Spectroscopic, Electrochemical, and Computational Studies on an [FeFe]-Hydrogenase Active Site Mimic with a Terthiophene Bridging the 2Fe2S Core

Abstract

As a means of reducing the dependence on fossil fuels, generation of hydrogen (H₂) has been proposed as a route for storing energy in a chemical bond. To access this energy, H₂ can be combusted with oxygen or used in a fuel to release the energy stored in the chemical bond, while generating water as the byproduct. To generate the hydrogen necessary to fuel a hydrogen economy, an energy efficient and stable catalyst needs to be designed. The work presented in this thesis describes the investigation of a catalytic mimic inspired by the [FeFe]-hydrogenase enzyme. The design of this and similar mimics have been pursued as the active site of the enzyme is composed of readily available and abundant elements, and has a turnover rate of 6000-9000 molecules of H₂ s⁻¹. The catalyst in this work was studied via cyclic voltammetry and density functional theory calculations to determine the catalytic activity as well as a mechanism for H₂ production of the complex. The complex 2,5-bis-(2',2"-thiophen-2-yl)-thiophene-µ-3,4-dithiolato)diiron hexacarbonyl, 1, was prepared and found to catalyze the production of molecular hydrogen in CH₂Cl₂, however the overpotential for catalysis was not determined as the standard potential of acetic acid in CH₂Cl₂ is not known. Comparison of the catalytic potentials of terthiophene-cat to µ-(1,2-benzenedithiolato)diiron hexacarbonyl, 2, and µ-(3,4-thiophenedithiolato)diiron hexacarbonyl, 3, in CH₂Cl₂ showed that 1 had a less negative potential (0.14 V and 0.16 V, respectively) for the catalytic reduction of protons to H₂. Electrochemical investigations combined with density functional theory (DFT) indicated that 1 has an ECEC mechanism for the reduction of protons, where E is an electrochemical step and C is a chemical process. The proposed mechanism for 1 is similar to that of 2 and 3, with 1 catalyzing the production of H₂ using acetic acid at a less negative potential than 2 and 3.