Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India.
Dalton Trans. 2018 Dec 4;47(47):17086-17101. doi: 10.1039/c8dt03257b.
Aerobic reaction of the ligand H2L1, 2-(2-phenylazo)-anilino-4,6-di-tert-butylphenol, CoCl2·6H2O and Et3N in MeOH under refluxing conditions produces, after work-up and recrystallization, black crystals of [Co(L1)2] (1). When examined by cyclic voltammetry, 1 displays in CH2Cl2 three one-electron redox responses: two oxidative, E11/2 = 0.30 V (peak-to-peak separation, ΔEp = 100 mV) and E21/2 = 1.04 V (ΔEp = 120 mV), and one reductive E1/2 = -0.27 V (ΔEp = 120 mV) vs. SCE. Consequently, 1 is chemically oxidized by 1 equiv. of [FeIII(η5-C5H5)2][PF6], affording the isolation of deep purple crystals of [Co(L1)2][PF6]·2CH2Cl2 (2), and one-electron reduction with [CoII(η5-C5H5)2] yielded bluish-black crystals of [CoIII(η5-C5H5)2][Co(L1)2]·MeCN (3). A solid sample of 1 exhibits temperature-independent (50-300 K) magnetism, revealing the presence of a free radical (S = 1/2), which exhibits an isotropic EPR signal (g = 2.003) at 298 K and at 77 K an eight-line feature characteristic of hyperfine-interaction of the radical with the Co (I = 7/2) nucleus. Based on X-ray structural parameters of 1-3 at 100 K, magnetic and EPR spectral behaviour of 1, and variable-temperature (233-313 K) 1H NMR spectral features of 1-3 and 13C NMR spectra at 298 K of 2 and 3 in CDCl3 point to the electronic structure of the complexes as either [CoIII{(LAP)2-}{(LISQ)}˙-] or [CoIII{(L1)2}˙3-] (delocalized nature favours the latter description) (1), [CoIII{(LISQ)˙-}2][PF6]·2CH2Cl2 (2) and [CoIII(η5-C5H5)2][CoIII{(LAP)2-}2]·MeCN (3) [(LAP)2- and (LISQ)˙- represent the redox-level of coordinated ligands o-amidophenolate(2-) ion and o-iminobenzosemiquinonate(1-) π-radical ion, respectively]. Notably, all the observed redox processes are ligand-centred. To the best of our knowledge, this is the first time that six-coordinate complexes of a common tridentate o-aminophenolate-based ligand have been structurally characterized for the parent 1, its monocation 2 and the monoanion 3 counterparts. Temperature-dependent 1H NMR spectra reveal the existence of valence-tautomeric equilibria in 1-3. Density Functional Theory (DFT) calculations at the B3LYP-level of theory corroborate the electronic structural assignment of 1-3 from experimental data. The origins of the observed UV-VIS-NIR absorptions for 1-3 have been assigned, based on time-dependent (TD)-DFT calculations.
配体 H2L1（2-（2-苯基偶氮）-4,6-二叔丁基苯酚）、CoCl2·6H2O 和 Et3N 在 MeOH 中的有氧反应在回流条件下进行，反应后经过处理和重结晶，得到黑色晶体[Co(L1)2]（1）。通过循环伏安法检测，1 在 CH2Cl2 中显示出三个单电子氧化还原响应：两个氧化，E11/2 = 0.30 V（峰-峰分离，ΔEp = 100 mV）和 E21/2 = 1.04 V（ΔEp = 120 mV），以及一个还原 E1/2 = -0.27 V（ΔEp = 120 mV）相对于 SCE。因此，1 通过 1 当量的[FeIII(η5-C5H5)2][PF6]被化学氧化，得到深紫色晶体[Co(L1)2][PF6]·2CH2Cl2（2）的分离，并且用[CoII(η5-C5H5)2]进行一电子还原得到蓝黑色晶体[CoIII(η5-C5H5)2][Co(L1)2]·MeCN（3）。1 的固体样品表现出温度无关（50-300 K）的磁性，表明存在自由基（S = 1/2），其在 298 K 时表现出各向同性的 EPR 信号（g = 2.003），在 77 K 时表现出自由基与 Co（I = 7/2）核的超精细相互作用的八条线特征。基于 1-3 在 100 K 时的 X 射线结构参数、1 的磁性和 EPR 光谱行为以及 1-3 在 233-313 K 时的 1H NMR 光谱特征和 298 K 时的 13C NMR 光谱在 CDCl3 中，指出了配合物的电子结构为[CoIII{(LAP)2-}{(LISQ)}˙-]或[CoIII{(L1)2}˙3-]（离域性质有利于后者描述）（1）、[CoIII{(LISQ)˙-}2][PF6]·2CH2Cl2（2）和[CoIII(η5-C5H5)2][CoIII{(LAP)2-}2]·MeCN（3）[（LAP）2-和（LISQ）˙-分别代表配位配体邻-氨基苯酚盐（2-）离子和邻-亚氨基苯醌（1-）π-自由基离子的氧化还原水平]。值得注意的是，所有观察到的氧化还原过程都是配体中心的。据我们所知，这是首次对基于常见三齿 o-氨基苯酚的配体的六配位配合物进行结构表征，包括母体 1、其单正离子 2 和单负离子 3 对应物。温度依赖的 1H NMR 光谱揭示了 1-3 中存在价互变异构平衡。基于密度泛函理论（DFT）的 B3LYP 理论水平的计算证实了从实验数据得出的 1-3 的电子结构分配。基于时间相关（TD）-DFT 计算，对 1-3 的观察到的紫外可见近红外吸收的起源进行了分配。
