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1. Non-covalent assembly of proton donors and p- benzoquinone anions for co-electrocatalytic reduction of dioxygen

   https://doi.org/10.1039/D1SC01271A  

   Hooe, Shelby L.; Cook, Emma N.; Reid, Amelia G.; Machan, Charles W. (July 2021, Chemical Science)

   The two-electron and two-proton p -hydroquinone/ p -benzoquinone (H 2 Q/BQ) redox couple has mechanistic parallels to the function of ubiquinone in the electron transport chain. This proton-dependent redox behavior has shown applicability in catalytic aerobic oxidation reactions, redox flow batteries, and co-electrocatalytic oxygen reduction. Under nominally aprotic conditions in non-aqueous solvents, BQ can be reduced by up to two electrons in separate electrochemically reversible reactions. With weak acids (AH) at high concentrations, potential inversion can occur due to favorable hydrogen-bonding interactions with the intermediate monoanion [BQ(AH) m ]˙ − . The solvation shell created by these interactions can mediate a second one-electron reduction coupled to proton transfer at more positive potentials ([BQ(AH) m ]˙ − + n AH + e − ⇌ [HQ(AH) (m+n)−1 (A)] 2− ), resulting in an overall two electron reduction at a single potential at intermediate acid concentrations. Here we show that hydrogen-bonded adducts of reduced quinones and the proton donor 2,2,2-trifluoroethanol (TFEOH) can mediate the transfer of electrons to a Mn-based complex during the electrocatalytic reduction of dioxygen (O 2 ). The Mn electrocatalyst is selective for H 2 O 2 with only TFEOH and O 2 present, however, with BQ present under sufficient concentrations of TFEOH, an electrogenerated [H 2 Q(AH) 3 (A) 2 ] 2− adduct (where AH = TFEOH) alters product selectivity to 96(±0.5)% H 2 O in a co-electrocatalytic fashion. These results suggest that hydrogen-bonded quinone anions can function in an analogous co-electrocatalytic manner to H 2 Q. 

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2. C–H bond activation via concerted metalation–deprotonation at a palladium( iii ) center

   https://doi.org/10.1039/D3SC00034F  

   Bouley, Bailey S.; Tang, Fengzhi; Bae, Dae Young; Mirica, Liviu M. (April 2023, Chemical Science)

   Herein we report the direct observation of C–H bond activation at an isolated mononuclear Pd( iii ) center. The oxidation of the Pd( ii ) complex ( Me N4)Pd II (neophyl)Cl (neophyl = –CH 2 C(CH 3 ) 2 Ph; Me N4 = N , N ′-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) using the mild oxidant ferrocenium hexafluorophosphate (FcPF 6 ) yields the stable Pd( iii ) complex [( Me N4)Pd III (neophyl)Cl]PF 6 . Upon the addition of an acetate source, [( Me N4)Pd III (neophyl)Cl]PF 6 undergoes Csp 2 –H bond activation to yield the cyclometalated product [( Me N4)Pd III (cycloneophyl)]PF 6 . This metalacycle can be independently prepared, allowing for a complete characterization of both the starting and final Pd( iii ) complexes. The C–H activation step can be monitored directly by EPR and UV-Vis spectroscopies, and kinetic isotope effect (KIE) studies suggest that either a pre-association step such as an agostic interaction may be rate limiting, or that the C–H activation is partially rate-limiting in conjunction with ligand rearrangement. Density functional theory calculations support that the reaction proceeds through a κ 3 ligand coordination and that the flexible ligand structure is important for this transformation. Overall, this study represents the first example of discrete C–H bond activation occurring at a Pd( iii ) center through a concerted metalation–deprotonation mechanism, akin to that observed for Pd( ii ) and Pd( iv ) centers. 

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3. Electrochemistry of Polymeric Cobaloxime-Containing Assemblies in Organic and Aqueous Solvents

   https://doi.org/10.1149/2162-8777/aba1ff  

   Wadsworth, Brian L.; Nishiori, Daiki; Nguyen, Nghi P.; Reyes Cruz, Edgar A.; Moore, Gary F. (July 2020, ECS Journal of Solid State Science and Technology)

   We report on the structural and electrochemical properties of a heterogeneous-homogeneous assembly composed of molecular cobaloxime catalysts immobilized onto graphite electrodes via an intervening polyvinylpyridine surface coating. When these modified electrodes are immersed in an organic solvent (propylene carbonate containing 0.1 M tetrabutylammonium perchlorate as a supporting electrolyte) or basic aqueous solutions (0.1 M NaOH), cyclic voltammetry measurements enable determination of the Co^III/IIpeak potentials and Co^II/Imidpoint potentials of cobaloximes embedded within the polymeric architectures. Additionally, voltammetry measurements recorded using pH neutral aqueous solutions (0.1 M phosphate buffer) confirm the immobilized cobaloximes remain catalytically active for hydrogen production and operate at a turnover frequency of 1.6 s⁻¹when polarized at –0.35 V vs the H⁺/H₂equilibrium potential. Waveform analysis of redox features associated with immobilized cobaloximes indicates more repulsive interactions within the polymer film at pH neutral vs basic conditions, which is attributed to the increased fraction of pyridinium species at lower pH values. Our measurements also show the number of electrochemically active sites changes when measured in different solvent environments, indicating that electroactive loadings determined under non-catalytic solvent conditions are not necessarily representative of those under catalytic conditions and could thereby lead to misrepresentations of catalytic turnover frequencies. 

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4. Dynamic aqueous transformations of lithium cobalt oxide nanoparticle induce distinct oxidative stress responses of B. subtilis

   https://doi.org/10.1039/D0EN01151G  

   Gari, Metti K.; Lemke, Paul; Lu, Kelly H.; Laudadio, Elizabeth D.; Henke, Austin H.; Green, Curtis M.; Pho, Thomas; Hoang, Khoi Nguyen; Murphy, Catherine J.; Hamers, Robert J.; et al (January 2021, Environmental Science: Nano)

   null (Ed.)

   Lithium cobalt oxide (LiCoO 2 ), an example of nanoscale transition metal oxide and a widely commercialized cathode material in lithium ion batteries, has been shown to induce oxidative stress and generate intracellular reactive oxygen species (ROS) in model organisms. In this study, we aimed to understand the time-dependent roles of abiotic ROS generation and Co ions released in aqueous medium by LiCoO 2 NPs, and examined the induced biological responses in model bacterium, B. subtilis upon exposure. We found that the redox-active LiCoO 2 NPs produced abiotic ROS primarily through H 2 O 2 generation when freshly suspended. Subsequently, the freshly-suspended LiCoO 2 NPs induced additional DNA breakage, and changes in expression of oxidative stress genes in B. subtilis that could not be accounted for by the released Co ions alone. Notably, in 48 hour old LiCoO 2 suspensions, H 2 O 2 generation subsided while higher concentrations of Co ions were released. The biological responses in DNA damage and gene expression to the aged LiCoO 2 NPs recapitulated those induced by the released Co ions. Our results demonstrated oxidative stress mechanisms for bacteria exposed to LiCoO 2 NPs were mediated by the generation of distinct biotic and abiotic ROS species, which depended on the aqueous transformation state of the NPs. This study revealed the interdependent and dynamic nature of NP transformation and their biological consequences where the state of NPs resulted in distinct NP-specific mechanisms of oxidative injury. Our work highlights the need to capture the dynamic transformation of NPs that may activate the multiple routes of oxidative stress responses in cells. 

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5. Robust redox-reversible perovskite type steam electrolyser electrode decorated with in situ exsolved metallic nanoparticles

   https://doi.org/10.1039/c9ta06309a  

   Liu, Tong; Zhao, Yiqian; Zhang, Xiaoyu; Zhang, Hong; Jiang, Guang; Zhao, Wen; Guo, Jiayi; Chen, Fanglin; Yan, Mufu; Zhang, Yanxiang; et al (January 2020, Journal of Materials Chemistry A)

   Redox stabilities of the hydrogen electrode with in situ exsolved Fe–Ni nanoparticles from Sr 2 Fe 1.4 Ni 0.1 Mo 0.5 O 6−δ (SFMNi) perovskite are studied by analyzing the evolution of the phase composition and morphology during the redox cycles. It is found that certain amount of the exsolved nanoparticles have been oxidized to the transition metal oxide (Ni,Fe)O instead of reincorporating into the parent perovskite lattice upon re-oxidizing at 800 °C in air. However, the (Ni,Fe)O secondary phases show no adverse effect on the subsequent reduction treatment. The redox reversibility mechanism is explained by the regular-solution model. The electrodes are almost fully recovered in the reducing atmosphere, and the symmetrical cells measured under 9.7% H 2 –3% H 2 O–87.3% N 2 conditions show a stable specific area polarization resistance of around 1.93 Ω cm 2 at 800 °C during 13 redox cycles. Single cells using the Ni–Fe nanoparticles structured electrode exhibit a stable electrode polarization resistance of about 0.598 Ω cm 2 at 800 °C under open circuit voltage conditions and a steady electrolysis current density of about −653 mA cm −2 at 1.5 V during the steam electrolysis process over 5 redox cycles. These results indicate that the SFMNi material is a very promising electrode candidate for steam electrolysis application with robust redox reversibility. 

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