Bistable Copper Complexes of Bis-thia-bis-quinoline Ligands

The new ligands R,R-trans-S,S‘-bis[methyl(2‘-quinolyl)]-1,2-dithiacyclohexane, cis-S,S‘-bis[methyl(2‘-quinolyl)]-1,2-dithiacyclohexane, and 1,6-bis(2‘-quinolyl)-2,5-dithiahexane have been synthesized and their complexes with CuI
and CuII prepared. The ligand/metal systems are bistable, as the complexes with copper in both its oxidation states
are stable under the same conditions as solids and in solution. The crystal and molecular structure of [CuI(1,6-bis(2‘-quinolyl)-2,5-dithiahexane)]ClO4 has been determined by X-ray diffraction and reveals that the complex is
monomeric, with the ligand folding around the Cu+ cation, imparting to it a tetrahedral coordination. UV−vis, MS-ESI, and NMR data indicate that the same is found for the CuI complexes of all three ligands. Also, the CuII
complexes are monomeric, but with a square arrangement of the ligands around Cu2+. On changing the oxidation
state, the change in the geometrical arrangement is fast and complete in less than 80 ms, as demonstrated by
cyclic voltammetry experiments. In the CV profiles, the oxidation and reduction events take place at separated Eox
and Ered values, with no return wave even at the fastest scan rates. In the Eox−Ered interval (which ranges from 450
to 650 mV, depending on the ligand), the ligand/copper system can thus exist in one of its two states, depending
on its history, and thus display electrochemical hysteretical behavior. The electrochemical cycle leading from the
tetrahedral [CuI(ligand)]+ to the square [CuII(ligand)]2+ complex (and vice versa) is reversible and repeatable without
degradation, as checked by coupled UV−vis-controlled potential coulometry experiments.

本研究合成了三种新型配体（ligand），分别为R,R-反式-S,S'-双[甲基(2'-喹啉基)]-1,2-二硫环己烷、顺式-S,S'-双[甲基(2'-喹啉基)]-1,2-二硫己烷与1,6-双(2'-喹啉基)-2,5-二硫己烷，并成功制备了它们与铜(I)（CuI）及铜(II)（CuII）的配合物。该配体-金属体系呈现双稳态特性：无论是固态还是溶液形态，对应两种铜氧化态的配合物在相同条件下均可稳定存在。通过X射线衍射（X-ray diffraction）解析了[Cu(I)(1,6-双(2'-喹啉基)-2,5-二硫己烷)]ClO4的晶体与分子结构，结果表明该配合物为单核（monomeric）结构，配体围绕Cu+阳离子发生折叠，使Cu+中心形成四面体配位（tetrahedral coordination）构型。紫外-可见光谱（UV-vis）、电喷雾质谱（MS-ESI）与核磁共振（NMR）表征数据显示，三种配体的铜(I)配合物均具有一致的结构特征。而铜(II)配合物同样为单核结构，但配体围绕Cu2+中心形成四方配位排布。循环伏安法（cyclic voltammetry）实验证实，改变铜的氧化态时，配位几何构型的转变快速且完全，耗时不足80 ms。在循环伏安图谱（CV profiles）中，氧化与还原过程分别于分离的氧化电位（Eox）与还原电位（Ered）处发生，即便采用最快扫描速率，也未观测到返回波。在450 mV至650 mV（因配体不同而存在差异）的Eox-Ered区间内，该配体-铜体系可根据其历史状态以两种稳态之一存在，因此表现出电化学滞后（hysteretical）行为。由四面体构型的[Cu(I)(配体)]+向四方构型的[Cu(II)(配体)]2+配合物（及反向转化）的电化学循环具备可逆性与可重复性，且无降解现象，该结论通过联用紫外-可见光谱与控制电位库仑法（controlled potential coulometry）的实验得到了验证。