Decaborane
Decaborane, also known as decaborane(14), is an inorganic borane cluster compound with the chemical formula B₁₀H₁₄, consisting of ten boron atoms arranged in a nido polyhedral structure resembling a boat, featuring four bridging B-H-B bonds and ten terminal B-H bonds.[1] This air-stable, malodorous, colorless crystalline solid has a melting point of 99.7 °C and boils with decomposition at 213 °C, with a dipole moment ranging from 3.17 to 3.62 D and a standard enthalpy of formation of -66.1 kJ/mol.[1] First synthesized over a century ago by Alfred Stock through the pyrolysis of diborane at elevated temperatures, decaborane attracted significant interest in the post-World War II era for its potential as a high-energy rocket fuel component, though toxicity concerns limited practical deployment.[1] Today, it finds applications as a precursor for carborane synthesis, boron-based nanomaterials, neutron capture therapy agents, and fuel cell components, while also serving as a mild, selective reducing agent in organic reactions such as reductive amination and etherification, often in protic solvents with catalysts.[1][2] Despite its utility, decaborane is highly toxic, capable of causing severe neurological effects upon exposure, necessitating stringent safety protocols in handling.[1]Structure and Properties
Molecular Structure
Decaborane has the chemical formula \ce{B10H14} and is systematically named decaborane(14), reflecting the 14 hydrogen atoms attached to the boron cluster. It is classified as a neutral nido-borane, belonging to the family of polyhedral boron hydrides characterized by an open cage structure.[3][4] The molecular structure features 10 boron atoms arranged in a nido geometry, derived from an incomplete icosahedron by removal of two adjacent vertices, creating an open pentagonal face. This 10-vertex cluster adheres to Wade's rules, possessing 12 skeletal electron pairs that support the delocalized bonding framework typical of nido species with the general formula \ce{B_nH_{n+4}}. The bonding within the cluster involves three-center two-electron (3c-2e) bonds among the boron atoms, which stabilize the polyhedral skeleton. Additionally, the structure includes 10 terminal B-H bonds, each a conventional two-center two-electron bond, and 4 bridging B-H-B bonds across the open face, contributing to the overall electron count and structural integrity.[5][6] The structure of decaborane was first elucidated in the 1950s through X-ray crystallography by William N. Lipscomb and collaborators, marking a pivotal advancement in understanding boron cluster bonding and earning Lipscomb the 1976 Nobel Prize in Chemistry for his work on boranes. This determination revealed the precise atomic positions, confirming the nido arrangement and the presence of both terminal and bridging hydrogens essential to the molecule's stability.[5]Physical Properties
Decaborane appears as a white crystalline solid with an intense, bitter, chocolate-like odor.[7][8] It has a molar mass of 122.2 g/mol.[9] The compound melts at 99.6 °C and boils at 213 °C, at which point it decomposes.[9] Its density is 0.94 g/cm³ at 20 °C.[10] The dipole moment ranges from 3.17 to 3.62 D, and the standard enthalpy of formation is −66.1 kJ/mol.[1] Decaborane exhibits low solubility in cold water, approximately 0.1 g/100 mL, but is more soluble in organic solvents such as benzene, toluene, tetrahydrofuran, diethyl ether, and alcohols.[9][10] The compound is highly flammable and burns with a characteristic green flame attributable to its boron content.[8] Its vapor pressure is 0.15 mmHg at 20 °C.[10]Chemical Properties and Handling
Decaborane(14), with the formula B₁₀H₁₄, is air-stable as a crystalline solid at room temperature, exhibiting good thermal stability up to its decomposition point.[1] However, it undergoes slow hydrolysis in moist air, with the reaction accelerating significantly in the presence of water. In boiling water, hydrolysis proceeds rapidly as a first-order reaction, yielding boric acid and hydrogen gas according to the stoichiometry B₁₀H₁₄ + 30 H₂O → 10 B(OH)₃ + 22 H₂.[11] Decaborane(14) acts as a weak Brønsted acid, with a pKₐ in the range of 2.41–3.21 in aqueous ethanol, allowing deprotonation by strong bases to form the nido-[B₁₀H₁₃]⁻ anion.[1] Purification of decaborane(14) is typically achieved by vacuum sublimation at temperatures of 50–80 °C under reduced pressure (e.g., 0.1 mmHg) to remove impurities and evolved gases.[12] For safe laboratory handling, decaborane(14) should be stored in tightly sealed containers under an inert or dry atmosphere in a cool, well-ventilated area to minimize hydrolysis.[13] Operations involving the compound must be conducted in a chemical fume hood due to its toxicity and volatility, and it is incompatible with strong oxidizing agents such as chlorates, nitrates, or peroxides, which can lead to violent reactions.[14]Synthesis
Pyrolysis Methods
The primary method for synthesizing decaborane involves the pyrolysis of diborane (B₂H₆) at temperatures of 200–250 °C under low pressure conditions, typically in a continuous flow reactor to facilitate product separation.[15] This thermal decomposition proceeds via the overall reaction: $5 \mathrm{B_2H_6} \rightarrow \mathrm{B_{10}H_{14}} + 8 \mathrm{H_2} where decaborane (B₁₀H₁₄) forms as a key byproduct alongside hydrogen and lower yields of intermediate boranes.[16] Yields of decaborane based on boron conversion from diborane are relatively low, with the process optimized for industrial scalability during the mid-20th century.[17] This approach was developed in the 1950s as part of U.S. rocket programs, such as Projects Hermes, Zip, and High Energy Fuels (HEF), to produce high-energy borane compounds for propulsion systems.[17] These pyrolysis methods offer scalability for larger production volumes but are energy-intensive due to the high temperatures required and often result in mixed borane products that necessitate additional purification steps.[18]Borohydride-Based Synthesis
One prominent borohydride-based method for decaborane synthesis entails the reaction of sodium borohydride (NaBH₄) with boron trifluoride (BF₃), typically as its diethyl etherate complex, in diglyme solvent at 100–120 °C under an inert atmosphere. This process generates the tetradecahydroundecaborate(1-) anion ([B₁₁H₁₄]⁻) intermediate in situ, which is subsequently oxidized—often with potassium permanganate in acidic media—to yield decaborane (B₁₀H₁₄). The net stoichiometry can be approximated as 10 NaBH₄ + 5 BF₃ → B₁₀H₁₄ + 10 NaF + 6 H₂, though the reaction proceeds stepwise with hydrogen evolution and fluoride salt precipitation aiding product isolation via extraction into benzene or similar solvents.[19][20] An alternative approach utilizes the tetradecahydroundecaborate(1-) intermediate prepared directly from NaBH₄ via pyrolysis at elevated temperatures or acidification with boric acid in ethereal solvents, followed by the same mild oxidation step to afford decaborane. This variant avoids the direct use of BF₃, reducing handling hazards associated with the Lewis acid, and has been optimized for one-step formation of the intermediate from borohydride ions.[21][22] Simplified protocols for these methods, emphasizing scalability and safety, were detailed in Inorganic Syntheses (Volume 22), reporting yields up to 50% based on boron content, with the product typically isolated as colorless crystals after filtration and solvent evaporation. Recent refinements have focused on using mild organic oxidants such as aldehydes or ketones for the [B₁₁H₁₄]⁻ to B₁₀H₁₄ conversion, enhancing efficiency and purity through subsequent chromatographic separation on silica gel.[23][1] These solution-based routes are preferred for laboratory-scale production due to their lower temperatures and reduced volatility risks compared to gas-phase pyrolysis, enabling higher control over reaction conditions and facilitating downstream applications where high-purity decaborane is essential, such as in boron neutron capture therapy precursors.[1][24]Reactions
Adduct and Salt Formation
Decaborane(14), with its nido cluster structure featuring bridging hydrogens, exhibits pronounced Lewis acid character, enabling the formation of coordination compounds with Lewis bases. These adducts typically arise through the displacement of the B-H-B bridging hydrogens at the 6,9-positions, resulting in arachno-B₁₀H₁₂L₂ species where L represents the base ligand. For example, reaction with ammonia in benzene or toluene yields arachno-6,9-(NH₃)₂B₁₀H₁₂, a stable bis(ammonia) adduct characterized by X-ray crystallography showing coordination to apical boron atoms and a charge transfer from the ammonia ligands to the boron cluster. Similarly, triphenylphosphine forms arachno-6,9-(PPh₃)₂B₁₀H₁₂, where the bulky phosphine ligands occupy the same positions, as confirmed by single-crystal X-ray diffraction revealing a distorted square pyramidal coordination geometry around the ligand-bound borons. In addition to adduct formation, decaborane undergoes facile deprotonation to generate the nido-[B₁₀H₁₃]⁻ anion, a key ionic species in boron hydride chemistry. Strong bases such as sodium hydride in diethyl ether promote this reaction, liberating dihydrogen and forming sodium nido-decaboranate(13):\ce{B10H14 + NaH -> Na+[B10H13]- + H2}
This process exploits the relatively acidic bridging hydrogens on the boron cluster.[25] Milder organic bases like triethylamine also achieve deprotonation in non-aqueous media, yielding the triethylammonium salt:
\ce{B10H14 + Et3N -> [Et3NH]+[B10H13]-}
The resulting [B₁₀H₁₃]⁻ anion features a nido geometry with mobile, tautomeric bridging hydrogens that contribute to its reactivity as a synthetic intermediate.[25] Beyond coordination chemistry, decaborane functions as a mild reducing agent in organic transformations, notably reductive amination of carbonyl compounds. In methanol under nitrogen at room temperature, it reacts with aldehydes or ketones in the presence of primary or secondary amines to afford the corresponding amines in high yields (typically 80–95%), proceeding via in situ imine formation followed by selective hydride transfer from the boron cluster without over-reduction. This method's neutrality and compatibility with sensitive functional groups distinguish it from harsher metal hydride reagents.[26]