Facile Construction of New Hybrid Conjugation via Boron Cage Extension

Regiocontrollable B(2/3)-H Alkenylation of nido-Carboranes

Anionic nido-carboranes, as open-cage analogues of closo-carboranes with strong hydrophilicity and higher potential in the development of biomedicines, have been notably more challenging because of their strong interaction with transition metals. While the exo-cage B-H activation reactions of nido-carboranes have been widely studied, there are few reports on the direct functionalization of B-H bonds located on a closed polyhedral sphere. Here, we report an efficient palladium-catalyzed regioselective B(2/3)-H alkenylation of nido-carboranes with various alkenes and alkyne coupling partners, enabled by 3-methylpyridine directing groups, to achieve a regiocontrollable functionalization of B(2/3)-H vertices over highly reactive exo-cage B11-H vertex in nido-carboranes.

Intercluster B-H and B-B aggregation in iso- and trans-[B20H18]2-. Spherical aromaticity in borane dimers

The formation of molecular-based functional materials is a key step towards the development of technologies at the nanoscale. Recently, it has been shown that after oxidation of closo-[B10H10]2- anions, an induced aggregation of two cluster units is achieved, retaining their parent B10 backbones as persistent building blocks. Such characterization provides an interesting scenario to further understand relevant factors leading to aggregation in a minimal structure involving two units. Here, we explore the interaction between closo-[B10H10]2- units in two isomers, namely, iso- and trans-[B20H18]2-, involving different intercluster contacts based on B-B and B-H interactions, respectively. Our results show that the inherent spherical aromatic characteristics of the parent closo-[B10H10]2- cluster are persistent in both iso- and trans-[B20H18]2- isomers as an interplay between the spherical aromatic properties from both B10 motifs, leading to an overlap of the shielding regions from shielding cone properties, ascribed as a dual spherical-spherical aromatic cluster. From 11B-NMR features, it came out that trans-[B20H18]2- involves larger differences in comparison to closo-[B10H10]2-, owing to the variation of the B10-B10 backbone provided by the intercluster B-interaction, thus resulting in a more effective aggregation connecting such building units, towards boron-based cluster materials.

Full Electron Delocalization across the Cluster in 1,12-bisBMes2-p-carborane Radical Anion

Conjugation between three-dimensional (3D) carboranes and the attached substituents is commonly believed to be very weak. In this paper, we report that reducing 1,12-bis(BMes2)-p-carborane (B2pCab) with one electron gives a radical anion with a centrosymmetric semiquinoidal structure. This radical anion shows extensive electron delocalization between the two boron centers over the p-carborane bridge due to the overlap of carborane lowest unoccupied molecular orbital (LUMO) and the BMes2 LUMO. Unlike dianions of other C2B10H12 carboranes, which rearrange to a nido-form, two-electron reduction of B2pCab leads to a rearrangement into a basket-shaped intermediate.

Selective B(3)-H Activation Affording Multinuclear Ir(III) Complexes with (o-Carboranyl)dithioester Ligands

A method was developed to link two or three o-carborane moieties to form a series of carboranyl dithioester bridging ligands via in situ substitution of haloalkanes by tetraphenylphosphonium carboranyldithiocarboxylates. Based on these ligands, direct B-H activation without the assistance of Ag(I) and alkali was successfully achieved with half-sandwich Ir(III) substrate [Cp*IrCl2]2 to yield corresponding bimetallic or trimetallic complexes. Single crystal structure analyses of the B-H activated complexes and corresponding SnCl2-inserted derivatives confirm the selective B(3)-H activation in these complexes.

Boosting 2000-Fold Hypergolic Ignition Rate of Carborane by Substitutes Migration in Metal Clusters

Hypergolic propellants rely on fuel and oxidizer that spontaneously ignite upon contact, which fulfill a wide variety of mission roles in launch vehicles and spacecraft. Energy-rich carboranes are promising hypergolic fuels, but triggering their energy release is quite difficult because of their ultrastable aromatic cage structure. To steer the development of carborane-based high-performance hypergolic material, carboranylthiolated compounds integrated with atomically precise copper clusters are presented, yielding two distinct isomers, Cu14B-S and Cu14C-S, both possessing similar ligands and core structures. With the migration of thiolate groups from carbon atoms to boron atoms, the ignition delay (ID) time shortened from 6870 to 3 ms when contacted with environmentally benign oxidizer high-test peroxide (HTP, with a H2O2 concentration of 90%). The extraordinarily short ignition ID time of Cu14B-S is ranking among the best of HTP-active hypergolic materials. The experimental and theoretical findings reveal that benefitting from the migration of thiolate groups, Cu14B-S, characterized by an electron-rich metal kernel, displays enhanced reducibility and superior charge transfer efficiency. This results in exceptional activation rates with HTP, consequently inducing carborane combustion and the simultaneous release of energy. This fundamental investigation shed light on the development of advanced green hypergolic propulsion systems.

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.

Expanding Luminescence Horizons in Macropolyhedral Heteroboranes

Luminescence is observed in three novel macropolyhedral nineteen- and eighteen-vertex chalcogenaboranes: Se2B17H17 (1), SeB17H19 (3) and SeB18H20 (4). This led us to the recognition that previously published macropolyhedral heteroborane species might also exhibit luminescence. Thus, the known nineteen- and eighteen-vertex dithiaboranes S2B17H17 (2), n-S2B16H16 (5) and i-S2B16H16 (6) were synthesised and also found to exhibit a range of luminescent properties. These macropolyhedral species are very different from the previously unique fluorescent binary borane B18H22 in terms of their structural architectures, by the presence of borane cluster hetero atoms, and, as in the cases of 5 and 6, that their synthetic origins are not derived simply through the modification of B18H22 itself. They consequently greatly expand the possibilities of finding new luminescent inorganic borane species.

A 12-Vertex Metallacarborane of Silver(I)

The discovery of ferrocene in 1951 was a significant landmark in the field of organometallic chemistry, and since then, numerous sandwich- or half-sandwich metallic complexes have been reported. However, silver stands as an intriguing exception in this regard, and knowledge of its bonding situation has remained undisclosed. Herein, unprecedented 12-vertex metallacarboranes of Ag(I) (2a and 2b) were synthesized through the reaction of sodium hexamethyldisilazide (NaHMDS) with the mixture of nido-C2B9 carborane anion-supported N-heterocyclic carbene precursors (1a and 1b) and [Ag(PPh3)Cl]4. The X-ray structural analysis of the resulting metallacarboranes revealed a unique "slipped" half-sandwich structure, which is a rarity among cyclopentadienyl analogues. DFT calculations provided insights into the asymmetric π-interactions between the pentagonal C2B3 face and the silver ion.

Photoinduced Selective B-H Activation of nido-Carboranes

The development of new synthetic methods for B-H bond activation has been an important research area in boron cluster chemistry, which may provide opportunities to broaden the application scope of boron clusters. Herein, we present a new reaction strategy for the direct site-selective B-H functionalization of nido-carboranes initiated by photoinduced cage activation via a noncovalent cage···π interaction. As a result, the nido-carborane cage radical is generated through a single electron transfer from the 3D nido-carborane cage to a 2D photocatalyst upon irradiation with green light. The resulting transient nido-carborane cage radical could be directly probed by an advanced time-resolved EPR technique. In air, the subsequent transformations of the active nido-carborane cage radical have led to efficient and selective B-N, B-S, and B-Se couplings in the presence of N-heterocycles, imines, thioethers, thioamides, and selenium ethers. This protocol also facilitates both the late-stage modification of drugs and the synthesis of nido-carborane-based drug candidates for boron neutron capture therapy (BNCT).

Borirenes and Boriranes: Development and Perspectives

Strained compounds constitute a highly topical area of research in chemistry. Borirene and borirane both feature a BC2 three-membered ring. They can be viewed as the structural analogues of cyclopropane and cyclopropene, where a CH2 unit of the carbonaceous counterparts is replaced with BH, respectively. Indeed, this structural variation introduces numerous intriguing aspects. For instance, borirane and borirene are both Lewis acidic due to the presence of a tricoordinate borane center. In addition, borirene is 2π aromatic according to Hückel's rule. In addition to their ability to form adducts with Lewis bases and the capacity of borirenes to act as ligands in coordination with metals, both borirenes and boriranes exhibit ring-opening reactivity due to the considerable ring strain. Under specific conditions, coordinated boriranes can even cleave two BC bonds to serve as formal borylene sources (although the reaction mechanisms are quite complex). On the other hand, recent successful syntheses of benzoborienes and their carborane-based three-dimensional analogues (also referred to as carborane-fused boriranes) have introduced novel perspectives to this field. For instance, they display excellent ring-expanding reactivity, possibly attributed to the boosted ring strain arising from the fusion of borirenes with benzene and boriranes with o-carborane. Importantly, their applications as valuable "BC2 " synthons have become increasingly evident along with the newly disclosed reactivity. Additionally, the boosted Lewis acidity of carborane-fused boriranes, thanks to the potent electron-withdrawing effect of o-carborane, combined with their readiness for ring enlargement, makes them promising candidates as electron-accepting building blocks in the construction of chemically responsive luminescent materials. This review provides a summary of the synthesis and reactivity of borirene and borirane derivatives, with the aim of encouraging the design of new borierene- and borirane-based molecules and inspiring further exploration of their potential applications.

Carboranylphosphines: B9-Substituted Derivatives with Enhanced Reactivity for the Anchoring to Dendrimers

Several ortho-carboranes bearing a phenoxy or a phenylamino group in the B9 position were prepared employing various protection and deprotection strategies. Following established protocols, dendritic compounds were synthesized from a hexachlorocyclotriphosphazene or thiophosphoryl chloride core, and possible anchoring options for the B9-substituted ortho-carboranes were investigated experimentally and theoretically (DFT). Furthermore, 1- or 1,2-phosphanyl-substituted carborane derivatives were obtained. The resulting diethyl-, diisopropyl-, di-tert-butyl-, diphenyl- or diethoxyphosphines bearing a tunable ortho-carborane moiety are intriguing ligands for future applications in homogeneous catalysis or the medicinal sector.

9,9'-Bis-o-carboranes: synthesis and exploration of properties

A highly efficient Pd-catalyzed B(9)-H/B(9)-H oxidative dehydrogenation coupling of carboranes to synthesize 9,9'-bis-o-carboranes has been developed. The properties and derivatization of 9,9'-bis-o-carborane were also examined, which provided diverse bis-o-carborane derivatives and bis-nido-carborane.

Synthesis of Charge-Compensated nido-Carboranyl Derivatives of Sulfur-Containing Amino Acids and Biotin

A new group of charge-compensated nido-carboranyl derivatives of sulfur-containing amino acids and biotin has been synthesized in which the boron atom in position 9 or 10 of carborane is attached to a positively charged sulfur atom. The possibilities of obtaining symmetrical B(10)-substituted and asymmetric B(9)-substituted nido-carboranes were studied. Using the example of (S)-methionine and D-biotin derivatives, water-soluble S-substituted charge-compensated nido-carboranes with free functional groups were prepared. The results obtained open up prospects for the development of potential boron delivery agents for BNCT as well as new bioactive compounds containing a negatively charged nido-carboranyl fragment bearing a positive charge on the sulfur atom associated with the boron cluster.

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.

Iridium(III)-Catalyzed Regioselective B(4)-H Amination of o-Carboranes with Sufilimines

Iridium(III)-catalyzed regioselective B(4)-H amination is developed from the reaction of o-carborane acids with sulfilimines without any oxidants under mild conditions, which leads to a wide range of B(4)-H aminated o-carboranes in good yields with a broad substrate scope. Moreover, the selective B(3,6)-diamination reaction of the o-carborane acid was achieved. The present reaction is attractive from a practical point of view because dibenzothiophene is quantitatively recovered and reused.

Redox-active carborane clusters in bond activation chemistry and ligand design

Icosahedral closo-dodecaboranes have the ability to accept two electrons, opening into a dianionic nido-cluster. This transformation can be utilized to store electrons, drive bond activation, or alter coordination to metal cations. In this feature article, we present cases for each of these applications, wherein the redox activity of carborane facilitates the generation of unique products. We highlight the effects of exohedral substituents on reactivity and the stability of the products through conjugation between the cluster and exohedral substituents. Futher, the utilization of the redox properties and geometry of carborane clusters in the ligand design is detailed, both in the stabilization of low-valent complexes and in the tuning of ligand geometry.

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.