X‑Shaped Oligomeric Pyromellitimide Polyradicals

The synthesis of stable organic polyradicals is important for the development of magnetic materials. Herein we report the synthesis, isolation, and characterization of a series of X-shaped pyromellitimide (PI) oligomers (Xn-R, n = 2–4, R = Hex or Ph) linked together by single C–C bonds between their benzenoid cores. We hypothesize that these oligomers might form high-spin states in their reduced forms because of the nearly orthogonal conformations adopted by their PI units. 1H and 13C nuclear magnetic resonance (NMR) spectroscopies confirmed the isolation of the dimeric, trimeric, and tetrameric homologues. X-ray crystallography shows that X2-Ph crystallizes into a densely packed superstructure, despite the criss-crossed conformations adopted by the molecules. Electrochemical experiments, carried out on the oligomers Xn-Hex, reveal that the reductions of the PI units occur at multiple distinct potentials, highlighting the weak electronic coupling between the adjacent redox centers. Finally, the chemically generated radical anion and polyanion states, Xn-Hex•– and Xn-Hexn(•−), respectively, were probed extensively by UV–vis–NIR absorption, EPR, and electron nuclear double resonance (ENDOR) spectroscopies. The ENDOR spectra of the radical monoanions Xn-Hex•– reveal that the unpaired electron is largely localized on a single PI unit. Further reductions of Xn-Hex•– yield EPR signals (in frozen solutions) that can be assigned to spin–spin interactions in X2-Hex2(•−), X3-Hex3(•−), and X4-Hex4(•−). Taken together, these findings demonstrate that directly linking the benzene rings of PIs with a single C–C bond is a viable method for generating stabilized high-spin organic anionic polyradicals.稳定有机多自由基的合成对于磁性材料的发展具有重要意义。本文报道了一系列X形均苯四甲酰亚胺（pyromellitimide, PI）低聚物（Xₙ-R，n=2~4，R=Hex或Ph）的合成、分离与表征，该类低聚物的各苯环核心之间通过单根C-C键相互连接。我们假设：由于其PI单元采取近乎正交的构象，这类低聚物在还原态下可能形成高自旋态。通过¹H与¹³C核磁共振（nuclear magnetic resonance, NMR）光谱证实，我们成功分离得到了二聚、三聚及四聚同系物。X射线晶体学研究表明，尽管X₂-Ph分子采取十字交叉构象，其晶体仍形成了紧密堆积的超结构。针对低聚物Xₙ-Hex开展的电化学实验显示，PI单元的还原过程发生在多个不同的电势下，这表明相邻氧化还原中心之间仅存在微弱的电子耦合作用。最后，我们通过紫外-可见-近红外（UV–vis–NIR）吸收光谱、电子顺磁共振（EPR）及电子核双共振（electron nuclear double resonance, ENDOR）光谱，对化学生成的自由基单阴离子Xₙ-Hex•⁻与多阴离子态Xₙ-Hexₙ(•−)进行了广泛表征。针对自由基单阴离子Xₙ-Hex•⁻的ENDOR光谱结果显示，未成对电子主要局域在单个PI单元上。对Xₙ-Hex•⁻进行进一步还原后，在冷冻溶液中得到的EPR信号可归因为X₂-Hex₂(•−)、X₃-Hex₃(•−)及X₄-Hex₄(•−)中的自旋-自旋相互作用。综上，上述研究结果表明，通过单根C-C键直接连接PI的苯环，是制备稳定高自旋有机阴离子多自由基的可行方法。