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Carbon-13 Kinetic Isotope Effects of Catalytic CO2 Reduction by Polypyridyl Transition Metal Complexes University of Connecticut Masthead Logo OpenCommons@UConn Doctoral Dissertations University of Connecticut Graduate School 12-18-2018 Carbon-13 Kinetic Isotope Effects of Catalytic CO2 Reduction by Polypyridyl Transition Metal Complexes Taylor William Schneider University of Connecticut - Storrs, [email protected] Follow this and additional works at: https://opencommons.uconn.edu/dissertations Recommended Citation Schneider, Taylor William, "Carbon-13 Kinetic Isotope Effects of Catalytic CO2 Reduction by Polypyridyl Transition Metal Complexes" (2018). Doctoral Dissertations. 2142. https://opencommons.uconn.edu/dissertations/2142 Carbon-13 Kinetic Isotope Effects of Catalytic CO2 Reduction by Polypyridyl Transition Metal Complexes Taylor Schneider, Ph.D. University of Connecticut, 2019 Abstract 13 Herein, we have used carbon-13 kinetic isotope effects ( C KIEs) to probe the catalytic CO2 reduction mechanisms of two different polypyridyl transition metal complexes, Re(bpy)(CO)3Cl (1), which will convert CO2 to CO, and [Ru(tpy)(bpy)Cl]PF6 (2) (bpy = 2,2’-bipyridine; tpy = 2,2’:6’,2”- terpyridine), which will convert CO2 to a mixture of CO and formic acid. We used several sets of reaction conditions to drive CO2 reduction. These included sodium mercury (Na(Hg)) amalgam as a sacrificial reducing agent, photocatalytic conditions in which triethanolamine (TEOA) was a sacrificial electron donor, and electrocatalytic conditions in which the reaction potential was applied by a potentiostat. Under some photocatalytic reactions with 1 and all photocatalytic reactions with 2, the photosensitizer [Ru(bpy)3]Cl2 was included in the reaction mixture. Furthermore, photocatalytic CO2 reduction by 1 was performed in two different solvents, acetonitrile (ACN) and dimethylformamide (DMF), whereas all experiments by 2 were performed in ACN. The reduction of CO2 by complex 1 driven by Na(Hg) and electrocatalytic conditions, respectively, produced 13C KIEs of 1.0169 ± 0.0012 and 1.022 ± 0.004. The 13C KIEs from photocatalytic reduction in ACN, in DMF, and in ACN with [Ru(bpy)3]Cl2, respectively, were 1.0718 ± 0.0036, 1.0685 ± 0.0075, and 1.0703 ± 0.0043, which all agree with each other within their error values. This implies that the reaction mechanisms have the same first irreversible step and proceed with similar reactive intermediates upon reduction. Meanwhile, the photocatalytic reduction of CO2 by 2 in ACN with [Ru(bpy)3]Cl2, first in dry solvent, and then in a reaction mixture containing 1% water, 13 produced C KIEs of 1.052 ± 0.004 and 1.044 ± 0.006, respectively. Electrocatalytic CO2 reduction by Taylor Schneider University of Connecticut, 2019 2 yielded a 13C KIE of 1.014 ± 0.004, but this was accompanied by a low R-value, and an inability to reproduce the results. Our initial analysis of the 13C KIE from 1 and Na(Hg) ruled out the possibility of an outer- sphere electron transfer mechanism, and indicated that CO2 binding to the reduced rhenium complex is the rate determining step. Subsequent experiments under photocatalytic and electrocatalytic conditions were accompanied with density functional theory (DFT) calculations, which gave us insight into the nature of the transition states. Additionally, the similar 13C KIEs from the different solvent systems under photocatalytic conditions are all consistent with the calculated isotope effect of CO2 binding to I •− the one-electron reduced [Re (bpy )(CO)3] species. These findings provide strong evidence that the reactions in the two different solvents have the same first irreversible step and proceed with similar reactive intermediates upon reduction. Theoretically, we found that the major contribution for the large 13C isotope effects comes from a dominant zero point energy (ZPE) term. Meanwhile, the same process under electrocatalytic conditions had a 13C KIE that was markedly lower than the corresponding theoretical KIE of 1.077. This may be due to the use of an applied potential that was more negative than necessary, which would lead to a decrease in the isotope effect. Our 13C KIEs from 2, both in dry solvent and with 1% added water, are in good agreement with the theoretical value of 1.058, which was obtained from a weighted average of the values corresponding to the productions of CO and formic acid. The first irreversible steps of these processes are the binding II •− •− 0 of CO2 to the two-electron reduced [Ru (tpy )(bpy )] for CO production, and the electrophilic attack II •− •− + of CO2 onto the hydride-containing complex [Ru (tpy )(bpy )H] for the production of formic acid. A theoretical analysis of the electrocatalytic reduction of CO2 by 2 remains to be done. We believe the poor reproducibility from electrocatalysis is due to a limitation in the current design of the Taylor Schneider University of Connecticut, 2019 electrochemical cell which we designed for handling small volumes of isotopically sensitive gas samples. We have instead, proposed some redesigns, which should make possible a more comprehensive analysis of the effects of applied potential on the reaction mechanisms for these families of CO2 reduction catalyst. Although the electrocatalytic experimental design must be adjusted, the overall results obtained from both catalysts and a range of experimental conditions have laid the groundwork for combined experimental and theoretical approaches for analysis of competitive isotope effects toward understanding CO2 reduction catalyzed by other complexes. Carbon-13 Kinetic Isotope Effects of Catalytic CO2 Reduction by Polypyridyl Transition Metal Complexes Taylor Schneider B.S., University of Massachusetts, Lowell, 2012 A Dissertation Submitted in Partial Fulfillment of the Requirement for the Degree of Doctor of Philosophy at the University of Connecticut 2019 i Copyright by Taylor Schneider 2019 ii Approval Page Doctor of Philosophy Dissertation Carbon-13 Kinetic Isotope Effects of Catalytic CO2 Reduction by Polypyridyl Transition Metal Complexes Presented by Taylor Schneider Major Advisor Dr. Alfredo M. Angeles-Boza Associate Advisor Dr. Christian Brückner Associate Advisor Dr. Jose Gascon Associate Advisor Dr. Rebecca Quardokus Associate Advisor Alfredo M. Angeles-Boza Dr. Edward J. Neth University of Connecticut 2019 iii Dedicated to my parents, Stephen and Devon Schneider, for all your love and support. iv Acknowledgments Thank you to my parents, Stephen and Devon Schneider, for instilling in me a strong work ethic, encouraging me to always do better, and for helping me become the person I am today. Thank you for moral support in the face of many stressful years and for always giving me a place to come home. More than anyone else, I could not have done this without you. I would like to thank my advisor, Dr. Alfredo Angeles-Boza, for your patience, guidance, and for pushing me to develop my skills as a researcher. Thank you to my lab mates, John Ng’ang’a, Sam Juliano, Scott Pierce, Jasmin Portelinha, Searle Duay, Murphy Jennings, Daben Libardo, and Kankana Mullick for your sense of humor, wacky conversations, and making the lab a fun place to work. Also, thank you to my collaborators, to Dr. Mehmed Z. Ertem for your computational expertise, and to Dr. Michael Hren, for letting me to use your dual-inlet isotope ratio mass spectrometer. v Chapter Specific Acknowledgments Chapter 1 Much of the material from this chapter comes directly from the introductory sections of manuscripts cited for Chapters 2, 3, and 4. Additional contents were compiled to give the reader a solid background in the material presented throughout this dissertation. Chapter 2. Much of the material from this chapter comes directly from a manuscript entitled 13 18 “Competitive C and O kinetic isotope effects on CO2 reduction catalyzed by Re(bpy)(CO)3Cl” by Taylor W. Schneider and Alfredo M. Angeles-Boza, which has been published in Dalton Trans, 2016 44, 8484-8487. Taylor W. Schneider collected the data. Both authors are equal contributors to the analysis of the data and preparation of the manuscript. Chapter 3. Much of the material from this chapter comes directly from a manuscript entitled “Mechanism of Photocatalytic Reduction of CO2 by Re(bpy)(CO)3Cl from Differences in Carbon Isotope Discrimination” by Taylor W. Schneider, Mehmed Z. Ertem, James T. Muckerman, and Alfredo M. Angeles-Boza, which has been published in ACS Catal, 2016, 6, 5473-5481. Taylor W. Schneider collected the experimental data, and Mehmed Z. Ertem collected the computational data. All authors are equal contributors to the analysis of the data and preparation of the manuscript. Chapter 4. Much of the material from this chapter comes directly from a manuscript entitled “[RuII(tpy)(bpy)Cl]+-Catalyzed reduction of carbon dioxide. Mechanistic insights by carbon-13 kinetic isotope effects” by T. W. Schneider, M. T. Hren, M. Z. Ertem, and A. M. Angeles-Boza, which has been published in Chem Commun, 2018, 54, 8518-8521. T. W. Schneider collected the experimental data, and M. Z. Ertem collected the computational data. M. T. Hren supplied the instrumentation for measuring isotope ratios. All authors are equal contributors to the analysis of the data and preparation of the manuscript. Chapter 5. The material in this chapter is unpublished work. Taylor W. Schneider collected the experimental data, and Mehmed Z. Ertem collected the computational data. M. T. Hren supplied the instrumentation for measuring isotope ratios. All authors are equal contributors to the analysis of the data and preparation of the manuscript. Chapter 6. The
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