Wojdyla Zuzanna, Srnec Martin
J. Heyrovský Institute of Physical Chemistry, The Czech Academy of Sciences Dolejškova 3 Prague 8 18223 Czech Republic
Chem Sci. 2024 May 10;15(22):8459-8471. doi: 10.1039/d4sc01507j. eCollection 2024 Jun 5.
Here, we demonstrate that the relationship between reactivity and thermodynamics in radical ligand transfer chemistry can be understood if this chemistry is dissected as concerted ion-electron transfer (cIET). Namely, we investigate radical ligand transfer reactions from the perspective of thermodynamic contributions to the reaction barrier: the diagonal effect of the free energy of the reaction, and the off-diagonal effect resulting from asynchronicity and frustration, which we originally derived from the thermodynamic cycle for concerted proton-electron transfer (cPET). This study on the OH transfer reaction shows that the three-component thermodynamic model goes beyond cPET chemistry, successfully capturing the changes in radical ligand transfer reactivity in a series of model Fe-OH⋯(diflouro)cyclohexadienyl systems. We also reveal the decisive role of the off-diagonal thermodynamics in determining the reaction mechanism. Two possible OH transfer mechanisms, in which electron transfer is coupled with either OH and OH transfer, are associated with two competing thermodynamic cycles. Consequently, the operative mechanism is dictated by the cycle yielding a more favorable off-diagonal effect on the barrier. In line with this thermodynamic link to the mechanism, the transferred OH group in OH/electron transfer retains its anionic character and slightly changes its volume in going from the reactant to the transition state. In contrast, OH/electron transfer develops an electron deficiency on OH, which is evidenced by an increase in charge and a simultaneous decrease in volume. In addition, the observations in the study suggest that an OH/electron transfer reaction can be classified as an adiabatic radical transfer, and the OH/electron transfer reaction as a less adiabatic ion-coupled electron transfer.
在此，我们证明，如果将自由基配体转移化学解析为协同离子 - 电子转移（cIET），那么就可以理解自由基配体转移化学中反应活性与热力学之间的关系。具体而言，我们从热力学对反应势垒的贡献角度研究自由基配体转移反应：反应自由能的对角效应，以及由非同步性和受挫导致的非对角效应，这是我们最初从协同质子 - 电子转移（cPET）的热力学循环推导出来的。对OH转移反应的这项研究表明，三组分热力学模型超越了cPET化学，成功捕捉了一系列模型Fe - OH⋯(二氟)环己二烯基体系中自由基配体转移反应活性的变化。我们还揭示了非对角热力学在确定反应机理中的决定性作用。两种可能的OH转移机制，即电子转移与OH转移或OH转移耦合，与两个相互竞争的热力学循环相关。因此，起作用的机制由对势垒产生更有利非对角效应的循环决定。与这种与机理的热力学联系一致，OH/电子转移中转移的OH基团在从反应物到过渡态的过程中保持其阴离子特性，并略微改变其体积。相比之下，OH/电子转移在OH上产生电子不足，这通过电荷增加和体积同时减小得以证明。此外，该研究中的观察结果表明，OH/电子转移反应可归类为绝热自由基转移，而OH/电子转移反应为绝热性较低的离子耦合电子转移。
