Editorial Cite This: Inorg. Chem. 2019, 58, 1721−1723
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Actinide Chemistry at the Extreme
Inorg. Chem. 2019.58:1721-1723. Downloaded from pubs.acs.org by 46.148.120.181 on 02/04/19. For personal use only.
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World War II. In the 1990s, almost no one worked with elements heavier than uranium. Many nations lost all of their actinide chemists who were not working within government-run nuclear facilities, and even some of the elements themselves, such as curium, berkelium, californium, einsteinium, and fermium, stopped being prepared in nuclear reactors. This atrophy of the field is no longer true today. The field has been reborn from its ashes. This phoenix effect for the actinides came about for many reasons, including because we simply remain curious about the universe in which we live. We reach out to the stars and into the depths of the atom for the same reasonbecause we wonder and have an unquenchable thirst for knowledge. However, the rebirth of actinide chemistry was primarily practical in origin. Governments of nuclear-enabled nations realized that they had an aging workforce with no youth rising through the ranks. They recognized that capabilities in national security, energy production, and mitigation of the environmental effects of Cold War nuclear weapons development and testing, and, yes, accidents such as Chernobyl and Fukushima, would soon be lost. Committees were assembled, recommendations were made, and new radiochemistry laboratories were created.10 In academia, new ideas are being coupled to new characterization tools and significant advances in theory to allow researchers to demonstrate that these elements are not humdrum metals with uninteresting chemistry and properties but rather quite the opposite. Now we understand that actinide elements display a vast array of structures, bonding, reactivity, and physical properties that we were seemingly unaware of before. Yet, if we look back to the first explorers of extreme actinide chemistry, we find that, even with microgram quantities of material, photographic X-ray techniques, limited spectroscopy, and theories based largely on data fitting, these pioneers, albeit with some trepidation, predicted much of what we are verifying today, such as the increasing stability of the divalent oxidation state late in the actinide series. Actinides represent the final elements where bulk chemical and physical properties can be measured with our hands and are not limited to speculation on paper. Yet, two challenges are always present. First, all actinides are radioactive, some so much so that truly exotic samples only last a few hours. Second, elements beyond americium made synthetically are seldom available in quantities beyond a few milligrams. Both challenges have been overcome during the past 20 years with the advent of laboratory-based instrumentation that allows for the detailed interrogation of small samples. X-ray microsources coupled with area detectors allow for structure elucidation from microcrystals, and microspectrophotometers allow for detailed spectroscopic measurements on micrometer-sized samples. Using these techniques, as well as those that have come online at synchrotrons, neutron spallation sources, and high magnetic field facilities, we have begun to uncover the fine details of actinide compound properties.
he radioactive actinide elements residing underneath the main body of the Periodic Tablewhat we might consider its foundationconjure images of mushroom clouds, glowing test tubes, and superheroes awakening. While there is some truth to this view, as chemists working beyond the edge of nuclear stability, we have peered into the abyss to find elements whose realities are so complex and mystifying that we do not need to invent fanciful tales to remain a captive audience. This ACS Select Virtual Issue, “Actinide Chemistry at the Extreme”, serves as a snapshot to capture some of the breadth of basic and applied research on these f-block elements that the American Chemical Society and especially the journal Inorganic Chemistry have helped to communicate in recent years. All actinides share one common feature: They are all firmly in the relativistic regime. Their inner electrons move at significant fractions of the speed of light. Their high nuclear charge creates both large scalar relativistic effects and massive spin−orbit coupling.1 Toward the end of the actinide series, the 1s electrons are approaching velocities of about 70% of the speed of light with increases in their effective mass of about 40%. Some actinide compounds, such as Cm3+ bound in the iron-transport protein transferrin or Cf3+ dipicolinate complexes, even possess pronounced ligand-field splitting akin to that found in the dblock transition metals.2,3 Moreover, these effects increase in magnitude in a nonlinear manner, yielding step changes in the chemical and physical properties between neighboring actinides, as was found to occur between plutonium and americium during the Manhattan Project and, more recently, between curium and californium.4−8 The results include alterations and splitting of orbital energies and shapes in ways that chemists are unaccustomed to considering. Instead of probing a single type of valence orbital that might play a role in chemistry as one would do in the d block, a host of frontier orbitals, from filled semicore 6p orbitals (with staggering spin−orbit splitting of about 10 eV) to more traditionally addressed 5f and 6d orbitals and all the way to the outlying 7s and 7p orbitals, must all be contemplated when describing the nature of the electronic structure in actinide compounds and complexes. We have become inured into perceiving bonds in the traditional way that an organic chemist thinks about them using chalkboard cartoons of overlapping balloons, which makes sense for hydrocarbons. Yet, actinides lie in an extreme realm where simple concepts concerning the nature of chemical bonds break down7 and where Occam’s razorthe philosophical approach that the simplest solution tends to be the correct oneleads to failure. Thus, our ability to use periodic trends as a predictive tool tends to unravel as we traverse the series from actinium to lawrencium.9 Perhaps the most difficult part of creating this Virtual Issue is the down-selection process needed to stay within the article number limit constraints. Herein lies an important aspect of the field: 20 years ago, few f-block chemists remained from the original cadre that explored these elements as we sought to understand and tame radioactivity just before, during, and after © 2019 American Chemical Society
Published: February 4, 2019 1721
DOI: 10.1021/acs.inorgchem.8b03603 Inorg. Chem. 2019, 58, 1721−1723
Inorganic Chemistry
Editorial
While the theoretical framework for incorporating relativistic effects into quantum mechanics has existed for almost a century, thanks to Paul Dirac,11 actually carrying out these calculations on molecules and materials containing actinides using ab initio wave-function methods that capture relativistic effects has only recently become practical.12 The coupling of these experimental and computational efforts has not only changed how our peers view the chemistry of actinides but also captured the public’s imagination in a way that has not happened in decades.13−15 We find ourselves now looking back at tables in research tomes that have gathered dust to find molecules once thought of as welldefined that are actually far more perplexingfor example, benchmark complexes such as [AmCl6]3−, the earliest harbinger of 5f orbital bonding.16,17 We are revisiting simple metal complexants, such as borohydrides developed during the Manhattan Project, for quite different purposes, including everything from precursors to reactive molecules to agents for delivering life-saving radiopharmaceuticals. In only two dozen articles that we present here, it is challenging to capture all of the facets of these unusual elements. In a humble attempt to highlight the breadth of actinide chemistry, this collection includes articles describing muchneeded developments in closed nuclear fuel cycles that could enhance utilization of energy resources and limit our impact on the environment, tracking biological uptake of actinides using the actinides themselves as luminescent probes, revisiting the chemistry of the most poorly understood naturally occurring actinide, protactinium, fine-tuning the reactivity in thorium and uranium complexes for activating strong bonds, and preparing compounds with the later actinides, where it becomes a race against time to capture their chemistry before, like a dying star, they violently fade away, leaving only a remnant of themselves to remind us of what was once there.
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FSU, serving as a member of The Department of Energy’s Chemical Sciences Council. Among his awards and recognitions, AlbrechtSchmitt has received the ACS Nobel Laureate Signature Award for Graduate Education in Chemistry, the ACS Southern Chemist Award, and ACS Glenn T. Seaborg Award for Nuclear Chemistry. His research focuses on lanthanide and actinide chemistry, especially with transuranium elements, structure and bonding, radiochemistry, crystallography, materials chemistry, and nuclear energy.
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REFERENCES
(1) Pyykkö, P. Relativistic Effects in Chemistry: More Common Than You Thought. Annu. Rev. Phys. Chem. 2012, 63, 45−64. (2) Cary, S. K.; Silver, M. A.; Liu, G.; Wang, J. C.; Bogart, J. A.; Stritzinger, J. T.; Arico, A. A.; Hanson, K.; Schelter, E. J.; AlbrechtSchmitt, T. E. Spontaneous Partitioning of Californium from Curium: Curious Cases from the Crystallization of Curium Coordination Complexes. Inorg. Chem. 2015, 54, 11399−11404. (3) Sturzbecher-Hoehne, M.; Goujon, C.; Deblonde, G. J.-P.; Mason, A. B.; Abergel, R. A. Sensitizing Curium Luminescence through an Antenna Protein To Investigate Biological Actinide Transport Mechanisms. J. Am. Chem. Soc. 2013, 135, 2676−2683. (4) Heathman, S.; Le Bihan, T.; Yagoubi, S.; Johansson, B.; Ahuja, R. Structural investigation of californium under pressure. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 214111. (5) Deblonde, G. J.-P.; Kelley, M. P.; Su, J.; Batista, E. R.; Yang, P.; Booth, C. H.; Abergel, R. J. Spectroscopic and Computational Characterization of Diethyltriaminepentaacetic Acid/Transplutonium Chelates: Evidencing Heterogeneity in the Heavy Actinide(III) Series. Angew. Chem., Int. Ed. 2018, 57, 4521−4526. (6) Cary, S. K.; Vasiliu, M.; Baumbach, R. E.; Stritzinger, J. T.; Green, T. D.; Diefenbach, K.; Cross, J. N.; Knappenberger, K. L.; Liu, G.; Silver, M. A.; DePrince, A. E., III; Polinski, M. J.; Van Cleve, S. M.; House, J. H.; Kikugawa, N.; Gallagher, A.; Arico, A. A.; Dixon, D. A.; Albrecht-Schmitt, T. E. Emergence of Californium as the Second Transitional Element in the Actinide Series,. Nat. Commun. 2015, 6, 6827−34. (7) Polinski, M. J.; Garner, E. B., III; Maurice, R.; Planas, N.; Stritzinger, J. T.; Parker, T. G.; Cross, J. N.; Green, T. D.; Alekseev, E. V.; Van Cleve, S. M.; Depmeier, W.; Gagliardi, L.; Shatruk, M.; Knappenberger, K. L.; Liu, G.; Skanthakumar, S.; Soderholm, L.; Dixon, D. A.; Albrecht-Schmitt, T. E. Unusual Structure, Properties, and Bonding in a Californium Borate,. Nat. Chem. 2014, 6, 387−392. (8) Silver, M. A.; Cary, S. K.; Johnson, J. A.; Baumbach, R. E.; Arico, A. A.; Luckey, M.; Urban, M.; Wang, J. C.; Polinski, M. J.; Chemey, A.; Liu, G.; Chen, K.-W.; Van Cleve, S. M.; Marsh, M. L.; Eaton, T. M.; van de Burgt, B.; Gray, A. L.; Hobart, D. E.; Hanson, K.; Maron, L.; Gendron, F.; Autschbach, J.; Speldrich, M.; Kogerler, P.; Yang, P.; Braley, J.; Albrecht-Schmitt, T. E. Characterization of berkelium(III) dipicolinate and borate compounds in solution and the solid state,. Science 2016, 353, aaf3762−aaf3762. (9) Sato, T. K.; Asai, M.; Borschevsky, A.; Stora, T.; Sato, N.; Kaneya, Y.; Tsukada, K.; Düllmann, Ch. E.; Eberhardt, K.; Eliav, E.; Ichikawa, S.; Kaldor, U.; Kratz, J. V.; Miyashita, S.; Nagame, Y.; Ooe, K.; Osa, A.; Renisch, D.; Runke, J.; Schädel, M.; Thörle-Pospiech, P.; Toyoshima, A.; Trautmann, N. Measurement of the first ionization potential of lawrencium, element 103,. Nature 2015, 520, 209−211. (10) Assuring a Future U.S.-Based Nuclear and Radiochemistry Expertise; National Academies Press, 2012; DOI: 10.17226/13308. (11) Dirac, P. A. M. On the Theory of Quantum Mechanics. Proc. R. Soc. London, Ser. A 1926, 112, 661−677. (12) Knecht, S.; Jensen, H. J. A.; Saue, T. Relativistic quantum chemical calculations show that the uranium molecule U2 has a quadruple bond. Nat. Chem. 2019, in press. 1140. (13) Science Daily, April 16, 2016, “Discovery Changes How Scientists Examine Rarest Elements of Periodic Table”, www. sciencedaily.com/releases/2015/04/150416083353.htm. Accessed Dec 21, 2018.
Thomas E. Albrecht-Schmitt* AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Thomas E. Albrecht-Schmitt: 0000-0002-2989-3311 Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS. Biography
Thomas E. Albrecht-Schmitt holds the Gregory R. Choppin Chair in Chemistry at Florida State University (FSU) and is Director of the Department of Energy’s Center for Actinide Science & Technology at 1722
DOI: 10.1021/acs.inorgchem.8b03603 Inorg. Chem. 2019, 58, 1721−1723
Inorganic Chemistry
Editorial
(14) Kozimor, S. A. Actinide Chemistry Reveals Unusual Bonds and Offers a Novel Form of Cancer Treatment. ACS Cent. Sci. 2017, 3 (6), 510−511. (15) MIT Technology Review: Innovators under 35; Biotechnology & Medicine, Rebecca Abergel, A pill to Decontaminate People after a Radiation Exposure,https://www.innovatorsunder35.com/the-list/ rebecca-abergel/. Accessed Dec 21, 2018. (16) Cross, J. N.; Su, J.; Batista, E. R.; Cary, S. C.; Evans, W. J.; Kozimor, S. A.; Mocko, V.; Scott, B. L.; Stein, B. W.; Windorff, C. J.; Yang, P. Covalency in Americium(III) Hexachloride. J. Am. Chem. Soc. 2017, 139, 8667−8677. (17) Diamond, R. M.; Street, K., Jr.; Seaborg, G. T. An Ion-exchange Study of Possible Hybridized 5f Bonding in the Actinides. J. Am. Chem. Soc. 1954, 76, 1461−1469.
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DOI: 10.1021/acs.inorgchem.8b03603 Inorg. Chem. 2019, 58, 1721−1723
