Usually viewed as intermediates that become long-lived at low temperatures in superacidic media, simple alkyl carbocations such as the tert-butyl cation can be isolated as stable salts at room temperature when partnered with inert carborane anions. The X-ray crystal structures of tert-butyl (depicted), tert-pentyl, and methyl-cyclopentyl cations have been determined with CHB 11Me5X6- counterions (X= Br or Cl). C blue; H white; B yellow; X green.

The boron subphthalocyanine cation, B(SubPc)+, has been prepared as a salt of a weakly coordinating carborane anion, CHB11Me 5Br6-, by a metathesis reaction of Et 3Si(CHB11Me5Br6) with B(SubPc)Cl. The separation of the cation and anion in the X-ray structure indicates coordinative unsaturation at the boron center, and this is corroborated by DFT calculations. A strongly Lewis acidic nature for the B(SubPc)+ cation is indicated by its hydrolysis to an unusual product, the di-meso-N-protonated -oxo dimer, [H(SubPc)B-O-B(SubPc)H]2+. ©2006 Wiley Periodicals, Inc.

The weakly coordinating carborane anion in ion-like trialkylsilyl species R3Si(CHB11Cl11) can be displaced by nucleophiles as weak as ortho-dichlorobenzene, SO2 and trialkylsilanes, the latter forming nearly linear hydride bridges in R3Si-H-SiR3+ cations.

A new class of potent electrophilic "R(+)" alkylating agents has been developed using weakly nucleophilic carborane anions as leaving groups. These reagents, R(CHB(11)Me(5)X(6)) (R = Me, Et, and i-Pr; X = Cl, Br), are prepared via metathesis reactions with conventional alkylating agents such as alkyl triflates, using the high oxophilicity of silylium ion-like species, Et(3)Si(carborane), as the driving force to obtain increased alkyl electrophilicity. The crystal structure of the isopropyl reagent, i-Pr(CHB(11)Me(5)Br(6)), has been determined, revealing covalence in the alkyl-carborane bonding. This contrasts with the free i-Pr(+) carbocation observed when the anion is less coordinating (e.g. Sb(2)F(11)(-)) or with tertiary alkyl centers, as in [tert-butyl][carborane] salts. In solution, the reagents exist as equilibrating isomers with the alkyl group at the 7-11 or 12 halide positions of the CB(11) icosahedral carborane anion. These alkylating agents are so electrophilic that they (a) react with alkanes at or below room temperature via hydride extraction to produce carbenium ions, (b) alkylate benzene without a Friedel-Crafts catalyst to give arenium ions, and (c) alkylate electron-deficient phosphorus compounds that are otherwise inert to conventional alkylating agents such as methyl triflate.