Metal–Ligand Cooperation in N–H Activation: Bridging Electron-Pushing Formalism and Energy Descriptors

The activation of N–H bonds is a fundamental step in the synthesis of industrially relevant compounds but remains a challenging process. A promising strategy to address it, introduced by Milstein and co-workers, relies on metal–ligand cooperation, in which N–H activation is coupled with an aromatization–dearomatization process of a pincer ligand. In this work, we employ state-of-the-art theoretical methods grounded in quantum chemical topology (QCT) to gain deeper insights into this process. Using the archetypal PNP–Ru(II) complex reported by Milstein (JACS 2010, 132, 8542), we analyze the electron density rearrangements during N–H activation through the electron localization function and bonding evolution theory. Interacting quantum atoms energy decomposition is further applied to quantify interactions between key groups. The study covers substrates from ammonia to primary amines, revealing that hydrogen transfer occurs as a quasi-protonic species, yielding a Ru–amido complex. The mechanism remains consistent across substrates, with electron-withdrawing groups facilitating the process by stabilizing the NH–R interaction. Additionally, modifying the ligand scaffold with electron-donating substituents enhances charge accumulation at the reactive carbon, improving both kinetics and thermodynamics. Overall, our findings highlight QCT as a powerful framework for guiding the rational design of improved systems.

N–H键活化是合成工业相关化合物的关键步骤，但该过程仍颇具挑战性。Milstein及其同事提出的极具前景的解决策略，依托金属-配体协同作用：在此机制中，N–H键活化与钳形配体的芳构化-去芳构化过程相耦合。本研究采用基于量子化学拓扑学（quantum chemical topology, QCT）的前沿理论方法，以期对该过程获得更深入的认知。以Milstein团队2010年发表于《美国化学会志》（JACS, 2010, 132, 8542）的典型PNP-钌(II)配合物为研究对象，我们通过电子定域函数（electron localization function）与成键演化理论（bonding evolution theory），分析了N–H键活化过程中的电子密度重排行为。进一步应用相互作用量子原子能量分解法（interacting quantum atoms energy decomposition），对关键基团间的相互作用进行量化表征。本次研究覆盖了从氨到伯胺的各类底物，结果显示氢转移以类质子物种的形式进行，最终生成钌-酰胺配合物。不同底物的反应机制保持一致，吸电子基团可通过稳定NH–R相互作用促进反应进程。此外，在配体骨架上引入给电子取代基，可增强反应活性碳位点的电荷积累，同时优化反应的动力学与热力学性能。综上，本研究结果证实，量子化学拓扑学（QCT）是指导合理设计更高效催化体系的有力研究框架。