Nido-carborane

Milder conditions provide a route to nido-carboranes, e.g. ... 

Metal-vapor synthesis also prepares metallaboranes and metallacarboranes with oxidation of the metal. Thermally generated Ni, Co and Fe atoms react with the nido-carborane 2,6-C2B7H, and either cyclopentadiene, toluene, mesitylene, or... 

The original synthesis of metallacarborancs is effected by deprotonation of nido-carborane anions in a manner like that employed to prepare ferrocene, e.g. ... 

Figure 68 The neutral ortho- or anionic nido-carborane attached pyrrole molecule (111, 112,113, and 114) used in the synthesis of conducting organic polymers 115, 116, etc. (Adapted from refs. 141 and 142.)...

For small nido-carboranes according to the following reaction35 ... 

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 nido-carborane anion, 7,8-C2B9H12, and derivatives are generally formed by deboronating ort o-carboranes with suitable bases (COMC (1982) 5.4.2.6.6) but a direct route to two derivatives of the anion has been developed in good yields from decaborane and polarized alkynes.78... 

However, the duster compounds differ greatly in their water solubility. Whereas cioso-dodecaborate (as the sodium salt) is readily water soluble, o-carborane and its thermal rearrangement products m-carborane and p-carborane show poor water solubility. The nido-carborane system, because of its negative charge, is water soluble. The azanonaborane duster is neutral, but still shows a certain degree of water solubility. 

Oligonucleotides bearing nido-carborane have been synthesized as phospho-diesters using an automated DNA synthesizer (see Scheme 2.2-14) [39]. These oligophosphates are homogeneous, very hydrophilic and are readily taken up into cells. Fluorescein-labeled nido-carboranyl oligomeric phosphate diesters accumulate in the cell nucleus [40]. 

Scheme 2.2-13. Examples of an amino acid and a porphyrin with nido-carborane moieties.

A one pot route to nido-1,6-diiodo-tctracarba-nuio-hcxaborane derivatives is depicted in Scheme 3.2-32. After iodoboration of the alkynes with BI3 the dehaloge-nation of the alkenyl derivatives presumably leads to dimerization and rearrangement to give structurally characterized 1,6-diiodo-nido-carboranes 56d [81]. 

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. 

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... 

The next family to be discussed, i.e., the nine-vertex nido-carboranes, includes the complications of the B10H14 family (i.e., dilFeringly coordinated edge positions) as well as one additional complication not encountered in the B10H14, and B5H9 families. The additional... 

Two different types of gallacarboranes with a closo structure have been reported in the literature. They can also be regarded as n complexes of a nido-carborane dianion and a RGa2+ center. 

As mentioned previously, the first metallocarborane synthesized, (l,2-C2B9Hu)2Fe(II)2- (Fig. 3), was prepared in a manner similar to the synthesis of ferrocene, e.g., reaction of anhydrous FeCl2 with the nido-carborane dianion 7, S-C BgHn2, which itself was formed from 1,2-C2BioHi2... 

Three-dimensional aromaticity has been demonstrated, for instance in the case of Cram s cyclophane in 1951 [46, 47] and, more recently, for von Rague Schleyer s adamantane dication [48], half-sandwich carbocations, and nido-carboranes. Ferrocene and various other or-ganometallic compounds also are three-dimensional aromatics. Spherical aromaticity is exhibited by closo-carboranes, borane anions, and, to a lesser extent, C60, although these compounds are isotropic. 

The mechanism for the reductive insertion is not known since the corresponding neutral nido-carborane is also produced, it would seem to involve the THF solvent. 

The unsubstituted nido-carborane C2B7Hn exhibits an NMR spectrum indicating a structure similar to that of the dimethyl species, in terms of vertex positions, but the hydrogen positions are different133 . Apparently there is a BH2 group and only one bridging hydrogen. 

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