It is possible to introduce reactive centers into the cycles, either as linker-group modifiers or by functionalization of the carborane icosahedra, thus opening the way to large multivalent species capable of interaction with either transition metal centers or other electrophilic guests. Further reactions of this sort are currently under investigation. 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. 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. The dependence of the product stereochemistry upon halide ion identity supports the template ion mechanism of mercuracarborand formation shown in Figure 2. This use of the boron vertices of the carboranes for the skeletal linkage within the mercuracarborand array allows the peripheral carbon vertices to be derivatized. The boron vertices attached to the mercury atoms are electron-rich and the mercuracarborand does not coordinate guest anions. As mentioned above, the enhancement of hydrocarbon solubility of mercuracarborands was attained by the attachment of hydrophobic organic substituents to the boron vertices of the component carborane cages.