Among the actinides, uranium and thorium are the most abundant in the earth’s crust due their long-lived isotopes (thorium-232 and uranium-238). These tend to be found in the form of oxides such as thoria (ThO2) and uraninite (UO2). These properties have allowed the chemistry of both elements to flourish over the years, garnering a variety of applications. Within the scope of thorium, its complexes have found use in catalysis and materials science, sparking wider interest from the scientific community.[1-3] Within the scope of anhydrous thorium chemistry, ThBr4(thf)4 and ThCl4 were the most typically used precursors. However, their syntheses were not practical, since the former requires access to thorium metal (not easy to obtain), and the latter requires ThO2 under heat-intensive conditions unamenable to the average laboratory.
In 2010, the group of Jaqueline Kiplinger at Los Alamos National Laboratory, reported a highly practical synthesis for ThCl4(dme)2; an easily derivatizable, soluble Th(IV) precursor. Their procedure starts from commercially available thorium(IV) nitrate and involves mild reaction conditions that furnish the product in multigram scales.[4]
Procedure:[4] Target compound was synthesized in two steps:
Synthesis of tetrachlorothorium(IV) tetrahydrate: In air, a 500mL round-bottom flask was charged with Th(NO3)4(H2O)5 (20.0g, 35.1mmol), and 100mL of concentrated hydrochloric acid were added under magnetic stirring. After dissolution, the flask was heated under reflux for 6h causing evolution of NOx fumes. Once the reaction mixture became colorless, the solvent was removed in vacuo, yielding ThCl4(H2O)4 as a white solid (reported yield: 8.1g, 100%).
Synthesis of tetrachlorobis(dimethoxyethane)thorium(IV): A 500mL two-neck round-bottom flask with a stir bar was charged with ThCl4(H2O)4 (15.5g, 34.8mmol) and 100mL of anhydrous DME. The flask was then attached to a swivel-frit apparatus on a dual-manifold vacuum/Schlenk line and kept under argon. Trimethylsilyl chloride (70mL, 557mmol) was added dropwise (via syringe or dropping funnel), which led to formation of a white precipitate. The mixture was then stirred at 50degC overnight, and allowed to cool to room-temperature. Solvent was concentrated in vacuo down to ca. 20mL, and 50mL of anhydrous hexanes were added to precipitate the product. Filtration and drying in vacuo gave ThCl4(dme)2 as a white crystalline solid (reported yield: 18.3g, 95%). The solid was then transferred into the glovebox for storage.
Notes:
- Thorium-232 is a weak emitter of alpha-radiation, with a half-life of 1.4E10 years. Proper radiation equipment should be used when handling these types of compounds. ThCl4(dme)2 is a paramagnetic, moisture sensitive, white solid and should be stored under argon/nitrogen to prevent decomposition. It is soluble in arene and ethereal solvents, and insoluble in hexane. Furthermore, its thermal stability is in stark contrast to the THF-bound congener (ThCl4(thf)3.5). The latter is thermally unstable at room temperature and decomposes via ring-opening and polymerization of THF.
- Intermediate ThCl4(H2O)4 is insoluble in most organic solvents. It is moderately soluble in 1,4-dioxane.
- Anhydrous DME can be purchased from chemical vendors, or dried with four-Angstrom, activated molecular sieves for at least 48h prior to synthesis (for best results, dry for 48h, transfer solvent into a fresh batch of sieves, and dry for another 48h). Molecular sieves should be activated by heating under dynamic vacuum at 250degC for 48h prior to use. Thorium nitrate can be purchased from chemical vendors (Millipore-Sigma, Strem, American Elements).
- This synthetic protocol is patented by the authors until 2031.[5]
[1] Burns, C. J.; Eisen, M. S. The Chemistry of the Actinide and Transactinide Elements, 3rd edition, ed. Morss, L. R.; Edelstein, N. M.; Fuger, J. Springer, The Netherlands, 2006, 2911-3012.
[2] Arnold, J.; Gianetti, T. L.; Kashtan, Y. Thorium lends a fiery hand. Nature Chemistry 2014, 6, 554.
[3] Tutson, C. D.; Gorden, A. E. V. Thorium coordination: A comprehensive review based on coordination number. Coord. Chem. Rev. 2017, 333, 27-43.
[4] Cantat, T.; Scott, B. L.; Kiplinger, J. L. Chem. Commun. 2010, 46, 919-921.
[5] Kiplinger, J. L.; Cantat, T. Method of synthesis of anhydrous thorium(iv) complexes. U.S. Patent US20110282039A1, November 17, 2011.