Boron Vertices Research Articles

The First Discovery of Spherical Carborane Molecular Ferroelectric Crystals

AbstractCarborane compounds, known for their exceptional thermal stability and non‐toxic attributes, have garnered widespread utility in medicine, supramolecular design, coordination/organometallic chemistry, and others. Although there is considerable interest among chemists, the integration of suitable carborane molecules into ferroelectric materials remains a formidable challenge. In this study, we employ the quasi‐spherical design strategy to introduce functional groups at the boron vertices of the o‐carborane cage, aiming to reduce molecular symmetry. This approach led to the successful synthesis of the pioneering ferroelectric crystals composed of cage‐like carboranes: 9‐OH‐o‐carborane (1) and 9‐SH‐o‐carborane (2), which undergo above‐room ferroelectric phase transitions (Tc) at approximately 367 K and 347 K. Interestingly, 1 and 2 represent uniaxial and multiaxial ferroelectrics respectively, with 2 exhibiting six polar axes and as many as twelve equivalent polarization directions. As the pioneering instance of carborane ferroelectric crystals, this study introduces a novel structural archetype for molecular ferroelectrics, thereby providing fresh insights into the exploration of molecular ferroelectric crystals with promising applications.

A Pd-catalyzed route to carborane-fused boron heterocycles.

Due to the expanding applications of icosahedral carboranes in medicinal and materials chemistry research, their functionalizations have become one of the central themes in boron-rich cluster chemistry. Although several strategies for incorporating nitrogen-containing nucleophiles on a single boron vertex of the icosahedral carboranes (C2B10H12) have been developed, methods for preparing clusters with vicinal B-N moieties are still lacking. The steric bulk of icosahedral carboranes and disparate electronic and steric nature of the N-containing groups have rendered the vicinal diamination challenging. In this article, we show how a developed Pd-catalyzed process is used to incorporate an array of NH-heterocycles, anilines, and heteroanilines with various electronic and steric profiles onto the vicinal boron vertices of a meta-carborane cluster via sequential or one-pot fashion. Importantly, oxidative cyclizations of the cross-coupling products with indoles and pyrroles appended to boron vertices generate a previously unknown class of all-boron-vertex bound carborane-fused six- and seven-membered ring heterocycles. Photophysical studies of the meta-carborane-fused heterocycles show that these structures can exhibit luminescence with high quantum yields and are amenable to further manipulations.

Direct B-H Functionalization of Icosahedral Carboranes via Hydrogen Atom Transfer.

The efficient and selective functionalization of icosahedral carboranes (C2B10H12) at the boron vertexes is a long-standing challenge owing to the presence of 10 inert B-H bonds in a similar chemical environment. Herein, we report a new reaction paradigm for direct B-H functionalization of icosahedral carboranes via B-H homolysis enabled by a nitrogen-centered radical-mediated hydrogen atom transfer (HAT) strategy. Both the HAT process of the carborane B-H bond and the resulting boron-centered carboranyl radical intermediate have been confirmed experimentally. The reaction occurs at the most electron-rich boron vertex with the lowest B-H bond dissociation energy (BDE). Using this strategy, diverse carborane derivatization, including thiolation, selenation, alkynylation, alkenylation, cyanation, and halogenation, have been achieved in satisfactory yields under a photoinitiated condition in a metal-free and redox-neutral fashion. Moreover, the synthetic utility of the current protocol was also demonstrated by both the scale-up reaction and the construction of carborane-based functional molecules. Therefore, this methodology opens a radical pathway to carborane functionalization, which is distinct from the B-H heterolytic mechanism in the traditional strategies.

Innenrücktitelbild: Site‐Selective Functionalization of Carboranes at the Electron‐Rich Boron Vertex: Photocatalytic B−C Coupling via a Carboranyl Cage Radical (Angew. Chem. 37/2022)

Die regioselektive Funktionalisierung von Carboranen mit mehreren BH-Zentren ist eine seit langem bestehende Herausforderung. In ihrem Forschungsartikel (e202205672) entwickelten Deshuang Tu, Changsheng Lu, Hong Yan und Mitarbeiter ein effizientes Reaktionsprotokoll, um eine selektive Alkylierung, Alkenylierung und Heteroarylierung von Carboranen unter milden Bedingungen über Bor-zentrierte Carboranylradikale zu erreichen, die durch photoinduzierte Decarboxylierung von borsubstituierten Carborancarbonsäuren erzeugt werden.

Inside Back Cover: Site‐Selective Functionalization of Carboranes at the Electron‐Rich Boron Vertex: Photocatalytic B−C Coupling via a Carboranyl Cage Radical (Angew. Chem. Int. Ed. 37/2022)

Inside Back Cover: Site‐Selective Functionalization of Carboranes at the Electron‐Rich Boron Vertex: Photocatalytic B−C Coupling via a Carboranyl Cage Radical (Angew. Chem. Int. Ed. 37/2022)

Site‐Selective Functionalization of Carboranes at the Electron‐Rich Boron Vertex: Photocatalytic B−C Coupling via a Carboranyl Cage Radical

AbstractFunctionalization 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.

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.

Visible-Light-Promoted Nickel-Catalyzed Cross-Coupling of Iodocarboranes with (Hetero)Arenes via Boron-Centered Carboranyl Radicals.

A general strategy for the generation of hypervalent boron-centered carboranyl radicals at the B(3), B(4), and B(9) positions has been developed for the first time via visible-light-promoted iodine atom abstraction from iodo-o-carboranes by low-valent nickel complex. These radicals react with various (hetero)arenes to afford a wide range of cage B-arylated carborane derivatives at room temperature in very good to excellent yields with a broad substrate scope. Their electrophilicities are dependent on the vertex charges of the cage and follow the order B(3) > B(4) > B(9). Both visible light and nickel catalyst are proved critical to the generation of boron-centered carboranyl radicals. The involvement of boron radicals is supported by control experiments. A reaction mechanism associated with these reactions is also proposed. This strategy offers a new protocol for the generation of boron-centered carboranyl radicals at the selected boron vertex, leading to a facile synthesis of a large class of cage boron substituted carborane molecules.

A Strategy for Selective Catalytic B-H Functionalization of o-Carboranes.

Carboranes are a class of polyhedral carbon-boron molecular clusters featuring three-dimensional aromaticity, which are often considered as 3D analogues of benzene. Their unique structural and electronic properties make them invaluable building blocks for applications ranging from functional materials to versatile ligands to pharmaceuticals. Thus, selective functionalization of carboranes has received tremendous research interest. In earlier days, the vast majority of the works in this area were focused on cage carbon functionalization via facile deprotonation of cage CH, followed by reaction with electrophiles. On the contrary, cage B-H activation is very challenging since the 10 B-H bonds on o-carborane are very similar, and how to achieve the desired transformation at specific boron vertex is a long-standing issue.As carbon is considered more electronegative than boron, this property results in different vertex charges on the o-carborane cage, which follow the order B(3,6)-H ≪ B(4,5,7,11)-H < B(8,10)-H < B(9,12)-H. We thought that this difference may trigger the favorite interaction of a proper transition metal complex with a specific B-H bond of carborane, which could be utilized to solve the selectivity issue. Accordingly, our strategy is described as follows: (1) electron-rich transition metal catalysts are good for the activation of the most electron-deficient B(3,6)-H bonds (connected to both cage C-H vertices); (2) electron-deficient transition metal catalysts are good for the activation of the relatively electron-rich B(8,9,10,12)-H bonds (with no bonding to either cage C-H vertices); and (3) directing-group-assisted transition metal catalysis is appropriate for the activation of the B(4,5,7,11)-H bonds (connected to only one cage C-H vertex), whose vertex charges lie in the middle of the range for the 10 B-H bonds. This strategy has been successfully applied by our laboratory and other groups in the development of a series of synthetic routes for catalytic selective activation of B-H bonds of the carborane cage, resulting in the synthesis of a large number of cage-boron-functionalized carborane derivatives in a regioselective and catalytic fashion. Subsequently, significant progress in this emerging area has been made.In 2013 we reported the selective tetrafluorination of o-carboranes at the B(8,9,10,12)-H bonds using an electron-deficient Pd(II) salt, [Pd(MeCN)4][BF4], as the catalyst. In 2014 we disclosed the first example of carboxy-directed alkenylation of o-carboranes at the B(4) vertex promoted by an Ir(III) catalyst. Subsequently, in 2017 we presented an electron-rich Ir(I)-catalyzed diborylation of o-carboranes at the B(3,6)-H bonds. We also uncovered the first example of Pd-catalyzed asymmetric synthesis of chiral-at-cage o-carboranes in 2018. These proof-of-principle studies have greatly stimulated research activities in selective B-H activation of carboranes and boron clusters enabled by transition metal catalysts. We have so far developed a toolbox of synthetic methods for selective catalytic cage B-olefination, -arylation, -alkenylation, -alkynylation, -oxygenation, -sulfenylation, -borylation, -halogenation, and -amination. We have recently expanded our research to base metal catalysis. As the field progresses, we expect that other methods for regioselective cage B-H activation will be invented, and the results detailed in this Account will promote these efforts.

Free Radical Polymerization of o‐Carborane Functionalized Styrene: Effect of Substitution Position at Carbon and Boron

AbstractThe AIBN initiated radical polymerization of o‐carborane functionalized styrenes has been developed. The styrene decorated on the cage boron or cage carbon vertex of o‐carboranes were all compatible with this polymerization. The Mw of the desired P[4‐(B3‐o‐carborane)‐styrene] and P[C1‐methylene‐o‐carborane‐4‐vinylbenzene] could up to 3.5×105 and 1.7×105, respectively. This work offers an efficient method for synthesis of o‐carborane functionalized polystyrenes, which has important reference for synthesis of o‐carborane containing polymers.

Rh-Catalyzed Decarbonylative Cross-Coupling between o-Carboranes and Twisted Amides: A Regioselective, Additive-Free, and Concise Late-Stage Carboranylation.

The convenient cross-coupling of sp2 or sp3 carbons with a specific boron vertex on carborane cage represents significant synthetic values and insurmountable challenges. In this work, we report an Rh-catalyzed reaction between o-carborane and N-acyl-glutarimides to construct various Bcage -C bonds. Under the optimized condition, the removable imine directing group (DG) leads to B(3)- or B(3,6)-C couplings, while the pyridyl DG leads to B(3,5)-Ar coupling. In particular, an unexpected rearrangement of amide reagent is observed in pyridyl directed B(4)-C(sp3 ) formation. This scalable protocol has many advantages, including easy access, the use of cheap and widely available coupling agents, no requirement of an external ligand, base or oxidant, high efficiency, and a broad substrate scope. Leveraging the RhI dimer and twisted amides, this method enables straightforward access to diversely substituted and therapeutically important carborane derivatives at boron site, and provides a highly valuable vista for carborane-based drug screening.

Boron-vertex modification of carba-closo-dodecaborate for high-performance magnesium-ion battery electrolyte

A new family of Mg salts of carba-closo-dodecaborate was developed for Mg ion battery applications. Modification at the boron vertices drastically improves the solubility without compromising the chemical and redox stability.

"POLYHEDRAL CONTRACTION" OF TETRAMETHYLAMMONIUM CLOSO-CARBOUNDECABORATE UNDER ACTION OF TRIS(TRIPHENYLPHOSPHINE)RUTHENIUM DICHLORIDE

In this paper the objectives were not only to investigate new promising methods leading to a deep structural rearrangement of carboranes and metallacarboranes but also to try to fix, to isolate and to characterize the intermediates. At the same time considerable attention is paid to the so-called "anti-wade" clusters formed during the reactions, the electronic structure of which does not correspond to their actually observed geometry. It is shown that the interaction of 11-vertex monocarbon closo-carboundecaborane with tris(triphenylphosphine)ruthenium dichloride leads to a series of new metallacarborane complexes. For the first time it was possible to control the process of "polyhedral contraction" of carborane {CB10} → (RuCB10} → {RuCB9} → {RuCB8} → {RuCB6} on the example of ruthenium complex and also to confirm experimentally each stage of the whole process. As a result of the reaction clusters of classical type and electron-deficient isocloso-/hypercloso-rutenacarborans {RuCB8}, {RuCB9} with 2n skeletal electrons were isolated. It is found that the interaction of tetramethylammonium closo-carboundecaborate with tris(triphenylphosphine)ruthenium dichloride proceeds in several stages. The first intermediate 12-vertex cluster is formed by mixing the initial reagents under heterogeneous conditions (methanol-THF, K2CO3, triethylbenzylammonium chloride) at room temperature for 10 hours. When the resulting complex is stirred in a mixture of methanol-THF solvents for 7 days at 20 °C, its almost quantitative transition to 11-vertex rutenacarborane was observed. Further transformation with the formation of a 10-vertex isocloso-rutenacarborane occurs when the methanol solution of this complex is boiled for 15 minutes. When the complex is boiled in methanol for 8 hours we obtained another extraordinary metallacarborane belonging to the rare 8-vertex pileo-clusters with a boron vertex built over the triangular face.

Studies on synthesis of boron and carborane cage derivatives

Weakly coordinating anions are extremely versatile species for capturing and characterizing cationic intermediates. Among these anions, carboranes and some boranes are known to be the most weakly coordinating anions. In this study, the carboranes and boranes were synthesized in order to use in different fields. B 11 H 14 - was synthesized from NaBH4 and used to synthesize CB 11 H 12 -. CB 11 H 12 - was synthesized by carbene addition to undecaborate. B 11 H 14 - was converted to B 10 H 14 by oxidation and extrusion of one boron vertex. CB11H12- was methylated on boron vertex to get HCB 11 Me 11 -. The methylated carborane cages are more hydrophobic and weakly coordinating anions.

Periphery Decorated and Core Initiated Neutral and Polyanionic Borane Large Molecules: Forthcoming and Promising Properties for Medicinal Applications.

A mini-review based on radial growing macromolecules and core initiated Borane periphery decorated with o-carboranes and metallacarboranes that has been developed in the authors laboratories is reported. The review is divided into four sections; three of them are related to the design and synthesis of these large boron-containing molecules and the fourth deals with the unique properties of anionic metallacarborane molecules that provide a glimpse of their potential for their promising use in medicinal applications. Their unique stability along with their geometrical and electronic properties, as well as the precise steric structure of 1,2-closo-C2B10H12 (o-carborane) that has the potential for the incorporation of many substituents: at the carbon (Cc), at the boron and at both carbon and boron vertices, suggests this cluster as an innovative building block or platform for novel applications that cannot be achieved with organic hydrocarbon compounds. Poly(aryl-ether) dendrimers grown from fluorescent cores, such as 1,3,5-triarylbenzene or meso-porphyrins, have been decorated with boron clusters to attain rich boron containing dendrimers. Octasilsesquioxane cubes have been used as a core for its radial growth to get boron-rich large molecules. The unique properties of cobaltabisdicarbollide cluster, which include: i) self-assembly in water to produce monolayer nano-vesicles, ii) crossing lipid bilayer membranes, iii) interacting with membrane cells, iv) facilitating its visualization within cells by Raman and fluorescence techniques and v) their use as molecular platform for "in vivo" imaging are discussed in detail.

Synthesis, structure, and physicochemical properties of triply-bridged binuclear copper(II) complex [Cu2Phen2(µ-CH3CO2)2(µ-OH)]2[B10Cl10

The reaction between copper(II) acetate and Phen in the presence of [B10Cl10]2− anion affords mononuclear copper(II) complex [CuPhen2(CH3CO2)]2[B10Cl10]·[CuPhen(CH3CO2)2]. Continuous heating of the latter in DMSO results in triply-bridged binuclear copper(II) complex [Cu2Phen2(CH3CO2)2OH]2[B10Cl10] isolated from the reaction solution as a DMSO solvate hydrate [Cu2Phen2(CH3CO2)2OH]2[B10Cl10]·3DMSO·0.5H2O. Both complexes are studied by IR, 11B NMR, and X-ray diffraction; the binuclear complex is studied by 35Cl NQR and EPR spectroscopies as well as X-ray powder diffraction technique. Despite the short Cu−Cu distance revealed by X-ray diffraction analysis in the binuclear cationic complex (3.234 Å), no interactions between copper atoms are detected by EPR spectroscopy. According to 35Cl NQR studies, chlorine atoms bound with equatorial boron vertices of the boron cage participate in non-covalent interactions with H atoms of Phen molecules.

Improved synthesis of icosahedral carboranes containing exopolyhedral B[sbnd]C and C[sbnd]C bonds

Carboranes are boron-rich molecular clusters possessing electronic characteristics that allow for orthogonal approaches to vertex-selective modifications. We report improved functionalization methods utilizing orthogonal chemistry to achieve efficient substitution at electron-rich B-vertices and electron-poor C-vertices of carborane. Functionalization of B-vertices with alkyl and (hetero)aryl groups using the corresponding Grignard reagents has been improved through the use of a Pd-based precatalyst featuring an electron-rich biaryl phosphine ligand, resulting in reduced reaction times. Importantly, this method is tolerant towards alkyl-based Grignard reagents containing β-hydrogens. Furthermore, a transition metal-free approach to the substitution of carborane C-vertices with (hetero)aryl substrates has been developed under nucleophilic aromatic substitution (SNAr) conditions. The selective substitution of carboranes afforded by these methods holds potential for the rational synthesis of heterofunctionalized boron clusters with substituents on both boron and carbon-based vertices.

Palladium-Catalyzed Selective Five-Fold Cascade Arylation of the 12-Vertex Monocarborane Anion by B-H Activation.

A series of cage penta-arylated carboranes have been synthesized by palladium-catalyzed intermolecular coupling of the C-carboxylic acid of the monocarba- closo-dodecaborate anion [CB11H12]- with iodoarenes by direct cage B-H bond functionalization. These transformations set a record in terms of one-pot directing group-mediated activation of inert bonds in a single molecule. The methodology is characterized by high yields, good functional group tolerance, and complete cage regioselectivity. The directing group COOH can be easily removed during or after the intermolecular coupling reaction. The mechanistic pathways were probed using density functional theory calculations. A Pd(II)-Pd(IV)-Pd(II) catalytic cycle is proposed, in which initial coupling is followed by preferred B-H activation of the adjacent boron vertex, and continuation of this selectivity results in a continuous walking process of the palladium center. The methodology opens a new avenue toward building blocks with 5-fold symmetry.

Palladium catalyzed selective arylation of o-carboranes via B(4)-H activation: amide induced regioselectivity reversal.

By changing the charge distribution of boron vertices via introducing an amide on cage B(9), the selective B(4) arylation of o-carboranes via Suzuki-Miyaura coupling has been developed. A series of o-carborane derivatives decorated with diverse active groups have been synthesized with moderate to good yields, which have been proved to be further transformed to a novel kind of tri-substituted nido-carborane fused oxazole with potential application in boron neutron capture therapy, organometallic as well as coordination chemistry.

Metal–metal bonding in deltahedral dimetallaboranes and trimetallaboranes: a density functional theory study

Abstract The skeletal bonding topology as well as the Re=Re distances and Wiberg bond indices in the experimentally known oblatocloso dirhenaboranes Cp*2Re2B n−2H n−2 (Cp*=η5Me5C5, n=8–12) suggest formal Re=Re double bonds through the center of a flattened Re2B n−2 deltahedron. Removal of a boron vertex from these oblatocloso structures leads to oblatonido structures such as Cp2W2B5H9 and Cp2W2B6H10. Similar removal of two boron vertices from the Cp2Re2B n−2H n−2 (n=8–12) structures generates oblatoarachno structures such as Cp2Re2B4H8 and Cp2Re2B7H11. Higher energy Cp2Re2B n−2H n−2 (Cp=η5-C5H5, n=8–12) structures exhibit closo deltahedral structures similar to the deltahedral borane dianions B n H n 2−. The rhenium atoms in these structures are located at adjacent vertices with ultrashort Re≣Re distances similar to the formal quadruple bond found in Re2Cl8 2− by X-ray crystallography. Such surface Re≣Re quadruple bonds are found in the lowest energy PnRe2B n−2H n−2 structures (Pn=η5,η5-pentalene) in which the pentalene ligand forces the rhenium atoms to occupy adjacent deltahedral vertices. The low-energy structures of the tritungstaboranes Cp3W3(H)B n−3H n−3 (n=5–12), related to the experimentally known Cp*3W3(H)B8H8, have central W3B n−3 deltahedra with imbedded bonded W3 triangles. Similar structures are found for the isoelectronic trirhenaboranes Cp3Re3B n−3H n−3. The metal atoms are located at degree 6 and 7 vertices in regions of relatively low surface curvature whereas the boron atoms are located at degree 3–5 vertices in regions of relatively high surface curvature. The five lowest-energy structures for the 11-vertex tritungstaborane Cp3W3(H)B8H8 all have the same central W3B8 deltahedron and differ only by the location of the “extra” hydrogen atom. The isosceles W3 triangles in these structures have two long ~3.0 Å W–W edges through the inside of the deltahedron with the third shorter W–W edge of ~2.7 to ~2.8 Å corresponding to a surface deltahedral edge.