Carborane acid
Carborane acids H(CHB11X11) are a class of superacids at least one million times stronger than 100% sulfuric acid in terms of their Hammett acidity function (H0) values, which measure the ability of a medium or solvent to donate protons. The most well known example is chlorine based (X = Cl) [Figure 1]. The reason for their high acidities is that their conjugate bases, anions of the form (CHB
11X−
11), are highly delocalized and stable species substituted with electronegative groups (the (pseudo)halogen X). They are the only acids known to protonate C60fullerene without decomposing it. Additionally, they form stable, isolable salts with protonated benzene, C
6H+
7.
In coordination chemistry carboranes can be used as unique bulky ligand scaffolds. It was recently demonstrated that the same carboranyl moiety can act either as strongly electron-withdrawing or electron-donating substituent, depending on the positional attachment of the cluster to the heteroatom.
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, [H(CO2)2]+.
As defined in chemistry, a Brønsted-Lowry acid’s strength corresponds with its ability to release a hydrogen ion [Figure 2]. This superacid is known to be one million times stronger than sulfuric acid whose H0 value is −12. However, the crystalline 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 have pKa values well below −20 in 1,2-dichloroethane, with the (yet unknown) fully fluorinated analog H(CB11F12) having a calculated pKa of −46.
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