Anomalous One-Electron Processes in the Chemistry of Uranium Nitrogen Multiple Bonds

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.