Borane closo

The structures of boranes can be grouped into several classifications. If the structure contains a complete polyhedron of boron atoms, it is referred to as a closo borane (closo comes from a Greek word meaning closed ). If the structure has one boron atom missing from a comer of the polyhedron, the structure is referred to as a nido borane (nido comes from a Latin word for nest ). In this type of structure, a polyhedron having n comers has (n - 1) comers that are occupied by boron atoms. A borane in which two comers are unoccupied is referred to as an arachno structure (arachno comes from a Greek word for web ). Other types of boranes have structures that are classified in different ways, but they are less numerous and will not be described. 

There are many neutral and ionic species composed of boron and hydrogen, far too many to describe in this text. For the purposes of illustrating parallels between these species and transition metal clusters, we will first consider of one category of boranes, closo (cagelike) boranes that have the formula B H . These boranes consist of closed polyhedra with n corners and all triangular faces (triangulated polyhedra). Each corner is occupied by a BH group. 

Atoms in Cluster Vertices in Parent Polyhedron Framework Electron Pairs Valence Electrons (Boranes) Closo Nido Arachno Examples Valence Electrons (Transitlon-Metal Clusters) Closo Nido Arachno Examples ... 

Fig. 2. Idealized deltahedra and deltahedral fragments for closo, nido and arachno boranes and heteroboranes. From left to right the vertical columns give generic closo, nido, and arachno frameworks bridge hydrogens and BH2 groups are not shown, but when appropriate they are placed around the open...

Nido Clusters 2n + 4 Systems). Many closo boranes and heteroboranes add two electrons and undergo a concomitant stmctural transformation from a deltahedron to a deltahedral fragment. For instance, closo-2 ()-(Z, [17764-89-OJ, (2n + 2 = 24e ), is readily reduced to... 

Arachno Clusters 2n + 6 Systems). In comparison to the number of known closo and nido boranes and heteroboranes, there are relatively fewer arachno species. Partly because of the lack of a large number of stmctures on which to base empirical rules, arachno stmctures appear to be less predictable than their closo and nido counterparts. For example, there are two isomeric forms of one with the arachno [19465-30-6] framework shown... 

Closo Borane Anions. This group contains a homologous series of very stable polyhedral anions, [doso —, n = 6-12. Just as the... 

In aqueous solution, closo borane anions are very stable as their conjugate acids, which possess acidity similar to sulfuric acid, yet their chemistry is remarkably different. Large unipositive cations, such as Tl", Cs", Rb", [(CH2)4N]", and [(CH2)3S]", yield water-iasoluble salts of ... 

Pyrolysis of CsjB Hg] at 230°C gives CS2IB2H2] (60%) along with some CS2IB2QH2Q], CS2IB22H22], and CsBH (93). The sensitivity of polyhedral expansion reactions to solvent, temperature, and pressure is further exemplified by the results ia dioxane at 120°C under pressure. To obtain the closo borane, NajB H J is first converted to Cs2[B2 H23], which can be pyrolyzed to give Cs2[B2 H2J (89). 

Nonicosahedral carboranes can be prepared from the icosahedral species by similar degradation procedures or by reactions between boranes such as B H q and B H with acetylenes. The degradative reactions for intermediate C2B H 2 species (n = 6-9) have been described in detail (119). The small closo-Qr Yi 2 species (n = 3-5 are obtained by the direct thermal reaction (500—600°C) of B H using acetylene in a continuous-flow system. The combined yields approach 70% and the product distribution is around 5 5 1 of 2,4-C2B3H2 [20693-69-0] to l,6-C2B Hg [20693-67-8] to 1,5-C2B3H3 [20693-66-7] (120). A similar reaction (eq. 60) employing base catalysts, such as 2 6-dimethylpyridine at ambient temperature gives nido-2 >-(Z, ... 

As with the simple boranes, the closo carboranes are generally more thermally stable than the corresponding nido and arachno species. Thermal decomposition of nido and arachno carboranes often leads to one or more closo carborane. For example, pyrolysis of 2,3-C2B4Hg is another route to 2,3-C2B3H2 [30347-95-6], l,2-C2B4Hg [20693-68-9] and l,6-C2B4Hg [20693-67-8], and 1,5-C2B3H3 [20693-66-7] (123). 

In most cases the prolonged treatment of a closo carborane or borane with strong base results in the removal of a single-boron vertex to yield a nido cluster, inert to further degradation. This principle is exploited in the polyhedral contraction and subrogation synthetic strategies. In the prototypical case. 

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. ... 

The structural interrelationship of all the various closo-, nido- and arachno-boranes thus becomes evident a further example is shown at the foot of the page. 

For example, pyrolysis of the previously mentioned nido-2,3-C2B4Hg gives the 3 c/o o-species shown above, whereas under the milder conditions of photolytic closure the less-stable isomer closo-, 2-C2 i is obtained. Pyrolysis of alkyl boranes at 500-600° is a related route which is particularly useful to monocarbaboranes though the yields are often low, e.g. ... 

Typical borane clusters incorporating As or Sb atoms are c/oso-l,2-BioHioCHAs and closo-... 

The discovery of polyhedral boranes and polyhedral heteroboranes, which contain at least one atom other than in the cage, initiated a new era in boron chemistry.1-4 Most commonly, of the three commercially available isomeric dicarba-closo-dodecaborane carboranes(l,2-, 1,7-, and 1,12-), the 1,2-isomer 1 has been used for functionalization and connection to organic molecules. The highly delocalized three-dimensional cage bonding that characterizes these carboranes provides extensive thermal and kinetic stabilization as well as photochemical stability in the ultraviolet and visible regions. The unusual icosahedral geometry of these species provides precise directional control of all exopolyhedral bonds. 

Consider the closo-BnHn2 (6 < n < 12) boranes (Figure 1-2). Such deltahedral boranes cannot have any terminal BH2 groups or three-center two-electron B-H-B bonds but acquire two extra electrons from the —2 charge on the ion. Therefore s = x = 0 in the equations of balance (la) and (lb) these reduce to (2a) and (2b) in which n is the number of boron atoms in the deltahedron corresponding to p in (la) and (lb) ... 

Building on and extending earlier studies, Schleyer, Najafian, et al. [57] employed computed geometric, energetic, and magnetic properties to quantify the aromaticity of the closo boranes B H 2 (6 < n < 12), and their isoelectronic counterparts, the CB iH and C2B 2H carboranes, and the NB H azaboranes [34]. All possible heteroatom placements were considered. The most stable iso-... 

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