BN versus CC: How Similar Are They?

Pd-Senphos Catalyzed trans-Selective Cyanoboration of 1,3-Enynes

The first trans-selective cyanoboration reaction of an alkyne, specifically a 1,3-enyne, is described. The reported palladium-catalyzed cyanoboration of 1,3-enynes is site-, regio-, and diastereoselective, and is uniquely enabled by the 1,4-azaborine-based Senphos ligand structure. Tetra-substituted alkenyl nitriles are obtained providing useful boron-dienenitrile building blocks that can be further functionalized. The utility of our method has been demonstrated with the synthesis of Satigrel, an anti-platelet aggregating agent.

Boron-Induced Nitrogen Fixation in 3D Carbon Materials for Supercapacitors

Nitrogen-rich carbon materials attract great attention because of their admirable performance in energy storage and electrocatalysis. However, their conductivity and nitrogen content are somehow contradictory because good conductivity requires high-temperature heat treatment, which decomposes most of the nitrogen species. Herein, we propose a facile method to solve this problem by introducing boron (B) to fix the nitrogen in a three-dimensional (3D) carbon material even at 1000 °C. Besides, this N-rich carbon material has a high content of pyrrolic nitrogen due to the selective stabilization of B, which is favorable in electrochemical reactions. Density functional theory (DFT) investigation demonstrates that B reduces the energy level of neighboring N species (especially pyrrolic nitrogen) in the graphene layer, making it difficult to escape. Thus, this carbon material simultaneously, achieves high conductivity (30 S cm^-1) and nitrogen content (7.80 atom %), thus showing an outstanding capacitance of 412 F g^-1 and excellent rate capability.

BNB-Doped Phenalenyls: Modular Synthesis, Optoelectronic Properties, and One-Electron Reduction

A highly modular synthesis of BNB- and BOB-doped phenalenyls is presented. Treatment of the 1,8-naphthalenediyl-bridged boronic acid anhydride 1 with LiAlH₄/Me₃SiCl afforded the corresponding 1,8-naphthalenediyl-supported diborane(6) 2, which served as the starting material for all subsequent transformations. Upon addition of MesMgBr/Me₃SiCl, 2 was readily converted to the tetraorganyl diborane(6) 5. The further heteroatoms were finally introduced through the reaction of 2 with (Me₃Si)₂NR' or 5 with H₂NR' or H₂O (R' = H, Me, p-Tol). A helically twisted, fully BNB-embedded PAH 11 was prepared by combining 2 with a dibrominated m-terphenylamine, followed by a Grignard-mediated double ring-closure reaction. All compounds devoid of B-H bonds show favorable optoelectronic properties, such as luminescence and reversible reduction behavior. In the case of the BNB-phenalenyl 7 (BMes, NMe), the radical-anion salt K[7^•] was generated through chemical reduction with K metal and characterized by EPR spectroscopy. K[7^•] is not long-term stable in a THF/c-hexane solution, but abstracts an H atom with formation of the diamagnetic BNB-doped 1H-phenalene K[7H].

Boron: Its Role in Energy-Related Processes and Applications

Boron's unique position in the Periodic Table, that is, at the apex of the line separating metals and nonmetals, makes it highly versatile in chemical reactions and applications. Contemporary demand for renewable and clean energy as well as energy-efficient products has seen boron playing key roles in energy-related research, such as 1) activating and synthesizing energy-rich small molecules, 2) storing chemical and electrical energy, and 3) converting electrical energy into light. These applications are fundamentally associated with boron's unique characteristics, such as its electron-deficiency and the availability of an unoccupied p orbital, which allow the formation of a myriad of compounds with a wide range of chemical and physical properties. For example, boron's ability to achieve a full octet of electrons with four covalent bonds and a negative charge has led to the synthesis of a wide variety of borate anions of high chemical and electrochemical stability-in particular, weakly coordinating anions. This Review summarizes recent advances in the study of boron compounds for energy-related processes and applications.

BN-Embedded Polycyclic Aromatic Hydrocarbon Oligomers: Synthesis, Aromaticity, and Reactivity

BN-embedded oligomers with different pairs of BN units were synthesized by electrophilic borylation. Up to four pairs of BN units were incorporated in the large polycyclic aromatic hydrocarbons (PAHs). Their geometric, photophysical, electrochemical, and Lewis acidic properties were investigated by X-ray crystallography, optical spectroscopy, and cyclic voltammetry. The B-N bonds show delocalized double-bond characteristics and the conjugation can be extended through the trans-orientated aromatic azaborine units. Calculations reveal the relatively lower aromaticity for the inner azaborine rings in the BN-embedded PAH oligomers. The frontier orbitals of the longer oligomers are delocalized over the inner aromatic rings. Consequently, the inner moieties of the BN-embedded PAH oligomers are more active than the outer parts. This is confirmed by a simple oxidation reaction, which has significant effects on the aromaticity and the intramolecular charge-transfer interactions.

Probing Peripheral H-Bonding Functionalities in BN-Doped Polycyclic Aromatic Hydrocarbons

The replacement of carbon atoms at the zigzag periphery of a benzo[fg]tetracenyl derivative with an NBN atomic triad allows the formation of heteroatom-doped polycyclic aromatic hydrocarbon (PAH) isosteres, which expose BN mimics of the amidic NH functions. Their ability to form H-bonded complexes has never been touched so far. Herein, we report the first solution recognition studies of peripherally NBN-doped PAHs to form H-bonded DD·AA- and ADDA·DAAD-type complexes with suitable complementary H-bonding acceptor partners. The first determination of K_a in solution showed that the 1:1 association strength is around 27 ± 1 M^-1 for the DD·AA complexes in C₆D₆, whereas it rises to 1820 ± 130 M^-1 for the ADDA·DAAD array in CDCl₃. Given the interest of BN-doped polyaromatic hydrocarbons in supramolecular and materials chemistry, it is expected that these findings will open new possibilities to design novel materials, where the H-bonding properties of peripheral NH hydrogens could serve as anchors to tailor the organizational properties of PAHs.

Synthesis of bis-BN-Naphthalene-Fused Oxepins and Their Photoluminescence Including White-Light Emission

A series of novel bis-BN-naphthalene-fused oxepin derivatives were synthesized via a Pd-catalyzed tandem reaction from brominated 2,1-borazaronaphthalenes and cis-bis(boryl)alkenes. X-ray crystallographic analysis revealed that bis-BN-naphthalene-fused oxepins feature a planar framework. The electronic and photophysical properties of the novel BN-naphthalene-fused oxepins were investigated by UV-vis and fluorescence spectroscopies and density functional theory (DFT) calculations, which disclosed the distinct electronic and photophysical properties of the analogous hydrocarbon system. Interestingly, dual-fluorescent emissions were observed upon dissolving N-substituted derivatives 10-14 in dimethyl sulfoxide. Tunable emission colors especially for white-light emissions can be achieved by controlling the ratio of solvents, concentration, or temperature using only a single-molecule compound.

Expanding the BN-embedded PAH family: 4a-aza-12a-borachrysene

Previously unknown 4a-aza-12a-borachrysene has been synthesized in only four steps.

Borazatruxenes

We report the synthesis and characterization of a series of arene-borazine hybrids called borazatruxenes. These molecules are BN-isosteres of truxene whereby the central benzene core has been replaced by a borazine ring. The straightforward three step synthesis, stability and their chiroptical and electronic properties recommend them as new scaffolds for BN-carbon hybrid materials. Computational studies at DFT level, closely matching the experimental data, provided insights in the electronic structure of these molecules.

Controllable syntheses of B/N anionic aminoborane chain complexes by the reaction of NH3BH3 with NaH and the mechanistic study

B/N anionic aminoborane linear or branched chain complexes, [BH3(NH2BH2)nH]- (n = 1, 2, and 3) and [BH(NH2BH3)3]-, have been synthesized through the controlled reactions of ammonia borane (NH3BH3) with NaH by adjusting the reactant ratios and reaction temperatures. The possible reaction mechanisms were elucidated based on experimental and theoretical studies.

Triplet-State Structures, Energies, and Antiaromaticity of BN Analogues of Benzene and Their Benzo-Fused Derivatives

It is well known that benzene is aromatic in the ground state (the Hückel's rule) and antiaromatic in the first triplet (T₁) excited state (the Baird's rule). Whereas its BN analogues, the three isomeric dihydro-azaborines, have been shown to have various degrees of aromaticity in their ground state, almost no data are available for their T₁ states. Thus, the purpose of this work is to theoretically [B3LYP/6-311+G(d,p)] predict structures, energies, and antiaromaticity of T₁ dihydro-azaborines and some benzo-fused derivatives. Conclusions are based on spin density analysis, isogyric and hydrogenation reactions, HOMA, NICS, and ACID calculations. The results suggest that singlet-triplet energy gaps, antiaromaticity, and related excited-state properties of benzene, naphthalene, and anthracene could be tuned and controlled by the BN substitution pattern. While all studied compounds remain (nearly) planar upon excitation, the spin density distribution in T₁ 1,4-dihydro-azaborine induces a conformational change by which the two co-planar C-H bonds in the ground state become perpendicular to each other in the excited state. This predicted change in geometry could be of interest for the design of new photomechanical materials. Excitation of B-CN/N-NH₂ 1,4-azaborine would have a few effects: intramolecular charge transfer, aromaticity reversal, rotation, and stereoelectronic Umpolung of the amino group.

Boron-nitrogen substituted dihydroindeno[1,2-b]fluorene derivatives as acceptors in organic solar cells

The electrophilic borylation of 2,5-diarylpyrazines results in the formation of boron-nitrogen doped dihydroindeno[1,2-b]fluorene which can be synthesized using standard Schlenk techniques and worked up and handled readily under atmospheric conditions. Through transmetallation via diarylzinc reagents a series of derivatives were synthesized which show broad visible to near-IR light absorption profiles that highlight the versatility of this BN substituted core for use in optoelectronic devices. The synthesis is efficient, scalable and allows for tuning through changes in substituents on the planar heterocyclic core and at boron. Exploratory evaluation in organic solar cell devices as non-fullerene acceptors gave power conversion efficiencies of 2%.

A DFT Study of the Modulation of the Antiaromatic and Open-Shell Character of Dibenzo[a,f]pentalene by Employing Three Strategies: Additional Benzoannulation, BN/CC Isosterism, and Substitution

Dibenzo[a,f]pentalene ([a,f]DBP) is a highly antiaromatic molecule having appreciable open-shell singlet character in its ground state. In this work, DFT calculations at the B3LYP/6-311+G(d,p) level of theory were performed to explore the efficiency of three strategies, that is, BN/CC isosterism, substitution, and (di)benzoannulation of [a,f]DBP, in controlling its electronic state and (anti)aromaticity. To evaluate the type and extent of the latter, the harmonic oscillator model of aromaticity (HOMA) and aromatic fluctuation (FLU) indices were used, along with the nucleus-independent chemical shift NICS-XY-scan procedure. The results suggest that all three strategies could be employed to produce either the closed-shell system or open-shell species, which may be in the singlet or triplet ground state. Triplet states have been characterized as aromatic, which is in accordance with Baird's rule. All the singlet states were found to have weaker global paratropicity than [a,f]DBP. Additional (di)benzo fusion adds local aromatic subunit(s) and mainly retains the topology of the paratropic ring currents of the basic molecule. The substitution of two carbon atoms by the isoelectronic BN pair, or the introduction of substituents, results either in the same type and very similar topology of ring currents as in the parent compound, or leads to (anti)aromatic and nonaromatic subunits. The triplet states of all the examined compounds are also discussed.

Alkaline-earth metallacyclic complexes bearing a diborane-bridged tetraamide ligand: synthesis, structure and fluorescence property

A series of alkaline-earth (Mg, Ca, and Sr) metallacyclic complexes bearing a diborane-bridged tetraamide ligand were achieved for the first time through a clean one-step approach. All of these metallacycles were characterized by single-crystal X-ray diffraction analyses. UV-Vis absorption/emission spectroscopy showed deep blue fluorescence of these complexes.

1,2-Azaborine's Distinct Electronic Structure Unlocks Two New Regioisomeric Building Blocks via Resolution Chemistry

Two new 1,2-azaborine building blocks that enable the broad diversification of previously not readily accessible C4 and C5 ring positions of the 1,2-azaborine heterocycle are developed. 1,2-Azaborine's distinct electronic structure allowed the resolution of a mixture of C4- and C5-borylated 1,2-azaborines. The connection between the electronic structure of C4 and C5 positions of 1,2-azaborine and their distinct reactivity patterns is revealed by a combination of reactivity studies and kinetic measurements that are supported by DFT calculations. Specifically, we show that oxidation by N-methylmorpholine N-oxide (NMO) is selective for the C4-borylated 1,2-azaborine, and the Ir-catalyzed deborylation is selective for the C5-borylated 1,2-azaborine via kinetically controlled processes. On the other hand, ligand exchange with diethanolamine takes place selectively with the C4-borylated isomer via a thermodynamically controlled process. These results represent the first examples for chemically distinguishing a mixture of two aryl mono-Bpin-substituted isomers.

Structural Properties of Highly Doped Borazino Polyphenylenes Obtained through Condensation Reaction

Here we describe the synthesis and spectroscopic and structural characterization of various borazine-doped polyphenylenes displaying high doping dosages (16-18%). Capitalizing on the condensation reaction approach, the desired products were formed using a mixture of p-phenylendiamine and aniline with BCl₃, followed by the addition of an aryl lithium derivative. The use of mesityl lithium (MesLi) yields strained multiborazine derivatives, which proved to be unstable in the presence of moisture. However, when xylyl lithium (XylLi) was used, chemically stable multiborazines were obtained, with oligomers showing molecular weight up to 10⁴, corresponding to 16-18 monomer units. While the dimer, trimer, and tetramer could be isolated as pure products and their structure characterized by mass and NMR analysis, higher oligomers could only be isolated as mixtures of B-hydroxy-substituted derivatives and characterized by gel permeation chromatography. The structures of the dimer and trimer derivatives were confirmed by X-ray analysis, which nicely showed the presence of the two and three borazine rings spaced by one and two 1,4-aryl bridges, respectively. Notably, the trimer forms a porous crystalline clathrate. The peripheral xylyl and phenyl moieties of each molecule intramolecularly embrace each other through C-H and π-π stacking interactions. Steady-state UV-vis absorption characterization suggested that the molecules are UV absorbers, with the extinction coefficient linearly scaling with the degree of oligomerization. On the other hand, low-emission quantum yields were obtained for all derivatives (<7%), suggesting that high BN-doping dosages dramatically affect the emission properties of the doped polyphenylenes.

A New Member of the BN-Phenanthrene Family: Understanding the Role of the B-N Bond Position

3,4-Dihydro-4-aza-3-boraphenanthrene, which shows the highest fluorescence quantum yield of all nonsubstituted BN-phenanthrenes reported to date (ϕ_F = 0.61), has been synthesized in only three steps (76% overall yield) from easily accessible 1-bromo-2-vinylnaphthalene, along with several substituted derivatives. The reactivity of these previously unknown BN-aromatic compounds toward organolithium compounds and bromine has been studied. This latter reaction affords bromo-substituted compounds that are suitable for further functionalization via Suzuki and Sonogashira couplings, with complete regioselectivity. The optical properties and excited state deactivation mechanisms of selected compounds were studied using computational methods.

Polymorphism and pseudosymmetry of 10,10'-oxybis(9-thia-10-hydro-10-boraanthracene)

We have encountered two polymorphs of the title compound, C24H16B2OS2, both of which display almost the same unit-cell parameters. Compound (I) crystallizes in the non-centrosymmetric space group P21 with four mol-ecules in the asymmetric unit. These mol-ecules are related by pseudosymmetry. As a result, the space group looks like P21/c, but the structure cannot be refined successfully in that space group. Compound (II) on the other hand crystallizes in the centrosymmetric space group P21/c with only two mol-ecules in the asymmetric unit. The crystals studied for (I) and (II) were both non-merohedral twins.

Chlorine-Substituted 9,10-Dihydro-9-aza-10-boraanthracene as a Precursor for Various Boron- and Nitrogen-Containing π-Conjugated Compounds

Chlorine-substituted 9,10-dihydro-9-aza-10-boraanthracene was synthesized. Derivatization of this compound by taking advantage of the transformable B-Cl moiety gave 9,10-dihydro-9-aza-10-boraanthracene derivatives with various aryl substituents. In addition, further functionalization on NH groups by alkylation and Buchwald-Hartwig amination was demonstrated. Photophysical properties of the resulting 9,10-dihydro-9-aza-10-boraanthracene derivatives were also discussed.

Superelectrophilicity of 1,2-Azaborine: Formation of Xenon and Carbon Monoxide Adducts

The BN analogue of ortho-benzyne, 1,2-azaborine, is shown to bind carbon monoxide and a xenon atom under matrix isolation conditions, demonstrating its strongly Lewis acidic superelectrophilic nature. The Lewis acid-base complexes involving CO and Xe can be cleaved photochemically and reformed by mildly annealing the matrices. The interaction energy of 1,2-azaborine with Xe is 3 kcal mol^-1 according to quantum chemical computations, and is similar to that of the superelectrophilic carbene difluorovinylidene.

Unravelling a general mechanism of converting ionic B/N complexes into neutral B/N analogues of alkanes: Hδ+⋯Hδ– dihydrogen bonding assisted dehydrogenation

Different mechanisms for [NH₃BH₂NH₃]⁺[BH₄]⁻ → NH₃BH₃ + [NH₂BH₂]_n and [NH₄]⁺[BH₃NH₂BH₃]⁻ → NH₃BH₂NH₂BH₃ have been elucidated, which leads to the proposal of a general mechanism as below.

Planarized B,N-phenylated dibenzoazaborine with a carbazole substructure: electronic impact of the structural constraint

Structural constraint in B,N-phenylated dibenzoazaborine alters the π-conjugation mode and furnishes intense deep-blue emission and intriguing electrochemical properties.

Photo electron transfer induced desilylation of N,N-bis(trimethylsilyl)aminodibenzoborole to aminodibenzoborole

Photoinduced desilylation proceeds by single electron transfer and yields the first example of an unsubstituted aminoborole derivative.

Synthesis, characterization and photophysical studies of a novel polycyclic diborane

A long polycyclic diborane (4) with a dihedral angle of 21.75° between the two naphthalene rings has been successfully prepared and fully characterized.