Reports: G3
47013-G3 Molecular Mechanism of Hydrogen-formation in Fe-only Hydrogenases
After starting as an assistant professor at the University of Michigan, I became aware of the strong program in energy-related research at this institution, fostered by the Michigan Memorial Phoenix Energy Institute (MMPEI). My interactions with other scientists through this Institute got me interested in hydrogen-based approaches for alternative energies. The PRF-G funding that I received in 2007 ultimately allowed me to start research in this area by providing funds for a postdoctoral fellow, Dr. Grace Galinato. Hence, the PRF-G grant has laid the foundation for my engagement in energy-related research, and my research program in this area has since been growing. For example, I have obtained funding from the University of Michigan to start additional research in this area, and I am participating in multi-PI applications in energy research spearheaded by the MMPEI. Finally, I have recently applied for DOE and NSF funding in collaboration with a group from Chemical Engineering, University of Michigan, for the development of electrochemical cells for hydrogen production and consumption. The PRF-G grant has enabled me to produce a number of preliminary results, and in this way, to compete for grants at major funding institutions.
While a significant amount of work has been performed on analyzing the experimental and theoretical vibrational spectra of the simplest hydrogenase model, Fe2(edt)(CO)6, as a starting point to completely understand the vibrational properties of the active site of [FeFe] hydrogenase (submitted), part of the research program in the second year focused on investigating the spectroscopic properties and electronic structures of the terminal and bridging hydride isomers of [Fe2(edt)(PMe3)4(CO)2(H)]+. This work was performed in collaboration with Prof. Thomas Rauchfuss (University of Illinois).[1,2] The larger complex [Fe2(pdt)(dppv)2(CO)2(H)]+ studied initially showed weak Raman scattering and very complex IR spectra (PRF report 2008). The analogous complex [Fe2(edt)(PMe3)4(CO)2(H)]+ also models the key protonated intermediate of the active site in [FeFe] hydrogenases, but contains the smaller phosphine ligand PMe3. Our central hypothesis is that only the terminal hydride isomer is catalytically active leading to production of H2. We have studied both isomers of [Fe2(edt)(PMe3)4(CO)2(H)]+ using (resonance) Raman, IR, and UV-Vis absorption spectroscopy, and DFT calculations. These studies constitute the first detailed vibrational investigations on protonated hydrogenase model complexes, and hence, will provide the necessary groundwork to understand the spectroscopic and electronic-structural differences between the two isomers in detail.
AIM #1: Spectroscopic Investigation of [Fe2(edt)(PMe3)4(CO)2(H)]+
Room temperature (RT) Raman spectra of the solid form of the bridging (μ-H) and terminal (H-term) hydride complexes [Fe2(edt)(PMe3)4(CO)2(H)]+ were obtained at laser excitation wavelengths of 785 and 1064 nm, whereas initial studies at 77K on frozen solutions of the complexes using 647 nm excitation showed only a very low signal-to-noise ratio. The RT conditions have limitations on the less stable, more important terminal hydride isomer, as will be briefly discussed. For the μ-H and the deuterated complex (μ-D), distinct signals at 1928 and 1934 cm-1 are observed, which correspond to (C=O)term stretching modes. A peak at 1213 cm-1 is observed for μ-H, which shifts to 891/1008 cm-1 in μ-D. DFT calculations (B3LYP/TZVP) on μ-H predict the bridging nu(Fe-H) stretch at 1260 cm-1, which is ~50 cm-1 overestimated relative to the experimental ν(Fe-H) energy. The lower energy region (200-740 cm-1) of the μ-H and μ-D spectra are similar, indicating that this region is dominated by inner ligand vibrations of the coordinated phosphine and thiolate ligands. Unlike the μ-H complex, H-term was unfortunately unstable under the experimental conditions. Imaging the solid H-term species before and after 785 nm excitation showed decomposition upon irradiation, which prevented us from obtaining useable Raman spectra of the H-term complex. Raman spectra were obtained for D-term, showing a distinct band at 1931 cm-1, which corresponds to a ν(C=O)term mode based on comparison with the IR spectrum of this complex. However, the terminal ν(Fe-H) stretch, which is predicted at 1904 cm-1 (B3LYP/TZVP), could not be observed experimentally.
Since ν(Fe-H) is not available from Raman experiments, diffuse reflectance FT-IR spectroscopy was utilized. The IR data of μ-H and H-term show interesting differences in the ν(C=O) stretching region, but no other distinct differences in theν (Fe-H) stretching region (vide supra) could be observed. Future Raman experiments therefore have to utilize a cryostat for sample cooling to obtain useable spectra of H-term.
Additionally, we have started low-temperature protonation experiments with H-term. This is feasible, given the DFT-calculated stability of the intermediate H2 complex in non-coordinating solvents (Aim # 2). UV-Vis absorption spectra of H-term show a prominent peak at 261 nm, which is analogous to the band we observe in Fe2(edt)(CO)6 at 323 nm, and which is therefore assigned to a Fe to CO(π*) charge transfer transition.[3] An initial study on the protonation of H-term has been performed at -60oC, but experimental conditions are currently being optimized to potentially observe the [Fe2(edt)(PMe3)4(CO)2(H-H)] intermediate.
AIM #2: DFT Calculations
DFT calculations were performed to test whether adding one equivalent of a strong acid to the H-term species leads to a stable H2 intermediate. We computed the binding energy of H2 to the optimized H-term complex to be 23 kcal/mol, corresponding to a surprisingly stable species. The experimental low-temperature reaction is monitored using in-situ UV-Vis absorption and IR spectroscopy. This work is in progress.
In addition, DFT calculations are currently being performed to determine the potential energy surface (PES) for the reaction of H-term with a proton source. Several acids were screened computationally to optimize the thermodynamics of the formation of the H-term-H2 complex. Nitropyridinium was computationally determined to be the best proton source (Delta(E) for proton transfer = -7.5 kcal/mol). Partial geometry optimizations will be performed on H-term and nitropyridinium in a fixed arrangement, where the energy will be monitored at each step of the path as the proton is transferred to form the H-term-H2 complex.
Citations
[1] Barton, B. E.; Rauchfuss, T. B., Inorg. Chem. 2008, 47, 2261-2263.
[2] Justice, A. K.; Linck, R. C.; Rauchfuss, T. B.; Wilson, S. R., J. Am. Chem. Soc. 2004, 126, 13214-13215.
[3] Fiedler, A. T.; Brunold, T. C., Inorg. Chem. 2005, 44, 1794-1809.
