Basicity Carboranes

Carboranes (or more correctly carbaboranes) are compounds having as the basic structural unit a number of C and B atoms arranged on... 

The reactions of the Group VIII di- and triborolyl or carboranes sandwiches are directed basically to their extension to multidecker derivatives. 

The selective basic degradation of 1,2-dicarbaclovodode-carborane(12) and its C-methyl and C-phenyl derivatives has been recently reported.3 The ethanolic potassium hydroxide degradation of C, C"-dimethyl-l, 2-dicarbaclovododecaborane (12) and the isolation of the trimethylammonium salt of the resulting B9C2Hio(CH3)2 anion are described here as an illustration of this general reaction. The properties of the trimethylam-... 

The reaction is carried out in a 500-ml. three-necked flask equipped with a reflux condenser, mechanical stirrer, heating mantle, and nitrogen inlet. The equipment is similar to that pictured in Fig. 11, except that an addition funnel is not required. In the reaction flask 20 g. (0.36 mol, 100% excess) of potassium hydroxide is dissolved in 300 ml. of absolute ethanol. The spare neck is closed with a ground-glass stopper, and the solution is stirred until it reaches room temperature. Addition of the carborane to the warm basic solution may result in an initial vigorous reaction. To this solution is added 30.0 g. (0.175 mol) of solid dimethylcarborane. The solution is stirred for one hour at room temperature and is then heated at the reflux temperature for 14 hours or until hydrogen evolution has stopped. 

By no means do all metallocarboranes have the metal atoms occupying vertices of the basic polyhedra. Apart from many derivatives in which o-bonded metal residues occupy exo sites attached to particular skeletal atoms, several metalloboranes and -carboranes are known in which the metal occupies an edge-bridging site, effectively replacing a bridging hydrogen atom of the parent borane. Many are nido species related to BeHio, for example, the /x-silyl and /i-germyl carboranes. 

Simple tetrahedral or octahedral models are useful in connection with basic structural questions (as, for example, the first time you try to convince yourself that the two enantiomers of CHFCIBr or of [Co(en)3]+3 are really nonsuperimposable). If stick models are rot available, such simple models can be constructed in a few minutes from paper. In addition, models having bond angles not normally found in ball-and-stick kits—for example, the icosahedral boranes and carboranes—can also be reudily constructed from paper. Paper models are especially useful when large numbers of models are necessary as, for example, in constructing models of the iso-end heteropolyanions. 

The borane polymers discussed were never fully developed because of several basic drawbacks. They were resinous and acted like strong reducing agents consequently, they could not be safely formulated with oxidants. A major development took place when the dicarbaclovododeca-borane (carborane) compounds were discovered because these materials were extremely stable and organophilic in character. (The formula —C-----C—is used for carborane to symbolize the delocalized bonding... 

This is basically the same as the one in procedure A but using the closo species instead of the nido one.l8, It is far simpler than the former one since the nido species does not need to be synthesized. For [(rf-NC4H4)Co(C2B,Hu)] the procedure is as simple as the reaction of o-carborane with KCNCJHJ and CoCl2 in the ratio 1 12 5 in dme yields after working up [(rf-NC4H4)Co(C2B,Hll)] in 76% yield. The reaction is shown in Figure 3. 

BORON-DERIVATIZED CARBORANES WEAKLY BASIC ANIONS [CBuHeXs]-... 

Boranes and carboranes have structures in which their skeletal B- or C-atoms form triangular-faced polyhedra. There are basically three structural types, namely the closo- (an euphonious modification of the Greek clovo = cage, i. e., a complete or closed polyhedron), the nido (from Latin nest-like ) and the arachno- (from Greek cob-web ) structure. Each of these three types is adopted by cluster compounds of specific atomic ratios. c/o o-Structures occur in borane dianions B H , in car-borane anions (CB iH ) , and carboranes (C2B 2H ). Each skeletal atom has a single H-atom terminally attached by a bond directed outwards, away from the polyhedron center (see the example of BioHio in Fig. 3-1 below). Wo-Structures are adopted by boranes B H +4 and their related carboranes CB iH +3, C2H 2H +2 etc., and amc/z/2o-structures by boranes B H +6 and related carboranes CB iH +5, C2B 2H +4 etc. In other words, carboranes have the general formula [(CH) (BH) Hc] , where the sum a + c + x) is equal to 2 for a closo-structure, 4 for a /do-structure, and 6 for an amc/z o-structure. 

It is interesting to note that one of the C-H vertices of each carborane fragment interacts with the strontium. Since the B—H vertices of the carborane fragment are certainly more basic than the C-H vertices, one would anticipate that a second M-H-B interaction would be preferred over the observed M-H-C interaction. Consequently, the M-H-C interaction is not yet understood. 

Figure 3.15. Some siloxane chemistry (a) Silicates react with organic silane derivatives to form siloxanes. (b) Siloxanes may also be formed by partial hydrolysis of substituted silanes followed by heat treatment with a basic catalyst, (c) A highly refractory polymer consisting of a carborane-substituted siloxane.

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