Investigation of Ligand Modification Effects on Electronic Properties and Hydrogen Evolution Reactivity of Fe, Co-IminoBpy Complexes

Abstract

A fundamental understanding of the hydrogen evolution reaction (HER) is essential for designing catalysts capable of promoting hydrogen generation with high activity. Among various homogeneous transition-metal systems, Fe and Co complexes incorporating Imbpy (imine–bipyridine) ligands have been reported to exhibit distinct electron-transfer pathways depending on the spin state of the central metal. These previous findings indicate that fine-tuning the spin state of the metal center and the electronic structure of the ligand plays a crucial role in improving HER efficiency. In this study, Fe and Co complexes incorporating Imbpy (imine–bipyridine) ligands were designed and synthesized to investigate the substituent effects and electronic influences on hydrogen evolution reaction (HER) catalysis. Electron-withdrawing (EWG) and electron-donating (EDG) groups were introduced at the phenyl ring of the Imbpy ligand to modulate the orbital energy levels and to compare their effects on the metal center and catalytic activity. NMR analyses confirmed the structural and electronic variations induced by the substituents, and their electrochemical properties were evaluated. Furthermore, a π-extended pyrene- substituted Imbpy complex was newly designed, where enhanced π–π* resonance and electron delocalization are proposed to facilitate charge transfer and improve HER activity.