Use of weakly coordinating anions to develop an integrated approach to the tuning of ΔE1/2 values by medium effects
Use of weakly coordinating anions to develop an integrated approach to the tuning of ΔE1/2 values by medium effects
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DOI：
10.1021/ja058171x
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发表时间：
2006-03-29
影响因子：
15
通讯作者：
Geiger, WE
中科院分区：
文献类型：
--
作者：
Barrière, F;Geiger, WE
通过介质效应，对涉及阳离子产物的多步氧化还原反应描述了一种控制δ E-1/2值的综合方法，从而描述了比例平衡。在45种不同的溶剂和支撑电解质条件下，测定了双（fulvalene）二镍1的两种单电子氧化反应的E-1/2值(定义为E-1/2（2+/1+) - E-1/2(1+/0)）。苯甲醚/0.1 M [NBU4]Cl中最小值为212 mV， CH2Cl2/0.02 M Na[B(C6H3(CF3)(2))(4)]中最大值为850 mV。通过系统地改变溶剂性质、支撑电解质离子的离子配对强度和电解质浓度，我们发现了一组理想的性质，可以使带正电的电极产物的E-1/2值最大化。最重要的是(i)溶剂必须具有较低的极性和较低的施主强度；（ii）支撑电解质必须具有弱配位阴离子（WCA）。1（2+）与WCAs（弱侧）和卤化物（强侧）的离子配对趋势的对比类似于THF中的diions，其中长链四烷基铵离子（弱离子配对）与碱金属离子（强离子配对）的对比。介质效应的总体图景是影响多电子系统E-1/2值调谐的溶剂和电解质性质的“镜像”。对四铁新基配合物Ni(S2C2FC2)(2), Fc = Fe(C5H5)(C5H4)， 2的四步氧化法和两步还原法进行了应用。根据介质的不同，可以观察到双电子过程2（0/2-）作为单个双电子伏安波或作为两个分离良好的单电子波。讨论了混合价化学中E-1/2值解释的结果。
An integrated approach to the control of Delta E-1/2 values, and therefore comproportionation equilibria, through medium effects was delineated for multi-step redox reactions involving cationic products. Delta E-1/2 values (defined as E-1/2(2+/1+) - E-1/2(1+/0)) of the two one-electron oxidations of bis(fulvalene)dinickel, 1, were measured under 45 different conditions of solvent and supporting electrolyte. The smallest value, 212 mV, was found in anisole/0.1 M [NBU4]Cl and the largest, 850 mV, in CH2Cl2/0.02 M Na[B(C6H3(CF3)(2))(4)]. By systematically changing the solvent properties, the degree of ion-pairing strengths of the supporting electrolyte ions, and the concentration of the electrolytes, a set of ideal properties was found for maximizing Delta E-1/2 values involving positively charged electrode products. Most importantly (i) the solvent must be of lower polarity and low donor strength and (ii) the supporting electrolyte must have a weakly coordinating anion (WCA). The contrast in ion-pairing tendencies of 1(2+) with WCAs (on the weak side) and halides (on the strong side) mimics that of dianions in THF, where longer chain tetraalkylammonium ions (weak ion pairing) contrast with alkali metal ions (strong ion pairing). The broad picture of medium effects is that of a "mirror image" of solvent and electrolyte properties that influence the tuning of Delta E-1/2 values for multi-electron systems. Application was made both to the four-step oxidation process and to the two-step reduction process of the tetraferrocentyl complex Ni(S2C2FC2)(2), Fc = Fe(C5H5)(C5H4), 2. The two-electron process 2(0/2-) is observed either as a single two-electron voltammetric wave or as two well-separated one-electron waves, depending on the medium. The consequences of the present model for the interpretation of Delta E-1/2 values in mixed-valence chemistry are discussed.