Platinum is one of the best catalysts to employ H2 as an energy carrier in the industry, yet limited availability prompts us to find alternative catalysts. In nature, a diverse group of hydrogenases can catalyze H2 evolution. Among them, [FeFe] hydrogenase has the highest turnover rate up to 104 s-1. Current understanding of the catalytic cycle for [FeFe] hydrogenase has inspired chemists to develop di-iron model complexes to mimic the function of enzymes. Herein, we focus our study on a different type of hydrogenase model complexes, which consist of mix-valent tetranuclear iron clusters. Using Mössbauer spectroscopy and Infrared spectroscopy assisted by density functional theory (DFT) calculations, we seek to identify and characterize electronic structures of intermediate complexes to further test the reaction mechanism for H2 evolution by these complexes, where the reaction can proceed through either a “CCEE” or an “EECC” route in the catalytic cycles. By comparing DFT results and spectroscopic data, a unique intermediate spin state (S = 1) of Fe(II) site in the tetranuclear iron clusters is confirmed under the sulfur dominated ligand environment. DFT calculations suggests that the S = 0 Fe(II) site in the mix-valent dinuclear Fe(I)Fe(II) precursors change the spin state to S = 1 state upon formation of the tetranuclear iron cluster. This spectroscopic/theoretical result is further confirmed in the single/double protonated and one electron reduced complexes. The implication of an S = 1 Fe site in the tetranuclear iron cluster to its chemical reactivity is further discussed.

Mechanistic and spectroscopic studies of [FeFe] Hydrogenase model complexes

ACS Fall 2021

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