O-carborane clusters

FIGURE 22.6 Deboronation of a cZo o-carborane cluster to the nido cluster compound. 

Many novel cluster compounds have now been prepared in this way, including mixed metal clusters. Further routes involve the oxidative fusion of dicarbon metallacarborane anions to give dimetal tetracarbon clusters such as (103) and (104) O (jjg insertion of isonitriles into inetallaborane clusters to give monocarbon meiallacarboranes such as (105) and the reaction of small ii/t/o-carboranes with alane adducts such as Et3NAlH3 to give the commo species (106) ... 

It is known that a carborane framework involves three-center two-electron bonding and is thus an elelctron-deficient cluster. Shatenshtein76 reported that the pKa value of the C—H proton of carborane is 23. Therefore, a proton attached to the carbon of o-carborane could be deprotonated easily by a weak base, and the resulting... 

Precise placement of metal complexing sites within the infrastructure of a cascade molecule is of importance from a variety of perspectives. In the construction of the above noted Micellane family (cf. Sect. 3.1), we reported the construction of dendrimers with four alkyne moieties at sites equidistant from each other in the interior (17, Fig. 8) [60]. These were treated with decaborane (B10H14) to afford 1,2-dicarba-closo-dodecaboranes (o-carboranes) [71]. Rendering boron clusters soluble in water is of interest because of their use in cancer treatment by Boron Neutron Cancer Therapy. First and second generation water-soluble dendrimers containing four and twelve precisely located boron cluster sites, respectively, were synthesized (e.g., 18). These water soluble dendrimers and their precursors were characterized by 13C-, and nB-NMR spectroscopy (Fig. 8). 

For compounds to be useful in BNCT, they must allow administration to a patient (usually through the blood stream), accumulate or be retained in the tumor, be of low toxicity, and on the basis of the required accumulation, carry as many boron atoms as possible. To achieve the latter, boron clusters are of great value. In this chapter, the chemistry of some clusters is described that have found application in BNCT (see Scheme 2.2-1). The focus is on the o-carborane system dicarba-doso-dodecaborane, C2B1oH12, and its degradation product nido-carborate,... 

The polyalkyne precursors to the hydrocarbon-based unimolecular micelles 179 allowed the testing of chemical modification at specific sites within the interior of a cascade infrastructure. 180 Thus, treatment of the alkynes 153 or 154 with decaborane afforded excellent yields of the 1,2-dicarba-cfoso-dodecaboranes 181 (o-carboranes) or with Co2(CO)8 afforded the desired poly(dicobalt carbonyl) clusters. 182 Details are given in Chapter 8. 

In 1977 we reported a method based on graph theory for study of the skeletal bonding topology in polyhedral boranes, carboranes, and metal clusters Q). Subsequent work has shown this method to be very effective In relating electron count to cluster shape for diverse metal clusters using a minimum of computation. Discrete metal clusters treated effectively by this method Include post-transition metal clusters (, ) > osmium carbonyl clusters (O, gold clusters, platinum carbonyl clusters (J., 7 ) > and... 

In analogy to compounds linked to the carbon atoms of the cluster through a spacer, B-substituted phosphonates bearing the ort/to-carborane cluster in the ester moiety were also reported. These compounds were prepared from o-carborane-9-thiol or -selenol. The phosphonates can be prepared either by reaction with methylchlorophosphonates or by Michaelis-Arbuzov reaction of phosphonothioites or -selenoites (Scheme 2.6). 

Sivaev and Bregadze discussed the major types of boron clusters used in biomedical applications (Sivaev and Bregadze, 2009). These can be classified into two groups (1) neutral clusters, which include ortho-, meta-, andpara-dicarbadodecaboranes (doso-carboranes, (1-3, Figure 6.1), and (2) anionic clusters, which include dicarbaundecaborates (1-) ( i /o-carborane anions, 4-6), 1-carbado-decaborate (1-) (7), and the twofold negatively charged dodecahydro-c/oio-dodecaborate (2-) (8) and decahydro-c/oio-decaborate (2-) clusters (9) (Sivaev and Bregadze, 2009). 

FIGURE 6.1 Neutral and anionic boron clusters, o-carborane (1), m-carborane (2), p-carborane (3), nido-o-carborane (4), nido-m-caiborane (5), nido-p-carboiane (6), 1-carbadodecaborate anion (7), dodecahydro-c/oso-dodecaborate anion (8), and decahydro-cZoso-decaborate anion (9). 

BSH contains a thiol group and the dodecahydro-cto o-dodecaborate anion [Bi2Hi2] as the boron cluster this cluster is easily prepared °B-enriched for use in BNCT, and it is readily mono-functionalized. However, further functionalization of the boron cluster in BSH is challenging due to lack of chemo-regioselectivity. Therefore, most derivatives of BSH prepared to date involve either deriva-tization via the thiol group, or the replacement of the boron cage by a more hydrophobic carborane cluster or the polyhedral anion which has nearly double the boron content. Carboranes... 

A molecule could be prepared for the first time in which both clusters, dodecaborate as donor and o-carborane as acceptor substituent, were connected via a stable Ji-conjngated systan. The advantage of this compound is a high transparency in the wavelength region above 400 mn, yet the hyper-polarizabilities P of these compounds remain to be determined. 

Cobalt-catalyzed cycloaddition reactions of symmetrical alkynes usually generate Cg-symmetric hexaphenylbenzenes (Pisula et al., 2004 Chebny et al., 2006). Synthesis of the star-shaped compound 14, which contains six ort/io-carborane clusters surrounding a hexaphenylbenzene core (Figure 26.3) was synthesized in a like manner. Heximer 14 was then decapitated in a methanolic sodium hydroxide solution to convert the neutral cZoio-carboranes into their corresponding [nido-C2B9H,o] cages, as shown for 15, imparting water solubility (Dash et al., 2009). 

In 1990, Gabel and coworkers reported the synthesis of carboranylporphyrins in which the carborane cluster is linked to the porphyrin via carbon-carbon bonds through a benzyl group. Later, Vicente et al. used a similar strategy to prepare carboranylporphyrins. In both cases, tetraarylpor-phyrins (para- and mew-substituted) containing four or eight l-CH3-l,2-c/oso-C2B,oH,o, not directly... 

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