Reath Alexander H, Ziller Joseph W, Tsay Charlene, Ryan Austin J, Yang Jenny Y
Department of Chemistry, University of California , Irvine, California 92697, United States.
Inorg Chem. 2017 Mar 20;56(6):3713-3718. doi: 10.1021/acs.inorgchem.6b03098. Epub 2017 Feb 27.
Redox inactive Lewis acidic cations are thought to facilitate the reactivity of metalloenzymes and their synthetic analogues by tuning the redox potential and electronic structure of the redox active site. To explore and quantify this effect, we report the synthesis and characterization of a series of tetradentate Schiff base ligands appended with a crown-like cavity incorporating a series of alkali and alkaline earth Lewis acidic cations (1M, where M = Na, K, Ca, Sr, and Ba) and their corresponding Co(II) complexes (2M). Cyclic voltammetry of the 2M complexes revealed that the Co(II/I) redox potentials are 130 mV more positive for M = Na and K and 230-270 mV more positive for M = Ca, Sr, and Bacompared to Co(salen-OMe) (salen-OMe = N,N'-bis(3-methoxysalicylidene)-1,2-diaminoethane), which lacks a proximal cation. The Co(II/I) redox potentials for the dicationic compounds also correlate with the ionic size and Lewis acidity of the alkaline metal. Electronic absorption and infrared spectra indicate that the Lewis acid cations have a minor effect on the electronic structure of the Co(II) ion, which suggests the shifts in redox potential are primarily a result of electrostatic effects due to the cationic charge.
氧化还原惰性的路易斯酸性阳离子被认为可通过调节氧化还原活性位点的氧化还原电位和电子结构来促进金属酶及其合成类似物的反应活性。为了探索和量化这种效应，我们报道了一系列四齿席夫碱配体的合成与表征，这些配体带有一个冠状空腔，其中包含一系列碱金属和碱土金属路易斯酸性阳离子（1M，其中M = Na、K、Ca、Sr和Ba）及其相应的Co(II)配合物（2M）。2M配合物的循环伏安法表明，与缺乏近端阳离子的Co(salen-OMe)（salen-OMe = N,N'-双(3-甲氧基水杨醛)-1,2-二氨基乙烷）相比，对于M = Na和K，Co(II/I)氧化还原电位更正130 mV，对于M = Ca、Sr和Ba则更正230 - 270 mV。二价阳离子化合物的Co(II/I)氧化还原电位也与碱金属的离子大小和路易斯酸性相关。电子吸收光谱和红外光谱表明，路易斯酸阳离子对Co(II)离子的电子结构影响较小，这表明氧化还原电位的变化主要是由于阳离子电荷引起的静电效应。
