Probing the Chemistry, Electronic Structure and Redox Energetics in Organometallic Pentavalent Uranium Complexes

A series of organometallic pentavalent uranium complexes of the general formula (C5Me5)2U(=N-2,6-iPr2-C6H3)(Y) (Y = monoanionic, non-halide ligand) have been prepared using a variety of routes. Utilizing the direct oxidation of (C5Me5)2U(=N-2,6-iPr2-C6H3)(THF) (2) with the appropriate copper(I) salt yielded the triflate (Y = OTf (OSO2CF3), 11), thiolate (Y = SPh, 12), and acetylide (Y = CCPh, 13) complexes, while a salt metathesis route between the UV-imido (C5Me5)2U(=N-2,6-iPr2-C6H3)(I) (10) and various alkali salts gave the diphenylamide (Y = NPh2, 14), aryloxide (Y = OPh, 15), alkyl (Y = Me, 16), and aryl (Y = Ph, 17) complexes. Paired with 13, the isolation of 16 and 17 shows that UV can support the full range of carbon anions (sp, sp2, and sp3), and these are, to the best of our knowledge, the first examples of pentavalent uranium complexes with anionic carbon moieties other than carbocyclic (C5R5, C7H7, C8H8) ligands. Finally, both protonolysis and insertion pathways afforded the UV-imido ketimide complex (C5Me5)2U(=N-2,6-iPr2-C6H3)(N=CPh2) (18). The complexes have been isolated in good yield and characterized using various combinations of 1H NMR spectroscopy, elemental analysis, mass spectrometry, single crystal X-ray diffraction, cyclic voltammetry, UV-visible-NIR absorption spectroscopy, and magnetic susceptibility measurements. All (C5Me5)2U(=N−Ar)(X) (X = F, Cl, Br, I) and (C5Me5)2U(=N−Ar)(Y) complexes exhibit UVI/UV and UV/UIV redox couples by voltammetry. The potential separation between these couples remains essentially constant at ∼1.50 V, but both processes shift in tandem in potential by ∼700 mV across the series of X/Y ligands. No significant differences between μeff values or temperature dependencies in the magnetic susceptibility were observed for these complexes regardless of the identity of the ancillary X/Y ligand. However, an excellent linear correlation was observed between the chemical shift values of C5Me5 ligand protons in the 1H NMR spectra and the oxidation potentials of (C5Me5)2U(=N-2,6-iPr2-C6H3)(X/Y), suggesting that there is a common origin, overall σ-/π-donation from the ancillary X/Y ligand to the metal, contributing to both observables. Combined, these data confer the following trend in increasing σ/π-donating ability of the X/Y ligand to the UV metal center: OTf < I < Br < Cl < SPh < CCPh < F < [OPh ∼ Me ∼ Ph] ≪ NPh2 < N=CPh2. These (C5Me5)2U(=N-2,6-iPr2-C6H3)(X/Y) complexes also show distinct hallmarks of a covalent bonding interaction between the metal and the imide ligand that is modulated to varying degrees by the interaction between the X/Y ancillary ligand and the UV metal center. These signatures of covalency include stabilization of multiple metal oxidations states [UVI, UV, and UIV] and enhanced intensities in the intraconfiguration (f-f) transitions. Of particular note in this regard is the more than 20-fold enhancement in the f-f intensities observed for Y = CCPh and N=CPh2, which is a clear reflection of the covalent metal-ligand bonding interactions sustained by the acetylide and ketimide ligands in these pentavalent systems.一系列通式为(五甲基环戊二烯基(C5Me5))₂U(=N-2,6-二异丙基苯基(2,6-iPr₂-C₆H₃))(Y)（其中Y为单阴离子非卤配体）的有机金属五价铀配合物，已通过多种合成路线制备得到。通过以合适的铜(I)盐对(五甲基环戊二烯基(C5Me5))₂U(=N-2,6-二异丙基苯基(2,6-iPr₂-C₆H₃))(四氢呋喃(THF))(2)进行直接氧化，可得到三氟甲磺酸根(OTf, OSO₂CF₃)配合物（Y=OTf，11）、苯硫基(SPh)配合物（Y=SPh，12）以及炔化物(C≡CPh)配合物（Y=C≡CPh，13）；而通过五价铀亚胺配合物(五甲基环戊二烯基(C5Me5))₂U(=N-2,6-二异丙基苯基(2,6-iPr₂-C₆H₃))(I)(10)与多种碱金属盐之间的盐复分解路线，则可得到二苯胺基(NPh₂)配合物（Y=NPh₂，14）、芳氧基(OPh)配合物（Y=OPh，15）、烷基(Me)配合物（Y=Me，16）以及芳基(Ph)配合物（Y=Ph，17）。结合配合物13来看，16与17的分离结果表明，五价铀可承载所有类型的碳阴离子（sp杂化、sp²杂化及sp³杂化）；且据我们所知，这是首例不含碳环配体（C5R5、C7H7、C8H8）的五价铀阴离子碳基团配合物。此外，通过质子解与插入两条反应路径，均可得到五价铀亚胺酮亚胺配合物(五甲基环戊二烯基(C5Me5))₂U(=N-2,6-二异丙基苯基(2,6-iPr₂-C₆H₃))(N=CPh₂)(18)。上述配合物均以良好产率分离得到，并通过氢核磁共振波谱、元素分析、质谱、单晶X射线衍射、循环伏安法、紫外-可见-近红外吸收光谱以及磁化率测量等多种手段组合进行了表征。所有通式为(五甲基环戊二烯基(C5Me5))₂U(=N-芳基(Ar))(X)（X=F、Cl、Br、I）与(五甲基环戊二烯基(C5Me5))₂U(=N-芳基(Ar))(Y)的配合物，在伏安测试中均展现出U(VI)/U(V)与U(V)/U(IV)两组氧化还原电对。这两组氧化还原电对的电位差基本维持在约1.50 V，但其电位均会随着X/Y配体系列的变化而同步偏移约700 mV。无论辅助配体X/Y的种类如何，上述配合物的有效磁矩(μeff)数值或磁化率的温度依赖性均未出现显著差异。然而，氢核磁共振谱中五甲基环戊二烯基配体质子的化学位移数值，与(五甲基环戊二烯基(C5Me5))₂U(=N-2,6-二异丙基苯基(2,6-iPr₂-C₆H₃))(X/Y)的氧化电位之间存在极佳的线性相关性，这表明辅助配体X/Y向金属中心的整体σ/π给电子作用，是这两个观测物理量的共同成因。综合上述数据，可得到X/Y配体向五价铀金属中心的σ/π给电子能力递增的如下顺序：OTf < I < Br < Cl < SPh < C≡CPh < F < [OPh ≈ Me ≈ Ph] ≪ NPh₂ < N=CPh₂。这类(五甲基环戊二烯基(C5Me5))₂U(=N-2,6-二异丙基苯基(2,6-iPr₂-C₆H₃))(X/Y)配合物，同样展现出金属与亚胺配体之间共价键相互作用的显著特征，且该相互作用的程度会因辅助配体X/Y与五价铀金属中心的相互作用而发生不同程度的调控。这类共价作用的标志性特征包括：稳定多种金属氧化态[U(VI)、U(V)及U(IV)]，以及增强电子组态内(f-f)跃迁的吸收强度。在此方面尤其值得关注的是，当Y为C≡CPh与N=CPh₂时，f-f跃迁的吸收强度提升了20倍以上，这清晰地反映出，在该类五价铀体系中，炔化物与酮亚胺配体可形成稳定的金属-配体共价键相互作用。