Structural, Electrochemical, and Spectroscopic Investigation of Acetate Bridged Dinuclear Tetrakis-Schiff Base Macrocycles of Mn and Zn

The synthesis of Mn 2 LAc +, Zn 2 LAc +, and H 4 L 2+ is described, where L is a tetrakis-Schiff base macrocycle formed using 4-tert-butyl-2,6-diformylphenol and 2,2′-diamino-N-methyldiethylamine resulting in an N6O2 coordination environment. In Mn 2 LAc + and Zn 2 LAc +, the two metal centers are bridged by an acetate ligand. [Mn2LAc]­(ClO4)·(DMF)0.5, [Mn2LAc]­(ClO4)·(ACN)0.5, and [Zn2LAc]­(PF6) crystallized in the space group P21/c, with nearly identical unit-cell dimensions and geometric structures. Electrochemical analysis of Zn 2 LAc +, and H 4 L 2+ by cyclic voltammetry (CV) revealed two irreversible anodic waves that were assigned to oxidations of the phenolate ligands. CVs of Mn 2 LAc + displayed two chemically reversible anodic waves corresponding to MnII/III oxidations, followed by irreversible oxidations of the phenolate ligands. Interfacial electron transfer rates for the single electron oxidations from Mn 2 II LAc + to Mn II Mn III LAc 2+ to Mn 2 III LAc + determined from digital simulation of the CVs were 0.6 and 1.1 × 10–3 cm s–1, respectively. The sluggish interfacial electron transfer rates observed in electrochemical scans of Mn 2 LAc + are consistent with broken symmetry density functional theory electronic structure calculations (B3LYP/6-311G­(2df)/6-311G­(d,p)) that predict large structural rearrangements of the Mn coordination environment upon oxidation to MnIII with associated Jahn–Teller distortions. Titration of Mn 2 LAc +, Zn 2 LAc +, and H 4 L 2+ with NOPF6 in acetonitrile allowed for the isolation and spectroscopic examination of higher oxidations and were consistent with electrochemical assignments. The electrochemical and spectroscopic analysis of these complexes will aid in future studies involving electrocatalytic processes with related dinuclear macrocycles.本文报道了Mn₂LAc⁺、Zn₂LAc⁺与H₄L²⁺的合成方法，其中配体L为以4-叔丁基-2,6-二甲酰基苯酚与2,2'-二氨基-N-甲基二乙胺缩合得到的四希夫碱大环配体，其可提供N6O2配位环境。在Mn₂LAc⁺与Zn₂LAc⁺中，两个金属中心以乙酸根配体作为桥联配体。[Mn₂LAc](ClO₄)·(DMF)₀.₅、[Mn₂LAc](ClO₄)·(ACN)₀.₅与[Zn₂LAc](PF₆)均以空间群P2₁/c结晶，且晶胞参数与几何构型几乎完全一致。 采用循环伏安法（Cyclic Voltammetry, CV）对Zn₂LAc⁺与H₄L²⁺开展电化学分析，结果观测到两处不可逆阳极峰，可归属于酚氧配体的氧化过程。Mn₂LAc⁺的循环伏安曲线则呈现两处化学可逆的阳极峰，对应Mn(II)/Mn(III)的氧化过程，后续则为酚氧配体的不可逆氧化过程。通过对循环伏安曲线进行数字模拟，得到Mn₂IILAc⁺依次氧化为MnIIMnIIILAc²⁺、再氧化为Mn₂IIILAc⁺的单电子氧化界面电子转移动力学速率，分别为0.6与1.1×10⁻³ cm·s⁻¹。 Mn₂LAc⁺电化学扫描中观测到的缓慢界面电子转移动力学速率，与对称性破缺密度泛函理论（B3LYP/6-311G(2df)/6-311G(d,p)）电子结构计算结果相符：该计算预测Mn配位环境在氧化为Mn(III)时会发生显著结构重排，并伴随姜-泰勒畸变。以六氟磷酸亚硝（NOPF₆）在乙腈中对Mn₂LAc⁺、Zn₂LAc⁺与H₄L²⁺进行滴定，可分离得到更高氧化态的产物并开展光谱表征，所得结果与电化学分析的归属一致。 对上述配合物的电化学与光谱分析，将为后续开展涉及相关双核大环配合物的电催化过程研究提供有益参考。