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.