Date of Completion

12-18-2018

Embargo Period

12-17-2018

Keywords

Catalysis, Mechanisms, Isotope Effects, CO2, Rhenium, Ruthenium, Photochemistry

Major Advisor

Dr. Alfredo Angeles-Boza

Associate Advisor

Dr. Christian Brueckner

Associate Advisor

Dr. Jose Gascon

Associate Advisor

Dr. Rebecca Quardokus

Associate Advisor

Dr. Edward Neth

Field of Study

Chemistry

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Herein, we have used carbon-13 kinetic isotope effects (13C 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, produced 13C KIEs of 1.052 ± 0.004 and 1.044 ± 0.006, respectively. Electrocatalytic CO2 reduction by 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 the one-electron reduced [ReI(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 of CO2 to the two-electron reduced [RuII(tpy•−)(bpy•−)]0 for CO production, and the electrophilic attack of CO2 onto the hydride-containing complex [RuII(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 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.

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