Carborane geometry

Ab initio computations have so far proved to be reliable structural tools for neutral carboranes and their anions. They have been used to predict the carborane geometries for six-vertex, eight-vertex, and 11-vertex nido-carboranes,67-69 and for 5-12 vertex r/aro-carboranes.70,71 However, carborane anions in the presence of a group I metal cation may form ion-pair anions with one cation tightly bound to the carborane anion (see Chapter 3.03).72,73 Even computations on a salt with a carborane anion in a non-coordinating solvent would need to include the cation for computed shifts that tally with observed NMR data.74... 

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. 

Ab initio computations, particularly at MP2 and (the computationally less intensive) B3LYP levels of theory, have been largely responsible for the rapid progress in carborane structural research, permitting the determination of the molecular geometry of a novel carborane from solution-state NMR data where X-ray diffraction could not be used (such as those in trace amounts or for which suitable crystals could not be obtained). Earlier... 

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 solid-state structure of ort o-carborane was determined for the first time in 1996 featuring HMPA as a weak hydrogen bond adduct in the solid state.569 The geometry of ort o-carborane, I T BjoH, is a distorted icosahedron with a bond of 1.62 A between the cage carbons (C1-C2). This bond is remarkably elastic being elongated up to 1.85 A... 

Unlike the subicosahedral geometrical pattern, the supraicosahedral pattern is complex as shown in Figure 13 (COMC (1982) 5.4.2.7.2 and COMC (1995) 6.2.3.1, 6.2.4.4 and 6.2.5.1). For example, there are eight distinct geometries known with a nido-12-vertex count. This pattern is expected to expand further as the interest in 13-and 14-vertex closo-carboranes, which were only identified in the last three years, grows. 

Constrained-geometry heterocarbaboranes with silicon and germanium have been synthesized in moderate yield starting with the o-carborane derivative r,/6u,o-l-(CH2OH)-2-(Me)-l,2-C2B1oH10.47... 

The carbonylation of (Cp )(77S-C2B9H11)TiMe resulted in an unusual B-H activation, generating novel complexes containing linked carborane-alkoxide ligands.30 However, dicarbollide analogs of the constrained-geometry complex,... 

As discussed in the introduction (Section 3.2.1), derivatives of the diboracyclo-propane 1C are non-classical organoboranes having 8 SE, and according to the 2n+2 SE rule may be classified as the simplest doso-carboranes of the series CH(BH) H ( = 2). Compounds IB and 1C have been computed [5] to be 17.5 and 47.6 kcal mol-1 lower in energy, respectively, than the classical diboracyclopropane 1A. They are 2e aromatics and possess planar-tetracoordinate centers in IB this unusual geometry is found at the carbon and one boron atom, in 1C at both boron atoms (Scheme 3.2-2). 

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... 

FIGURE 67. Molecular geometry of the magnesium C2B4H4 carboranes 140 and 141... 

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