Influence of Ring-Expanded N‑Heterocyclic Carbenes on the Structures of Half-Sandwich Ni(I) Complexes: An X‑ray, Electron Paramagnetic Resonance (EPR), and Electron Nuclear Double Resonance (ENDOR) Study

Potassium graphite reduction of the half-sandwich Ni­(II) ring-expanded diamino/diamidocarbene complexes CpNi­(RE-NHC)Br gave the Ni­(I) derivatives CpNi­(RE-NHC) (where RE-NHC = 6-Mes (1), 7-Mes (2), 6-MesDAC (3)) in yields of 40%–50%. The electronic structures of paramagnetic 1–3 were investigated by CW X-/Q-band electron paramagnetic resonance (EPR) and Q-band 1H electron nuclear double resonance (ENDOR) spectroscopy. While small variations in the g-values were observed between the diaminocarbene complexes 1 and 2, pronounced changes in the g-values were detected between the almost isostructural species (1) and diamidocarbene species (3). These results highlight the sensitivity of the EPR g-tensor to changes in the electronic structure of the Ni­(I) centers generated by incorporation of heteroatom substituents onto the backbone ring positions. Variable-temperature EPR analysis also revealed the presence of a second Ni­(I) site in 3. The experimental g-values for these two Ni­(I) sites detected by EPR in frozen solutions of 3 are consistent with resolution on the EPR time scale of the disordered components evident in the X-ray crystallographically determined structure and the corresponding density functional theory (DFT)-calculated g-tensor. Q-band 1H ENDOR measurements revealed a small amount of unpaired electron spin density on the Cp rings, consistent with the calculated SOMO of complexes 1–3. The magnitude of the 1H A values for 3 were also notably larger, compared to 1 and 2, again highlighting the influence of the diamidocarbene on the electronic properties of 3.通过石墨钾还原半夹心型二价镍扩环二氨基/二酰胺卡宾配合物溴化(环戊二烯基（Cyclopentadienyl, Cp）)(扩环氮杂环卡宾（ring-expanded N-heterocyclic carbene, RE-NHC）)合镍(II)（CpNi(RE-NHC)Br），得到了一价镍衍生物CpNi(RE-NHC)（其中RE-NHC = 6-Mes (1)、7-Mes (2)、6-MesDAC (3)），产率为40%~50%。研究人员采用连续波（Continuous Wave, CW）X波段与Q波段电子顺磁共振（Electron Paramagnetic Resonance, EPR）以及Q波段1H电子核双共振（Electron Nuclear Double Resonance, ENDOR）光谱，对顺磁性配合物1~3的电子结构展开了探究。尽管二氨基卡宾配合物1与2的g值仅存在小幅差异，但近乎同构的1与二酰胺卡宾配合物3之间的g值却呈现出显著变化。上述结果表明，一价镍中心的电子顺磁共振g张量对骨架环位置引入杂原子取代基所引发的电子结构变化具有极高敏感性。变温电子顺磁共振分析还发现，配合物3中存在第二种一价镍位点。在配合物3的冷冻溶液中，电子顺磁共振检测到的两个一价镍位点的实验g值，与X射线晶体衍射解析结构中无序组分在电子顺磁共振时间尺度上的可分辨特征，以及密度泛函理论（Density Functional Theory, DFT）计算得到的g张量均保持一致。Q波段1H电子核双共振（ENDOR）测量结果显示，环戊二烯基环上存在少量未成对电子自旋密度，这与配合物1~3的计算单电子占据分子轨道（Singly Occupied Molecular Orbital, SOMO）结果相符。相较于配合物1与2，配合物3的1H超精细耦合常数A值量级明显更大，进一步凸显了二酰胺卡宾对配合物3电子性质的影响。