Primary and Secondary Coordination Sphere Control of Push-Pull Reaction Dynamics on Fe Porphyrins
Margarit, Charles Gerrit
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CitationMargarit, Charles Gerrit. 2019. Primary and Secondary Coordination Sphere Control of Push-Pull Reaction Dynamics on Fe Porphyrins. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
AbstractThe looming threats associated with global climate change have inspired study of proton reduction to hydrogen gas for use in fuel cells, and the reduction of carbon dioxide for use as fuel in a closed carbon cycle. One class of molecules that has shown electrochemical competence to catalyze both the hydrogen evolution reaction (HER) and the CO2 reduction reaction (CO2RR) are metallated porphyrins. These reduction processes can be described using a push-pull description of the mechanism whereby a reduced metal center “pushes” electrons onto a substrate, forming a chemical bond. Proton transfer and bond cleavage events then serve to "pull" the product from the catalytic intermediate and close the catalytic cycle.
This laboratory has championed the synthesis and application of the hangman porphyrin scaffold in which Brønsted acidic or basic moieties are strategically placed in the secondary coordination sphere of the metal center to facilitate proton coupled electron transfer processes for energy related redox reactions. Until the work of this thesis, the Nocera laboratory had yet to approach CO2RR using the hangman scaffold. The application of an Fe hangman porphyrin system to manipulate reaction kinetics for reduction of CO2 to CO in this work is described. Fe hangman porphyrins containing pendant phenol, guanidine, and sulfonic acid groups were synthesized and used in cyclic voltammetry and bulk electrolysis experiments. Supplemental quantum chemical calculations revealed that greater stabilization of the CO2 reduction intermediate yielded enhanced catalysis.
In their fully reduced state Fe(0) porphyrins are competent to bind CO2 on either face of the macrocycle. Thereby, it became necessary to synthesize an α,β–double hangman porphyrin to modify both faces at the same time to better ascertain the hangman effect for CO2RR. The modification of these porphyrins was arduous and instead were simply used as pendant carboxylic acids in electrochemical CO2 reduction. Their reactivity toward CO2 was found to be hindered with respect to the non-hangman and single hangman congeners and a coulombic effect for repulsion of negatively charged intermediates was diagnosed as detrimental to the reaction by in-situ generated carboxylate moieties.
Where the hangman scaffold serves to facilitate the "pull" of the reaction dynamic, little attention has been paid to the “push”. The only examples of such control include the substitution of the porphyrin meso-groups to lower the overpotential of the catalytic reaction. Cyclic voltammetry and bulk electrolysis are used to investigate the role of co-substrates in the modulation of push side reaction dynamics for electrochemical HER and CO2RR catalyzed by FeTPP. Addition of nitrogen containing bases to FeTPP with acetic acid under argon shows dramatic changes in current response and quantitative analysis demonstrates acceleration of rates for hydride formation and hydrogen release during HER. Application of these co-substrates to electrochemical CO2 reduction catalyzed by FeTPP using weak acids showed different effects on reaction dynamics. Bulk electrolysis experiments reveal divergent pathways for CO2 reduction where formate rather than CO can be produced with up to 80% faradaic efficiency. These results introduce a new mechanism of formate production from CO2 that does not proceed via hydride insertion and shines light on the interplay of multiple system components in the design of improved catalytic systems.
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