Carboracycles were originally prepared for use as precursors to cyclic metallacarboranes and in molecular recognition studies involving localized hydrophobic bonding. A novel application under investigation is the interaction of water-soluble carboracycles with hydrophobic organic guests in aqueous media. Thus water-soluble carboracycles with hydrophobic cavities may support the solubilization of hydrophobic carborods (rotaxane formation), fullerenes, hydrocarbons, and other species through localized hydrophobic bonding (see Figure 2).

In addition, carboracycles may be used as molecular scaffolding for the support of functional groups (i.e., hydroxyl, carboxyl, phosphonium or ammonium) which may demonstrate novel association phenomena in solution through both covalent (metal complexation) and coulombic means. Hydrogen-bonding or coulombic interactions between cycles could also be manifested in the solid sate as either edge-pairing or ring-stacking (see Figure 3).

          The size and geometry of the cycles, the identity of the carborane fragments, and the nature of the linking groups can all be varied in a controlled fashion to give a variety of novel structures. 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. For example, the linker alpha,alpha'-2,6-lutidylene-bridged meta-carborane cyclic dimer was prepared. Subsequent oxidation of the lutidylene produced the bis(N-oxide) carboracycle derivative. Further reactions of this sort are currently under investigation.

          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).

A strategically different synthetic scheme leads to an "inside-out" mercuracarborand in which the mercury atoms are bonded to the boron vertices and the carbon vertices of the meta-carborane are directed away from the internal cavity. 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. Although structurally similar to the carbon-linked mercuracarborand trimer, the electronic properties of the "inside-out" trimer are very different. 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. This solubility in weakly coordinating lipophlic solvents was desirable for the study of electrophilic catalysis by mercuracarborands or the coordination of weakly-bound electron donors with the same acceptors. An equally important challenge is finding a means of solubilizing mercuracarborands and their complexes in aqueous media by introducing hydrophilic substituents. The purpose of moving this chemistry toward aqueous media is several-fold. First, biomolecules such as amino acids, peptides, proteins, nucleic acids, DNA and RNA may be examined as their mercuracarborand complexes using ¹⁹⁹Hg and perhaps ¹¹B NMR studies coupled, where possible, to crystallographic structural determinations. Second, it should be possible to quantitatively evaluate ionic and related equilibria involving the formation of a broad range of mercuracarborand complexes and thereby achieve a quantitative understanding of guest-binding selectivities and quantitative activity-structure relationships; the formation constant, K_a, for the [12]-mercuracarborand-4^.Cl^- complex shown in Figure 1 is estimated to be 10⁷. Third, aqueous organic reactions which are subject to electrophilic catalysis may be assisted with mercuracarborand catalysts. Recently, we reported mercuracarborand-catalyzed Diels-Alder reactions of a thioester with cyclopentadiene in methylene chloride (see Figure 8).

          Anion complexation chemistry has recently received increasing attention due to its chemical and biological importance. Currently, we are investigating the complexation ability of the mercuracarborands towards anions. The cyclic trimer ([9]mercuracarborand-3) has been utilized in sequestering I^-, Br^-, Cl^-, SCN^-, NO₃^-, and ClO₄^-, and as the active component in chloride-sensitive membrane electrodes for clinical use. Analogously, cyclic tetramer ([12]mercuracarborand-4) has been employed in sequestering I^-, Br^-, Cl^-, CN^-, NO₃^-, O₂^2-, CH₃COO^-, C₆H₅S^-, OH^-, and closo-B₁₀H₁₀^2-. The first example of octahedral coordination of a halide ion in an electrophilic sandwich was demonstrated using iodide ion and two [9]mercuracarborand-3 ligands (see Figure 9).

Selected References:

Hans Lee, Martin Diaz, Carolyn Knobler and M. Frederick Hawthorne, "Octahedral Coordination of Iodide in an Electrophilic Sandwich," Angew. Chem., 39, 776 (2000).

Hans Lee, Martin Diaz and M. Frederick Hawthorne, "Mercuracarborand-Catalyzed Diels-Alder Reaction of a Thionoester with Cyclopentadiene," Tetahedron Lett., 40, 7651 (1999).

Ibrahim H. A. Badr, Martin Diaz, M. Frederick Hawthorne and Leonidas G. Bachas, "Mercuracarborand "Anti-Crown Ether" Based Chloride Sensitive Liquid/Polymeric Membrane Electrodes," Anal. Chem., 71, 1371 (1999).

M. Frederick Hawthorne and Zhiping Zheng, "Recognition of Electron-Donating Guests by Carborane-Supported Multidentate Macrocyclic Lewis Acid Host," Accounts of Chemical Research, 30, 267 (1997).

Wei Jiang, Igor T. Chizhevsky, Mark D. Mortimer, Weilin Chen, Carolyn B. Knobler, S. E. Johnson, F. A. Gomez, and M. Frederick Hawthorne, "Carboracycles: Macrocyclic Compounds Composed of Carborane Iscosahedra Linked by Organic Bridging Groups," Inorg. Chem., 35, 5417 (1996)

View All