Controlling Electron and Proton Transfer Pathways in Transition Metal Complexes Jueun Lee College of Natural Sciences Gwangju Institute of Science and Technology

Abstract

This dissertation demonstrates how proton-coupled electron transfer (PCET) can be controlled in transition metal complexes through systematic modulation of electron and proton transfer processes. The study focuses on cobalt (Co) and iron (Fe) complexes supported by imino-bipyridyl (Imbpy) ligands, where metal–ligand orbital interactions and spin configurations define the accessible reaction pathways. First, Imbpy-Co complexes were investigated to establish how exchange coupling between the metal and redox-active ligand controls electron transfer behavior. Distinct ferromagnetic and antiferromagnetic coupling states govern different PCET pathways, resulting in either concerted or stepwise mechanisms during proton reduction. Second, PCET in Imbpy-Co complexes are controlled not only by spin interactions but also by the pKa of the proton source, which determines the overall reaction pathway. Third, Imbpy-Fe complexes were examined to evaluate the role of spin state in electron transfer. A low-spin Fe(II) complex enables rapid consecutive two-electron transfer through ligand π* orbitals, resulting in high catalytic activity for hydrogen evolution with a turnover frequency of 224,643 s⁻¹. These results collectively show that PCET pathways can be systematically controlled by independently regulating electron transfer through spin interactions and proton transfer through acid strength. This work provides a general framework for understanding and controlling PCET processes in molecular catalysts based on transition metal complexes.