Recent Research Developments

Index of Recent Research News
Feb 19, 2003
    New Homoleptic Anthracene Complexes of Anionic Metals: Promising Isolable Synthons for Atoms of Niobium(1-), Tantalum(1-) and Iron(1-)

    William W. Brennessel, John E. Ellis, Sergey N. Roush, Brian R. Strandberg, Victor J. Sussman, Eugenijus Urnezius, Oliver E. Woisetschlaeger and Victor G. Young, Jr.

    In a previous "Research News" report of January 9, 2002, the synthesis, isolation and chemical properties of bis(1-4-η4-anthracene)cobaltate(1-) was announced. This was a remarkable new compound for several reasons, i.e., it was the only known homoleptic or "pure" anthracene transition metal complex to have been prepared by a conventional synthesis, the first anionic complex of a late (groups 7-11) transition metal containing only bound hydrocarbons, and perhaps most interesting, it functioned as an isolable synthon for atomic cobalt anion in its reactions with a variety of ligands. Subsequently, this work was published in Angewandte Chemie International Edition (2002, vol. 41, pp. 1211-1215). The unusual character of this substance was of sufficient general interest to warrant mention in the April 1, 2002 issue of Chemical and Engineering News in the "Science and Technology Concentrates" section on page 52 under the title "Naked atomic cobalt anions." On the basis of the promising character of this species, recently we have extended this study to transition metals throughout the periodic table and have obtained the first such anthracene complexes of niobium, tantalum, and iron. All have been structurally characterized by single crystal X-ray crystallography and show some striking features. In particular, the niobium and tantalum complexes, shown in Fig. 1 for tantalum, are the first tris(anthracene) complexes to be reported for any metal and readily shed the relatively weakly bound anthracenes in the presence of carbon monoxide, 1,3,5,7-cycloctatetraene, and similar ligands to provide high yields of corresponding anionic species, e.g., hexacarbonylniobate(1-) and tris(cyclooctatetraene)tantalate(1-), respectively. Preliminary communications on the niobium and tantalum complexes have been recently reported in Chemical Communications (2002, pp. 2356 - 2357) and Journal of the American Chemical Society (2002, vol. 124, pp. 10258-10259), respectively.

    Iron also provided a surprise in that the very first paramagnetic anthracene complex of this type was produced: bis(1-4-η 4-anthracene)ferrate(1-). Its structure, Fig. 2, shows that the coordination environment about the iron is close to tetrahedral and the substance is the first available synthon for atomic Fe(1-). For example, it reacts with carbon monoxide and 2,2'-bipyridyl to afford high yields of octacarbonyldiferrate(2-), a dimer of the unstable and paramagnetic iron tetracarbonyl anion, and tris(bipyridyl)ferrate(1-), a long known substance, but one that previously had not been structurally characterized. This study will be submitted for publication soon.

    These studies underscore the particular utility of anthracene in accessing previously unknown classes of hydrocarbon-transition metal complexes, which also promise to be effective sources of highly unsaturated transition metal fragments in synthesis. We anticipate that other polycyclic aromatic hydrocarbons will also provide significant new results. One of the objectives in our research program, which is supported by grants from the National Science Foundation and the Petroleum Research Fund, Administered by the American Chemical Society, is to obtain molecular species containing metals in unusual or previously unknown low formal oxidation states and ligand environments, i.e., "textbook molecules," to maximize our chances for the discovery of unprecedented "chemistry of the elements." Basically if it isn't new and exciting then we're not interested!

    Figure 1 Tris(1-4-η4-anthracene)tantalate(1-).

    Figure 2 Bis(1-4-η4-anthracene)ferrate(1-).
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