Eric J. Schelter ponders on cerium's rather puzzling redox reactivity, and the varied practical applications that have emerged from it.
Cerium is one of the seventeen rare-earth metals (scandium, yttrium and the lanthanides La–Lu) but, despite the group's name, is fairly abundant — only slightly less so than copper in the Earth's crust. Element 58 is much more present in modern life than it may seem. Aqueous slurries consisting of ceria (CeO2) are used for the chemical–mechanical polishing of surfaces, including microelectronic device wafers, electronic displays, eye-glass lenses and other optical materials. Through chemical attack of basic sites on surfaces, ceria provides greater polishing rates than simple mechanical techniques.
The use of cerium in many applications comes from the interconversion between 4f1–Ce(III) and 4f0–Ce(IV) oxidation states — a unique behaviour among the rare-earth metals. Cerium(III/IV) redox chemistry makes oxides useful in heterogeneous catalysis through the storage and release of oxygen. The non-stoichiometric cerium oxide system CeO2−x has unusually high ion mobilities owing in part to octahedral oxygen vacancies in its lattice1.
Cerium oxides also serve in the promotion of the industrially important water-gas-shift reaction, and in solid-oxide fuel cells. Hydrocarbon fuels encounter element 58 at both the beginning and the end of their useful life: a zeolite (faujasite) impregnated with cerium and lanthanum serves as a petroleum-cracking catalyst in refining. And harmful fuel exhaust gases are converted into N2, CO2 and H2O using ceria and precious metals in automotive three-way catalytic converters. Owing to their uptake of reactive oxygen species, ceria nanoparticles are also being explored in medicinal applications as antioxidant therapeutics.
For the synthetic chemist, cerium is most familiar as a potent oxidizing agent in the ubiquitous ceric ammonium nitrate (CAN) — a drastic 'nuclear option' for oxidation reactions. In contrast to the utility of cerium oxides and the widespread use of CAN as a one-electron oxidant in both organic and inorganic chemistry, the coordination and organometallic chemistry of cerium(IV) is not particularly developed.
This lack of studies is likely to be linked to the fact that it is unexpectedly difficult to oxidize a cerium(III) coordination compound and isolate a cerium(IV) product in good yield. One possible reason is the slow rates of these oxidation reactions, which arise from the steric hindrance necessary to prepare discrete complexes comprising a single cerium cation. In our group, we recently tackled this issue by attempting to control the metal coordination sphere through heterobimetallic complexes2 (generalized structure pictured). Surrounding the cerium atom with an interlocked, flexible structure of lithium cations and aryloxide ligands kept it accessible and enabled its quick and easy conversion to cerium(IV), lending support to the idea that cerium oxidation reactions are under kinetic control.
Another fun aspect of element 58 engendered by its redox activity is the somewhat unconventional and controversial electronic structures of its compounds, such as cerocene (Ce(C8H8)2), an 8-fold symmetric, eclipsed sandwich complex. The most accurate picture of the valence at its cerium cation has remained somewhat ambiguous3. Energy decomposition analysis suggests a strong ionic interaction between the cerium centre and each cyclooctatetraene ring, and X-ray absorption spectroscopy indicates that cerocene has a ground state that is strongly multiconfigurational4 — so much so that the compound is now described as 'intermediate valent'. It is trapped between configurations of Ce(III) and Ce(IV) character that are quantum-mechanically admixed and comprise a strongly stabilized open-shell singlet ground state4,5.
This deceptively simple compound represents a stimulating case where the very human concept of a formal oxidation state fails to capture the essential essence of a molecule. The simultaneous local/non-local character of the 4f-electron in cerocene is reminiscent of f-element superconductors' behaviour6, and investigations on cerium compounds can provide insight on how local behaviour gives rise to exotic materials properties.
Alongside all this fertile academic inquiry are also excellent practical motivations for studying cerium chemistry. Cerium is found naturally in bastnaesite and monazite ores, together with other light rare-earth elements such as neodymium. Neodymium presents a high value owing to its parentage of Nd2Fe14B — a hard magnetic material that finds diverse uses, such as in wind-power generators. In a typical rare-earth-metal separations process, cerium, in as much as three-fold excess of neodymium by mass, is removed and discarded as a byproduct. This is an excellent opportunity to search for new applications and add value to cerium 'waste' — even more so when the tight economic margins of rare-earth-metals mining are taken into account.