Authors: Alistair S. P. Frey*, F. Geoffrey N. Cloke*, Martyn P. Coles*, Laurent Maron#, and Thomas Davin #
Journal: Angewandte Chemie International Edition
Affiliation: *Division of Chemistry, School of Life Sciences University of Sussex, Brighton, #LPCNO, INSA Toulouse
The area of Fisher-Tropsch chemistry seems to have always attracted much interest, but it is especially attractive during oil shortages and spikes in energy costs. The basic idea is to take syngas and produce liquid fuels, a process that could potentially cut down on the amount of oil drilling that we need to sustain our standard of living. Another area of chemistry which has seen a recent rise in popularity is the chemistry of Uranium, as people involved in Uranium enrichment would like something to do with the non-U235 isotopes, which make up a large percentage of Uranium ore. We find this paper at the intersection of these two interesting subjects.
The Uranium (U) species under investigation is a U(III) sandwich complex consisting of a dianionic cyclooctadenide containing two bulky triisopropylsilyl groups and an anionic pentamethylcyclopentadienide (both of which are aromatic). The researchers previously found that the Uranium(III) (U(III)) complex pictured above reacts with excess CO and forms a cyclic trimer of CO bound between two U centers or one equivalent of CO to form an yne-diolate. The current paper was a result of their attempt to probe the reactivity of proposed U-bound CO by exposing the U complex to stoichiometric quantities of CO and H2. To their surprise, the resulting species that they were able to isolate was a Uranium(IV)-methoxide (U(IV)-OMe) complex (pictured above), this is exciting because they selectively made a useful fuel (methanol) from CO and H2 (which are rarely react this way) without having to force the reaction to proceed with light, elevated temperatures, or chemical redox agents. This conversion occurred upon warming a vessel containing a 1:1:2 molar ratio of U complex:CO:H2 from -78oC to room temperature.
The authors state that they attempted to make methanol directly from the U(IV)-OMe complex by adding protic acids, but they were unsuccessful. Addition of the oxophilic trimethylsilyltrifluoromethanesulfonate (TMSOTf) resulted in the isolation of TMS-OMe and a U(IV)-OTf complex, which could be reduced by potassium amalgam to return the starting U(III) complex. The inactivity of the U(IV)-OMe complex could be due to the atypical mode of binding that occurs. Computational studies suggest that the U(IV) and the O on the methoxide participate in two pi-bonds, which can be seen in the crystal structure above as the nearly 180o U-O-C angle.
The mechanism of reaction is not known, but the fact that the U(III) starting material only contributes one electron to the overall four electron reduction of the C in CO is puzzling. The authors suggest that initial formation of a species containing a C-C bond (which is known to occur) could be reductively cleaved by H2, resulting in new C-H bonds, computational and mechanistic studies are ongoing.
If you are interested in Uranium chemistry, one of the contributors to the Periodic Table of Videos is engaged in Uranium chemistry and has made a few informative videos about what is needed if one wants to work with Uranium in the chemistry lab.