o‐Carborane‐based Biphenyl and p‐Terphenyl Derivatives

The synthesis and properties of biphenyl‐ and p‐terphenyl‐fused o‐carboranes are described. Aryl rings in the biphenyl and p‐terphenyl skeletons are highly coplanar because of the presence of the o‐carborane unit. o‐Carborane exhibits an electron‐withdrawing character via the inductive effect, resulting in a decrease in both the HOMO and LUMO levels of oligophenyls without causing electronic perturbation.     

Nickel‐Catalyzed Cross‐Coupling Reactions of o‐Carboranyl with Aryl Iodides: Facile Synthesis of 1‐Aryl‐o‐Carboranes and 1,2‐Diaryl‐o‐Carboranes

A nickel‐catalyzed arylation at the carbon center of o‐carborane cages has been developed, thus leading to the preparation of a series of 1‐aryl‐o‐carboranes and 1,2‐diaryl‐o‐carboranes in high yields upon isolation. This method represents the first example of transition metal catalyzed C,C′‐diarylation by cross‐coupling reactions of o‐carboranyl with aryl iodides.     

Reaction of o‐Carboryne with Furans: Facile Synthesis of Carborane‐ Fused Oxanorbornenes and Their Derivatives

o‐Carboryne (1,2‐dehydro‐o‐carborane) is a very useful synthon for the synthesis of a variety of carborane‐functionalized molecules. Diels‐Alder reaction of o‐carboryne with furans gave a series of carborane‐fused oxanorbornenes in moderate to high yields using 1‐OTf‐1,2‐C₂B₁₀H₁₁ as carboryne precursor. The resultant cycloadducts can undergo hydrogenation, cyclic oxidation, bromination, [4 + 2]/[2 + 2] cycloaddition and nucleophilic ring opening reaction to afford a variety of highly functionalized carboranes that may find applications as useful basic units in medicine and materials science.     

Facial Synthesis of o‐Carborane‐Substituted Alkenes and Allenes by a Regioselective Ene Reaction of 1,3‐Dehydro‐o‐carborane

1,3‐Dehydro‐o‐carborane is a useful synthon for selective cage boron functionalization of o‐carboranes. It reacts readily with alkenes or alkynes to give a variety of cage B(3)‐alkenyl/allenyl o‐carboranes by ene reactions in very high yields and excellent regioselectivity. This can be ascribed to the highly polarized cage C?B multiple bond, which lowers the activation barriers of the ene reaction.     

Visible‐Light‐Promoted Photocatalytic B−C Coupling via a Boron‐Centered Carboranyl Radical: Facile Synthesis of B(3)‐Arylated o‐Carboranes

A visible‐light‐mediated in situ generation of a boron‐centered carboranyl radical (o‐C₂B₁₀H₁₁ ^. ) has been described. With eosin Y as a photoredox catalyst, 3‐diazonium‐o‐carborane tetrafluoroborate [3‐N₂‐o‐C₂B₁₀H₁₁][BF₄] was converted into the corresponding boron‐centered carboranyl radical intermediate, which can undergo efficient electrophilic substitution reaction with a wide range of (hetero)arenes. This general and simple procedure provides a metal‐free alternative for the synthesis of 3‐(hetero)arylated‐o‐carboranes.     

Palladium‐Catalyzed Regioselective Diarylation of o‐Carboranes By Direct Cage B−H Activation

Palladium‐catalyzed intermolecular coupling of o‐carborane with aromatics by direct cage B?H bond activation has been achieved, leading to the synthesis of a series of cage B(4,5)‐diarylated‐o‐carboranes in high yields with excellent regioselectivity. Traceless directing group ‐COOH plays a crucial role for site‐ and di‐selectivity of such intermolecular coupling reaction. A Pd^II–Pd^IV–Pd^II catalytic cycle is proposed to be responsible for the stepwise arylation.     

1,3‐Dehydro‐o‐Carborane: Generation and Reaction with Arenes

Like the importance of benzyne, witnessed in modern arene chemistry for decades, 1,2‐dehydro‐o‐carborane (o‐carboryne), a three‐dimensional relative of benzyne, has been used as a synthon for generating a wide range of cage, carbon‐functionalized carboranes over the past 20 years. However, the selective B functionalization of the cage still represents a challenging task. Disclosed herein is the first example of 1,3‐dehydro‐o‐carborane featuring a cage C? B bond having multiple bonding characters, and is successfully generated by treatment of 3‐diazonium‐o‐carborane tetrafluoroborate with non‐nucleophilic bases. This presents a new methodology for simultaneous functionalization of both cage carbon and boron vertices.     

Pd‐Catalyzed Selective Bifunctionalization of 3‐Iodo‐o‐Carborane by Pd Migration

A palladium‐catalyzed highly selective 3,4‐bifunctionalization of 3‐I‐o‐carborane has been developed, leading to the preparation of 3‐alkenyl‐4‐R‐o‐carboranes (R=alkyl, alkynyl, aryl, allyl, CN, and amido) in high to excellent yields. This protocol combines the sequential activation of cage B(3)?I and B(4)?H bonds by Pd migration from exo‐alkenyl sp² C to cage B(4), which is driven by thermodynamic force. This represents a brand‐new strategy for selective bifunctionalization of carboranes with two different substituents.     

Aggregation‐Induced Delayed Fluorescence Based on Donor/Acceptor‐Tethered Janus Carborane Triads: Unique Photophysical Properties of Nondoped OLEDs

Luminescent materials consisting of boron clusters, such as carboranes, have attracted immense interest in recent years. In this study, luminescent organic–inorganic conjugated systems based on o‐carboranes directly bonded to electron‐donating and electron‐accepting π‐conjugated units were elaborated as novel optoelectronic materials. These o‐carborane derivatives simultaneously possessed aggregation‐induced emission (AIE) and thermally activated delayed fluorescence (TADF) capabilities, and showed strong yellow‐to‐red emissions with high photoluminescence quantum efficiencies of up to 97 % in their aggregated states or in solid neat films. Organic light‐emitting diodes utilizing these o‐carborane derivatives as a nondoped emission layer exhibited maximum external electroluminescence quantum efficiencies as high as 11 %, originating from TADF.     

Direct Nucleophilic Substitution Reaction of Cage B−H Bonds by Grignard Reagents: A Route to Regioselective B4‐Alkylation of o‐Carboranes

Direct nucleophilic substitution reaction of cage B−H bonds of o ‐carboranes by Grignard reagents in the absence of any transition metals has been achieved for the first time, and leads to the regioselective synthesis of a series of 4‐alkyl‐1,2‐diaryl‐o ‐carboranes in very high yields. The presence of two electron‐withdrawing aryl groups on the cage carbon atoms is crucial to realizing the reaction. The regioselectivity is controlled by both electronic and steric factors. This work represents a new strategy for the development of methods for carborane functionalization.     

From Mono‐ to Poly‐Substituted Frameworks: A Way of Tuning the Acidic Character of Cc?H in o‐Carborane Derivatives

The incorporation of iodine atoms onto the boron vertices of the o‐carborane framework causes, according to spectroscopic data, a uniform increase in the acidic character of the C_c? H (C_c= cluster carbon) vertices, whereas the incorporation of methyl groups onto the boron vertices of the o‐carborane framework reduces their acidity. Methyl groups when attached to boron are electron‐withdrawing in boron clusters, whereas iodine atoms bonded to boron act as electron donors. This has been proven on B‐methyl and B‐iodinated o‐carboranes with NMR spectroscopy measurements and DFT calculations of natural bond orbital (NBO) charges, which show a cumulative buildup of positive cluster‐only total charge (CTC) on B‐methyl o‐carboranes and a cumulative buildup of negative cluster‐only total charge for B‐iodinated o‐carboranes.     

Carborane‐Induced Excimer Emission of Severely Twisted Bis‐o‐Carboranyl Chrysene

The synthesis of a highly twisted chrysene derivative incorporating two electron deficient o‐carboranyl groups is reported. The molecule exhibits a complex, excitation‐dependent photoluminescence, including aggregation‐induced emission (AIE) with good quantum efficiency and an exceptionally long singlet excited state lifetime. Through a combination of detailed optical studies and theoretical calculations, the excited state species are identified, including an unusual excimer induced by the presence of o‐carborane. This is the first time that o‐carborane has been shown to induce excimer formation ab initio, as well as the first observation of excimer emission by a chrysene‐based small molecule in solution. Bis‐o‐carboranyl chrysene is thus an initial member of a new family of o‐carboranyl phenacenes exhibiting a novel architecture for highly‐efficient multi‐luminescent fluorophores.     

Exceptionally Long C−C Single Bonds in Diamino‐o‐carborane as Induced by Negative Hyperconjugation

The synthesis of a series of 1,2‐diamino‐o‐carboranes ( 1 – 4 ) is reported. The molecular structures of these diamino‐o‐carboranes are remarkable as the inner‐cluster C?C bonds are all ultra‐long (162.7–193.1 pm) and vary substantially with small variations in the substituents. The results of quantum mechanical investigations suggest that the origin of the bond elongation is significant in‐plane negative hyperconjugation of lone pairs of the nitrogen substituents with the σ* orbitals of the C?C bonds in o‐carboranes.     

Rhodium‐Catalyzed Regioselective Hydroxylation of Cage B−H Bonds of o‐Carboranes with O2 or Air

A rhodium‐catalyzed hydroxylation of a cage B4?H bond in o‐carboranes with either O₂ or air as the oxygen source is described, and serves as a new methodology for the regioselective generation of a series of 4‐OH‐o‐carboranes in a one‐pot process. The use of either O₂ or air as both the oxidant and the oxygen source makes this protocol very environmentally friendly and practical.     

Luminescent Polymer Consisting of 9,12‐Linked o‐Carborane

The synthesis of novel luminescent polymer containing p‐phenylene‐ethynylene and 9,12‐linked o‐carborane units alternately in the main chain is reported. The obtained polymer exhibits intense blue photoluminescence, providing the first insights into the optical properties of a 9,12‐disubstituted o‐carborane dye.  π‐Conjugated substituent at 9 and/or 12‐positions in o‐carborane is electrically independent, and both the HOMO and the LUMO levels slightly increase, whereas LUMO of the π‐conjugated substituent at 1 and/or 2‐positions in o‐carborane decrease.  Thus, it is deduced that polymers consisting of the 9,12‐linked o‐carborane unit are able to be applied as light‐emitting materials.

     

Luminescence Color Tuning from Blue to Near Infrared of Stable Luminescent Solid Materials Based on Bis‐o‐Carborane‐Substituted Oligoacenes

Aryl‐substituted o ‐carboranes have shown highly efficient solid‐state emission in previous studies. To demonstrate color tuning of the solid‐state emission in an aryl‐o ‐carborane‐based system, bis‐o ‐carborane‐substituted oligoacenes were synthesized and their properties were systematically investigated. Optical and electrochemical measurements revealed efficient decreases in energy band gaps and lowest unoccupied molecular orbital (LUMO) levels by adding a number of fused benzene rings for the extension of π‐conjugation. As a consequence, bright solid‐state emission was observed in the region from blue to near infrared (NIR). Furthermore, various useful features were obtained from the modified o ‐carboranes as an optical material. The naphthalene derivatives exhibited aggregation‐induced emission (AIE) and almost 100 % quantum efficiency in the crystalline state. Furthermore, it was shown that the tetracene derivative with NIR‐emissive properties had high durability toward photo‐bleaching under UV irradiation.     

Simple Electrochemical Characterization of ortho‐Carborane and some of its exo‐Skeletal Derivatives

Electrochemical behaviour of 12 icosahedral carboranes – ortho‐, meta‐ and para‐carborane with different exo‐skeletal substituents was investigated. The study was done using phosphate buffers, in some cases with 20 % and 30 % dimethyl sulfoxide (DMSO) addition to increase the solubility of studied compounds. Commercial glassy carbon electrode or home‐made screen printed electrodes were used. Distinct electrochemical responses were observed only for the ortho‐carborane and its exo‐skeletal derivatives. The study of different exo‐skeletal substituents on the electrochemical behaviour of the carboranes is crucial for the intended use of these compounds as electrochemical labels of biomolecules.     

Crystal Structures of the Carborane Dianions [1,4‐(PhCB10H10C)2C6H4]2− and [1,4‐(PhCB10H10C)2C6F4]2− and the Stabilizing Role of the para‐Phenylene Unit on 2 n+3 Skeletal Electron Clusters

While carboranes with 2 n+2 and 2 n+4 (n=number of skeletal atoms) skeletal electrons (SE) are widely known, little has been reported on carboranes with odd SE numbers. Electrochemical measurements on two‐cage assemblies, where two C‐phenyl‐ortho‐carboranyl groups are linked by a para‐phenylene or a para‐tetrafluorophenylene bridge, revealed two well separated and reversible two‐electron reduction waves indicating formation of stable dianions and tetraanions. The salts of the dianions were isolated by reduction with sodium metal and their unusual structures were determined by X‐ray crystallography. The diamagnetic dianions contain two 2 n+3 SE clusters where each cluster has a notably long carborane C–carborane C distance of ca 2.4 Å. The π conjugation within the phenylene bridge plays an important role in the stabilization of these carboranes with odd SE counts.     

Two isostructural carboranes: 3‐phenyl‐1,2‐dicarba‐closo‐dodecaborane(12) and 1‐phenyl‐1,7‐dicarba‐closo‐dodecaborane(12)

Two phenyl‐substituted carboranes, 3‐phenyl‐1,2‐dicarba‐closo‐dodecaborane(12), C₈H₁₆B₁₀, (I), and 1‐phenyl‐1,7‐dicarba‐closo‐dodecaborane(12), C₈H₁₆B₁₀, (II), were found to be isostructural. Comparison of the bond angles at the ipso‐C atoms of the phenyl substituent for (I) and (II) [117.71 (3) and 118.45 (10)°, respectively] indicates that electron donation of the carborane cage for B‐ and C‐substituted carboranes is different.     

Fluorescence of New o‐Carborane Compounds with Different Fluorophores: Can it be Tuned?

Two sets of o‐carborane derivatives incorporating fluorene and anthracene fragments as fluorophore groups have been successfully synthesized and characterized, and their photophysical properties studied. The first set, comprising fluorene‐containing carboranes 6 – 9 , was prepared by catalyzed hydrosilylation reactions of ethynylfluorene with appropriate carboranylsilanes. The compound 1‐[(9,9‐dioctyl‐fluorene‐2‐yl)ethynyl]carborane ( 11 ) was synthesized by the reaction of 9,9‐dioctyl‐2‐ethynylfluorene and decaborane (B₁₀H₁₄). Furthermore, reactions of the lithium salt of 11 with 1 equivalent of 4‐(chloromethyl)styrene or 9‐(chloromethyl)anthracene yielded compounds 12 and 13 . Members of the second set of derivatives, comprising anthracene‐containing carboranes, were synthesized by reactions of monolithium or dilithium salts of 1‐Me‐1,2‐C₂B₁₀H₁₁, 1‐Ph‐1,2‐C₂B₁₀H₁₁, and 1,2‐C₂B₁₀H₁₂ with 1 or 2 equivalents of 9‐(chloromethyl)anthracene, respectively, to produce compounds 14 – 16 . In addition, 2 equivalents of the monolithium salts of 1‐Me‐1,2‐C₂B₁₀H₁₁ (Me‐o‐carborane) and 1‐Ph‐1,2‐C₂B₁₀H₁₁ (Ph‐o‐carborane) were reacted with 9,10‐bis(chloromethyl)anthracene to produce compounds 17 and 18 , respectively. Fluorene derivatives 6 – 9 exhibit moderate fluorescence quantum yields (32–44 %), whereas 11 – 13 , in which the fluorophore is bonded to the C_cluster (C_c), show very low emission intensity (6 %) or complete fluorescence quenching. The anthracenyl derivatives containing the Me‐o‐carborane moiety exhibit notably high fluorescence emissions, with ?_F=82 and 94 %, whereas their Ph‐o‐carborane analogues are not fluorescent at all. For these compounds, we have observed a correlation between the C_c?C_c bond length and the fluorescence intensity in CH₂Cl₂ solution, comparable to that observed for previously reported styrene‐containing carboranes. Thus, our hypothesis is that for systems of this type the fluorescence may be tuned and even predicted by changing the substituent on the adjacent C_c.