Carborane acid

Carborane acids H(CXB11Y5Z6) (X, Y, Z = H, Alk, F, Cl, Br, CF3) are a class of superacids, some of which are estimated to be at least one million times stronger than 100% 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. The most well studied example is the highly chlorinated derivative H(CHB11Cl11). The acidity of H(CHB11Cl11) was found to vastly exceed that of triflic acid, CF3SO3H, and bistriflimide, (CF3SO2)2NH, compounds previously regarded as the strongest isolable acids. Carborane acids H(CXB11Y5Z6) (X, Y, Z = H, Alk, F, Cl, Br, CF3) are a class of superacids, some of which are estimated to be at least one million times stronger than 100% 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. The most well studied example is the highly chlorinated derivative H(CHB11Cl11). The acidity of H(CHB11Cl11) was found to vastly exceed that of triflic acid, CF3SO3H, and bistriflimide, (CF3SO2)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. 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(CHB11F11), 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, +, 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. 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). 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(CHB11Cl11) 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). 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, with the (yet unknown) fully fluorinated analog H(CB11F12) having a calculated pKa of −46. The known acid H(CHB11F11) with one fewer fluorine is expected to be only slightly weaker. In the gas phase, H(CHB11F11) 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(CHB11Cl11). 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(CHB11F11) is likely the most acidic substance so far synthesized in bulk, in terms of its gas phase acidicity. 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 and approaching the measured acidity of helionium ion (gas phase acidity 178 kcal/mol), HeH+, the most acidic species possible. 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). Despite being the strongest acid, the boron-based carborane acids are described as being 'gentle', cleanly protonating weakly basic substances without further side reactions. 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. 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. Carborane acid was first discovered and synthesized by Professor Christopher Reed and his colleagues in 2004 at the University of California, Riverside. Prior to carborane acid's discovery, the long-standing record of “strongest acids as single isolable compounds” was held by the two superacids, fluorosulfonic acid and trifluoromethanesulfonic acid, with pKas of −14 and −16 respectively. The parent molecule from which carborane acid is derived, an icosahedral carboranate anion, HCB11H−11, was first synthesized at DuPont in 1967 by Walter Knoth. Research into this molecules 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. The process consists of treating Cs+– with SO2Cl2, refluxing under dry argon to fully chlorinate the molecule yielding carborane acid, but this has been shown to fully chlorinate only under select conditions. In 2010, Reed published a guide giving detailed procedures for the synthesis of carborane acids and their derivatives. 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(CHB11Cl11) 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: