Synthesis, structure, and reactivity of 13- and 14-vertex carboranes

Redox-Active Boron Clusters

ConspectusIn this Account, we discuss our group's research over the past decade on a class of functionalized boron clusters with tunable chemical and physical properties, with an emphasis on accessing and controlling their redox behavior. These clusters can be thought of as three-dimensional aromatic systems that have distinct redox behavior and photophysical properties compared to their two-dimensional organic counterparts. Specifically, our lab has studied the highly tunable, multielectron redox behavior of B12(OR)12 clusters and applied these molecules in various settings. We first discuss the spectroscopic and electrochemical characterization of B12(OR)12 clusters in various oxidation states, followed by their use as catholytes and/or anolytes in redox flow batteries and chemical dopants in conjugated polymers. Additionally, the high oxidizing potential and visible light-absorbing nature of fluoroaryl-functionalized B12(OR)12 clusters have been leveraged by our group to generate weakly coordinating, photoexcitable species that can promote photooxidation chemistry.We have further translated these solution-phase studies of B12(OR)12 clusters to the solid state by using the precursor [B12(OH)12]2- cluster as a robust building block for hybrid metal oxide materials. Specifically, we have shown that the boron cluster can act as a thermally stable cross-linking material, which enhances electron transport between metal oxide nanoparticles. We applied this structural motif to create TiO2- and WO3-containing materials that showed promising properties as photocatalysts and electroactive materials for supercapacitors. Building on this concept, we later discovered that B12(OCH3)12, the smallest of the B12(OR)12 family, could retain its redox behavior in the solid state, a previously unseen phenomenon. We successfully harnessed this unique behavior for solid-state energy storage by implementing this boron cluster as a cathode-active material in a Li-ion prototype cell device. Recently, our group has also explored how to tune the redox properties of clusters other than B12(OR)12 species by synthesizing a library of vertex-differentiated clusters containing both B-OR and B-halogen groups. Due to the additive qualities of different functional groups on the cluster, these species allow access to a region of electrochemical potentials previously inaccessible by fully substituted closo-dodecaborate alkoxy-based derivatives.Lastly, we discuss our research into smaller-sized redox-active polyhedral boranes (B6- and B10-based cluster cores). Interestingly, these clusters show significantly less redox stability and reversibility than their dodecaborate-based counterparts. While exploring the functionalization of closo-hexaborate to create fully substituted derivates (i.e., [B6R6Hfac]-), we observed unique oxidative decomposition pathways for this cluster system. Consequently, we leveraged this oxidative instability to generate useful alkyl boronate esters via selective chemical oxidation. We further explored a closo-decaborate cluster as a platform to access electrophilic [B10H13]+ species capable of directly borylating arene compounds with unique regioselectivity. Upon chemical oxidation of the arylated decaborate clusters, we successfully synthesized various aryl boronate esters, establishing the generality of the oxidative cluster deconstruction concept.Overall, our work shows that boron clusters are an appealing class of redox-active molecules, and this fundamental and understudied property can be leveraged for constructing novel materials with tunable physical and electrochemical properties, as well as producing unique chemical reagents for small molecule synthesis.

Hetero-trimetallic complexes comprising bridging boryl and borylene ligands: an experimental and theoretical study

In an effort to explore the coordination chemistry of the coordinative sulfur centers in arachno-ruthenaborane [(Cp*Ru)2(B3H8)(CS2H)] (arachno-1), we have thermolyzed arachno-1 with group-6 metal carbonyls [M(CO)5·THF] (M = Cr, Mo and W). The reaction of arachno-1 with [Cr(CO)5·THF] resulted in the formation of hetero-trimetallic triply bridging borylene [(Cp*Ru)2(μ-CO)(μ3-CH2S2-κ2S':κ2S''){Cr(CO)3}(μ3-BH)] (2), bridging boryl-borylene [(Cp*Ru)2(μ-CO){(μ3-BH(CH2S2)-κ2B:κ2S':κ1S'')}{Cr(CO)3}(μ3-BH)] (3), and sulfido bridged hetero-trimetallic complex [(Cp*Ru)2(μ-CO)3{Cr(CO)3}(μ3-S)] (4). In 2, one side of Ru2Cr-triangle features a μ3-BH ligand while the other side is quadruply bridged by a methanedithiolato ligand in an unsymmetrical fashion. Unlike 2, in complex 3, one side of the Ru2Cr-triangle has a μ3-BH ligand while the opposite side is bridged by a boryl ligand BH(CH2S2) in an unsymmetrical way (μ3-κ2:κ2:κ1) to the metal centers. Interestingly, when the similar reactions of arachno-1 were performed with heavier group-6 metal carbonyls [M(CO)5·THF] (M = Mo and W), it led to the formation of methanedithiolato bridged hetero-trimetallic chain complexes, [{Cp*Ru(CO)}2(μ-CO)2(μ3-CH2S2-κ2S':κ2S''){M(CO)2}] (5, M = Mo; 6, M = W) and sulfido-bridged hetero-trimetallic complexes [(Cp*Ru)2(μ-CO)3{M(CO)3}(μ3-S)] (7, M = Mo; 8, M = W), analogous to 4. In complexes 5 and 6, a Ru2M-chain is symmetrically bridged by a methanedithiolato ligand. On the other hand, in complexes 4, 7, and 8, a sulfido ligand coordinates to two ruthenium and one group-6 metal atoms in μ3-fashion. All the complexes have been characterized by 1H NMR, 13C NMR, UV-vis, IR spectroscopy, and mass spectrometry and their structural architectures have been unambiguously established by single crystal X-ray diffraction studies. In addition, theoretical investigations provided valuable insights into their electronic structures and bonding properties.

N-Ligand-Enabled Aromatic Nucleophilic Amination of 1,2-Diaryl-o-Carboranes with (R2 N)2 Mg for Selective Synthesis of 4-R2 N-o-Carboranes and 2-R2 N-m-Carboranes

The nucleophilic aromatic BH substitution reaction of carboranes is uncommon, compared to the electrophilic one. This work reported a pyridine-enabled transition-metal-free regioselective nucleophilic aromatic cage B(4)-H amination of 1,2-diaryl-o-carboranes with magnesium bisamides, giving a series of B(4)-aminated o-carboranes. DFT calculations showcased a stepwise B-N formation/B-H cleavage process, in which Mg-H formation/cage closure is the rate-determining step. Unprecedentedly, in the presence of 4,4'-di-tert-butyl-2,2'-dipyridyl (dtbpy), a tandem B(4)-amination/cage isomerization reaction of o-carboranes was discovered for the facile preparation of B(2)-aminated m-carboranes. Control experiments indicated that magnesium complex, bidentate ligand (dtbpy) and reaction temperature were crucial in the cage isomerization process. This direct nucleophilic aromatic cage B-H amination reaction offers an alternative strategy for selective amination of o- and m-carboranes.

Functionalization of o-carboranes via carboryne intermediates

Carborynes (1,2-dehydro-o-carborane and 1,3-dehydro-o-carborane), three-dimensional analogues of benzyne, can be generated in situ from the precursors 1-X-2-Li-1,2-C₂B₁₀H₁₀ (X = Br, I, OTs, OTf), or 1-Me₃Si-2-[IPh(OAc)]-1,2-C₂B₁₀H₁₀ or [1-Li-3-N₂-1,2-C₂B₁₀H₁₀][BF₄]. They are a class of very useful synthons for the synthesis of a large variety of functionalized carborane derivatives for potential application in medicine, materials science and organometallic/coordination chemistry. The experimental data demonstrate that there is a correspondence between the reactions of carborynes and those of benzyne with alkenes, dienes, alkynes, aromatics or heteroaromatics in a pericyclic reaction fashion. On the other hand, carborynes have unique properties of their own owing to their steric/electronic features. They undergo regioselective sp²/sp³ C-H bond and N-Li bond insertion reactions, which has not been observed for benzyne. This review provides a comprehensive overview of recent advances in this interesting research field with considerable attention devoted to the reaction modes and the mechanisms involved.

Conjugation between 3D and 2D aromaticity: does it really exist? The case of carborane-fused heterocycles

Although several synthesized icosahedral carborane fused 2D π-ring systems are known, and even considerable conjugation has been noted between them in some cases, the phenomenon itself is not fully understood. Based on the results of our computational study, it can be concluded that the 2D aromatic character of the fused (exo) five-membered ring is low, even in cases where significant conjugation was proposed in previous studies. Moreover, the carborane moiety constricts the bonding properties of the exo ring, thus prohibiting or promoting different Lewis resonance structures. These results will shed further light on the design and electronic modulation of new carborane-based materials.

We have demonstrated that carborane fused heterocycles can definitely be considered as non-2D aromatic compounds; therefore, their aromatic character was significantly overestimated in earlier studies.

Fusing 10-vertex closo-Carborane Anions with N-Heterocyclic Carbenes

Discovered by Knöth in 1964, the 10-vertex closo-carborane anion [HCB9H91-] is a classical bicapped square antiprism that contains an unusual pentacoordinate carbon center. Compared to its larger icosahedral cousin [HCB11H111-],...

Regioselectivity of Pd-catalyzed o-carborane arylation: a theoretical view

B(3)-Arylation is unfavorable because the steric repulsion between the substituent group on C(2) and the metal moiety would lead to significant distortion of o-carborane and would result in a higher activation energy for reductive elimination.

Light-enabled alkenylation of iodocarboranes with unactivated alkenes

The synthesis of alkenylated-o-carboranes via photoalkenylation of iodocarboranes with unactivated alkenes has been achieved. This strategy features a transition metal-free protocol, a light-promoted reaction under mild reaction conditions, broad substrate scope and good functional group tolerance. Control experiments suggest that the reaction may involve the cage C-centered radical species.

Theoretical investigation of substituent effects on the relative stabilities and electronic structure of [BnXn]2- clusters

In this study, we provide a theoretical evaluation of relative stabilities and electronic structure for [B_nX_n]^2- clusters (n = 10, 12, 13, 14, 15, 16). Structural and electronic characteristics of [B_nX_n]^2- clusters are examined by comparison with the [B₁₂X₁₂]^2- counterparts with a focus on the substituent effects (X = H, F, Cl, Br, CN, BO, OH, NH₂) on the electronic structure, electron detachment energies, formation enthalpies, and charge distributions. For the electronic structure and electron detachment energies, substituent effects on boron clusters are shown to follow a very similar trend to the mesomeric and inductive effects (± M and ± I) of π-conjugated systems, and the most stable derivatives in terms of HOMO/LUMO and electron detachment energies are calculated for CN and BO substituents due to strong -M effects. In the case of formation enthalpies for larger boron clusters (n ≥ 13), the icosahedral barrier is shown to increase with the halogen and CN substitution, whereas it is possible to reduce the icosahedral barrier for the cases of X = OH and NH₂. It is shown that this reduction results from destabilizing the [B₁₂X₁₂]^2- cluster with electronic (+ M) and symmetry effects induced by OH and NH₂ ligands.

Access to cationic polyhedral carboranes via dynamic cage surgery with N-heterocyclic carbenes

Polyhedral boranes and heteroboranes appear almost exclusively as neutral or anionic species, while the cationic ones are protonated at exoskeletal heteroatoms or they are instable. Here we report the reactivity of 10-vertex closo-dicarbadecaboranes with one or two equivalents of N-heterocyclic carbene to 10-vertex nido mono- and/or bis-carbene adducts, respectively. These complexes easily undergo a reaction with HCl to give cages of stable and water soluble 10-vertex nido-type cations with protonation in the form of a BHB bridge or 10-vertex closo-type cations containing one carbene ligand when originating from closo-1,10-dicarbadecaborane. The reaction of a 10-vertex nido mono-carbene adduct with phosphorus trichloride gives nido-11-vertex 2-phospha-7,8-dicarbaundecaborane, which undergoes an oxidation of the phosphorus atom to P = O, while the product of a bis-carbene adduct reaction is best described as a distorted C₂B₆H₈ fragment bridged by the (BH)₂PCl₂⁺ moiety.

In comparison to their neutral or anionic counterparts, examples of cationic boron clusters remain scarce. Here, the authors prepare a variety of cationic polyhedral boranes by reacting closo-10-vertex carboranes with N-heterocyclic carbenes; the resulting open-cage cationic nido- arachno- or closo- derivatives are water soluble, which may enable unprecedented applications.

Directed Syntheses of CS2- and CS3-Bridged Decaborane-14 Analogues

To establish a procedure for single-cage cluster expansion of open cage dimetallaoctaboranes(12), we have investigated the chemistry of nido-[(Cp*M)₂B₆H₁₀] (η⁵-C₅Me₅ = Cp*, 1: M = Co; 2: M = Rh), with diverse chalcogen-based borate ligands. As a result, treatment of nido-1 and nido-2 with Li[BH₂E₃] (E = S, Se, or Te) yielded 10-vertex nido-[(Cp*Co)₂B₇EH₉] (3: E = S; 4: E = Se; 5: E = Te) along with known 10-vertex nido-[(Cp*M)₂B₆H₆E₂] (6: E = S, M = Co; 7: E = Se, M = Co; 8: E = Te, M = Co; 9: E = Se, M = Rh). The geometries of dimetallachalcogenaboranes, 3-9, are isostructural with decaborane(14). Thermolysis of nido-1 and nido-2 with an intermediate, generated from CS₂ and [LiBH₄]·THF reaction in THF, produced nido-[(Cp*M)₂B₆S₂H₄(CH₂S₂)] (10: M = Co; 11: M = Rh) and nido-[(Cp*M)₂B₆S₂H₄(CS₃)] (12: M = Co; 13: M= Rh). Clusters 10-13 are rare species in which one of the B-B bonds is coordinated with a {CS₂}^2- or {CS₃}^2- ligand, generating di(thioborolane) {B₂S₂CH₂} or di(thioboralane)-thione {B₂CS₃} moieties. To examine further the coordination chemistry of CS₂-bridged decaborane(14) analogue nido-10, photolysis was carried out with {M(CO)₅·THF} (M = Mo or W) that led to the isolation of [(Cp*Co)₂B₆S₂H₄(CH₂S₂){M(CO)₅}] (14: M = Mo; 15: M = W), where the {CH₂S₂} moiety is coordinated with one {M(CO)₅} moiety in η¹-fashion. All the synthesized clusters have been characterized by ESI-mass, multinuclear NMR spectroscopy, and IR spectroscopy and structurally solved by single-crystal XRD. Furthermore, DFT calculations probe the bonding of these CS₂- and CS₃-bridged decaborane analogues.

Ene Reaction of o-Carboryne with Alkynes and Alkenes at Room Temperature: Synthesis of o-Carboranyl Allenes and Alkenes

o-Carboryne undergoes at room temperature an efficient ene reaction with a large variety of alkynes and alkenes possessing an α-CH proton to give a series of o-carboranyl allenes and alkenes in good to high isolated yields. This reaction has a broad substrate scope from alkyl and aryl to silyl substituents. This protocol provides a facile synthetic method for accessing cage C-substituted carboranyl allenes and alkenes, which may be utilized as useful starting materials to synthesize multifunctionalized carboranes.

Metal-Rich Metallaboranes: Synthesis, Structures and Bonding of Bi- and Trimetallic Open-Faced Cobaltaboranes

Synthesis, isolation, and structural characterization of unique metal rich diamagnetic cobaltaborane clusters are reported. They were obtained from reactions of monoborane as well as modified borohydride reagents with cobalt sources. For example, the reaction of [Cp*CoCl]2 with [LiBH4·THF] and subsequent photolysis with excess [BH3·THF] (THF = tetrahydrofuran) at room temperature afforded the 11-vertex tricobaltaborane nido-[(Cp*Co)3B8H10] (1, Cp* = η5-C5Me5). The reaction of Li[BH2S3] with the dicobaltaoctaborane(12) [(Cp*Co)2B6H10] yielded the 10-vertex nido-2,4-[(Cp*Co)2B8H12] cluster (2), extending the library of dicobaltadecaborane(14) analogues. Although cluster 1 adopts a classical 11-vertex-nido-geometry with one cobalt center and four boron atoms forming the open pentagonal face, it disobeys the Polyhedral Skeletal Electron Pair Theory (PSEPT). Compound 2 adopts a perfectly symmetrical 10-vertex-nido framework with a plane of symmetry bisecting the basal boron plane resulting in two {CoB3} units bridged at the base by two boron atoms and possesses the expected electron count. Both compounds were characterized in solution by multinuclear NMR and IR spectroscopies and by mass spectrometry. Single-crystal X-ray diffraction analyses confirmed the structures of the compounds. Additionally, density functional theory (DFT) calculations were performed in order to study and interpret the nature of bonding and electronic structures of these complexes.

Tandem [4 + 2]/[2 + 2] cycloaddition of o-carboryne with enynes: facile construction of carborane-fused tricyclics

o-Carboryne (1,2-dehydro-o-carborane) is a very useful synthon for the synthesis of a variety of carborane-functionalized molecules. With 1-Li-2-OTf-o-C₂B₁₀H₁₀ as the precursor, o-carboryne undergoes an efficient [4 + 2] cycloaddition with various conjugated enynes, followed by a subsequent [2 + 2] cycloaddition at room temperature, generating a series of carborane-fused tricyclo[6.4.0.0^2,7]dodeca-2,12-dienes in moderate to high isolated yields. This reaction is compatible with many functional groups and has a broad substrate scope. A reactive carborane-fused 1,2-cyclohexadiene intermediate is involved, which is supported by experimental results and DFT calculations. This protocol offers a convenient strategy for the construction of complex carborane-functionalized tricyclics.

An unprecedented tandem [4 + 2]/[2 + 2] cycloaddition of o-carboryne with enynes has been disclosed for the efficient synthesis of various carborane-fused tricyclics, in which a reactive carborane-fused 1,2-cyclohexadiene intermediate is involved.

Synthesis and structure of metallacarboranes bearing carborane-fused cyclopentenyl and dicarbollyl-fused cyclopentene ligands

New types of ligands, carborane-fused cyclopentenyl and dicarbollyl-fused cyclopentene, were conveniently prepared in high yields. Treatment of the lithium salt of carborane-fused cyclopentenyl anion, [1,2-[C(Me)CHC(Ph)]-1,2-C2B10H10]Li, with [Cp*RuCl]4 generated an 18-electron complex [η3:η2-(PhCCHCMe)C2B10H10]Ru(η5-Cp*). On the other hand, reaction of the dicarbollide salt, [7,8-(CH2CH[double bond, length as m-dash]CMe)-7,8-C2B9H9]Na2 with [(η6-p-cymene)RuCl2]2 or CoCl2/CpNa, afforded the salt metathesis product [η5-(CH2CH[double bond, length as m-dash]CMe)C2B9H9]Ru(η6-p-cymene) or [η5-(CH2CH[double bond, length as m-dash]CMe)C2B9H9]Co(η5-C5H5) in good yields. In contrast, [7,8-(CH2CH[double bond, length as m-dash]CMe)-7,8-C2B9H9]Na2 reacted with Ni(dppp)Cl2 (dppp = 1,3-bis(diphenylphosphino)propane) to give another 18-electron metallacarborane [η5-(CH2CH[double bond, length as m-dash]CMe)C2B9H9]Ni(dppp), in which one of the cage carbon atoms has been isomerized to the lower belt of the cage due to steric reasons. All these new metallacarborane compounds were fully characterized by NMR spectroscopy, high-resolution mass spectroscopy and single-crystal X-ray analyses.

Synthesis and X-ray characterization of 15- and 16-vertex closo-carboranes

Carboranes are a class of carbon-boron molecular clusters with three-dimensional aromaticity, and inherent robustness. These endowments enable carboranes as valuable building blocks for applications ranging from functional materials to pharmaceuticals. Thus, the chemistry of carboranes has received tremendous research interest, and significant progress has been made in the past decades. However, many attempts to the synthesis of carboranes with more than 14 vertices had been unsuccessful since the report of a 14-vertex carborane in 2005. The question arises as to whether these long sought-after molecules exist. We describe in this article the synthesis and structural characterization of 15- and 16-vertex closo-carboranes as well as 16-vertex ruthenacarborane. Such a success relies on the introduction of silyl groups to both cage carbons, stabilizing the corresponding nido-carborane dianions and promoting the capitation reaction with HBBr₂·SMe_2. This work would shed some light on the preparation of carboranes with 17 vertices or more, and open the door for studying supercarborane chemistry.

The synthesis of closo-carboranes with more than 14 vertexes is challenging, and no examples have been reported to date. Herein, the authors present the long-sought 15- and 16-vertex closo-carboranes, in which the introduction of silyl groups to the two cage carbons is crucial; this finding might enable the synthesis of even larger carborane analogs in the future.

Magnesium-mediated sp3 C-H activation in cascade cyclization of 1-arylethynyl-2-alkyl-o-carboranes: efficient synthesis of carborane-fused cyclopentanes

This work reports an unprecedented cascade cyclization of 1-arylethynyl-2-alkyl-o-carboranes promoted by magnesium-mediated sp3 C-H activation. Treatment of 1-arylethynyl-2-alkyl-o-carboranes with MeMgBr gives a series of carborane-fused cyclopentanes in very good yields. Deuterium labelling and control experiments suggest that HMgBr, resulting in situ from the nucleophilic substitution of cage B-H bonds with Grignard reagent, initiates the reaction, in which magnesium-promoted intramolecular sp3 C-H activation serves as a key step. This work not only offers a new route for the synthesis of carborane-fused cyclopentanes, but also sheds some light on Mg-mediated C-H activation and functionalization.

Thiaboranes on Both Sides of the Icosahedral Barrier: Retaining and Breaking the Barrier with Carbon Functionalities

The concept of icosahedral barrier has been expanded from the chemistry of carbaboranes to the area of thiaboranes. Both representatives of this barrier, i. e., closo-1,2-C₂ B₁₀ H₁₂ and closo-1-SB₁₁ H₁₁ , are similar in their electron distribution, which is dominated by positive charge in the midpoint of the C-C vector and on the sulfur atom with experimentally determined dipole moments of 4.50 D and 3.64 D, respectively. This is a driving force for their reactivity as exemplified by their reactions with different carbon functionalities. Icosahedral closo-1-SB₁₁ H₁₁ reacts both with an electron sextet containing carbon (in the form of N-heterocyclic carbenes), reported earlier, and with methyl iodide with an electron octet on the carbon. The latter reaction provides hexamethylated thiaborane on the basis of methylation so far unknown in this area of heteroborane chemistry. The computations of the heat of formation (ΔH_f ²⁹⁸ ) make it possible to estimate the height of the barrier as well as to propose closo-thiaboranes beyond the barrier. Eleven and twelve vertex thiaboranes with nido electron count are known experimentally for breaking the barrier. These computations also suggest that the larger nido-thiaboranes are promising candidates for the corresponding experimental availability, i. e., the ΔH_f ²⁹⁸ of a 13-vertex nido-thiaborane cluster has been computed to be more negative than that of the well-known nido-SB₁₀ H₁₁ ^- cluster (-6.7 and -5.6 kcal mol^-1 per vertex, respectively).

Anthracene–styrene-substituted m-carborane derivatives: insights into the electronic and structural effects of substituents on photoluminescence

Efficient light emitters based on anthracene derivatives were developed. We prove that m-carborane is a perfect scaffold for coupling to anthracene giving rise to superb emitters in solution, while keeping the emission properties in aggregate state.

Organoaluminum Compounds as Catalysts for Monohydroboration of Carbodiimides

The effective catalytic activity of organoaluminum compounds for the monohydroboration of carbodiimides has been demonstrated. Two aluminum complexes, 2 and 3, were synthesized and characterized. The efficient catalytic performances of four aluminum hydride complexes L1 AlH2 (L1 =HC(CMeNAr)2 , Ar=2,6-Et2 C6 H3 ; 1), L2 AlH2 (NMe3 ) (L2 =o-C6 H4 F(CH=N-Ar), Ar=2,6-Et2 C6 H3 ; 2), L3 AlH (L3 =2,6-bis(1-methylethyl)-N-(2-pyridinylmethylene)phenylamine; 3), and L4 AlH(NMe3 ) (L4 =o-C6 H4 (N-Dipp)(CH=N-Dipp), Dipp=2,6-iPr2 C6 H3 ; 4), and an aluminum alkyl complex L1 AlMe2 (5) were used for the monohydroboration of carbodiimides investigated under solvent-free and mild conditions. Compounds 1-3 and 5 can produce monohydroborated N-borylformamidine, whereas 4 can afford the C-borylformamidine product. A suggested mechanism of this reaction was explored, and the aluminum formamidinate compound 6 was characterized by single-crystal X-ray, also a stoichiometric reaction was investigated.

Slow-spin relaxation of a low-spin S = 1/2 FeIII carborane complex

In this communication, we report the first evidence of slow-spin relaxation of a low-spin FeIII carborane complex. Iron S = 1/2 complexes showing such behaviour are particularly appealing as qubit candidates because they fulfil some of the main requirements to reach long decoherence times, such as moderate magnetic anisotropy, small spin, metal element mainly with zero-nuclear spin and furthermore, large versatility to introduce chemical modifications.

Investigation of Thiaborane closo- nido Conversion Pathways Promoted by N-Heterocyclic Carbenes

The 12-X- closo-SB₁₁H₁₀ (X = H or I) thiaboranes react with one or two molar equivalents of various N-heterocyclic carbenes (NHCs) to give the deprotonated 12-vertex species of [12-X-SB₁₁H₉·NHC]^-[NHC-H]⁺composition as kinetic products. The use of one molar equivalent of a sterically more hindered NHC reactant leads to the formation of 12-X-SB₁₁H₁₀·NHC adducts with a heavily distorted cage and the nido electron count. Further reaction of 12-I-SB₁₁H₁₀·NHC to deboronated 12-X-SB₁₀H₉·NHC proceeds in acetone to complete the closo- nido reaction pathway under the thermodynamic control. The structures of all compounds have been investigated by NMR spectroscopy and diffraction techniques. The results are supported by theoretical methods.

Electronic Effect-Guided, Palladium-Catalyzed Regioselective B-H Activation and Multistep Diarylation of o-Carboranes with Aryl Iodides

Density functional theory calculations at IDSCRF-B3LYP/DZVP computational level were conducted on palladium-catalyzed regioselective B-H activation and diarylation of o-carboranes with aryl iodides in solution. Computational results indicate that this reaction follows a multistep mechanism and needs to get over several transition states before the final B(4,5)-diarylated o-carborane derivatives are formed. B-H activation, oxidation addition, and successive reduction of the Pd(II) catalyst involving a Pd(II)-Pd(IV)-Pd(II) catalytic cycle has been confirmed, in which AgOAc plays a crucial role. Electron-donating group on the cage carbon of o-carboranes is verified to be beneficial for its B-H activation and diarylation, while steric hindrance between the aryl and o-carboranyl groups retards it. Natural population analysis and Gibbs free energetic results predict consistent regioselectivities with experiments and manifest the pivotal role of electronic effect in controlling regioselective B-H activation of o-carboranes. These results are expected to shed some light on further improvement of experimental conditions and better controlling of regioselectivities.

Efficient blue light emitting materials based on m-carborane–anthracene dyads. Structure, photophysics and bioimaging studies

Linking m-carborane to the anthracene dye produces an exceptional enhancement of the fluorescence properties, with quantum efficiencies close to 100% in solution. Dyads were internalized by HeLa cells through endocytosis showing intense blue emission.

Amphiphiles without Head-and-Tail Design: Nanostructures Based on the Self-Assembly of Anionic Boron Cluster Compounds

Anionic boron cluster compounds (ABCCs) are intrinsically amphiphilic building blocks suitable for nanochemistry. ABCCs are involved in atypical weak interactions, notably dihydrogen bonding, due to their peculiar polyhedral structure, consisting of negatively charged B-H units. The most striking feature of ABCCs that differentiates them from typical surfactants is the lack of head-and-tail structure. Furthermore, their structure can be described as intrinsically amphiphilic or aquaneutral. Therefore, classical terms established to describe self-assembly of classical amphiphiles are insufficient and need to be reconsidered. The opinions and theories focused on the solution behavior of ABCCs are briefly discussed. Moreover, a comparison between ABCCs with other amphiphilic systems is made focusing on the explanation of enthalpy-driven micellization or relations between hydrophobic and chaotropic effects. Despite the unusual structure, ABCCs still show self- and coassembly properties comparable to classical amphiphiles such as ionic surfactants. They self-assemble into micelles in water according to the closed association model. The most typical features of ABCCs solution behavior is demonstrated on calorimetry, NMR spectroscopy, and tensiometry experiments. Altogether, the unique features of ABCCs makes them a valuable inclusion into the nanochemisty toolbox to develop novel nanostructures both alone and with other molecules.

Synthesis, structure and aromaticity of carborane-fused carbo- and heterocycles

The results of molecular structures, NMR data, and NICS (nucleus-independent chemical shift) and ISE (isomerization stabilization energy) values as well as molecular orbital analyses clearly suggest the presence of considerable aromatic character in the exo five-membered ring of carborane-fused carbo- and heterocycles and considerable conjugation between a 3-D carborane and a fused 2-D π-ring system.

Conjugation between a 3-D icosahedral carborane and a fused 2-D π-ring system is ambiguous. To address this issue, we prepared several carborane-fused carbo- and heterocycles. Detailed studies on their molecular structures, NMR data, and NICS (nucleus-independent chemical shift) and ISE (isomerization stabilization energy) values as well as molecular orbital analyses clearly suggest the presence of (1) considerable aromatic character in the exo five-membered ring of carborane-fused carbo- and heterocycles and (2) considerable conjugation between a 3-D carborane and a fused 2-D π-ring system. These results will shed some light on the design of new carborane-based materials.

The effect of the embedded o-carborane ligand on the photophysical properties of a cyclometalated Pt(ii) complex: a theoretical investigation

The embedded o-carborane ligands will have an impact on the photophysical properties and quantum efficiency of these highly efficient blue phosphorescent materials.

Cyclic amino(carboranyl) silylene: synthesis, structure and reactivity

A carbene-stabilized cyclic amino(carboranyl) silylene has been prepared and structurally characterized, which can form a Lewis acid–base adduct with borane and undergo cycloaddition reactions with unsaturated molecules such as diphenylacetylene and benzophenone.

Synthesis of 13-vertex dimetallacarboranes by electrophilic insertion into 12-vertex ruthenacarboranes

The electrophilic insertion of organometallic species into metallacarboranes was studied in detail for the model compound - the 12-vertex closo-ruthenacarborane anion [Cp*Ru(C₂B₉H₁₁)]^- (1). Reactions of the anion 1 with the 12-electron cationic species [M(ring)]⁺ (M(ring) = RuCp, RuCp* and Co(C₄Me₄)) gave the 13-vertex closo-dimetallacarboranes Cp*Ru(C₂B₉H₁₁)M(ring). Similar reactions of the neutral ruthenacarborane Cp*Ru(Me₂S-C₂B₉H₁₀) produce the cationic dimetallacarboranes [Cp*Ru(Me₂S-C₂B₉H₁₀)M(ring)]⁺. The symmetrical 13-vertex diruthenacarboranes (C₅R₅)Ru(R₂C₂B₉H₉)Ru(C₅R₅) can be prepared by the direct reactions of Tl₂[7,8-R₂-7,8-C₂B₉H₉] (R = H and Me) with two equivalents of [CpRu(MeCN)₃]⁺ or [Cp*RuCl]₄. The insertions of the 14-electron cationic species [M(ring)]⁺ (M(ring) = NiCp, NiCp* and Co(C₆Me₆)) into 1 gave the 13-vertex dimetallacarboranes Cp*Ru(C₂B₉H₁₁)M(ring), which have a distorted framework with one open face. The structures of Cp*Ru(C₂B₉H₁₁)Co(C₄Me₄) and Cp*Ru(C₂B₉H₁₁)NiCp were established by X-ray diffraction. Some of the 13-vertex dimetallacarboranes have two electrons less than required by Wade's rules. This violation is explained by the absence of the appropriate pathway for the distortion of the framework.