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Uranium chemistry is heating up. That's not just because the element is radioactive. It's also because a cadre of chemists has been branching out from traditional uranium oxide chemistry and discovering that the f orbitals in uranium (and thorium) allow for unprecedented structure and bonding in organometallic complexes. Two research groups that are a part of this new thrust in actinide chemistry now report the synthesis and characterization of a new round of unique uranium-nitrogen compounds.
William J. Evans and his colleagues at the University of California, Irvine, recently reported the first molecular uranium compounds containing bimetallic nitride linkages (Science 2005, 309, 1835). The compounds are made up of uranium metallocene groups joined together by nitride (N3-) ligands, and these units are in turn coupled together by azide (N3-) groups. A sequence of four of these UR2-N-UR2-NNN moieties (where R = cyclopentadienyl) forms the edges of a 24-membered uranium-nitrogen molecular square.
In a related development, Margaret-Jane Crawford at the University of Munich, in Germany, and her coworkers report the synthesis and characterization of an ammonium salt of a uranium polyazide, U(N3)73- (Angew. Chem. Int. Ed., published online, dx.doi.org/10.1002/anie.200502484). This polyazide is the first structurally characterized heptaazide compound for any element, and it's the first known example of a binary actinide azide-a compound containing only an actinide metal and azide groups.
These uranium-nitrogen compounds eventually could be important as precursors to a sought-after uranium nitride nuclear reactor fuel or perhaps as high explosives. They also could be a segue to new materials with interesting optical, magnetic, and electronic properties, as well as to a broader array of uranium-based catalysts for C-H bond activation, dinitrogen activation, and more.
There has been a recent resurgence of interest in the chemistry of the early actinides, thorium through plutonium, notes Carol J. Burns, group leader of isotope and nuclear chemistry at Los Alamos National Laboratory. The interest is driven in part by technical needs but also by the relative lack of sophistication with which we have treated f-electron systems in the past, she says.
The role of actinide metal d- and f-valence orbitals in single and multiple covalent bonding of ligands in nonaqueous systems hasn't been fully explored, Burns explains. Organouranium compounds having nitrogen, oxygen, sulfur, and phosphorus ligands with the potential for multiple bonding have been reported. But some compounds with key nitrogen-based ligands are still missing and remain synthetic targets. Novel chemistry with nitrogen-based ligands provides the most striking range of possibilities for varying metal-ligand orbital overlap, she notes.
The list of firsts in this area has grown quickly during the past few years, Burns says. Examples include novel amido and organoimido complexes, ketimido ligands, hydrazido derivatives, and dinitrogen adducts. The Evans and Crawford papers add to this array of firsts, she points out, and highlight the bewildering variation in behavior that can arise with actinide metals.
Evans' group has developed a number of organouranium complexes as part of its ongoing work on lanthanide and actinide chemistry. One set of compounds is a class of uranium metallocenes, [(C5Me4R)2U][(-Ph)2BPh2], where R is H or Me (Me = methyl, Ph = phenyl). In these compounds, which function as reducing agents, two of the phenyl rings on the boron anion are coordinated to the uranium center of the metallocene.
Evans, graduate student Stosh A. Kozimor, and crystallographer Joseph W. Ziller reacted the metallocenes with NaN3, expecting to obtain a simple substituted azide product, such as (C5Me5)2UN3. The researchers were delighted to discover that the large (UNUN3)4 rings formed, Evans says.
They believe an azide group first replaces the boron ligand, and then half of the azide groups are reduced to nitride groups, forming [(C5Me4R)2UN]-. Oligomerization of these intermediates then occurs with the remaining azide-containing metallocenes, [(C5Me4R)2UN3]+, or through other possible intermediates. It's not yet clear to the researchers why the assemblies form the molecular square structure rather than linear oligomers.
Evans' group previously prepared uranium cyclopentadienyl complexes that have a dinitrogen ligand bound end-on to the uranium atom, the first such example of a monometallic f-element complex of N2 (C&EN, Nov. 17, 2003, page 16). Only a couple of other linear uranium-N2 complexes are known.
Uranium nitrides are rare, Evans says. Few experimental examples are available for comparison with the calculations on uranium nitrides and related materials. Hence, the existence of the nitride ligands in our compounds, their nearly linear U-N-U linkages, and their short U-N bond distances characteristic of multiple bonding are what is most unusual.
The large polymetallic assembly also is unusual, he adds. Few molecular assemblies as large as our 24-atom octametallic ring complexes have been made with f-element metallocenes because f orbitals are not thought to have the rigidity needed to make the edges and corners to construct supramolecular systems, Evans notes. The inherent flexibility allows the rings to adopt either a chairlike or a boatlike conformation, which may affect the properties.
Crawford, crystallographer Peter Mayer of the University of Munich, and their coworkers have been extending knowledge of uranium chemistry during the past few years. They teamed up to report the first unambiguously characterized compound with a uranium(VI)-iodine bond, UO2I2(OH2)2, and separately reported a complete characterization of UO2I42-, which had been another missing piece of actinide chemistry (Inorg. Chem. 2005, 44, 5547).
These compounds are useful starting materials for synthesis of a broader range of uranium compounds, Crawford notes. For example, Crawford, Mayer, and their coworkers used a UO2I2 salt to make the first structurally characterized uranium(VI) isocyanate (NCO-) compounds. Preparing the isocyanates alerted Crawford to the fact that a binary uranium azide compound had not been made, although three azide-containing uranium compounds were known before now.
Crawford synthesized U(N3)73- by reacting (Bu4N)2UCl6 and (Bu4N)Br (Bu = butyl) with AgN3 in propionitrile solvent at room temperature. The octaazide was the expected product, but instead, air-sensitive emerald green crystals of [Bu4N]3[U(N3)7] were isolated. Modifying the starting materials, solvent, and reaction stoichiometry still resulted only in the heptaazide, she notes. The crystal structure of U(N3)73-, determined by Crawford, Mayer, and crystallographer Arkady Ellern at Iowa State University, reveals a pentagonal bipyramid arrangement of the seven azide groups centered on the uranium atom.
The relatively large size of actinide ions and the large number of valence shell orbitals available mean that high coordination numbers, especially 8 and 9 substituents, are common. For example, the uranium isothiocyanate compound, U(NCS)84-, has been reported. But the azide group is a harder anion than isothiocyanate and is less effective at delocalizing its negative charge, Burns points out. Thus, ligand-ligand repulsion may limit the number of azide groups that can be put around a uranium atom.
A number of binary polyazide anions of main-group and transition-metal elements have been synthesized in recent years. While these are high-energy-density compounds that tend to decompose explosively by forming N2 gas, their salts formed with bulky organic cations are somewhat stable. Some of these compounds might potentially be suitable for use as clean explosives, propellants, or fireworks. The azides also could have potential uses in the fabrication of electronic devices.
A friendly competition in this azide renaissance has been waged in the past couple of years between the groups of Thomas M. Klaptke at the University of Munich (Crawford's Ph.D. adviser and now department chairman) and Karl O. Christe at the University of Southern California. These groups and others have been making high-energy nitrogen compounds with azide anions, as well as marching across the periodic table trying to make azides of elements in as many of the table's groups as possible (C&EN, Oct. 11, 2004, page 44).
Christe, Ralf Haiges, and their coworkers at the University of Southern California and the Air Force Research Lab at Edwards Air Force Base, in California, were able to make the first reported heptaazides earlier this year (Angew. Chem. Int. Ed. 2005, 44, 1860). The work, part of the team's effort to prepare the first group 6 polyazides, led to synthesis of Mo(N3)7- and W(N3)7- salts, as well as the neutral Mo(N3)6 and W(N3)6. The heptaazides exploded upon warming up to room temperature, however, so the researchers were unable to structurally characterize them.
The U(N3)73- anion gives hope for the synthesis of a stable octaazide, Crawford says. Making the heptaazides also raises the question of whether other uranium azides can be made in a lower metal oxidation state or as a neutral species, such as U(N3)4 and U(N3)6, she adds.
Uranium chemistry has traditionally revolved around the aqueous chemistry of the uranyl ion, UO22+, and related molecular species, Burns notes. One important use of this chemistry is for preparation of uranium oxide ceramic pellets that are used as the fuel in light-water nuclear reactors. Often the fuel materials start to break down before the uranium is completely spent, however. Thus, the fuel must be removed prematurely and reprocessed.
Proposed efforts to reprocess spent fuel more efficiently, or to increase the use of fast reactors that burn up the fuel more completely, will require fuel materials with better performance than oxides, Burns explains. The high density of uranium and nitrogen atoms in the new compounds could make them or their derivatives useful precursors.
Actinide catalysts have been used in a few industrial processes, she adds, but most people would be highly skeptical of proposing broad uses of organometallic actinide complexes because of the radioactivity. Although practical catalysis applications might be limited, that doesn't relegate nonaqueous actinide chemistry to curiosity status, she says.
There is a genuine beauty and fundamental interest in the Evans and Crawford complexes, comments Sandro Gambarotta, a chemistry professor at the University of Ottawa, in Ontario. Gambarotta's research has included work on dinitrogen activation using actinide complexes.
The reactivity of actinide metals is absolutely first class, he says, and the number of recent papers reporting surprising transformations gives the impression that just the tip of the iceberg is being unveiled.
Actinide chemistry may in fact teach us new tricks in classical organometallic chemistry, Burns contends. We've spent a lot of time trying to make f-element species behave' more covalently like d-orbital transition-metal complexes. But in doing so, we have identified reactivity patterns that are pretty unique. It may be interesting now to try to turn the chemistry around and see if we can induce these types of actinide reactions in appropriately designed transition-metal species or in lanthanide species.
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