The template reaction of dilithiated ortho-carborane with mercury salts results in the self-assembly of cyclic arrays (mercuracarborands) composed of alternating carborane icosahedra and mercury atoms linked by C-Hg-C moieties. The carbon vertices of the carborane icosahedra are strongly electron-withdrawing and enhance the Lewis acidity of the mercury centers. This electronic feature enables the mercuracarborands to function as cyclic hosts for anionic and other electron-rich guest species as in the chloride ion complex shown in Figure 4.

          Anion template effects have been shown to play a critical role in determining the ring size of the mercuracarborands which are produced during synthesis (See Figure 5).

Consequently, use of a mercury salt with a larger, more weakly donating anion like acetate in the reaction with dilithio-ortho-carborane affords a guest-free cyclic mercuracarborand trimer. However, use of a mercury salt with the smaller, more strongly donating halide counter ions provides kinetically controlled formation of a cyclic mercuracarborand tetramer with one or two centrally coordinated halide ion guests. Thiocyanate ion templates a cyclic pentamer which has been observed by mass spectroscopy. Halide ions in the tetrameric host can be easily removed with sliver tetrafluoroborate thereby providing a guest-free, electrophilic host molecule. Substitution of the 9- and 12- boron vertices of the carborane icosahedron (those furthermost removed from the carbon vertices) with alkyl or aryl groups enhances the solubility of the guest-free mercuracarborands in less polar, weakly donating solvents and provides flexibility in molecular design and function (see Figure 6).

          Attachment of substituents to the 3- (or 6-) boron vertex of the carborane cage creates stereoisomeric mercuracarborand-halide complexes. The dependence of the product stereochemistry upon halide ion identity supports the template ion mechanism of mercuracarborand formation shown in Figure 2. This class of sterically encumbered mercuracarborand-halide ion complexes may be employed as weakly coordinating anions (see Figure 7).

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