Redox Agents Research Articles

Photochemical separation of plutonium from uranium.

Plutonium-based technologies would benefit if chemical hazards for purifying plutonium were reduced. One critical processing step where improvements could be impactful is the adjustment of plutonium oxidation-states during separations. This transformation often requires addition of redox agents. Unfortunately, many of the redox agents used previously cannot be used today because their properties are deemed incompatible with modern day processing facilities and waste stream safety requirements. We demonstrated herein that photochemistry can be used as an alternative to those chemical agents. We observed that (1) Pu4+ → Pu3+ and UO22+ → U4+ photoreduction proceeded in HCl(aq) and HNO3(aq) and (2) photogenerated Pu3+(aq) and U4+(aq) could be separated using anion exchange chromatography (high yield, >90%; good separation factor, 322).

Vapor generation of mercury and methylmercury in aqueous microdroplets produced by pneumatic nebulization

The vapor generation process of Hg2+ and MeHg+ to form volatile Hg0 was found in aqueous microdroplets produced by pneumatic nebulization without any added redox agent, electricity or radiation.

Stimuli-sensitivity and dynamics in the self-assembly structure of TEMPO-containing nonamphiphilic nanoparticles and their triggering hydrophobic drug release

Understanding the dynamics of stimuli-sensitive self-assembled nanomaterials is crucial for their potential applications, however, it is relatively difficult to quantitatively probe the dynamics using conventional analytical techniques because of the low sensitivity and selectivity of these techniques. In this research, TEMPO-containing nonamphiphilic copolymers are synthesized and highly sensitive, selective and non-destructive electron spin resonance (ESR) spectroscopy is applied to study their dynamics in stimuli-sensitive self-assembled nanostructures in different microenvironments. ESR spectroscopy reveals the structural changes of the smart supramolecular nanoparticles (SNPs) in their controlled assemblies by observing the molecular-level interactions. In order to obtain the dynamical evidence, several spectroscopic parameters such as rotational correlation time, hyperfine splitting constant, intensity increase/decrease and anisotropy parameters are determined from the ESR spectra. The stimuli-sensitivity and dynamics strongly depend on the SNPs’ microenvironment correlated to several chemical and physiological parameters such as radical concentration, pH, redox agent, temperature, polarity and viscosity. The results provide quantitative insights into the interface of the supramolecular assemblies. The nanoparticles showed good biocompatibility confirmed by the hemolysis test and the MTT assay. Furthermore, proof-of-concept experiments were carried out to demonstrate the SNPs’ stimuli-sensitive performance as a hydrophobic anticancer drug nanocarrier. This effort can be used to design other colloidal nano-platforms for the potential application in cancer treatment.

Dynamics in Controllable Stimuli-Responsive Self-Assembly of Polymer Vesicles with Stable Radical Functionality.

Self-assembled polymer vesicles have emerged as exciting and promising materials for their potential application in drug delivery, but the dynamics of stimuli-responsive polymers in these areas with pendant functionality in order to understand the structure-property relationship under different physicochemical conditions is still open to discussion. In this work, nitroxide radical-containing copolymers were synthesized and utilized to investigate local dynamics in their vesicular assemblies. Herein, electron paramagnetic resonance (EPR) spectroscopy was applied to reveal the smart supramolecular vesicular structure and polymer chain dynamics in stimuli-responsive controlled assemblies by considering molecular-level interactions. These interactions and dynamics were dependent on the microenvironment of the assemblies, which might be affected by physicochemical parameters such as radical concentration, pH, redox agent, polarity, and viscosity. These observations help to accomplish quantitative insights into the stimuli-responsive colloidal vesicular assemblies. The vesicles were used as an anticancer drug carrier, which showed high drug loading efficiency (63.65%). The reduction-responsive prompt disassembly accelerated the release. Furthermore, the biocompatibility and anticancer activity were examined by cellular experiments against normal fibroblasts (L929) and human cervical cancer (HeLa) cell lines, respectively. The results demonstrate that this effort provides an easy strategy for designing controllable stimuli-responsive polymer nanosystems which promotes their promising application in cancer treatment.

Novel Type of Li-Ion Capacitor with Improved Energy/Power Performance

Constantly, the world is looking for the perfect energy storage and conversion system. Two energy storage technologies are currently the most popular - electric double-layer capacitors (EDLCs) and lithium-ion batteries (LIBs). On the one hand, EDLCs cannot (and is not obliged to) compete with Li-based technology in terms of energy density; this feature is quite often sought as the reason for the limited commercialization of EDLCs. On the other hand, LIBs have shorter lifespan and lower power values comparing to EDLCs. Therefore, it seems reasonable to merge both technologies into one compact device, called a hybrid lithium-ion capacitor (LIC). The hybridization of energy storage mechanisms allows for acquiring higher energy density values than EDLCs while maintaining high specific power and a great number of charge/discharge cycles. The major problem occurring during LICs assembling is the pre-lithiation stage. Auxiliary metallic lithium electrode is one of the solutions – in fact, this approach was encouraging; however, after the pre-lithiation process, the metallic lithium electrode should be entirely utilized or removed and become replaced by a carbon electrode. This raises technical and safety issues because the exchange requires an inert environment. Another approach uses composite materials as positive electrodes, allowing full intercalation of lithium into graphite structure.Nonetheless, after the pre-lithiation process, the dead mass of the composite stays in the system and deteriorates the efficiency [1]. A different method of pre-lithiation using lithium from electrolyte was presented by Beguin et al. [2]. In this approach, the main problem identified was the consumption of lithium ions from the electrolyte and the diminished electrolytic conductivity of the solution. In our work, we propose another concept that allows for full intercalation of the negative electrode without an auxiliary electrode or less-conductive composite on the positive side. In principle, the redox-active lithium salt dissolved in 1 mol∙L-1 LiPF6 in EC:DMC electrolyte served as a charge-balancer. Thus, additional Li+ in the redox agent structure made the Li+ concentration in the electrolyte non-affected. Full electrochemical tests, including cyclic voltammetry, galvanostatic charge/discharge and impedance spectroscopy, constant power discharge, were carried out. In further studies, optimal LIC operating conditions, i.e., voltage (2.2 – 4.2 V) and current density (0.2 A⸱g-1), were determined. The system has also been compared with commercial products. Both, redox and non-redox processes occurring on both electrodes were identified. The capacitor's cycle life and the effect of the addition of inorganic salt on the conductivity were also investigated. Furthermore, detailed analysis has been conducted using operando mass spectrometry coupled with gas chromatography, SEM/EDS, XRD, Raman and FTIR spectroscopy.

Design of Experiments for Optimization of Polyoxometalate Syntheses

Design of experiments (DOE) is a key method for optimizing physical processes by altering multiple variables at once to assess their effect. In chemistry, DOE explores a wider parameter space than the dominant “One Factor at a Time” (OFAT) method providing greater opportunity to explore the factors that can be used to optimize yield, purity, and to explore chemical space for new compounds. One area of chemistry that suffers from low yields and poor reproducibility but is full of hard to predict and interesting materials is polyoxometalate cluster science. Herein, we developed a DOE analysis methodology to explore the parameter space of polyoxometalate cluster formation to explore the subtle input effects that are known to have an impact on the product discovery, purity, and stability under the preparation conditions. Using a Plackett–Burman screening design, we analyzed the effect of six synthetic parameters in only 12 experiments, following up with a full factorial analysis of the three most significant factors to identify the key parameters in the successful synthesis of each. Based on this, we provide a useful template that produces the input data for automated synthesis based on DOE on other synthetic procedures. In our POM test cases, redox agent stoichiometry was found in three of the four systems studied to be significant factors with pH and temperature, which also found to be commonly important. The insights derived from this analysis were applied to design optimized synthetic procedures and improve the yield of the product by on average >33% from the highest reported literature yield. Thus, the DOE methodology outlined here is shown to yield insights into reaction optimization rapidly with facile experimental design and analysis even for complex multivariate synthetic procedures.

Electrochemical Heteroatom-Heteroatom Bond Construction.

Heteroatom-heteroatom linkage, with S-S bond as a presentative motif, served a crucial role in biochemicals, pharmaceuticals, pesticides, and material sciences. Thus, preparation of the privileged scaffold has always been attracting tremendous attention from the synthetic community. However, classic protocols suffered from several drawbacks, such as toxic and unstable agents, poor functional group tolerance, multiple steps, and explosive oxidizing regents as well as the transitional metal catalysts. Electrochemical organic synthesis exhibited a promising alternative to the traditional chemical reaction due to the sustainable electricity can be employed as the traceless redox agents. Hence, toxic and explosive oxidants and/or transitional metals could be discarded under mild reaction with high efficiency. In this context, a series of electrochemical approaches for the construction of heteroatom-heteroatom bond were reviewed. Notably, most of the cases illustrated the dehydrogenative feature with the clean energy molecules hydrogen as the sole by-product.

Tempol differently affects cellular redox changes and antioxidant enzymes in various lung-related cells

Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) is a potential redox agent in cells. The present study investigated changes in cellular reactive oxygen species (ROS) and glutathione (GSH) levels and in antioxidant enzymes, in Tempol-treated Calu-6 and A549 lung cancer cells, normal lung WI-38 VA-13 cells, and primary pulmonary fibroblasts. Results demonstrated that Tempol (0.5–4 mM) either increased or decreased general ROS levels in lung cancer and normal cells at 48 h and specifically increased O2•− levels in these cells. In addition, Tempol differentially altered the expression and activity of antioxidant enzymes such as superoxide dismutase, catalase, and thioredoxin reductase1 (TrxR1) in A549, Calu-6, and WI-38 VA-13 cells. In particular, Tempol treatment increased TrxR1 protein levels in these cells. Tempol at 1 mM inhibited the growth of lung cancer and normal cells by about 50% at 48 h but also significantly induced cell death, as evidenced by annexin V-positive cells. Furthermore, down-regulation of TrxR1 by siRNA had some effect on ROS levels as well as cell growth inhibition and death in Tempol-treated or -untreated lung cells. In addition, some doses of Tempol significantly increased the numbers of GSH-depleted cells in both cancer cells and normal cells at 48 h. In conclusion, Tempol differentially increased or decreased levels of ROS and various antioxidant enzymes in lung cancer and normal cells, and induced growth inhibition and death in all lung cells along with an increase in O2•− levels and GSH depletion.

Thiolated amphiphilic β-cyclodextrin-decorated gold colloids: Synthesis, supramolecular nanoassemblies and controlled release of dopamine

We report a hybrid nanoassembly based on gold colloids decorated with a new thiolated amphiphilic cyclodextrin complexing dopamine (AuNPs@SC16SH/DA). The novel amphiphilic β-cyclodextrin SC16SH, bearing on average one thiol group at the end of an oligoethylene glycol chain per CyD unit, was obtained in a multi-step synthesis and fully characterized by NMR spectroscopy and MALDI analysis. The AuNPs@SC16SH hybrid assembly was prepared by mixing AuNPs and SC16SH in aqueous solution and investigated by UV/Vis and TEM measurements. The AuNPs@SC16SH/DA supramolecular assembly, bearing both thiol-stabilized gold nanoparticles and the neurotransmitter dopamine, has been conceived as stimuli-responsive delivery system able to control the release of dopamine upon proper stimuli. Two redox functionalities (i.e. the Au-S bond and the redox behaviour of dopamine) have been exploited to detach SC16SH/DA complex from the AuNPs@SC16SH/DA progenitor platform, using dithiothreitol (DTT), and to detect the complex by cyclic voltammetry (CV). Following, dopamine was finally released from the SC16SH/DA assembly upon treatment with Brij® S20 and the free dopamine was monitored by CV. Overall, our hybrid nanoassemblies turned out to be able to release dopamine, upon combined action of redox and digestion agents in medium emulating biological environment.

Synthesis and characterization of lysozyme-conjugated Ag.ZnO@HA nanocomposite: A redox and pH-responsive antimicrobial agent with photocatalytic activity

Hydroxyapatite (HA) is extensively used for implantable device coating; however, it lacks antibacterial property, leading to potential bacterial infection during orthopedic implantation surgery. Herein, to enhance the antibacterial activity of HA, a redox- and pH-responsive HA nanocomposite with photocatalytic activity was designed. A photosensitive heterostructure, zinc oxide/hydroxyapatite (ZnO.HA), was coated with Ag nanoparticles (AgNPs) with assisted gallic acid using the UV-irradiation method. An antibacterial enzyme, lysozyme, was then conjugated on the surface of the nanocomposite by a cleavable disulfide linker, resulting in a redox-sensitive nanoplatform. In comparison with bare HA, the designed nanocomposites as Lyso.CAGZ@HA displayed much higher antibacterial activity (> 5-fold) toward Escherichia coli (E. coli) owing to the synergistic antibacterial effects of ZnONPs, AgNPs, gallic acid, and lysozyme on the surface of the nanocomposite. However, antibacterial and antifouling effects are much more enhanced in Lyso.CAGZ@HA-treated bacteria as they were subjected to UVA irradiation. Moreover, the cellular uptake of nanocomposite and intracellular glutathione depletion enhanced in the presence of UVA light, resulting in reactive oxygen specious generation enhancement. Further, in vitro cytotoxicity experiments on mammalian cells (human foreskin fibroblast) revealed that nanocomposite has no cytotoxic effects. Hence, this study demonstrated that Lyso.CAGZ@HA could be considered as a potential therapeutic approach against bacterial infectious diseases.

Stimuli-Responsive Polymeric Nanoplatforms for Cancer Therapy.

Polymeric nanoparticles have been widely used as carriers of drugs and bioimaging agents due to their excellent biocompatibility, biodegradability, and structural versatility. The principal application of polymeric nanoparticles in medicine is for cancer therapy, with increased tumor accumulation, precision delivery of anticancer drugs to target sites, higher solubility of pharmaceutical properties and lower systemic toxicity. Recently, the stimuli-responsive polymeric nanoplatforms attracted more and more attention because they can change their physicochemical properties responding to the stimuli conditions, such as low pH, enzyme, redox agents, hypoxia, light, temperature, magnetic field, ultrasound, and so on. Moreover, the unique properties of stimuli-responsive polymeric nanocarriers in target tissues may significantly improve the bioactivity of delivered agents for cancer treatment. This review introduces stimuli-responsive polymeric nanoparticles and their applications in tumor theranostics with the loading of chemical drugs, nucleic drugs and imaging molecules. In addition, we discuss the strategy for designing multifunctional polymeric nanocarriers and provide the perspective for the clinical applications of these stimuli-responsive polymeric nanoplatforms.

Universal Self-Healing Poly(dimethylsiloxane) Polymer Crosslinked Predominantly by Physical Entanglements.

Harsh conditions are inevitable for long-term use of self-healing polymers. However, the majority of reported self-healing materials cannot remain stable under harsh conditions due to the presence of vulnerable dynamic crosslinking sites. Herein, a universal self-healing poly(dimethylsiloxane) (PDMS) polymer is reported. In our design, the PDMS polymer chains are crosslinked predominantly through physical entanglements. Owing to the invulnerable nature of the entanglement junctions and high mobility of polymer chains, the as-synthesized polymer exhibits autonomous self-healing capabilities not only under ambient conditions but also in a variety of harsh environments, including aqueous solutions, organic solvents, and extreme conditions (strong acid/alkali, redox agents, freezing temperature). Moreover, this polymer can be easily integrated with a eutectic gallium-indium (EGaIn) alloy to achieve layer-by-layer self-healing electronic skin sensors, which realize the combination of excellent electrical conductivity, long-term sensing stability, and universal self-healing capability.

Probing the Conformational States of a pH-Sensitive DNA Origami Zipper via Label-Free Electrochemical Methods.

DNA origami structures represent an exciting class of materials for use in a wide range of biotechnological applications. This study reports the design, production, and characterization of a DNA origami “zipper” structure, which contains nine pH-responsive DNA locks. Each lock consists of two parts that are attached to the zipper’s opposite arms: a DNA hairpin and a single-stranded DNA that are able to form a DNA triplex through Hoogsteen base pairing. The sequences of the locks were selected in a way that the zipper adopted a closed configuration at pH 6.5 and an open state at pH 8.0 (transition pKa 7.6). By adding thiol groups, it was possible to immobilize the zipper structure onto gold surfaces. The immobilization process was characterized electrochemically to confirm successful adsorption of the zipper. The open and closed states were then probed using differential pulse voltammetry and electrochemical impedance spectroscopy with solution-based redox agents. It was found that after immobilization, the open or closed state of the zipper in different pH regimes could be determined by electrochemical interrogation. These findings pave the way for development of DNA origami-based pH monitoring and other pH-responsive sensing and release strategies for zipper-functionalized gold surfaces.

(Invited) On the Effective Design of Electrochemical Interfaces for Solar Production of Fuels

Extensive research has been conducted on man-made systems for the solar production of fuels in which many different chemical and physical constructs and prototypes have been assembled to demonstrate the possibility of creating fuels from sunlight. These efforts comprise a broad array of chemical and electrochemical work that originates in the Fujishima and Honda work on photoelectrochemical water splitting and has moved on to molecular-based catalysts for water splitting as well as designed nanostructures of semiconductors for light absorption and charge separation.This presentation examines how the mechanistic design of these systems for solar fuels production constrain and define the chemistry and electrochemistry of the solid and molecular components required for effective operation. It will also identify alternative approaches that show promise. The discussion will begin with design analyses of the photosynthetic system for carbon fixation and man-made photosystems for the oxidation of water as a sacrificial electron donor for fuels production. It will be shown how the kinetics inherent in their operational design imposes demands upon the photosystem components, in particular the catalyst TOF and TON figures. With this collection of designs, full structures for water or carbon dioxide reduction can be composed and consequent demands upon catalysts can be quantified.While many research systems for solar photoconversion are given as examples of “artificial photosynthesis,” few actually take full advantage of the important design principles that underlie the effectiveness of photosynthesis as a means of transducing solar energy into the biochemistry of a life form. A tracing of the flow of energy and charge through the photosynthetic reaction centers and then to RuBisCO for CO2 fixation reveals the significance of impedance matching at various coupling points in the system. A direct coupling of Photosystem II with the oxygen evolving catalyst is evident as is indirect coupling at other points where protons and redox agents are produced and their diffusion creates a Nernstian concentration potential at distant locations to initiate further reaction. These types of coupling are of consequence in man-made systems.The oxidation of water provides the electrons and protons for solar driven reduction reactions, whether it be natural or artificial. Nature has its own oxygen evolving complex in photosynthesis to effect this reaction and there are many man-made structures that have been assembled and studied to oxidize water as well. A kinetic design of the natural system will be drawn in this presentation that will be compared with the kinetic designs of representative artificial systems and a critique will be made. These include semiconductor photoelectrolysis of water, dye sensitized electrochemical water oxidation, molecular water oxidation catalysts, and particulate and nanostructured systems for water splitting. The relationship between the kinetic design of the system and the component solids, catalysts and associated species will be shown.The catalytic reduction of CO2 or water to a fuel are wide spread and dynamic areas of research. Whereas the electrochemistry of water reduction to hydrogen has long been an object of study, the catalysis of CO2 reduction is much less developed. Molecular catalysts are being sought for this role as are interfacial metal deposits with both being accompanied by theoretical backing. Given the recognition and analysis of the various designs identified in this discussion, a more effective effort can be made in the development of photosystems for production of these fuels. They can help define not only the form of the catalyst, be it solid or molecular, but also its required reaction velocity and durability.

Evolution of Earth-Abundant 3 d-Metallaelectro-Catalyzed C-H Activation: From Chelation-Assistance to C-H Functionalization without Directing Groups.

Catalyzed C-H functionalizations have emerged as a transformative platform for molecular syntheses. Despite of indisputable advances, oxidative C-H activations have been largely restricted to precious transition metals and stoichiometric amounts of chemical oxidants. In contrast, we herein discuss the potential of earth-abundant, environmentally-benign 3d transition metals for C-H activation, which has recently gained major momentum. Thus, a strategy for full resource economy has been established in our group, with green electricity as a renewable redox agent, giving valuable hydrogen as the sole byproduct under redox mediator-free conditions. In this account, we detail our accomplishments in 3d metallaelectrocatalysis towards green syntheses until March 2021.

Synthesis and Properties of Multistimuli Responsive Shape Memory Polyurethane Bioinspired from α‐Keratin Hair

AbstractHerein, a multistimuli responsive shape memory polyurethane (multiresponsive SMPU) containing disulfide bonds (–S–S–), bioinspired from α‐keratin hair component, was synthesized using a two‐step reaction and activated via four‐varied stimuli, including water, heat, redox agent, and UV‐light. First, the prepolymer was prepared by the reaction between the comonomers of hexamethylene diisocyanate and polycaprolactone‐2000, and once ended, 1,4‐butylene glycol and 3‐mercapto‐1,2‐propanediol (C3H8O2S) have been appended to the prepolymer to initiate the chain‐extension reaction. The chemical structure and properties of the multi‐responsive SMPU were decoded by FT‐IR, XRD, DSC, and Raman spectra. The correlation between microstructure, dynamic‐mechanical‐analysis, and shape‐memory properties of the multi‐responsive SMPU were systemically discussed. As a primary clue, the opening/closing of H‐bonds and disulfide bonds approved that they dictated the shape memory performance. The clear‐cut results clarified that the multiresponsive SMPU endowed composition‐dependent stress relaxation, and its reversible variation of storage modulus was examined by changing temperature. Owing to such molecular switches, the strip pieces were able to represent a multistimuli responsive shape memory effect —induced by water, heat, redox agent (NaHSO3‐H2O2 solutions), and UV‐light. This research paves a feasible road to prepare a novel multistimuli responsive shape memory actuator with high potential features in flexible sensor and robotic applications.

A novel electrochemical approach for the ultrasensitive detection of fluoroquinolones based on a double-labelled aptamer to surpass complementary strands of aptamer lying flat

Abstract This study develops a novel electrochemical detection strategy for determination of fluoroquinolones (FQs) which was formed with a double-labelled aptamer as the detection element, methylene blue (MB) as redox agent and complementary strands of aptamer (CSs). This method exhibited excellent sensitivity for detection of ciprofloxacin as a member of FQs due to its ability to outstrip CSs lying flat on the electrode surface as a major challenge. Simplicity is another characteristic of this approach. The dynamic detection range was from 300 pM to 450 nM and the detection limit was 100 pM for ciprofloxacin. Also, the system displayed high selectivity towards FQs. The aptasensor succeeded in detection of ciprofloxacin in the spiked real samples, including human serum, milk and water samples. Altogether, the designed approach can introduce an effective and functional foundation for small molecules analysis to ensure food and human safety by applying corresponding aptamer sequences.

The Redox Journey of Iconic Ferrocene: Ferrocenium Dications and Ferrocenate Anions.

The recent discoveries of both dicationic and monoanionic ferrocene derivatives throw light on the effect of the substituents on the C5 ring as well as the choice of redox agents and solvent system in the preparation of previously believed to be difficult synthetic targets. These oxidized and reduced forms of ferrocene are of interest to spectroscopists, magnetochemists, and theoreticians.

The Redox Journey of Iconic Ferrocene: Ferrocenium Dications and Ferrocenate Anions

The recent discoveries of both dicationic and monoanionic ferrocene derivatives throw light on the effect of the substituents on the C5 ring as well as the choice of redox agents and solvent system in the preparation of previously believed to be difficult synthetic targets. These oxidized and reduced forms of ferrocene are of interest to spectroscopists, magnetochemists, and theoreticians.

Dark Reduction Drives Evasion of Mercury From the Ocean

Much of the surface water of the ocean is supersaturated in elemental mercury (Hg0) with respect to the atmosphere, leading to sea-to-air transfer or evasion. This flux is large, and nearly balances inputs from the atmosphere, rivers and hydrothermal vents. While the photochemical production of Hg0 from ionic and methylated mercury is reasonably well-studied and can produce Hg0 at fairly high rates, there is also abundant Hg0 in aphotic waters, indicating that other important formation pathways exist. Here, we present results of gross reduction rate measurements, depth profiles and diel cycling studies to argue that dark reduction of Hg2+ is also capable of sustaining Hg0 concentrations in the open ocean mixed layer. In locations where vertical mixing is deep enough relative to the vertical penetration of UV-B and photosynthetically active radiation (the principal forms of light involved in abiotic and biotic Hg photoreduction), dark reduction will contribute the majority of Hg0 produced in the surface ocean mixed layer. Our measurements and modeling suggest that these conditions are met nearly everywhere except at high latitudes during local summer. Furthermore, the residence time of Hg0 in the mixed layer with respect to evasion is longer than that of redox, a situation that allows dark reduction-oxidation to effectively set the steady-state ratio of Hg0 to Hg2+ in surface waters. The nature of these dark redox reactions in the ocean was not resolved by this study, but our experiments suggest a likely mechanism or mechanisms involving enzymes and/or important redox agents such as reactive oxygen species and manganese (III).