Thorium Lends a Fiery Hand John Arnold, Thomas L

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in your element Thorium lends a fiery hand John Arnold, Thomas L. Gianetti and Yannai Kashtan look back on thorium’s chemistry, and look forward to harnessing its nuclear potential.

n 1828, the chemist Jöns Jacob Berzelius exclusively 232Th, thus no costly isotopic received for inspection an unusual ore, enrichment process would be necessary, and Iwhich he named thoria, after Thor, the this would present a significant potential Scandinavian god of thunder and war. The benefit over today’s uranium-based reactors. metal later isolated from it was named Perhaps the biggest advantages of thorium, which has persisted to the present thorium reactors are their safety and their day, even though the ore has changed its relatively reduced environmental impact. name slightly to thorite. Although natural Unlike uranium-based reactors, which thorium is radioactive, almost all of its produce waste that remains harmful for

present uses exploit its chemical, rather than ALEX WING thousands of years, 83% of waste from a its nuclear, properties. proposed liquid fluoride thorium reactor The first practical use for thorium was Interestingly, experimental and computational will become safe within 10 years, and the found 63 years after Berzelius’s discovery. evidence suggests the single unpaired remaining 17% after 300 years. Thorium

Carl Auer von Welsbach found that a ‘mantle’, electron in the ThCp3 complex resides not reactors aren’t just an abstract concept 2 made of 99% ThO2 and 1% CeO2, surrounding in a 5f orbital, but in the 6dz (refs 6,7). either: the Indian government has a strong a gas flame would radiate white light more Unfortunately, despite the existence of a interest in thorium power because India has efficiently than an open flame, and this gas- formally trivalent species, no redox chemistry approximately a third of the world’s thorium lamp technology quickly spread throughout has yet been observed with thorium. reserves, and in 2002 the government issued Europe. Although these lamps have since been Recent research has led to promising approval to start construction of a prototype superseded, thorium compounds are still used thorium-based materials — such as thorium fast-breeder plant. today as catalysts for petroleum cracking, thorium- and copper-doped magnetite — to Thorium has proved itself as a catalyst sulfuric acid synthesis and in the Ostwald catalyse the activation of small molecules. and a great refractive material. With the process for nitric acid synthesis. Thorium These materials could eventually replace the effects of global climate change increasing also features, because of its strength and creep more traditional chromium- and copper- with every passing year, hopefully more resistance at high temperatures, in the Mag– doped magnetite catalysts used to form research and allocated resources will help Thor alloy used in aircraft and rocket engines. carbon dioxide and hydrogen from carbon thorium realize its great untapped potential 8 Additionally, ThO2 has a high refractive index monoxide and water . Along similar lines, and become a truly revolutionary material and low dispersion and is therefore used in several research groups have synthesized in our energy economy. ❐ high-quality optical lenses1. well-defined thorium complexes to study Among its remarkable chemical and their bonding and reactivity. Recent reports JOHN ARNOLD, THOMAS L. GIANETTI and physical properties, thorium metal is liquid include the synthesis and characterization YANNAI KASHTAN are at The University over the widest temperature range of any of thorium complexes using corrole ligands9 of California, Department of Chemistry,

element, and ThO2 has the highest melting and a dihalide thorium complex supported Berkeley, California 9470-1460, USA. point of any known oxide2. Being the first true by trans-calix[2]benzene[2]pyrrolide, e-mail: [email protected] actinide, thorium still has an empty 5f orbital, which on reduction undergoes double aryl and therefore Th(iv) is the most stable and metallation of the ligand10. References plentiful oxidation state. Because of its large Although most of the current research 1. Patnaik, P. Handbook of Inorganic Chemicals 931 (McGraw-Hill, 2003). size, thorium’s coordination chemistry is also is focused on its chemical characteristics, 2. Gray, T. & Field, S. Elements Vault 88–89 (2011). quite remarkable, with Th(NO3)4.5H2O being perhaps thorium’s most revolutionary 3. Greenwood, N. N. & Earnshaw, A. Chemistry of the Elements the first recorded example of an 11-coordinate potential use involves its nuclear chemistry. 2nd edn, 1276 (Butterworth-Heinemann, 1997). 3 4. Klapötke, T. M. & Schulz, A. Polyhedron 16, 989–991 (1997). complex . Additionally, Th(iii) is also an The theoretical feasibility of a thorium- 5. Blake, P. C., Lappert, M. F., Atwood, J. L. & Zhang, H. J. Chem. accessible oxidation state and has been based nuclear reactor has long been Soc. Chem. Commun. 1148–1149 (1986). reported4 as free Th3+ in an aqueous solution recognized. However, technical difficulties 6. Kot, W. K., Shalimoff, G. V., Edelstein N. M., Edelman, M. A. & Lappert, M. F. J. Am. Chem. Soc. 110, 986–987 (1988). of ThCl4 and HN3. Ligand-stabilized Th(iii) as well as prioritized interest in uranium 7. Bursten, B. E., Rhodes, L. F. & Strittmatter, R. J. J. Am. Chem. Soc. complexes are also known and include ThCp3 reactors (which some posit stems from 111, 2756–2758 (1988). and the bis(trimethylsilyl) Cp analogue5. uranium reactors’ greater ability to 8. Costa, J. L., Marchetti, G. S. & Rangel, M. C. Catal. Today breed plutonium for fission bombs) have 77, 205–213 (2002). 9. Ward, A. L., Buckley, H. L., Lukens, W. W. & Arnold, J. J. Am. prevented the development of commercial Chem. Soc. 135, 13965–13971 (2013). thorium reactors. Natural thorium is almost 10. Arnold, P. L. et al. Chem. Sci. 5, 756–765 (2014). Fr Ra Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Rf Db Sg Bh 554 NATURE CHEMISTRY | VOL 6 | JUNE 2014 | www.nature.com/naturechemistry