Carboranes, icosahedral

The three isomeric icosahedral carboranes (76-78) are unique both in their ease of preparation and their great stability in air, and consequently their chemistry has been the most fully studied. The 1,2-isomer in particular is available on the multikilogram scale. It is best prepared in bulk by the direct reaction of ethyne with decaborane in the presence of a Lewis base, preferably Et2S ... 

An extensive derivative chemistry of the icosahedral carboranes has been developed, especially for l,2-C2BioHi2. Terminal H atoms attached to B undergo facile electrophilic substitution and the sequence of reactivity follows the sequence of negative charge density on the BHt group ... 

The structure of the bimetallic 10-vertex cluster was shown by X-ray diffraction to be (84). When the icosahedral carborane l,2-C2BioHi2 was used, the reaction led to the first supraicosahedral metallocarboranes with 13- and 14-vertex polyhedral structures (85)-(89). Facile isomerism of the 13-vertex monometallodicarbaboranes was observed as indicated in the scheme above (in which = CH and O = BH). 

Hawthorne and co-workers have also produced a series of macrocyclic Lewis acid hosts called mercuracarborands (156, 157, and 158) (Fig. 84) with structures incorporating electron-withdrawing icosahedral carboranes and electrophilic mercury centers. They were synthesized by a kinetic halide ion template effect that afforded tetrameric cycles or cyclic trimers in the presence or absence of halide ion templates, respectively.163 These complexes, which can bind a variety of electron-rich guests, are ideal for catalytic and ion-sensing applications, as well as for the assembly of supramolecular architectures. 

Figure 84 Macrocyclic mercuracarborands (156, 157, and 158) with structures incorporating electron-withdrawing icosahedral carboranes and electrophilic mercury centers. (Adapted from ref. 163.)... 

Since icosahedral carboranes are generally viewed as 3D energy sinks as well as polymer building blocks, the thermal and radiation stability that these units impart on the base rubbery material needs to be explored. While the thermal stability of these materials has been studied and reported by a number of workers, there is little reported work on the stability of these materials to ionizing radiation. A brief account of our work within this area is detailed below. 

ICOSAHEDRAL CARBORANES AND INTERMEDIATES LEADING TO THE PREPARATION OF CARBAMETALLIC BORON HYDRIDE DERIVATIVES... 

The series of syntheses reported here presents (1) detailed procedures based on previously published work for the preparation of Bi0C2Hi2 and some of its common C-substituted derivatives, (2) a procedure for conversion of a C-substituted derivative of the icosahedral carborane to the corresponding derivative of B9C2Hi2-, and (3) methods suitable for the preparation of (ir-B9C2Hn)2Fe and (ir-B9C2Hii)2Co. ... 

A new deboronating agent for icosahedral carboranes, dry tetrabutylammonium fluoride (TBAF), was found to degrade some small carboranes, ortho- and t( / -carborane(COMC (1995 ) 6.2.4.6).55 As it was clear that water plays a part in the process, this reagent in its wet form was used to deboronate a large number of z< /carborane derivatives.257-259 264 Wet TBAF proved to be superior to alcoholic potassium hydroxide for effective deboronation of meta-... 

Although legions of icosahedral carboranes 49 are known, the formation of supra-icosahedral boron-containing clusters was restricted to metal carboranes (MxC2B10 or MxC4Bg, x = 1,2) [62, 63]. 

Recently, Welch et al. [64] have shown that it is possible to expand icosahedral carborane to yield the new C2B11 framework. This was achieved by reacting the dianion [7,8-/<- C<,H4(CH2)2 -7,8-uido-C2B 0H o 2 50 [65] with PI1BCI2 to form the 13-vertex carborane 51 1,2-/<-[Q H4(CH2)2 -3-Ph-1,2-C2Bn H10 (Scheme 3.2-25). The shape of the cluster is that of a henicosahedron. 

Applied to icosahedral carboranes as the starting materials, it leads to metallocarborane based on 13- or 14-vertex polyhedra, e.g. (35, 70),... 

Following Custelcean and co-workers, this great advantage of dihydrogen bonds opens new avenues to the rational assembly of extended covalent materials with controlled architecture [4]. As examples, we describe in this chapter some derivatives of icosahedral carboranes widely applied as building blocks in supramolecular systems, as well as NaNH4 poly(2-hydroxyethyl)cyclen building blocks. 

Carborane Superacids H(CB11HR X.6). Recently, new carbon superacids, icosahedral carboranes H(CBnHR5X6) (where X = chlorine, bromine or iodine R = H, Me, Cl), have been described by Reed et al.34, whose conjugate base, the carborane anion (CBnHR5X6), is quite inert due to low nucleophilicity. 

Icosahedral carbaboranes, deboronating agents for, 3, 67 Icosahedral carboranes w -carboranes, 3, 74 0 0-carboranes, 3, 71 /> ra-carboranes, 3, 74 coordinated anions and derivatives, 3, 68 derivatization, 3, 67... 

Poly(bis-alkoxysilylene)s, preferential screw sense, 3, 617 Polyboranes, in subicosahedral and icosahedral carborane synthesis... 

Fig. 1. Structures and numbering of the three isomeric icosahedral carboranes, and the degradation of 1,2-C.2BioHi2 to 7,8-C2B9Hir. The bridging hydrogen is shown in one of the two equivalent bridging positions.

The phosphine 1-(1,2-C2BxoHii)P(CHs)2 was prepared and reacted with an iridium (I) complex, as shown in Fig. 27. Spectroscopic evidence strongly supported the view that the complexed Ir(I) had inserted into a B—H bond with the formation of an iridium-boron bond and an iridium-hydride link (58). Specific deuterium labels attached to the carborane moiety of the phosphine clearly proved this point. Thus, the B-H vertices of the icosahedral carborane group that are nearest the carbon atom bearing phosphorus were shown to be involved, although a distinction could not be made between the 3,6 or 4,5 sets of BH groups. 

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