Dodecaborate (or closo-dodecaborate, or dodecahydro-closo-dodecaborate) is a ionic molecule containing a symmetrical cluster of boron and hydrogen atoms with the molecular formula B12H122−. Formally it can be derived from a hypothetical borane called dodecaborane B12H14. It is isoelectronic in structure with the dicarborane with formula B10C2H12. The dodecaborate has been of great theoretical interest to the chemistry community.
The [B12H12]2− anion's B12 core is a regular icosahedron. The [B12H12]2− as a whole also has icosahedral molecular symmetry, and it belongs to the molecular point group Ih. Its icosahedral shape is consistent with the classification of this cage as "closo" in polyhedral skeletal electron pair theory. (B12H14 - 2H+ → (B12H122-.)
The molecular weight = 143.6 g/mol and and pKa = 28.
The dodecaborate anion was first predicted to exist by H. C. Longuet-Higgins and M. de V. Roberts in 1955. They predicted that only the dianion would be stable. Hawthorne and Pitochelli first made it in 1960, but only with a yield of 4%. Later other synthesis techniques found had yields up to 90%.
The dodecaborate anion was first prepared in modest yield by Pitochelli and Hawthorne from iododecarborane by refluxing it with triethylamine in benzene. 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:
- 4 NaBH4 + BF3 → NaB3H8 + 3 NaF + 4 H2
Pyrolysis of the triborate gives the twelve boron cluster as the sodium salt.
Pyrolysis of the triborate compounds of different anions produces various different hydroborate compounds with different results for different metals and temperatures. Other cations include [B10H10]2−, [B9H9]2− and [B11H14]−. The metals and organic ions that can be pyrolised in this way include sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, tetramethyammonium, tetraethyammonium, tetrapropyammonium, and tetrabutyammonium ions.
Another high yield method heats triethylamine borane with diborane at a temperature of 180 °C. Yet another heats pentaborane at 125° with the same triethylamine borane. In both cases the dodecaborate salt of trimethylamine is produced.
The cheapest synthesis route involves a potassium tetrafluoroborate reaction with calcium hydride.
Lithium, sodium, potassium, rubidium, cesium, copper(i), silver, thallium(i), beryllium, magnesium, calcium, strontium, barium, zinc, cadmium, aluminium, scandium, yttrium, numerous rare earth metals, zirconium, hafnium, uranium, ammonium, tetramethyammonium, tetraethyammonium, tetrapropyammonium, tetrabutyammonium, triethylammonium (pictured), (and other amines) guanidinium, tetraphenylphosphonium and trimethylsulfonium salts have all been made. Mixed salts with chloride, bromide and iodide also exist. Large cations can also form mixed nitrates and hydroborates.
Dodecaborate is quite stable, and survives heating with sodium hydroxide or hydrochloric acid. Dodecaborate rarely forms a ligand in an inner sphere position, but has been found to do so with some compounds of mercury, ruthenium or copper.
It can be electrochemically oxidised to [B24H23]3−.
The hydrogen atoms can be substituted by the halogens to form different isomers with different degrees of substitution of from one to twelve hydrogen atoms. To label the products the boron atoms are numbered. 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. The icosahedron of boron atoms is aromatic in nature, and substituents can cause more reactivity at other positions which can be called the ortho, meta or para positions.
Fluorine is a deactivating meta direction substituent. Halogens can react directly, or from a dihalomethane, or from hydrogen fluoride. The degree of replacement of hydrogen depends on temperature and acidity. The perhalo ions form strong acids in water.
Under kilobar pressure carbon monoxide [B12H12]2- can react to form the carbonyl ion [B12H11CO]- and 1,12-B12H11(CO)2 with a small amount of 1,7-B12H10(CO)2. The para disubstitution at the 1,12 is unusual. In water the dicarbonyls appear to form carboxylic ions: [B12H10(CO)COOH]- and [B12H10(COOH)2]2-.
Scientists have studied metal borohydrides as a possible hydrogen storage medium. However, rehydrogenation of one of the intermediate compounds, metal dodecaborate, was difficult until the discovery of a solvent-free synthetic route:
- (2MBH4) + (B10H14) → (M2B12H12) + 5H2
where M = Li, Na, K.
The distribution of water around the dodecaborate anion [B12H12]2- depends on the substituents of the hydroborane cluster. The anion [B12H122-] has a negatively charged outer shell due to hydrogen’s greater electronegativity, thus attracting the positively charged hydrogens of water. With substituents like [B12H11NH3-], the charge distribution is more positive on the top hydrogens near Nitrogen and more negative on the lower hydrogens. As a result of this, the water shell around the molecule is arranged in layers of corresponding charges.
Salts of B12H122− are precursors to related derivatives including B12(OH)122− and B12(CH3)122−. This closo boron hydride resists degradation more so than the isoelectronic carboranes.
Treatment of nuclear waste
Radioactive waste contains long lived radionuclides, such as 152Eu and 241Am, of which disposal is difficult, expensive and has the potential to produce unnecessary byproducts; however, specific selection requires a way to separate these trivalent actinides from other fission products. Hydroborate derivatives based on [B12H12]2- anion form neutral compounds (ion pairs) with 152Eu and 241Am, which are cationic trivalent radionuclides. These neutral compounds can be extracted with an organic solvent with medium polarity.
Icosahedral borane anion [B12H12]2- and its dodecahydroxyl derivative [B12(OH)12]2- are capable of forming closomers, which have been used to make nontargeted high-performance MRI contract agents. These agents have shown to be more permeable and possess a retention effect that allows excellent persistence in tumor tissue. The particular synthesis of a monoether derivative of [B12(OH)12]2- allowed the attachment of a target-specific moiety, which becomes extremely useful in constructing a targeted delivery system. However, its synthesis and purification relies on multiple purification steps through size-exclusion chromatography and ultimately results in relatively low yields.
Boron neutron capture therapy
Closo-dodecaborate is a potential therapeutic agent cancer treatment. In boron neutron capture therapy (BNCT), derivatives of closo-dodecaborate are encapsulated within a liposome and injected into the subject, increasing the specificity of neutron irradiation treatment as the encapsulated dodecaborate disperses to different organs within the body at different concentrations. Neutron irradiation converts nonradioactive dodecaborate containing 10B to radioactive 11B, which then decays and emits an alpha particle near tumor site. Clinical trials are underway for the treatment of human brain tumors using this agent.
A related dodecaborane ion (B12H14+) can be produced using an external quadrupole static attraction ion trap (EQSIT), which comprises four electrodes and a surrounding electrode (cage) that traps ions using a nonuniform electric field . Decaborane gas (B10H14) and diborane gas (B2H6) are introduced into the EQSIT (both gases are commercially available). Electron irradiation on B10H14 produces B10Hx+ (x = 6, 8, 10, 12, 14). B10H8+ (which is trapped) then reacts with B2H6 molecules in the EQSIT, producing dodecaborane ions.
- (B10H8+) + (B2H6) → (B12H14+)
The energy of this reaction is ∆E = 2.88 eV, as calculated per the equation:
- Er (B12Hn+) = Etot(B12Hn+) + (14 – n)/2 • Etot(H2) + Etot(H) – Etot(B12H14+) where n=14
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