carborane

carborane
/kahr"beuh rayn'/, n.
any of the crystalline compounds obtained by the substitution of carbon for boron in borane.
[b. CARBON and BORANE]

* * *

Introduction

      any member of a class of organometallic compounds containing carbon (C), boron (B), and hydrogen (H). The general formula of carboranes is represented by C2BnHn + 2, in which n is an integer; carboranes with n ranging from 3 to 10 have been characterized.

      The first carboranes were produced in the 1950s, but the results were not declassified and published until 1962–63. Since then, many thousands of carboranes have been prepared, and they have been combined with transition metals (metal) to yield derivatives called metallacarboranes, some of which show catalytic activity.

Structure and bonding of carboranes
      The carboranes have polyhedral molecular structures based on networks of boron and carbon atoms, in which the carbon atoms occupy adjacent positions. As a result, the structures of carboranes and their derivatives are similar to those of the isoelectronic (possessing the same number of electrons) polyhedral boranes, and, like the boranes, they involve three-centre bonds as well as ordinary two-centre bonds. Their most significant structural feature is the covalent bonding of carbon simultaneously to five or six other atoms.

      In addition, the nomenclature of carboranes employs the same structural prefixes (closo-, arachno-, etc.) as those of borane. The numbering of the atoms in carborane compounds begins with the apex atom of lowest coordination (i.e., with the fewest bonds), and polyhedral vertex atoms are numbered in a clockwise direction with the carbon atoms being given the lowest possible numbers. The best-studied carborane is ortho-carborane, C2B10H12, made by reaction of acetylene with decaborane in the presence of diethyl sulfide. Its molecular structure resembles an icosahedron with the 10 boron atoms and two adjacent carbon atoms forming the apices.

Reactions and synthesis of carboranes
      Carboranes are generally prepared by reaction of acetylene or acetylene derivatives with boron hydrides (hydride).The first three carboranes—trigonal bipyramidal 1,5-C2B3H5; the 1,2- and 1,6- isomers of octahedral C2B4H6; and pentagonal pyramidal 2,4-C2B5H7—that were produced in the 1950s were generated in low yield by the reaction of pentaborane(9) with acetylene in a silent electric discharge. As is the case with boranes, the nido- and arachno-carboranes are less thermally stable and reactive toward air and chemical reagents than the corresponding closo-carboranes, most of which are stable to 400 °C (750 °F), although they may rearrange to more stable isomeric forms.

      The three isomeric icosahedral closo-carboranes of formula C2B10H12 are unusual both in their ease of preparation and their stability in air. Not only has their chemistry been the most extensively studied of all carboranes, but their discovery ushered in the rapid development of the field. Although their systematic International Union of Pure and Applied Chemistry (IUPAC) name is closo-dicarbadodecaborane(12), the three isomers are often simply called ortho-, meta-, and para-carborane.

      The most common carborane, the ortho-isomer, has been available in multikilogram quantities since the early 1970s and is best prepared by the reaction of acetylene, C2H2, with decaborane(14) in the presence of a Lewis base such as diethyl sulfide, (C2H5)2S.

nido-B10H14 + 2(C2H5)2S → B10H12{S(C2H5)2}2 + H2
B10H12{S(C2H5)2}2 + C2H2closo-1,2-C2B10H12 + 2(C2H5)2S + H2
The two other isomers are prepared by thermal isomerization of the 1,2- (ortho-) isomer.

      Numerous derivatives of icosahedral closo-carboranes, especially of 1,2-C2B10H12, have been prepared by replacement of hydrogen atoms by other groups. Despite their stability toward heat and reagents, the American chemist M. Frederick Hawthorne in 1964 showed that they could be degraded to nido-carborane anions (anion) by reaction with strong bases in protonic solvents (those capable of donating H+ ions); for example,

closo-1,2-C2B10H12 + RONa + 2ROH → Na[nido-7,8-C2B9H12] + B(OR)3 + H2
(where R is an alkyl group). These nido-anionic cages, called dicarbollide ions (from Spanish olla, meaning “bowl”) led to the preparation of metallacarboranes with their own extensive chemistry.

      The first hypho-carborane, C3B4H12, was reported in 1993 by Robert Greatrex, Norman N. Greenwood, and their colleagues.

George B. Kauffman

Additional Reading
N.N. Greenwood and A. Earnshaw, Chemistry of the Elements (1984), provides basic coverage of carboranes; and Geoffrey Wilkinson, F. Gordon A. Stone, and Edward W. Abel (eds.), Comprehensive Organometallic Chemistry, 9 vol. (1982), contains several chapters dealing with carboranes and other heteroboranes. Russell N. Grimes, Carboranes (1970), is the first book-length account of these compounds.

* * *


Universalium. 2010.

Игры ⚽ Поможем написать реферат

Look at other dictionaries:

  • carborane — ● carborane nom masculin Nom générique des composés de bore, de carbone et d hydrogène. (Leurs molécules ont une forme de cage.) …   Encyclopédie Universelle

  • Carborane — Carborane,   Singular Carboran das, s, Borverbindungen …   Universal-Lexikon

  • Carborane — Ball and stick model of o carborane …   Wikipedia

  • Carborane — Modèle Ball and stick du o carborane Un carborane est une molécule constituée d atomes de bore et de carbone. Comme beaucoup de boranes, les carboranes sont polyédriques. Ils sont classifiés comme closo , nido , arachno , hyph …   Wikipédia en Français

  • Carborane — 3D Modell der Carboransäure Carborane (englisch: Carboranes oder Carbaboranes) sind Borane, bei denen ein oder mehrere Boratome durch Kohlenstoffatome ersetzt sind, zum Beispiel das luftstabile B10C2H12. Carborane bilden auch Anionen, etwa das… …   Deutsch Wikipedia

  • carborane — karboranas statusas T sritis chemija formulė Formulę žr. priede. priedas( ai) Grafinis formatas atitikmenys: angl. carborane rus. карборан …   Chemijos terminų aiškinamasis žodynas

  • carborane — noun Any of a class of organic compounds of boron, mostly having a polyhedral structure. Syn: carbaborane …   Wiktionary

  • carborane — car·bo·rane …   English syllables

  • carborane — ˈkärbəˌrān noun ( s) Etymology: blend of carbon and borane : any of a class of thermally stable compounds BnC2Hn+2 that are used in the synthesis of polymers and lubricants …   Useful english dictionary

  • metal carborane — metalo karboranas statusas T sritis chemija apibrėžtis Karboranas, kurio molekulės poliedro viršūnėse yra metalo atomų. atitikmenys: angl. metal carborane rus. карболлидный комплекс; металлокарборан ryšiai: sinonimas – karbolidinis kompleksas …   Chemijos terminų aiškinamasis žodynas

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”