Nido

The proved reserves and levels of production for Japan, Myanmar (formerly Burma), Pakistan, Taiwan, and Thailand are insignificant by world standards. In 1979, the Philippines estabUshed the first commercial production in the small offshore South Nido field. This success came after more than 75 years of wildcat drilling in the Philippines. After several additional discoveries, production rose to 0.3 x 10 m (1.7 x 10 bbl) in 1991. 

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... 

Proton Abstraction. Although the exopolyhedral hydrogens of nido and arachno boranes are generally considered hydridic, the bridge hydrogens are acidic as first demonstrated by titration of and deuterium exchange (71). Some typical reactions are... 

The discovery (116) of the base-promoted degradation of the isomeric closo-Q,. . cages provided one of the most important carborane anion systems, the isomeric nido-Q,. ][ , anions,... 

Protonation of the nido-lanion with strong acids leads to the neutral highly acidic C2B2H23 molecule. 

Aside from their extensive use in metaHacarborane chemistry, the dicarboUide anions are important intermediates in the synthesis of other carborane compounds. For example, aqueous ferric chloride oxidation of the 1 anion results in the 10-vertex cage nido- b ()-(Z, 2 (H8) and the... 

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). 

A readily accessible carborane is nido-1 -(]5 i])- l- 1( 12 [I d-5], a 2witterionic species formally derived from [CB2qH23] by replacement of a by NH3. It has been shown (124) that this monocarbaborane can be obtained in excellent yield by treatment of B2QH24 with CN followed by passage through an acidic ion-exchange column (eq. 61). 

Other large monocarbaboranes include /<7( -6-(NR3)-6-CB2H [f/oj o-l-CB H J [38192-43-7] and closo-C ]H. ][ [39102-46-0]. The closo monocarbaboranes can be functionalized at carbon via lithiation using reagents such as -butyl lithium in a manner similar to the dicarbaboranes. The small monocarbaboranes /oj o-l-CB H [25301-90-0], nido-2-C [12385-35-2], and a variety of their alkylated derivatives are also known (127,128). 

As the C B series of tetracarbaboranes is classified in the electron-counting formaUsm as nido, these molecules are expected to have open stmctures even though extra hydrogens are absent. Spectroscopic studies (130) have confirmed this expectation for 2,3,4,5-C4B2H3 [28323-17-3]. One isomer of (CH3)4C4BgHg has the open nonicosahedral stmcture shown in Figure 11 and another isomer, the 1,2,3,8-tetramethyl compound [54387-54-1], is apparently even more open (131). Other tetracarbaboranes include isomers of nido-Q]. and (132). 

Other Heteroboranes. Other weU-documented families of heteroboranes include the azaboranes, phosphaboranes, arsenaboranes, stibaboranes, selenaboranes, and teUuraboranes. Table 5 Hsts representative examples of heteroboranes from Groups 15 (V) and 16 (VI). The thiaboranes are the most extensively developed class of heteroboranes after the carboranes. The thiaboranes arachno-()-S fri f , [45979-10-0] and nido-()S fri ... 

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