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 U^V(NSiMe₃)­[N­(SiMe₃)₂]₃ (<b>1</b>), the synthesis of a uranium­(V) tritylimido complex, U^V(NCPh₃)­[N­(SiMe₃)₂]₃ (<b>4</b>), from U^III[N­(SiMe₃)₂]₃ and Ph₃CN₃ was found to proceed through multiple one-electron steps. Whereas the oxidation of <b>1</b> with copper­(II) salts produced the uranium­(VI) monoimido complexes U^VI(NSiMe₃)­X­[N­(SiMe₃)₂]₃ (X = Cl, Br), the reaction of <b>4</b> with CuBr₂ undergoes sterically induced reduction to form the uranium­(VI) monoimido complex U^VI(NCPh₃)­Br₂[N­(SiMe₃)₂]₂, demonstrating a striking difference in reactivity based on imido substituent. The facile reduction of compounds <b>1</b> and <b>4</b> with KC₈ allowed for the synthesis of the uranium­(IV) monoimido derivatives, K­[U^IV(NSiMe₃)­[N­(SiMe₃)₂]₃] (<b>1-K</b>) and K­[U^IV(NCPh₃)­[N­(SiMe₃)₂]₃] (<b>4-K</b>), respectively. In contrast, an analogous uranium­(IV) monoimido complex, K­[U^IV(NPh^F)­[N­(SiMe₃)­Ph^F]], Ph^F = -pentafluorophenyl (<b>6</b>), was prepared through a loss of N­(SiMe₃)₂Ph^F 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]­[U^VIO­(NSiMe₃)­[N­(SiMe₃)₂]₃]. The energetics and electrochemical processes involved in the various oxidation reactions are discussed. Finally, comparison of the U^VI(NSiMe₃)­X­[N­(SiMe₃)₂]₃, X = Cl, Br, complexes with the previously reported U^VIOX­[N­(SiMe₃)₂]₃, X = Cl, Br, complexes suggested that the donor strength of the trimethylsilylimido ligand is comparable to the oxo ligand