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Scholarly Works (13 results)

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UC Berkeley Electronic Theses and Dissertations (2022)

Abstract

Biologically Driven Design of Molecular and Molecular-Materials Platforms for Electrochemical Carbon Dioxide Reduction

by

Jeffrey S. Derrick

Doctor of Philosophy in Chemistry

University of California, Berkeley

Professor Christopher J. Chang, Chair

Chapter 1. An overview of the field of electrochemical carbon dioxide reduction with homogenous, molecular catalyst. Discussion in this chapter is focused on the motivation, challenges, and strategies to convert carbon dioxide into value-added chemicals and ends with a brief primer on electrochemistry.

Chapter 2. Inspired by the role of redox reservoirs in nature, we explore the application of the chelate effect to adhere molecular catalysts onto the surface of a palladium cathode to give a new molecular-material hybrid catalyst platform. These palladium functionalized electrodes show marked improvements in catalyst stability and selectivity for the electrochemical reduction of carbon dioxide into C1 products compared to both bare, unfunctionalized palladium electrodes and palladium surfaces modified with monodentate carbenes.

Chapter 3. Biological systems utilize precisely positioned hydrogen bonding networks to direct redox reactions with high selectivity and efficiency. Inspired by this, we report the design, synthesis, and characterization of a series of iron porphyrin complexes that are equipped with amide pendants at various positions along the periphery of the metal core. We show through electrochemical analysis that proper positioning of these hydrogen bond donors greatly affects the electrocatalytic activity for carbon dioxide reduction and that these second coordination spheres can be utilized to break the electronic scaling relationships that often limit catalyst performance by entangling the overpotential with the turnover frequency.

Chapter 4. The application of appropriately positioned hydrogen bonding networks into the second coordination sphere of molecular porphyrin catalysts is further explored. Here, we exchange the single-point hydrogen bond donor amides for two-point urea-appended porphyrin analogs. We report the design, synthesis, and characterization of iron porphyrin complexes that incorporate a bis-aryl urea moiety at ortho or para positions of the meso aryl ring of tetraphenylporphyrin. We show that these urea-complexes can template bicarbonate into the second coordination sphere resulting in a large enhancement in electrochemical carbon dioxide reduction catalysis. This work illustrates the importance of understanding bicarbonate speciation in organic electrolytes and demonstrates how these equilibria can be exploited to enhance the performance of homogeneous catalysts.

Chapter 5. Biological systems often exhibit a high degree of electronic delocalization that serves to minimize energy input and maximize selectivity for desired chemical transformations. Here we report the design, synthesis, and characterization of a molecular iron catalyst that captures this design concept through the use of a redox non-innocent terpyridine-based polypyridine ligand. Due to strong metal–ligand exchange coupling between an intermediate-spin Fe(II) center an a doubly reduced ligand, this iron complex displays redox behavior at extremely mild potentials. This electronic interaction enables electrochemical reduction of CO2 to CO at low overpotentials with high selectivity and fast turnover frequencies in organic and aqueous electrolytes. This work provides a starting point for the design of systems that exploit metal–ligand cooperativity for electrocatalysis where the electrochemical potential of redox non-innocent ligands can be tuned through secondary metal-dependent interactions.

Chapter 6. The origins of the exceptional catalytic performance of the iron polypyridyl complex developed in the previous chapter are explored here though the use of a combined computational and experimental study. This work establishes two distinct mechanistic pathways for electrochemical CO2 reduction catalyzed by [Fe(tpyPY2Me)]2+ ([Fe]2+) as a function of applied overpotential. Comparison of experimental kinetic data to rates obtained from the energetic span model support a PCET pathway as the most likely mechanism. This study serves as a foundation from which we build upon this mechanistic understanding to propose the design of an improved ligand framework. Taken together, this work highlights the value of synergistic computational/experimental approaches to decipher mechanisms of new electrocatalysts and direct the rational design of improved platforms.

Chapter 7. The work presented in this final chapter seeks to explore the extent to which the concept of metal–ligand exchange coupling from Chapter 5 can be utilized to reduce carbon dioxide by more than two electrons to give access to more value-added products. The design, synthesis, characterization, and analysis of two new redox-active ligand platforms for electrochemical carbon dioxide reduction is discussed.

Appendix A. The synthesis, characterization, and screening of a library of nickel bis(iminopyridine) complexes as catalysts for the electrochemical reduction of CO2.

Appendix B. Metal–ligand cooperativity enable by the tpyPY2Me ligand from Chapter 5 is leveraged for controlling the product selectivity of single-electron transfers to alkyl halides through an outer sphere pathway.

    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2018)
    The development of catalysts for electrochemical reduction of carbon dioxide offers an attractive approach to transforming this greenhouse gas into value-added carbon products with sustainable energy input. Inspired by natural bioinorganic systems that feature precisely positioned hydrogen-bond donors in the secondary coordination sphere to direct chemical transformations occurring at redox-active metal centers, we now report the design, synthesis, and characterization of a series of iron tetraphenylporphyrin (Fe-TPP) derivatives bearing amide pendants at various positions at the periphery of the metal core. Proper positioning of the amide pendants greatly affects the electrocatalytic activity for carbon dioxide reduction to carbon monoxide. In particular, derivatives bearing proximal and distal amide pendants on the ortho position of the phenyl ring exhibit significantly larger turnover frequencies (TOF) compared to the analogous para-functionalized amide isomers or unfunctionalized Fe-TPP. Analysis of TOF as a function of catalyst standard reduction potential enables first-sphere electronic effects to be disentangled from second-sphere through-space interactions, suggesting that the ortho-functionalized porphyrins can utilize the latter second-sphere property to promote CO2 reduction. Indeed, the distally-functionalized ortho-amide isomer shows a significantly larger through-space interaction than its proximal ortho-amide analogue. These data establish that proper positioning of secondary coordination sphere groups is an effective design element for breaking electronic scaling relationships that are often observed in electrochemical CO2 reduction.
      Cover page: Positional effects of second-sphere amide pendants on electrochemical CO 2 reduction catalyzed by iron porphyrins
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2020)
      Difluoromethylene-containing compounds have attracted substantial research interest over the past decades for their ability to mimic biological functions of traditional functional groups while providing a wide variety of pharmacological benefits bestowed by the C-F bond. We report a novel strategy to access RCF2Br-containing heterocycles by regio- and enantioselective bromocyclization of difluoroalkenes enabled by chiral anion phase-transfer catalysis. The utility of this methodology was highlighted through a synthesis of an analogue of efavirenz, a drug used for treating HIV. Additionally, the synthetic versatility of the CF2Br intermediates was showcased through functionalization to a variety of enantioenriched α,α-difluoromethylene-containing products.
        Cover page: Regio- and Enantioselective Bromocyclization of Difluoroalkenes as a Strategy to Access Tetrasubstituted Difluoromethylene-Containing Stereocenters
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2022)
        [Fe(tpyPY2Me)]2+ ([Fe]2+) is a homogeneous electrocatalyst for converting CO2 into CO featuring low overpotentials of <100 mV, near-unity selectivity, and high activity with turnover frequencies faster than 100 000 s-1. To identify the origins of its exceptional performance and inform future catalyst design, we report a combined computational and experimental study that establishes two distinct mechanistic pathways for electrochemical CO2 reduction catalyzed by [Fe]2+ as a function of applied overpotential. Electrochemical data shows the formation of two catalytic regimes at low (ηTOF/2 of 160 mV) and high (ηTOF/2 of 590 mV) overpotential plateaus. We propose that at low overpotentials [Fe]2+ undergoes a two-electron reduction, two-proton-transfer mechanism (electrochemical-electrochemical-chemical-chemical, EECC), where turnover occurs through the dicationic iron complex, [Fe]2+. Computational analysis supports the importance of the singlet ground-state electronic structure for CO2 binding and that the rate-limiting step is the second protonation in this low-overpotential regime. When more negative potentials are applied, an additional electron-transfer event occurs through either a stepwise or proton-coupled electron-transfer (PCET) pathway, enabling catalytic turnover from the monocationic iron complex ([Fe]+) via an electrochemical-chemical-electrochemical-chemical (ECEC) mechanism. Comparison of experimental kinetic data obtained from variable controlled potential electrolysis (CPE) experiments with direct product detection with calculated rates obtained from the energetic span model supports the PCET pathway as the most likely mechanism. Moreover, we build upon this mechanistic understanding to propose the design of an improved ligand framework that is predicted to stabilize the key transition states identified in our study and explore their electronic structures using an energy decomposition analysis. Taken together, this work highlights the value of synergistic computational/experimental approaches to decipher mechanisms of new electrocatalysts and direct the rational design of improved platforms.
          Cover page: Deciphering Distinct Overpotential-Dependent Pathways for Electrochemical CO2 Reduction Catalyzed by an Iron–Terpyridine ComplexCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2022)
          Bicarbonate-based electrolytes are ubiquitous in aqueous electrochemical CO2 reduction, particularly in heterogenous catalysis, where they demonstrate improved catalytic performance relative to other buffers. In contrast, the presence of bicarbonate in organic electrolytes and its roles in homogeneous electrocatalysis remain underexplored. Here, we investigate the influence of bicarbonate on iron porphyrin-catalyzed electrochemical CO2 reduction. We show that bicarbonate is a viable proton donor in organic electrolyte (pKa = 20.8 in dimethyl sulfoxide) and that urea pendants in the second coordination sphere can be used to template bicarbonate in the vicinity of a molecular iron porphyrin catalyst. The templated binding of bicarbonate increases its acidity, resulting in a 1500-fold enhancement in catalytic rates relative to unmodified parent iron porphyrin. This work emphasizes the importance of bicarbonate speciation in wet organic electrolytes and establishes second-sphere bicarbonate templating as a design strategy to harness this adventitious acid and enhance CO2 reduction catalysis.
            Cover page: Templating Bicarbonate in the Second Coordination Sphere Enhances Electrochemical CO2 Reduction Catalyzed by Iron PorphyrinsCreative Commons 'BY-NC' version 4.0 license
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2022)
            Developing strategies to study reactivity and selectivity in flexible catalyst systems has become an important topic of research. Herein, we report a combined experimental and computational study aimed at understanding the mechanistic role of an achiral DABCOnium cofactor in a regio- and enantiodivergent bromocyclization reaction. It was found that electron-deficient aryl substituents enable rigidified transition states via an anion-π interaction with the catalyst, which drives the selectivity of the reaction. In contrast, electron-rich aryl groups on the DABCOnium result in significantly more flexible transition states, where interactions between the catalyst and substrate are more important. An analysis of not only the lowest-energy transition state structures but also an ensemble of low-energy transition state conformers via energy decomposition analysis and machine learning was crucial to revealing the dominant noncovalent interactions responsible for observed changes in selectivity in this flexible system.
              Cover page: A Combined DFT, Energy Decomposition, and Data Analysis Approach to Investigate the Relationship Between Noncovalent Interactions and Selectivity in a Flexible DABCOnium/Chiral Anion Catalyst System
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2022)
              Catalysts promoting multielectron charge delocalization offer selectivity for the CO2reduction reaction (CO2RR) over the competing hydrogen evolution reaction. Here, we show metal-ligand exchange coupling as an example of charge delocalization that can determine the efficiency for photocatalytic CO2RR. A comparative evaluation of iron and cobalt complexes supported by the redox-Active ligand tpyPY2Me establishes that the two-electron reduction of [Co(tpyPY2Me)]2+([Co]2+) occurs at potentials 770 mV more negative than the [Fe(tpyPY2Me)]2+([Fe]2+) analogue by maximizing the exchange coupling in the latter compound. The positive shift in the reduction potential promoted by metal-ligand exchange coupling drives [Fe]2+to be among the most active and selective molecular catalysts for photochemical CO2RR reported to date, maintaining up to 99% CO product selectivity with total turnover numbers (TONs) and initial turnover frequencies exceeding 30,000 and 900 min-1, respectively. In contrast, [Co]2+shows much lower CO2RR activity, reaching only ca. 600 TON at 83% CO product selectivity under similar conditions accompanied by rapid catalyst decomposition. The spin density plots of the two-electron reduced [Co]0complex implicate a paramagnetic open-shell doublet ground state compared to the diamagnetic open-shell singlet ground state of reduced [Fe]0, rationalizing the observed negative shift in two-electron reduction potentials from the [M]2+species and lowered CO2RR efficiency for the cobalt complex relative to its iron congener. This work emphasizes the contributions of multielectron metal-ligand exchange coupling in promoting effective CO2RR and provides a starting point for the broader incorporation of this strategy in catalyst design.
                Cover page: Exchange Coupling Determines Metal-Dependent Efficiency for Iron- and Cobalt-Catalyzed Photochemical CO2 ReductionCreative Commons 'BY-NC' version 4.0 license
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2021)
                To facilitate computational investigation of intermolecular interactions in the solution phase, we report the development of ALMO-EDA(solv), a scheme that allows the application of continuum solvent models within the framework of energy decomposition analysis (EDA) based on absolutely localized molecular orbitals (ALMOs). In this scheme, all the quantum mechanical states involved in the variational EDA procedure are computed with the presence of solvent environment so that solvation effects are incorporated in the evaluation of all its energy components. After validation on several model complexes, we employ ALMO-EDA(solv) to investigate substituent effects on two classes of complexes that are related to molecular CO2 reduction catalysis. For [FeTPP(CO2-κC)]2- (TPP = tetraphenylporphyrin), we reveal that two ortho substituents which yield most favorable CO2 binding, -N(CH3)3 + (TMA) and -OH, stabilize the complex via through-structure and through-space mechanisms, respectively. The coulombic interaction between the positively charged TMA group and activated CO2 is found to be largely attenuated by the polar solvent. Furthermore, we also provide computational support for the design strategy of utilizing bulky, flexible ligands to stabilize activated CO2 via long-range Coulomb interactions, which creates biomimetic solvent-inaccessible "pockets" in that electrostatics is unscreened. For the reactant and product complexes associated with the electron transfer from the p-terphenyl radical anion to CO2, we demonstrate that the double terminal substitution of p-terphenyl by electron-withdrawing groups considerably strengthens the binding in the product state while moderately weakens that in the reactant state, which are both dominated by the substituent tuning of the electrostatics component. These applications illustrate that this new extension of ALMO-EDA provides a valuable means to unravel the nature of intermolecular interactions and quantify their impacts on chemical reactivity in solution.
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2021)
                  Electrocatalysis enables the construction of C-C bonds under mild conditions via controlled formation of carbon-centered radicals. For sequences initiated by alkyl halide reduction, coordinatively unsaturated Ni complexes commonly serve as single-electron transfer agents, giving rise to the foundational question of whether outer- or inner-sphere electron transfer oxidative addition prevails in redox mediation. Indeed, rational design of electrochemical processes requires the discrimination of these two electron transfer pathways, as they can have outsized effects on the rate of substrate bond activation and thus impact radical generation rates and downstream product selectivities. We present results from combined synthetic, electroanalytical, and computational studies that examine the mechanistic differences of single electron transfer to alkyl halides imparted by Ni metal-ligand cooperativity. Electrogenerated reduced Ni species, stabilized by delocalized spin density onto a redox-active tpyPY2Me polypyridyl ligand, activates alkyl iodides via outer-sphere electron transfer, allowing for the selective activation of alkyl iodide substrates over halogen atom donors and the controlled generation and sequestration of electrogenerated radicals. In contrast, the Ni complex possessing a redox-innocent pentapyridine congener activates the substrates in an inner-sphere fashion owning to a purely metal-localized spin, thereby activating both substrates and halogen atom donors in an indiscriminate fashion, generating a high concentration of radicals and leading to unproductive dimerization. Our data establish that controlled electron transfer via Ni-ligand cooperativity can be used to limit undesired radical recombination products and promote selective radical processes in electrochemical environments, providing a generalizable framework for designing redox mediators with distinct rate and potential requirements.
                    Cover page: Controlled Single-Electron Transfer via Metal–Ligand Cooperativity Drives Divergent Nickel-Electrocatalyzed Radical Pathways
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2020)
                    Biological and heterogeneous catalysts for the electrochemical CO2 reduction reaction (CO2RR) often exhibit a high degree of electronic delocalization that serves to minimize overpotential and maximize selectivity over the hydrogen evolution reaction (HER). Here, we report a molecular iron(II) system that captures this design concept in a homogeneous setting through the use of a redox non-innocent terpyridine-based pentapyridine ligand (tpyPY2Me). As a result of strong metal-ligand exchange coupling between the Fe(II) center and ligand, [Fe(tpyPY2Me)]2+ exhibits redox behavior at potentials 640 mV more positive than the isostructural [Zn(tpyPY2Me)]2+ analog containing the redox-inactive Zn(II) ion. This shift in redox potential is attributed to the requirement for both an open-shell metal ion and a redox non-innocent ligand. The metal-ligand cooperativity in [Fe(tpyPY2Me)]2+ drives the electrochemical reduction of CO2 to CO at low overpotentials with high selectivity for CO2RR (>90%) and turnover frequencies of 100 000 s-1 with no degradation over 20 h. The decrease in the thermodynamic barrier engendered by this coupling also enables homogeneous CO2 reduction catalysis in water without compromising selectivity or rates. Synthesis of the two-electron reduction product, [Fe(tpyPY2Me)]0, and characterization by X-ray crystallography, Mössbauer spectroscopy, X-ray absorption spectroscopy (XAS), variable temperature NMR, and density functional theory (DFT) calculations, support assignment of an open-shell singlet electronic structure that maintains a formal Fe(II) oxidation state with a doubly reduced ligand system. This work provides a starting point for the design of systems that exploit metal-ligand cooperativity for electrocatalysis where the electrochemical potential of redox non-innocent ligands can be tuned through secondary metal-dependent interactions.
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                    Cover page: Metal–Ligand Cooperativity via Exchange Coupling Promotes Iron- Catalyzed Electrochemical CO2 Reduction at Low Overpotentials
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