Boron-rich Clusters Research Articles

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.

Deep Blue Phosphorescence from Platinum Complexes Featuring Cyclometalated N-Pyridyl Carbazole Ligands with Monocarborane Clusters (CB11H12-).

The utilization of deep blue phosphorescent materials in high-performance displays and solid-state lighting requires high quantum efficiencies and color purities. Here, we describe the preparation and luminescent properties of novel platinum triplet emitters featuring cyclometalated N-pyridyl-carbazole ligands functionalized with closo-monocarborane clusters [CB11H12]-. All reported complexes were fully characterized by using standard small molecule techniques (UV-vis, cyclic voltammetry, nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HRMS)), and their solid-state structures were elucidated by X-ray diffraction. These platinum phosphors emit in the blue region of the visible wavelength spectrum in both the solid and solution states. Complex 4a exhibits the highest luminous efficiency at λem = 439 nm with a photoluminescent quantum yield (PLQY) of 60% by dispersing in a PMMA matrix. Electrochemical and computational studies of complexes 4a and 4b revealed that the blue phosphorescence originates mainly from intraligand 3π → π* (3ILCT) transitions with relatively small 3MLCT mixing. A deep-blue OLED containing 4a as the light-emitting dopant was successfully fabricated using a solution-processed method, and the device exhibited blue photoluminescence with CIE coordinates of (0.17, 0.15) and a maximum external quantum efficiency (EQEmax) value of 6.2%. This article represents the pioneering study of a deep blue PhOLED using a Pt complex bearing a closo-monocarborane anion substituent, providing a new avenue into the preparation of novel triplet emitters based on boron-rich cluster anions.

Structures and chemical bonding of boron-based B12O and B11Au clusters. A counterexample in boronyl chemistry.

Boron oxide clusters have structural diversity and unique chemical bonding, and recent literature has shown that boronyl complexes dominate boron-rich oxide clusters. A counterexample in boronyl chemistry is presented in this work. Using global structural searches, electronic structure calculations, and chemical bonding analyses, we shall report on the computational design of two boron-based quasi-planar or planar clusters: B12O and B11Au. Contrary to expectation, the B12O cluster has a circular quasi-planar shape with a peripheral B-O-B bridge, which resembles bare B12 cluster. It does not contain a boronyl ligand. The isomeric boronyl complex turns out to be 10.32 kcal mol-1 higher in energy at the single-point CCSD(T) level. In contrast, B11Au cluster behaves normally with an elongated B11 moiety and a terminal Au ligand. Chemical bonding analyses reveal three-fold π/σ aromaticity in circular B12O cluster, including global 6π aromaticity, as well as spatially isolated inner 2σ aromaticity and outer 10σ aromaticity. The three-fold 6π/2σ/10σ aromaticity underlies the stability of B12O cluster. This bonding picture is unknown for bare B12 cluster and its derivatives. The elongated B11Au cluster has conflicting π/σ aromaticity (with 6π versus 8σ electron-counting). The B12O cluster is actually isoelectronic with bare B12 cluster in terms of delocalized π/σ bonding, which inherits the structural and electronic robustness of the latter.

An Organometallic Strategy for Cysteine Borylation.

Synthetic bioconjugation at cysteine (Cys) residues in peptides and proteins has emerged as a powerful tool in chemistry. Soft nucleophilicity of the sulfur in Cys renders an exquisite chemoselectivity with which various functional groups can be placed onto this residue under benign conditions. While a variety of reactions have been successful at producing Cys-based bioconjugates, the majority of these feature sulfur-carbon bonds. We report Cys-borylation, wherein a benchtop stable Pt(II)-based organometallic reagent can be used to transfer a boron-rich cluster onto a sulfur moiety in unprotected peptides forging a boron-sulfur bond. Cys-borylation proceeds at room temperature and tolerates a variety of functional groups present in complex polypeptides. Further, the bioconjugation strategy can be applied to a model protein modification of Cys-containing DARPin (designed ankyrin repeat protein). The resultant bioconjugates show no additional toxicity compared to their Cys alkyl-based congeners. Finally, we demonstrate how the developed Cys-borylation can enhance the proteolytic stability of the resultant peptide bioconjugates while maintaining the binding affinity to a protein target.

Carborane-Containing Matrix Metalloprotease (MMP) Ligands as Candidates for Boron Neutron-Capture Therapy (BNCT).

Based on the previously reported potent and selective sulfone hydroxamate inhibitors SC-76276, SC-78080 (SD-2590), and SC-77964, potent MMP inhibitors have been designed and synthesized to append a boron-rich carborane cluster by employing click chemistry to target tumor cells that are known to upregulate gelatinases. Docking against MMP-2 suggests binding involving the hydroxamate zinc-binding group, key H-bonds by the sulfone moiety with the peptide backbone residues Leu82 and Leu83, and a hydrophobic interaction with the deep P1' pocket. The more potent of the two triazole regioisomers exhibits an IC50 of 3.7 nM versus MMP-2 and IC50 of 46 nM versus MMP-9.

Comparative Study of inorganic, Boron-Rich Cluster and Organic, Phenyl Adenosine Modifications: Synthesis and Properties

Background: Nucleoside analogs are important class of chemotherapeutics. One of the original openings in the nucleoside medicinal chemistry was derivatives comprising a boron cluster component. Results: A series of adenosine derivative pairs containing inorganic boron cluster or alternatively its mimic, organic phenyl modification were synthesized and their physicochemical and biological properties compared. Marked effects of boron clusters, which are qualitatively and quantitatively different from the phenyl group effects, were detected. The studied characteristics include syn/anti conformation, lipophilicity, cytotoxicity and antiviral activity, as well as phosphorylation by adenosine kinase. Conclusion: The obtained results demonstrate usefulness of the boron clusters for tuning properties of biomolecules and prove their potential as modifying units in design of future therapeutics based on nucleoside structures.

Studies towards the development of a PET radiotracer for imaging of the P2Y1 receptors in the brain: synthesis, 18F-labeling and preliminary biological evaluation

Purine nucleotides such as ATP and ADP are important extracellular signaling molecules in almost all tissues activating various subtypes of purinoreceptors. In the brain, the P2Y1 receptor (P2Y1R) subtype mediates trophic functions like differentiation and proliferation, and modulates fast synaptic transmission, both suggested to be affected in diseases of the central nervous system. Research on P2Y1R is limited because suitable brain-penetrating P2Y1R-selective tracers are not yet available. Here, we describe the first efforts to develop an 18F-labeled PET tracer based on the structure of the highly affine and selective, non-nucleotidic P2Y1R allosteric modulator 1-(2-[2-(tert-butyl)phenoxy]pyridin-3-yl)-3-[4-(trifluoromethoxy)phenyl]urea (7). A small series of fluorinated compounds was developed by systematic modification of the p-(trifluoromethoxy)phenyl, the urea and the 2-pyridyl subunits of the lead compound 7. Additionally, the p-(trifluoromethoxy)phenyl subunit was substituted by carborane, a boron-rich cluster with potential applicability in boron neutron capture therapy (BNCT). By functional assays, the new fluorinated derivative 1-{2-[2-(tert-butyl)phenoxy]pyridin-3-yl}-3-[4-(2-fluoroethyl)phenyl]urea (18) was identified with a high P2Y1R antagonistic potency (IC50 = 10 nM). Compound [18F]18 was radiosynthesized by using tetra-n-butyl ammonium [18F]fluoride with high radiochemical purity, radiochemical yield and molar activities. Investigation of brain homogenates using hydrophilic interaction chromatography (HILIC) revealed [18F]fluoride as major radiometabolite. Although [18F]18 showed fast in vivo metabolization, the high potency and unique allosteric binding mode makes this class of compounds interesting for further optimizations and investigation of the theranostic potential as PET tracer and BNCT agent.

Metal-catalyzed cross-coupling chemistry with polyhedral boranes.

Over the past several decades, metal-catalyzed cross-coupling has emerged as a very powerful strategy to functionalize carbon-based molecules. More recently, some of the cross-coupling methodologies have been adapted to inorganic compounds including boron-rich clusters. The development of this chemistry relies on the ability to synthesize halogenated boron-rich clusters which can serve as electrophilic cross-coupling partners with nucleophilic substrates in the presence of a metal catalyst. While the cross-coupling chemistry with boron-clusters is conceptually reminiscent of that of its hydrocarbon counterparts, several key aspects including the spheroidal bulk of clusters and the distinct nature of boron-halogen/boron-heteroatom bonds make this chemistry unique. The utility of metal-catalyzed cross-coupling can be extended to several classes of polyhedral boranes including neutral and anionic carboranes, metallaboranes, and carbon-free boranes. Importantly, cross-coupling enables a suite of boron-heteroatom (C, N, O, P, S) couplings to prepare boron cluster-based systems that can be used for ligand design, medicinal chemistry, and materials applications.

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.

A theoretical study of substitutional boron–nitrogen clusters in diamond

Substitutional clusters of multiple light element dopants are a promising route to the elusive shallow donor in diamond. To understand the behaviour of co-dopants, this report presents an extensive first principles study of possible clusters of boron and nitrogen. We use periodic hybrid density functional calculations to predict the geometry, stability and electronic excitation energies of a range of clusters containing up to five N and/or B atoms. Excitation energies from hybrid calculations are compared to those from the empirical marker method, and are in good agreement.When a boron-rich or nitrogen-rich cluster consists of three to five atoms, the minority dopant element—a nitrogen or boron atom respectively—can be in either a central or peripheral position. We find B-rich clusters are most stable when N sits centrally, whereas N-rich clusters are most stable with B in a peripheral position. In the former case, excitation energies mimic those of the single boron acceptor, while the latter produce deep levels in the band-gap. Implications for probable clusters that would arise in high-pressure high-temperature co-doped diamond and their properties are discussed.

B-N, B-O, and B-CN Bond Formation via Palladium-Catalyzed Cross-Coupling of B-Bromo-Carboranes.

Carboranes are boron-rich molecules that can be functionalized through metal-catalyzed cross-coupling. Here, for the first time, we report the use of bromo-carboranes in palladium-catalyzed cross-coupling for efficient B-N, B-O, and unprecedented B-CN bond formation. In many cases bromo-carboranes outperform the traditionally utilized iodo-carborane species. This marked difference in reactivity is leveraged to circumvent multistep functionalization by directly coupling small nucleophiles (-OH, -NH2, and -CN) and multiple functional groups onto the boron-rich clusters.

Luminescent Metal Complexes Featuring Photophysically Innocent Boron Cluster Ligands.

We report the synthesis and characterization of a series of d8 metal complexes featuring robust and photophysically innocent strong-field chelating 1,1'-bis(o-carborane) (bc) ligand frameworks. A combination of UV-Vis spectroscopy, single crystal X-ray structural analysis, and DFT calculations of these species suggest that the dianionic bc ligand does not contribute to any visible metal-to-ligand charge transfer (MLCT) transitions, yet it provides a strong ligand field in these complexes. Furthermore, a bc-based Pt(II) complex containing a 4,4'-di-tert-butyl-2,2'-bipyridine ligand (dtb-bpy) has been prepared and was found to display blue phosphorescent emission dominated by MLCT from the Pt(II) center to the dtb-bpy ligand. Importantly, the bulky three-dimensional nature of the bc ligand precludes intermolecular Pt(II)⋯Pt(II) interactions in the solid state where the resulting compounds retain their emission properties. This study opens a potentially new avenue for designing organic light-emitting diode (OLED) materials with tunable properties featuring photophysically innocent boron-rich cluster ligands.

B2(BO)6 0/- and B 2(BS) 6 0/- doubly bridged structures containing BO or BS as ligands.

The investigation on the geometrical and electronic properties of B(2)(BO)(6) (0/-) and B(2)(BS)(6) (0/-) has been performed by density functional theory (DFT) using the B3LYP and BP86 methods. The chemical bonding in B(2)A(6) (A = H, BO, and BS) series is elucidated through the recently developed adaptive natural density partitioning (AdNDP). D(2h) B(2)(BO)(6) and B(2)(BS)(6) were found to possess two bridging η (2)-BO or η (2)-BS groups, as well as four terminal BO or BS groups that are analogs of diborane B(2)H(6). D(2)h B(2)(BO)(6) (-) and B(2)(BS)(6) (-) with two bridging η (2)-BO or η (2)-BS groups which are more stable than their corresponding D(3d) structures. The binding energy of B(2)(BO)(6) and B(2)(BS)(6) with respect to B(2)(BO)(6) (D2h) → 2B(BO)(3) (D(3h)) and B(2)(BS)(6)(D(2h)) → 2B(BS)(3) (D(3h)) are estimated to be (△)E = 19.8 and 40.6 kcal mol(-1) at CCSD(T)//B3LYP level, respectively. This finding advances the boronyl chemistry and helps establish the isolobal analogy between boron-rich oxide clusters and boranes.

Structural changes in the series of boron-carbon mixed clusters CxB10-x− (x = 3-10) upon substitution of boron by carbon

We report a theoretical investigation on the ten-atom boron-carbon mixed clusters C(x)B(10-x)(-) (x = 3-10), revealing a molecular wheel to monocyclic ring and linear species structural change as a function of x upon increasing the number of carbon atoms in the studied series. The unbiased searches for the global minimum structures of the clusters with x ranging from 3 to 9 were conducted using the Coalescence Kick program for different spin multiplicities. Subsequent geometry optimizations with follow-up frequency calculations at the hybrid density functional B3LYP∕6-311+G(d) level of theory along with the single point coupled-cluster calculations (UCCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(d) and RCCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(d)) revealed that the C3B7(-) and C4B6(-) clusters possess planar distorted wheel-type structures with a single inner boron atom, similar to the recently reported CB9(-) and C2B8(-). Going from C5B5(-) to C9B(-) inclusive, monocyclic and ring-like structures are observed as the most stable ones on the PES. The first linear species in the presented series is found for the C10(-) cluster, which is almost isoenergetic with the one possessing a monocyclic geometry. The classical 2c-2e σ bonds are responsible for the peripheral bonding in both carbon- and boron-rich clusters, whereas multicenter σ bonding (nc-2e bonds with n > 2) on the inner fragments in boron-rich clusters is found to be the effective tool to describe their chemical bonding nature. It was shown that the structural transitions in the C(x)B(10-x)(-) series occur in part due to the preference of carbon to form localized bonds, which are found on the periphery of the clusters. Chemical bonding picture of C10(-) is explained on the basis of the geometrical structures of the C10 and C10(2-) clusters and their chemical bonding analyses.

New ligand platforms featuring boron-rich clusters as organomimetic substituents.

200 years of research with carbon-rich molecules have shaped the development of modern chemistry. Research pertaining to the chemistry of boron-rich species has historically trailed behind its more distinguished neighbor (carbon) in the periodic table. Notably, a potentially rich and, in many cases, unmatched field of coordination chemistry using boronrich clusters remains fundamentally underdeveloped. Our work has been devoted to examining several basic concepts related to the functionalization of icosahedral boron-rich clusters and their use as ligands, aimed at designing fundamentally new hybrid molecular motifs and materials. Particularly interesting are icosahedral carboranes, which can be regarded as 3D analogs of benzene. These species comprise a class of boron-rich clusters that were discovered in the 1950s during the "space race" while researchers were developing energetic materials for rocket fuels. Ultimately, the unique chemical and physical properties of carborane species, such as rigidity, indefinite stability to air and moisture, and 3D aromaticity, may allow one to access a set of properties not normally available in carbon-based chemistry. While technically these species are considered as inorganic clusters, the chemical properties they possess make these boron-rich species suitable for replacing and/or altering structural and functional features of the organic and organometallic molecules-a phenomenon best described as "organomimetic". Aside from purely fundamental features associated with the organomimetic chemistry of icosahedral carboranes, their use can also provide new avenues in the development of systems relevant to solving current problems associated with energy production, storage, and conversion.

Bridging η2‐BO in B2(BO)3− and B3(BO)3− Clusters: Boronyl Analogs of Boranes

Anion photoelectron spectroscopy and theoretical calculations are combined to probe the structures and chemical bonding of two boron-rich oxide clusters, B(5)O(3)(-) and B(6)O(3)(-), which are shown to be appropriately formulated as B(2)(BO)(3)(-) and B(3)(BO)(3)(-), respectively. The anion clusters are found to each possess a bridging η(2)-BO group, as well as two terminal BO groups and are analogs of B(2)H(3)(-) and B(3)H(3)(-). This finding advances the boronyl chemistry and helps establish the isolobal analogy between boron-rich oxide clusters and boranes.

Structural and energetic exploration of a boron-rich sulfide cluster B6S

Boron and mixed-boron clusters have received considerable attention because of their wide applications and their essential roles in advancing chemical bonding models. Bearing the bright prospects as building blocks to form novel polymeric materials, the sulfur-rich boron sulfides have been greatly studied. However, the knowledge of the boron-rich boron sulfides is much rare. In this article, we report an extensive theoretical study on the structural, energetic, and stability features of a hitherto unknown septa-atomic cluster B6S at the CCSD(T)/6-311+G(2df)//B3LYP/6-311+G(d) level. The local minimum isomers were obtained through our recently developed program “grid-based comprehensive isomeric search algorithm.” The results show that the planar knife-like isomer B5(−BS) 01 (0.0 kcal/mol) containing the −BS moiety is the lowest energy, followed by the quasi-planar belt-like isomer B6(>S) 02 (6.7 kcal/mol) and the pyramid-like isomer B6(>S) 03 (8.4 kcal/mol). Notably, the three singlet isomers all have good kinetic stability on the basis of the potential energy surface analysis. The B/S-centered wheel-like isomers are unfavorable in thermodynamics and kinetics. The triplet state structures generally can not compete with the singlet ones. The results are compared to the analogous and isoelectronic cluster B6O. Our work is expected to provide useful information for understanding the structures and stability of boron sulfides. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

Collision-induced gas-phase reactions of perhalogenated closo-dodecaborate clusters – a comparative study

The gas phase reactivity of perhalogenated closo-dodecaborate clusters [B(12)X(12)](2-) (X = F, Cl, Br, I) with N-tetraalkylated ammonium counter ions was investigated by electrospray ionization ion trap mass spectrometry (ESI-IT-MS). Collisions with the background gases introduced a broad variety of gas phase reactions. This study represents the first experimental approach to a new class of boron-rich boron clusters that are not accessible in the condensed phase. The anionic ion pair [B(12)X(12) + N(C(n)H(2n+1))(4)](-) is generally found as the ion of highest mass. Its reaction sequence starts with an alkyl transfer from the ammonium ion to the dodecaborate cluster. Subsequently, the alkylated intermediate [B(12)X(12) + C(n)H(2n+1)](-) decomposes to give very reactive ions of the general formula [B(12)X(11)](-). These ions possess a free boron vertex and immediately bind to the residual gases N(2) and H(2)O in the ion trap by formation of the corresponding adducts [B(12)X(11) + N(2)](-) and [B(12)X(11) + H(2)O](-). Subsequent fragmentations of the water adduct repetitively substitute halogen atoms by hydroxyl groups. The fragmentation process of the free anion [B(12)X(12)](2-) depends on the applied excitation energy and on the halogen substituent X. A radical dehalogenation of the B(12) unit is observed for X = I, whereas for X = Cl or F the loss of small molecules (mainly BX(3)) dominates. The different reaction behavior is explained by the different electron affinity of the halogens and the strength of the boron-halogen-bonds. Surprisingly, isolation of the fragment ion [B(12)I(9)](-) in the ion trap yields the highly stable [B(24)I(18)](2-) dianion. This observation suggests a reaction between two negative ions in the gas phase.

Porosity tuning of carborane-based metal–organic frameworks (MOFs) via coordination chemistry and ligand design

Two new metal–organic framework (MOF) materials based on boron-rich cluster struts ( p-carborane) are reported herein. Cu(I) catalyzed coupling chemistry was used to synthesize carboxylate-based ligands, which are substantially longer than the previously studied dicarboxylated p-carborane, leading to structures with greater porosity. Solvothermal syntheses involving these ligands and Zn salts were used to prepare two new Zn(II)-based MOFs with 2D and 3D open framework structures. Upon thermal activation, these MOFs retain the chemical identity of their frameworks, leading to highly porous materials.

Doping Effects in Rare-Earth Borides

Boron-rich cluster compounds are studied for their potential as high-temperature thermoelectric materials with low thermal conductivity. Two B 12 icosahedra systems with attractive thermoelectric properties have recently been discovered: p-type REB 44 Si 2 (where RE stands for rare earth) and the anomalous n-type series of RE-B-C(N) homologous compounds. Three different doping effects of these compounds were investigated in this work in an attempt to control the properties: doping disorder into REB 44 Si 2 , which is a variable-range hopping system, V-doping of YB 44 Si 2 , and C-doping (carbon composition control) of YB 28.5 C 4 . The attempt to enhance the Seebeck coefficient in TbB 44 Si 2 through disorder was inconclusive, while V-doping of YB 44 Si 2 had a negative effect on the thermoelectric properties. Controlling the carbon composition of YB 28.5 C 4 appeared to be a promising route for increasing the absolute value of the Seebeck coefficient.