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.