boranes and carboranes

Boranes, or boron hydrides, are any of a homologous series of inorganic compounds of boron (B) and hydrogen (H) or their derivatives. During the period 1912 to roughly 1937, German chemist Alfred Stock first systematically synthesized and characterized by them, calling them boranes in analogy to the alkanes (saturated hydrocarbons), the hydrides of carbon (C), boron’s neighbor in the periodic system. Because the lighter boranes were sensitive to air and moisture, volatile, and toxic, Stock developed high-vacuum methods and apparatus for studying them. In 1931, Hermann I. Schlesinger and Anton B. Burg initiated American work on boranes, which remained primarily of academic interest. This changed in the Second World War, when the U.S. government supported research to find volatile uranium compounds (borohydrides) for isotope separation, and in the 1950s,when it supported programs to develop high-energy fuels for rockets and jet aircraft. (The heats of combustion of boranes and their derivatives are much higher than hydrocarbon fuels.) The boranes that were prepared by Stock had the general composition BnHn + 4 and BnHn + 6, but more complex species, both neutral and negative (anionic), are now known. Boron’s hydrides are more numerous than are those of any other element except carbon. The simplest isolable borane, B2H6, diborane(6), is one of the most extensively studied and most synthetically useful chemical intermediates. For years, many boranes and their derivatives were prepared either directly or indirectly from this commercially available compound. Although free BH3 (and B3H7) are very unstable, they can be isolated as stable adducts (addition products) with Lewis bases (electron-donor molecules)—e.g., BH3 9 N(CH3)3. Boranes may be solids, liquids, or gases. Their melting and boiling points generally increase with increasing complexity and molecular weight. Heteroboranes are compounds in which one or more nonmetal atoms—e.g., carbon (C), nitrogen (N), silicon (Si), phosphorus (P), arsenic (Ar), selenium (Se), or antimony (Sb)—or one or more metal atoms form part of a borane polyhedron. The most common heteroboranes are those in which the hetero atom is carbon. These compounds, (commonly called carboranes in American usage but more correctly termed carbaboranes according to IUPAC standards) or sulfur (S, called thiaboranes), belong to a class of organometallic compounds having the general formula C2BnHn ¿2, in which C, B, and H represent, respectively, carbon, boron, and hydrogen atoms and n an integer. Carboranes with n values ranging from 3 to 10 have been characterized. The carboranes have polyhedral molecular structures based on networks of boron and carbon atoms, the carbon atoms occupying adjacent positions. The first (closo-) carboranes—trigonal bipyramidal 1,5-C2B3H5, the 1,2- and 1,6- isomers of octahedral C2B4H6, and pentagonal pyramidal 2,4-C2B5H7—were prepared during the late 1950s, but not until 1962-62 were the results declassified and published. (Isomers are compounds with the same molecular formula but different atomic arrangements.) Several thousand carboranes have since been prepared, and they have been combined with transition metals to yield derivatives called metallacarboranes, some of which show catalytic activity. The molecular structure of the best-studied carborane, o-carborane, C2B10H12, resembles an icosahedron with the 10 boron atoms and 2 adjacent carbon atoms forming the apices. The study of boranes and carboranes has become one of the most rapidly expanding areas of inorganic research. The 1976 Nobel Prize for Chemistry was awarded to William Nunn Lipscomb, Jr., "for his studies on the structure of boranes illuminating the problems of chemical bonding." The 1979 prize went to Herbert Charles Brown, one of Schlesinger's students, for his hydroboration reaction. First achieved in 1956, this represented the remarkably easy addition of BH3 (in the form of BH3 9 S) to unsaturated organic compounds (i.e., alkenes and alkynes) in ether solvents (S) at room temperature to yield organoboranes quantitatively). The hydroboration reaction, in turn, led to new research frontiers in the area of stereospecific organic synthesis.