Computational, Photoelectron Spectroscopic, and Electrochemical Studies of the Electronic Structures in Hydrogenase-Inspired Molecules

Abstract

This dissertation focuses on the study of [FeFe]-Hydrogenase active-site mimics, which are utilized for electrocatalytic generation of H₂ from acetic acid. Infrared and photoelectron spectroscopy, cyclic voltammetry, and DFT computations are used in order to gain insights into the mechanistic pathway of selected catalysts. While many active-site mimics have been synthesized, few have been studied electrochemically, and even fewer have had mechanistic studies performed. This work attempts to focus on a few parent mimics, on which many derivatives have been based, and to provide a clear understanding of how the catalytic mechanisms produce molecular hydrogen. First, a derivative of (μ-1,2-benzenedithiolato)[Fe(CO)₃]₂ (1) is studied, where the 3,6-hydrogen atoms on the benzene ring are substituted with alcohol groups, [μ-4-substituted-3,6-dihydroxy-1,2-benzenedithiolato][Fe(CO)₃]₂, X = H, OMe, Me, ᵗBu, Cl, or Br (2a-f). A control complex, [μ-3,6-dimethoxy-1,2-benzenedithiolato][Fe(CO)₃]₂ (3), is also studied. It was found that both the hydroquinone and dimethylhydroquinone derivatives operate in an E-ECEC mechanism, and the rate-limiting step in all cases remains the protonation of the dianionic species. Internal hydrogen-bonding in the hydroquinone complexes stabilizes the dianionic species, thereby slowing the rate of protonation. Three complexes, [μ-SCH₂XCH₂S][Fe(CO)₃]₂, X = CH₂, NH, and O (4CH₂, 4NH, and 4₀), are studied to better understand the implications of different atoms in the bridgehead of the [FeFe]-Hydrogenase enzyme. A rotated cationic structure, where a terminal COapical has moved to bridge the two Fe atoms, similar to the resting state of the active site is seen and unique properties of 4NH, outside of changes in electronegativity, are seen. The electrocatalytic reduction of acetic acid using 4CH₂, 4NH, and 4₀ is studied. Using cyclic voltammetry with aid from DFT computations, all complexes are found to generate H₂ by either an EECC or EECE-C mechanism. Two additional mechanistic pathways are available for 4NH, involving protonation of the anion: ECEC or E-CECE. The parent compound, 4NH, was decorated with either a methyl (4Me) or a ᵗbutyl group (4tBu) on the nitrogen bridgehead, adding electron density. Both substituted compounds are found to undergo an initial two-electron reduction, which decreases the overpotential of H₂ catalysis. 4tBu was found to have the both the lowest overpotential and the highest catalytic activity of the series. DFT computations have been vital throughout this work, which lead to an in-depth examination of how functionals handle the pKₐ values of five [FeFe]-Hydrogenase active-site mimics: [μ-SCH₂XCH₂S][Fe(CO)₃]₂, X = NHH⁺ and NMe⁺ (4NHH⁺ and 4NMeH⁺), [μ-SCHMeNHHCHMeS][Fe(CO)₃]₂ (5NHH⁺), [μ-SCH₂NHHCH₂S][Fe(CO)₂](PMe₃)]₂ (6NHH⁺), and [μ-SCHMeNHHCHMeS][Fe(CO)₂](PMe₃)]₂ (7NHH⁺). Twelve exchange correlation functionals are evaluated for use in predicting the pKₐ values and oxidation potentials of the active-site mimics in addition to 12 small amines. None of the functionals were able to correctly predict the ΔG of the de-protonation reaction or the ΔG of oxidation for the amines or the iron complexes. The findings indicate that none of the functionals are able to adequately account for stabilizing positive charges, but the underestimation of this stabilization becomes less important in larger molecules. Combining IR and photoelectron spectroscopies, cyclic voltammetry, and DFT computations, it is possible to gain a thorough understanding of the mechanism a catalyst follows when reducing acids to H₂. With this understanding one can hope to design more efficient catalysts for hydrogen production.