Conversion of Americia to Anhydrous Trivalent Americium Halides

Communication Cite This: Organometallics XXXX, XXX, XXX−XXX

pubs.acs.org/Organometallics

Conversion of Americia to Anhydrous Trivalent Americium Halides Shane S. Galley,† Joseph M. Sperling,‡ Cory J. Windorff,‡ Matthias Zeller,† Thomas E. Albrecht-Schmitt,‡ and Suzanne C. Bart*,† †

H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States

‡

Organometallics Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/18/19. For personal use only.

S Supporting Information *

ABSTRACT: Anhydrous americium(III) starting materials were prepared from americia (AmO2) using inexpensive, commercially available reagents and mild reaction conditions. [AmCl(μ-Cl)2(THF)2]n (1-Am) and AmBr3(THF)4 (2-Am) were isolated and characterized by electronic absorption spectroscopy and X-ray diffraction. These new starting materials are soluble in organic solvents, making them useful synthons for nonaqueous organometallic americium chemistry.

O

synthesis of UI3(THF)4, and generally only plutonium is available in this form. Over the past decade, efforts have been made in synthesizing anhydrous transuranium materials such as AnCl4(DME)2 (An: Np, Pu).7,9−12 For instance, Gaunt and co-workers reported anhydrous actinide halide starting materials made from a stock solution of AnIV in HCl (An: Np, Pu).13 Prior to this, Pu(III) starting material was only accessible from Pu0 metal. Americium predominately exists in the trivalent oxidation state, but limited access to metallic americium therefore makes americia (AmO2) the only viable Am source. Previously, [AmCl6]3− was synthesized as its tetraphenylphosphonium salt, but it is not a good candidate as a synthon for organoamericium chemistry due to the stability of this anion.14 Thus, generation of a low-valent americium starting material from the corresponding oxide would be a significant advance that could facilitate development of americium organometallic and coordination chemistry. This was previously explored by Pappalardo, Carnall, and Fields, who were able to generate AmX3 (X = Cl, Br, I) by heating AmO2 in molten AlX3.15 While this supplied pure material, this method is cumbersome and required sublimation of the corresponding americium halides at 750 °C. Herein, we report an effective synthetic pathway to generate anhydrous americium halide synthons starting from americia under mild conditions using commercially available, inexpensive reagents. Synthesis of halide derivatives from AmO2 was achieved by first dissolving AmO2 in concentrated hydrochloric acid (Scheme 1).16 After swirling, the solution was slowly evaporated to dryness, yielding a yellow residue. Following transfer to an argon atmosphere glovebox, the yellow residue

rganometallic chemistry has long been dominated by dblock metals on the periodic table. An interest in organoactinide derivatives was ignited during the last century with the Manhattan Project,1 as it was proposed that organouranium derivatives could have increased volatility that would be useful in isotope enrichment.2,3 Several years later, the discovery of ferrocene (1951) marked a rebirth in organometallic chemistry for all metals,4 prompting the synthesis of the first organouranium species, Cp3UCl, from the laboratory of Geoffrey Wilkinson.5 Over the next 40 years, the organometallic chemistry of uranium focused on the +4 oxidation state, due to the thermodynamic stability of this form, as well as its availability as UCl4.6 The organometallic chemistry of uranium(III) has lagged behind its tetravalent counterpart, mainly due to this material only being accessible from alkali-metal reduction of UCl4. In 1994, Clark and Sattelberger reported the synthesis of UI3(THF)4, a convenient trivalent precursor generated under mild conditions from available uranium metal.7 This material revolutionized the fields of low-valent uranium organometallic and coordination chemistries, as it facilitated the creation of a wide array of new low-valent uranium compounds.6 Relative to uranium, the chemistry of the later actinides has been underexplored.8 While these elements are traditionally difficult to handle due to their increased levels of radioactivity in comparison to (depleted) uranium, new technologies and renewed interests have enabled handling of these elements safely in a laboratory environment. A continued hindrance to further develop organoactinide chemistry in this part of the periodic table is the lack of available starting materials.8 These rare metals are sourced as their metal oxide forms, AnO2 (An: Np, Pu, Am), which must be transformed to their corresponding halide forms by cleavage of the strong metal− oxygen bonds. The lack of pure metal sources of transuranium elements precludes use of the same strategy proven for the © XXXX American Chemical Society

Received: November 16, 2018

A

DOI: 10.1021/acs.organomet.8b00840 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

AmCl(μ-Cl)2(THF)2 is analogous to those of its lanthanide derivatives, LnCl(μ-Cl)2(THF)2 (Ln: La, Ce, Nd, Pr, Sm, Dy, Yb) .17−19 The optimal synthesis for LnCl3(THF)n was found to be sonication of lanthanide metals with hexachloroethane, rather than treatment with Me3SiCl as was done here for americium.19 Synthesis of the bromide derivative was attempted in a manner analogous to that for 1-Am. AmO2 was dissolved in concentrated hydrobromic acid; subsequent concentration and evaporation of the solution to dryness revealed a yellow-orange residue. Under an argon environment, this residue was treated with bromotrimethylsilane, Me3SiBr, to remove any trace amounts of water. After workup the remaining residue was dissolved in THF and layered with pentane, eventually yielding peach-colored crystals of AmBr3(THF)4 (2-Am). X-ray diffraction of one of these crystals followed by refinement of the data revealed that 2-Am is isomorphous with both UI3(THF)4 and PuBr3(THF)4.7,10 2-Am crystallizes in the triclinic space group P1̅ and, like 1-Am, exhibits a sevencoordinate distorted-pentagonal-bipyramidal geometry around the americium. All four THF ligands along with one bromide are coordinated in the equatorial plane, while the other two bromides occupy the axial positions at an angle of 166.65(2)°. This distortion from linearity arises from the repulsion of the bromide in the equatorial position and is on par with what has been observed in the Pu analogue (164.960(17)°). The average Am−Br bond distance is 2.8426(6) Å, whereas that of the Am−O distances is 2.490(4) Å. Once again, no literature comparison exists for the Am−Br distances, but the Am−O average is similar to that in 1-Am. In order to assess the electronic structures of 1-Am and 2Am, electronic absorption spectroscopy was utilized. UV−vis− NIR absorption spectra were collected from a single crystal of both 1-Am and 2-Am (Figure 2). For both compounds,

Scheme 1. Synthethic Route of 1-Am and 2-Am

was treated with chlorotrimethylsilane, Me3SiCl, to generate a completely anhydrous material. After stirring/standing, the volatiles were removed in vacuo, completely drying the material. The residue was then brought up in THF. Slow evaporation of the THF into a layer of polybutenes produced yellow crystals assigned as [AmCl(μ-Cl)2(THF)2]n (1-Am). Although quantification of the crystals was not possible, they were evenly spread over the bottom of the 7 mL vial, indicating that a good yield was obtained from the 5 mg of AmO2 used. To establish the structural parameters for this new americium compound, one of these crystals was characterized using X-ray diffraction (Figure 1). Crystallographic data of 1-

Figure 1. Thermal ellipsoid plots of [AmCl(μ-Cl)2(THF)2]n (1-Am) (left) and AmBr3(THF)4 (2-Am) (right) (pink, Am; green, Cl; orange, Br; red, O; black, C) shown at 30% probability. Hydrogen atoms and cocrystallized solvents have been omitted for clarity.

Am reveal an extended structure made up of chloride-bridged [AmCl(μ-Cl)2(THF)2]n units which crystallizes in the triclinic space group P1̅. The molecular structure shows a sevencoordinate Am center in a pentagonal-bipyramidal geometry with five chlorides and two THF molecules, one of each in the axial positions featuring an O−Am1−Cl angle of 174.11(11)°. Interestingly, the structure of 1-Am contains bridging chlorides in a μ2 fashion in the equatorial plane, ultimately generating 1D chains in an extended structure. The Am−Cl distances range from 2.6414(14) to 2.8075(13) Å (Table 1), in correspondence with the previously reported Am−Cl bonds in trivalent [AmCl6]3− that has distances ranging from 2.713(3) to 2.752(3) Å. The Am−O bond lengths are 2.473(4) and 2.488(4) Å. No dative Am−O bonds have been previously reported for an appropriate comparison. This structure for Table 1. Selected Bond Lengths (Å) of 1-Am and 2-Ama 1-Am Am−Cl1 Am−Cl1i Am−Cl2ii Am−Cl2 Am−Cl3 Am−O1 Am−O2

2-Am 2.7808(13) 2.8321(13) 2.7856(14) 2.8075(13) 2.6414(14) 2.473(4) 2.488(4)

Am−Br1 Am−Br2 Am−Br3 Am−O1 Am−O2 Am−O3 Am−O4

Figure 2. Electronic absorption spectra of 1-Am and 2-Am collected on single crystals from 350 to 1100 nm. 2.8222(6) 2.8445(6) 2.8610(6) 2.466(4) 2.467(4) 2.493(4) 2.533(4)

absorptions visible at 506 and 818 nm are similar to those observed for [AmCl6]3−, which are found at 504 and 815 nm.14 All are consistent with data previously recorded for the solventfree AmX3 series at liquid helium temperature, which showed absorbances for AmCl3 at 509 and 812 nm, as well as 510 and 823 nm for AmBr3.15 These absorptions are fingerprint peaks that are characteristic of Am(III) ions15,20 and are associated

Symmetry operators: (i) −x, −y, −z; (ii) −x; 1 − y, −z.

a

B

DOI: 10.1021/acs.organomet.8b00840 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics with excitations from Am(III) 7F0′ to 5L6′ (∼500 nm) and 7F5′ (∼800 nm).14,15,20 Both 1-Am and 2-Am contribute to a rather small family of americium coordination compounds and give broader insight into the fundamental electronic, bonding, and structural properties of nonaqueous halide derivatives of the actinides.21−29 With the analogous nature of the structures of LnCl(μ-Cl)2(THF)2 and 1-Am, comparisons of chemical and physical properties can now be made between lanthanides and actinides.18,19 Similarly, 2-Am can be studied alongside PuBr3(THF)4 and LnCl3(THF)x compounds to highlight periodic trends as the actinide series is traversed. Currently the predictive tools to expand our knowledge of the periodic table are in their infancy; thus, generation of experimental data is essential. In summary, we have reported a new synthetic route for generation of two anhydrous Am(III) starting materials from the corresponding AmO2, along with structural and spectroscopic characterization. Even though 1-Am is an extended network rather than a molecular structure like 2-Am, both materials are readily soluble in organic solvents. These americium halide solvates represent valuable starting materials to facilitate the development of the nonaqueous organometallic and coordination chemistry of americium.

■

the Radiochemical Engineering and Development Center at Oak Ridge National Laboratory.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00840. Experimental and synthetic details and X-ray crystallographic data (PDF) Accession Codes

CCDC 1557708 and 1877647 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

REFERENCES

(1) Gilman, H. Personal Notes on More Than One-half Century of Organometallic Chemistry. Adv. Organomet. Chem. 1969, 7, 1−52. (2) Gilman, H. Adv. Organomet. Chem. 1969, 7, 1−57. (3) Gilman, H.; Jones, R. G.; Bindschadler, E.; Blume, D.; Karmas, G.; Martin, G. A.; Nobis, J. F.; Thirtle, J. R.; Yale, H. L.; Yoeman, F. A. Organic Compounds of Uranium. I. 1,3-Dicarbonyl Chelates. J. Am. Chem. Soc. 1956, 78, 2790−2792. (4) Kealy, T. J.; Pauson, P. L. A New Type of Organo-Iron Compound. Nature 1951, 168, 1039−40. (5) Reynolds, L. T.; Wilkinson, G. π-Cyclopentadienyl Compounds of Uranium (IV) and Thorium (IV). J. Inorg. Nucl. Chem. 1956, 2, 246−53. (6) Johnson, S. A.; Bart, S. C. Achievements in Uranium Alkyl Chemistry: Celebrating Sixty Years of Synthetic Pursuits. Dalton Trans. 2015, 44, 7710−7726. (7) Avens, L. R.; Bott, S. G.; Clark, D. L.; Sattelberger, A. P.; Watkin, J. G.; Zwick, B. D. A Convenient Entry into Trivalent Actinide Chemistry: Synthesis and Characterization of AnI3(THF)4 and an[N(SiMe3)2]3 (An = U, Np, Pu). Inorg. Chem. 1994, 33, 2248−56. (8) Jones, M. B.; Gaunt, A. J. Recent Developments in Synthesis and Structural Chemistry of Nonaqueous Actinide Complexes. Chem. Rev. 2013, 113, 1137−1198. (9) Gaunt, A. J.; Enriquez, A. E.; Reilly, S. D.; Scott, B. L.; Neu, M. P. Structural Characterization of Pu[N(SiMe3)2]3, a Synthetically Useful Nonaqueous Plutonium(III) Precursor. Inorg. Chem. 2008, 47, 26−28. (10) Gaunt, A. J.; Reilly, S. D.; Enriquez, A. E.; Hayton, T. W.; Boncella, J. M.; Scott, B. L.; Neu, M. P. Low-Valent Molecular Plutonium Halide Complexes. Inorg. Chem. 2008, 47, 8412−8419. (11) Minasian, S. G.; Boland, K. S.; Feller, R. K.; Gaunt, A. J.; Kozimor, S. A.; May, I.; Reilly, S. D.; Scott, B. L.; Shuh, D. K. Synthesis and Structure of (Ph4P)2MCl6 (M = Ti, Zr, Hf, Th, U, Np, Pu). Inorg. Chem. 2012, 51, 5728−5736. (12) Reilly, S. D.; Scott, B. L.; Gaunt, A. J. [N(NBu)4]2[Pu(NO3)6] and [N(NBu)4]2[PuCl6]: Starting Materials to Facilitate Nonaqueous Plutonium(IV) Chemistry. Inorg. Chem. 2012, 51, 9165−9167. (13) Reilly, S. D.; Brown, J. L.; Scott, B. L.; Gaunt, A. J. Synthesis and Characterization of NpCl4(DME)2 and PuCl4(DME)2 Neutral Transuranic An(IV) Starting Materials. Dalton Trans. 2014, 43, 1498−1501. (14) Cross, J. N.; Su, J.; Batista, E. R.; Cary, S. K.; 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. (15) Pappalardo, R. G.; Carnall, W. T.; Fields, P. R. LowTemperature Optical Absorption of Americium Halides. J. Chem. Phys. 1969, 51, 1182−1200. (16) Caution! 243Am (t1/2 = 7370 y) is an intense α-emitter and also emits penetrating γ-rays up to 142 keV in energy. Of equal importance, its α-decay product is 239Np, which is both short-lived (t1/2 = 2.35 d) and is an even more potent γ-ray emitter with energies up to 278 keV. (17) Evans, W. J.; Shreeve, J. L.; Ziller, J. W.; Doedens, R. J. Structural Diversity in Solvated Lanthanide Halide Complexes. Inorg. Chem. 1995, 34, 576−85. (18) Wu, S.-H.; Ding, Z.-B.; Li, X.-J. A Facile Method for Preparation of Tetrahydrofuran Complexes of Lanthanide Trichlorides. Polyhedron 1994, 13, 2679−2681. (19) Deacon, G. B.; Feng, T.; Junk, P. C.; Skelton, B. W.; Sobolev, A. N.; White, A. H. Preparation and X-Ray Crystal Structures of Tetrahydrofuran-Complexed Rare Earth Chlorides - a Structurally Rich Series. Aust. J. Chem. 1998, 51, 75−89. (20) 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.; Polinski, M. J.; Van Cleve, S. M.;

AUTHOR INFORMATION

Corresponding Author

*E-mail for S.C.B.: [email protected]. ORCID

Cory J. Windorff: 0000-0002-5208-9129 Matthias Zeller: 0000-0002-3305-852X Thomas E. Albrecht-Schmitt: 0000-0002-2989-3311 Suzanne C. Bart: 0000-0002-8918-9051 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This material is based on work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Elements Chemistry Program, under Awards DE-SC0008479 (S.C.B.) and DE-FG02-13ER16414 (T.E.A.-S.). The 243Am used in this work was supplied by the U.S. Department of Energy, Office of Science, by the Isotope Program in the Office of Nuclear Physics and provided to Florida State University via the Isotope Development and Production for Research and Applications Program through C

DOI: 10.1021/acs.organomet.8b00840 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics 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. (21) Fischer, E. O.; Laubereau, P.; Baumgärtner, F.; Kanellakopulos, B. Tricyclopentadienylneptunium Chloride. J. Organomet. Chem. 1966, 5, 583−584. (22) Baumgärtner, F.; Fischer, E. O.; Kanellakopulos, B.; Laubereau, P. Tetrakis(cyclopentadienyl)neptunium(IV). Angew. Chem., Int. Ed. Engl. 1968, 7, 634. (23) Bohlander, R. Organometallic Chemistry of Neptunium; 0303− 4003; Universität (TH) Karlsruhe: 1986; 168 pp. (24) De Ridder, D. J. A.; Rebizant, J.; Apostolidis, C.; Kanellakopulos, B.; Dornberger, E. Bis(cyclooctatetraenyl)neptunium(IV). Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1996, C52, 597−600. (25) Magnani, N.; Apostolidis, C.; Morgenstern, A.; Colineau, E.; Griveau, J.-C.; Bolvin, H.; Walter, O.; Caciuffo, R. Magnetic Memory Effect in a Transuranic Mononuclear Complex. Angew. Chem., Int. Ed. 2011, 50, 1696−1698. (26) Bursten, B. E.; Rhodes, L. F.; Strittmatter, R. J. The Bonding in Tris(η5-cyclopentadienyl)actinide Complexes. IV: Electronic Structural Effects in AnCl3 and (η5-C 5H5)3An (An = ThoriumCalifornium) Complexes. J. Less-Common Met. 1989, 149, 207−11. (27) Dutkiewicz, M. S.; Apostolidis, C.; Walter, O.; Arnold, P. L. Reduction Chemistry of Neptunium Cyclopentadienide Complexes: From Structure to Understanding. Chem. Sci. 2017, 8, 2553−2561. (28) Seaborg, G. T.; Wahl, A. C.; Kennedy, J. W. Radioactive Element 94 from Deuterons on Uranium. Phys. Rev. 1946, 69, 367. (29) Windorff, C. J.; Chen, G. P.; Cross, J. N.; Evans, W. J.; Furche, F.; Gaunt, A. J.; Janicke, M. T.; Kozimor, S. A.; Scott, B. L. Identification of the Formal + 2 Oxidation State of Plutonium: Synthesis and Characterization of Pu(II)[C5H3(SiMe3)2]3. J. Am. Chem. Soc. 2017, 139, 3970−3973.

D

DOI: 10.1021/acs.organomet.8b00840 Organometallics XXXX, XXX, XXX−XXX