids

Herein, we report the syntheses and electronic structures of crystalline dianionic as well as neutral diboron-centered classical diradicaloids as boron analogues of classical Thiele, Chichibabin, and Müller (this only for dianionic diradicaloids!) hydrocarbons. These are based on borane radical anion and NHC-stabilized boryl radical spin carriers, respectively. All these dianionic diboron-centered diradicaloids exhibit triplet population at room temperature regardless of the π-conjugated spacer: p-phenylene, p,p'-biphenylene, or p,p″-terphenylene. In the case of neutral diboron-centered diradicaloids, the employed π-conjugated spacer plays a crucial role for the triplet population at room temperature: EPR inactive for p-phenylene vs EPR active for p,p'-biphenylene. The findings emphasize the importance of the spin carriers for the resulting ground-state: borane radical anion vs NHC-stabilized boryl radical along with the pivotal role of the π-conjugated spacer as spin-coupler between two spins. Notably, 100 years (a century) after the first report by Krause of the triphenyl borane radical-anion, being isoelectronic to the triphenylmethyl radical, we convey borane radical anion-based diradicaloids. Furthermore, while donor-stabilized boryl radicals were introduced in the 1980s by Giles and Roberts, said concept is herewith being extended to NHC-stabilized boryl radical-based diradicaloids.

Double axial stabilization of a carbenium ion via convergent PO → C+ tetrel bonding

Our efforts in carbocation chemistry have led us to target examples of such species stabilized intramolecularly by tetrel bonding. Described here is an example of such a compound in which a triaryl carbenium is stabilized intramolecularly by two convergent PO → Ccarbenium interactions, as confirmed by structural studies. The formation of this new motif favorably impacts the reversibility of the first and second reduction of the carbenium center. It also has profound influence on the Lewis acidity of the carbenium center which is lower than that of the unsubstituted parent carbenium by seven orders of magnitude.

Structure-property-function relationships of stabilized and persistent C- and N-based triaryl radicals

Structurally similar C- and N-based triaryl radicals are among the most commonly used structural motifs in stable, open-shell, organic molecules. The application of such species is associated with their stability, properties and structural design. This study summarizes the basic stabilization and persistence principles of C- and N-based triaryl radicals and highlights recent advances in design strategies of radicals tailored for specific applications.

Filling the gap with a bulky diaryl boron group: fluorinated and non-fluorinated copper pyrazolates fitted with a dimesityl boron moiety on the backbone

Successful synthesis has been reported of 4-Mes2B-3,5-(CF3)2PzH and 4-Mes2B-3,5-(CF3)2PzH bearing sterically demanding diarylboron moieties at the pyrazole ring 4-position, and their corresponding copper(I) pyrazolate complexes. They show visible blue photoluminescence in solution. The X-ray crystal structures revealed that the fluorinated {[4-BMes2-3,5-(CF3)2Pz]Cu}3 crystallizes as discrete trinuclear molecules whereas as the non-fluorinated {[4-BMes2-3,5-(CH3)2Pz]Cu}3 forms dimers of trimers with two close inter-trimer Cu⋯Cu separations. The solid {[4-BMes2-3,5-(CF3)2Pz]Cu}3 featuring a sterically confined Cu3N6 core displays bright blue phosphorescence while {[4-BMes2-3,5-(CH3)2Pz]Cu}3, which is a dimer of a trimer, is a red phosphor at room temperature. This work illustrates the modulation of photo-physical properties of metal pyrazolates by adjusting the supporting ligand steric features and introducing secondary diarylboron luminophores. Computational analysis of the structures and photophysical properties of copper complexes are also presented.

A Neutral Borylene and its Conversion to a Radical by Selective Hydrogen Transfer

A successful selective reduction of X₂B-Tip (Tip = 1,3,5-ⁱPr₃-C₆H₂, X = I, Br) with KC₈ and Mg metal, respectively, in the presence of a hybrid ligand (C₆H₄(PPh₂)LSi) leads to a stable low-valent five-membered ring as a boryl radical [C₆H₄(PPh₂)LSiBTip][Br] (1) and neutral borylene [C₆H₄(PPh₂)LSiBTip] (2). Compound 2 reacts with 1,4-cyclohexadiene, resulting in hydrogen abstraction to afford the radical [C₆H₄(PPh₂)LSiB(H)Tip] (3). Quantum chemical studies reveal that compound 1 is a B-centered radical, and compound 2 is a phosphane and silylene stabilized neutral borylene in a trigonal planar environment, whereas compound 3 is an amidinate-centered radical. Although compounds 1 and 2 are stabilized by hyperconjugation and π-conjugation, they display high H-abstraction energy and basicity, respectively.

Neutral Boryl Radicals in Mixed-Valent B(III) Br-B(II) Adducts

The general strategies to stabilize a boryl radical involve single electron delocalization by π-system and the steric hinderance from bulky groups. Herein, a new class of boryl radicals is reported, with intramolecular mixed-valent B^(III) Br-B^(II) adducts ligated by a cyclic (alkyl)(amino)carbene (CAAC). The radicals feature a large spin density on the boron center, which is ascertained by EPR spectroscopy and DFT calculations. Structural and computational analyses revealed that the stability of radical species was assisted by the CAAC ligand and a weak but significant B(III)Br-B(II) interaction, suggesting a cooperative avenue for stabilization of boryl radicals. Two-electron reduction of these new boryl radicals provides C-H insertion products via a borylene intermediate.

Synthesis and isolation of dinuclear N,C-chelate organoboron compounds bridged by neutral, anionic, and dianionic 4,4'-bipyridine via reductive coupling of pyridines

The reaction of Bppy(Mes)2 (BN1; ppy = 2-phenylpyridine) and BCH₂ppy(Mes)₂ (BN3) with the reducing reagent KC₈ resulted in C-C bond formation via intermolecular radical coupling to generate the 4,4'-bipyridyl ligand compounds BN2 and BN4. Adding 1 equivalent of KC₈ to a THF solution of BN2 and BN4 generated the 4,4'-bipyridyl radical anions BN2K and BN4K. The dianion species BN2K2 and BN4K2 could be obtained by adding 2 equivalents of KC₈ to the THF solution of BN2 and BN4. In the presence of 2,2,2-cryptand or 18-crown-6, the radical anion salt BN2K(crypt) and the dianion salt BN2K2(18c6)2 were isolated for single-crystal X-ray diffraction analysis. Structural, spectroscopic, and computational studies were performed on the three species of BN2 derivatives (neutral, radical anion, and dianion species). BN2 and BN4 were stable and did not undergo photoisomerization or photoelimination under UV light irradiation.

Fluoride ion sensing with an acridinium borane

With our continuing interest in the chemistry of cationic boranes, we have synthesizedthe tetrafluoroborate salt of 1-dimesitylboron-4-(N-methyl-9-acridinium)-phenylene which acts as a turn-on fluoride anion sensor, visibly changing from yellow to orange upon binding fluoride. To understand this reactivity, we spectroscopically and computationally analyzed the cation and triarylfluoroborate adduct. UV-vis spectroscopy and TD-DFT revealed the basis of the color change to be a red shift in a low-energy absorption band resulting from intramolecular charge-transfer. Electrochemical studies were undertaken to further probe this system. Cyclic voltammetry indicated a reversible one-electron reduction for the cation and a cathodic shift of -0.12 V in the first reduction wave upon fluoride binding. Chemical reduction of the cation yielded the acridine borane radical which was verified by EPR spectroscopy.

Predicting Dinitrogen Activation by Five-Electron Boron-Centered Radicals

Due to the high bond dissociation energy (945 kJ mol^-1) and the large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap (10.8 eV), dinitrogen activation under mild conditions is extremely challenging. On the other hand, the conventional Haber-Bosch ammonia synthesis under harsh conditions consumes more than 1% of the world's annual energy supply. Thus, it is important and urgent to develop an alternative approach for dinitrogen activation under mild conditions. In comparison with transition metals, main group compounds are less explored for nitrogen activation. Here, we carry out density functional theory calculation to screen boron radicals for dinitrogen activation. As a result, the experimentally available seven-electron boron-centered radicals are found to be inactive to N₂ activation, whereas some five-electron boron-centered radicals become favorable for dinitrogen activation, inviting experimental chemists' examination. The principal interacting spin-orbital analyses suggest that a five-electron boron-centered radical can mimic a transition metal on a synergic interaction with dinitrogen in the transition states.

NHC induced radical formation via homolytic cleavage of B–B bonds and its role in organic reactions

New borylation methodologies have been reported recently, wherein diboron(4) compounds apparently participate in free radical couplings via the homolytic cleavage of the B–B bond. We report herein that bis-NHC adducts of the type (NHC)₂·B₂(OR)₄, which are thermally unstable and undergo intramolecular ring expansion reactions (RER), are sources of boryl radicals of the type NHC–BR₂˙, exemplified by Me₂Im^Me·Bneop˙ 1a (Me₂Im^Me = 1,3,4,5-tetramethyl-imidazolin-2-ylidene, neop = neopentylglycolato), which are formed by homolytic B–B bond cleavage. Attempts to apply the boryl moiety 1a in a metal-free borylation reaction by suppressing the RER failed. However, based on these findings, a protocol was developed using Me₂Im^Me·B₂pin₂3 for the transition metal- and additive-free boryl transfer to substituted aryl iodides and bromides giving aryl boronate esters in good yields. Analysis of the side products and further studies concerning the reaction mechanism revealed that radicals are likely involved. An aryl radical was trapped by TEMPO, an EPR resonance, which was suggestive of a boron-based radical, was detected in situ, and running the reaction in styrene led to the formation of polystyrene. The isolation of a boronium cation side product, [(Me₂Im^Me)₂·Bpin]⁺I⁻7, demonstrated the fate of the second boryl moiety of B₂pin₂. Interestingly, Me₂Im^Me NHC reacts with aryl iodides and bromides generating radicals. A mechanism for the boryl radical transfer from Me₂Im^Me·B₂pin₂3 to aryl iodides and bromides is proposed based on these experimental observations.

Bis-NHC adducts of the type (NHC)₂·B₂(OR)₄ are sources of boryl radicals of the type NHC–BR₂˙, which are formed by homolytic B–B bond cleavage.

One- and two-electron reduction of triarylborane-based helical donor-acceptor compounds

One-electron chemical reduction of 10-(dimesitylboryl)-N,N-di-p-tolylbenzo[c]phenanthrene-4-amine (3-B(Mes)2-[4]helix-9-N(p-Tol)2) 1 and 13-(dimesitylboryl)-N,N-di-p-tolyldibenzo[c,g]phenanthrene-8-amine (3-B(Mes)2-[5]helix-12-N(p-Tol)2) 2 gives rise to monoanions with extensive delocalization over the annulated helicene rings and the boron p z orbital. Two-electron chemical reduction of 1 and 2 produces open-shell biradicaloid dianions with temperature-dependent population of the triplet states due to small singlet-triplet gaps. These results have been confirmed by single-crystal X-ray diffraction, EPR and UV/vis-NIR spectroscopy, and DFT calculations.

Advances in chemistry of N-heterocyclic carbene boryl radicals

Boron-centred radicals (boryl radicals) are potential and attractive species in main group chemistry and synthetic chemistry. Recently, the development of boron compounds ligated by N-heterocyclic carbenes (NHCs) has sparked off advavnces in boryl radical chemistry because NHCs can highly stabilise boryl radicals by electronic and steric factors. This review highlights recent synthesis and reactions of such NHC-boryl radicals. From the standpoint of main group chemistry, examples of isolation or detection of unique NHC-boryl radicals are presented. From the standpoint of synthetic chemistry, on the other hand, the development of reactions of user-friendly NHC-boryl radicals, which has contributed to radical chemistry, organoboron chemistry and polymer science, is comprehensively described.

Conformational Switching through the One-Electron Reduction of an Acridinium-based, γ-Cationic Phosphine Gold Complex

Our efforts in the chemistry of gold complexes featuring ambiphilic phosphine-carbenium L/Z-type ligand have led us to consider the reduction of the carbenium moiety as a means to modulate the gold-carbenium interaction present in these complexes. Here, it was shown that the one-electron reduction of [(o-Ph₂ P(C₆ H₄ )Acr)AuCl]⁺ (Acr=9-N-methylacridinium) produces a neutral stable radical, the structure of which showed a marked increase in the Au-Acr distance. Related structural changes were observed for the phosphine oxide analogue [(o-Ph₂ P(O)(C₆ H₄ )Acr]⁺ , the reduction of which interfered with the P=O→carbenium interaction. These structural effects, driven by a reduction-induced change in the electronic and electrostatic characteristics of the compounds, showed that the charge and accepting properties of the carbenium unit can be modulated. These results highlight the redox-noninnocence of carbenium Z-type ligand, a feature that can be exploited to induce specific conformational changes.

Anionic Boron- and Carbon-Based Hetero-Diradicaloids Spanned by a p-Phenylene Bridge

Herein we report the synthesis and characterization of anionic boron- and carbon-based Kekulé diradicaloids spanned by a p-phenylene bridge. In contrast to Thiele's hydrocarbon, a closed-shell singlet system, they show an appreciable population of the triplet state at room temperature, as evidenced by both NMR and EPR spectroscopy. Moreover, en route to these anionic boron- and carbon-based hetero-diradicaloids, the formation of an isolable diamino(4-diarylboryl-phenyl)methyl radical was observed.

Phenylpyridyl-Fused Boroles: A Unique Coordination Mode and Weak B-N Coordination-Induced Dual Fluorescence

Using 4-phenylpyridine or 2-phenylpyridine in place of biphenyl, two electron-poor phenylpyridyl-fused boroles, [TipPBB1]4 and TipPBB2 were prepared. [TipPBB1]4 adopts a unique coordination mode and forms a tetramer with a cavity in both the solid state and solution. The boron center of TipPBB2 is 4-coordinate in the solid state but the system dissociates in solution, leading to 3-coordinate borole species. Compared to its borafluorene analogues, the electron-accepting ability of TipPBB2 is largely enhanced by the pyridyl group. TipPBB2 exhibits dual fluorescence in solution due to an equilibrium between free TipPBB2 and a weak intermolecular coordination adduct with a second molecule. This equilibrium was further investigated by low-temperature NMR spectroscopy and photophysical studies. Theoretical studies indicate that the highest occupied molecular orbital (HOMO) of TipPBB2 localizes at the Tip group, in contrast to its borafluorene derivatives, wherein the HOMOs are localized on the borafluorene cores.

Two derivatives of phenylpyridyl-fused boroles with contrasting electronic properties: decreasing and enhancing the electron accepting ability

Two derivatives of phenylpyridyl-fused boroles were prepared via functionalization of the pyridyl groups, namely to an electron-rich dihydropyridine moiety (compound 1) and an electron-deficient N-methylpyridinium cation (compound 2). Due to strong conjugation between the dihydropyridine moiety and the boron atom, the reduction potential of compound 1 shifts cathodically. In contrast, compound 2 exhibits three reduction processes with a first reversible reduction potential anodically shifted in comparison to its precursor (TipPBB2) or the non-borylated framework 1-methyl-2-phenylpyridin-1-ium triflate (3). The significantly anodically shifted reduction potential indicates the extreme electron deficiency of compound 2, which also leads to the reversible coordination of THF. Photophysical properties of both compounds in different solvents were investigated. Theoretical studies further support the strong conjugation in the ground state of compound 1 and the electron-deficient property of compound 2.

N-Substituted Acridinium as a Multi-Responsive Recognition Unit in Supramolecular Chemistry

The N-substituted acridinium motif is an electron-deficient unit with appealing multi-responsive properties which have been exploited in the field of supramolecular chemistry. This building block reversibly alters its shape, with its chemical and optical properties in response to a chemical or redox signal. In this Review, we discuss selected examples where the switchable properties of 9-aryl-N-methyl-acridinium lead to actuators, multi-input and multi-output systems, host or guest systems, and to interlocked systems with controllable motion.

A Super-Oxidized Radical Cationic Icosahedral Boron Cluster

While the icosahedral closo-[B₁₂H₁₂]^2- cluster does not display reversible electrochemical behavior, perfunctionalization of this species via substitution of all 12 B-H vertices with alkoxy or benzyloxy (OR) substituents engenders reversible redox chemistry, providing access to clusters in the dianionic, monoanionic, and neutral forms. Here, we evaluated the electrochemical behavior of the electron-rich B₁₂(O-3-methylbutyl)₁₂ (1) cluster and discovered that a new reversible redox event that gives rise to a fourth electronic state is accessible through one-electron oxidation of the neutral species. Chemical oxidation of 1 with [N(2,4-Br₂C₆H₃)₃]^•+ afforded the isolable [1]^•+ cluster, which is the first example of an open-shell cationic B₁₂ cluster in which the unpaired electron is proposed to be delocalized throughout the boron cluster core. The oxidation of 1 is also chemically reversible, where treatment of [1]^•+ with ferrocene resulted in its reduction back to 1. The identity of [1]^•+ is supported by EPR, UV-vis, multinuclear NMR (¹H, ¹¹B), and X-ray photoelectron spectroscopic characterization.

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).

Stable Radical Cation and Dication of an N-Heterocyclic Carbene Stabilized Digallene: Synthesis, Characterization and Reactivity

One- and two-electron oxidation of a digallene stabilized by an N-heterocyclic carbene afforded the first stable gallium-based radical cation and dication salts, respectively. Structural analysis and theoretical calculations reveal that the oxidation occurs at the Ga=Ga double bond, leading to removal of π electrons of the double bond and a decrease of the bond order. The spin density of the radical cation mainly locates at the two gallium centers as demonstrated by EPR spectroscopy and theoretical calculations. Moreover, the reactivity of the radical cation salt toward nBu₃ SnH and cyclo-S₈ was studied; a digallium-hydride cation salt containing a Ga-Ga single bond and a gallium sulfide cluster bearing an unprecedented ladder-like Ga₄ S₄ core structure were obtained, respectively.

Persistent Borafluorene Radicals

N‐Heterocyclic carbene (NHC)‐ and cyclic (alkyl)(amino)carbene (CAAC)‐stabilized borafluorene radicals have been isolated and characterized by elemental analysis, single‐crystal X‐ray diffraction, UV/Vis absorption, cyclic voltammetry (CV), electron paramagnetic resonance (EPR) spectroscopy, and theoretical studies. Both the CAAC–borafluorene radical (2) and the NHC–borafluorene radical (4) have a considerable amount of spin density localized on the boron atoms (0.322 for 2 and 0.369 for 4). In compound 2, the unpaired electron is also partly delocalized over the CAAC ligand ^carbeneC and N atoms. However, the unpaired electron in compound 4 mainly resides throughout the borafluorene π‐system, with significantly less delocalization over the NHC ligand. These results highlight the Lewis base dependent electrostructural tuning of materials‐relevant radicals. Notably, this is the first report of crystalline borafluorene radicals, and these species exhibit remarkable solid‐state and solution stability.

One-electron bonds are not "half-bonds"

Despite the success of the molecular orbital (MO) and valence-bond (VB) models to describe the electronic structure and properties of molecules, neither MO nor VB provides an explanation for the nature of the chemical bond. The first to address this problem was Ruedenberg, who showed that chemical bonds result from quantum interference. He developed a method to calculate the interference contribution to the total electronic energy and density and applied it to molecules containing typical two-centre two-electron (2c-2e) covalent bonds. To test the generality of Ruedenberg's hypothesis, we developed a powerful Interference Energy Analysis (IEA) method to calculate the interference contributions of individual chemical bonds to the total energy of diatomic and polyatomic molecules, and showed that any two-electron bond, despite its polarity, results from quantum interference. Nevertheless, many stable molecules are experimentally known whose chemical structures clearly indicate the existence of two-centre one-electron bonds (2c-1e). Therefore, the question remains if quantum interference will be the dominant effect for these systems. This work describes the extension of the IEA for treating two-centre one-electron bonds, making use of a Generalised Product Function (GPF) built from spin coupled wave functions of N electrons in M orbitals, SC(N,M). Several diatomic and polyatomic molecules were analysed and whenever possible the results were compared with the analogous case of a two-electron bond. The results indicate that interference is the dominant effect for the one-electron bonds, which reinforces the role of quantum interference as the central element in chemical bonding theory.

A Main-Group Element Radical Based One-Dimensional Magnetic Chain

The first main-group element radical based one-dimensional magnetic chain (1K)_n was realized by one-electron reduction of the pyridinyl functionalized borane 1 with elemental potassium in THF in the absence of 18-crown-6 (18-c-6). The electron spin density of (1K)_n mainly resides at the boron centers with a considerable contribution from central benzene and pyridine moieties. The spin centers exhibit an antiferromagnetic interaction as demonstrated by magnetic measurements and theoretical calculations. In contrast, the reduction in the presence of 18-c-6 afforded the separated radical anion salt 1K(Crown), in which the potassium cation was trapped by THF and 18-c-6 molecules. Further one-electron reduction of 1K(Crown) and (1K)_n led to the diamagnetic monomer and polymer, respectively.

Stable cyclic (alkyl)(amino)carbene (cAAC) radicals with main group substituents

Recent attempts to isolate cyclic (alkyl)(amino)carbene stabilized radicals of p-block elements have been described here.

Isolation and characterization of stable radicals has been a long-pursued quest. While there has been some progress in this field particularly with respect to carbon, radicals involving heavier p-block elements are still considerably sparse. In this review we describe our recent successful efforts on the isolation of stable p-block element radicals particularly those involving aluminum, silicon, and phosphorus.

Isolable diboryl radicals acting as highly efficient reaction intermediates under mild conditions

Activation of the B–B bond in diborane with dimesitylpyridyl boranes afforded the stable diboryl radicals in moderate yields.

5- Endo Cyclizations of NHC-Boraallyl Radicals Bearing Ester Substituents: Characterization of Derived 1,2-Oxaborole Radicals and Boralactones

EPR studies of radical hydrogen abstraction reactions of N-heterocyclic carbene (NHC) complexes of alkenylboranes bearing two ester substituents revealed not the expected boraallyl radicals but instead isomeric 1,2-oaxborole radicals. Such radicals are new, and DFT calculations show that they arise from the initially formed boraallyl radicals by a rapid, exothermic 5- endo cyclization. These spectroscopic and computational discoveries prompted a series of preparative experiments that provided access to a novel family of robust NHC-boralactones. A one-pot procedure was developed to access the boralactones directly from an NHC-borane (NHC-BH₃) and dimethyl acetylenedicarboxylate.

Destabilizing Character of a π-Conjugated Boron Center in Bisphenol Radicals

Although boron-containing radicals are promising materials for molecular electronic devices, the electronic effect of the σ-donating yet π-accepting boron center on the stability of open-shell species has been less discussed. In this work, the role of a tricoordinate boron center in π-conjugated radicals was explored through electron paramagnetic resonance measurement of several boron-linked bisphenol radicals and diradicals. Replacing the bridging methine fragment of a neutral Galvinoxyl radical with an arylboryl group led to the corresponding boron-linked radical anion that requires excessive steric protection at the boron center to be persistent in solution. Experimental and theoretical investigations revealed that the introduction of boron would diminish the quinoidal character of the phenoxyl radical and increase both the electrophilicity and nucleophilicity of the open-shell species. Therefore, it is important to consider the steric protection of the boron center in boron-containing π-conjugated organic radicals.

Selective one- and two-electron reductions of a haloborane enabled by a π-withdrawing carbene ligand

A carbene-stabilised neutral boryl radical and a boryl anion are isolated via selective one- and two-electron reduction of a diamidocarbene (DAC) adduct of dibromo(pentafluorophenyl)borane. Both the radical and the anion have been characterised by various spectroscopic techniques in solution, while the structures have been ascertained by single-crystal X-ray diffraction. In contrast, the reduction of the analogous cyclic (alkyl)(amino) carbene (CAAC) adduct yields a C-H activation product.

Difluorination at Boron Leads to the First Electrophilic Ligated Boryl Radical (NHC-BF2. )

1,3-Dimethylimidazol-2-ylidene difluoroborane (NHC-BF₂ H) was prepared in a one-pot, two-step reaction from the parent ligated borane (NHC-BH₃ ). The derived difluoroboryl radical (NHC-BF₂^. ) was generated by laser flash photolysis experiments and characterized by UV spectroscopy and rate-constant measurements. It is transient and reacts quickly with O₂ . Unusually, it also reacts more rapidly with ethyl vinyl ether than with methyl acrylate. By this measure, it is the first electrophilic ligated boryl radical. Both NHC-BH₃ and NHC-BF₂ H serve as co-initiators in bulk photopolymerizations, converting both electron-poor and electron-rich monomers at roughly similar rates. However, the difluorinated coinitiator provides polymers with dramatically increased chain lengths from both monomers.

Stable Organic Neutral Diradical via Reversible Coordination

We report the formation of a stable neutral diboron diradical simply by coordination of an aromatic dinitrogen compound to an ortho-phenyldiborane. This process is reversible upon addition of pyridine. The diradical species is stable above 200 °C. Computations are consistent with an open-shell triplet diradical with a very small open-shell singlet-triplet energy gap that is indicative of the electronic disjointness of the two radical sites. This opens a new way of generating stable radicals with fascinating electronic properties useful for a large variety of applications.

Preparation, Properties, and Structures of the Radical Anions and Dianions of Azapentacenes

A series of diazapentacenes (5,14-diethynyldibenzo[b,i]phenazine, 6,13-diethynylnaphtho[2,3-b]phenazine) and tetraazapentacenes (7,12-diethynylbenzo[g]quinoxalino[2,3-b]quinoxaline, 6,13-diethynylquinoxalino[2,3-b]phenazine) were reduced to their radical anions and dianions, employing either potassium anthracenide or lithium naphthalenide in THF. The anionic species formed were investigated by UV-vis-NIR, fluorescence and EPR spectroscopy, spectroelectrochemistry, and quantum chemical calculations. Single crystal X-ray structures of three of their radical anions and of three of their dianions were obtained. In contrast to the acenes, the anions of the azapentacenes are persistent and, in some cases, even moderately stable toward air, and were characterized.