Arachno-carboranes

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). 

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 ... 

A convenient route to three-carbon carboranes is the hydroboration of an alkyne with a preformed dicarbaborane. For example, reaction of ethyne (or propyne) with arachno-4,5-C2B7Hi3 (70) in hexane at 120°C gives a mixture of tri- and tetra-carbaboranes, e.g. (71), (72), (73), (74) in modest yield. Access to other... 

Figure 1 shows the known experimental geometries of carboranes. For discussion of closo-, nido-, and arachno-zlectron counts and their expected geometries, see COMC (1982) 5.4.2.2 and 5.4.2.3. Arachno-six- and seven-vertex geometries are not known experimentally for carboranes. 

The Brellochs reaction represents a significant breakthrough in monocarbaborane chemistry, the exploration of which was previously limited by multistep routes. For example, the important carborane anion CBnFI12 can now be made in two steps with an overall yield of 66%.88 Its synthesis previously required three steps involving undesirable compounds such as hydrogen cyanide, dimethylsulfate, and trialkylamine. (COMC (1982) 5.4.2.6.2) The arachno-cztbotznc... 

While the parent carboranes can be made from formaldehyde and decaborane, carboranes with substituents at the cage carbon can easily be synthesized from the appropriate aldehyde. For example, benzaldehyde and B10H14 give the zW(9-6-Ph-6-CB9Hn anion in 94% yield.92 The phenyl carborane formed is nido- not arachno- but subsequent carborane products can be made using similar reagents as for the parent arachno-6-CB9H14-.93... 

Two carborane cations, zV/ -2,3,4,5,6-McsC5BBr+ and the tentatively proposed arachno-N ci WX I71, were reported. The identity and pyramidal structure of the pentacarbaborane cation was determined by X-ray crystallography.112 The small arachno-carborane was detected on the basis of ab initio computations and 11B NMR data in the mixture from reaction of the 2-propyl cation with diborane.113 Two neutral adducts based on the pentacarbaborane cation, ///V/9-2,3,4,5,6-McsCsBBKCI2 (R = Cl or SiClj), were made from MesCsSiMe3 or (MesCs Si with B2CI4, respectively.114 The BCI3 adduct was structurally characterized by X-ray crystallography. 

The controlled synthesis of carbons adjacent nido- and arachno-carborane anions can be achieved by the reduction of C (cage)-C (cage)-linked o-carboranes with group 1 metals.234-237 Typical examples are illustrated in Scheme 22. These have been found to undergo metallation reactions similar to their carbons apart analogs, forming full- and half-sandwich complexes of lanthanide metals.238... 

C2B9H12, the isomers m- and p-carborane (see Chapter 3.2), the cioso-dodecaborate system B,2H,22 (see Chapter 2.1.5.6), and an arachno-azanonaborane system BXN H n (see Chapter 3.4.4.3). All these dusters have in common the fact that they are stable in aqueous solution, and thereby would allow administration to patients via the blood stream. Their chemistry, and thus the way in which they can be attached to organic molecules, as well as their physiologically relevant parameters of hydrophobicity, vary considerably, however. For a review of the chemistry of BNCT, see Refs. [9-11]. 

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

It turned out that the [B-H]-catalyzed condensation of two molecules of the intermediate 24 proceeds slowly with elimination of BEt3, when compared with bis(diethylboryl)-substituted hydrocarbons. Thus, a route to the arachno-carboranes 27 is provided [36] by heating the in situ formed 24 and 26 in the "hydride bath . The carboranes 27 were obtained as mixtures of isomers (Scheme 3.2-14). 

Scheme 3.2-33. 2e reductive opening of the nido-carborane 56g to the arachno-dianion 62 (R = SiMe3) and the reverse reaction by 2e oxidation. 

Scheme 3.2-40. Et2 BH-catalyzed condensation arachno-C6B6 carborane derivative 76, of which reactions of three borylated C2-units (see 77) the structure differs greatly from that of the in the hydride bath , starts from bis- parent compound C6B6Hi2 78.

Scheme 3.2-46. Closo- (39a, 49) and n/c/o-carboranes (56a, 64d, 71) and drum-like 76, an arachno-carborane.

The neutral double zwitterions 139-142 are also susceptible to oxidation and treatment of 141 with I2 (1 equiv) gave a surprising result.In the product, [3,4-(OEt)2-l,l,l-(CO)3-2-Ph-/zy crc/oA o-l,2-ReCB8H6] (143), the rhenium center may be considered to be in the -I- III oxidation state, as in 132-138, but the cluster in 143 has also lost one BH vertex. It is known" that oxidation of [6-Ph- /Jo-6-CB9H11], the 10-vertex carborane parent of 129 and 130, with FeCls affords 9-vertex [4-Ph-arac/z o-4-CBgHi3] and indeed the carborane subunit of 143 also has the arachno-A-CB geometry. In addition, two I ions would be liberated following the oxidation reaction and nucleophilic attack of these upon the positively... 

The same polyhedra serve as the basis for the structures of nido and arachno compounds, too, although for these boranes and carboranes the polyhedra are incomplete. Nido structures are adopted by neutral boranes BjjHw j.4, carboranes CBy 4Hn.. 3, CsBji—... 

The problems outlined in the previous section can be avoided if, instead of allocating the skeletal bonding electron pairs to localized bonds, one simply compares their number with the number of skeletal bonding MO s (199). The closo, nido, and arachno structures of boranes and carboranes can then be seen to reflect the numbers of skeletal bond pairs that are available to hold their skeletal atoms together. 

The alkaline degradation of closo-carboranes, used to prepare smaller carboranes from icosahedral starting materials, also occurs by closo- to nido-, possibly even arachno-, cage opening, e.g. 107),... 

In 1971, a note 164) was published favoring the hypothesis that the carboranes, boranes, their isoelectronic anions, Lewis base adducts, and heteroatom-substituted analogs should be viewed as constructed about the vertices of either the most spherical series of triangular-faceted polyhedra (deltahedra) found to be characteristic of the dicarba-cZoao-carboranes (Fig. 1) or, with one lone exception, fragments of the series of deltahedra produced by the successive removal of the highest coordinated vertices that sequentially define the nido and arachno classes. This position was in conflict with the then prevalent shibboleth that all nido and arachno compounds [except B5H9 (I-N5)] had or would prove to have icosahedral fragment structures. 

Fig. 20. The [BioHia] family of arachno-carboranes. (LB, Lewis base.)...

Wade also incorporates electron-rich species such as C5H5 and C4H42- as arachno species derived by removal of two nonadjacent high-coordination vertices from the pentagonal and tetragonal bipyramids, respectively, and has, moreover, correlated many aspects of seemingly unrelated classes of compounds beyond the recognized borders of carborane chemistry. 

Since a rather unstable C2BaHio ido-carborane had been known for several years to exist, this isomer of C2BeHio was structurally identified (41) as having (probably) the arachno structure of V-N8 instead of the nido structure X-N8, but this is not yet sure. 

For boranes and carboranes, the terms do so, nido, arachno, and hypho are used to describe their molecular skeletons with reference to a series of increasingly open deltahedra. Boranes of the nido, arachno, and hypho types are compounds based on formulas B H +4, B H +6, and B Il +8, respectively. The structures... 

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