Mechanistic Studies on Proton and Electron Transfer Pathways in Transition Metal Complexes

Abstract

This dissertation aims to control electron transfer in transition metal complexes by utilizing the proton reactivity of coordinating ligands, based on an understanding of metal-ligand orbital interactions. Three π- conjugated ligands, phenanthroline, dithiolene, and selenoureato, were used to prepare cobalt (Co), tungsten (W), and palladium (Pd) complexes. First, in cobalt complexes, introducing phenanthroline alongside an NNN-pincer ligand enabled metal-ligand mixed redox behavior, enhancing CO₂ reduction efficiency through effective two-electron transfer. Second, in a bis(dithiolene) W-oxo complex where the strongly σ-donating oxo ligand renders the system resistant to reduction, hydrogen bonding was used to induce structural distortion, thereby strengthening orbital overlap and lowering the lowest unoccupied molecular orbital energy level. This strategy facilitates reduction of WIV to WIII, thereby enabling the first spectroscopic identification of a WIII–OH species. Finally, a Pd-based H2 evolution catalyst using a selenoureato ligand showed high efficiency and air stability. Analysis of the Pd–Se bonding and reaction energetics revealed that the selenoureato ligand stabilizes the otherwise unstable Pd ion under electrochemical conditions, accounting for its superior performance. Throughout the dissertation, combined experimental and theoretical studies provided fundamental insights into electron transfer processes in transition metal complexes, and the research findings are discussed herein.