Savéant Jean-Michel
Laboratoire d'Electrochimie Moléculaire, Université de Paris 7-Denis Diderot, 2 place Jussieu, 75251 Paris Cedex 05, France.
J Am Chem Soc. 2008 Apr 9;130(14):4732-41. doi: 10.1021/ja077480f. Epub 2008 Mar 18.
Ion-pairing with electro-inactive metal ions may change drastically the thermodynamic and kinetic reactivity of electron transfer in chemical and biochemical processes. Besides the classical stepwise pathways (electron-transfer first, followed by ion-pairing or vice versa), ion-pairing may also occur concertedly with electron transfer. The latter pathway avoids high-energy intermediates but a key issue is that of the kinetic price to pay to benefit from this thermodynamic advantage. A model is proposed leading to activation/driving force relationships characterizing such concerted associative electron transfers for intermolecular and intramolecular homogeneous reactions and for electrochemical reactions. Contrary to previous assertions, the driving force of the reaction (defined as the opposite of the reaction standard free energy), as well as the intrinsic barrier, does not depend on the concentration of the ion-pairing agent, which simply plays the role of one of the reactants. Besides solvent and intramolecular reorganization, the energy of the bond being formed is the main component of the intrinsic barrier. Application of these considerations to reactions reported in recent literature illustrates how concerted ion-pairing electron-transfer reactions can be diagnosed and how competition between stepwise and concerted pathways can be analyzed. It provided the first experimental evidence of the viability of concerted ion-pairing electron-transfer reactions.
与电惰性金属离子形成离子对可能会极大地改变化学和生物化学过程中电子转移的热力学和动力学反应活性。除了经典的分步途径（先进行电子转移，随后形成离子对，或者反之亦然），离子对也可能与电子转移协同发生。后一种途径避免了高能中间体，但一个关键问题是要从这种热力学优势中获益需要付出的动力学代价。本文提出了一个模型，该模型得出了活化/驱动力关系，可用于表征分子间和分子内均相反应以及电化学反应中这种协同缔合电子转移的特征。与之前的论断相反，反应的驱动力（定义为反应标准自由能的相反数）以及内在势垒并不取决于离子配对剂的浓度，离子配对剂仅仅起到反应物之一的作用。除了溶剂和分子内重排外，正在形成的键的能量是内在势垒的主要组成部分。将这些考虑因素应用于近期文献报道的反应，说明了如何诊断协同离子对电子转移反应以及如何分析分步途径和协同途径之间的竞争。它提供了协同离子对电子转移反应可行性的首个实验证据。
