Carborane acids. New “strong yet gentle” acids for organic and inorganic chemistry

Abstract

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.