Arachno-pentaborane

Figure 7 Diaza-aracb/io-pentaborane and thiaaza-arachno-pentaborane.

Carbaboranes with an arachno-CB4 skeleton, that is derivatives of 1-carba-arachno-pentaborane(lO), were identified as side products (Scheme 3.2-11) 18 in the reaction of dichloroethylborane with elemental Li [29], and 19 was obtained from pro-pyne and tetraborane(lO) [30], The reaction of dimethyl(dipropyn-l-yl)silane with a large excess of tetraethyldiborane(6) [ hydride bath which contains Et2BH2BEt2, other ethyldiboranes(6) and triethylborane] [31, 32] resulted in the formation of 20 in good yield [33]. 

Scheme 3.2-12. Exchange of chlorine for hydrogen with LiBH4 in the diborabenzofulvene 21 followed by hydroboration and coordination with BH3 as well as protonation to give 1-carba-arachno-pentaborane(lO) 22.

Scheme 3.2-13. Hydroboration of diethyl (propyn-l-yl)borane in the hydride bath via 23 and 24, followed by Et2BH-catalyzed condensation and self-assembly, leads to the 1-carba-arachno-pentaborane(lO) 25, having the Et2B substituent at the exopolyhedral carbon atom.

Fig. 3.4-12. Two derivatives of the hypothetical parent diaza-arachno-pentaborane N2B3H7 with an ar-5 and an ar-5 structure in a topological representation and in one with localized bonds.

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. ... 

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. 

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