Hydrides in [FeFe]-hydrogenase model complexes

As humans continue to rely heavily on fossil fuels for our energy sources, many scientists are researching renewable energy sources. One potential energy source is hydrogen fuel cells, in which the anode reduces hydrogen to protons and electrons while the cathode forms H2O. Historically, Pt has been used as an efficient hydrogen production or reduction catalyst; however, it is expensive. Hydrogen fuel cells will not become abundant until a cheaper, stable hydrogen catalyst can be synthesized. Hydrogenase enzymes are able to produce or reduce hydrogen efficiently using first-row transition metals (Ni and Fe). This work investigates emulating the active site of [NiFe]- and [FeFe]-H2ase complexes. [NiFe]- and [FeFe]-H2ase enzymes evolved separately but have very similar features in the active site. Both active sites contain iron bound to a carbonyl moiety, a cyanide ligand, and bridging thiolates. Fe4S4 clusters are found in both enzymes and deliver electrons to and from the active site. The [FeFe]-H2ase enzyme is more active for hydrogen production and reduction, therefore this work will focus on modeling the [FeFe]-H2ase active site. Specifically, this work addresses the fact that these mimics are not as active as the protein. One major reason for this lower activity is that upon protonation of the metal center, a stable terminal hydride species is formed. The terminal hydride species upon further protonation, spontaneously releases H2. However, when a terminal hydride species is formed in a model complex, the hydride quickly isomerizes to a µ-hydride. The µ-hydride species is not active because the azadithiolate ligand, which delivers protons directly to the metal center, cannot interact with the µ-hydride due to the long distance. Therefore, the first three chapters of this thesis will be addressing how to activate the µ-hydride or stabilize the terminal hydride. The second chapter focuses on activating the µ-hydride by incorporating proton-responsive ligands. During the catalytic cycle of [NiFe]-H2ase enzymes, a µ-hydride is formed and then is further protonated through a protonatable ligand (a terminal thiolate). Utilizing a well-studied proton-responsive ligand, 2-diphenylposphinoaniline (PNH2), a new diiron complex was synthesized with two PNH2 ligands. A neutral µ-hydride species forms through oxidative addition immediately upon addition of the second PNH2 ligand. It was possible to protonate and deprotonate the neutral µ-hydride species and the ΔνCO shifts matched well with protonation of terminal thiolate ligands in the [NiFe]-H2ase. The third chapter addresses the stabilization of the terminal hydride by increasing the steric bulk at the bridgehead position. It was proposed that the steric interaction between the terminal hydride and the bridgehead would slow isomerization. 3,3-bis-methylpropanedithiolate, Me2pdt2, was used as the bridgehead since the addition of acid could not protonate the bridgehead and could only protonate the metal. The terminal hydride was stabilized for several hours; therefore, the rate of isomerization could be studied at various temperatures. Due to the longevity of the terminal hydride species, a new, never-before-seen intermediate, a symmetric terminal hydride species, was identified using 1H and 31P NMR. The fourth chapter focusing on stabilizing a catalytically active model complex. Due to the success in increasing the lifetime of a terminal hydride species in chapter three, a diiron complex containing a sterically bulk azadithiolate was investigated. In the azadithiolate, the steric bulk at the 3-position could not be increased without affecting the catalysis. The steric bulk at the methylene positions was therefore increased. The terminal hydride species was increased significantly to almost an hour at 30 °C. The isomerization occurred by a different mechanism than in the previous chapter; only one terminal hydride species was formed that eventually isomerized to two µ-hydride species. The fifth and final chapter of this thesis focuses on a joint project between the Rauchfuss and Vura-Weis groups that is currently ongoing. The long-term objectives are to study the electron transfers associated with geometry changes within a simple [NiFe]-H2ase model complex, (dppe)Ni(pdt)Fe(CO)3, and DuBois’ catalyst. Specifically of interest are the oxidations from NiI to NiII, where the geometry changes from tetrahedral to square planar. These electron transfers and geometry changes will be studied using extreme ultraviolet (XUV) spectroscopy which is element-, oxidation-, and spin-state specific. Two simple Ni complexes are currently being investigated: Ni(NCS)2(PPh3)2, square planar, and NiI2(PPh3)2, tetrahedral. Preliminary XUV data suggests that the edge position of the NiII shifts depending on the geometry of the nickel. To prepare for transient XUV spectroscopy, time dependent-density functional theory was performed to better understand the metal ligand charge transfer band, MLCT, that is being excited. Also, transient absorption spectroscopy has been completed to understand the timescales of the initial excitation and back-electron transfer