The dodecaborate(12) anion, ₁₂H₁₂sup>2−, is a

borane Trihydridoboron, also known as borane or borine, is an unstable and highly reactive molecule with the chemical formula . The preparation of borane carbonyl, BH3(CO), played an important role in exploring the chemistry of boranes, as it indicated ...

with an icosahedral arrangement of 12

boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the '' boron group'' it has t ...

atoms, with each boron atom being attached to a hydrogen atom. Its symmetry is classified by the molecular point group I_h.

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 Triethylamine is the chemical compound with the formula N(CH2CH3)3, commonly abbreviated Et3N. It is also abbreviated TEA, yet this abbreviation must be used carefully to avoid confusion with triethanolamine or tetraethylammonium, for which TEA ...

in benzene solution at 80 °C. It is more conveniently prepared in two steps from

sodium borohydride Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula Na BH4. This white solid, usually encountered as an aqueous basic solution, is a reducing agent that finds applica ...

. First the borohydride is converted into a triborate anion using the etherate of

boron trifluoride Boron trifluoride is the inorganic compound with the formula BF3. This pungent, colourless, and toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds. Structure and bond ...

: : 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 In chemistry, hydroxylation can refer to: *(i) most commonly, hydroxylation describes a chemical process that introduces a hydroxyl group () into an organic compound. *(ii) the ''degree of hydroxylation'' refers to the number of OH groups in a ...

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 chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

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−.

Potential applications

Compounds based on the ion ₁₂H₁₂sup>2− have been evaluated for solvent extraction of the

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...

ions ¹⁵²Eu³⁺ and ²⁴¹Am³⁺. ₁₂H₁₂sup>2−, ₁₂(OH)₁₂sup>2− and ₁₂(OMe)₁₂sup>2− show promise for use in

drug delivery Drug delivery refers to approaches, formulations, manufacturing techniques, storage systems, and technologies involved in transporting a pharmaceutical compound to its target site to achieve a desired therapeutic effect. Principles related to dr ...

. They form "closomers", which have been used to make nontargeted high-performance

MRI contrast agent MRI contrast agents are contrast agents used to improve the visibility of internal body structures in magnetic resonance imaging (MRI). The most commonly used compounds for contrast enhancement are gadolinium-based. Such MRI contrast agents shorten ...

s which are persistent in

tumor A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...

tissue.0> Salts of ₁₂H₁₂sup>2− are potential therapeutic agents in cancer treatment. For applications in

boron neutron capture therapy Neutron capture therapy (NCT) is a type of radiotherapy for treating locally invasive malignant tumors such as primary brain tumors, recurrent cancers of the head and neck region, and cutaneous and extracutaneous melanomas. It is a two-step pro ...

, derivatives of ''closo''-dodecaborate increase the specificity of neutron irradiation treatment. Neutron irradiation causes

boron-10 Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has t ...

to emit an

alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be prod ...

near the tumor.

References

{{reflist Boranes Anions Substances discovered in the 1960s