Boron clusters in luminescent materials

Carborane-arene fused boracyclic analogues of polycyclic aromatic hydrocarbons accessed by intramolecular borylation

Arenes are 2D aromatics while dicarbadodecaborane clusters are branded as 3D aromatic molecules. In this work we prepare molecules that feature fused 2D/3D aromatic systems that represent boron-doped analogues of polycyclic aromatic hydrocarbons. The electron withdrawing nature of the ortho-carborane substituent enables swift arene borylation on boron bromide or hydride precursors to furnish five- and six-membered boracycles in conjugation with the arene. The mechanism was modeled by DFT computations implying a concerted transition state and analyzing the photophysical properties revealed high quantum yields in the six-membered systems.

Unveiling Non-Covalent Interactions in Novel Cooperative Photoredox Systems for Efficient Alkene Oxidation in Water

A new cooperative photoredox catalytic system, [RuII(trpy)(bpy)(H2O)][3,3'-Co(8,9,12-Cl3-1,2-C2B9H8)2]2, 5, has been synthesized and fully characterized for the first time. In this system, the photoredox catalyst [3,3'-Co(8,9,12-Cl3-1,2-C2B9H8)2]-[Cl6-1]-, a metallacarborane, and the oxidation catalyst [RuII(trpy)(bpy)(H2O)]2+, 2 are linked by non-covalent interactions. This compound, along with the one previously synthesized by us, [RuII(trpy)(bpy)(H2O)][(3,3'-Co(1,2-C2B9H11)2]2, 4, are the only examples of cooperative molecular photocatalysts in which the catalyst and photosensitizer are not linked by covalent bonds. Both cooperative systems have proven to be efficient photocatalysts for the oxidation of alkenes in water through Proton Coupled Electron Transfer processes (PCETs). Using 0.05 mol% of catalyst 4, total conversion values were achieved after 15 min with moderate selectivity for the corresponding epoxides, which decreases with reaction time, along with the TON values. However, with 0.005 mol% of catalyst, the conversion values are lower, but the selectivity and TON values are higher. This occurs simultaneously with an increase in the amount of the corresponding diol for most of the substrates studied. Photocatalyst 4 acts as a photocatalyst in both the epoxidation of alkenes and their hydroxylation in aqueous medium. The hybrid system 5 shows generally higher conversion values at low loads compared to those obtained with 4 for most of the substrates studied. However, the selectivity values for the corresponding epoxides are lower even after 15 min of reaction. This is likely due to the enhanced oxidizing capacity of CoIV in catalyst 5, resulting from the presence of more electron-withdrawing substituents on the metallacarborane platform.

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.

Ortho-Carborane Decorated Multi-Resonance TADF Emitters: Preserving Local Excited State and High Efficiency in OLEDs

A novel class of o-carboranyl luminophores, 2CB-BuDABNA (1) and 3CB-BuDABNA (2) is reported, in which o-carborane moieties are incorporated at the periphery of the B,N-doped multi-resonance thermally activated delayed fluorescence (MR-TADF) core. Both compounds maintain the inherent local emission characteristics of their MR-emitting core, exhibiting intense MR-TADF with high photoluminescence quantum yields in toluene and rigid states. In contrast, the presence of the dark lowest-energy charge transfer state, induced by cage rotation in THF, is suggested to be responsible for emission quenching in a polar solvent. Despite the different arrangement of the cage on the DABNA core, both 1 and 2 show red-shifted emissions compared to the parent compound BuDABNA (3). By utilizing 1 as the emitter, high-efficiency blue organic light-emitting diodes (OLEDs) are achieved with a remarkable maximum external quantum efficiency of 25%, representing the highest reported efficiency for OLEDs employing an o-carboranyl luminophore as the emitter.

Synthesis of Carboranylated Dihydropyrrolo[1,2-a]quinoxalines and Dihydroindolo[1,2-a]quinoxalines by BF3·OEt2-Catalyzed Heterocyclization of C-Formyl-o-carboranes and Investigation of Their Oxidation Stability

A BF3·OEt2-catalyzed synthesis of carboranylated dihydropyrrolo[1,2-a]quinoxalines and dihydroindolo[1,2-a]quinoxalines in 30-99% yields is presented through the heterocyclization of various C-modified C-formyl-o-carboranes with 1-(2-aminophenyl)-pyrroles/indoles. A systematic comparative investigation of their oxidation stability in air confirmed that 4-carboranyl-4,5-dihydropyrrolo[1,2-a]quinoxaline had better stability than the 4-phenyl analogue. A cage-deboronation reaction for N-acetyl-substituted carboranylated dihydropyrrolo[1,2-a]quinoxaline produced the corresponding 7,8-nido-carborane cesium salt. A kinetic resolution was also realized to obtain an optically pure carboranylated N-heterocycle scaffold bearing a carborane cage carbon-bonded chiral stereocenter.

Palladium-Catalyzed Regioselective B(3,5)-Dialkenylation and B(4)-Alkenylation of o-Carboranes

Picolyl group directed B(3,5)-dialkenylation and B(4)-monoalkenylation of o-carboranes has been developed with a very low palladium catalyst loading. The degree of substitution is determined by the cage C(2)-substituents due to steric reasons. On the basis of experimental results, a plausible mechanism is proposed including electrophilic palladation and alkyne insertion followed by protonation.

Iridium-catalyzed selective arylation of B(6)-H of 3-aryl-o-carboranes with arylboronic acid via direct B-H activation

This work discloses an iridium-catalyzed selective arylation of B(6)-H of 3-Ar-o-carboranes with arylboronic acid via direct B-H activation for the first time. A series of unsymmetric and symmetric 3,6-diaryl-o-carboranes decorated with diverse active groups have been synthesized with moderate to excellent yields under mild conditions. This work offers an efficient approach for selective arylation of B(6)-H with arylboronic acid and has important value for selective functionalization of o-carboranes.

Vertex Differentiation Strategy for Tuning the Physical Properties of closo-Dodecaborate Weakly Coordinating Anions

We report the synthesis and characterization of various compounds containing the 1,7,9-hydroxylated closo-dodecahydrododecaborate (B12H9(OH)32-) cluster motif. Specifically, we show how the parent compound can be synthesized on the multigram scale and further perhalogenated, leading to a new class of vertex-differentiated weakly coordinating anions. We show that a postmodification of the hydroxyl groups by alkylation affords further opportunities for tailoring these anions' stability, steric bulk, and solubility properties. The resulting dodecaborate-based salts were subjected to a full thermal and electrochemical stability evaluation, showing that many of these anions maintain thermal stability up to 500 °C and feature no redox activity below ∼1 V vs Fc/Fc+. Mixed hydroxylated/halogenated clusters show enhanced solubility compared to their purely halogenated analogs and retain weakly coordinating properties in the solid state, as demonstrated by ionic conductivity measurements of their Li+ salts.

Reaction of 4-Bpin-o-Carborane with Ketones: Sequential Carbon Vertex Alkylation and B-B Bond Activation

Diboron compounds are important reagents in a series of transition metal catalyzed or metal-free borylation reactions. We describe herein a unique reactivity of 4-Bpin-o-carborane with ketones under basic conditions, leading to sequential cage carbon alkylation, B-B bond activation and unexpected O-migration. The reaction was compatible with a good substrate scope including dialkyl or alkyl aryl ketones. The reaction mechanism is also proposed, involving cage CH deprotonation, nucleophilic attack of ketone, and O-migration along with B-B bond cleavage.

Macropolyhedral syn-B18H22, the "Forgotten" Isomer

The chemistry and physics of macropolyhedral B18H22 clusters have attracted significant attention due to the interesting photophysical properties of anti-B18H22 (blue emission, laser properties) and related potential applications. We have focused our attention on the "forgotten" syn-B18H22 isomer, which has received very little attention since its discovery compared to its anti-B18H22 isomer, presumably because numerous studies have reported this isomer as nonluminescent. In our study, we show that in crystalline form, syn-B18H22 exhibits blue fluorescence and becomes phosphorescent when substituted at various positions on the cluster, associated with peculiar microstructural-dependent effects. This work is a combined theoretical and experimental investigation that includes the synthesis, separation, structural characterization, and first elucidation of the photophysical properties of three different monothiol-substituted cluster isomers, [1-HS-syn-B18H21] 1, [3-HS-syn-B18H21] 3, and [4-HS-syn-B18H21] 4, of which isomers 1 and 4 have been proved to exist in two different polymorphic forms. All of these newly substituted macropolyhedral cluster derivatives (1, 3, and 4) have been fully characterized by NMR spectroscopy, mass spectrometry, single-crystal X-ray diffraction, IR spectroscopy, and luminescence spectroscopy. This study also presents the first report on the mechanochromic shift in the luminescence of a borane cluster and generally enriches the area of rather rare boron-based luminescent materials. In addition, we present the first results proving that they are useful constituents of carbon-free self-assembled monolayers.

Charge Transfer Complex-Enabled Synthesis of (Hetero)arylated m-Carboranes from m-Carborane Phosphonium Salts

A photoinduced charge transfer complex (CTC)-enabled photoreduction of carborane phosphonium salts for the cage carbon (hetero)arylation of carboranes was developed. It offers a convenient approach for introducing a wide range of aryl and heteroaryl groups, such as pyrroles, thiophenes, indoles, thianaphthenes, benzofurans, pyridines, and benzenes, into carboranes. This strategy offers operational simplicity, mild reaction conditions, and a broad substrate scope, making it highly advantageous.

Improvement in Radiative Efficiency Via Intramolecular Charge Transfer in ortho-Carboranyl Luminophores Modified with Functionalized Biphenyls

In this study, we found that the electronic effects of the functional groups on aromatic units attached to o-carboranyl species can enhance the efficiency of intramolecular charge transfer (ICT)-based radiative decay processes. Six o-carboranyl-based luminophores having attached functionalized biphenyl groups with CF₃, F, H, CH₃, C(CH₃)₃, and OCH₃ substituents were prepared and fully characterized by multinuclear magnetic resonance spectroscopy. In addition, their molecular structures were determined by single-crystal X-ray diffractometry, which revealed that the distortion of the biphenyl rings and the geometries around the o-carborane cages were similar. All compounds exhibited ICT-based emissions in the rigid state (solution at 77 K and film). Intriguingly, the quantum efficiencies (Φ_em) of five compounds (that of the group with CF₃ could not be measured because of its extremely weak emissions) in the film state increased gradually as the electron-donating power of the terminal functional group modifying the biphenyl moiety increased. Furthermore, the nonradiative decay constants (k_nr) for the group with OCH₃ were estimated to be one-tenth of those for the group with F, whereas the radiative decay constants (k_r) for the five compounds were similar. The dipole moments (μ) calculated for the optimized first excited state (S₁) structures gradually increased, from that of the group with CF₃ to that of the group with OCH₃, implying that the inhomogeneity of the molecular charge distribution was enhanced by electron donation. The electron-rich environment formed as a result of electron donation led to efficient charge transfer to the excited state. Both experimental and theoretical findings revealed that the electronic environment of the aromatic moiety in o-carboranyl luminophores can be controlled to accelerate or interrupt the ICT process in the radiative decay of excited states.

Catalytic asymmetric synthesis of carboranylated diols bearing two adjacent stereocenters located at the α,β-position of o-carborane cage carbon

Despite the great interest in carborane-containing molecules, there is a lack of literature on the generation of central chiralities, via catalytic asymmetric transformations using prochiral carboranyl substrates. Herein, we have synthesized novel optically active icosahedral carborane-containing diols via Sharpless catalytic asymmetric dihydroxylation of carborane-derived alkenes, under mild conditions. The reaction showed a good substrate scope with 74-94% yields and 92->99% ee. This synthetic approach facilitated the creation of two adjacent stereocenters respectively located at the α,β-position of o-carborane cage carbon, with a single syn-diastereoisomer. In addition, the obtained chiral carborane-containing diol product can be transformed to cyclic sulfate and can subsequently undergo a nucleophilic substitution and reduction to obtain the unexpected nido-carboranyl derivatives of chiral amino alcohols in the form of zwitterions.

Facile Construction of New Hybrid Conjugation via Boron Cage Extension

Aromatic polycyclic systems have been extensively utilized as structural subunits for the preparation of various functional molecules. Currently, aromatics-based polycyclic systems are predominantly generated from the extension of two-dimensional (2D) aromatic rings. In contrast, polycyclic compounds based on the extension of three-dimensional (3D) aromatics such as boron clusters are less studied. Here, we report three types of boron cluster-cored tricyclic molecular systems, which are constructed from a 2D aromatic ring, a 3D aromatic nido-carborane, and an alkyne. These new tricyclic compounds can be facilely accessed by Pd-catalyzed B-H activation and the subsequent cascade heteroannulation of carborane and pyridine with an alkyne in an isolated yield of up to 85% under mild conditions without any additives. Computational results indicate that the newly generated ring from the fusion of the 3D carborane, the 2D pyridyl ring, and an alkyne is non-aromatic. However, such fusion not only leads to a ¹H chemical shift considerably downfield shifted owing to the strong diatropic ring current of the embedded carborane but also devotes to new/improved physicochemical properties including increased thermal stability, the emergence of a new absorption band, and a largely red-shifted emission band and enhanced emission efficiency. Besides, a number of bright, color-tunable solid emitters spanning over all visible light are obtained with absolute luminescence efficiency of up to 61%, in contrast to aggregation-caused emission quenching of, e.g., Rhodamine B containing a 2D-aromatics-fused structure. This work demonstrates that the new hybrid conjugated tricyclic systems might be promising structural scaffolds for the construction of functional molecules.

Regiodivergent metal-catalyzed B(4)- and C(1)-selenylation of o-carboranes

Regiodivergent transition metal-catalyzed B(4)- and C(1)-selenylation reactions of o-carboranes have been demonstrated. Namely, Ru(ii)-catalysis selectively generated B(4)-selenylated o-carboranes from the reaction of o-carborane acids with arylselenyl bromides with the release of carbon dioxide. In contrast, Pd(ii) catalysis provided exclusively C(1)-selenylated o-carboranes from the decarboxylative reaction of o-carborane acids with diaryl diselenides. In contrast to previous milestones in this area, these reactions demonstrate broad substrate scope with excellent yields. Combination of these methods leads to the formation of B(4)-C(1)-diselenylated o-carboranes. DFT studies revealed the mechanism of the Ru-process, with initial selenylation of the carborane cluster discovered to be essential for an energetically reasonable decarboxylation. This results in selenylation on the B(4) position prior to the decarboxylation event at C(1). This contrasted with the Pd-process in which the ready decarboxylation at C(1) leads to selenylation at C(1).

Iridium-catalyzed regioselective B(3,6)-dialkenylation or B(4)-alkenylation of o-carboranes via B-H activation and 1,2-carbon migration of alkynes

An efficient Ir-catalyzed cage boron alkenylation of 1-(2'-picolyl)-o-carboranes with diarylacetylenes has been developed, leading to a wide variety of B-H geminal addition products via 1,2-carbon migration of alkynes. The steric effect of cage carbon substituents has a great impact on the regioselectivity of such alkenylation reactions.

Relationship between the Molecular Geometry and the Radiative Efficiency in Naphthyl-Based Bis-Ortho-Carboranyl Luminophores

The efficiency of intramolecular charge transfer (ICT)-based emission on π-aromatic-group-appended closo-ortho-carboranyl luminophores is known to be affected by structural fluctuations and molecular geometry, but investigation of this relationship has been in progress to date. In this study, four naphthyl-based bis-o-carboranyl compounds, in which hydrogen (15CH and 26CH) or trimethysilyl groups (15CS and 26CS) were appended at the o-carborane cage, were synthesized and fully characterized. All the compounds barely displayed an emissive trace in solution at 298 K; however, 15CH and 26CH distinctly exhibited a dual emissive pattern in rigid states (in solution at 77 K and in films), attributed to locally excited (LE) and ICT-based emission, while 15CS and 26CS showed strong ICT-based greenish emission. Intriguingly, the molecular structures of the four compounds, analyzed by single X-ray crystallography, showed that the C-C bond axis of the o-carborane cage in the trimethysilyl group-appended compounds 15CS and 26CS were more orthogonal to the plane of the appended naphthyl group than those in 15CH and 26CH. These features indicate that 15CS and 26CS present an efficient ICT transition based on strong exo-π-interaction, resulting in a higher quantum efficiency (Φ_em) for ICT-based radiative decay than those of 15CH and 26CH. Moreover, the 26CS structure revealed most orthogonal geometry, resulting in the highest Φ_em and lowest k_nr values for the ICT-based emission. Consequently, all the findings verified that efficient ICT-based radiative decay of aromatic group-appended o-carboranyl luminophores could be achieved by the formation of a specific geometry between the o-carborane cage and the aromatic plane.

Metal-Catalyzed and Metal-Free Nucleophilic Substitution of 7-I-B18H21

In this work, two pathways of reactivity are investigated to generate site-specific substitutions at the B7 vertex of the luminescent boron cluster, anti-B₁₈H₂₂. First, a palladium-catalyzed cross-coupling reaction utilizing the precursor 7-I-B₁₈H₂₁ and a series of model nucleophiles was developed, ultimately producing several B-N- and B-O-substituted species. Interestingly, the B-I bond in this cluster can also be substituted in an uncatalyzed fashion, leading to the formation of various B-N, B-O, and B-S products. This work highlights intricate differences corresponding to these two reaction pathways and analyzes the role of solvents and additives on product distributions. As a result of our synthetic studies, seven new B₁₈-based clusters were synthesized, isolated, and characterized by mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. The photoluminescence properties of two structurally similar ether and thioether products were further investigated, with both exhibiting blue fluorescence in solution at 298 K and long-lived green or yellow phosphorescence at 77 K. Overall, this work shows, for the first time, the ability to perform substitution of a boron-halogen bond with nucleophiles in a B₁₈-based cluster, resulting in the formation of photoluminescent molecules.

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.

N-Ligand-Enabled Aromatic Nucleophilic Amination of 1,2-Diaryl-o-Carboranes with (R2 N)2 Mg for Selective Synthesis of 4-R2 N-o-Carboranes and 2-R2 N-m-Carboranes

The nucleophilic aromatic BH substitution reaction of carboranes is uncommon, compared to the electrophilic one. This work reported a pyridine-enabled transition-metal-free regioselective nucleophilic aromatic cage B(4)-H amination of 1,2-diaryl-o-carboranes with magnesium bisamides, giving a series of B(4)-aminated o-carboranes. DFT calculations showcased a stepwise B-N formation/B-H cleavage process, in which Mg-H formation/cage closure is the rate-determining step. Unprecedentedly, in the presence of 4,4'-di-tert-butyl-2,2'-dipyridyl (dtbpy), a tandem B(4)-amination/cage isomerization reaction of o-carboranes was discovered for the facile preparation of B(2)-aminated m-carboranes. Control experiments indicated that magnesium complex, bidentate ligand (dtbpy) and reaction temperature were crucial in the cage isomerization process. This direct nucleophilic aromatic cage B-H amination reaction offers an alternative strategy for selective amination of o- and m-carboranes.

Cu(OTf)2/NBS-Mediated Tandem Reaction of 1-Cinnamyl Alcohol-o-Carboranes via Ring Opening of Oxetane with Arenes: An Alternative Approach for the Synthesis of C-Alkenyl-o-Carboranes

The Cu(OTf)₂/NBS-mediated tandem reaction of 1-cinnamyl alcohol-o-carboranes for the synthesis of C-alkenyl-o-carboranes has been developed. Mechanism studies demonstrated that the Cu(OTf)₂-promoted ring opening of oxetane with electron-rich arenes as soft nucleophiles was involved and was the key step for the transformation. This work provided an alternative strategy for the synthesis of C-alkenyl-o-carboranes, which has an important reference for the synthesis of o-carborane derivatives with diversity.

Direct Phenylation of nido-B10H14

As a preliminary step toward its condensation into the porous polymer Activated Borane, the thermolysis of nido-B₁₀H₁₄ (1) in benzene at 200 °C results in the generation of a number of phenylated borane molecular species. The principal product is the new monophenylated compound 5-Ph-nido-B₁₀H₁₃ (2), isolated in 48% yield (based on consumption of 1) and structurally characterized by single-crystal X-ray diffraction analysis, NMR, and mass spectrometry along with other minor products, such as 6-Ph-nido-B₁₀H₁₃ (3), for which we observe UV-light-driven conversion into 2 via a "vertex-flip" mechanism, and novel diphenylated 5,8-Ph₂-nido-B₁₀H₁₂ (4). Together, the phenylated derivatives provide a valuable insight into the assembly of Activated Borane and ultimately inform on its structure. The new compounds also display strong blue fluorescence in both solid-state and in solution and are the first examples of the direct phenylation of nido-B₁₀H₁₄, thus opening the door to the straight-forward synthesis of highly luminescent organic-borane hybrid systems.

Carboranes in drug discovery, chemical biology and molecular imaging

There exists a paucity of structural innovation and limited molecular diversity associated with molecular frameworks in drug discovery and biomolecular imaging/chemical probe design. The discovery and exploitation of new molecular entities for medical and biological applications will necessarily involve voyaging into previously unexplored regions of chemical space. Boron clusters, notably the carboranes, offer an alternative to conventional (poly)cyclic organic frameworks that may address some of the limitations associated with the use of novel molecular frameworks in chemical biology or medicine. The high thermal stability, unique 3D structure and aromaticity, kinetic inertness to metabolism and ability to engage in unusual types of intermolecular interactions, such as dihydrogen bonds, with biological receptors make carboranes exquisite frameworks in the design of probes for chemical biology, novel drug candidates and biomolecular imaging agents. This Review highlights the key developments of carborane derivatives made over the last decade as new design tools in medicinal chemistry and chemical biology, showcasing the versatility of this unique family of boron compounds.

Water-Stable Carborane-Based Eu3+/Tb3+ Metal-Organic Frameworks for Tunable Time-Dependent Emission Color and Their Application in Anticounterfeiting Bar-Coding

Luminescent lanthanide metal-organic frameworks (Ln-MOFs) have been shown to exhibit relevant optical properties of interest for practical applications, though their implementation still remains a challenge. To be suitable for practical applications, Ln-MOFs must be not only water stable but also printable, easy to prepare, and produced in high yields. Herein, we design and synthesize a series of m CB-Eu _y Tb _1-y (y = 0-1) MOFs using a highly hydrophobic ligand mCBL1: 1,7-di(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane. The new materials are stable in water and at high temperature. Tunable emission from green to red, energy transfer (ET) from Tb³⁺ to Eu³⁺, and time-dependent emission of the series of mixed-metal m CB-Eu _y Tb _1-y MOFs are reported. An outstanding increase in the quantum yield (QY) of 239% of mCB-Eu (20.5%) in the mixed mCB-Eu_0.1Tb_0.9 (69.2%) is achieved, along with an increased and tunable lifetime luminescence (from about 0.5 to 10 000 μs), all of these promoted by a highly effective ET process. The observed time-dependent emission (and color), in addition to the high QY, provides a simple method for designing high-security anticounterfeiting materials. We report a convenient method to prepare mixed-metal Eu/Tb coordination polymers (CPs) that are printable from water inks for potential applications, among which anticounterfeiting and bar-coding have been selected as a proof-of-concept.

Functionalization of o-carboranes via carboryne intermediates

Carborynes (1,2-dehydro-o-carborane and 1,3-dehydro-o-carborane), three-dimensional analogues of benzyne, can be generated in situ from the precursors 1-X-2-Li-1,2-C₂B₁₀H₁₀ (X = Br, I, OTs, OTf), or 1-Me₃Si-2-[IPh(OAc)]-1,2-C₂B₁₀H₁₀ or [1-Li-3-N₂-1,2-C₂B₁₀H₁₀][BF₄]. They are a class of very useful synthons for the synthesis of a large variety of functionalized carborane derivatives for potential application in medicine, materials science and organometallic/coordination chemistry. The experimental data demonstrate that there is a correspondence between the reactions of carborynes and those of benzyne with alkenes, dienes, alkynes, aromatics or heteroaromatics in a pericyclic reaction fashion. On the other hand, carborynes have unique properties of their own owing to their steric/electronic features. They undergo regioselective sp²/sp³ C-H bond and N-Li bond insertion reactions, which has not been observed for benzyne. This review provides a comprehensive overview of recent advances in this interesting research field with considerable attention devoted to the reaction modes and the mechanisms involved.

Synthesis and Crystal Structure of 9,12-Dibromo-ortho-Carborane

Synthesis, NMR spectral data and crystal structure of 9,12-dibromo derivative of ortho-carborane are reported.

Color-tuning and manipulation of aggregation-induced emission efficiency of o-carborane–tetraphenylethylene dyads through substituted o-carboranes

Two new carborane-containing tetraphenylethylene derivatives (1 and 2) were designed and synthesized. Together with our previously published compounds (3–5), we studied structure–activity relationships in detail. Results showed that compounds 1...

Benchmarking the dynamic luminescent properties and UV stability of B18H22-based materials

The dynamic photoluminescent properties, and potential quenching mechanisms, of anti-B18H22, 4,4’-Br2-anti-B18H20, and 4,4’-I2-anti-B18H20 are investigated in solution and polymer films. UV stability studies of the neat powders show no decomposition...

(7,8-Dicarba-nido-undecaboran-7-yl)acetic acid: synthesis of individual enantiomers and the first example of the determination of the absolute configuration of chiral monosubstituted nido-carborane

Starting from achiral closo-carborane and using natural amino acid as chiral auxiliary, individual (RP)- and (SP)-enantiomers of C-monosubstituted nido-carboranes were obtained.

Alkene insertion reactivity of a o-carboranyl-substituted 9-borafluorene

The synthesis of 9-borafluorene with an electron-withdrawing o-carboranyl substituent and its reactions with a series of alkenes are described. The o-carboranyl substituent is bonded via one of the cluster carbon atoms to the boron atom of the 9-borafluorene moiety. In all cases, the reactions afford partly saturated analogs of borepins (i.e. 6,7-dihydroborepins) by unprecedented alkene insertion into the endocyclic B–C bond of the borole ring. Comparative studies with 9-bromo-9-borafluorene illustrate the superior insertion reactivity of the carboranyl-substituted derivative. A suite of experimental and computational techniques disclose the unique properties of the 9-borafluorene and provide insight into how the 9-carboranyl substituent affects its chemical reactivity.

A 9-carboranyl-substituted 9-borafluorene is reported, which is capable of undergoing efficient ring expansion to 6,7-dihydroborepins by a previously unknown alkene insertion.

Regioselectivity of Pd-catalyzed o-carborane arylation: a theoretical view

B(3)-Arylation is unfavorable because the steric repulsion between the substituent group on C(2) and the metal moiety would lead to significant distortion of o-carborane and would result in a higher activation energy for reductive elimination.

Manipulation of electronic and structural effects on the solid-state emission of multiple linked anthracenyl-o-carborane dyads

A direct boost of emission efficiency is presented based on the strategy of linking multiple weak AIE-active anthracenyl-o-carborane dyads together for improving solid-state luminescence.

Rhodium-catalyzed sequential B(3)-, B(4)-, and B(5)-trifunctionalization of o-carboranes with three different substituents

A rhodium-catalyzed one-pot trifunctionalization of o-carboranes with three different substituents via a carboxy group directed sequential B(5)-alkenylation, B(4)-alkyne annulation and B(3)-acyloxylation has been developed for the first time, leading to the synthesis of a new class of B(3,4,5)-trisubstituted o-carborane derivatives. Treatment of 1-COOH-2-CH₃-o-C₂B₁₀H₁₀ with ArCCAr in the presence of a [Cp*RhCl₂]₂ catalyst and a Cu(OPiv)₂ oxidant gave 1,4-[COOC(Ar)C(Ar)]-2-Me-3-OPiv-5-[C(Ar)CH(Ar)-o-C₂B₁₀H₇ in good to high yields. This protocol represents a new strategy for the catalytic selective polyfunctionalization of carboranes with different substituents.

Synthesis of 3-Aryl-ortho-carboranes with Sensitive Functional Groups

A simple and efficient method was developed for the one-pot synthesis of 3-aryl derivatives of ortho-carborane with sensitive functional groups using 3-iodo-ortho-carborane and aryl zinc bromides that were generated in situ. A series of 3-aryl-ortho-carboranes, including those containing nitrile and ester groups, 3-RC₆H₄-1,2-C₂B₁₀H₁₁ (R = p-Me, p-NMe₂, p-OCH₂OMe, p-OMe, o-CN, p-CN, o-COOEt, m-COOEt, p-COOEt) was synthesized using this approach. The solid-state structures of 3-RC₆H₄-1,2-C₂B₁₀H₁₁ (R = p-OMe, o-CN, and p-CN) were determined by single crystal X-ray diffraction. The intramolecular hydrogen bonding involving the ortho-substituents of the aryl ring and the CH and BH groups of carborane was discussed.

Cage-···Cage- Interaction: Boron Cluster-Based Noncovalent Bond and Its Applications in Solid-State Materials

Carboranes are boron-carbon clusters with important applications in the fields of materials, catalysis, pharmaceuticals, etc. However, the noncovalent interactions that could determine the solid-state structures and properties of such boron clusters have rarely been investigated. Herein, inspired by the coordinate bond in metallacarborane or ferrocene, the boron cluster-based noncovalent interaction (denoted as cage^-···cage^- interaction) between two nido-carborane clusters was successfully realized by using a pyridinium-based molecular barrier. The X-ray diffraction studies uncover that the cage^-···cage^- interaction has a contacting distance of 5.4-7.0 Å from centroid to centroid in the systems reported here. Theoretical calculations validate the formation of the noncovalent interaction and disclose its repulsive bonding nature that is overcome thanks to the positively charged pyridinium-based framework. Interestingly, such bulk crystalline materials containing the cage^-···cage^- interaction show relevant properties such as full-color absorption in the visible light range and important photothermal effect, which are absent for the control compound without carboranes. This study may offer fundamental insights into the boron cluster-based noncovalent interactions and open a new research avenue to rationally design boron cluster-based materials.

Synthesis of Derivatives of closo-Dodecaborate Anion Based on Amino Acid Esters

Abstract

This work proposes a new method for the synthesis of N-borylated amino acids based on nucleophilic substitution reactions in the [B12H11IPh]– anion. Esters of glycine and L-phenylalanine were used as nucleophiles. The structure of the products has been determined by multinuclear NMR spectroscopy, IR spectroscopy, and ESI mass spectrometry.

Molecular design and application of luminescent materials composed of group 13 elements with an aggregation-induced emission property

Complexation of π-conjugated ligands by metal or semimetal ions leads to the enhancement of the planarity and rigidity of π-conjugated systems. Boron, especially, has played a central role in the design of luminescent main-group complexes. However, these complexes still suffer the disadvantage of aggregation-caused quenching as well as typical organic fluorophores. It has recently been reported that some types of boron complexes exhibit the aggregation-induced emission (AIE) property. Moreover, AIE behavior from complexes and organometallic compounds composed of the other group 13 elements, such as aluminum and gallium, has emerged in this decade. These observations greatly encourage us to develop advanced functional materials based on the group 13 elements. Indeed, recent research has demonstrated that these classes of materials are potentially versatile scaffolds for constructing chromic luminophores, efficiently emissive π-conjugated polymers and so on. This review mainly describes AIE-active group 13 complexes with four-coordinate structures and their application as photo-functional materials. Proposed mechanisms of the origins of AIE behavior are briefly discussed.

Electrooxidative o-carborane chalcogenations without directing groups: cage activation by copper catalysis at room temperature

Copper-catalyzed electrochemical direct chalcogenations of o-carboranes was established at room temperature. Thereby, a series of cage C-sulfenylated and C-selenylated o-carboranes anchored with valuable functional groups was accessed with high levels of position- and chemo-selectivity control. The cupraelectrocatalysis provided efficient means to activate otherwise inert cage C-H bonds for the late-stage diversification of o-carboranes.

Iridium-Catalyzed Annulation of o-Carboranyl Carboxylic Acids with Alkynes: Synthesis of Carborano-Isocoumarins

An efficient iridium-catalyzed formal [4+2] annulation of carboranyl carboxylic acids with alkynes is developed, resulting in the facile synthesis of a new class of carborano-isocoumarin derivatives. The carboxyl group not only serves as a directing group to control the regioselectivity but also ingeniously becomes a part of the final products. The reaction mechanism involves sequential carboxyl-directed B(4)-H metalation, alkyne insertion, and reductive elimination.