Nido-pentaborane

Figure 1 gives an overview of some calculational results which clearly shows that silylboranes with a B=B double bond are less stable than those with a B=Si double bond. The most stable species is a nido-pentaborane. Consequently, the quest for a silyleneborane needs a strategy in which electronic as well as a steric effects must play a decisive factor. 

Polyhedral Boron Hydrides. Although relatively large quantities of polyhedral boron hydrides and carboranes have been produced under various government contracts, these materials are not currendy produced on a regular commercial basis. Pentaborane(9) nido- and decaborane(14) and carboranes, such as the ortho and meta-carboranes closo-1 1 well as various other derivatives, are... 

Boranes are usually named by indicating the number of B atoms with a latin prefix and the number of H atoms by an arabic number in parentheses, e.g. B5H9, pentaborane(9) B5H11, pentaboranefl 1). Names for anions end in ate rather than ane and specify both the number of H and B atoms and the charge, e.g. BsHs" octahydropentaboratefl—). Further information can be provided by the optional inclusion of the italicized descriptors closo-, nido-, arachno-, hypho- and conjuncto-, e.g. ... 

Many derivatives of isomeric 2,3-dicarba-nido-hexaborane(8) are known, available from the reaction of pentaborane(9) with alkynes [59, 60]. In contrast, the carboranes of type 43 with non-adjacent carbon atoms are rare. 

Reduction of the pentaalkyl-1,5-dicarba-closo-pentaboranes(5) 44 with elemental K opens the cluster and subsequent reaction with I2 enforces oxidative fusion of two C2B3 units to give a C4B6 cage (Scheme 3.2-36). The peralkylated hexaboraadamantane derivatives 70 (R = Me, Et X-ray structure analysis for R = Me [89]) rearrange irreversibly into the carboranes 71 with a wido-C4B6 framework [89] (X-ray structural analysis for R = Et [90]). The structure of the nido-C Bf, cluster is fluxional in solution [91]. 

If we remove two vertex boron atoms, the resulting framework is an arachno (Gr., spider s web ) structure. With two vertices missing, the structure is even more open than is the nido case and the resemblance to the parent closo structure is less appurent. Arachno structures obey the electronic formula 2n + 6 (or n + 3 electron pairs). Pentaborane(l I), BjB,. must therefore have an arachno structure. In the arachno series the extra hydrogen atoms form ertdo B—H bonds (lying dose to (he framework) as well as bridges. 

The syntheses of the heteroboranes often involves the addition of heteromolecules to nido-boranes, in so called cage expansion reactions. Two of the most used borane precursors are pentaborane(9) (nido-BsHg) and decab-orane(14) (niT/o-BioHn) both of which are shown in Figure 2. 

Conversion of the I-Carba-nracAno-pentaborane( 10) Derivative 7 into a nido-... 

Figure 3.3. Nido carborane families with five or six skeletal atoms, (a) Species with five skeletal atoms, related to pentaborane(9), B5H9 (b) species with six skeletal atoms, related to hexaborane(lO), BeHio.

Alkyl derivatives of the nido-carborane, 2-CB5H9, are found as side products from the thermal (200°C) reaction between pentaborane (9) and acetylenes (see 5.3.2.7.3). These alkyl derivatives of 2-CB5H9 are formed from the internal 1,1 addition of two B—H bonds of the polyborane to the acetylene. 

The construction of C—B bonds in carboranes is effected by reaction of aikynes with polyboron hydrides. High-T and electric discharge-induced reactions between these reagents yield close cage systems, whereas milder conditions can lead to nido-carboranes. The preferred route to the parent closo-l,6-C2B4Hg and -2,4-C2B5H7 compounds involves the direct thermal reaction (500-600°C) of pentaborane(9) and acetylene in a continuous-flow system. Also formed is the smallest known closo-carborane, 1,5-0263115, in which the combined yields of products approach 70% and the product distribution of 0265117 0264115 0263115 is ca. 5 5 1 ... 

The preferred preparation of the nido-carborane 2,3-C2B4Hg is effected from pentaborane(9) with acetylene at ca. 200°C or in the presence of base catalyst, such as... 

Adams, L., S. N. Hosmane, J. E. Eklund, J. Wang, and N. S. Hosmane. 2002. Novel route to boron-10 enriched pentaborane(9) from boric acid and conversion to nido- °BioHi4 and anti- BigH22 Synthetic advance in polyhedral borane chemistry and in BNCT research. In Proceedings of the First International Boron Symposium, ed. K. Erarslan, pp. 129-133. Kiitahya, Turkey Dumlupinar University Press. 

As part of studies concerned with the stabilisation of pentaborane(9) towards aerial oxidation, the reaction of B5H9 with EtsNH and 3-hexyne or phenylacetylene in THF at room temperature has been reported. The products are mdo-2,3-Et2-2,3-C2B4H6 80% yield) or nido-... 

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