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

Abstract

Icosahedral carborane anions such as CHB 11 Cl 11 2 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% H 2 SO 4 ). Acidity scale measurements indicate that H(CHB 11 Cl 11 ) 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 (C 6 H 7 + ), protonated C 60 (HC 60 +), tertiary carbocations (R 3 C + ), vinyl cations (R 2 CLC + -R), silylium ions (R 3 Si + ) and discrete hydronium ions (H 3 O + , H 5 O 2 + etc.) can be readily isolated as carborane salts and characterized at room temperature by X-ray crystallography.

The Low Basicity of Phosphabenzenes: First Examples of Protonation, Alkylation, and Silylation Reactions

“…[4,5] We now show that protonation, alkylation, and trialkylsilylation of phosphabenzenes can be achieved by exploiting the "strong yet gentle" characteristics of electrophilic reagents based on carborane anions. [6] The phosphabenzene of choice for this work is 2,4,6-tri-tertbutyl-1,3,5-triphosphabenzene 4 because of its solubility and synthetic accessibility. [7] We also report on the protonation chemistry of the structurally related 2,4,6-tri-tert-butyl-1-phosphabenzene 5.…”

“…To place the phosphabenzenes on a qualitative basicity scale with common arenes (mesitylene > C 60 > xylene > toluene > benzene), [6] bracketing experiments were carried out with arenium ion salts in CD 2 Cl 2 solution. Treatment of phosphabenzenes 4 and 5 with the mesitylenium salt of the [CHB 11 Me 5 Cl 6 ] À ion gave homogeneous solutions that showed no evidence of protonation in the 31 P NMR spectrum.…”

Unexpected photochemistry and charge-transfer complexes of [CB11H12]−carborane

“…[1][2][3][4] Carborane CB 11 H 12 2 results from the apical isomorphous substitution of a boron in dodecaborane with a carbon atom. 5 As result of this substitution and the extra carbon electron, a negative charge is introduced in the system and the carborane becomes an anionic species.…”

“…[1][2][3][4] The chemistry and properties of carboranes have recently been reviewed. [5][6][7][8][9] Carborane does not exhibit any absorption band in the UV/Vis region and, therefore, no photochemistry is expected upon irradiation in this wavelength region. 5 Also, carborane cannot be oxidized in the potential range commonly available in conventional organic solvents, 5 although oxidation could occur at higher potentials.…”

The Strongest Acid: Protonation of Carbon Dioxide

“…[16] On the other hand, carborane acids are solids and rely on the νNH scale. [25] We draw attention to the poorly recognized phenomenon of "basicity suppression" [26] whereby the Lewis acid in a mixed acid can form an adduct with an added base, thereby lowering the intrinsic basicity of that base, and making it harder to protonate. An alternative way to view this phenomenon is to recognize that in any mixed acid there will always be a competition between H + and the Lewis acid for an added base.…”