Division of Chemistry and Chemical Engineering, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, United States.
Department of Chemistry and Biochemistry, Suffolk University, Boston, Massachusetts 02108, United States.
J Am Chem Soc. 2022 Apr 13;144(14):6516-6531. doi: 10.1021/jacs.2c01356. Epub 2022 Mar 30.
Ni 2,2'-bipyridine (bpy) complexes are commonly employed photoredox catalysts of bond-forming reactions in organic chemistry. However, the mechanisms by which they operate are still under investigation. One potential mode of catalysis is via entry into Ni(I)/Ni(III) cycles, which can be made possible by light-induced, excited-state Ni(II)-C bond homolysis. Here, we report experimental and computational analyses of a library of Ni(II)-bpy aryl halide complexes, Ni(bpy)(Ph)Cl (R = MeO, -Bu, H, MeOOC; R' = CH, H, OMe, F, CF), to illuminate the mechanism of excited-state bond homolysis. At given excitation wavelengths, photochemical homolysis rate constants span 2 orders of magnitude across these structures and correlate linearly with Hammett parameters of both bpy and aryl ligands, reflecting structural control over key metal-to-ligand charge-transfer (MLCT) and ligand-to-metal charge-transfer (LMCT) excited-state potential energy surfaces (PESs). Temperature- and wavelength-dependent investigations reveal moderate excited-state barriers (Δ ∼ 4 kcal mol) and a minimum energy excitation threshold (∼55 kcal mol, 525 nm), respectively. Correlations to electronic structure calculations further support a mechanism in which repulsive triplet excited-state PESs featuring a critical aryl-to-Ni LMCT lead to bond rupture. Structural control over excited-state PESs provides a rational approach to utilize photonic energy and leverage excited-state bond homolysis processes in synthetic chemistry.
镍 2,2'-联吡啶（bpy）配合物通常被用作有机化学中形成键反应的光氧化还原催化剂。然而，它们的作用机制仍在研究中。一种潜在的催化模式是通过进入 Ni(I)/Ni(III) 循环，这可以通过光诱导的、激发态 Ni(II)-C 键均裂来实现。在这里，我们报告了一系列 Ni(II)-bpy 芳基卤化物配合物的实验和计算分析，Ni(bpy)(Ph)Cl（R = MeO，-Bu，H，MeOOC；R' = CH，H，OMe，F，CF），以阐明激发态键均裂的机制。在给定的激发波长下，光化学均裂速率常数在这些结构中跨越 2 个数量级，与 bpy 和芳基配体的哈米特参数线性相关，反映了结构对关键金属-配体电荷转移（MLCT）和配体-金属电荷转移（LMCT）激发态势能面（PES）的控制。温度和波长依赖性研究分别揭示了中等激发态势垒（Δ∼4 kcal mol）和最小能量激发阈值（∼55 kcal mol，525 nm）。与电子结构计算的相关性进一步支持了一种机制，即具有关键芳基到 Ni 的 LMCT 的排斥三重态激发态 PES 导致键断裂。对激发态 PES 的结构控制为利用光子能量和在合成化学中利用激发态键均裂过程提供了一种合理的方法。
