Vanadium Bisimide Bonding Investigated by X‑ray Crystallography, 51V and 13C Nuclear Magnetic Resonance Spectroscopy, and V L3,2-Edge X‑ray Absorption Near-Edge Structure Spectroscopy

Syntheses of neutral halide and aryl vanadium bisimides are described. Treatment of VCl2(NtBu)­[NTMS­(NtBu)], 2, with PMe3, PEt3, PMe2Ph, or pyridine gave vanadium bisimides via TMSCl elimination in good yield: VCl­(PMe3)2(NtBu)2 3, VCl­(PEt3)2(NtBu)2 4, VCl­(PMe2Ph)2(NtBu)2 5, and VCl­(Py)2(NtBu)2 6. The halide series (Cl–I) was synthesized by use of TMSBr and TMSI to give VBr­(PMe3)2(NtBu)2 7 and VI­(PMe3)2(NtBu)2 8. The phenyl derivative was obtained by reaction of 3 with MgPh2 to give VPh­(PMe3)2(NtBu)2 9. These neutral complexes are compared to the previously reported cationic bisimides [V­(PMe3)3(NtBu)2]­[Al­(PFTB)4] 10, [V­(PEt3)2(NtBu)2]­[Al­(PFTB)4] 11, and [V­(DMAP)­(PEt3)2(NtBu)2]­[Al­(PFTB)4] 12 (DMAP = dimethylaminopyridine, PFTB = perfluoro-tert-butoxide). Characterization of the complexes by X-ray diffraction, 13C NMR, 51V NMR, and V L3,2-edge X-ray absorption near-edge structure (XANES) spectroscopy provides a description of the electronic structure in comparison to group 6 bisimides and the bent metallocene analogues. The electronic structure is dominated by π bonding to the imides, and localization of electron density at the nitrogen atoms of the imides is dictated by the cone angle and donating ability of the axial neutral supporting ligands. This phenomenon is clearly seen in the sensitivity of 51V NMR shift, 13C NMR Δδαβ, and L3-edge energy to the nature of the supporting phosphine ligand, which defines the parameters for designing cationic group 5 bisimides that would be capable of breaking stronger σ bonds. Conversely, all three methods show little dependence on the variable equatorial halide ligand. Furthermore, this analysis allows for quantification of the electronic differences between vanadium bisimides and the structurally analogous mixed Cp/imide system CpV­(NtBu)­X2 (Cp = C5H51–).

本文报道了中性卤代及芳基取代钒双亚胺配合物的合成方法。以VCl₂(NtBu)[NTMS(NtBu)]（化合物2，其中NtBu为叔丁基亚氨基（tert-butylimido，NtBu）、TMS为三甲基硅基（trimethylsilyl，TMS））为原料，分别与三甲基膦（trimethylphosphine，PMe₃）、三乙基膦（triethylphosphine，PEt₃）、二甲基苯基膦（dimethylphenylphosphine，PMe₂Ph）或吡啶（pyridine，Py）反应，通过消除三甲基氯硅烷（TMSCl）即可高产率得到相应钒双亚胺配合物：VCl(PMe₃)₂(NtBu)₂（3）、VCl(PEt₃)₂(NtBu)₂（4）、VCl(PMe₂Ph)₂(NtBu)₂（5）以及VCl(Py)₂(NtBu)₂（6）。
通过使用三甲基溴硅烷（TMSBr）与三甲基碘硅烷（TMSI），可合成卤族系列（Cl–I）配合物，分别得到VBr(PMe₃)₂(NtBu)₂（7）与VI(PMe₃)₂(NtBu)₂（8）。将化合物3与二苯基镁（MgPh₂）反应，可得到芳基取代衍生物VPh(PMe₃)₂(NtBu)₂（9）。
将上述中性配合物与此前已报道的阳离子双亚胺配合物[V(PMe₃)₃(NtBu)₂][Al(PFTB)₄]（10）、[V(PEt₃)₂(NtBu)₂][Al(PFTB)₄]（11）以及[V(DMAP)(PEt₃)₂(NtBu)₂][Al(PFTB)₄]（12）进行对比；其中DMAP为二甲基氨基吡啶（dimethylaminopyridine，DMAP），PFTB为全氟叔丁氧基（perfluoro-tert-butoxide，PFTB）。
通过X射线衍射、¹³C核磁共振（¹³C NMR）、⁵¹V核磁共振（⁵¹V NMR）以及钒L₃,₂边X射线吸收近边结构（X-ray absorption near-edge structure，XANES）光谱对上述配合物进行表征，并结合第6族双亚胺配合物与弯曲型茂金属类似物的电子结构开展分析。
该类配合物的电子结构主要由与亚胺配体的π成键作用主导，而亚胺氮原子处的电子密度局域化程度则由轴向中性辅助配体的锥角与给电子能力决定。这一现象可从⁵¹V NMR化学位移、¹³C NMR Δδ_αβ以及L₃边吸收能随辅助膦配体性质的变化得到清晰体现，该结果为设计可断裂更强σ键的阳离子第5族双亚胺配合物提供了设计参数。
与之相反，上述三种表征方法均显示配合物的电子性质几乎不受可变的平伏位卤代配体影响。
此外，本研究还通过该分析实现了钒双亚胺配合物与结构类似的混合Cp/亚胺体系CpV(NtBu)X₂（Cp为环戊二烯基（cyclopentadienyl，Cp），即C₅H₅⁻）之间电子性质差异的定量表征。