Large, inert, weakly basic carborane anions of the icosahedral type CHB11R5X6 - (R = H, Me; X = Cl, Br) allow ready isolation and structural characterization of discrete salts of the solvated proton, [H(solvent) x ][CHB11R5X6], (solvent = common O-atom donor). These oxonium ion Brønsted acids are convenient reagents for the tuned delivery of protons to organic solvents with a specified number of donor solvent molecules and with acidities leveled to those of the chosen donor solvent. They have greater thermal stability than the popular [H(OEt2)2][BArF] acids based on fluorinated tetraphenylborate counterions because carborane anions can sustain much higher levels of acidity. When organic O-atom donors such as diethyl ether, tetrahydrofuran, benzophenone, and nitrobenzene are involved, the coordination number of the proton (x) in [H(solvent) x ]+ is two. A mixed species involving the [H(H2O)(diethyl ether)]+ ion has also been isolated. These solid-state structures provide expectations for the predominant molecular structures of solvated protons in solution and take into account that water is an inevitable impurity in organic solvents. The O···O distances are all short, lying within the range from 2.35 to 2.48 Å. They are consistent with strong, linear O···H···O hydrogen bonding. Density functional theory calculations indicate that all H(solvent)2 + cations have low barriers to movement of the proton within an interval along the O···H···O trajectory, i.e., they are examples of so-called SSLB H-bonds (short, strong, low-barrier). Unusually broadened IR bands, diagnostic of SSLB H-bonds, are observed in these H(solvent)2 + cations.

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Silicon and fluorine are an unequal pair, and yet they form the strongest couple of all elements. The unmatched SiÀF bond dissociation energy [1] of 662 kJ mol À1 offers the possibility of abstracting fluoride from almost any fluorine-containing species, given a suitable silyl Lewis acid. In particular, fluoride abstraction from organofluorine compounds by a silylium ion R 3 Si + is a thermodynamically favored process. [2] Silylium ion-based Lewis acids have been shown to effect CÀF activation in aliphatic fluorocarbons, affording isolable carbocationic intermediates [3] or hydrocarbons R 3 C-H in the presence of a reducing agent. [4] The extension of such reactivity to the heterolytic activation of fluoroarenes, however, remains a fundamental challenge. [