Acidity
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, ''H''0. Based on its ability to quantitatively protonate benzene, the chlorinated carborane acid was conservatively estimated to have an ''H''0 value at or below −18, leading to the common assertion that carborane acids are at least a million times stronger than 100% sulfuric acid (''H''0 = −12). However, since the ''H''0 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 p''K''a, 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 p''K''a values below −20, even without electron-withdrawing substituents on the boron atoms (e.g., is estimated to have a p''K''a of −24), with the (yet unknown) fully fluorinated analog having a calculated p''K''a of −46. The known acid with one fewer fluorine is expected to be only slightly weaker (p''K''a < −40). In the gas phase, has a computedHistory
Carborane acid was first discovered and synthesized by Professor Christopher Reed and his colleagues in 2004 at the University of California, Riverside. The parent molecule from which carborane acid is derived, an icosahedral carboranate anion, , 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. The process consists of treating Cs+ 11H11">CB11H11sup>− with , 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 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: :: 3Si–H–SiEt3">t3Si–H–SiEt3sup>+ 11Cl11">CB11Cl11sup>− + 2HCl → + 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). Although the reactions used in the synthesis are analogous, obtaining a pure sample of the more acidic turned out to be even more difficult, requiring extremely rigorous procedures to exclude traces of weakly basic impurities.Structure
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. : 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. 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. The chlorinated carba-''closo''-dodecaborate anion is an outstandingly stable anion with what has previously been described as "substitutionally inert" B–Cl vertices. The descriptor ''closo'' indicates that the molecule is formally derived (by B-to-C+ replacement) from a borane of stoichiometry and charge ''n''H''n''">''n''H''n''sup>2− (''n'' = 12 for known carborane acids). 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. 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.Usage
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. In inorganic synthesis, their unparalleled acidity may allow for the isolation of exotic species like salts of protonated xenon.References
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