Boron triiodide (CAS#13517-10-7)
Boron triiodide (BI₃), also known as triiodoborane, is a trigonal-planar boron trihalide supplied as a colorless to reddish-brown crystalline solid with a pungent odor. It has a molecular weight of 391.52 g/mol, a melting point of approximately 43–51 °C depending on the measurement, and a boiling point of 210 °C. The compound is extremely moisture-sensitive and reacts violently with water, hydrolyzing in moist air; it is therefore handled and stored under an inert atmosphere of dry nitrogen or argon in light-protected containers at 2–8 °C.
BI₃ is the strongest Lewis acid among the boron trihalides, a property that arises from the weak π-donation from iodine to the empty boron p-orbital. This high Lewis acidity, combined with the lability of the nucleophilic iodide ligands, underpins its distinctive reactivity in organic synthesis. It also exhibits reductive behavior that is not fully explained by Lewis acidity alone, enabling transformations that are not accessible with BF₃, BCl₃, or BBr₃.
Applications
Boron triiodide is a versatile reagent in organic and materials chemistry, although its use has historically been constrained by cost and handling difficulty. Its primary synthetic applications include the cleavage of C–O bonds in ethers, esters, and alcohols, the conversion of alcohols to alkyl iodides, and the cleavage of silanes and halides. It also converts sulfonyl and sulfinyl compounds to disulfides and enables the borylation of triarylamines.
Beyond these classical transformations, BI₃ has emerged as a powerful reagent for reductive deoxygenation. It fully deoxygenates aromatic aldehydes and ketones, reducing aryl ketones to the corresponding methylene compounds and aromatic aldehydes to benzylic halides, through a metal-free mechanism that proceeds without heating. BI₃ also mediates the reduction of nitroarenes to anilines and the partial hydrogenation of acenes, further demonstrating its unique reducing capability. In materials science and industrial chemistry, BI₃ is used in coal liquefaction to facilitate the production of liquid hydrocarbons from coal, in the synthesis of transition metal borides for electrocatalysis, and in the preparation of boron carbonitride (BCN) nanowires.
Preparation Method
Two principal routes are used to prepare boron triiodide.
The direct combination of the elements can be performed by reacting boron with iodine at elevated temperatures. Yields as high as 70% have been reported from the reaction of boron and iodine at 900 °C, with the equilibrium limiting the yield to approximately 49% at 1050 °C. A related approach reacts boron trichloride with hydroiodic acid at high temperature: 3 HI + BCl₃ → BI₃ + 3 HCl.
The more practical laboratory and scalable method uses potassium borohydride (KBH₄) and molecular iodine (I₂) . High-purity BI₃ can be prepared by treating a solution of iodine in heptane at 80 °C with recrystallized KBH₄, affording the product in approximately 25% yield. A more recent one-pot, gram-scale protocol optimizes the KBH₄:I₂ ratio at 1:2 in cyclohexane, using modular glassware designed to manage the elevated pressure generated during the reaction; this procedure delivers BI₃ with a purity of approximately 93% and is suitable for routine laboratory production.
Safety Information
Boron triiodide is classified as a corrosive substance under GHS, with the signal word Danger and hazard statement H314 (Causes severe skin burns and eye damage) . It is assigned to Skin Corrosion Category 1B, Storage Class 8A (combustible corrosive hazardous materials), and WGK 3 (severe water hazard). The compound reacts violently with water, releasing hydrogen iodide and heat, and is moisture- and light-sensitive.
For safe handling, all operations should be conducted in a well-ventilated fume hood or inert-atmosphere glovebox. Personal protective equipment must include chemical splash goggles and a face shield, protective gloves, protective clothing, and respiratory protection suitable for acid gases and particulates (e.g., P3 filter cartridges). In case of eye contact, rinse cautiously with water for several minutes and seek immediate medical attention (P305+P351+P338, P310). For skin contact, remove contaminated clothing immediately and flush the affected area with copious water. Do not induce vomiting if swallowed; seek medical attention immediately. BI₃ must be stored in tightly sealed, corrosion-resistant containers under dry nitrogen or argon, protected from light and humidity, in a cool, dry, well-ventilated area separated from water, strong bases, and other incompatible substances. Release to the aquatic environment must be avoided.
