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Resonance Raman Investigations of [NiFe] Hydrogenase Models THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Shelby Lee Behnke Graduate Program in Chemistry The Ohio State University 2016 Master's Examination Committee: David Nagib, Committee Member Hannah Shafaat, Advisor Copyrighted by Shelby Lee Behnke 2016 Abstract Hydrogenase (H2ases) enzymes carry out bidirectional hydrogen production and oxidation reactions. To better understand the mechanism of hydrogen conversion, spectroscopic studies on small molecule mimics provide important metrics to correlate structure and function of the native enzymes. In this work, a series of molecular complexes that mimic the [NiFe] hydrogenase have been synthesized with different phosphine ligand substituents and analyzed using multiple analytical techniques, including nuclear magnetic resonance, Fourier transform infrared, and resonance Raman spectroscopies. Three model compounds have been selected as the focus of this investigation into the vibrational structure of the [NiFe] active site: [Ni(dppe)(µ-pdt)(μ-H)Fe(CO)3][BF4], [Ni(dcpe)(µ- pdt)(μ-H)Fe(CO)3][BF4], and [Ni(dppbz)(µ-pdt)(μ-H)Fe(CO)3][BF4]. (dppe= diphenylphosphinoethane, dcpe= dicyclohexylphosphinoethane, and dppbz= diphenylphosphinobenzene). These compounds have been previously synthesized and shown to exhibit high activity for proton reduction but have not been fully characterized spectroscopically to assess the solution-phase structure of the metal-hydride core. (Barton et al., 2010, JACS. 132, 14877-14885.) Specifically, resonance Raman (RR) spectroscopy was used to study the vibrational bands of each of the molecules, which were then assigned to specific normal modes of the molecule. The [Ni(dppe)(μ-pdt)(μ-H)Fe(CO)3]BF4 compound has been previously studied in great detail by RR, and this molecule was used as a template from which to understand the structures of the series. It was found that ii perturbations of ligands on the metal center alter the molecular vibrations, particularly of the metal-hydride core. This work provides important metrics for comparison between synthetic and the natural enzyme systems, with the intention of better understanding the mechanisms of native hydrogenases and informing next-generation catalyst design. iii This thesis is dedicated to my husband and Revan. iv Acknowledgments I would like to thank my advisor Hannah Shafaat for all of her help and guidance. I am especially grateful to my group who are always there to lend a hand. I would also like to thank the Nagib and Turro group for their generosity and helpfulness. I would also like to acknowledge the support of the OSU Department of Chemistry and Biochemistry, OSU Institute for Materials Research, and the National Science Foundation. v Vita 2010................................................................North Augusta High School 2014................................................................B.S. Chemistry, University of South Carolina Aiken 2014 to present ..............................................Graduate Teaching Associate, Department of Chemistry, The Ohio State University Publications Behnke, Shelby. Shafaat, Hannah.; Heterobimetallic Models of the [NiFe] Hydrogenases: A Structural and Spectroscopic Comparison. Comments on Inorganic Chemistry. 2016, 36, 123-140. Fields of Study Major Field: Chemistry vi Table of Contents Abstract ............................................................................................................................... ii Acknowledgments............................................................................................................... v Vita ..................................................................................................................................... vi List of Figures .................................................................................................................. viii Chapter 1: Background and Introduction ........................................................................... 1 Chapter 2: Experimental ................................................................................................... 13 Chapter 3 Results and Discussion ..................................................................................... 22 3.1 Synthesis……………..………...…………………………………………….22 3.2 Characterization of Ni(dppe)(μ-pdt)(μ-H)Fe(CO)3 .………………………....22 3.3 Characterization of Ni(dcpe)(μ-pdt)(μ-H)Fe(CO)3 .………………...……….33 3.4 Characterization of Ni(dppbz)(μ-pdt)(μ-H)Fe(CO)3 …..…………………….56 Chapter 4: Conclusion....................................................................................................... 61 References ......................................................................................................................... 63 vii List of Figures Figure 1. The worldwide energy consumption from 2010 and estimated until 2050.1 ....... 1 Figure 2. Crystal structure of [NiFe] hydrogenase (PDB ID:4U9H) with active site structure highlighted. .......................................................................................................... 3 Figure 3. Proposed catalytic mechanism and discrete intermediates for hydrogen oxidation by the [NiFe] H2ases. ......................................................................................... 5 Figure 4. [NiFe] hydrogenase compound synthesized by Darensbourg group.21 ............... 7 Figure 5. The Pohl group synthesized a [NiFe] hydrogenase mimic in 1997.29 ................. 8 Figure 6. The Schröder group was able to synthesize one of the first models with distorted geometry around the nickel site.31 ....................................................................... 8 Figure 7. General structure of the Rauchfuss-type compounds, where L is typically CO or 33 a phosphine ligand and R is typically an alkyl or aryl group. .......................................... 9 34 Figure 8. The first model with redox activity on nickel center. ..................................... 10 Figure 9. Compounds from left to right: Ni(dcpe)(μ-pdt)(μ-H)Fe(CO)3, Ni(dppe)(μ- pdt)(μ-H)Fe(CO)3, and Ni(dppbz)(μ-pdt)(μ-H)Fe(CO)3. The bond lengths for M-H are listed below each compound.26,36 A crystal structure has not been determined for Ni(dppbz)(μ-pdt)(μ-H)Fe(CO)3. ........................................................................................ 11 Figure 10. Synthetic scheme to generate Ni(dppe)Cl2 ([1]). ............................................ 23 31 Figure 11. P-NMR of 1 (600 MHz, CDCl3). .................................................................. 24 viii Figure 12. Synthetic scheme to generate Ni(dppe)(pdt) ([2]). .......................................... 25 31 Figure 13. P-NMR of 2 (600 MHz, CDCl3). .................................................................. 25 Figure 14. Synthetic scheme to generate Ni(dppe)(μ-pdt)Fe(CO)3 ([3]). ......................... 26 31 Figure 15. P-NMR of 3 (600 MHz, CDCl3), the signal corresponding to Ni(dppe)(μ- pdt)Fe(CO)3 at 45.7 ppm................................................................................................... 27 Figure 16. FT-IR spectra of the reaction mixture in Fig. 14 at t = 0 hrs (black), t = 3.5 hrs (blue) and t = 6 hrs (red). .................................................................................................. 27 Figure 17. FT-IR spectrum of [3]. The * denotes a peak due to decomposition of the product. ............................................................................................................................. 28 Figure 18. Synthetic scheme to generate Ni(dppe)(μ-pdt)(μ-H)Fe(CO)3 ([4]). ................ 29 31 1 Figure 19. P-NMR and (inset) H-NMR of 4 (600 MHz, CD2Cl2). ............................... 29 Figure 20. Solution-phase FT-IR spectrum of [4] (298 K, CD2Cl2). ................................ 30 Figure 21. Synthetic scheme to generate Ni(dppe)(μ-pdt)(μ-D)Fe(CO)3 ([5]). ................ 31 1 Figure 22. The H-NMR spectrum of 5 (600 MHz, CDCl3). ............................................ 31 31 Figure 23. The P-NMR spectrum of 5 (600 MHz, CDCl3). ........................................... 32 Figure 24. FT-IR of ([5]). ................................................................................................. 32 Figure 25. Resonance Raman (514.9nm excitation, 77K) of [3] black, [4] blue, and [5] red in dichloromethane. The large solvent peaks have been removed for clarity and are indicated with *. RR spectra are normalized to the peak intensity at 217 cm-1 due to varying concentrations used. ............................................................................................. 33 Figure 26. Synthetic scheme to generate Ni(dcpe)Cl2 ([6]). ............................................. 34 31 Figure 27. P-NMR of 6 (600 MHz, CDCl3) ([6]). .......................................................... 34 ix Figure 28. Synthetic scheme to generate Ni(dcpe)(pdt) ([7]). .......................................... 35 31 Figure 29. P-NMR of [7] (600 MHz, CDCl3) ([7]). ....................................................... 35 Figure 30. Synthetic scheme to generate ([8]). ................................................................. 36 Figure 31. FT-IR of ([8]). ................................................................................................. 36 Figure 32. Synthetic scheme to generate Ni(dcpe)(μ-pdt)(μ-I)Fe(CO)3. .......................... 37 Figure 33. Synthetic
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