Icosahedral carborane anions such as CHB11Cl11− are amongst the least coordinating, most chemically inert anions known. They are also amongst the least basic, so their conjugate acids, H(carborane), are superacids (i.e. stronger than 100% H2SO4). Acidity scale measurements indicate that H(CHB11Cl11) is the strongest pure Brønsted acid presently known, surpassing triflic and fluorosulfuric acid. Nevertheless, it is also an extremely gentle acid—because its conjugate base engages in so little chemistry. Carborane acids separate protic acidity from anion nucleophilicity and destructive oxidative capacity in the conjugate base, to a degree not previously achieved. As a result, many long-sought, highly acidic, reactive cations such as protonated benzene (C6H7+), protonated C60
(HC60+), tertiary carbocations (R3C+), vinyl cations (R2CC+–R), silylium ions (R3Si+) and discrete hydronium ions (H3O+, H5O2+etc.) can be readily isolated as carborane salts and characterized at room temperature by X-ray crystallography.
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