Boron‐Cluster‐Enhanced Ultralong Organic Phosphorescence

Although carborane‐based luminescent materials have been studied for years, no persistent phosphor has been reported so far. Herein, we describe boron‐cluster‐based persistent phosphors obtained by linking a σ‐aromatic carboranyl cage to the π system of a carbazolyl group. The carboranes were found to promote intersystem crossing from a singlet to a triplet state. The rigid boron cluster was able to stabilize the ultralong triplet excitons through multiple nonclassical hydrogen bonds, such as B?H???π interactions, thus leading to a long lifetime of up to 0.666 s and an absolute phosphorescence quantum yield of 7.1 %, which is outstanding for an organic phosphor without heavy atoms. These phosphors can be excited by visible light and show dynamic emission behavior, including thermochromism and mechanochromism. This study demonstrates that non‐metal/heavy‐atom boron clusters can be used to develop multifunctional high‐performance phosphors for potential applications.     

Boron‐Based Drug Design

The use of the element boron, which is not generally observed in a living body, possesses a high potential for the discovery of new biological activity in pharmaceutical drug design. In this account, we describe our recent developments in boron‐based drug design, including boronic acid containing protein tyrosine kinase inhibitors, proteasome inhibitors, and tubulin polymerization inhibitors, and ortho‐carborane‐containing proteasome activators, hypoxia‐inducible factor 1 inhibitors, and topoisomerase inhibitors. Furthermore, we applied a closo‐dodecaborate as a water‐soluble moiety as well as a boron‐10 source for the design of boron carriers in boron neutron capture therapy, such as boronated porphyrins and boron lipids for a liposomal boron delivery system.     

Photosynthetic Antenna‐Reaction Center Mimicry with a Covalently Linked Monostyryl Boron‐Dipyrromethene–Aza‐Boron‐Dipyrromethene–C60 Triad

An efficient functional mimic of the photosynthetic antenna‐reaction center has been designed and synthesized. The model contains a near‐infrared‐absorbing aza‐boron‐dipyrromethene (ADP) that is connected to a monostyryl boron‐dipyrromethene (BDP) by a click reaction and to a fullerene (C₆₀) using the Prato reaction. The intramolecular photoinduced energy and electron‐transfer processes of this triad as well as the corresponding dyads BDP‐ADP and ADP‐C₆₀ have been studied with steady‐state and time‐resolved absorption and fluorescence spectroscopic methods in benzonitrile. Upon excitation, the BDP moiety of the triad is significantly quenched due to energy transfer to the ADP core, which subsequently transfers an electron to the fullerene unit. Cyclic and differential pulse voltammetric studies have revealed the redox states of the components, which allow estimation of the energies of the charge‐separated states. Such calculations show that electron transfer from the singlet excited ADP (¹ADP*) to C₆₀ yielding ADP^.+‐C₆₀^.? is energetically favorable. By using femtosecond laser flash photolysis, concrete evidence has been obtained for the occurrence of energy transfer from ¹BDP* to ADP in the dyad BDP‐ADP and electron transfer from ¹ADP* to C₆₀ in the dyad ADP‐C₆₀. Sequential energy and electron transfer have also been clearly observed in the triad BDP‐ADP‐C₆₀. By monitoring the rise of ADP emission, it has been found that the rate of energy transfer is fast (≈10¹¹ s^?1). The dynamics of electron transfer through ¹ADP* has also been studied by monitoring the formation of C₆₀ radical anion at 1000 nm. A fast charge‐separation process from ¹ADP* to C₆₀ has been detected, which gives the relatively long‐lived BDP‐ADP^.+C₆₀^.? with a lifetime of 1.47 ns. As shown by nanosecond transient absorption measurements, the charge‐separated state decays slowly to populate mainly the triplet state of ADP before returning to the ground state. These findings show that the dyads BDP‐ADP and ADP‐C₆₀, and the triad BDP‐ADP‐C₆₀ are interesting artificial analogues that can mimic the antenna and reaction center of the natural photosynthetic systems.     

Metal‐Free Boron‐Containing Heterogeneous Catalysts

Metal‐free catalysts have distinct advantages over metal and metal oxide catalysts, such as lower cost as well as higher reliability and sustainability. Among the nonmetal compounds used in catalysis, boron‐containing compounds with a few unique properties have been developed. In this Minireview, the recent advances in the field of boron‐containing metal‐free catalysts are presented, including binary and ternary boron‐containing catalytic materials. Additionally, the three main applications in catalysis are considered, namely, electrocatalysis, thermal catalysis, and photocatalysis, with the role of boron discussed in depth for each specific catalytic application. Boron‐containing compounds could have a substantial impact on the field of metal‐free catalysts in the future.     

Boron‐Doped Diamond Dual‐Plate Deep‐Microtrench Device for Generator‐Collector Sulfide Sensing

A BDD‐BDD dual‐plate microtrench electrode with 6 μm inter‐electrode spacing is investigated using generator‐collector electrochemistry and shown to give microtrench depth‐dependent sulfide detection down to the μM levels. The effect of the microtrench depth is compared for a “shallow” 44 μm and a “deep” 180 μm microtrench and linked to the reduction of oxygen to hydrogen peroxide which interferes with sulfide redox cycling. With a deeper microtrench and a fixed collector potential at ?1.4 V vs. SCE, two distinct redox cycling potential domains are observed at 0.0 V vs. SCE (2‐electron) and at 1.1 V vs. SCE (6‐electron).     

A Phosphine‐Coordinated Boron‐Centered Gomberg‐Type Radical

The P‐coordinated boryl radical [Ph₂P(naphthyl)BMes]^. (Mes=mesityl) was prepared by (electro)chemical reduction of the corresponding borenium salt or bromoborane. Electron paramagnetic resonance (EPR) analysis in solution and DFT calculations indicate large spin density on boron (60–70 %) and strong P–B interactions (P→B σ donation and B→P negative hyperconjugation). The radical is persistent in solution and participates in a Gomberg‐type dimerization process. The associated quinoid‐type dimer has been characterized by single‐crystal X‐ray diffraction.     

Boron‐Functionalized Graphene Oxide‐Organic Frameworks for Highly Efficient CO2 Capture

The capture and storage of CO₂ have been suggested as an effective strategy to reduce the global emissions of greenhouse gases. Hence, in recent years, many studies have been carried out to develop highly efficient materials for capturing CO₂. Until today, different types of porous materials, such as zeolites, porous carbons, N/B‐doped porous carbons or metal‐organic frameworks (MOFs), have been studied for CO₂ capture. Herein, the CO₂ capture performance of new hybrid materials, graphene‐organic frameworks (GOFs) is described. The GOFs were synthesized under mild conditions through a solvothermal process using graphene oxide (GO) as a starting material and benzene 1,4‐diboronic acid as an organic linker. Interestingly, the obtained GOF shows a high surface area (506 m² g⁻¹) which is around 11 times higher than that of GO (46 m² g⁻¹), indicating that the organic modification on the GO surface is an effective way of preparing a porous structure using GO. Our synthetic approach is quite simple, facile, and fast, compared with many other approaches reported previously. The synthesized GOF exhibits a very large CO₂ capacity of 4.95 mmol g⁻¹ at 298 K (1 bar), which is higher those of other porous materials or carbon‐based materials, along with an excellent CO₂/N₂ selectivity of 48.8.     

Electrochemical Determination of Sulphur‐containing Pharmaceuticals Using Boron‐doped Diamond Electrodes

N‐acetylcysteine (NAC) and gentamicin sulfate (GS) are biologically and pharmaceutically relevant thiol‐containing compounds. NAC is well known for its antioxidant properties, whereas GS is an aminoglycoside that is used as a broadband antibiotic. Both pharmaceuticals play a significant role in the treatment of bacterial infections by suppressing the formation of biofilms. According to the European Pharmacopeia protocol, GS is analyzed by high performance liquid chromatography (HPLC) using gold electrodes for electrochemical detection. Here, we report the electrochemical detection of these compounds at NH₂‐terminated boron‐doped diamond electrodes, which show significantly reduced electrode passivation, an issue commonly known for gold electrodes. Cyclic voltammetry experiments performed for a period of 70 minutes showed that the peak current decreased only by 1.6 %/7.4 % for the two peak currents recorded for GS, and 6.6 % for the oxidation peak of NAC, whereas at gold electrodes a decrease in peak current of 14.2 % was observed for GS, and of 64 %/30 % for the two peak currents of NAC. For their quantitative determination, differential pulse voltammetry was performed in a concentration range of 2–49 µg/mL of NAC with a limit of detection (LOD) of 1.527 µg/mL, and a limit of quantification (LOQ) of 3.624 µg/mL, respectively. The quantification of GS in a concentration range of 0.2–50 µg/mL resulted in a LOD of 1.714 µg/mL, and a LOQ of 6.420 µg/mL, respectively.     

1,2‐Azaborine: The Boron‐Nitrogen Derivative of ortho‐Benzyne

The BN analogue of ortho‐benzyne, 1,2‐azaborine, is generated by flash vacuum pyrolysis, trapped under cryogenic conditions, and studied by direct spectroscopic techniques. The parent BN aryne spontaneously binds N₂ and CO₂, thus demonstrating its highly reactive nature. The interaction with N₂ is photochemically reversible. The CO₂ adduct of 1,2‐azaborine is a cyclic carbamate which undergoes photocleavage, thus resulting in overall CO₂ splitting.     

Highly Fluorescent Non‐Conventional Boron‐Difluoride‐Based π Organogel with Gelation‐Assisted Piezochromism

Triphenylamine‐functionalized boron 2‐(2′‐pyridyl)imidazole complex bearing no alkyl chains or H‐bond unit was found to be able to gelate a series of solvents, and the balanced intermolecular π–π interactions play an important role in its supramolecular self‐assembly. The gelator molecule is piezochromic, and the dried gel responded to pressure more sensitively than regular crystalline powder.     

Boron‐Catalyzed N‐Alkylation of Amines using Carboxylic Acids

A boron‐based catalyst was found to catalyze the straightforward alkylation of amines with readily available carboxylic acids in the presence of silane as the reducing agent. Various types of primary and secondary amines can be smoothly alkylated with good selectivity and good functional‐group compatibility. This metal‐free amine alkylation was successfully applied to the synthesis of three commercial medicinal compounds, Butenafine, Cinacalcet. and Piribedil, in a one‐pot manner without using any metal catalysts.     

Boron‐Based Catalysts for C−C Bond‐Formation Reactions

Because the construction of the C?C bond is one of the most significant reactions in organic chemistry, the development of an efficient strategy has attracted much attention throughout the synthetic community. Among various protocols to form C?C bonds, organoboron compounds are not just limited to stoichiometric reagents, but have also made great achievements as catalysts because of the easy modification of the electronic and steric impacts on the boron center. This review presents recent developments of boron‐based catalysts applied in the field of C?C bond‐formation reactions, which are classified into four kinds on the basis of the type of boron catalyst: 1) highly Lewis acidic borane, B(C₆F₅)₃; 2) organoboron acids, RB(OH)₂, and their ester derivatives; 3) borenium ions, (R₂BL)X; and 4) other miscellaneous kinds.     

Homogenous Boron‐doping in Self‐sensitized Carbon Nitride for Enhanced Visible‐light Photocatalytic Activity

We report a solvothermal approach for the preparation of homogeneously B‐doped self‐sensitized carbon nitride (B‐SSCN) composed of a core of B‐doped carbon nitride microspheres and a covalently linked shell of s‐triazine oligomers. Compared to the undoped structure, the obtained B‐SSCN photocatalyst exhibits an enhanced visible‐light activity, excellent stability for photocatalytic hydrogen generation due to a reduced band‐gap, enhanced charge‐separation efficiency, and better surface reactivity of B‐SSCN. This work provides a new strategy to uniformly insert heteroatoms into the polymeric carbon nitride framework for the development of metal‐free photocatalysts towards efficient production of solar fuels.     

Boron‐Metallated Borirenes and Bis(Borirenes)

Heating the metalloborylene complex [{(η⁵‐C₅Me₅)Fe(CO)₂}(μ‐B){Cr(CO)₅}] with alkynes and diynes leads to the formation of B‐metallated borirenes and a bis(B‐metallated borirene) through formal transfer of the metalloborylene moiety [(η⁵‐C₅Me₅)(OC)₂Fe(B:)]. By using this protocol, a range of B‐metallated borirenes with electron‐donating and electron‐withdrawing substituents are prepared, and these are studied spectroscopically, structurally, and computationally. The yellow‐orange color of the complexes is additionally explained through time‐dependent density functional theory calculations.     

Synthesis of Previously Inaccessible Borylated Heterocycle Motifs Using Novel Boron‐Containing Amphoteric Molecules

The photoredox‐organocatalyzed α‐alkylation of the α‐MIDA boryl aldehyde with a range of α‐bromoketones resulted in the first examples of boron‐containing 1,4‐dicarbonyl compounds. These novel trifunctional amphoteric molecules, which bear an additional, strategically placed electrophilic site compared to the starting amphoteric α‐boryl aldehyde, were subjected to double‐condensation reactions in the presence of various nucleophiles. As a result, a variety of synthetically challenging 3‐borylated pyrroles and furans and 4‐borylated pyridazines were generated. The borylated regioisomers accessible with this condensation‐based strategy are distinctly different from those arising from the well‐known lithiation and C? H activation processes.     

Prediction of Boron–Boron Triple‐Bond Polymers Stabilized by Janus‐Type Bis(N‐heterocyclic) Carbenes

A class of polymeric compounds containing boron–boron triple bonds stabilized by N‐heterocyclic biscarbenes is proposed. Since a triply bonded B₂ is related to its third excited state, the predicted macromolecule would be composed by several units of an electronically excited first‐row homonuclear dimer. Moreover, it is shown that the replacement of biscarbene with N₂ or CO as spacers could change the bonding profile of the boron–boron units to a cumulene‐like structure. Based on these results, different types of diboryne polymers are proposed, which could lead to an unprecedented set of boron materials with distinct physical properties. The novel diboryne macromolecules could be synthesized by the reaction of Janus‐type biscarbenes with tetrabromodiborane, B₂Br₄, and sodium naphthalenide, [Na(C₁₀H₈)], similarly to Braunschweig’s work on the room temperature stable boron–boron triple bond compounds (Science, 2012 , 336, 1420).     

A Neutral and Aromatic Boron‐Rich Inorganic Benzene

A neutral 1,4‐diaza‐2,3,5,6‐tetraborinine derivative ( 3 ) featuring a 6π ring system has been synthesized and structurally characterized. The prospective aromatic nature of 3 was assessed by single‐crystal X‐ray analysis, NMR, and UV/Vis absorption spectroscopy as well as computational studies. Furthermore, preliminary reactivity investigations showed that 3 does not only react with diphenyl acetylene but also readily captures CO₂ in a [4+2] cycloaddition under ambient conditions.     

Regio‐ and Enantioselective Synthesis of Trifluoromethyl‐Substituted Homoallylic α‐Tertiary NH2‐Amines by Reactions Facilitated by a Threonine‐Based Boron‐Containing Catalyst

A method for catalytic regio‐ and enantioselective synthesis of trifluoromethyl‐substituted and aryl‐, heteroaryl‐, alkenyl‐, and alkynyl‐substituted homoallylic α‐tertiary NH₂‐amines is introduced. Easy‐to‐synthesize and robust N‐silyl ketimines are converted to NH‐ketimines in situ, which then react with a Z‐allyl boronate. Transformations are promoted by a readily accessible l ‐threonine‐derived aminophenol‐based boryl catalyst, affording the desired products in up to 91 % yield, >98:2 α:γ selectivity, >98:2 Z:E selectivity, and >99:1 enantiomeric ratio. A commercially available aminophenol may be used, and allyl boronates, which may contain an alkyl‐, a chloro‐, or a bromo‐substituted Z‐alkene, can either be purchased or prepared by catalytic stereoretentive cross‐metathesis. What is more, Z‐trisubstituted allyl boronates may be used. Various chemo‐, regio‐, and diastereoselective transformations of the α‐tertiary homoallylic NH₂‐amine products highlight the utility of the approach; this includes diastereo‐ and regioselective epoxide formation/trichloroacetic acid cleavage to generate differentiated diol derivatives.     

Synthesis and Electronic Structure of Boron‐Graphdiyne with an sp‐Hybridized Carbon Skeleton and Its Application in Sodium Storage

Boron‐graphdiyne (BGDY), which has a unique π‐conjugated structure comprising an sp‐hybridized carbon skeleton and evenlydistributed boron heteroatoms in a well‐organized 2D molecular plane, is prepared through a bottom‐up synthetic strategy. Excellent conductivity, a relatively low band gap and a packing mode of the planar BGDY are observed. Notably, the unusual bonding environment of the all sp‐carbon framework and the electron‐deficient boron centers generates affinity to metal atoms, and thus provides extra binding sites. Furthermore, the expanded molecule pores of the BGDY molecular plane can also facilitate the transfer of metal ions in the perpendicular direction. The practical effect of the all sp‐carbon structure and boron heteroatoms on the properties of BGDY are demonstrated in its performance as the anode in sodium‐ion batteries.     

A Versatile Methodology for the Controlled Synthesis of Photoluminescent High‐Boron‐Content Dendrimers

Fluorescent star‐shaped molecules and dendrimers with a 1,3,5‐triphenylbenzene moiety as the core and 3 or 9 carborane derivatives at the periphery, have been prepared in very good yields by following different approaches. One procedure relies on the nucleophilic substitution of Br groups in 1,3,5‐tris(4‐(3‐bromopropoxy)phenyl)benzene with the monolithium salts of methyl and phenyl‐o‐carborane. The second method is the hydrosilylation reactions on the peripheral allyl ether functions of 1,3,5‐tris(4‐allyloxy‐phenyl)benzene and 1,3,5‐tris(4‐(3,4,5‐trisallyloxybenzyloxy)phenyl)benzene with suitable carboranyl‐silanes to produce different generations of dendrimers decorated with carboranyl fragments. This approach is very versatile and allows one to introduce long spacers between the fluorescent cores and the boron clusters, as well as to obtain a high loading of boron clusters. The removal of one boron atom from each cluster leads to high‐boron‐content water‐soluble macromolecules. Thermogravimetric analyses show a higher thermal stability for the three‐functionalized compounds than for those containing 9 clusters. All compounds exhibit photoluminescent properties at room temperature under ultraviolet irradiation with high quantum yields; these depend on the nature of the cluster and the substituent on the C_cluster. Cyclic voltammetry indicates that there is no electronic communication between the core and the peripheral carboranyl fragments. Due to the high boron content of these molecules, we currently focus our research on their biocompatibility, biodistribution in cells cultures, and potential applications for boron neutron capture therapy (BNCT).