Carborane Anions Research Articles

Electrolyte Engineering with Carboranes for Next-Generation Mg Batteries.

To realize an energy storage transition beyond Li-ion competitive technologies, earth-abundant elements, such as Mg, are needed. Carborane anions are particularly well-suited to realizing magnesium-ion batteries (MIBs), as their inert and weakly coordinating properties beget excellent electrolyte performance. However, utilizing these materials in actual electrochemical cells has been hampered by the reliance on the Mg2+ salts of the commercially available [HCB11H11]- anion, which is not soluble in more weakly binding solvents apart from the higher glymes. Herein, we demonstrate it is possible to iteratively engineer the [HCB11H11]- anion surface synthetically to address previous solubility issues and yield a highly conductive (up to 7.33 mS cm-1) and electrochemically stable (up to +4.2 V vs Mg2+/0) magnesium electrolyte that surpasses the state of the art. This novel non-nucleophilic electrolyte exhibits highly dissociative behavior regardless of concentration and is tolerant of prolonged periods of cycling in symmetric cells at high current densities (up to 2.0 mA cm-2, 400 h). The hydrocarbon functionalized carborane electrolyte presented here demonstrates >96% Coulombic efficiency when paired with a Mo6S8 cathode. This approach realizes a needed candidate to discover next-generation cathode materials that can enable the design of practical and commercially viable Mg batteries.

Interaction of Vinyl-Type Carbocations, C3H5+ and C4H7+ with Molecules of Water, Alcohols, and Acetone.

X-ray diffraction analysis and IR spectroscopy were used to study the products of the interaction of vinyl cations C3H5+ and C4H7+ (Cat+) (as salts of carborane anion CHB11Cl11-) with basic molecules of water, alcohols, and acetone that can crystallize from solutions in dichloromethane and C6HF5. Interaction with water, as content increased, proceeded via three-stages. (1) adduct Cat+·OH2 forms in which H2O binds (through the O atom) to the C=C+ bond of the cation with the same strength as seen in the binding to Na in Na(H2O)6+. (2) H+ is transferred from cation Cat+·OH2 to a water molecule forming H3O+ and alcohol molecules (L) having the CH=CHOH entity. The O- atom of alcohols is attached to the H atom of the C=C+-H moiety of Cat+ thereby forming a very strong asymmetric H-bond, (C=)C+-H⋅⋅⋅O. (3) Finally all vinyl cations are converted into alcohol molecule L and H3O+ cations, yielding proton disolvates L-H+-L with a symmetric very strong H-bond. When an acetone molecule (Ac) interacts with Cat+, H+ is transferred to Ac giving rise to a reactive carbene and proton disolvate Ac-H+-Ac. Thus, the alleged high reactivity of vinyl cations seems to be an exaggeration.

Synthesis of fluorinated aminium cations coupled with carborane anions for use as strong one-electron oxidants.

Synthesis of a series of hydrocarbon-soluble triarylamines bearing F, CF3, and Br substituents showing quasi-reversible redox events in the 0.59-1.32 V range is reported. Chemical oxidation of the amines was carried out with 0.5PhI(OAc)2/Me3SiX/Na[RCB11Cl11] (X = Cl or OTf, R = H or Me), and a few aminium salts were isolated as pure solids.

Cyclic Carbonate Synthesis from Epoxides and CO2 Catalyzed by Aluminum-Salen Complexes Bearing a nido-C2B9 Carborane Ligand.

The active and well-designed Schiff base ligands are considered "privileged ligands". The so-called salen ligands, i.e., the tetradentate [O, N, N, O] bis-Schiff base ligands, have also found broad applications in many homogeneous catalytic reactions. Modification of the salen ligands has concentrated on altering the substituents in the phenolate rings and variations in the diamine backbones. Herein, o-carborane-supported salen ligands (2) were designed and prepared. A series of aluminum-salen complexes (3·(sol)2), which were supported by the nido-C2B9 carborane anions, were synthesized. These Al(III) complexes showed high activities (TOF up to 1500 h-1) in catalyzing the cycloaddition of epoxides and CO2 at atmospheric pressure and near room temperature. Complexes 3·(sol)2 are one of the rare examples of Al-based catalysts capable of promoting cycloaddition at 1 bar pressure of CO2. Density functional theory (DFT) studies combined with the catalytic results reveal that the catalytic cycles occur on two axial sites of the Al(III) center.

Cesium carbonate mediated C-H functionalization of perhalogenated 12-vertex carborane anions.

C-H functionalization of undecahalogenated carborane anions, [HCB11X11-] (X = Cl, Br, I), is performed with Cs2CO3 in acetonitrile. We show that the requisite Cl, Br and I carborane dianions can all be efficiently accessed with Cs2CO3. The utilization of Cs2CO3 eliminates the complications associated with competing E2 elimination reactions providing an efficient, more functional group tolerant, and broader scope than previously reported. The ensuing functionalized cages provide potential synthons for constructing advanced materials and other molecular architectures for various applications.

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-], few investigations have been made into its use as a weakly coordinating anion or as a ligand substituent. Here we show that it is possible to prepare both a dianionic N-heterocyclic carbene (NHC) Li+ adduct as well as a trianionic C-2, C-5 dilithio species featuring two 10-vertex carborane anion substituents. All compounds were characterized via multinuclear NMR spectroscopy, single crystal X-ray diffraction, and HRMS when possible.

Lewis Superacidic Tellurenyl Cation-Induced Electrophilic Activation of an Inert Carborane.

The aryltellurenyl cation [2‐(tBuNCH)C6H4Te]+, a Lewis super acid, and the weakly coordinating carborane anion [CB11H12]−, an extremely weak Brønsted acid (pK a=131.0 in MeCN), form an isolable ion pair complex [2‐(tBuNCH)C6H4Te][CB11H12], in which the Brønsted acidity (pK a 7.4 in MeCN) of the formally hydridic B−H bonds is dramatically increased by more than 120 orders of magnitude. The electrophilic activation of B−H bonds in the carborane moiety gives rise to a proton transfer from boron to nitrogen at slightly elevated temperatures, as rationalized by the isolation of a mixture of the zwitterionic isomers 12‐ and 7‐[2‐(tBuN{H}CH)C6H4Te(CB11H11)] in ratios ranging from 62 : 38 to 80 : 20.

Isomers of the Allyl Carbocation C3H5 + in Solid Salts: Infrared Spectra and Structures.

Three isomers of the allyl cation C3H5+ were obtained in salts with the carborane anion CHB11Cl11–. Two of them, angular CH3–CH=CH+ (I) and linear CH3–C+=CH2 (II), were characterized by X-ray crystallography, and the third one, (CH2CHCH2)+ (III), is formed in an amorphous salt. The stretch vibration of the charged double bond C=C+ of I and II is decreased by 162 cm–1 (I) or 76 cm–1 (II) as compared to that of neutral propene. This result contradicts the prediction of DFT and MP2 calculations with the 6-311G++(d,p) basis set that the appearance of the positive charge on the C=C bond should increase its stretch vibration by 200 cm–1 (I) or 210 cm–1 (II). According to infrared spectra, the CC bonds in isomer III have one-and-a-half bond status. Isomers I and II in the crystal lattice are stabilized due to uniform ionic interactions with neighboring anions with partial transfer of a positive charge to them. Additional stabilization of II is provided by a weak hyperconjugation effect. Isomer III is stabilized in the amorphous phase due to ion paring with a counterion and a strong intramolecular hyperconjugation effect.

N‐Heterocyclic Carbenes with a Nido‐C2B9 Carborane Backbone

Main observation and conclusionThe steric and electronic properties of N‐heterocyclic carbenes (NHCs) can be modified by the exocyclic substituents at the nitrogen atoms, by 1—3 atoms’ replacements of the five‐membered imidazolium skeleton and by the changes of the backbones. Herein, we report the usage of nido‐C2B9 carborane anions as the backbones of NHCs. Stirring the mixture of secondary amino o‐carboranes (1b—1e), triethyl orthoformate and HBF4∙Et2O results in the unexpected cage‐opening of o‐carboranes to afford the nido‐C2B9 carborane anions supported N‐heterocyclic carbene precursors (4b—4e). Deprotonation of 4b with sodium hexamethyldisilazide at −78 oC affords the N‐heterocyclic carbene ligand, which was used to form an Au(I) NHC complex (5). DFT calculations revealed a high‐lying lone pair orbitals of the carbene ligands, predicting their strong σ‐donating abilities.

Unsaturated Vinyl-Type Carbocation [(CH3)2C=CH]+ in Its Carborane Salts.

The isobutylene carbocation (CH3)2C=CH+ was obtained in amorphous and crystalline salts with the carborane anion CHB11Cl11–. The cation was characterized by X-ray crystallography and IR spectroscopy. Its crystal structure shows a relatively uniform ionic interaction of the cation with the surrounding anions, with a slightly shortened distance between the C atom of the =CH group and the Cl atom of the anion, pointing to a higher positive charge on this group. In the amorphous phase, the asymmetric interaction of the cation with the anion increases, approaching ion pairing. This gives rise to a strong hyperconjugation between the two CH3 groups and the 2pz orbital of the central carbon sp2 atom (the red shift of the CH stretch is 150 cm–1); this effect stabilizes the cation. Over time, as the structure of the amorphous phase becomes more ordered, the hyperconjugation weakens and disappears in the crystalline phase with the disappearance of ion pairing. The carbocation stabilization in the crystalline phase is achieved due to the transfer of a portion of the charge to the neighboring anions, whereas the charge on the C=C bond becomes the strongest: the C=C stretch frequency drops to ∼160 cm–1 relative to neutral isobutylene. The collected IR spectra for the optimized cation under vacuum (in the 6-311G ++ (d, p) basis for all HF, MP2, and DFT calculations) predict that a positive charge on the C=C bond increases its stretching frequency; this computational result contradicts the experimental data, perhaps because it does not take into account the significant impact of the environment.

Determination of acidity constants, ionic mobilities, and hydrodynamic radii of carborane-based inhibitors of carbonic anhydrases by capillary electrophoresis.

Capillary electrophoresis (CE) has been applied for determination of the thermodynamic acidity constants (pKa ) of the sulfamidoalkyl and sulfonamidoalkyl groups, the actual and limiting ionic mobilities and hydrodynamic radii of important compounds, eight carborane-based inhibitors of carbonic anhydrases, which are potential new anticancer drugs. Two types of carboranes were investigated, (i) icosahedral cobalt bis(dicarbollide)(1-) ion with sulfamidoalkyl moieties, and (ii) 7,8-nido-dicarbaundecaborate with sulfonamidoalkyl side chains. First, the mixed acidity constants, pKamix , of the sulfamidoalkyl and sulfonamidoalkyl groups of the above carboranes and their actual ionic mobilities were determined by nonlinear regression analysis of the pH dependences of their effective electrophoretic mobility measured by capillary electrophoresis in the pH range 8.00-12.25, at constant ionic strength (25mM), and constant temperature (25°C). Second, the pKamix were recalculated to the thermodynamic pKa s using the Debye-Hückel theory. The sulfamidoalkyl and sulfonamidoalkyl groups were found to be very weakly acidic with the pKa s in the range 10.78-11.45 depending on the type of carborane cluster and on the position and length of the alkyl chain on the carborane scaffold. These pKa s were in a good agreement with the pKa s (10.67-11.27) obtained by new program AnglerFish (freeware at https://echmet.natur.cuni.cz), which provides thermodynamic pKa s and limiting ionic mobilities directly from the raw CE data. The absolute values of the limiting ionic mobilities of univalent and divalent carborane anions were in the range 18.3-27.8 TU (Tiselius unit, 1 × 10-9 m2 /Vs), and 36.4-45.9 TU, respectively. The Stokes hydrodynamic radii of univalent and divalent carborane anions varied in the range 0.34-0.52 and 0.42-0.52nm, respectively.

Free three-dimensional carborane carbanions.

Carbon atom functionalization via generation of carbanions is the cornerstone of carborane chemistry. In this work, we report the synthesis and structural characterization of free ortho-carboranyl [C2B10H11]−, a three-dimensional inorganic analog of the elusive phenyl anion that features a “naked” carbanion center. The first example of a stable, discrete C(H)-deprotonated carborane anion was isolated as a completely separated ion pair with a crown ether-encapsulated potassium cation. An analogous approach led to the isolation and structural characterization of a doubly deprotonated 1,1′-bis(o-carborane) anion [C2B10H10]22−, which is the first example of a discrete molecular dicarbanion. These reactive carbanions are key intermediates in carbon vertex chemistry of carborane clusters.

C(1)-Phenethyl Derivatives of [closo-1-CB11 H12 ]- and [closo-1-CB9 H10 ]- Anions: Difunctional Building Blocks for Molecular Materials.

C(1)-vinylation of [closo-1-CB9 H10 ]- (A) and [closo-1-CB11 H12 ]- (B) with 4-benzyloxystyryl iodide followed by hydrogenation of the double bond and reductive deprotection of the phenol functionality led to C(1)-(4-hydroxyphenethyl) derivatives. The phenol functionality was protected as the acetate. The esters were then treated with PhI(OAc)2 and the resulting isomers were separated kinetically (for derivatives of anion A) or by chromatography (for derivatives of anion B) giving the difunctionalized building blocks in overall yields of 9 % and 50 %, respectively. A similar series of reactions was performed starting with anions A and B and 4-methoxystyryl bromide and iodide. Significant differences in the reactivity of derivatives of the two carborane anions were rationalized with DFT computational results. Application of the difunctionalized carboranes as building blocks was demonstrated through preparation of two ionic liquid crystals. The extensive synthetic work is accompanied by single crystal XRD analysis of six derivatives.

Phase Transitions and Crystal Structures of Ionic Plastic Crystals Comprising Quaternary Ammonium Cations and Carborane Anion

Quaternary ammonium salts with the carborane anion CB11H12− ([cation][CB11H12]; cations: Me4N+, Et4N+, (C4H8)2N+, Bu4N+, and MeEt3(MeOCH2CH2)N+) were prepared. Each salt displayed several solid pha...

Recent Progress in the Synthesis of the Monocarba-closo-dodecaborate(-) Anions.

This Concept article focuses on the rapid growth in studies of the chemistry of the monocarba-closo-dodecaborate(-) anion (C1 carborane anion). As one of the most stable anions known, the C1 carborane anion has been useful for exploring the chemistry of highly reactive cations. On the other hand, development of novel functional molecules utilizing the unique properties of C1 carborane anion (e.g., σ-aromaticity, rigid spherical skeleton) has progressed more slowly. The main reason for this is the relatively undeveloped state of synthetic chemistry in this area. Recent advances in the synthetic chemistry of C1 carborane anion are highlighted in this Concept article, focusing on cross-coupling reactions at the carbon vertex, direct conversion of B-H bonds, and the synthesis of multivalent weakly coordinating anions. These progressions move this species beyond its well-established role of highly stable "counter" monocharged anion.

Mercury derivatives of polyhedral boranes, carboranes, and metallacarboranes

The review concerns the synthesis, structure, and chemical transformations of mercury derivatives of various polyhedral boron hydrides (carboranes, metallacarboranes, and boranes) and covers a period of time since the pioneering studies of Prof. V. I. Bregadze on chemistry of C- and B-substituted derivatives of icosahedral carboranes C2B10H12 up to date. The emphasis is placed on recent results obtained in this field, in particular, Hg-promoted substitution in nido-carboranes as well as the synthesis and properties of mercury derivatives of monocarbon carborane anion [CB11H12]–.

Teaching an old dog new tricks: new directions in fundamental and applied closo-carborane anion chemistry.

This feature article covers new directions in the fundamental and applied chemistry of the closo-carborane anions [HCB11H11]- and [HCB9H9]-, as well as some related chemistry with the dicarbolide ion [H2C2B9]2-. Specifically the manuscript will focus on summarizing the authors' as well as related novel contributions to the field. The application of such clusters as solution based electolytes for Mg batteries and related materials for ionic liquids will be discussed. In addition, the preparation of heterocycles and radicals fused to carborane anions will be discussed as well as various novel chemical transformations. Furthermore, new developments in anionic carboranyl phosphines and N-heterocyclic carbenes in the context of catalysis will be summarized.

Simple and scalable synthesis of the carborane anion CB11H12.

We report a method to obtain carba-closo-dodecaborate anion CB11H12- from boron cluster B11H14- in up to 95% yield using difluorocarbene to complete the carborane cluster. Difluorocarbene itself comes from readily available Ruppert-Prakash reagent CF3SiMe3. The synthesis is straightforward to carry out in heavy wall glass pressure tubes without the need for a glove-box and is easily scalable to 15 g scale.

Ethylene Oligomerization and Polymerization by Palladium(II) Methyl Complexes Supported by Phosphines Bearing a Perchlorinated 10-Vertex closo-Carborane Anion Substituent

The synthesis and ethylene reactivity of the zwitterionic Pd methyl complexes (κ2-P,Cl-PR2CB9Cl9)PdMe(THF) (7, R = iPr; 8, R = Ph) and (κ2-P,O-P(o-OMe-Ph)2CB9Cl9)PdMe(THF) (9), which contain the first phosphines appended with anionic 10-vertex perchlorinated closo-carboranes, are described. Complexes 7 and 8 oligomerize ethylene (23 °C, 2 atm) to a Schulz–Flory distribution of C4–C10 olefins with TOFs of ca. 8000 and 1800 t.o./h, respectively. 8 is ca. 4 times more active than the analogous (κ2-P,F-ortho-PPh2C6H4BF3)PdMe(L) (L = pyridine or collidine) system reported by Jordan and Piers, which produces butenes. Complex 9 reacts with ethylene to yield polyethylene wax (Mn ca. 1000, Đ ca. 1.5) that is similar to commercial Fischer–Tropsch waxes. The activities of 7–9 are independent of ethylene pressure and the presence of B(C6F5)3, suggesting that the catalyst resting state is the corresponding (PR2CB9Cl9)PdR(H2C═CH2) adduct. The molecular weights of the oligomer/polymer products are independent of ethylen...

Josef Michl

Abstract“The Michl group deals with Singlet fission for solar cells, molecular rotors, direct alkylation of gold surfaces, and fluorinated carborane anions.” Find out more about this author's research at https://doi.org/10.1002/ejoc.201800507.