Synthesis, Characterization, and Multielectron Reduction Chemistry of Uranium Supported by Redox-Active α-Diimine Ligands

Uranium compounds supported by redox-active α-diimine ligands, which have methyl groups on the ligand backbone and bulky mesityl substituents on the nitrogen atoms {MesDABMe = [ArNC(Me)C(Me)NAr], where Ar = 2,4,6-trimethylphenyl (Mes)}, are reported. The addition of 2 equiv of MesDABMe, 3 equiv of KC8, and 1 equiv of UI3(THF)4 produced the bis(ligand) species (MesDABMe)2U(THF) (1). The metallocene derivative, Cp2U(MesDABMe) (2), was generated by the addition of an equimolar ratio of MesDABMe and KC8 to Cp3U. The bond lengths in the molecular structure of both species confirm that the α-diimine ligands have been doubly reduced to form ene-diamide ligands. Characterization by electronic absorption spectroscopy shows weak, sharp transitions in the near-IR region of the spectrum and, in combination with the crystallographic data, is consistent with the formulation that tetravalent uranium ions are present and supported by ene-diamide ligands. This interpretation was verified by U LIII-edge X-ray absorption near-edge structure (XANES) spectroscopy and by variable-temperature magnetic measurements. The magnetic data are consistent with singlet ground states at low temperature and variable-temperature dependencies that would be expected for uranium(IV) species. However, both complexes exhibit low magnetic moments at room temperature, with values of 1.91 and 1.79 μB for 1 and 2, respectively. Iodomethane was used to test the reactivity of 1 and 2 for multielectron transfer. While 2 showed no reactivity with CH3I, the addition of 2 equiv of iodomethane to 1 resulted in the formation of a uranium(IV) monoiodide species, (MesDABMe)(MesDABMe2)UI {3; MesDABMe2 = [ArNC(Me)C(Me2)NAr]}, which was characterized by single-crystal X-ray diffraction and U M4- and M5-edge XANES. Confirmation of the structure was also attained by deuterium labeling studies, which showed that a methyl group was added to the ene-diamide ligand carbon backbone.

本研究报道了一类由氧化还原活性α-二亚胺（redox-active α-diimine）配体负载的铀化合物，该配体在主链上带有甲基，且氮原子上连有大位阻均三甲苯基取代基，其中MesDABMe = [ArN=C(Me)C(Me)=NAr]，Ar为2,4,6-三甲基苯基（2,4,6-trimethylphenyl，简称Mes）。将2当量的MesDABMe、3当量的KC8与1当量的UI₃(THF)₄（四氢呋喃，tetrahydrofuran）混合反应，得到双配体物种(MesDABMe)₂U(THF)（化合物1）。通过将等摩尔比的MesDABMe与KC8加入至三茂铀（Cp₃U）中，成功合成了茂金属衍生物二茂铀-MesDABMe加合物（Cp₂U(MesDABMe)，化合物2）。两种物种的分子结构键长数据均证实，α-二亚胺配体已被双还原为烯二酰胺（ene-diamide）配体。电子吸收光谱表征显示，光谱近红外区存在弱而尖锐的吸收峰；结合晶体学数据，该结果与四价铀离子被烯二酰胺配体负载的配位模式一致。该结论通过U L₃边X射线吸收近边结构（X-ray absorption near-edge structure, 简称XANES）光谱以及变温磁性测试得到了验证。磁性数据显示，低温下体系呈现单重态基态，且具有铀(IV)物种应有的变温磁学行为。但两种配合物在室温下均表现出较低的磁矩，化合物1和2的磁矩分别为1.91 μB和1.79 μB。以碘甲烷为反应底物，测试了化合物1和2的多电子转移反应活性。化合物2与CH₃I未表现出明显反应活性，而向1中加入2当量碘甲烷后，生成了铀(IV)单碘化物物种(MesDABMe)(MesDABMe₂)UI（化合物3；其中MesDABMe₂ = [ArN=C(Me)C(Me₂)NAr]），该产物通过单晶X射线衍射以及U M₄和M₅边XANES光谱进行了表征。通过氘代标记实验进一步确认了产物结构，结果显示一个甲基被加成至烯二酰胺配体的碳主链上。