RON DAGANI
Anew twist on the chemistry of uranium and dinitrogen (N2) has been revealed by researchers at the University of California, Irvine. They have found that a simple complex of uranium, (C5Me5)3U, can be forced, under pressure, to take on what for an actinide is an unlikely ligand—neutral N2—and bind it end-on [J. Am. Chem. Soc., 125, 14264 (2003)].
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CROWDED In (C5Me5)3U(N2), the U–N bond is at right angles to the ring centroid–U bonds.
COURTESY OF STOSH A. KOZIMOR |
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The resulting metal complex, (C5 Me5)3U(N2), is a first in several respects, says chemistry professor William J. Evans, who led the research effort. Terminal end-on binding of N2 to transition metals is well documented, Evans and coworkers Stosh A. Kozimor and Joseph W. Ziller note in their paper. But, to their knowledge, it has never before been observed in complexes of an f element (an element having an f orbital—that is, a lanthanide or actinide). In the three uranium-N2 complexes reported previously, either two uranium atoms or a uranium and a molybdenum atom are bridged by the anionic (N2)2– ligand. Thus, the new U-N2 complex “appears to be the first monometallic f element complex of N2 of any kind,” the team points out.
“Part of the significance of this discovery is that dinitrogen may coordinate to f elements (and possibly other electropositive elements) to a greater extent than previously thought,” Evans tells C&EN. Most f-element reactions are run under dinitrogen because chemists assume that it is an inert atmosphere. “Perhaps it is not inert in all cases,” he suggests. For example, if N2 blocks a coordination position in an f-element complex, it could affect the chemistry.
“These results are surprising in another way,” Evans continues. “It is unexpected that the extremely sterically crowded molecule (C5Me5)3U would even [accept] another ligand.” To force it to take on dinitrogen, Kozimor subjected the complex, in solution, to 80 psi of nitrogen gas, which led to the formation of crystals of the U-N2 complex. “It is remarkable,” Evans says, that the crowded metal center binds another ligand of any type—particularly neutral N2, which has little affinity for the usually highly ionic f elements.
The (C5Me5)3U precursor is so crowded that when the dinitrogen is added, the positions of the three C5Me5 ligands do not change—“they probably have no room to move,” Evans says. As a result, the N2-containing product has an unusual structure in which the U–N bond is at right angles to the bonds formed between U and the center of the C5Me5 rings.
Other inorganic chemists are equally enthusiastic about Evans’ creation. “I think this is a spectacular molecule [that] raises fundamental questions on structure and bonding,” comments professor Geoff Cloke of the University of Sussex, in England. And professor Michael D. Fryzuk of the University of British Columbia, Vancouver, hails the complex as “a remarkable trophy in dinitrogen coordination chemistry.”
The work could help lead to insights into nitrogen fixation, Evans says.
