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The dodecaborate(12) anion, 12H12sup>2−, is a borane with an icosahedral arrangement of 12 boron atoms, with each boron atom being attached to a hydrogen atom. Its symmetry is classified by the molecular point group Ih.


Synthesis and reactions

The existence of the dodecaborate(12) anion, 12H12sup>2−, was predicted by H. C. Longuet-Higgins and M. de V. Roberts in 1955. Hawthorne and Pitochelli first made it 5 years later, by the reaction of 2-iododecaborane with triethylamine in benzene solution at 80 °C. It is more conveniently prepared in two steps from sodium borohydride. First the borohydride is converted into a triborate anion using the etherate of boron trifluoride: : 5 NaBH4 + BF3 → 2 NaB3H8 + 3 NaF + 2 H2 Pyrolysis of the triborate gives the twelve-boron cluster as the sodium salt. A variety of other synthetic methods have been published. Salts of the dodecaborate ion are stable in air and do not react with hot aqueous sodium hydroxide or hydrochloric acid. The anion can be electrochemically oxidised to 24H23sup>3−.


Substituted derivatives

Salts of undergo hydroxylation with hydrogen peroxide to give salts of 12(OH)12sup>2−. The hydrogen atoms in the ion 12H12sup>2− can be replaced by the halogens with various degrees of substitution. The following numbering scheme is used to identify the products. The first boron atom is numbered 1, then the closest ring of five atoms around it is numbered anticlockwise from 2 to 6. The next ring of boron atoms is started from 7 for the atoms closest to number 2 and 3, and counts anticlockwise to 11. The atom opposite the original is numbered 12. A related derivative is 12(CH3)12sup>2−. The icosahedron of boron atoms is aromatic in nature. Under kilobar pressure of carbon monoxide 12H12sup>2− reacts to form the carbonyl derivatives 12H11COsup>− and the 1,12- and 1,7-isomers of B12H10(CO)2. The para disubstitution at the 1,12 is unusual. In water the dicarbonyls appear to form carboxylic ions: 12H10(CO)CO2Hsup>− and 12H10(CO2H)2sup>2−. A perfluoroborane derivative (with the hydrogen atoms replaced by fluorine atoms) is also known.


Potential applications

Compounds based on the ion 12H12sup>2− have been evaluated for solvent extraction of the radioactive ions 152Eu3+ and 241Am3+. 12H12sup>2−, 12(OH)12sup>2− and 12(OMe)12sup>2− show promise for use in drug delivery. They form "closomers", which have been used to make nontargeted high-performance
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tissue.0> Salts of 12H12sup>2− are potential therapeutic agents in cancer treatment. For applications in boron neutron capture therapy, derivatives of ''closo''-dodecaborate increase the specificity of neutron irradiation treatment. Neutron irradiation of boron-10 leads to the emission of an alpha particle near the tumor.


References

{{Boron compounds Boranes Anions Substances discovered in the 1960s