Boron-containing radical species

Borafluorene-Mediated Sulfur Activation: Isolation of Boryl-Linked S7 and S8 Catenates and Related Chalcogenide Molecules

Although the activation of elemental sulfur by main group compounds is well-documented in the literature, the products of such reactions are often heterocyclic in nature. However, the isolation and characterization of sulfur catenates (i.e., acyclic sulfur chains) is significantly less common. In this study, we report the activation of elemental sulfur by the 9-CAAC-9-borafluorene radical (1) and anion (2) (CAAC = (2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene) to form boron-sulfur catenates (3-6). From the isolation of the octasulfide-bridged compound 3, a sulfur extrusion reaction using 1,3,4,5-tetramethylimidazol-2-ylidene (IMe4) was used to decrease the sulfide chain length from eight to seven (4). Bonding analysis of compounds 3-6 was performed using density functional theory, which elucidated the nature of the sulfur-sulfur bonding observed within these compounds. We also report the synthesis of a series of borafluorene-chalcogenide species (7-9), via diphenyl dichalcogenide activation, which portray characteristics described by an internal heavy atom effect. Compounds 7-9 each exhibit blue fluorescence, with the lowest energy emissive process (S2 → S0) at 436 nm (7 and 8) and 431 nm (9). The S1 → S0 emission is not observed experimentally due to a Laporte forbidden transition. Density functional theory was employed to investigate the frontier molecular orbitals and absorption and emission profiles of compounds 7-9.

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.

Aromatic 1,4,2,3-Diazadiborole Featuring an Unsymmetrical B=B Entity: A Versatile Synthon for Unusual Boron Heterocycles

The replacement of a CC unit with an isoelectronic BN unit in aromatic systems can give rise to molecules and materials with fascinating properties. We report here the synthesis, characterization, and reactivity of a 1,4,2,3-diazadiborole species, 2, featuring an unprecedented 6π-aromatic BN-heterocyclic moiety that is isoelectronic to cyclopentadienide (Cp-). Bearing an unsymmetrical B=B entity, 2 exhibits reactivity toward oxidants, protic reagents, electrophiles, and unsaturated substrates. This reactivity facilitates the synthesis of a variety of novel mono- and bicyclic organoboron derivatives through mechanisms including ring retention, cleavage/recombination, annulation, and expansion. These findings reveal innovative synthetic routes to BN-embedded aromatic compounds via desymmetrization, affording unique building blocks for synthetic chemistry.

Crystalline Radical Anion of a Diboratriazole and Its Conversion to a Neutral Radical Driven by a Carbene

One-electron reduction of diboratriazole 1 with potassium graphite (KC8) generates the radical anion 1•-•K+, which undergoes a salt (KCl) elimination reaction upon addition of an N-heterocyclic carbene (NHC) to afford the neutral diboratriazole radical 3. An X-ray diffraction analysis, electron paramagnetic resonance spectroscopy, and computational studies revealed that an unpaired electron in radical species 1•-•K+ and 3 is delocalized over the π-system of the B2N3 and carbene rings. Reversible oxidation of 3 gives rise to a diboratriazole cation 4 featuring a 6π aromatic character. Moreover, treating 1•-•K+ with a half equivalent of a bis(NHC) produces a biradical species 5, in which there is little interaction between two radical moieties separated by the bis(NHC) linker, suggesting the dis-biradical property. 5 undergoes stepwise and reversible two-electron oxidation, establishing three formal oxidation states.

Synthesis and Properties of Cyclic π-Conjugated Molecules and Their Dication and Monoradical Cation

We synthesized cyclic π-conjugated molecules by double Friedel-Crafts reaction of amino group-substituted 1,2-bis(2-phenylethynyl)benzene with Meldrum's acid derivative. The structures of the cyclic π-conjugated molecules were determined by single-crystal X-ray structure analysis. The oxidation of the dimethylamino group-substituted π-conjugated molecule with NOBF4 gave a closed-shell dication that is stable at >210 °C. The monoradical cation of the di(4-methoxyphenyl)amino group-substituted π-conjugated molecule is stable in dichloromethane solution (half-life of nearly 15 days) and shows near-infrared absorption.

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.

Redox- and Charge-State Dependent Trends in 5, 6, and 7-Membered Boron Heterocycles: A Neutral Ligand Coordination Chemistry Approach to Boracyclic Cations, Anions, and Radicals

ConspectusBoron heterocycles represent an important subset of heteroatom-incorporated rings, attracting attention from organic, inorganic, and materials chemists. The empty pz orbital at the boron center makes them stand out as quintessential Lewis acidic molecules, also serving as a means to modulate electronic structure and photophysical properties in a facile manner. As boracycles are ripe for extensive functionalization, they are used in catalysis, chemical biology, materials science, and continue to be explored as chemical synthons for conjugated materials and reagents. Neutral boron(III)-incorporated polycyclic molecules are some of the most studied types of boracycles, and understanding their redox transformations is important for applications relying on electron transfer and charge transport. While relevant redox species can often be electrochemically observed, it remains challenging to isolate and characterize boracycles where the boron center and/or polycyclic skeleton have been chemically reduced.We describe our recent work isolating 5-, 6-, and 7-membered boracyclic radicals, anions, and cations, focusing on stabilization strategies, ligand-mediated bonding situations, and reactivity. We present a versatile neutral ligand coordination chemistry approach that permits the transformation of boracycles from potent electrophiles to powerful nucleophilic heterocycles that facilitate diverse electron transfer and bond activation chemistry. Although there are a wide range of suitable stabilizing ligands, we have employed both diamino-N-heterocyclic carbenes (NHCs) and cyclic(alkyl)(amino) carbenes (CAACs), which led to boracycles with tunable electronic structures and aromaticity trends. We highlight successful isolation of borafluorene radicals and demonstrate their reversible redox behavior, undergoing oxidation to the cation or reduction to the anion. The borafluorene anion is a chemical synthon that has been used to prepare boryl main-group and transition-metal bonds, luminescent oxabora-spirocycles, borafluorenate-crown ethers, and CO-releasing molecules via carbon dioxide activation. We expanded to 6-membered boracycles and characterized neutral bis(NHC-supported 9-boraphenanthrene)s and the corresponding bis(CAAC-stabilized 9-boraphenanthrene) biradical. We detail the interconvertible multiredox states of boraphenalene, where the boraphenalenyl radical, anion, and cation mimic the charge-states of the all-hydrocarbon analogue. Reactivity studies of the boraphenalenyl anion displayed unusual nucleophilic reactivity at multiple sites on the periphery of the boraphenalenyl tricyclic scaffold. Reduced borepins, 7-membered boron containing heterocycles, have also been isolated. We used a stepwise one-pot synthesis combining the halo-borepin precursor, CAAC, and KC8 to afford the monomeric borepin radicals and anions. The π-system was extended to contain two borepin rings fused in a pentacyclic scaffold, which permitted isolation of diborepin biradicals and a diborepin containing a dibora-quinone core.Our goal is to provide a guide explaining the current structure-function trends and isolation strategies for redox-active boron-incorporated polycyclic molecules to initiate the rational design and use of these types of compounds across a vast chemical space.

Unlocking Lewis acidity via the redox non-innocence of a phenothiazine-substituted borane

We describe a new approach to enhancing Lewis acidity, through the single electron oxidation of a borane with a pendant phenothiazine. This results in the formation of a persistent radical cation with increased electrophilicity. Computational and experimental studies indicate this radical cation significantly enhances the Lewis acidity and catalytic activity compared to its neutral analog. These results illustrate the viability of this approach in turning on the Lewis acidity of relatively inert boranes.

Redox-Active Boron Clusters

ConspectusIn this Account, we discuss our group's research over the past decade on a class of functionalized boron clusters with tunable chemical and physical properties, with an emphasis on accessing and controlling their redox behavior. These clusters can be thought of as three-dimensional aromatic systems that have distinct redox behavior and photophysical properties compared to their two-dimensional organic counterparts. Specifically, our lab has studied the highly tunable, multielectron redox behavior of B12(OR)12 clusters and applied these molecules in various settings. We first discuss the spectroscopic and electrochemical characterization of B12(OR)12 clusters in various oxidation states, followed by their use as catholytes and/or anolytes in redox flow batteries and chemical dopants in conjugated polymers. Additionally, the high oxidizing potential and visible light-absorbing nature of fluoroaryl-functionalized B12(OR)12 clusters have been leveraged by our group to generate weakly coordinating, photoexcitable species that can promote photooxidation chemistry.We have further translated these solution-phase studies of B12(OR)12 clusters to the solid state by using the precursor [B12(OH)12]2- cluster as a robust building block for hybrid metal oxide materials. Specifically, we have shown that the boron cluster can act as a thermally stable cross-linking material, which enhances electron transport between metal oxide nanoparticles. We applied this structural motif to create TiO2- and WO3-containing materials that showed promising properties as photocatalysts and electroactive materials for supercapacitors. Building on this concept, we later discovered that B12(OCH3)12, the smallest of the B12(OR)12 family, could retain its redox behavior in the solid state, a previously unseen phenomenon. We successfully harnessed this unique behavior for solid-state energy storage by implementing this boron cluster as a cathode-active material in a Li-ion prototype cell device. Recently, our group has also explored how to tune the redox properties of clusters other than B12(OR)12 species by synthesizing a library of vertex-differentiated clusters containing both B-OR and B-halogen groups. Due to the additive qualities of different functional groups on the cluster, these species allow access to a region of electrochemical potentials previously inaccessible by fully substituted closo-dodecaborate alkoxy-based derivatives.Lastly, we discuss our research into smaller-sized redox-active polyhedral boranes (B6- and B10-based cluster cores). Interestingly, these clusters show significantly less redox stability and reversibility than their dodecaborate-based counterparts. While exploring the functionalization of closo-hexaborate to create fully substituted derivates (i.e., [B6R6Hfac]-), we observed unique oxidative decomposition pathways for this cluster system. Consequently, we leveraged this oxidative instability to generate useful alkyl boronate esters via selective chemical oxidation. We further explored a closo-decaborate cluster as a platform to access electrophilic [B10H13]+ species capable of directly borylating arene compounds with unique regioselectivity. Upon chemical oxidation of the arylated decaborate clusters, we successfully synthesized various aryl boronate esters, establishing the generality of the oxidative cluster deconstruction concept.Overall, our work shows that boron clusters are an appealing class of redox-active molecules, and this fundamental and understudied property can be leveraged for constructing novel materials with tunable physical and electrochemical properties, as well as producing unique chemical reagents for small molecule synthesis.

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.

Sequential radical and cationic reactivity at separated sites within one molecule in solution

Distonic radical cations (DRCs) with spatially separated charge and radical sites are expected to show both radical and cationic reactivity at different sites within one molecule. However, such "dual" reactivity has rarely been observed in the condensed phase. Herein we report the isolation of crystalline 1λ2,3λ2-1-phosphonia-3-phosphinyl-cyclohex-4-enes 2a,b˙+, which can be considered delocalized DRCs and were completely characterized by crystallographic, spectroscopic, and computational methods. These DRCs contain a radical and cationic site with seven and six valence electrons, respectively, which are both stabilized via conjugation, yet remain spatially separated. They exhibit reactivity that differs from that of conventional radical cations (CRCs); specifically they show sequential radical and cationic reactivity at separated sites within one molecule in solution.

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.

Unlocking Biradical Character in Diborepins

Systems that possess open- and closed-shell behavior attract significant attention from researchers due to their inherent redox and charge transport properties. Herein, we report the synthesis of the first diborepin biradicals. They display tunable biradical character based on the steric and electronic profile of the stabilizing ligand and the resulting geometric deviation of the diborepin core from planarity. While there are numerous all-carbon-based biradical systems, boron-based biradical compounds are comparatively rare, particularly ones in which the radical sites are disjointed. Calculations using density functional theory (DFT) and multireference methods demonstrate that the fused diborepin scaffold exhibits high biradical character, up to 95%. Use of a nonsterically demanding diaminocarbene promotes the planarization of the pentacyclic framework, resulting in the synthetic realization of a diborepin containing a dibora-quinoidal core, which possesses a closed-shell ground state and thermally accessible triplet state. The biradicals were structurally authenticated and characterized by both solution and solid-state electron paramagnetic resonance (EPR) spectroscopy. Half-field transitions were observed at low temperatures (about 170 K), confirming the presence of the triplet state. Initial reactivity studies of the biradicals led to the isolation and structural characterization of bis(borepin hydride) and bis(borepin dianion).

Juggling Optoelectronics and Catalysis: The Dual Talents of Bench Stable 1,4-Azaborinines

Boron- and nitrogen-doped polycyclic aromatic hydrocarbons (B-PAHs) have established a strong foothold in the realm of organic electronics. However, their catalytic potential remains largely untapped. In this study, we synthesise and characterise two bench stable B,N-doped PAH derivatives based on a 1,4-azaborinine motif. Most importantly, the anthracene derived structure is an efficient catalyst in the reduction of various carbonyls and imines. These results underscore the potential of B,N-PAHs in catalytic transformations, setting the stage for deeper exploration in this chemical space.

A Discrete, Boron-Containing Triangular Triradical

A neutral boron-containing triangular triradical based on a triptycene derivative has been designed and synthesized. Its structure, bonding and physical property have been studied by EPR spectroscopy, SQUID magnetometry and single crystal X-ray diffraction, as well as theoretical calculations. The triradical has a series of isosceles triangle conformations in the solution due to the Jahn-Teller distortion, leading to the splitting of the two low-lying doublet states. This factor together with negligible spin-orbit coupling (SOC) of composing light atoms quenches the spin frustration. The work represents a rare example of a neutral through-space triangular triradical.

Syntheses and Structures of 5-Membered Heterocycles Featuring 1,2-Diphospha-1,3-Butadiene and Its Radical Anion

LGa(P2 OC)cAAC 2 features a 1,2-diphospha-1,3-butadiene unit with a delocalized π-type HOMO and a π*-type LUMO according to DFT calculations. [LGa(P2 OC)cAAC][K(DB-18-c-6)] 3[K(DB-18-c-6] containing the 1,2-diphospha-1,3-butadiene radical anion 3⋅- was isolated from the reaction of 2 with KC8 and dibenzo-18-crown-6. 3 reacted with [Fc][B(C6 F5 )4 ] (Fc=ferrocenium) to 2 and with TEMPO to [L-H Ga(P2 OC)cAAC][K(DB-18-c-6)] 4[K(DB-18-c-6] containing the 1,2-diphospha-1,3-butadiene anion 4- . The solid state structures of 2, 3K(DB-18-c-6], and 4[K(DB-18-c-6] were determined by single crystal X-ray diffraction (sc-XRD).

Facile Borylation of Alkenes, Alkynes, Imines, Arenes and Heteroarenes with N-Heterocyclic Carbene-Boranes and a Heterogeneous Semiconductor Photocatalyst

Although the development of radical chain and photocatalytic borylation reactions using N-heterocyclic carbene (NHC)-borane as boron source is remarkable, the persistent problems, including the use of hazardous and high-energy radical initiators or the recyclability and photostability issues of soluble homogeneous photocatalysts, still leave great room for further development in a sustainable manner. Herein, we report a conceptually different approach toward highly functionalized organoborane synthesis by using recoverable ultrathin cadmium sulfide (CdS) nanosheets as a heterogeneous photocatalyst, and a general and mild borylation platform that enables regioselective borylation of a wide variety of alkenes (arylethenes, trifluoromethylalkenes, α,β-unsaturated carbonyl compounds and nitriles), alkynes, imines and electron-poor aromatic rings with NHC-borane as boryl radical precursor. Mechanistic studies and density functional theory (DFT) calculations reveal that both photogenerated electrons and holes on the CdS fully perform their own roles, thereby resulting in enhancement of photocatalytic activity and stability of CdS.

Geometrically Constrained Organoboron Species as Lewis Superacids and Organic Superbases

This report unveils an advancement in the formation of a Lewis superacid (LSA) and an organic superbase by the geometrical deformation of an organoboron species towards a T-shaped geometry. The boron dication [2]2+ supported by an amido diphosphine pincer ligand features both a large fluoride ion affinity (FIA>SbF5 ) and hydride ion affinity (HIA>B(C6 F5 )3 ), which qualifies it as both a hard and soft LSA. The unusual Lewis acidic properties of [2]2+ are further showcased by its ability to abstract hydride and fluoride from Et3 SiH and AgSbF6 respectively, and effectively catalyze the hydrodefluorination, defluorination/arylation, as well as reduction of carbonyl compounds. One and two-electron reduction of [2]2+ affords stable boron radical cation [2]⋅+ and borylene 2, respectively. The former species has an extremely high spin density of 0.798e at the boron atom, whereas the latter compound has been demonstrated to be a strong organic base (calcd. pKBH + (MeCN)=47.4) by both theoretical and experimental assessment. Overall, these results demonstrate the strong ability of geometric constraining to empower the central boron atom.

A Dynamic Triradical: Synthesis, Crystal Structure, and Spin Frustration

Polyradicals, i.e., multispin organic molecules, are playing important roles in radical-based material applications for their spin-spin interaction. A dynamic covalently bonded multispin molecule may endow materials with added function such as memory and switching. However, such a species has yet to be reported. We here report the synthesis, characterization, and crystal structure of a dynamic triradical species. It is generated by the self-assembly of two molecules through a Lewis acid coupled electron transfer. The crystalline species is spin-frustrated without Jahn-Teller distortion at low temperature, while it dissociates back to diamagnetic starting material in solution at high temperature. The reversible process is tracked by variable-temperature NMR, EPR, and UV-vis-NIR spectroscopy. Isolation, property study, and dynamic bonding investigation on such a species lay the foundation for the design of functional polyradicals with potential application as memory or switching devices.

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.

Monosubstituted Doublet Sn(I) Radical Featuring Substantial Unquenched Orbital Angular Momentum

Due to their intrinsic high reactivity, isolation of heavier analogues of carbynes remains a great challenge. Here, we report the synthesis and characterization of a neutral monosubstituted Sn(I) radical (2) supported by a sterically hindered hydrindacene ligand, which represents the first tin analogue of a free carbyne. Different from all Sn(I/III) species reported thus far, the presence of a sole Sn-C σ bond in 2 renders the remaining two Sn 5p orbitals energetically almost degenerate, of which one is singly occupied and the other is empty. Consequently, its S = 1/2 ground state possesses two-fold orbital pseudo-degeneracy and substantial unquenched orbital angular momentum, as evidenced by one component of its g matrix (1.957, 1.896, and 1.578) being considerably less than 2. Consistent with this unique electronic structure, 2 can bind to an N-heterocyclic carbene to afford a neutral two-coordinate Sn(I) radical and initiate a one-electron transfer to benzophenone to furnish a Sn(II)-ketyl radical anion adduct. As a manifestation of its Sn-centered radical nature, 2 reacts with diphenyl diselenide and p-benzoquinone to form Sn-S and Sn-O bonds, respectively.

Selective [2σ + 2σ] Cycloaddition Enabled by Boronyl Radical Catalysis: Synthesis of Highly Substituted Bicyclo[3.1.1]heptanes

In contrast to the traditional and widely-used cycloaddition reactions involving at least a π bond component, a [2σ + 2σ] radical cycloaddition between bicyclo[1.1.0]butanes (BCBs) and cyclopropyl ketones has been developed to provide a modular, concise, and atom-economical synthetic route to substituted bicyclo[3.1.1]heptane (BCH) derivatives that are 3D bioisosteres of benzenes and core skeleton of a number of terpene natural products. The reaction was catalyzed by a combination of simple tetraalkoxydiboron(4) compound B2pin2 and 3-pentyl isonicotinate. The broad substrate scope has been demonstrated by synthesizing a series of new highly functionalized BCHs with up to six substituents on the core with up to 99% isolated yield. Computational mechanistic investigations supported a pyridine-assisted boronyl radical catalytic cycle.

Lewis Base-Boryl Radicals Enabled Borylation Reactions and Selective Activation of Carbon-Heteroatom Bonds

ConspectusThe past decades have witnessed tremendous progress on radical reactions. However, in comparison with carbon, nitrogen, oxygen, and other main group element centered radicals, the synthetic chemistry of boron centered radicals was less studied, mainly due to the high electron-deficiency and instability of such 3-center-5-electron species. In the 1980s, Roberts and co-workers found that the coordination of a Lewis base (amines or phosphines) with the boron center could form 4-center-7-electron boryl radicals (Lewis base-boryl radicals, LBRs) that are found to be more stable. However, only limited synthetic applications were developed. In 2008, Curran and co-workers achieved a breakthrough with the discovery of N-heterocyclic carbene (NHC) boryl radicals, which could enable a range of radical reduction and polymerization reactions. Despite these exciting findings, more powerful and valuable synthetic applications of LBRs would be expected, given that the structures and reactivities of LBRs could be easily modulated, which would provide ample opportunities to discover new reactions. In this Account, a summary of our key contributions in LBR-enabled radical borylation reactions and selective activation of inert carbon-heteroatom bonds will be presented.Organoboron compounds have shown versatile applications in chemical society, and their syntheses rely principally on ionic borylation reactions. The development of mechanistically different radical borylation reactions allows synthesizing products that are inaccessible by traditional methods. For this purpose, we progressively developed a series of NHC-boryl radical mediated chemo-, regio-, and stereoselective radical borylation reactions of alkenes and alkynes, by which a wide variety of structurally diverse organoboron molecules were successfully prepared. The synthetic utility of these borylated products was also demonstrated. Furthermore, we disclosed a photoredox protocol for oxidative generation of NHC-boryl radicals, which enabled useful defluoroborylation and arylboration reactions.Selective bond activation is an ideal way to convert simple starting materials to value-added products, while the cleavage of inert chemical bonds, in particular the chemoselectivity control when multiple identical bonds are present in similar chemical environments, remains a long-standing challenge. We envisaged that finely tuning the properties of LBRs might provide a new solution to address this challenge. Recently, we disclosed a 4-dimethylaminopyridine (DMAP)-boryl radical promoted sequential C-F bond functionalization of trifluoroacetic acid derivatives, in which the α-C-F bonds were selectively snipped via a spin-center shift mechanism. This strategy enables facile conversion of abundantly available trifluoroacetic acid to highly valuable mono- and difluorinated molecules. Encouraged by this finding, we further developed a boryl radical enabled three-step sequence to construct all-carbon quaternary centers from a range of trichloromethyl groups, where the three C-Cl bonds were selectively cleaved by the rational choice of suitable boryl radical precursors in each step. Furthermore, a boryl radical promoted dehydroxylative alkylation of α-hydroxy carboxylic acid derivatives was achieved, allowing for the efficient conversion of some biomass platform molecules to high value products.

Captodative Effect Facilitates the Excitation in Diboron Molecule (CAAC)2 B2 (SH)2

Given the extraordinary versatility in chemical reactions and applications, boron compounds have gained increasing attentions in the past two decades. One of the remarkable advances is the unprecedented preparation of unsaturated boron species. Notably, Braunschweig et al. found that the cyclic (alkyl)(amino) carbenes (CAACs) stabilized diboron molecules (CAAC)₂ B₂ (SR)₂ host unpaired electrons and exist in the 90°-twisted diradical form, while other analogues, such as N-heterocyclic carbenes (NHCs), stabilized diboron molecules prefer a conventional B=B double bond. Since previous studies recognized the differences in the steric effect between CAAC and NHC carbenes, here we focused on the role of thiol substituents in (CAAC)₂ B₂ (SR)₂ by gradually localizing involved electrons. The co-planarity of the thiol groups and the consequent captodative effect were found to be the culprit for the 90°-twisted diradical form of (CAAC)₂ B₂ (SR)₂ . Computational analyses identified two forces contributing to the π electron movements. One is the "push" effect of lone pairs on the sulfur atoms which boosts the π electron delocalization between the BB center and CAACs. The other is the π electron delocalization within each (CAAC)B(SR) fragment where the pull effect originates from the π electron withdrawal by CAACs. There are two such independent and orthogonal push-pull channels which function mainly in individual (CAAC)B(SR) fragments. This enhanced π push-pull effect in the triplet state facilitates the electronic excitation in (CAAC)₂ B₂ (SR)₂ by reducing the singlet-triplet gap.

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.

Photoinduced Halogen-Atom Transfer by N-Heterocyclic Carbene-Ligated Boryl Radicals for C(sp3)-C(sp3) Bond Formation

Herein, we present a comprehensive study on the use of N-heterocyclic carbene (NHC)-ligated boryl radicals to enable C(sp³)-C(sp³) bond formation under visible-light irradiation via Halogen-Atom Transfer (XAT). The methodology relies on the use of an acridinium dye to generate the boron-centered radicals from the corresponding NHC-ligated boranes via single-electron transfer (SET) and deprotonation. These boryl radicals subsequently engage with alkyl halides in an XAT step, delivering the desired nucleophilic alkyl radicals. The present XAT strategy is very mild and accommodates a broad scope of alkyl halides, including medicinally relevant compounds and biologically active molecules. The key role of NHC-ligated boryl radicals in the operative reaction mechanism has been elucidated through a combination of experimental, spectroscopic, and computational studies. This methodology stands as a significant advancement in the chemistry of NHC-ligated boryl radicals, which had long been restricted to radical reductions, enabling C-C bond formation under visible-light photoredox conditions.

A linear Di-coordinate boron radical cation

The pursuit of di-coordinate boron radical has been continued for more than a half century, and their stabilization and structural characterization remains a challenge. Here we report the isolation and structural characterization of a linear di-coordinate boron radical cation, achieved by stabilizing the two reactive atomic orbitals of the central boron atom by two orthogonal π-donating and π-accepting functionalities. The electron deficient radical cation undergoes facile one-electron reduction to borylene and binds Lewis base to give heteroleptic tri-coordinate boron radical cation. The co-existence of half-filled and empty p orbitals at boron also allows the CO-regulated electron transfer to be explored. As the introduction of CO promotes the electron transfer from a tri-coordinate neutral boron radical to a boron radical cation, the removal of CO under vacuum furnishes the reverse electron transfer from borylene to yield a solution consisting of two boron radicals.

Modulating the frontier orbitals of L(X)Ga-substituted diphosphenes [L(X)GaP]2 (X = Cl, Br) and their facile oxidation to radical cations

Modulating the electronic structures of main group element compounds is crucial to control their chemical reactivity. Herein we report on the synthesis, frontier orbital modulation, and one-electron oxidation of two L(X)Ga-substituted diphosphenes [L(X)GaP]2 (X = Cl 2a, Br 2b; L = HC[C(Me)N(Ar)]2, Ar = 2,6-i-Pr2C6H3). Photolysis of L(Cl)GaPCO 1 gave [L(Cl)GaP]22a, which reacted with Me3SiBr with halide exchange to [L(Br)GaP]22b. Reactions with MeNHC (MeNHC = 1,3,4,5-tetramethylimidazol-2-ylidene) gave the corresponding carbene-coordinated complexes L(X)GaPP(MeNHC)Ga(X)L (X = Cl 3a, Br 3b). DFT calculations revealed that the carbene coordination modulates the frontier orbitals (i.e. HOMO/LUMO) of diphosphenes 2a and 2b, thereby affecting the reactivity of 3a and 3b. In marked contrast to diphosphenes 2a and 2b, the cyclic voltammograms (CVs) of the carbene-coordinated complexes each show one reversible redox event at E 1/2 = -0.65 V (3a) and -0.36 V (3b), indicating their one-electron oxidation to the corresponding radical cations as was confirmed by reactions of 3a and 3b with the [FeCp2][B(C6F5)4], yielding the radical cations [L(X)GaPP(MeNHC)Ga(X)L]B(C6F5)4 (X = Cl 4a, Br 4b). The unpaired spin in 4a (79%) and 4b (80%) is mainly located at the carbene-uncoordinated phosphorus atoms as was revealed by DFT calculations and furthermore experimentally proven in reactions with n Bu3SnH, yielding the diphosphane cations [L(X)GaPHP(MeNHC)Ga(X)L]B(C6F5)4 (X = Cl 5a, Br 5b). Compounds 2-5 were fully characterized by NMR and IR spectroscopy as well as by single crystal X-ray diffraction (sc-XRD), and compounds 4a and 4b were further studied by EPR spectroscopy, while their bonding nature was investigated by DFT calculations.

Site-Selective Functionalization of Carboranes at the Electron-Rich Boron Vertex: Photocatalytic B-C Coupling via a Carboranyl Cage Radical

Functionalization of carboranes in a vertex-specific manner is a perennial challenge. Here, we report a photocatalytic B-C coupling for the selective functionalization of carboranes at the boron site which is most distal to carbon. This reaction was achieved by the photo-induced decarboxylation of carborane carboxylic acids to generate boron vertex-centered carboranyl radicals. Theoretical calculations also demonstrate that the reaction more easily occurs at the boron site bearing higher electron density owing to the lower energy barrier for a single-electron transfer to generate a carboranyl radical. By using this strategy, a number of functionalized carboranes could be accessed through alkylation, alkenylation, and heteroarylation under mild conditions. Moreover, both a highly efficient blue emitter with a solid-state luminous efficiency of 42 % and a drug candidate for boron neutron capture therapy (BNCT) containing targeting and fluorine units were obtained.

Recent advances in stable main group element radicals: preparation and characterization

Radical species are significant in modern chemistry. Their unique chemical bonding and novel physicochemical properties play significant roles not only in fundamental chemistry, but also in materials science. Main group element radicals are usually transient due to their high reactivity. Highly stable radicals are often stabilized by π-delocalization, sterically demanding ligands, carbenes and weakly coordinating anions in recent years. This review presents the recent advances in the synthesis, characterization, reactivity and physical properties of isolable main group element radicals.

Isolation of Stable Borepin Radicals and Anions

Borepin, a 7-membered boron-containing heterocycle, has become an emerging molecular platform for the development of new materials and optoelectronics. While electron-deficient borepins are well-established, reduced electron-rich species have remained elusive. Herein we report the first isolable, crystalline borepin radical (2 a, 2 b) and anion (3 a, 3 b) complexes, which have been synthesized by potassium graphite (KC8 ) reduction of cyclic(alkyl)(amino) carbene-dibenzo[b,d]borepin precursors. Borepin radicals and anions have been characterized by EPR or NMR, elemental analysis, X-ray crystallography, and cyclic voltammetry. In addition, the bonding features have been investigated computationally using density functional theory.

Can a Wanzlick-like equilibrium exist between dicoordinate borylenes and diborenes?

Boron chemistry has experienced tremendous progress in the last few decades, resulting in the isolation of a variety of compounds with remarkable electronic structures and properties. Some examples are the singly Lewis-base-stabilised borylenes, wherein boron has a formal oxidation state of +I, and their dimers featuring a boron-boron double bond, namely diborenes. However, no evidence of a Wanzlick-type equilibrium between borylenes and diborenes, which would open a valuable route to the latter compounds, has been found. In this work, we combine DFT, coupled-cluster, multireference methods, and natural bond orbital/natural resonance theory analyses to investigate the electronic, structural, and kinetic factors controlling the reactivity of the transient CAAC-stabilised cyanoborylene, which spontaneously cyclotetramerises into a butterfly-type, twelve-membered (BCN)4 ring, and the reasons why its dimerisation through the boron atoms is hampered. The computations are also extended to the NHC-stabilised borylene counterparts. We reveal that the borylene ground state multiplicity dictates the preference for self-stabilising cyclooligomerisation over boron-boron dimerisation. Our comparison between NHC- vs. CAAC-stabilised borylenes provides a convincing rationale for why the reduction of the former always gives diborenes while a range of other products is found for the latter. Our findings provide a theoretical background for the rational design of base-stabilised borylenes, which could pave the way for novel synthetic routes to diborenes or alternatively non-dimerising systems for small-molecule activation.

Catalysis with Diboron(4)/Pyridine: Application to the Broad-Scope [3 + 2] Cycloaddition of Cyclopropanes and Alkenes

In contrast to the extensive but non-recyclable use of tetraalkoxydiboron(4) compounds as stoichiometric reagents in diverse reactions, this article reports an atom-economical reaction using a commercial diboron(4) as the catalyst. The key to success was designing a catalytic cycle for radical [3 + 2] cycloaddition involving a pyridine cocatalyst to generate from the diboron(4) catalyst and reversibly mediate the transfer of boronyl radicals. In comparison with known [3 + 2] cycloaddition with transition metal-based catalysts, the current reaction features not only metal-free conditions, inexpensive and stable catalysts, and simple operation but also remarkably broadened substrate scope. In particular, previously unusable cyclopropyl ketones without an activating group and/or alkenes with 1,2-disubstitution and 1,1,2-trisubstitution patterns were successfully used for the first time. Consequently, challenging cyclopentane compounds with various levels of substitution (65 examples, 57 new products, up to six substituents at all five ring atoms) were readily prepared in generally high to excellent yield and diastereoselectivity. The reaction was also successfully applied in concise formal synthesis of an anti-obesity drug and building natural product-like complex bridged or spirocyclic compounds. Mechanistic experiments and computational investigation support the proposed radical relay catalysis featuring a pyridine-assisted boronyl radical catalyst. Overall, this work demonstrates the first approach to use tetraalkoxydiboron(4) compounds as catalysts and may lead to the development of new, green, and efficient transition metal-like boron-catalyzed organic reactions.

Visible-Light-Promoted Nickel-Catalyzed Cross-Coupling of Iodocarboranes with (Hetero)Arenes via Boron-Centered Carboranyl Radicals

A general strategy for the generation of hypervalent boron-centered carboranyl radicals at the B(3), B(4), and B(9) positions has been developed for the first time via visible-light-promoted iodine atom abstraction from iodo-o-carboranes by low-valent nickel complex. These radicals react with various (hetero)arenes to afford a wide range of cage B-arylated carborane derivatives at room temperature in very good to excellent yields with a broad substrate scope. Their electrophilicities are dependent on the vertex charges of the cage and follow the order B(3) > B(4) > B(9). Both visible light and nickel catalyst are proved critical to the generation of boron-centered carboranyl radicals. The involvement of boron radicals is supported by control experiments. A reaction mechanism associated with these reactions is also proposed. This strategy offers a new protocol for the generation of boron-centered carboranyl radicals at the selected boron vertex, leading to a facile synthesis of a large class of cage boron substituted carborane molecules.

Recent Advances in the Domain of Cyclic (Alkyl)(Amino) Carbenes

Isolation of cyclic (alkyl) amino carbenes (cAACs) in 2005 has been a major achievement in the field of stable carbenes due to their better electronic properties. cAACs and bicyclic(alkyl)(amino)carbene (BicAAC) in essence are the most electrophilic as well as nucleophilic carbenes are known till date. Due to their excellent electronic properties in terms of nucleophilic and electrophilic character, cAACs have been utilized in different areas of chemistry, including stabilization of low valent main group and transition metal species, activation of small molecules, and catalysis. The applications of cAACs in catalysis have opened up new avenues of research in the field of cAAC chemistry. This review summarizes the major results of cAAC chemistry published until August 2021.

Utilization of BODIPY-based redox events to manipulate the Lewis acidity of fluorescent boranes

This report describes the implementation of a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dye into the ligand framework of a borane. The redox-active nature of the BODIPY dye is utilized to generate a family of molecular boranes that are capable of exhibiting tunable Lewis acidities through BODIPY-based redox events.

A cationic sulfur-hydrocarbon triradical with an excited quartet state

The triptycene-bridged tris(thianthrene) compound 1 was designed and synthesized. Three-electron oxidation of 1 by NO[Al(OC(CF₃)₃)₄], followed by crystallization at two different temperatures resulted in the triradical trication salts 2a and 2b respectively, which feature different crystal packing patterns. The triradical trications in 2a and 2b both feature a doublet ground state which can be thermally populated to a quartet state, representing the first examples of cationic main-group triradicals.

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.

Synthesis and redox activity of carbene-coordinated group 13 metal radicals

Carbenes are known to stabilize main group element compounds with unusual electronic properties. Herein, we report the synthesis of carbene-stabilized group 13 metal radicals (cAAC)MX2(IPr) (M = Al, X =...

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.

A crystalline radical cation derived from Thiele's hydrocarbon with redox range beyond 1 V

Thiele’s hydrocarbon occupies a central role as an open-shell platform for new organic materials, however little is known about its redox behaviour. While recent synthetic approaches involving symmetrical carbene substitution of the CPh₂ termini yield isolable neutral/dicationic analogues, the intervening radical cations are much more difficult to isolate, due to narrow compatible redox ranges (typically < 0.25 V). Here we show that a hybrid BN/carbene approach allows access to an unsymmetrical analogue of Thiele’s hydrocarbon 1, and that this strategy confers markedly enhanced stability on the radical cation. 1^•+ is stable across an exceptionally wide redox range (> 1 V), permitting its isolation in crystalline form. Further single-electron oxidation affords borenium dication 1²⁺, thereby establishing an organoboron redox system fully characterized in all three redox states. We perceive that this strategy can be extended to other transient organic radicals to widen their redox stability window and facilitate their isolation.

Organic molecules that can access various redox states have potential applications in electronics, batteries, catalysis, among others. Here the authors report the preparation of an unsymmetrical organoboron analogue of Thiele’s hydrocarbon and study its one- and two-electron oxidation reactions; remarkably, the radical cation is stable over a redox range of > 1 V and can also be isolated.

A stable triplet diradical emitter

Molecules with luminescence have been extensively investigated, but the luminescence of a stable molecule with a triplet ground state has not been observed. Synthesis of boron-containing radicals has attracted lots of interest because of their unique electronic structures and potential applications in organic semiconductors. Though some boron-based diradicals have been reported, neutral boron-containing diradicals with triplet ground states are rare. Herein two borocyclic diradicals with different substituents (3 and 4) have been isolated. Their electronic structures were investigated by EPR and UV spectroscopy, and SQUID magnetometry, in conjunction with DFT calculations. Both experiment and calculation suggest that 3 is an open shell singlet diradical while 4 is a triplet ground state diradical with a large singlet–triplet gap (0.25 kcal mol⁻¹). Both diradicals show multi fluorescence peaks (3: 414, 431, and 470 nm; 4: 420, 433, and 495 nm). 3 displays multiple redox steps and is a potential material towards the design of high-density memory devices. 4 represents the first example of a neutral triplet boron-containing diradical with a strong ferromagnetic interaction, and also is the first stable triplet diradical emitter.

Stable borocyclic diradical emitters with a tunable ground state.

Understanding the Unique Antioxidation Property of Boron-Based Catalysts during Oxidative Dehydrogenation of Alkanes

Boron-based catalysts show excellent performance in oxidative dehydrogenation (ODH) of light alkanes to alkenes with high selectivity and extremely good antioxidation properties. However, the anti-deep-oxidation mechanism remains unclear. Herein, we chose h-BN and B₂O₃ as representative boron-based catalysts to investigate their reactions with two important intermediates in the light alkane ODH, Et· (evolving to ethene) and EtO· (evolving to ethene or CO_x), to elucidate the origin of the antioxidation of alkanes. The density functional theory calculations reveal that surface boron sites could eliminate alkoxy in their vicinity, resulting in exceptional inhibition of alkane deep-oxidation. The analysis of the electronic and geometric structures of key stationary points showed that the oxophilicity of B determined the low deep-oxidation of alkanes, and the homoleptic coordination of B with all three ligating atoms being O moderately enhanced its oxophilicity. This work represents a novel conceptual advance in the mechanistic understanding of alkane ODH.