Preparation and Characterization of [FeFe]-Hydrogenase Active Site Mimics Studied by Gas-Phase Photoelectron Spectroscopy, Electrochemistry, and Computational Models

Abstract

Hydrogen is an attractive fuel because it is clean, carbon neutral, energy dense, and sustainable, but in order for a hydrogen economy to become a reality it is necessary to develop inexpensive and efficient methods of hydrogen production. The [FeFe]-hydrogenase enzymes are efficient at catalyzing the reduction of protons to dihydrogen. Nature-inspired functional active site mimics which feature sulfur and iron have been the focus of much research, yet there are still challenges to overcome. The challenges of the [FeFe]-H₂ase active site mimic catalysts which are addressed in this dissertation are (1) the reversibility of the catalyst to reduction and (2) the overpotential required to achieve catalytic activity. One of the active site-inspired catalysts is (μ-1,3-propanedithiolato)diironhexacarbonyl. When studied by cyclic voltammetry, CV, this catalyst produces hydrogen but also transforms into catalytically inactive products. (μ-2,4-pentanedithiolato)diironhexacarbonyl, 3, was prepared and found be fully reversible to reduction at all scan rates, indicating that it does not decompose on the CV timescale. Compound 3 is prepared as a mixture of cis and trans isomers. The trans isomer is able to undergo inversion of the bridgehead and the cis isomer is fixed with no evidence of bridgehead fluxionality. NMR studies verify proton assignments and the barrier of inversion for the fluxional trans compound. DFT studies indicate that multiple pathways to catalysis are possible for 1 depending upon the pKₐ of acid present and the potential applied. [FeFe]-H₂ase-inspired catalysts which produce hydrogen in the presence of a weak acid require a higher than desired overpotential. To overcome this, an ideal catalyst would use captured solar energy to produce hydrogen. The catalyst (μ-1,2-benzenedithiolato)diironhexacarbonyl 5, is well studied and understood. Thiophene has a π-system that is isoelectronic with benzene. Thiophenes are air-stable and may be polymerized into electrically conductive polymers which may be light active. The catalyst (μ-3,4-thiophenedithiolato)diironhexacarbonyl, 6, was prepared as a proof of concept model for catalysts with polythiophene features. As predicted, compound 6 was found to reduce protons at the same potential as 5. DFT computations indicate these catalysts go through the same catalytic mechanisms. X-ray crystal structures indicate similar bond lengths and angles.