Hubbry Logo
Carborane acidCarborane acidMain
Open search
Carborane acid
Community hub
Carborane acid
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Carborane acid
Carborane acid
Carborane acid
View on Wikipedia
from Wikipedia
Carborane acids H[CXB11Y5Z6]
Ball-and-stick model of [CHB11Cl11]− (Chlorinated carborane acid). (Acidic proton not displayed).

Colour scheme:
hydrogen − white,
chlorine − green,
boron − pink,
carbon − black.
Identifiers
3D model (JSmol)
  • InChI=1S/CHB11Cl11.H/c13-2-1-3(2,14)5(1,16)6(1,17)4(1,2,15)8(2,19)7(2,3,18)9(3,5,20)11(5,6,22)10(4,6,8,21)12(7,8,9,11)23;/h1H;
    Key: HUAJRBVWWAXTQN-UHFFFAOYSA-N
  • [H].[CH]1234[B]([Cl])56%10[B]([Cl])17%11[B]([Cl])28%12[B]([Cl])39%13[B]([Cl])415[B]([Cl])620[B]([Cl])37%10[B]([Cl])48%11[B]([Cl])59%12[B]([Cl])01%13[B]([Cl])2345
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Carborane acids H(CXB
11Y
5Z
6) (X, Y, Z = H, Alk, F, Cl, Br, CF3) are a class of superacids,[1] some of which are estimated to be at least one million times stronger than 100% pure sulfuric acid in terms of their Hammett acidity function values (H0 ≤ −18) and possess computed pKa values well below −20, establishing them as some of the strongest known Brønsted acids.[2][3][4] The best-studied example is the highly chlorinated derivative H(CHB
11Cl
11). The acidity of H(CHB
11Cl
11) was found to vastly exceed that of triflic acid, CF
3SO
3H, and bistriflimide, (CF
3SO
2)
2NH, compounds previously regarded as the strongest isolable acids.

Their high acidities stem from the extensive delocalization of their conjugate bases, carboranate anions (CXB11Y5Z6−), which are usually further stabilized by electronegative groups like Cl, F, and CF3. Due to the lack of oxidizing properties and the exceptionally low nucleophilicity and high stability of their conjugate bases, they are the only superacids known to protonate C60 fullerene without decomposing it.[5][6] Additionally, they form stable, isolable salts with protonated benzene, C6H7+, the parent compound of the Wheland intermediates encountered in electrophilic aromatic substitution reactions.

The fluorinated carborane acid, H(CHB
11F
11), is even stronger than chlorinated carborane acid. It is able to protonate butane to form tert-butyl cation at room temperature and is the only known acid to protonate carbon dioxide to give the bridged cation, [H(CO
2)
2]+
, making it possibly the strongest known acid. In particular, CO2 does not undergo observable protonation when treated with the mixed superacids HF-SbF5 or HSO3F-SbF5.[7][8][9]

The generic structure of a carborane acid exhibits up to three different types of substituents X, Y, and Z. The position of the acidic proton will depend on the substituents and is shown here in a generic location.

As a class, the carborane acids form the most acidic group of well-defined, isolable substances known, far more acidic than previously known single-component strong acids like triflic acid or perchloric acid. In certain cases, like the nearly perhalogenated derivatives mentioned above, their acidities rival (and possibly exceed) those of the traditional mixed Lewis-Brønsted superacids like magic acid and fluoroantimonic acid. (However, a head-to-head comparison has not been possible thus far, due to the lack of a measure of acidity that is suitable for both classes of acids: pKa values are ill-defined for the chemically complex mixed acids while H0 values cannot be measured for the very high melting carborane acids).

Acidity

[edit]
The carborane acid H(CHB
11Cl
11) was shown to be monomeric in the gas phase, with the acidic proton (shown in red) bound to Cl(12) and secondarily bonded to Cl(7). The monomeric form is metastable when condensed, but eventually polymerizes to give a structure with the acid proton bridging between carborane units.[10] (N.B.: The lines between the carbon and boron atoms of the carborane core show connectivity but should not be interpreted to be single bonds. The bond orders are less than one, due to electron deficiency.)

A Brønsted–Lowry acid's strength corresponds with its ability to release a hydrogen ion. One common measure of acid strength for concentrated, superacidic liquid media is the Hammett acidity function, H0. Based on its ability to quantitatively protonate benzene, the chlorinated carborane acid H(CHB
11Cl
11) was conservatively estimated to have an H0 value at or below −18, leading to the common assertion that carborane acids are at least a million times stronger than 100% sulfuric acid (H0 = −12).[11][12] However, since the H0 value measures the protonating ability of a liquid medium, the crystalline and high-melting nature of these acids precludes direct measurement of this parameter. In terms of pKa, a slightly different measure of acidity defined as the ability of a given solute to undergo ionization in a solvent, carborane acids are estimated to have pKa values below −20, even without electron-withdrawing substituents on the boron atoms (e.g., H(CHB
11H
11) is estimated to have a pKa of −24),[13] with the (yet unknown) fully fluorinated analog H(CB
11F
12) having a calculated pKa of −46.[4] The known acid H(CHB
11F
11) with one fewer fluorine is expected to be only slightly weaker (pKa < −40).

In the gas phase, H(CHB
11F
11) has a computed acidity of 216 kcal/mol, compared to an experimentally determined acidity of 241 kcal/mol (in reasonable agreement with the computed value of 230 kcal/mol) for H(CHB
11Cl
11). In contrast, HSbF6 (a simplified model for the proton donating species in fluoroantimonic acid) has a computed gas phase acidity of 255 kcal/mol, while the previous experimentally determined record holder was (C4F9SO2)2NH, a congener of bistriflimide, at 291 kcal/mol. Thus, H(CHB
11F
11) is likely the most acidic substance so far synthesized in bulk, in terms of its gas phase acidity. In view of its unique reactivity, it is also a strong contender for being the most acidic substance in the condensed phase (see above). Some even more strongly acidic derivatives have been predicted, with gas phase acidities < 200 kcal/mol.[14][15]

Carborane acids differ from classical superacids in being well-defined one component substances. In contrast, classical superacids are often mixtures of a Brønsted acid and Lewis acid (e.g. HF/SbF5).[6] Despite being the strongest acid, the boron-based carborane acids are described as being "gentle", cleanly protonating weakly basic substances without further side reactions.[11] Whereas conventional superacids decompose fullerenes due to their strongly oxidizing Lewis acidic component, carborane acid has the ability to protonate fullerenes at room temperature to yield an isolable salt.[16][6] Furthermore, the anion that forms as a result of proton transfer is nearly completely inert. This property is what makes the carborane acids the only substances that are comparable in acidity to the mixed superacids that can also be stored in a glass bottle, as various fluoride-donating species (which attack glass) are not present or generated.[17][16]

History

[edit]
Synthesis of Carborane acid from Cs+[HCB11H11]− to Cs+[HCB11Cl11]−.

Carborane acid was first discovered and synthesized by Professor Christopher Reed and his colleagues in 2004 at the University of California, Riverside.[6] The parent molecule from which carborane acid is derived, an icosahedral carboranate anion, HCB
11H−
11, was first synthesized at DuPont in 1967 by Walter Knoth. Research into this molecule's properties was put on hiatus until the mid 1980s when the Czech group of boron scientists, Plešek, Štíbr, and Heřmánek improved the process for halogenation of carborane molecules. These findings were instrumental in developing the current procedure for carborane acid synthesis.[16][18] The process consists of treating Cs+[HCB11H11]− with SO
2Cl
2, refluxing under dry argon to fully chlorinate the molecule yielding carborane acid, but this has been shown to fully chlorinate only under select conditions.[19][16][20]

In 2010, Reed published a guide giving detailed procedures for the synthesis of carborane acids and their derivatives.[21] Nevertheless, the synthesis of carborane acids remains lengthy and difficult and requires a well-maintained glovebox and some specialized equipment. The starting material is commercially available decaborane(14), a highly toxic substance. The most well-studied carborane acid H(CHB
11Cl
11) is prepared in 13 steps. The last few steps are especially sensitive and require a glovebox at < 1 ppm H2O without any weakly basic solvent vapors, since bases as weak as benzene or dichloromethane will react with carborane-based electrophiles and Brønsted acids. The final step of the synthesis is the metathesis of the μ-hydridodisilylium carboranate salt with excess liquid, anhydrous hydrogen chloride, presumably driven by the formation of strong Si–Cl and H–H bonds in the volatile byproducts:

[Et3Si–H–SiEt3]+[HCB11Cl11]− + 2HCl →H(CHB
11Cl
11) + 2Et3SiCl + H2

The product was isolated by evaporation of the byproducts and was characterized by its infrared (νCH = 3023 cm−1) and nuclear magnetic resonance (δ 4.55 (s, 1H, CH), 20.4 (s, 1H, H+) in liquid SO2) spectra (note the extremely downfield chemical shift of the acidic proton).[21] Although the reactions used in the synthesis are analogous, obtaining a pure sample of the more acidic H(CHB
11F
11) turned out to be even more difficult, requiring extremely rigorous procedures to exclude traces of weakly basic impurities.[7]

Structure

[edit]

Carborane acid consists of 11 boron atoms; each boron atom is bound to a chlorine atom. The chlorine atoms serve to enhance acidity and act as shields against attacks from the outside due to the steric hindrance they form around the cluster. The cluster, consisting of the 11 borons, 11 chlorines, and a single carbon atom, is paired with a hydrogen atom, bound to the carbon atom. The boron and carbon atoms are allowed to form six bonds due to boron's ability to form three-center, two-electron bonds.[18]

Boron has the ability to form "three-center-two-electron bond." Pictured here are the resonance structures of a 3c-2e bond in diborane.

Although the structure of the carborane acid differs greatly from conventional acids, both distribute charge and stability in a similar fashion. The carboranate anion distributes its charge by delocalizing the electrons throughout the 12 cage atoms.[22] This was shown in a single crystal X-ray diffraction study revealing shortened bond lengths in the heterocyclic portion of the ring suggesting electronic delocalization.[23]

The chlorinated carba-closo-dodecaborate anion HCB
11Cl−
11 is an outstandingly stable anion with what has previously been described as "substitutionally inert" B–Cl vertices.

While the C-vertex of the boron-based anion can de deprotonated and furthermore react with electrophiles, the B–Cl vertices remain substantially inert.

The descriptor closo indicates that the molecule is formally derived (by B-to-C+ replacement) from a borane of stoichiometry and charge [BnHn]2− (n = 12 for known carborane acids).[24] The cagelike structure formed by the 11 boron atoms and 1 carbon atom allows the electrons to be highly delocalized through the 3D cage (the special stabilization of the carborane system has been termed "σ-aromaticity"), and the high energy required to disrupt the boron cluster portion of the molecule is what gives the anion its remarkable stability.[24] Because the anion is extremely stable, it will not behave as a nucleophile toward the protonated substrate, while the acid itself is completely non-oxidizing, unlike the Lewis acidic components of many superacids like antimony pentafluoride. Hence, sensitive molecules like C60 can be protonated without decomposition.[25][26]

Usage

[edit]

There are many proposed applications for the boron-based carborane acids. For instance, they have been proposed as catalysts for hydrocarbon cracking and isomerization of n-alkanes to form branched isoalkanes ("isooctane", for example). Carborane acids may also be used as strong, selective Brønsted acids for fine chemical synthesis, where the low nucleophilicity of the counteranion may be advantageous. In mechanistic organic chemistry, they may be used in the study of reactive cationic intermediates.[27] In inorganic synthesis, their unparalleled acidity may allow for the isolation of exotic species like salts of protonated xenon.[17][18][28]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Carborane acid

View on Grokipedia
from Grokipedia
Carborane acids are a class of exceptionally strong Brønsted superacids based on icosahedral anions, such as CHB₁₁Cl₁₁⁻ or CHB₁₁F₁₁⁻, where the conjugate acid H(carborane) exhibits protic acidity far surpassing that of concentrated —over a million times stronger—while maintaining "gentle" characteristics due to the low nucleophilicity, weak coordinating ability, and minimal oxidative capacity of the anion. These acids were first developed in the early 2000s by researchers at the , led by Christopher Reed, with the chlorinated variant H(CHB₁₁Cl₁₁) identified as the strongest known neutral Brønsted acid in 2004, capable of stable storage in a bottle despite its potency. The fluorinated analog, H(CHB₁₁F₁₁), further demonstrates this extreme acidity by successfully protonating notoriously weak bases like to form H(CO₂)₂⁺, a feat unattainable by traditional mixed superacids such as due to competing Lewis acid-base interactions. Structurally, these acids feature a proton that, in the gas phase, bridges adjacent atoms on the cage via asymmetric hydrogen bonding, leading to metastable monomeric, dimeric, and polymeric phases upon condensation. The defining advantage of carborane acids lies in their separation of high acidity from the destructive tendencies of conventional superacids, enabling the isolation and crystallographic characterization of highly reactive cations—such as tropylium (C₇H₇⁺), fullerenium (HC₆₀⁺), and silylium ions—at without decomposition. This "strong yet gentle" profile has positioned them as valuable tools in for acidifying sensitive molecules like vitamins, as well as in for studying elusive species in petroleum refining, drug development, and even potential applications in protonating like . Their icosahedral boron cluster core, comprising 11 atoms and one carbon, contributes to the anion's exceptional stability and inertness, measured via Hammett acidity functions (H₀) that confirm their superiority over triflic and fluorosulfuric acids.

Overview

Definition and Classification

Carborane acids are a class of protic superacids characterized by the general formula \ceH(CHB11X11)\ce{H(CHB_{11}X_{11})}, where X denotes atoms or other electron-withdrawing substituents, and the anion is based on the icosahedral cluster, specifically the carba-closo-dodecaborate framework with a on the carbon and substituents on the atoms. These acids derive their exceptional strength from the highly delocalized negative charge on the boron-rich cluster anion, which stabilizes the proton and enables unprecedented Brønsted acidity. Unlike traditional superacids that rely on Lewis acid components, carborane acids are single-component, isolable solids, making them uniquely practical for laboratory use. The nomenclature "" stems from "carba-borane," highlighting the incorporation of a carbon atom into the cluster structure, which forms the basis of these polyhedral clusters. This etymology reflects their origin in closo-borane chemistry, where the carborane anion serves as an exceptionally weakly coordinating base. Carborane acids are thus classified within the broader category of cluster superacids, distinguished by their ability to achieve extreme acidity without the oxidative or nucleophilic drawbacks of earlier superacids like . A hallmark of carborane acids is their designation as "strong yet gentle" superacids, a term coined to describe their combination of high protonating power with the minimal Lewis basicity and reactivity of the conjugate base, allowing selective chemistry without decomposition of sensitive substrates. For instance, the prototypical example H(CHB11_{11}Cl11_{11}) exhibits Hammett acidity function (H0H_0) values below -20, as estimated from its capacity to fully protonate weak bases like benzene and from computational assessments, positioning it among the strongest known neutral Brønsted acids. This acidity surpasses that of 100% sulfuric acid (H0≈−12H_0 \approx -12) by orders of magnitude, yet the non-nucleophilic anion prevents side reactions common in other superacids.

Key Characteristics

Carborane acids are typically white, moisture-sensitive solids that appear colorless in pure form and are hygroscopic, requiring strict exclusion of during handling. They possess high melting points and low volatility, exemplified by H(CHB11_{11}Cl11)_{11}), which sublimes under at temperatures exceeding 150°C without . These acids exhibit high solubility in non-polar solvents, such as hydrocarbons like , where concentrations greater than 0.01 M can be achieved for related salts, unlike conventional superacids that demand fluorinated media for dissolution. Carborane acids display outstanding thermal and , resisting oxidation and decomposition up to 200°C in inert atmospheres due to the robust icosahedral cluster framework. Despite their exceptional acidity, carborane acids are notably non-corrosive, handling without violent reactions toward glass or metals under controlled conditions, enabling safer storage and use compared to traditional strong acids.

History

Discovery and Early Development

The development of carborane acids emerged from research conducted by Christopher A. Reed and his group at the , spanning the late to early 2000s. This effort built on pioneering work with closo- anions, such as [CB11_{11}H12_{12}]−^-, first synthesized in 1967 by W. H. Knoth through the reaction of with , which established these clusters as exceptionally stable and weakly coordinating species due to their icosahedral boron-carbon framework. By the , derivatized carborane anions had been explored for their low nucleophilicity and resistance to oxidation, laying the groundwork for their use in stabilizing highly reactive cations. Reed's 1998 review highlighted carboranes as a promising new class of anions for media, setting the stage for protonated variants. The primary motivation was to design Brønsted acids that could protonate extremely weak bases—such as hydrocarbons or fullerenes—without the conjugate base causing unwanted nucleophilic attacks or reactions, a common issue with mixed superacids like HF-SbF5_5. Drawing from closo-borane chemistry's emphasis on delocalized electron density for anion stability, Reed's team sought isolable, "strong yet gentle" acids that functioned in weakly coordinating solvents without added Lewis acids. This approach addressed limitations in traditional superacids, enabling cleaner isolation of protonated intermediates for structural and mechanistic studies. The inaugural carborane acid, H(CB11_{11}H6_6Cl6_6), was reported in 2000, prepared via metathesis of the cesium salt with HCl and used to protonate C60_{60} , yielding stable [HC60_{60}]+^+ and [C60_{60}]2+^{2+} salts unattainable with or other conventional systems due to anion interference. This milestone, published in Science, demonstrated the acids' utility in and chemistry. Further refinement culminated in 2004 with the synthesis of H(CHB11_{11}Cl11_{11}), obtained by treating Et3_3Si[CB11_{11}Cl11_{11}]−^- with anhydrous HCl to form a bench-stable, sublimable . Initial characterizations of H(CHB11_{11}Cl11_{11}) via 13^{13}C and 11^{11}B NMR in liquid SO2_2 showed deshielding shifts (Δδ = 84.0 ppm for ) far exceeding those of H2_2SO4_4 (Δδ = 64.3 ppm), alongside IR νNH_{NH} values indicating superior acidity. These measurements, adapted from Hammett H0_0 scales, positioned it as a with inferred H0_0 > -18, over a million times stronger than concentrated (H0_0 ≈ -12). The breakthrough received prominent coverage in Nature News in November 2004, underscoring its status as the strongest isolable neutral Brønsted acid and opening avenues for non-destructive protonations.

Advancements and Variants

Following the initial discovery of carborane acids in the early , significant advancements focused on halogenated variants to enhance acidity and stability. The fluorinated derivative H(CHB11_{11}F11_{11}), developed in 2013, represented a major breakthrough as the strongest known neutral Brønsted , capable of protonating alkanes such as n-hexane and at to form stable salts without requiring low temperatures or added Lewis acids. This compound's exceptional strength stems from its weakly coordinating conjugate base, enabling reactions that traditional superacids like HF/SbF5_5 cannot achieve under ambient conditions. In 2015, further characterization confirmed H(CHB11_{11}F11_{11})'s supremacy by demonstrating its ability to protonate , yielding the dication H(CO2_2)2+_2^{+}, a feat unattainable with conventional mixed Brønsted-Lewis superacids due to competitive binding. Computational and spectroscopic analyses supported gas-phase acidity estimates exceeding those of chlorinated analogs by over 20 kcal/mol, underscoring its role as a benchmark for superacidity. The perchlorinated analog H(CHB11_{11}Cl11_{11}), refined post-2005 through optimized protocols, served as a foundational structure for these improvements, offering robust monomeric gas-phase behavior and H0_0 values around -18. To achieve tuned reactivity, mixed-halogen derivatives have been synthesized by selective substitution on the cage, allowing modulation of nucleophilicity and properties for targeted applications in . These variants maintain superacidic character while providing steric and electronic fine-tuning, as evidenced by their use in stabilizing reactive intermediates. Recent progress from 2020 to 2025 has explored incorporation of motifs into polymeric frameworks, yielding solid-state materials with enhanced thermal stability. For instance, carborane-containing polyimides exhibit decomposition temperatures exceeding 500°C, addressing limitations in liquid superacid handling. Concurrently, computational modeling using has elucidated acidity trends across halogenated series, predicting that fluorination increases gas-phase acidity by over 20 kcal/mol compared to chlorinated analogs through reduced anion basicity. Early syntheses suffered from low yields (often below 20%) due to inefficient boron cluster , but post-2005 innovations, including difluorocarbene-mediated closure of B11_{11}H14−_{14}^{-} precursors, have boosted scalability to 95% for key anions, facilitating broader access to variants. These optimizations, combined with silver-catalyzed routes for mild conditions, have overcome sensitivity and multi-step inefficiencies.

Structure

Cluster Framework

The cluster framework of carborane acids is centered on the closo-CB11 anion, a highly stable icosahedral cage composed of 11 boron atoms and one carbon atom forming a 12-vertex polyhedron. This geometry arises from the three-dimensional arrangement of vertices connected by skeletal bonds, creating a robust, nearly spherical structure that exemplifies deltahedral borane cluster chemistry. The unsubstituted closo-CB11H12- anion exhibits icosahedral (Ih) point group symmetry, which imparts uniformity to the cage and underlies its resistance to distortion under various chemical conditions. Within this framework, bond lengths reflect the electronic differences between and carbon atoms. Boron-boron (B-B) bonds typically range from 1.75 to 1.80 , while carbon-boron (C-B) bonds are shorter, measuring approximately 1.70 to 1.75 , due to carbon's greater and its influence on distribution. These bonds support a density across the entire cage, contributing to the cluster's aromatic character and overall rigidity, as evidenced by crystallographic studies of salts like Tl[CB11H12]. Exohedral substituents, such as in perfunctionalized derivatives like [CHB11Cl11]-, are positioned on the vertices, preserving the core icosahedral symmetry while enabling tailored properties. The electronic structure adheres to Wade's rules for closo clusters, featuring a 2n+2 skeletal count where n=12, yielding 26 electrons that stabilize the anion and minimize its coordinating tendencies. This inherent stability of the cluster framework underpins the weakly coordinating nature essential to acids' performance.

Proton and Anion Components

Carborane acids are composed of an acidic proton paired with a anion, forming a weakly interacting ionic entity that defines their superacidic nature. The core anion is the closo-[CB11H12]−, a carba-dodecaborate cluster where the negative charge is highly delocalized across the icosahedral framework of one carbon and eleven atoms, effectively minimizing the anion's nucleophilicity and enhancing its utility as a weakly coordinating . Halogenated derivatives of the anion, represented as [CB11XnH12-n]− (where X = Cl, Br, or I and n = 6–11), feature substituent patterns that further disperse the charge; for partially halogenated forms (n=6–10), are often positioned on the lower pentagonal belt (vertices 7–11) to optimize delocalization and steric shielding, while in perhalogenated variants (n=11), they occupy all boron vertices (2–12). These modifications maintain the closo-icosahedral while tuning the anion's interaction potential with the proton. The acidic proton exists as a protic H+, loosely associated with the anionic rather than forming a strong . In structural depictions, particularly for superacidic forms like H(CHB11Cl11), it is shown as positioned externally without a direct B-H linkage, enabling high mobility and weak electrostatic binding to the delocalized charge. In the gas phase, the proton bridges adjacent atoms via asymmetric hydrogen bonding, leading to metastable monomeric, dimeric, and polymeric phases upon . This interaction results in a dynamic ionic pair, where the proton's position can shift, often bridging atoms in solid-state structures or remaining solvent-exposed in solution. The structural assembly of the acid can be conceptually represented by the pairing of the anion with a proton source, as in the general form CB11H12− + HX → H(CB11H12) + X−, emphasizing the non-covalent nature of the proton-cage association over reactive details. Spectroscopic supports these structural features. In 1H NMR spectra, the acidic proton appears as a broad, highly downfield-shifted signal reflecting its rapid exchange and weak binding, e.g., around 20 ppm in liquid SO₂. reveals characteristic B-H stretching vibrations for the cage hydrogens at approximately 2500 cm−1, confirming the integrity of the polyhedral framework and absence of perturbed bonds involving the acidic proton. The overall icosahedral geometry of the cluster underpins this delocalized charge distribution.

Properties

Acidity and Protonation Ability

Carborane acids demonstrate unparalleled proton-donating capability among isolable Brønsted acids, with the representative compound H(CHB11Cl11) estimated to have (H0) of at least -18, and possibly as low as -25, based on spectroscopic shifts in protonation equilibria, surpassing the acidity of the conventional superacid mixture HF/SbF5. Computational assessments further indicate pKa values well below -15 for these acids in , underscoring their extreme strength in polar solvents. This superior acidity allows carborane acids to protonate notoriously inert substrates that resist conventional superacids. Representative examples include the room-temperature protonation of alkanes, such as butane to yield the tert-butyl cation (C4H9+) and hydrogen gas using the fluorinated variant, enabling the isolation of alkyl carbocations otherwise unstable under milder conditions. Similarly, fullerenes like C60 are protonated to form the stable HC60+ species, which can be characterized in solution and the solid state without decomposition. A particularly striking demonstration is the of , the weakest known base, as shown in the reaction with the fluorinated variant: \ceH(CHB11F11)+2CO2−>[H(CO2)2]+[CHB11F11]−\ce{H(CHB11F11) + 2 CO2 -> [H(CO2)2]+ [CHB11F11]-} This process, reported in a 2015 study, highlights the acid's ability to generate elusive protonated species like H(CO2)2+, which traditional superacids fail to produce due to insufficient strength or anion interference. The enabling factor for this protonation prowess lies in the conjugate anion's extremely weak basicity, which provides robust stabilization to the protonated products while preventing deleterious back-reactions or coordination.

Stability and Non-Nucleophilicity

The non-nucleophilic character of carborane acids stems from the conjugate base, such as [CB11H12]- or [CB11Cl11]-, where the negative charge is delocalized over the 11 boron atoms of the icosahedral cluster, significantly reducing its basicity and coordination ability. This delocalization minimizes interactions with electrophiles, making the anion one of the least nucleophilic known, with fluoride ion affinity lower than that of [SbF6]-. Carborane anions exhibit exceptional oxidative stability, with redox potentials exceeding 2 V versus the (SCE), which prevents the decomposition of oxidation-sensitive cations paired with them. This high oxidative threshold arises from the robust cluster framework, allowing the acids to function without the destructive oxidizing power seen in traditional superacids. In comparison to [SbF6]-, the [CB11Cl11]- anion shows negligible coordination to metal centers and resists under ambient conditions, enhancing its utility as a weakly coordinating . Experimental includes the isolation of air-stable salts such as tropylium+ [CB11Cl11]-, which remain intact for months in air and have been characterized by at .

Synthesis

Preparation Methods

The synthesis of carborane acids typically begins with the preparation of salts of the parent closo-[CB11H12]- anion, such as the or cesium salts, which serve as precursors for subsequent derivatization. These salts are obtained through closo cluster formation involving the insertion of a carbon atom into the nido-[B11H14]- precursor, often via difluorocarbene insertion generated from (CF3SiMe3) with (NaH) in (THF). This method provides a simple, scalable route with yields of 70–90%, producing the [closo-CB11H12]- anion as the cesium or salt after and . Perhalogenation of the boron vertices in the [CB11H12]- anion follows, replacing the 11 B–H bonds with to yield [CHB11X11]- (X = Cl, Br, or I; for , see specialized methods). often proceeds via the dianionic precursor [CB11H11]2-, obtained by of [CB11H12]-, to enhance reactivity at vertices, followed by reprotonation at carbon. For chlorination, the cesium salt of [CB11H11]2- is treated with (ICl) in (HOTf) at 200 °C for 5 days in a sealed reactor, achieving undecachlorination with yields of 65–86% after recrystallization. Bromination employs similar conditions with (Br2) in glacial acetic acid at 80 °C, while uses gas (F2, 30% in N2) in anhydrous (aHF) at −40 °C with and 22 equivalents of F2, yielding [CHB11F11]- in 90%, though this variant is less common due to handling challenges. These improved protocols enhance efficiency over earlier methods, minimizing over-halogenation at the carbon vertex. The final step involves protonation of the halogenated anion to generate the neutral carborane acid H(CHB11X11). This is accomplished via acid-base metathesis, typically by suspending the cesium salt of [CHB11X11]- in (HCl) or (HBr) within a non-aqueous such as chlorosulfonyl fluoride (SO2ClF) or liquid SO2 at low temperatures (−78 °C to 0 °C), followed by warming to . The reaction proceeds quantitatively, precipitating the acid as a colorless solid upon and evaporation. For the chlorinated variant H(CHB11Cl11), this yields a stable, isolable product in near-quantitative conversion from the salt. Alternatively, can use a silylated precursor like [(Et3Si)2H][B11Cl11] treated with dry HCl gas at 0 °C, yielding 94%. Overall yields for the multi-step synthesis of carborane acids range from 50–70%, limited primarily by the efficiency, with the parent [CB11H12]- salt step contributing minimal losses. Purification is achieved by recrystallization from (CH2Cl2) under inert atmosphere, yielding analytically pure solids. Due to their superacidic nature (H0 ≈ −21), handling requires Schlenk techniques or conditions to avoid or of glassware; protective equipment and non-reactive materials are essential to prevent violent reactions with moisture or bases.

Key Reagents and Conditions

The synthesis of carborane acids relies on the initial assembly of the closo-carbaborate cluster framework, followed by to introduce substituents that enhance acidity, with all steps conducted under strictly conditions to prevent . Primary for cluster assembly include borohydride precursors such as (NaBH₄), which is used to form the parent [B₁₁H₁₄]⁻ intermediate from or related boron hydrides, enabling subsequent carbon insertion to yield [CB₁₁H₁₂]⁻ salts like Na[CB₁₁H₁₂] in yields up to 72% when combined with insertion agents like NaH and Me₃Si-CF₃ in (THF) solvent. Halogenation for substitution employs or halogenating agents tailored to the desired variant; for chlorinated carborane acids like H(CHB₁₁Cl₁₁), (ICl) serves as the key chlorinating reagent, reacting with Cs[CB₁₁H₁₁] in (HOTf) to achieve perchlorination, often requiring extended reaction times. Bromination follows analogous protocols using Br₂, while fluorination for H(CHB₁₁F₁₁) utilizes fluorine gas (F₂, 30% in N₂) as the primary agent, delivered in excess (22 equivalents) to ensure complete substitution at boron vertices. Solvents critical to these processes include anhydrous (aHF) for fluorination to maintain reactivity and prevent side reactions, (SO₂) for spectroscopic characterization or select halogenations, and fluorinated ethers like for handling moisture-sensitive intermediates; all manipulations occur under an inert atmosphere of dry (Ar) or (N₂) in a or to exclude oxygen and water. Reaction conditions vary by halogen: chlorination demands high temperatures of 200°C for 5 days in a sealed reactor to overcome kinetic barriers, yielding Cs[CHB₁₁Cl₁₁] at 65-86% after recrystallization from hot water, whereas fluorination proceeds at lower temperatures of -40°C with overnight stirring and in aHF under open-flow F₂ delivery, achieving 90% yield with complete (>90%) at the 11 positions while sparing the carbon vertex. The final to form the free acid, such as H(CHB₁₁Cl₁₁), involves treatment of a silylated precursor like [(Et₃Si)₂H][B₁₁Cl₁₁] with dry HCl gas at 0°C for 1 hour in a Schlenk tube, followed by cooling to -190°C, vacuum removal of HCl, and warming to , resulting in 94% yield of the moisture-sensitive solid. Shorter reaction times of 1-24 hours suffice for initial halogenations or , but perhalogenation often extends to days; sublimation under high vacuum at 150°C purifies the product. Alternative fluorination variants include electrochemical methods or XeF₂ as a milder agent for selective substitution, though F₂ remains preferred for and high in undecafluorinated species.

Applications

In Organic Synthesis

Carborane acids serve as powerful reagents in by protonating substrates such as arenes and alkenes to generate and isolate reactive carbocations as stable salts with the non-nucleophilic carborane anion, enabling their characterization and use in subsequent transformations. This approach allows for the manipulation of highly reactive species that are typically unstable under standard conditions. A key advantage of carborane acids in these applications is their compatibility with non-aqueous media, such as hydrocarbons, which circumvents the need for fluorinated solvents often required with other superacids like HF-SbF5, thereby reducing side reactions and improving substrate compatibility. The weakly coordinating nature of the carborane anion minimizes interactions with the cation, promoting high selectivity in and cation transfer steps; for example, hydride abstraction from alkanes using methyl carborane reagents produces isolable primary and secondary carbocations. Notable examples include the 1999 isolation of the protonated cation (C6H7+, a Wheland intermediate) as a crystalline salt using H(CHB11Me5Br6) in , marking the first room-temperature characterization of this intermediate. These isolations highlight the precision of acids in enabling stoichiometric carbocation-based methodologies with minimal anion interference.

In Catalysis and Material Science

Carborane acids serve as highly effective Brønsted acid catalysts in organic transformations due to their exceptional strength and ability to stabilize reactive intermediates without nucleophilic interference from the conjugate anion. In electrophilic aromatic substitutions, they facilitate the generation of stable benzenium ion salts, such as [C6H7+][CHB11Cl11-], enabling the isolation and study of Wheland intermediates that are typically fleeting under conventional acid conditions. This approach surpasses triflate-based systems, which often lead to decomposition, and has been applied to protonate and functionalize weakly basic arenes like and phosphabenzenes. For reactions, anions paired with metal centers, such as silver phosphanes, act as potent Lewis acid catalysts in hetero-Diels-Alder processes, promoting high-yield synthesis of bioactive heterocycles with turnover numbers exceeding 100 in some cases, owing to the anion's weak coordination and recyclability. A notable example of their utility in C-H activation is the room-temperature of alkanes using H(CHB11F11), the strongest known pure Brønsted acid, which forms alkyl salts and opens pathways for catalytic of linear alkanes to branched isomers without requiring elevated temperatures or metal co-catalysts. In material science, carborane anions are integrated into ion-conducting electrolytes for advanced batteries, exploiting their low nucleophilicity to prevent unwanted side reactions and enhance electrochemical stability. For instance, carborane salts like Mg(CB11H12)2 have been incorporated into glyme-based electrolytes for magnesium-ion batteries, achieving ionic conductivities up to 7.3 mS/cm at ambient temperature and enabling reversible Mg plating/stripping with over 96% Coulombic , far surpassing traditional electrolytes in voltage window (up to +4.2 V vs. Mg2+/Mg). As of 2024, further engineering of hydrocarbon-functionalized carborane electrolytes has improved stability for next-generation Mg batteries. Emerging applications from 2020 onward include doping polybenzimidazole membranes with metallacarborane salts such as H[Co(III)(C2B9H11)2] for high-temperature fuel cells, improving proton conductivity and mechanical stability under conditions. Carborane acids have shown activity in and reactions.
Add your contribution
Related Hubs
User Avatar
No comments yet.