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.

本研究阐释了铀(IV)、铀(V)和铀(VI)单亚胺配位化合物的合成反应路径。与铀(IV)三(三甲基硅基)亚胺配位化合物 UV(NSiMe3)[N-(三甲基硅基)2]3 (1) 的直接制备方法不同，从 UIII[N-(三甲基硅基)2]3 和 Ph3CN3 合成铀(V)三叔丁基亚胺配位化合物 UV(NCPh3)[N-(三甲基硅基)2]3 (4) 的过程被发现是通过多个单电子步骤进行的。1 与铜(II)盐的氧化反应生成了铀(VI)单亚胺配位化合物 UVI(NSiMe3)X[N-(三甲基硅基)2]3 (X = Cl, Br)，而 4 与 CuBr2 的反应则经历空间诱导的还原，形成铀(VI)单亚胺配位化合物 UVI(NCPh3)Br2[N-(三甲基硅基)2]2，展示了基于亚胺取代基的显著反应性差异。化合物 1 和 4 与 KC8 的简单还原反应分别合成了铀(IV)单亚胺衍生物 K[UIV(NSiMe3)[N-(三甲基硅基)2]3] (1-K) 和 K[UIV(NCPh3)[N-(三甲基硅基)2]3] (4-K)。相比之下，一个类似的铀(IV)单亚胺配位化合物 K[UIV(NPhF)[N-(三甲基硅基)-PhF]] (6)，是通过 N-(三甲基硅基)2PhF 的损失以及铀(III)中心的单电子氧化同时进行的。铀(IV)单亚胺配位化合物对亲电试剂表现出活性，证实了 N-C 和 N-Si 单键的形成。亚硝酸根的单电子还原提供了一条通往铀(VI)氧/亚胺配位化合物 [Ph4P][UVIO(NSiMe3)[N-(三甲基硅基)2]3] 的途径。讨论了各种氧化反应中涉及的能量学和电化学过程。最后，UVI(NSiMe3)X[N-(三甲基硅基)2]3，X = Cl, Br，与先前报道的 UVIOX[N-(三甲基硅基)2]3，X = Cl, Br，配位化合物的比较表明，三甲基硅基亚胺配体的供体强度与氧配体相当。