The para-carborods are only sparingly soluble. Thus one major goal of carborod chemistry is to increase their solubilities by substituting the B-H vertices of the molecular cage units with solubilizing substituents. This can be accomplished by using camouflaged carboranes, like per-B-methylated para-carborane, which will provide excellent solubility in hydrophobic hydrocarbon solvents. Per-B-hydroxylated para-carborane could be utilized to synthesize a hydrophilic rod. Figure 2 shows a universal para-carborod to demonstrate/summarize the research activities that are currently in progress. The investigations include the variation of the terminal substituents (X, Y), the type of linkage (R), the substituents at the B-H vertices of the cage (R'), and the isomeric carborane used (ortho-, meta-, or para-) in modular construction. In this manner, the dimensions, solubility, and function of the resulting carborod may be tuned to fulfill a particular purpose.

           If Y in figure 2a represents a polar group such as -COOH or -SiCl₃ the dimeric rod is suited for the generation of an ordered monolayer on water or silica. Polymeric rods resembling those in figure 2b represent potential candidates for liquid-crystal applications. Molecules of the type shown in figure 2c which are terminally substituted with metal centers in different oxidation states may serve as molecular wires allowing the metal centers to communicate electronically. Furthermore, cross-coupling the three isomeric icosahedral carboranes via suitable linkers will lead to mixed, three-dimensional carborod-like constructs (e.g. "carboboxes"). This endeavor, if successful, could provide a large advance toward a real molecular "tinker toy" construction set based upon aromatic organoborane components.

Selected References:

Axel Herzog, Andreas Maderna, George N. Harakas, Carolyn B. Knobler and M. Frederick Hawthorne, "A Camouflaged Nido-Carborane Anion: Facile Synthesis of Octa-B-methyl-1,2-dicarba-closo-dodecaborane(12) and Its Deboration Reaction," Chem. Eur. J., 5, 1212, (1999).

Toralf Peymann, Axel Herzog, Carolyn B. Knobler, and M. Frederick Hawthorne, "Aromatic Polyhedral Hydroxyborates: Bridging Boron Oxides and Boron Hydrides," Angew. Chem. Int. Ed., 38(8), 1061, (1999).

Laura J. Yeager, Fusayo Saeki, Kenneth Shelly, M. Frederick Hawthorne and Robin L. Garrell, "A New Class of Self Assembled Monolayers: closo-B₁₂H₁₁S^3- on Gold ," J. Am. Chem. Soc., 120, 9961, (1998).

Wei Jiang, David E. Harwell, Mark D. Mortimer, Carolyn B. Knobler and M. Frederick Hawthorne, "Palladium-Catalyzed Coupling of Ethynylated p-Carborane Derivatives: Synthesis and Structural Characterization of Modular Ethynylated p-Carborane Molecules," Inorg. Chem., 35, 4355, (1996).

Wei Jiang, Carolyn B. Knobler, Mark D. Mortimer, and M. Frederick Hawthorne, "A Camouflaged Icosahedral Carborane: Dodecamethyl-1,12-dicarba-closo-dodecarborane(12) and Related Compounds," Angew. Chem., 34, 1332, (1995).

Xiaoguang Yang, Wei Jiang, Carolyn B. Knobler and M. Frederick Hawthorne, "Rigid-Rod Molecules: Carborods. Synthesis of Tetrameric p-Carboranes and the Crystal Structure of Bis(tri-n-Butylsilyl)-Tetra-p-Carboranes," J. Am. Chem. Soc., 114, 9719, (1992).

Kenneth P. Callahan and M. Frederick Hawthorne, "Organometallic Reactions of 1-Ethynyl-1,2-carborane," J. Am. Chem. Soc., 95, 4574, (1973).

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