Biomimetic Oxygen Reduction by Cofacial Porphyrins at a Liquid–Liquid Interface

Bioinspired and Bioderived Aqueous Electrocatalysis

The development of efficient and sustainable electrochemical systems able to provide clean-energy fuels and chemicals is one of the main current challenges of materials science and engineering. Over the last decades, significant advances have been made in the development of robust electrocatalysts for different reactions, with fundamental insights from both computational and experimental work. Some of the most promising systems in the literature are based on expensive and scarce platinum-group metals; however, natural enzymes show the highest per-site catalytic activities, while their active sites are based exclusively on earth-abundant metals. Additionally, natural biomass provides a valuable feedstock for producing advanced carbonaceous materials with porous hierarchical structures. Utilizing resources and design inspiration from nature can help create more sustainable and cost-effective strategies for manufacturing cost-effective, sustainable, and robust electrochemical materials and devices. This review spans from materials to device engineering; we initially discuss the design of carbon-based materials with bioinspired features (such as enzyme active sites), the utilization of biomass resources to construct tailored carbon materials, and their activity in aqueous electrocatalysis for water splitting, oxygen reduction, and CO₂ reduction. We then delve in the applicability of bioinspired features in electrochemical devices, such as the engineering of bioinspired mass transport and electrode interfaces. Finally, we address remaining challenges, such as the stability of bioinspired active sites or the activity of metal-free carbon materials, and discuss new potential research directions that can open the gates to the implementation of bioinspired sustainable materials in electrochemical devices.

Lowering the Symmetry of Cofacial Porphyrin Prisms for Selective Oxygen Reduction Electrocatalysis

Cofacial porphyrin catalysts for the Oxygen Reduction Reaction (ORR) formed via coordination-driven self-assembly have so far been limited to designs with fourfold symmetry, where four molecular clips bridge two porphyrin sites. We have synthesized six Py_nPh_m (Py = pyridyl, Ph = phenyl) metalloporphyrin prisms (Co²⁺, Zn²⁺) bridged by molecular clips containing two Rh³⁺ centers. Four of these structures are lower symmetry, with the Py₃Ph and Py₂Ph₂ prisms containing three and two molecular clips, respectively. The Co²⁺ species were evaluated for their ORR activity. Cyclic and hydrodynamic voltammetry studies of heterogeneous catalyst inks in aqueous media revealed marked differences in selectivity from ∼5% (Py₃Ph) to ∼37% (Py₂Ph₂) for the formation of H₂O₂. The single-crystal X-ray structure of the Zn₂ Py₂Ph₂ prism shows an offset between the porphyrin faces. This structural feature may be responsible for the change in selectivity, consistent with previous studies of covalently tethered cofacial porphyrins that have shown that geometry is a critical determinant of two-electron/two-proton versus four-electron/four-proton pathways. Extraction of standard rate constants k_s for the ORR revealed a cofacial enhancement of ∼2 orders of magnitude over mononuclear Co²⁺ tetrapyridyl porphyrin. Even though all the prisms described here use the same molecular clip, the resultant structures, and thus the reactivity for the ORR, differ significantly based on the number and orientation of pyridyl donor groups on the porphyrins, highlighting how coordination-driven self-assembly can be used to rapidly tune dinuclear catalysts.

Potential‐Modulated Ion Distributions in the Back‐to‐Back Electrical Double Layers at a Polarised Liquid|Liquid Interface Regulate the Kinetics of Interfacial Electron Transfer

Biphasic interfacial electron transfer (IET) reactions at polarisable liquid|liquid (L|L) interfaces underpin new approaches to electrosynthesis, redox electrocatalysis, bioelectrochemistry and artificial photosynthesis. Herein, using cyclic and alternating current voltammetry, we demonstrate that under certain experimental conditions, the biphasic 2-electron O₂ reduction reaction can proceed by single-step IET between a reductant in the organic phase, decamethylferrocene, and interfacial protons in the presence of O₂. Using this biphasic system, we demonstrate that the applied interfacial Galvani potential difference Δ o w φ provides no direct driving force to realise a thermodynamically uphill biphasic IET reaction in the mixed solvent region. We show that the onset potential for a biphasic single-step IET reaction does not correlate with the thermodynamically predicted standard Galvani IET potential and is instead closely correlated with the potential of zero charge at a polarised L|L interface. We outline that the applied Δ o w φ required to modulate the interfacial ion distributions, and thus kinetics of IET, must be optimised to ensure that the aqueous and organic redox species are present in substantial concentrations at the L|L interface simultaneously in order to react.

Crucial Roles of a Pendant Imidazole Ligand of a Cobalt Porphyrin Complex in the Stoichiometric and Catalytic Reduction of Dioxygen

A cobalt porphyrin complex with a pendant imidazole base ([(L₁ )Co^II ]) is an efficient catalyst for the homogeneous catalytic two-electron reduction of dioxygen by 1,1'-dimethylferrocene (Me₂ Fc) in the presence of triflic acid (HOTf), as compared with a cobalt porphyrin complex without a pendant imidazole base ([(L₂ )Co^II ]). The pendant imidazole ligand plays a crucial role not only to provide an imidazolinium proton for proton-coupled electron transfer (PCET) from [(L₁ )Co^II ] to O₂ in the presence of HOTf but also to facilitate electron transfer (ET) from [(L₁ )Co^II ] to O₂ in the absence of HOTf. The kinetics analysis and the detection of intermediates in the stoichiometric and catalytic reduction of O₂ have provided clues to clarify the crucial roles of the pendant imidazole ligand of [(L₁ )Co^II ] for the first time.

Metal coordination in C2N-like materials towards dual atom catalysts for oxygen reduction

Single-atom catalysts, in particular the Fe-N-C family of materials, have emerged as a promising alternative to platinum group metals in fuel cells as catalysts for the oxygen reduction reaction. Numerous theoretical studies have suggested that dual atom catalysts can appreciably accelerate catalytic reactions; nevertheless, the synthesis of these materials is highly challenging owing to metal atom clustering and aggregation into nanoparticles during high temperature synthesis treatment. In this work, dual metal atom catalysts are prepared by controlled post synthetic metal-coordination in a C2N-like material. The configuration of the active sites was confirmed by means of X-ray adsorption spectroscopy and scanning transmission electron microscopy. During oxygen reduction, the catalyst exhibited an activity of 2.4 ± 0.3 A gcarbon -1 at 0.8 V versus a reversible hydrogen electrode in acidic media, comparable to the most active in the literature. This work provides a novel approach for the targeted synthesis of catalysts containing dual metal sites in electrocatalysis.

Defined Ion-Transfer Voltammetry of a Single Microdroplet at a Polarized Liquid/Liquid Interface

A strategy for the fast analysis of ion transfer/facilitated ion transfer toward a tiny (femtoliter) water-in-oil droplet has been established. This scenario is embodied by the fusion of a w/o microdroplet at the micro liquid/liquid (L/L) interface, with the use of Fourier transform fast-scan cyclic voltammetry (FT-FSCV) to express the apparent half-wave potentials of anions or cations encapsulated inside the w/o microdroplet. First, the half-wave potential is in strict accordance with the transfer Gibbs free energy of either cations or anions. Second, the half-wave potential has been found to be positively proportional to the logarithmic concentration of ions, shedding thermodynamic insight into ion transfer. Third, as an instance of multivalent biopolymers, the transfer of protamine inside the single w/o microdroplet has been investigated. Obvious discrepancies in the behaviors of the fusion impacts at different pH, as well as in the absence and presence of the cationic surfactant DNNS^-, are revealed. The internal mechanism of protamine transfer has been thoroughly investigated. This work proposes a strategy to sensitively and quickly determine the transfer Gibbs energy and the concentration of ions encapsulated in a single microdroplet, and it provides the possibility of analyzing the interfacial transfer properties of trace biomacromolecules inside an aqueous micro- or nanoscale droplet.

Modulating the pro-apoptotic activity of cytochrome c at a biomimetic electrified interface

Programmed cell death via apoptosis is a natural defence against excessive cell division, crucial for fetal development to maintenance of homeostasis and elimination of precancerous and senescent cells. Here, we demonstrate an electrified liquid biointerface that replicates the molecular machinery of the inner mitochondrial membrane at the onset of apoptosis. By mimicking in vivo cytochrome c (Cyt c) interactions with cell membranes, our platform allows us to modulate the conformational plasticity of the protein by simply varying the electrochemical environment at an aqueous-organic interface. We observe interfacial electron transfer between an organic electron donor decamethylferrocene and O₂, electrocatalyzed by Cyt c. This interfacial reaction requires partial Cyt c unfolding, mimicking Cyt c in vivo peroxidase activity. As proof of concept, we use our electrified liquid biointerface to identify drug molecules, such as bifonazole, that can potentially down-regulate Cyt c and protect against uncontrolled neuronal cell death in neurodegenerative disorders.

The Solvent Effect on H2 O2 Generation at Room Temperature Ionic Liquid|Water Interface

H₂ O₂ is a versatile chemical and can be generated by the oxygen reduction reaction (ORR) in proton donor solution in molecular solvents or room temperature ionic liquids (IL). We investigated this reaction at interfaces formed by eleven hydrophobic ILs and acidic aqueous solution as a proton source with decamethylferrocene (DMFc) as an electron donor. H₂ O₂ is generated in colorimetrically detectable amounts in biphasic systems formed by alkyl imidazolium hexafluorophosphate or tetraalkylammonium bis(trifluoromethylsulfonyl)imide ionic liquids. H₂ O₂ fluxes were estimated close to liquid|liquid interface by scanning electrochemical microscopy (SECM). Contrary to the interfaces formed by hydrophobic electrolyte solution in a molecular solvent, H₂ O₂ generation is followed by cation expulsion to the aqueous phase. Weak correlation between the H₂ O₂ flux and the difference between DMFc/DMFc⁺ redox potential and 2 electron ORR standard potential indicates kinetic control of the reaction.

Ion Selective Detection Based on the Nuances of the Kinetic Fingerprint for Ion Transfer at Soft Interfaces

A novel approach has been developed for the selective determination of cations or anions based on the application of Fourier transformed staircase sinusoidal voltammetry (FT-SC-SV) in combination with the interface between two immiscible electrolyte solutions (ITIES) in the four-electrode configuration. The electrochemistry at the ITIES provides a very simple yet sensitive platform for the detection of a broad spectrum of redox inactive ions and even the neutral (bio)molecules that can be charged (e.g., protonated in appropriate pH). FT-SC-SV employs a complex potential excitation, i.e., a large-amplitude sine wave superimposed onto a dc bias potential that is stepped/scanned throughout the potential window. The response current is subsequently analyzed in the frequency domain by FT. Although the ions have close standard/formal transfer potential, discrimination and selective detection can be achieved by the higher harmonics. Feasibility and reliability of the proposed approach were verified with two pairs of ions that have very close transfer potentials across the ITIES and were chosen as the model systems. Besides, the additivity of the ionic current magnitude on concentration measured either in the mixture of ionic analytes or in individually prepared solutions containing the separate ionic analyte was tested. The experimental results prove that the principle of additivity holds. Compared with the traditional voltammetry, FT-SC-SV is simpler and more efficient in discrimination and quantification of apparently indistinguishable ion transfer from the viewpoint of thermodynamics. This demonstration may provide a new way for analytical detection of a broad range of redox inactive ions in terms of both fundamentals and applications.

A charge transfer state induced by strong exciton coupling in a cofacial μ-oxo-bridged porphyrin heterodimer

Photosensitizers with high energy, long lasting charge-transfer states are important components in systems designed for solar energy conversion by multistep electron transfer. Here, we show that in a push-pull type, μ-oxo-bridged porphyrin heterodimer composed of octaethylporphyrinatoaluminum(iii) and octaethylporphyrinatophosphorus(v), the strong excitonic coupling between the porphyrins and the different electron withdrawing abilities of Al(iii) and P(v) promote the formation of a high energy CT state. Using, an array of optical and magnetic resonance spectroscopic methods along with theoretical calculations, we demonstrate photodynamics of the heterodimer that involves the initial formation of a singlet CT which relaxes to a triplet CT state with a lifetime of ∼130 ps. The high-energy triplet CT state (3CT = 1.68 eV) lasts for nearly 105 μs prior to relaxing to the ground state.

Porphyrin-based frameworks for oxygen electrocatalysis and catalytic reduction of carbon dioxide

The recent progress made on porphyrin-based frameworks and their applications in energy-related conversion technologies (e.g., ORR, OER and CO₂RR) and storage technologies (e.g., Zn–air batteries).

Electrocatalysis of molecular oxygen reduction reaction at liquid-liquid interface and DFT computational study of proton transfer from the conjugate acid of 2,2'-dipyridylamineto oxygen

In this study, the catalytic effect of 2,2'-dipyridylamine (DPA) on the reduction of oxygen (O₂) at the polarized water/1,2-dichloroethane (DCE) interface was investigated. Ferrocene (Fc) and tetrathiafulvalene (TTF) were weak electron donors used in this study. Slow reduction of O₂ at the interface containing Fc and TTF was significantly accelerated upon the addition of DPA. Voltammetry and biphasic shake flask experiments revealed that DPA acts as a proton ionophore to transfer protons between the aqueous and organic phases. The PA, GB, and pK_a values of all possible conjugate acids of DPA were calculated. Then, a mechanism was suggested to explain the interaction between protonated DPA and oxygen molecular. The mechanism was computationally analyzed by using density functional theory (DFT). Furthermore, DFT calculations at the B3LYP/6-31G** level of theory showed that the conjugate acid species of DPA transfer proton to O₂ at the interface. The results show that DPA-H²⁺ and DPA-H¹⁺ are the best species to transfer proton to molecular oxygen.

A Mechanistic Dichotomy in Two-Electron Reduction of Dioxygen Catalyzed by N,N'-Dimethylated Porphyrin Isomers

Selective two-electron reduction of dioxygen (O₂ ) to hydrogen peroxide (H₂ O₂ ) has been achieved by two saddle-distorted N,N'-dimethylated porphyrin isomers, an N21,N'22-dimethylated porphyrin (anti-Me₂ P) and an N21,N'23-dimethylated porphyrin (syn-Me₂ P) as catalysts and ferrocene derivatives as electron donors in the presence of protic acids in acetonitrile. The higher catalytic performance in an oxygen reduction reaction (ORR) was achieved by anti-Me₂ P with higher turnover number (TON=250 for 30 min) than that by syn-Me₂ P (TON=218 for 60 min). The reactive intermediates in the catalytic ORR were confirmed to be the corresponding isophlorins (anti-Me₂ Iph or syn-Me₂ Iph) by spectroscopic measurements. The rate-determining step in the catalytic ORRs was concluded to be proton-coupled electron-transfer reduction of O₂ with isophlorins based on kinetic analysis. The ORR rate by anti-Me₂ Iph was accelerated by external protons, judging from the dependence of the observed initial rates on acid concentrations. In contrast, no acceleration of the ORR rate with syn-Me₂ Iph by external protons was observed. The different mechanisms in the O₂ reduction by the two isomers should be derived from that of the arrangement of hydrogen bonding of a O₂ with inner NH protons of the isophlorins.

Meso-Tetrapyrenylporphyrins: Synthesis, structural, spectral, electrochemical properties and Förster energy transfer (FRET) studies

Meso-tetrapyrenylporphyrin and its metal (Co[Formula: see text], Cu[Formula: see text], Ni[Formula: see text] and Zn[Formula: see text]) complexes were synthesized, characterized and studied for their spectral, electrochemical and energy transfer properties. DFT optimization was carried out to gain an insight into the interactions between the porphyrin [Formula: see text]-system and the pyrenyl substituents. The pyrenyl substituents and the porphyrin core remain essentially orthogonal to each other in both the free base and the metallated porphyrins. Redox potentials of the pyrenylporphyrins are marginally shifted as compared to their corresponding phenyl derivatives. Förster resonance energy transfer (FRET) studies were carried out in toluene for free-base pyrenylporphyrin and its Zn(II) complex. Since pyrene is a good donor, an efficient energy transfer from pyrene (D) to the porphyrin core (A) on the order of 80–85% was observed for these two compounds. It was observed that energy transfer occurs mainly via ”through-bond” (TB) interaction rather than ”through-space” (TS) interaction.

Oxygen Reduction Reaction at Penta-Coordinated Co Phthalocyanines

From the early 60s, Co complexes, especially Co phthalocyanines (CoPc) have been extensively studied as electrocatalysts for the oxygen reduction reaction (ORR). Generally, they promote the 2-electron reduction of O2 to give peroxide whereas the 4-electron reduction is preferred for fuel cell applications. Still, Co complexes are of interest because depending on the chemical environment of the Co metal centers either promote the 2-electron transfer process or the 4-electron transfer. In this study, we synthetized 3 different Co catalysts where Co is coordinated to 5 N atoms using CoN4 phthalocyanines with a pyridine axial linker anchored to carbon nanotubes. We tested complexes with electro-withdrawing or electro-donating residues on the N4 phthalocyanine ligand. The catalysts were characterized by EPR and XPS spectroscopy. Ab initio calculations, Koutecky-Levich extrapolation and Tafel plots confirm that the pyridine back ligand increases the Co-O2 binding energy, and therefore promotes the 4-electron reduction of O2. But the presence of electron withdrawing residues, in the plane of the tetra N atoms coordinating the Co, does not further increase the activity of the compounds because of pull-push electronic effects.

Significantly improved electrocatalytic oxygen reduction by an asymmetrical Pacman dinuclear cobalt(ii) porphyrin-porphyrin dyad

Asymmetrical Pacman dinuclear Co bisporphyrin shows significantly improved activity and selectivity for catalytic reduction of O₂ to water in comparison with corresponding mononuclear Co porphyrins and symmetrical dinuclear Co bisporphyrins.

Pacman dinuclear Co^II triphenylporphyrin-tri(pentafluorophenyl)porphyrin 1 and dinuclear Co^II bis-tri(pentafluorophenyl)porphyrin 2, anchored at the two meso-positions of a benzene linker, are synthesized and examined as electrocatalysts for the oxygen reduction reaction (ORR). Both dinuclear Co bisporphyrins are more efficient and selective than corresponding mononuclear Co^II tetra(pentafluorophenyl)porphyrin 3 and Co^II tetraphenylporphyrin 4 for the four-electron electrocatalytic reduction of O₂ to water. Significantly, although the ORR selectivities of the two dinuclear Co bisporphyrins are similar to each other, 1 outperforms 2, in terms of larger catalytic ORR currents and lower overpotentials. Electrochemical studies showed different redox behaviors of the two Co sites of 1: the Co^III/Co^II reduction of the Co-TPP (TPP = triphenylporphyrin) site is well-behind that of the Co-TPFP (TPFP = tri(pentafluorophenyl)porphyrin) site by 440 mV. This difference indicated their different roles in the ORR: Co^II-TPFP is likely the O₂ binding and reduction site, while Co^III-TPP, which is generated by the oxidation of Co^II-TPP on electrodes, may function as a Lewis acid to assist the O₂ binding and activation. The positively charged Co^III-TPP will have through-space charge interactions with the negatively charged O₂-adduct unit, which will reduce the activation energy barrier for the ORR. This effect of Co-TPP closely resembles that of the Cu_B site of metalloenzyme cytochrome c oxidase (CcO), which catalyzes the biological reduction of O₂. This work represents a rare example of asymmetrical dinuclear metal catalysts, which can catalyze the 4e reduction of O₂ with high selectivity and significantly improved activity.

Mechanistic Study of Oxygen Reduction at Liquid/Liquid Interfaces by Hybrid Ultramicroelectrodes and Mass Spectrometry

Proton-coupled electron transfer (PCET) reactions at various interfaces (liquid/membrane, solid/electrolyte, liquid/liquid) lie at the heart of many processes in biology and chemistry. Mechanistic study can provide profound understanding of PCET and rational design of new systems. However, most mechanisms of PCET reactions at a liquid/liquid interface have been proposed based on electrochemical and spectroscopic data, which lack direct evidence for possible intermediates. Moreover, a liquid/liquid interface as one type of soft interface is dynamic, making the investigation of interfacial reactions very challenging. Herein a novel electrochemistry method coupled to mass spectrometry (EC-MS) was introduced for in situ study of the oxygen reduction reaction (ORR) by ferrocene (Fc) under catalysis from cobalt tetraphenylporphine (CoTPP) at liquid/liquid interfaces. The key units are two types of gel hybrid ultramicroelectrodes (agar-gel/organic hybrid ultramicroelectrodes and water/PVC-gel hybrid ultramicroelectrodes), which were made based on dual micro- or nanopipettes. A solidified liquid/liquid interface can be formed at the tip of these pipettes, and it serves as both an electrochemical cell and a nanospray emitter for mass spectrometry. We demonstrated that the solidified L/L interfaces were very similar to typical L/L interfaces. Key CoTPP intermediates of the ORR at the liquid/liquid interfaces were identified for the first time, and the four-electron oxygen reduction pathway predominated, which provides valuable insights into the mechanism of the ORR. Theoretical simulation has further supported the possibility of formation of intermediates. This type of platform is promising for in situ tracking and identifying intermediates to study complicated reactions at liquid/liquid interfaces or other soft interfaces.

Electrified Soft Interface as a Selective Sensor for Cocaine Detection in Street Samples

A straightforward, direct, and selective method is presented for electrochemical cocaine identification in street samples. The sensing mechanism is based on a simple ion transfer reaction across the polarized liquid–liquid interface. The interfacial behavior of a number of cutting agents is also reported. Interfacial miniaturization has led to improved electroanalytical properties of the liquid–liquid interface based sensor as compared with the macroscopic analogue. The reported method holds great potential to replace colorimetric tests with poor selectivity for on-site street sample analysis.

Oxygen Reduction by Homogeneous Molecular Catalysts and Electrocatalysts

The oxygen reduction reaction (ORR) is a key component of biological processes and energy technologies. This Review provides a comprehensive report of soluble molecular catalysts and electrocatalysts for the ORR. The precise synthetic control and relative ease of mechanistic study for homogeneous molecular catalysts, as compared to heterogeneous materials or surface-adsorbed species, enables a detailed understanding of the individual steps of ORR catalysis. Thus, the Review places particular emphasis on ORR mechanism and thermodynamics. First, the thermochemistry of oxygen reduction and the factors influencing ORR efficiency are described to contextualize the discussion of catalytic studies that follows. Reports of ORR catalysis are presented in terms of their mechanism, with separate sections for catalysis proceeding via initial outer- and inner-sphere electron transfer to O₂. The rates and selectivities (for production of H₂O₂ vs H₂O) of these catalysts are provided, along with suggested methods for accurately comparing catalysts of different metals and ligand scaffolds that were examined under different experimental conditions.

Remarkable influence of 'phane effect' on the excited-state properties of cofacially oriented coumarins

A comprehensive investigation of the photophysics of a cofacially oriented bis-coumarin based on naphthalene, i.e., Cou-Nap, designed and synthesized to examine the influence of π-electronic communication between the two fluorophores, reveals exceptional excited-state properties. While the anticipated [2 + 2] photocycloaddition is not observed despite the fact that the two reactive coumarin units are at a distance of 3.8 Å, the fluorescence quantum yields and singlet lifetimes in different solvents are found to be remarkably higher when compared to those of the parent coumarin and a mono-coumarin model system, i.e., Cou-Dur. In addition to large solvent-induced Stokes shifts, Cou-Nap displays intriguing temperature-dependent emission in a nonpolar solvent such as cyclohexane. The observed photophysical properties are reconciled based on the so-called 'phane effect' that is operative in cyclophanes. In the latter, an effective π-π interaction between the aromatic rings modifies the attributes of the chromophore in such a manner that the observed properties cannot be associated with the individual aromatic rings. The temperature-dependent emission is proposed to arise as a consequence of thermally activated ISC from the singlet-excited state to one of the higher energy triplet states. The results constitute, for the first time, the demonstration of modification of the excited-state properties of a fluorophore in a non-cyclophane system by 'phane effect'.

Cofacial porphyrin dimers assembled from N-heterocyclic carbene–metal bonds

Cofacial porphyrin dimers assembled from eight metal–carbene bonds were synthesized by using porphyrins with four imidazolium groups as precursors.

Redox Electrocatalysis of Floating Nanoparticles: Determining Electrocatalytic Properties without the Influence of Solid Supports

Redox electrocatalysis (catalysis of electron-transfer reactions by floating conductive particles) is discussed from the point-of-view of Fermi level equilibration, and an overall theoretical framework is given. Examples of redox electrocatalysis in solution, in bipolar configuration, and at liquid-liquid interfaces are provided, highlighting that bipolar and liquid-liquid interfacial systems allow the study of the electrocatalytic properties of particles without effects from the support, but only liquid-liquid interfaces allow measurement of the electrocatalytic current directly. Additionally, photoinduced redox electrocatalysis will be of interest, for example, to achieve water splitting.

A High-Valent Non-Heme μ-Oxo Manganese(IV) Dimer Generated from a Thiolate-Bound Manganese(II) Complex and Dioxygen

This study deals with the unprecedented reactivity of dinuclear non-heme MnII -thiolate complexes with O2 , which dependent on the protonation state of the initial MnII dimer selectively generates either a di-μ-oxo or μ-oxo-μ-hydroxo MnIV complex. Both dimers have been characterized by different techniques including single-crystal X-ray diffraction and mass spectrometry. Oxygenation reactions carried out with labeled 18 O2 unambiguously show that the oxygen atoms present in the MnIV dimers originate from O2 . Based on experimental observations and DFT calculations, evidence is provided that these MnIV species comproportionate with a MnII precursor to yield μ-oxo and/or μ-hydroxo MnIII dimers. Our work highlights the delicate balance of reaction conditions to control the synthesis of non-heme high-valent μ-oxo and μ-hydroxo Mn species from MnII precursors and O2 .

Interfacial kinetics in a model emulsion polymerisation system using microelectrochemical measurements at expanding droplets (MEMED) and time lapse microscopy

Physicochemical processes that take place at the oil-water interface of an epoxy-amine emulsion polymerisation system influence the properties and structural morphology of the polymeric microparticles formed. Investigating these processes, such as the transport of monomers across the liquid/liquid interface brings new understanding which can be used to tune polymeric morphology. Two different approaches are used to provide new insights on these processes. Microelectrochemical measurements at expanding droplets (MEMED) is used to measure the transfer of amine from an organic phase comprised of epoxide and amine into an aqueous receptor phase. The rate of amine transfer across the liquid/liquid interface is characterised using MEMED and finite element method modelling and kinetic values are reported. Time lapse microscopy of epoxide droplets held in deionised water or an aqueous amine solution heated to different temperatures is further used to characterise epoxide dissolution into the aqueous phase. Mass-transport of epoxide into the aqueous phase is shown to be temperature-dependent. Epoxide homopolymerisation at the droplet-water interface is found to influence the rate of epoxide droplet dissolution. The rate of the epoxy-amine cure reaction is shown to be faster than the rate of the epoxide homopolymerisation reaction. The combination of methods used here is not limited to emulsion polymerisation and should find application in a myriad of processes at liquid/liquid interfaces.

Halogen substitutions leading to enhanced oxygen evolution and oxygen reduction reactions in metalloporphyrin frameworks

Halogen substitutions modify the electronic structure of active sites to enhance the oxygen evolution and reduction reactions on metalloporphyrin frameworks.

In situ hybridization of CoOX nanoparticles on N-doped graphene through one step mineralization of co-responsive hydrogels

Involving GO into Co-PT hydrogels can enhance the strength of hydrogels. The electrocatalytic properties of CoO_X/N-rGO700 aerogels are improved by annealing.

Energy-Related Small Molecule Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based Systems

Globally increasing energy demands and environmental concerns related to the use of fossil fuels have stimulated extensive research to identify new energy systems and economies that are sustainable, clean, low cost, and environmentally benign. Hydrogen generation from solar-driven water splitting is a promising strategy to store solar energy in chemical bonds. The subsequent combustion of hydrogen in fuel cells produces electric energy, and the only exhaust is water. These two reactions compose an ideal process to provide clean and sustainable energy. In such a process, a hydrogen evolution reaction (HER), an oxygen evolution reaction (OER) during water splitting, and an oxygen reduction reaction (ORR) as a fuel cell cathodic reaction are key steps that affect the efficiency of the overall energy conversion. Catalysts play key roles in this process by improving the kinetics of these reactions. Porphyrin-based and corrole-based systems are versatile and can efficiently catalyze the ORR, OER, and HER. Because of the significance of energy-related small molecule activation, this review covers recent progress in hydrogen evolution, oxygen evolution, and oxygen reduction reactions catalyzed by porphyrins and corroles.