Ni(II) and Pd(II) complexes of a new redox-active pentadentate azo-appended 2-aminophenol ligand: Pd(II)-assisted intraligand cyclization forms a phenoxazinyl ring,Dalton Transactions

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Ni(II) and Pd(II) complexes of a new redox-active pentadentate azo-appended 2-aminophenol ligand: Pd(II)-assisted intraligand cyclization forms a phenoxazinyl ring
Dalton Transactions ( IF 3.5 ) Pub Date : 2024-07-24 , DOI: 10.1039/d4dt01513d
Saumitra Bhowmik ₁ , Arunava Sengupta ₂ , Rabindranath Mukherjee ₃

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Square planar complexes of Ni(II) and Pd(II) of a new redox-active pentadentate azo-appended 2-aminophenol ligand (H₄L = N,N′-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2′-diamino-ortho-azobenzene) in three accessible redox levels [amidophenolate(2−), semiquinonate(1−) π radical, and quinone(0)] were synthesized. The coordinated HL(3−) ligand provides four donor sites [two N(iminophenolates), an N′(azo), and an O(phenolate)], while the phenolic OH group remains free in the three complexes. Cyclic voltammetry on complex [Ni(L)] 1 and its corresponding Pd(II) analogue [Pd(L)] 2 in CH₂Cl₂ displayed three redox responses (two oxidative at E_1/2 = 0.06 V and E_pa (anodic peak potential) = 0.80 V and one reductive at −0.77 V for 1 and at E_1/2 = 0.08 V and E_pa = 0.85 V and at −0.74 V for 2 vs. Fc⁺/Fc). The chemical oxidation of 1 with AgSbF₆ afforded [Ni(L)]SbF₆·2CH₂Cl₂ (3·2CH₂Cl₂). Complex [Pd(L*)] 4, which is coordinated by a phenoxazinyl derivative of L(4−), was obtained via intraligand cyclization in the parent complex 2 under basic oxidizing conditions. The molecular structures of 1, 2, 3·2CH₂Cl₂ and 4 were elucidated through X-ray crystallography at 100 K. Characterization using ¹H NMR, X-band EPR, and UV–VIS–NIR spectroscopy established that the complexes have [Ni^II{(L^ISQ)˙²⁻}] 1, [Pd^II{(L^ISQ)˙²⁻}] 2, [Ni^II{(L^IBQ)⁻}]SbF₆/1⁺SbF₆⁻(3), and [Pd^II{(L*^AP)˙²⁻}] 4 electronic states. Complexes 1, 2, and 4 possess paramagnetic S_t (total spin) = 1/2 ground-state, whereas 3 is diamagnetic (S_t = 0). Density functional theory (DFT) electronic structural calculations at the B3LYP level rationalized the observed experimental results. Time-dependent (TD)-DFT calculations allowed us to identify the nature of the observed absorption spectra.

中文翻译：

新型氧化还原活性五齿偶氮连接的 2-氨基苯酚配体的 Ni(II) 和 Pd(II) 配合物：Pd(II) 辅助配体内环化形成吩恶嗪基环

新型氧化还原活性五齿偶氮连接的 2-氨基苯酚配体的 Ni( II ) 和 Pd( II ) 方形平面配合物 (H ₄ L = N , N '-双(2-羟基-3,5-二叔)合成了三种可氧化还原水平的[氨基酚盐(2−)、半醌酸盐(1−)π自由基和醌(0)]。配位的 HL(3−) 配体提供四个供体位点 [两个 N（亚氨基酚盐）、一个 N'（偶氮）和一个 O（酚盐）]，而酚 OH 基团在三个配合物中保持游离状态。 CH ₂ Cl ₂中配合物 [Ni(L)] 1及其相应的 Pd( II ) 类似物 [Pd(L)] 2的循环伏安法显示出三种氧化还原反应（两种氧化反应分别为E _1/2 = 0.06 V 和E _pa (阳极峰值电位) = 0.80 V， 1为 -0.77 V， E _1/2 = 0.08 V 且E _pa = 0.85 V， 2为 -0.74 V 时还原，相对于Fc ⁺ /Fc)。用AgSbF ₆化学氧化1得到[Ni(L)]SbF ₆ ·2CH ₂ Cl ₂ ( 3 ·2CH ₂ Cl ₂ )。络合物[Pd(L*)] 4是由L(4-) 的吩恶嗪基衍生物配位的，是通过在碱性氧化条件下母体络合物2中的配体内环化获得的。通过 100 K 下的 X 射线晶体学阐明了1 , 2 , 3 ·2CH ₂ Cl ₂和4的分子结构。使用¹ H NMR、X 波段 EPR 和 UV-VIS-NIR 光谱进行表征表明，这些配合物具有[Ni ^II {(L ^ISQ )˙ ²⁻ }] 1 , [Pd ^II {(L ^ISQ )˙ ²⁻ }] 2 , [Ni ^II {(L ^IBQ ) ⁻ }]SbF ₆ / 1 ⁺ SbF ₆ ⁻ ( 3 ) 和 [Pd ^II {(L* ^AP )˙ ²⁻ }] 4 个电子态。配合物1、2和4具有顺_磁性St （总自旋）= 1/2 基态，_而3是反磁性的（ St = 0）。 B3LYP 水平的密度泛函理论 (DFT) 电子结构计算合理化了观察到的实验结果。时间相关 (TD)-DFT 计算使我们能够识别所观察到的吸收光谱的性质。

更新日期：2024-07-24

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