Tetracarba-nido-hexaboranes

The halogeno derivatives 56b and 56d are excellent candidates for the synthesis of other derivatives of the nido-C4B2 cluster series bearing functional groups either in the 6- or the 1,6-positions. With few exceptions, the reactions of 56d with nu- 

The most intriguing substituent at the boron atom(s) of the nido-C4B2 framework is the amino group NR2 because of potential (N=B6)7t interactions which may weaken the B-B bonding in the cluster, because NPrl2 groups enforce a classical bicyclic structure 60 [79]. 

The bicyclic compound 58 was found to be extremely reactive towards traces of H-acidic reagents. Even secondary amines, e.g., Et2NH react with 58 by ring- 

C4B2 framework in 56e to 56f, which can be understood as an example of the diamand-square-diamond skeletal rearrangement [80]. 

The carborane 64c was characterized by a consistent set of NMR data, ab initio MO calculations, and by X-ray structure analysis [88]. The synthetic potential of 64c is indicated by its reaction with Fe(CO)3 fragments to give 69 [87]. The di-ferracarborane 69 is isostructural with nido-decaborane(14), because two BH and four BH(H) fragments were replaced by two isolobal Fe(CO)3 and four CH fragments, respectively. 

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Tetraalkyl-l,6-dihalogeno-2,3,4,5-tetracarba-nido-hexaboranes(6) 56d were obtained in high yield by another route (Scheme 3.2-30), taking advantage of the exchange reactions of the spirotin compounds 57 with boron halides. 

Reductive opening of a tetracarba-nido-hexaborane cluster to an arachno-dianion was described for 56g, which is accessible by thermal elimination of bis(trime-thylsilyl)ethyne from the bridged bishomoborirene 61 [82], Reaction of 56g with elemental lithium in THF gives [(Li thf)2]l,3-diboratabenzene 62, its reoxidation to 56g requires boiling 1,2-dibromoethane (Scheme 3.2-33). 

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