Western Washington University Western CEDAR WWU Graduate School Collection WWU Graduate and Undergraduate Scholarship Spring 2016 Synthesis & Reactivity of Iron (II) Pyridinediimine Complexes For the Reduction of Nitrite Yubin Kwon Western Washington University, [email protected] Follow this and additional works at: https://cedar.wwu.edu/wwuet Part of the Chemistry Commons Recommended Citation Kwon, Yubin, "Synthesis & Reactivity of Iron (II) Pyridinediimine Complexes For the Reduction of Nitrite" (2016). WWU Graduate School Collection. 474. https://cedar.wwu.edu/wwuet/474 This Masters Thesis is brought to you for free and open access by the WWU Graduate and Undergraduate Scholarship at Western CEDAR. It has been accepted for inclusion in WWU Graduate School Collection by an authorized administrator of Western CEDAR. For more information, please contact [email protected]. Synthesis & Reactivity of Iron (II) Pyridinediimine Complexes For the Reduction of Nitrite By Yubin Kwon Accepted in Partial Completion Of the Requirements for the Degree Master of Science Kathleen L. Kitto, Dean of the Graduate School ADVISORY COMMITTEE Chair, Dr. John D. Gilbertson Dr. Amanda Murphy Dr. David Rider MASTER’S THESIS In presenting this thesis in partial fulfillment of the requirements for a master’s degree at Western Washington University, I grant to Western Washington University the non-exclusive royalty-free right to archive, reproduce, distribute, and display the thesis in any and all forms, including electronic format, via any digital library mechanisms maintained by WWU. I represent and warrant this is my original work, and does not infringe or violate any rights of others. I warrant that I have obtained written permissions from the owner of any third party copyrighted material included in these files. 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Yubin Kwon April 28, 2016 Synthesis & Reactivity of Iron (II) Pyridinediimine Complexes For the Reduction of Nitrite A Thesis Presented to The Faculty of Western Washington University In Partial Fulfillment Of the Requirements for the Degree Master of Science By Yubin Kwon April 2016 ABSTRACT The activation of small molecules has been studied by the scientific field for many decades as it plays a key role in nature such as photosynthesis and respiration. Many of these reactions are catalyzed by metalloenzymes in nature where the transfer of electrons and protons are coupled for the reaction to move forward. Noncovalent interactions in the secondary coordination sphere of metalloenzymes play an important role in determining the activity and selectivity. Hydrogen bonds are the most common noncovalent interactions that metalloenzymes utilize to control the reactivity in the secondary coordination sphere. Therefore, it is important to develop compounds and catalysts that can move both protons and electrons. Recent studies have been done by several groups on the mechanism of nitrite reduction. Based on those findings, we developed a series of iron (II) pyridinediimine (PDI) complexes that contain pendant bases, with varying pKa values, located in the secondary coordination sphere. These ligands were synthesized, coordinated to iron (II) and reduced under carbon monoxide (CO) to store electrons within the ligand scaffold. These reduced complexes were then protonated to form hydrogen bonds and fine tune the reactivity. These PDI complexes that are capable of storing both electrons and protons were investigated to functionally mimic the metalloenzyme nitrite reductase. To date, the mechanism of nitrite reduction remains unknown. In an attempt to determine how nitrite binds to the metal of our PDI complex, we synthesized a dinitrosyl iron complex. The synthesis of this complex should help to determine the mechanism of nitrite reduction. iv ACKNOWLEDGMENTS Research Advisor: Dr. John D. Gilbertson Thesis Committee Members: Dr. Amanda Murphy Dr. David Rider Research Group Members: Mayra Delgado, Audrey Cheung, Josh Ziegler, Kyle Burns, Jared Chang, David Brewster Instrument Technicians: Dr. Hla Win-Piazza and Charles Wandler X-ray Crystallographer: Dr. Lev Zakharov Mössbauer: Dr. Takele Seda Financial Support: National Science Foundation Western Washington University Department of Chemistry v Table of Contents Abstract iv Acknowledgments v List of Figures & Tables viii List of Schemes & Equations xi List of Abbreviations xiii 1. Introduction 1 1.1 Small Molecule Activation 1 1.2 Proton-Coupled Electron Transfer 3 1.3 Nitrogen Cycle and the Role of Nitrite Reductase 5 1.4 Synthetic Models 8 1.5 Pyridinediimine Ligands 1 1.6 Research Objectives 13 2. Synthesis of Pendant Base Fe(II) PDI Complexes 14 2.1 Synthesis of PDI Ligand Scaffold 15 2.2 Metallation of PDI Complexes 17 2.2 Reduction of PDI Complexes 25 2.3 Protonation of PDI Complexes 32 3. Nitrite Chemistry 38 3.1 Mechanism of Nitrite Reduction 38 3.2 Dinitrosyl Iron Complexes 40 3.3 Synthesis of Fe(II) PDI Dinitrosyl Complex 41 3.4 FTIR Studies 42 3.5 Structural Studies 44 vi 3.6 Electrochemical Studies 48 3.7 Kinetic Studies 49 3.8 Proposal of Mechanism & Products 55 3.9 NO gas Reactivity 57 4. Conclusion 60 5. Experimental 63 6. References 82 vii List of Figures & Tables Figure 1.1 Common targets for small molecule activation 1 Figure 1.2 Active site of Fe[Fe] hydrogenase 2 Figure 1.3 Free energy curves of reactant and product parabolas 5 Figure 1.4 The Nitrogen Cycle 6 Figure 1.5 Active site of iron and copper nitrite reductase 7 Figure 1.6 Examples of tripodal ligands 9 Figure 1.7 Chemdraw and ORTEP view of Fe(didpa)Br2 12 Figure 2.1 Pendant bases with their pKa value 15 Figure 2.2 FTIR spectrum of 1 16 Figure 2.3 1H NMR spectrum of 1 17 Figure 2.4 Overlay FTIR spectra of 1 & 2 18 Figure 2.5 1H NMR spectrum of 2 19 Figure 2.6 FTIR spectrum of 3 20 Figure 2.7 ORTEP view of 3 21 Figure 2.8 Zero-field 57Fe Mössbauer spectrum of 3 22 Table 2.1 Mössbauer parameters of pendant Lewis base FeII PDI complexes 22 Figure 2.9 ORTEP view & Zero-field 57Fe Mössbauer spectrum of 4 24 Figure 2.10 ORTEP view & Zero-field 57Fe Mössbauer spectrum of 5 25 Figure 2.11 ORTEP view & Zero-field 57Fe Mössbauer spectrum of 6 27 Table 2.2 Carbonyl stretching frequencies of FeII PDI complexes 27 Figure 2.12 1H NMR spectrum of 6 28 Figure 2.13 13C NMR spectrum of 6 28 viii Figure 2.14 FTIR Spectrum of 6 29 Figure 2.15 ORTEP view & Zero-field 57Fe Mössbauer spectrum of 8 30 Figure 2.16 1H NMR spectrum of 8 31 Figure 2.17 13C NMR spectrum of 8 31 Figure 2.18 ORTEP view of [(Hdidpa)FeOH(NCC)][PF6] 32 Figure 2.19 ORTEP view & Zero-field 57Fe Mössbauer spectrum of 9 34 Figure 2.20 1H NMR spectrum of 9 35 Figure 2.21 13C NMR spectrum of 9 35 Figure 2.22 FTIR Spectrum of 9 36 Figure 2.23 Crude FTIR spectrum of reacting NH4PF6 with 8 37 Figure 3.1 Proposed mechanism of iron and copper nitrite reduction 38 Figure 3.2 ORTEP view of iPrPDI dinitrosyl iron complex 39 Figure 3.3 CO gas calibration curve & gas chromatograph of headspace analysis 42 Figure 3.4 Overlay FTIR spectra of 9 & 10 43 Figure 3.5 Calibration curves to calculate the yield of 10 44 Figure 3.6 ORTEP view of 10 45 Figure 3.7 Zero-field 57Fe Mössbauer spectrum of 10 46 14 15 Figure 3.8 Overlay FTIR spectra of 10( NO)2 and 10( NO)2 47 Figure 3.9 ORTEP view of 10 48 Figure 3.10 CV scan of 10 under N2 atm 49 Figure 3.11 UV-Vis Spectrum of 9 50 iPr MeO Figure 3.12 Structures of PDIFe(CO)2 & PDIFe(CO)2 50 iPr Figure 3.13 UV-Vis spectrum of PDIFe(CO)2 52 MeO Figure 3.14 UV-Vis spectrum of PDIFe(CO)2 52 ix Figure 3.15 Abs vs. Time (left) & Conc. Vs Time (right) of regions 400 & 700 53 Figure 3.16 Three Dimensional plot & IR band profiles when 9 is reacted with NaNO2 54 Figure 3.17 Nitric Oxide (NO) purification system 57 Figure 3.18 FTIR Spectrum of 9 reactivity with NO gas 58 Figure 3.19 FTIR Spectrum of 8 reactivity with NO gas 59 Figure 5.1 FTIR Spectrum of 1 76 Figure 5.2 FTIR Spectrum of 2 77 Figure 5.3 13C NMR Spectrum of 2 77 Figure 5.4 1H NMR Spectrum of 3 78 Figure 5.5 1H NMR Spectrum of 4 78 Figure 5.6 1H NMR Spectrum of 5 79 Figure 5.7 FTIR Spectrum of 6 79 Figure 5.8 FTIR Spectrum of 8 80 Figure 5.9 FTIR Spectrum of 10 80 15 Figure 5.10 FTIR Spectrum of 10( NO)2 81 x List of Schemes & Equations Scheme 1.1 Concerted vs sequential PCET mechanism 3 Scheme 1.2 Hydrogen atom transfer 4 Scheme 1.3 Electron-proton transfer 4 - Equation 1.1 NO2 to NO Reduction 7 Scheme 1.4 Three oxidation states of the PDI ligand scaffold 11 Scheme 1.5 PDI complex displayed as a direduced delocalized system 11 Scheme 1.6 Reversible one electron transfer of PDI ligand scaffold 12 i Equation 2.1 Synthesis of [(2,6- PrC6H3N=CMe)(O=CMe)C5H3N] 16 Equation 2.2 Synthesis of TEAPDI ligand scaffold 17 Equation 2.3 Metallation of f TEAPDI ligand scaffold 18 Scheme 2.1.