Enhanced Catalytic Activity via Rapid Two-Electron Transfer in Low-Spin Fe(II) Complex and Spin-State Dependent Proton Reduction Pathways

The growing interest in green hydrogen gas production has brought significant attention to the development of efficient proton reduction catalysts. A comprehensive understanding of proton and electron transfer processes within catalyst complexes is crucial for developing efficient catalysts. While the proton transfer process is influenced by the Brønsted acid used, electron transfer is an intrinsic property determined by the molecular orbitals and spin states of complexes. Complexes that rapidly transfer electrons are associated with high catalytic performance. In this study, we present a first example of low-spin Fe^II complex that utilizes the π* orbital of ligand for rapid two-electron transfer, resulting in exceptional catalytic performance for hydrogen gas evolution. The consecutive two-electron transfer rate was measured at 33.24 s^-1, and in combination with proton transfer, the catalyst achieved an extraordinarily high turnover frequency (TOF) of 224,643 s^-1 for hydrogen gas production. Conversely, a high-spin Fe(II) complex produced hydrogen gas at a relatively low TOF of 8848 s^-1. These comparative experiments confirmed that the observed high catalytic efficiency is unique to the low-spin Fe^II complex, attributed to its distinct electron transfer mechanism.