Dicarba species

The first carboranes were discovered " ° 2 among the products of reactions between alkynes and boron hydrides. Such reactions remain the best routes to many dicarba species (Scheme 3.1, 3, route Derivatives... 

The discovery of polyhedral boranes and polyhedral heteroboranes, which contain at least one atom other than in the cage, initiated a new era in boron chemistry.1-4 Most commonly, of the three commercially available isomeric dicarba-closo-dodecaborane carboranes(l,2-, 1,7-, and 1,12-), the 1,2-isomer 1 has been used for functionalization and connection to organic molecules. The highly delocalized three-dimensional cage bonding that characterizes these carboranes provides extensive thermal and kinetic stabilization as well as photochemical stability in the ultraviolet and visible regions. The unusual icosahedral geometry of these species provides precise directional control of all exopolyhedral bonds. 

In contemplating dicarba-wido-decaborane species, it is easy to predict the most stable dianion, C2BgHio" (VIII-NlO). With no bridge hydrogens to complicate the issue, the most stable isomer should be the isomer illustrated less stable isomers with carbons in five-coordinate sites should also be observed. 

The new compounds described in this paper have a rhenium atom incorporated into closo-l,2-dicarba-3-rhenadodecaborane or c/oro-l-carba-2-rhenadodecaboranc structures. However, to avoid a complicated nomenclature for the complexes reported, and to relate them to the many known rhenium species with 77-co-ordinated cyclopentadienyl ligands, we treat the cages as nido- 1 -vertex ligands with numbering as for an icosahedron from which the twelfth vertex has been removed. 

Compound C is synthesized from the pentaethyl-1,5-dicarba-c/oso-pentaborane(S) (A) by a dimerization that has not yet been fully clarified mechanistically 1 mol of A (5"B = + 13.5) reacts with 1.2 mol of metallic potassium in tetrahydrofuran (THF) at room temperature according to the above equation, slowly forming a dark brown solution. The metal does not react faster when the mixture is heated. If > 0.5 mol of potassium per mole of A ( "B = 13.5) are taken up, four new "B NMR signals of the weakly paramagnetic solution (ESR measurementsf) appear at = 26.8, — 14.2, — 16.2, and — 37.1 (see Fig. 1). The intensity of the "B NMR signal of A decreases to ca. 50% after reaction of > 1.0-1.2 mol of potassium per mol of A. Correspondingly, we presume that a potassium salt of the dianionic species B is formed in an equilibrium with A and a small amount of the radical anion salt B (see equation). 

The microwave spectrum of l,7-dicarba-c/oso-octaborane(8) has been studied. An assignment of the rotational spectra for nine isotopic species was achieved. The QBs skeleton has slightly distorted Djd geometry in the gas phase (see Figure 3). All of the bond distances were reported. ... 

Of all of the carboranes, by far the most studied are the icosahedral species, C2B10H12. As seen in Fig. 11, three isomers exist. Their systematic names are 1,2-dicarba-c/(95 (9-dodecaborane(12), 1,7-dicarba-c/o50-dodecabo-... 

Carboranes. The accumulation of charge observed in borane anions discussed above may also be afforded by changing some boron atoms by other elements with more valence electrons than boron. The best known examples of this kind of isoelectronic compound are the carboranes. The most important series of carboranes is that with molecular formulas C2B 2Hn, the dicarba-c/oso-boranes, where n is any integer from 5 to 12. These neutral species are isoelectronic with the borane anions BnH . The structures of these compounds are closed deltahedra with terminal hydrogen atoms attached to each carbon or... 

Another important series of carboranes are the double dianions [C2Bn 2Hn]. These species which have n cage atoms and (n + 2) pairs of skeleton electrons adopt ni /o-structures in which one corner of the polyhedron is left vacant as shown in the scheme of the dicarba-mVio-hexaborane (8) illustrated in Fig. 4.14. Dicarba-c/oso-boranes in general are unreactive in... 

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