Nido-anions

Therefore, the simplest strategy to increase the water solubility of C2B10-containing compounds is degradation of the closo-cage to the nido-anion, and this has been applied to C2Bio-containing amino acids and porphyrins (see Scheme 2.2-13). 

Scheme 2.2-12. Deboronation of o-C2Bio derivatives to the nido-anion.

Structural information about the nido-anion B4H3RT originates first from energy calculations for B4H7- (MP2(fc)/6-31G ).8 Four minima could be localized on the potential hyperface (Fig. 1). 

In the presence of air at 20°C, the reaction of the iminophosphorane 5 with ortho carborane 1 resulted in deboronation to yield the H2NP(NMe2)3+ salt 6 of the nido anion 7,8-C2B9Hi2 and boric acid.7... 

Crystals formed from the reaction mixture were identified by X-ray crystallography as a 1 1 polymeric adduct of [H2NP(NMe2)3+]2C032"-B(0H)3. Unlike HMPA 4, the iminophosphorane 5 absorbs carbon dioxide and water in the presence of air. Base 5 converts or/Ao-carborane into the nido-anion just as effectively in the absence of air indicating that water and C02 are not vital ingredients in the deboronation process. 

The breakthrough came with the discovery that the nido anion C 2B<)1 In p (an icosahedral fragment prepared by removal of a BI P unit from ortho-carborane l,2-C2BioHi2, by means of its reaction with sodium methoxide in methanol) could coordinate strongly to transition metal cations in a manner similar to the cyclopentadienide anion, (11 (Scheme 3.7)11.40,198-202... 

Closo cages have a p.e.c. of 4n + 2 electrons, whereas nido-carboranes require 4n + 4 electrons. The larger doso-carboranes can be reduced by alkali metals to nido anions. The latter can then often be converted into doso-metallocarboranes on treatment with a metal halide ... 

---