Effect of Geometrical Distortion on the Electronic Structure: Synthesis and Characterization of Monoradical-Coordinated Mononuclear Cu(II) Complexes

Ligand H3SamiMixed(tBu) was composed of two different compartments, a redox-active 2-aminophenol and a salen salicylidene. Both compartments were linked via a benzyl linker. The ligand reacted with CuCl2·2H2O under air in the presence of Et3N and provided the corresponding monoradical-coordinated mononuclear Cu­(II) complex (1). Complex 1, in solution, reacted with air and provided complex 2 via ligand-centered oxygenation at the benzyl-CH2 position. Both complexes were characterized via IR, mass spectrometry, X-ray single-crystal diffraction, variable-temperature magnetic susceptibility, cyclic voltammograms (CVs), and UV–vis/NIR spectroscopic techniques. X-ray crystallographic analyses clearly showed almost equally distorted square planar geometry around the Cu­(II) atom in both complexes. However, the bending of the radical-containing C6 ring compared to the N1–Cu1–O1 plane was different in both complexes. While complex 1 was paramagnetic and showed a ferromagnetic coupling between the dx2–y2 magnetic orbital of Cu­(II) ion and the pz orbital of coordinated π-radical, complex 2 was diamagnetic by experiencing a strong antiferromagnetic coupling between the two magnetic orbitals. UV–vis/NIR spectra of the complexes were dominated by charge-transfer transitions. CVs of the complexes showed two reversible one-electron oxidations and one reversible one-electron reduction. E1/2ox2 and E1/2red1 potentials were different in both complexes, while E1/2ox1 values were almost the same and the process corresponded to the formation of phenoxyl radical. Theoretical studies were also performed to understand the magnetic coupling phenomena, and TD-DFT calculations were employed for the assignment of charge-transfer absorption bands.

配体H3SamiMixed(tBu)由两个不同的结构域组成：一个具有氧化还原活性的2-氨基苯酚，以及一个salen型水杨醛亚胺配体结构单元。两个结构域通过苄基连接桥相连。
该配体在空气氛围下、三乙胺(Et3N)存在时与二水合氯化铜(II)(CuCl2·2H2O)反应，得到对应的单自由基配位单核铜(II)配合物(1)。配合物1在溶液中可与空气反应，通过苄基-CH2位点的配体中心氧化过程生成配合物2。
两种配合物均通过红外光谱(IR)、质谱法、X射线单晶衍射(X-ray single-crystal diffraction)、变温磁化率测试、循环伏安法(CVs)以及紫外-可见/近红外(UV–vis/NIR)光谱技术进行了表征。X射线晶体结构分析清晰显示，两种配合物中铜(II)原子周围均呈现近似程度一致的畸变平面正方形几何构型。但两种配合物中，含自由基的C6环相对于N1–Cu1–O1平面的弯折角度存在差异。
配合物1为顺磁性，其铜(II)离子的dx²–y²磁轨道与配位π自由基的pz轨道之间存在铁磁耦合作用；而配合物2则因两个磁轨道间发生强反铁磁耦合而呈现抗磁性。
两种配合物的紫外-可见/近红外光谱均以电荷转移跃迁为主要特征。两种配合物的循环伏安曲线均显示出两次可逆的单电子氧化过程与一次可逆的单电子还原过程。
两种配合物的E1/2ox2与E1/2red1电位存在差异，但E1/2ox1数值基本一致，该过程对应苯氧自由基的生成。
本研究还开展了理论计算以阐明磁耦合现象，并采用含时密度泛函理论(TD-DFT)计算对电荷转移吸收峰进行了归属。