Subtle Niobium Michael Tarselli Reflects on the Intriguing Characteristics of a Rather Underrated Element, Niobium, in Its ‘Missing’ and Existing Forms

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in your element Subtle niobium Michael Tarselli reflects on the intriguing characteristics of a rather underrated element, niobium, in its ‘missing’ and existing forms.

ity poor niobium — it’s suffered nanoparticles and clays serve as solid identity crises for the better part of supports for oxidative transformations. Pthe past two centuries. Originally Homogeneous catalysis with niobium(v) dubbed columbium by discoverer Charles chloride can deprotect methyl ethers, Hatchett in 1801, niobium was exceedingly perform Friedel–Crafts cyclizations and difficult to separate from its group 5 snap together multicomponent reactions. neighbour tantalum, as they possess very A recent study6 capitalizes on niobium(v)’s similar chemical properties. Nearly 50 highly oxidizing nature to decontaminate confused years passed before Heinrich warfare agents: combined with hydrogen Rose ‘rediscovered’ niobium in the mineral peroxide, niobium–saponite clays can tantalite in 1844. He named it after the addition–fragmentation chain transfer readily convert mustard gas simulants into

goddess Niobe, whose father, the mythical (RAFT) polymerizations under irradiation non-toxic products. WWW.AUSTRIAN-MINT.AT MINT, AUSTRIAN king Tantalus, is the namesake of its with visible light, yielding well-controlled Niobium also inspires beautiful chemical elemental cousin. The name columbium, chains of the versatile poly(methyl artistry. Pamela Zurer wistfully describes7 however, was to linger alongside niobium methacrylate) polymer. Doped niobium how refractive, shimmering surfaces can be until the 1950s. Several other elements oxides, called ‘niobates’, serve as thin-film ‘dialled in’ on the surface of bare niobium believed to have been isolated from tantalite capacitors for solar cells and promote metal using anodizing baths to generate were later shown to be either niobium biofuel refining on zinc supports. Alloyed thin coatings of niobium oxide. Different or mixtures of niobium and tantalum. In with tin or strontium, niobium exhibits layer thicknesses produce different colours. fact, Hatchett’s columbium itself was also superconductor-like behaviour promising This technology has served both art and probably an Nb–Ta mixture. for energy storage materials. Your mobile commerce, with Austria minting green, pink, Much of the niobium believed to exist on phone may well contain niobium nitride, or purple collectable Euro coins (pictured). Earth — from compositional comparison a superconductor used in some tiny What does the future hold for this with meteorites — is actually ‘missing’; piezoelectric devices. underappreciated metal? No doubt its uses geologists theorize1 that it may be contained With five electrons in its outer valence in energy storage and organic synthesis will in deep silicate reservoirs in the planet’s shell, niobium enjoys rich redox chemistry increase, but perhaps most exciting is its core. Unequal distribution in the Earth’s through oxidation states ranging from −1 role in building novel inorganic frameworks crust of the remainder — mostly in Brazil to +5. This shell can be plucked away to — zeolites, semicrystals and core–shell and Canada — has prompted a vigorous reveal a highly oxophilic [Kr] core, useful particles — for biomass conversion and search for new deposits. Many have recently as a halide transfer agent, oxidant, or even harvesting solar power. After two centuries, been found in geopolitically sensitive areas, to activate C–H bonds. In a recent catalytic niobium may finally receive some long such as southern Afghanistan2 and the effort, a Nb(iii) complex has been shown overdue recognition. ❐ Democratic Republic of the Congo. to activate fluoroarenes’ C–F bonds4 — Despite this avid interest, niobium probably through two niobium complexes MICHAEL A. TARSELLI is in the Chemistry manages to fly under the very radars it helps arranging in an ‘inverted sandwich’ around Information Systems division, Novartis to build. In practical applications — for the benzene ring, twisting it with η6 Institutes for BioMedical Research, example as a steel strengthener, a refining coordination into an almost cyclohexane- 186 Massachusetts Avenue, Cambridge, additive or a mineral acid for metalwork like geometry, thus removing its aromaticity Massachusetts 02139, USA. — it always plays second fiddle to some and facilitating C–F cleavage. e-mail: [email protected] other transition metal. Yet element 41 Ask most organic chemists about References endows many materials with noteworthy niobium-promoted reactions and you’ll get 1. Wade, J. & Wood, B. J. Nature 409, 75–78 (2001). 3 properties. Niobium–zirconium alloys blank stares — its organic chemistry just 2. Stone, R. Science 345, 725–727 (2014). have been incorporated in non-toxic, hasn’t been widely explored. But niobium 3. Mantripagada, V. P., Lecka-Czernik, B., Ebraheim, N. A. non-immunogenic dental amalgams and still finds synthetic use in surprising places. & Jayasuriya, A. C. J. Biomed. Mater. Res. A 11, 3349–3364 (2013). 4. Gianetti, T. L., Bergman, R. G. & Arnold, J. Chem. Sci. bone implants. Benzyl alcohol-modified For example, the niobium(v) to (iii) 5, 2517–2524 (2014). niobium nanoparticles can initiate reversible reduction can promote reductive couplings5 5. Lacerda V. Jr, Araujo dos Santos, D., da Silva-Filho, L. C., of alkynes and various electrophiles — Greco, S. J. & dos Santos, R. B. Aldrichim. Acta 45, 19–27 (2012). 6. Carniato, F., Bisio, C., Psaro, R., Marchese, L. & Guidotti, M. much like that of manganese, titanium, or Angew. Chem. Int. Ed. 53, 10095–10098 (2014). nickel. In heterogeneous catalysis, niobium 7. Zurer, P. S. Chem. Eng. News 81, 106 (2003). Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Ce 180 NATURE CHEMISTRY | VOL 7 | FEBRUARY 2015 | www.nature.com/naturechemistry