Electrochemical and Computational Analysis of Aryl-Bridged [2Fe-2S] Complexes for Hydrogen Production

Abstract

Advances in renewable energy storage are necessary before the technology can become widespread due to the intermittency of energy supply. Hydrogen is a promising energy storage solution due to its high energy density and carbon-free emissions. In nature, the [FeFe] hydrogenase enzyme efficiently catalyzes the reduction of protons to hydrogen. Inspired by this process, several aryl-bridged analogs were investigated which maintained the [2Fe-2S] cluster core found within the catalytically active site of the enzyme. Mechanistic studies on (μ-2,3-naphthalenedithiolato-1,4-bisisobutyrate)Fe2(CO)6 characterized the foundational catalytic properties of the [2Fe-2S] active site within the PDMAEMA-g-[2Fe-2S] metallopolymer. Spectroscopic and electrochemical characterization of (μ-3,4-thiophenedithiolato)Fe2(CO)6 and (μ-2,3-thiophenedithiolato)Fe2(CO)6 served to illustrate the impact that system variables such as substrate and catalyst concentration can have on the catalytic mechanism, observed current, and mitigation of degradation pathways. Comparison of these results to the well-known and fast (μ-1,2-benzenedithiolato)Fe2(CO)6 complex catalogued these results with respect to those in the broader aryl-bridged [2Fe-2S] cluster community. All studied complexes followed an E(ECEC) mechanism in which the neutral procatalyst must undergo an initial electron transfer event to enter the hydrogen evolution reaction catalytic cycle. Density functional theory (DFT) was used to better understand the structural changes each complex underwent in the initial reduction event as well as locate the thermodynamically favorable sites for protonation. Cyclic voltammetry revealed different equilibrium constants for the first protonation step, as well as allowed the measurement of slow heterogeneous electron transfer rates which proved significant in limiting the amount of observed catalytic current. Though the kinetics of aryl-bridged [2Fe-2S] cluster catalysts differed, the significance of this was diminished upon embedment into a polymer framework and transition to aqueous solution conditions as the switch from homogeneous to heterogeneous catalysis substantially impacted the rate of catalysis. The grounds for adsorption were assigned to the presence of the PDMAEMA backbone, with or without the protonation of its amines, in conjunction with its presence in an aqueous buffered solution.