Carboranes isoelectronic

A separate series of closo-carboranes, isoelectronic with the BnH2 dianions and the neutral QB 2H compounds, consists of the monocarbon anions CB iH (Table 5-3). The preparative routes to these species vary widely, but a commonly used method for incorporating a single carbon atom into a borane cage involves the conversion of cyanoboranes to amino derivatives the cyano carbon atom satisfies its valence requirements via absorption into the cluster framework. 

Fig. 1-2. The most nearly spherical deltahedra found in the boranes B H 2 (6 < n < 12) and isoelectronic carboranes.

Building on and extending earlier studies, Schleyer, Najafian, et al. [57] employed computed geometric, energetic, and magnetic properties to quantify the aromaticity of the closo boranes B H 2 (6 < n < 12), and their isoelectronic counterparts, the CB iH and C2B 2H carboranes, and the NB H azaboranes [34]. All possible heteroatom placements were considered. The most stable iso-... 

The polyhedra in Fig. 1 thus represent suitable shapes for cluster species with n skeletal atoms (each of which can furnish three AO s for use in skeletal bonding) and with (n + 1) skeletal bond pairs. Since it is the cluster symmetry that determines the number of bonding MO s, the same polyhedra can serve as the basis for the structures of a whole range of isoelectronic species, including neutral carboranes of formula C2B 2Hn, bismuth clusters, such as the trigonal-bipyramidal Bis " ",... 

One particularly interesting category of metallocarborane is that in which a single metal atom is shared between two polyhedra that have a vertex in eommon. In effect, the metal is sandwiched between two nido-carborane residues. Examples are shown in Fig. 17. For such commo compounds, the metal can be assumed to contribute three AO s to the skeletal bonding of each polyhedron, when the (n + 1) rule for closo clusters is found to be obeyed. For example, the isoelectronic... 

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. 

The next development in the understanding of structure and bonding in the Zintl ions recognized their relationship to the polyhedral boranes and the isoelectronic carboranes. Then the Wade-Mingos rules [13-16], which were developed to understand the structure and bonding in polyhedral boranes, could be extended to isovalent Zintl ions and related post-transition element clusters. 

Fig. 27. Examples of isostructural carboranes and metallocarboranes that are skele-tally isoelectronic (255).

The acetylene may be unsubstituted (R = H) or substituted, in which case the reaction proceeds even more readily. The resulting compound is known as 1.2-dicarbtw/aY -dodecaborane(l2), or the ortho carborane, and is isoelectronic and isostructur.il with [BI2HI2]2-. It is stable to both heat and air, but it isomerizes at high temperatures to the 1,7 ( meta or neo isomer) and the 1,12 ( para isomer) (Fig. 16.51). The mechanism of isomerization, thought to be intramolecular, has been discussed for many years.133... 

Explicitly included are species with carbonyl (>CO) groups such as the isomeric phthalimide (VIII) and isatin (IX), and with thiocarbonyl (>CS) groups such as the isomeric benzo-l,2-dithol-3-thione (X) and benzo-l,3-dithiol-2-thione (phenylene trithiocarbonate) (XI). Conversely, species with the isoelectronic >BF (and the related >BH) are ignored. Thus the question of the aromaticity in carboranes never arises in this chapter, even had we been explicitly interested in three-dimensional aromaticity (another issue we will ignore here). 

As later work would show (47, 77), an entire series of polyhedral B Hn2-and isoelectronic C2B 2H carboranes were synthetically accessible for n = 6 through 12. The generally decreased chemical reactivity of these polyhedral species over that of the boron hydrides suggested that they... 

It was correctly assumed and later determined unequivocally (64) that in each of these two reactions the BH vertex that was removed was always one of the two equivalent vertices found as the nearest neighbors of the two equivalent CH vertices. The fact that the carbon atoms in polyhedral surfaces of C2Bb 2H carboranes are electronic counterparts of boron atoms in the corresponding isoelectronic B H 2 polyhedral ions (62) requires these carbon atoms to resemble C+, a species present in carbonium ions. Consequently, the BH vertices which are nearest neighbors of two such carbon atoms will be activated for nucleophilic attack by base through the advent of a strong inductive effect. 

Boranes of the general formula B Hn2 and isoelectronic carboranes such as CB iHk- and C2B 2H have closo structures, in which n skeletal B and C atoms are located at the vertices of a polyhedron bounded by trianglular faces (deltahedron). For the BMH 2 cfo.vo-boranes... 

The smallest of these, nido- 1,2-C2B3H7 (Fig. 5-13a), an isoelectronic and iso-structural analogue of B5H9, is unstable, polymerizing in the liquid state to form a white solid. Parent nido-QB4H8 (Fig. 5-13c), the first carborane reported in the... 

The rapid development of carborane chemistry is mainly due to their practical applications. For instance, the potential utility of carborane polymers as gaskets, O-rings, and electrical connector inserts has been reported. Their functionality for solvent extraction of radionuchdes as well as the potential medicinal value of the isoelectronic and isostructural boron analogues of biologically important molecules has been the subject of many review articles. For example, a number of boron compounds have been found to possess anti-inflammatory and antiarthritic activity in animal model studies. Boron compounds have also been implicated in studies designed to probe the importance of the so-called anionic subsite of acetylcholine esterase and Ach receptors. But, by far the most interesting practical apphcations of carboranes are in areas of boron neutron capture therapy (BNCT) and supramolecular assembly. 

---