Deetz Alexander M, Meyer Gerald J
Department of Chemistry, University of North Carolina at Chapel Hill, Murray Hall 2202B, Chapel Hill, North Carolina 27599-3290, United States.
JACS Au. 2022 Apr 12;2(4):985-995. doi: 10.1021/jacsau.2c00088. eCollection 2022 Apr 25.
Stabilization of ions and radicals often determines reaction kinetics and thermodynamics, but experimental determination of the stabilization magnitude remains difficult, especially when the species is short-lived. Herein, a competitive kinetic approach to quantify the stabilization of a halide ion toward oxidation imparted by specific stabilizing groups relative to a solvated halide ion is reported. This approach provides the increase in the formal reduction potential, Δ°'(Χ), where X = Br and I, that results from the noncovalent interaction with stabilizing groups. The [Ir(dF-(CF)-ppy)(tmam)] photocatalyst features a dicationic ligand tmam [4,4'-bis[(trimethylamino)methyl]-2,2'-bipyridine] that is shown by H NMR spectroscopy to associate a single halide ion, = 7 × 10 M (Br) and = 1 × 10 M (I). Light excitation of the photocatalyst in halide-containing acetonitrile solutions results in competitive quenching by the stabilized halide and the more easily oxidized diffusing halide ion. Marcus theory is used to relate the rate constants to the electron-transfer driving forces for oxidation of the stabilized and unstabilized halide, the difference of which provides the increase in reduction potentials of Δ°'(Br) = 150 ± 24 meV and Δ°'(I) = 67 ± 13 meV. The data reveal that is a poor indicator of these reduction potential shifts. Furthermore, the historic and widely used assumption that Coulombic interactions alone are responsible for stabilization must be reconsidered, at least for polarizable halogens.
离子和自由基的稳定性常常决定反应动力学和热力学，但对稳定性大小的实验测定仍然困难，尤其是当这些物种寿命较短时。本文报道了一种竞争动力学方法，用于量化特定稳定基团相对于溶剂化卤离子对卤离子氧化的稳定作用。该方法提供了由于与稳定基团的非共价相互作用而导致的形式还原电位的增加量Δ°'(Χ)，其中X = Br和I。[Ir(dF-(CF)-ppy)(tmam)]光催化剂具有双阳离子配体tmam [4,4'-双[(三甲氨基)甲基]-2,2'-联吡啶]，通过核磁共振氢谱表明它能与单个卤离子缔合，其缔合常数K = 7×10⁴ M⁻¹(Br)和K = 1×10⁵ M⁻¹(I)。在含卤乙腈溶液中对光催化剂进行光激发会导致被稳定的卤离子和更容易被氧化的扩散卤离子之间的竞争猝灭。利用马库斯理论将速率常数与稳定和未稳定卤离子氧化的电子转移驱动力联系起来，两者的差值给出了还原电位的增加量，即Δ°'(Br) = 150±24 meV和Δ°'(I) = 67±13 meV。数据表明，K并不是这些还原电位变化的良好指标。此外，必须重新考虑历史上广泛使用的仅库仑相互作用导致稳定的假设，至少对于可极化卤素来说是这样。
