Carboranes, characteristics

At AWE, the Lewis acid-catalyzed bulk polymerization route has been the main synthesis route to poly(m-carborane-siloxane) elastomers. Our selection has been based on considerations of safety, availability of key reagents, and ease of scale-up operations. An understanding of the physical and chemical properties of these materials, and how these properties can be modified through the synthesis process, is essential in order to develop materials of controlled characteristics. 

The presence of four kinds of nuclear magnetic resonance (NMR) observable nuclei ( H, uB, 13C, and 29Si) allows poly(m-carborane-siloxane) to be readily investigated using NMR spectroscopy. In addition, H spin-echo NMR relaxation techniques can provide an insight into polymer segmental chain dynamics and therefore useful information on material viscoelastic characteristics. 

Multicenter bonding is the key to understanding carboranes. Classical multicenter n bonding gives rise to electron-precise structures characteristic of Hiickel aromatics, which are planar and have 4n + 2 n electrons. Clusters are defined here as ensembles of atoms connected by non-classical multicenter bonding , i.e., all... 

In 1971, a note 164) was published favoring the hypothesis that the carboranes, boranes, their isoelectronic anions, Lewis base adducts, and heteroatom-substituted analogs should be viewed as constructed about the vertices of either the most spherical series of triangular-faceted polyhedra (deltahedra) found to be characteristic of the dicarba-cZoao-carboranes (Fig. 1) or, with one lone exception, fragments of the series of deltahedra produced by the successive removal of the highest coordinated vertices that sequentially define the nido and arachno classes. This position was in conflict with the then prevalent shibboleth that all nido and arachno compounds [except B5H9 (I-N5)] had or would prove to have icosahedral fragment structures. 

CARBORANE. A cry stalline compound composed of boron, carbon, and hydrogen. It can be synthesized in various ways, chiefly by the reaction of a borane (penta-or deca-) with acetylene, either at high temperature in the gas phase or in the presence of a Lewis base. Alkylated derivatives have been prepared. Carborancs have different structural and chemical characteristics and should not be conTused with hydrocarbon derivatives or boron hydrides. The predominant structures arc the cage type, the nest type, and the web type, these terms being descriptive of the arrangement of atoms in the crystals. Active research on cargorane chemistry has been conducted under sponsorship of the U.S. Office of Naval Research, http //www.onr.navy.mil/... 

Macropolyhedral azaboranes, preparation, 3, 121 Macropolyhedral carboranes, synthesis and characteristics, 3, 101... 

Polyhedral carboranes (continued) seven-vertex carboranes, 3, 58 six-vertex carboranes, 3, 56 small carborane syntheses, 3, 54 subicosahedral carborane geometrical patterns, 3, 50 subicosahedral and icosahedral, 3, 51 supraicosahedral, characteristics, 3, 96 ten-vertex carboranes, 3, 60 thirteen-vertex carboranes, 3, 100 tricarbaborane synthesis, 3, 54 twelve-vertex carboranes, 3, 98 Polyhedral skeletal electron pair theory, Ru and Os tetranuclear clusters, 6, 874-875 Poly(iV-heterocyclic carbene) ligands... 

Boranes and carboranes are electron-deficient compounds with interesting molecular geometries and characteristic bonding features. 

Fig. 5-18i,13 has been followed in recent years by studies of early transition metal-carboranes as catalysts for olefin polymerization.14 In these applications, the metalla-carborane species resemble their metallocene-based analogues however, there are other areas, such as the construction of new types of electronic, magnetic, and optical materials, in which the unique structural and other characteristics of metalla-carboranes may be put to use. Venus flytrap linked-dicarbollide ligands such as that shown encapsulating a Co3+ ion in Fig. 5-18f have been prepared as a possible way to bind radiotransition metals to tumor-associated monoclonal antibodies for purposes of diagnosis and therapy.15... 

A potentially valuable characteristic of poly(ether-ketone-carborane)s is that they display enormously enhanced char-yields (up to 95% on pyrolysis in air), compared to the yields obtained from analogous all-aromatic polymers.7 This behaviour suggests that carborane-based polyketones such as 7 could eventually find application as fire-retardants and as precursor polymers for carbon-ceramic materials. 

---