Carborods [pdf]

          Icosahedral carborane modules can be linked with each other by direct C-C bond formation, through a linking moiety such as alkylene, alkenylene, alkynylene and arylene groups, or a metal center such as Hg. The intermodular linkage can be made at a carbon vertex or at a boron vertex of the carborane cage. The functionalization of the C-H vertices of carborane is easily accomplished via metallation. Electrophilic substitution reactions at the B-H vertices of the aromatic carborane modules are comparable to those observed with aryl groups. If the modular components are linked in a cyclic array the macromolecular construct is identified as a carboracycle, but if they are linked in a rigid linear assembly the resulting oligomer is called a carborod.
          The following graphic represents a survey of representative linear rigid rods based on carboranes which were synthesized by the Hawthorne group in the order in which they appeared in the chemical literature.

          Carborods are thick compared to known rigid-rod molecules such as [n]staffanes, [n]rodanes, and [n]cubanes. Their van der Waals diameter is approximately 7 Å. The high symmetry of the carborane cage results in uniformly filled space around the longitudinal axis of the carborod, thus providing a cylindrical shape. The lengths of carborods are commonly integral multiples of about 4.5 Å (the average length of the carborane, plus the incremental distance for the linker and endcap). In this manner, the span of a carborod can be tailored to match individual applications. The tetramer (17.2 Å long) depicted in Figure 1b is the longest para-carborod to be structurally characterized by X-ray diffraction.
          In the past 10 years, there has been a dramatic increase in both the syntheses and applications of rigid rod molecules which can be used as building blocks or modules to create supramolecular arrays. Carborod-based supramolecular structures have been demonstrated to be useful as building blocks for molecular scaffolding and supramolecular constructs, polymers for high temperature applications and semi-permeable membranes, self-assembled monolayers and Langmuir-Blodgett films, neutron shielding materials, ceramic precursors, and as materials for non-linear optics.

Page 1 2