Anomalous One-Electron Processes in the Chemistry of Uranium Nitrogen Multiple Bonds
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Novel reaction pathways are illustrated in the synthesis of uranium(IV), uranium(V), and uranium(VI) monoimido complexes. In contrast to the straightforward preparation of UV(NSiMe3)[N(SiMe3)2]3 (1), the synthesis of a uranium(V) tritylimido complex, UV(NCPh3)[N(SiMe3)2]3 (4), from UIII[N(SiMe3)2]3 and Ph3CN3 was found to proceed through multiple one-electron steps. Whereas the oxidation of 1 with copper(II) salts produced the uranium(VI) monoimido complexes UVI(NSiMe3)X[N(SiMe3)2]3 (X = Cl, Br), the reaction of 4 with CuBr2 undergoes sterically induced reduction to form the uranium(VI) monoimido complex UVI(NCPh3)Br2[N(SiMe3)2]2, demonstrating a striking difference in reactivity based on imido substituent. The facile reduction of compounds 1 and 4 with KC8 allowed for the synthesis of the uranium(IV) monoimido derivatives, K[UIV(NSiMe3)[N(SiMe3)2]3] (1-K) and K[UIV(NCPh3)[N(SiMe3)2]3] (4-K), respectively. In contrast, an analogous uranium(IV) monoimido complex, K[UIV(NPhF)[N(SiMe3)PhF]], PhF = -pentafluorophenyl (6), was prepared through a loss of N(SiMe3)2PhF concomitant with one-electron oxidation of a uranium(III) center. The uranium(IV) monoimido complexes were found to be reactive toward electrophiles, demonstrating N–C and N–Si single bond formation. One-electron reduction of nitrite provided a route to the uranium(VI) oxo/imido complex, [Ph4P][UVIO(NSiMe3)[N(SiMe3)2]3]. The energetics and electrochemical processes involved in the various oxidation reactions are discussed. Finally, comparison of the UVI(NSiMe3)X[N(SiMe3)2]3, X = Cl, Br, complexes with the previously reported UVIOX[N(SiMe3)2]3, X = Cl, Br, complexes suggested that the donor strength of the trimethylsilylimido ligand is comparable to the oxo ligand.
