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1. Use of Intramolecular Quinol Redox Couples to Facilitate the Catalytic Transformation of O ₂ and O ₂ -Derived Species

   https://doi.org/10.1021/acs.accounts.4c00645  

   Farnum, Byron H; Goldsmith, Christian R (January 2025, Accounts of Chemical Research)

   The redox reactivity of transition metal centers can be augmented by nearby redox-active inorganic or organic moieties. In some cases, these functional groups can even allow a metal center to participate in reactions that were previously inaccessible to both the metal center and the functional group by themselves. Our research groups have been synthesizing and characterizing coordination complexes with polydentate quinol-containing ligands. Quinol is capable of being reversibly oxidized by either one or two electrons to semiquinone or para-quinone, respectively. Functionally, quinol behaves much differently than phenol, even though the pKa values of the first O−H bonds are nearly identical. The redox activity of the quinol in the polydentate ligand can augment the abilities of bound redox-active metals to catalyze the dismutation of O2−• and H2O2. These complexes can thereby act as high-performing functional mimics of superoxide dismutase (SOD) and catalase (CAT) enzymes, which exclusively use redox-active metals to transfer electrons to and from these reactive oxygen species (ROS). The quinols augment the activity of redox-active metals by stabilizing higher-valent metal species, providing alternative redox partners for the oxidation and reduction of reactive oxygen species, and protecting the catalyst from destructive side reactions. The covalently attached quinols can even enable redox-inactive Zn(II) to catalyze the degradation of ROS. With the Zn(II)-containing SOD and CAT mimics, the organic redox couple entirely substitutes for the inorganic redox couples used by the enzymes. The ligand structure modulates the antioxidant activity, and thus far, we have found that compounds that have poor or negligible SOD activity can nonetheless behave as efficient CAT mimics. Quinol-containing ligands have also been used to prepare electrocatalysts for dioxygen reduction, functionally mimicking the enzyme cytochrome c oxidase. The installation of quinols can boost electrocatalytic activity and even enable otherwise inactive ligand frameworks to support electrocatalysis. The quinols can also shift the product selectivity of O2 reduction from H2O2 to H2O without markedly increasing the effective overpotential. Distinct control of the coordination environment around the metal center allows the most successful of these catalysts to use economic and naturally abundant first-row transition metals such as iron and cobalt to selectively reduce O2 to H2O at low effective overpotentials. With iron, we have found that the electrocatalysts can enter the catalytic cycle as either an Fe(II) or Fe(III) species with no difference in turnover frequency. The entry point to the cycle, however, has a marked impact on the effective overpotential, with the Fe(III) species thus far being more efficient. 

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2. Controlling Product Selectivity During Dioxygen Reduction with Mn Complexes Using Pendent Proton Donor Relays and Added Base

   https://doi.org/10.1039/D3SC02611F  

   Cook, Emma N.; Courter, Ian M.; Dickie, Diane A.; Machan, Charles W. (January 2024, Chemical Science)

   The catalytic reduction of dioxygen (O2) is important in biological energy conversion and alternative energy applications. In comparison to Fe- and Co-based systems, examples of catalytic O2 reduction by homogeneous Mn-based systems is relatively sparse. Motivated by this lack of knowledge, two Mn-based catalysts for the oxygen reduction reaction (ORR) containing a bipyridine-based non-porphyrinic ligand framework have been developed to evaluate how pendent proton donor relays alter activity and selectivity for the ORR, where Mn(p-tbudhbpy)Cl (1) was used as a control complex and Mn(nPrdhbpy)Cl (2) contains a pendent –OMe group in the secondary coordination sphere. Using an ammonium-based proton source, N,N′-diisopropylethylammonium hexafluorophosphate, we analyzed catalytic activity for the ORR: 1 was found to be 64% selective for H2O2 and 2 is quantitative for H2O2, with O2 binding to the reduced Mn(II) center being the rate-determining step. Upon addition of the conjugate base, N,N′-diisopropylethylamine, the observed catalytic selectivity of both 1 and 2 shifted to H2O as the primary product. Interestingly, while the shift in selectivity suggests a change in mechanism for both 1 and 2, the catalytic activity of 2 is substantially enhanced in the presence of base and the rate-determining step becomes the bimetallic cleavage of the O–O bond in a Mn-hydroperoxo species. These data suggest that the introduction of pendent relay moieties can improve selectivity for H2O2 at the expense of diminished reaction rates from strong hydrogen bonding interactions. Further, although catalytic rate enhancements are observed with a change in product selectivity when base is added to buffer proton activity, the pendent relays stabilize dimer intermediates, limiting the maximum rate. 

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3. Oxygen Reduction Electrocatalysis with Epitaxially Grown Spinel MnFe ₂ O ₄ and Fe ₃ O ₄

   https://doi.org/10.1021/acscatal.1c05172  

   Bredar, Alexandria R.; Blanchet, Miles D.; Burton, Andricus R.; Matthews, Bethany E.; Spurgeon, Steven R.; Comes, Ryan B.; Farnum, Byron H. (March 2022, ACS Catalysis)

   Nanocrystalline MnFe2O4 has shown promise as a catalyst for the oxygen reduction reaction (ORR) in alkaline solutions, but the material has been sparingly studied as highly ordered thin-film catalysts. To examine the role of surface termination and Mn and Fe site occupancy, epitaxial MnFe2O4 and Fe3O4 spinel oxide films were grown on (001)- and (111)-oriented Nb:SrTiO3 perovskite substrates using molecular beam epitaxy and studied as electrocatalysts for the oxygen reduction reaction (ORR). High-resolution X-ray diffraction (HRXRD) and X-ray photoelectron spectroscopy (XPS) show the synthesis of pure phase materials, while scanning transmission electron microscopy (STEM) and reflection high-energy electron diffraction (RHEED) analysis demonstrate island-like growth of (111) surface-terminated pyramids on both (001)- and (111)-oriented substrates, consistent with the literature and attributed to the lattice mismatch between the spinel films and the perovskite substrate. Cyclic voltammograms under a N2 atmosphere revealed distinct redox features for Mn and Fe surface termination based on comparison of MnFe2O4 and Fe3O4. Under an O2 atmosphere, electrocatalytic reduction of oxygen was observed at both Mn and Fe redox features; however, a diffusion-limited current was only achieved at potentials consistent with Fe reduction. This result contrasts with that of nanocrystalline MnFe2O4 reported in the literature where the diffusion-limited current is achieved with Mn-based catalysis. This difference is attributed to a low density of Mn surface termination, as determined by the integration of current from CVs collected under N2, in addition to low conductivity through the MnFe2O4 film due to the degree of inversion. Such low densities are attributed to the synthetic method and island-like growth pattern and highlight challenges in studying ORR catalysis with single-crystal spinel materials. 

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4. Benzoquinone Cocatalyst Contributions to DAF/Pd(OAc) ₂ -Catalyzed Aerobic Allylic Acetoxylation in the Absence and Presence of a Co(salophen) Cocatalyst

   https://doi.org/10.1021/acscatal.1c01074  

   Kozack, Caitlin V.; Tereniak, Stephen J.; Jaworski, Jonathan N.; Li, Bao; Bruns, David L.; Knapp, Spring M.; Landis, Clark R.; Stahl, Shannon S. (April 2021, ACS Catalysis)

   null (Ed.)

   Palladium(II)-catalyzed allylic acetoxylation has been the focus of extensive development and investigation. Methods that use molecular oxygen (O2) as the terminal oxidant typically benefit from the use of benzoquinone (BQ) and a transition-metal (TM) cocatalyst, such as Co(salophen), to support oxidation of Pd0 during catalytic turnover. We previously showed that Pd(OAc)2 and 4,5-diazafluoren-9-one (DAF) as an ancillary ligand catalyze allylic oxidation with O2 in the absence of cocatalysts. Herein, we show that BQ enhances DAF/Pd(OAc)2 catalytic activity, nearly matching the performance of reactions that include both BQ and Co(salophen). These observations are complemented by mechanistic studies of DAF/Pd(OAc)2 catalyst systems under three different oxidation conditions: (1) O2 alone, (2) O2 with cocatalytic BQ, and (3) O2 with cocatalytic BQ and Co(salophen). The beneficial effect of BQ in the absence of Co(salophen) is traced to the synergistic roles of O2 and BQ, both of which are capable of oxidizing Pd0 to PdII. The reaction of O2 generates H2O2 as a byproduct, which can oxidize hydroquinone to quinone in the presence of PdII. NMR spectroscopic studies, however, show that hydroquinone is the predominant redox state of the quinone cocatalyst in the absence of Co(salophen), while inclusion of Co(salophen) maintains oxidized quinone throughout the reaction, resulting in better reaction performance. 

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5. Direct Evidence of a Light-Dependent Sink of Superoxide within Chromophoric Dissolved Organic Matter

   https://doi.org/10.1021/acs.est.3c08254  

   Le_Roux, Danielle M; Powers, Leanne C; Blough, Neil V (December 2023, Environmental Science & Technology)

   Superoxide (O2• –) is produced photochemically in natural waters by chromophoric dissolved organic matter (CDOM) via the reaction of molecular oxygen with photoproduced one-electron reductants (OERs) within CDOM. In the absence of other sinks (metals or organic radicals), O2• – is believed to undergo primarily dismutation to produce hydrogen peroxide (H2O2). However, past studies have implicated the presence of an additional light-dependent sink of O2• – that does not lead to H2O2 production. Here, we provide direct evidence of this sink through O2• – injection experiments. During irradiations, spikes of O2• – are consumed to a greater extent (∼85–30% loss) and are lost much faster (up to ∼0.09 s–1) than spikes introduced post-irradiation (∼50–0% loss and ∼0.03 s–1 rate constant). The magnitude of the loss during irradiation and the rate constant are wavelength-dependent. Analysis of the H2O2 concentration post-spike indicates that this light-dependent sink does not produce H2O2 at low spike concentrations. This work further demonstrates that simply assuming that the O2• – production is twice the H2O2 production is not accurate, as previously believed. 

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