The Role of Quantum Tunneling in Metal–Ligand Proton Tautomerism

Proton transfer processes are fundamental to a wide range of chemical transformations, yet the role of quantum mechanical effects in these events remains largely unexplored. Herein, we report the first experimental demonstration of metal–ligand proton tautomerism (MLPT) proceeding via quantum mechanical tunneling (QMT) in a ruthenium complex supported by a chelating pyridine-based PN ligand. Due to their near isoenergetic nature, both tautomers can be spectroscopically observed, which enables a detailed mechanistic interrogation through isotopic labeling, kinetic studies, and comprehensive computational analysis. Remarkably, these tautomers display divergent reactivity in hydrogen atom abstraction and dihydrogen activation, revealing that MLPT profoundly modulates the chemical behavior of the complex. Computational investigations further indicate that QMT-facilitated MLPT may be widespread across pincer-type systems, suggesting an under-appreciated mechanistic paradigm in organometallic chemistry. These findings provide critical new insights into proton dynamics and open avenues for designing catalysts that harness quantum tunneling effects.