Exhaustive Electrolysis Research Articles

Aggregation and Capacity Limiting Effects in Anthraquinone-Based Flow Battery Negolytes

Anthraquinone-based molecules are promising electroactive materials for use in aqueous organic flow batteries. At high concentrations in aqueous solutions, the well-known negolyte molecule anthraquinone disulfonic acid (AQDS) molecule has been observed to aggregate under near-neutral and acidic conditions. Aggregation has been hypothesized to be directly linked to observed concurrent capacity reduction. In this study, we investigated three different water-soluble anthraquinones in electrolytes of varying compositions and pH to gain further insight into the possible causes of capacity loss. We used low-field benchtop 1H-NMR and diffusion NMR measurements directly in non-deuterated aqueous flow-battery electrolytes to investigate molecular aggregation. Single-cell testing was performed under exhaustive electrolysis conditions to determine the number of electrons exchanged per molecule. We observed a decrease in the number of electrons exchanged per molecule in the presence of carbonate ions due to CO2 adduct formation. The aggregation constants were determined from both concentration-dependent chemical shifts and self-diffusion coefficients. We show that aggregation of the oxidized form of all three molecules studied here occurs under near-neutral and alkaline conditions and does not affect the number of accessible e−.

Simple microfluidic device for spectroelectrochemistry

The design, fabrication, and performance of a simple microfluidic device permitting spectroelectrochemical measurements on submicroliter samples are described. Planar electrode chips were prepared from cast epoxy and assembled to yield a device with a single flow channel of approximate dimensions 25 mm × 1.3 mm × 0.2 mm. Spectral measurements were made by passing the light beam parallel to the 1.3-mm channel dimension, resulting in a relatively high optical sensitivity. Bare disk electrodes embedded in opposing channel walls permit spectroelectrochemical measurements on sample volumes as low as approximately 3 µL, corresponding to a solution plug just long enough to contact all three electrodes. Adding a solution-cast ionomeric film to the channel wall containing reference and auxiliary electrodes provided an effective salt bridge to significantly smaller sample volumes in contact with a bare working electrode on the opposite channel wall. As demonstration of submicroliter sample analysis, spectroelectrochemical measurements were carried out on a ∼ 0.3 µL aqueous ferricyanide solution and showed near exhaustive electrolysis of the optically sampled volume is achieved in approximately 3 min.

Dispensing Single Drops as Electrochemical Reactors

AbstractA strategy is proposed to generate single microdrops with femtoliter to microliter volumes as a liquid reactor and to guide them towards a reactive surface within a viscous carrier liquid. Reactions of the drop content with the surface are exemplified by exhaustive electrolysis of the reactor content, making use of an ion‐conducting pathway through the carrier liquid. The mechanisms by which the redox‐active species from the aqueous drop reach the electrode surface were established by a combination of time‐lapse microscopy of the drop shape, chronoamperometric recording of the single drop electrolysis, and modelling of the reaction‐transport problem, showing that partitioning over the liquid‐liquid interface must be a key step in this mechanism. The possibilities of reagent delivery and external triggering of a reaction are expanded by the possibility for controlled addition of further dissolved reactants to the liquid microreactor.

Vitamin-Based Organic Electrolytes for Redox Flow Batteries

With the current drive towards environmental sustainability, electrochemical energy storage system (EES) such as redox flow batteries (RFB) are thought to be a promising solution as they can play an important role in balancing energy generation and consumption to ensure power network stability and reliability. [1,2] Organic compounds possess tremendous potential for energy storage as they are structural diverse and synthetically tunable. In addition, organic compounds can be cost-effective and environmentally benign due to their abundancy in nature. However, many organic species experience stability issue in their charged form. [3,4] The present study reports an all-organic vitamin-based molecular system based on a non-aqueous, mixed-reactant electrolyte containing modified forms of the naturally occurring vitamin K1 and vitamin E as anolyte and catholyte, respectively. [5,6] In the presence of malonic ester (a relatively weak organic acid), the redox transformations of the organic species were proven chemically stable and reversible under exhaustive electrolysis conditions, as confirmed by UV-vis spectroscopy. [7] The pairing of both half-cells provides a desirable experimental cell voltage of 1.27 V. The proposed system also demonstrates good thermal and ambient stability via series of experiments. The overall findings presented demonstrate the practical significance of the vitamin-based organic electrolytes in the all-organic RFB technologies.

Bi–Sn nanoparticles for electrochemical denitrification: activity and selectivity towards N2 formation

Electrochemical denitrification is a promising approach to remove NO3− towards its conversion to innocuous N2. In this work, the electrocatalytic performance of Bi, Sn and BixSny carbon-supported nanoparticles (∼10 nm) was evaluated for the selective NO3− electroreduction (NER) to N2. The different electrocatalysts were synthesised using a simple chemical method, physicochemically characterised by TEM-EDS and XPS and, subsequently, air-brushed onto a Toray carbon paper to build electrodes. These electrodes were characterised by SEM-EDS and cyclic voltammetry. Electrolyses were carried out in a slightly alkaline media to find the most active electrocatalyst towards the NO3− removal. Among them, Bi60Sn40/C showed the best results with more than 50% of selectivity to N2 after exhaustive electrolyses, regardless of the cathode potential or the electric charge passed. Other detected non-desired secondary products were N2O, NO2− and NH4+/NH3. The results obtained point out that the combination of Sn and Bi gives rise to higher NER activity and selectivity to N2 than pure Bi and pure Sn. Therefore, these electrocatalysts are not only promising but also cost-effective for future practical applications like electrochemical denitrification of wastewater. Nevertheless, further investigation is still needed in order to assess the optimum operational conditions.

Intensifying Diffusion-Limited Reactions by Using Static Mixer Electrodes in a Novel Electrochemical Flow Cell

Surface-mediated reactions require effective mixing for them to be intense and efficient. In electrochemistry where the reactants must contact electrode surfaces for a sufficient time to enable electron tunnelling to occur, exhaustive electrolysis in poorly stirred reactors can take many hours to complete. As a consequence, electrochemical processing in many industries is restricted to batch processing which is more costly and labour intensive than continuous processing and results in significant down time between batches. This is not ideal for bulk production or treatment. Here we describe CSIRO’s novel axial flow electrochemical cell which aims to address these mixing problems by using a bespoke static mixer electrode (SME), designed by computational fluid dynamics (CFD) and manufactured using additive manufacturing (AM) technology to retain the fidelity of the original design. This electrode was characterized by SEM-EDS and electrosorption measurements. The performance of the electrochemical flow cell was evaluated by probing the ferricyanide ([Fe(CN)6]3−) reduction reaction on a platinum-coated SME under controlled mass transport conditions using cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometric measurements. This reaction was chosen because it has fast electron transfer kinetics, so the rate of reaction in dilute solutions is limited only by mass transport of the reactant to the electrode surface. Under the highest flow rate used (400 ml min−1) the cell enhanced the rate of reduction of ([Fe(CN)6]3− in a 10−3 M solution by 45 times compared with the no flow result. The impact the cell has diminishes as the concentration increases where mass transport plays a diminishing role in controlling the reaction rate. The performance of the cell is compared with a rotating disk electrode and a tubular flow cell and mass transfer coefficients are also presented for different reactant concentrations and flow conditions.

Confirmation and elimination of cyclic electrolysis of uranium ions in molten salts

The cathodic collection efficiency (CCE) for the exhaustive electrolysis of uranium is very low; however, determination of the mechanism responsible for this phenomenon has been hindered by the lack of information about the uranium species present in molten salts. Herein, we used in situ X-ray absorption spectroscopy and quasi in situ electronic absorption spectroscopy to study the chemical speciation of uranium during the electrolysis process. For the first time, uranium was revealed to be continuously converted between [UCl6]2− and [UCl6]3−, leading to problematic cyclic electrolysis. This cyclic electrolysis process was the main cause of the low CCE during exhaustive electrolysis, and also hindered the full recovery of uranium. To eliminate this cyclic electrolysis process, we adopted a sacrificial active lanthanum metal anode. The lanthanum metal dissolved at the anode to form La3+ ions, which hindered the conversion between [UCl6]2− and [UCl6]3−. Thus, the uranium cycle between the cathode and the anode was eliminated, resulting in a significant improvement in the CCE for exhaustive electrolysis.

A Novel Flow Electrochemical Reactor for Intensifying Diffusion-Limited Electrochemical Processes

Electrochemical processing is attracting renewed interest in industry for synthesis of chemicals or for the removal of a range of contaminants because it is cleaner than most alternatives. Redox reactions that are controlled by applying a potential between two electrodes avoid the use of potentially harmful auxiliary chemicals, an increasingly important advantage that offsets the electrical energy that is required. However, electrochemical processing is still limited in many applications by electrochemical engineering issues such as electrode material selection and construction, efficiency losses caused by mass transfer limitations and batch mode of operation. The CSIRO flow electrochemical cell aims to address these issues by using advanced computational fluid dynamics (CFD) to design, manufacture and optimize an electrode that can maximize mixing within an axial flow cell. The static mixer working electrode is made using additive manufacturing techniques that allows complex designs to be realised using a range of different metals with different surface roughnesses. The current design of the CSIRO electrochemical flow cell employs an additively manufactured, high surface area static mixer as a central working electrode that fits snugly within a tubular porous polymeric separator which defines a working compartment. Two ports at either end of the electrode, manufactured as an integral part of the design provide connections for fluid flow. As with all static mixers, the momentum of the solution induces mixing as it flows past the electrode surface. In the current version of the cell, an inert tubular counter electrode surrounds the working compartment at a small distance from the separator, creating a low volume counter compartment and formed the outside casing of the cell. Separate ports provide fluid flow into this latter compartment. Figure 1 shows a schematic drawing of one configuration of the electrochemical cell with the arrangement of the electrodes, the fluidic setup including the peristaltic pump(s) used to control the solution flow into and out of the cell and the power supply connection to the cell. The efficiency with which the cell works may be evaluated by comparing the limiting current measured at various flow rates with results from a Rotating disk electrode (RDE) in the same solution. These comparisons are useful indicators of performance only, and are not used to draw any conclusions about the hydrodynamic conditions at the static mixer surface. The CSIRO flow electrochemical cell has been characterized using the reduction of Ferricyanide, [Fe(CN)6]3+) at three different reactant concentrations and seven different flow rates. Results show that the cell can accelerate the rate of the reduction reaction by nearly forty-five times. As expected, the rate of this acceleration is highest when the reaction is diffusion limited, so the impact is seen most clearly in dilute solutions. By comparison, results obtained with an RDE were not as promising confirming the effectiveness of the CSIRO designed flow electrochemical cell. Exhaustive electrolysis experiments were also undertaken to show how effectively the cell can remove contaminants from a fixed volume of a contaminated aqueous solution. A two Litre solution of copper contaminated water (i.e. 100 pm CuSO4.4H2O in 0.01M H2SO4) was processed using the flow cell at a constant flow rate of 100 mL/min for 5 hours. SEM/EDS results confirmed the deposition of copper ions onto the static mixer working electrode. ICP results showed that a 50% reduction in the copper concentration from 100 ppm to 51 ppm was achieved in 5 hours which is encouraging. Figure 1

Electrocatalytic oxidation of water at a polyoxometalate nanoparticle modified gold electrode.

Spherical polyoxometalate nanoparticles, [HPMo]NPs, were synthesized from a very well known Keggin-type polyoxometalate [H3PMo12O40] in the presence of sodium dodecyl sulphate (SDS) and polyvinyl pyrrolidine (PVP) in aqueous medium and characterized by UV-Vis spectroscopy and Transmission Electron Microscopy (TEM). The [HPMo]NPs were used to modify a gold working electrode and they were characterized by SEM, EDX, elemental mapping, cyclic voltammetry and electrochemical impedance spectroscopy and applied for the electrocatalytic oxidation of water in a phosphate buffer solution at neutral pH. The modified electrode showed excellent electrocatalytic activity towards oxidation of water at an impressively low overpotential ∼350 mV with a high current density of around 1.7 mA cm−2, good stability under exhaustive electrolysis conditions and also showed long term stability.

Evidence of the Formation of Isoporphyrin in the Electrooxidation Processes of Meso-Tetraphenylporphyrin

In this research, the electrochemical oxidation of meso-tetraphenylporphyrin (H2TPP) has been studied in two solvents, benzonitrile and dichloromethane, with Bu4NClO4 as supporting electrolyte. Three oxidation waves are observed during the anodic sweep; the first two of nature quasi-reversible, and the third irreversible (Fig.1 a,b). The processes occurring at the first and second waves have been assigned to the formation of cation radical and dication, respectively1. Cyclic voltammograms show that when the switching potential occurred after peak IIa, a reduction peak is observed around -0.17 V vs Fc+/Fc (IIIc); this peak, along with the third oxidation wave (IIIa), might be associated to the formation of isoporphyrin. Our hypothesis is that traces of water in the solvent can act as nucleophiles at the meso position of porphyrin dication, giving rise to the isoporphyrin species2. Absorption bands at 445, 755, 794 and 883 nm observed during thin layer spectroelectrochemical UV-visible experiments (Fig.1 c,d,e) are also consistent with the formation of isoporphyrins following the formation of the dication3,4. Long-term experiments such as controlled potential electrolysis show that isoporphyrins can also be formed from the cation radical, however this reaction seems to be too slow to affect the voltammograms. Exhaustive electrolysis at the potential of the first oxidation peak Ia, consumed about one-electron per molecule, and at the end of the electrolysis an anodic sweep revealed the disappearance of peak Ia; in a cathodic sweep, peak IIIc was observed. When the electrolyzed solution containing the isoporphyrin was reduced to the starting porphyrin at a potential slightly cathodic from of wave IIIc, the original anodic signals of H2TPP were restored, showing the reversibility of the process. Similar results were obtained in both solvents. This demonstrates the existence of at least two reaction pathways, one for the formation of isoporphyrins from the radical cation H2TPP•+ at longer time scales, and other from H2TPP+2 at shorter times. This results contrast with previous studies that have shown that in benzonitrile the radical cation of the free-base porphyrins undergoes hydrogen atom abstractions leading to diprotonated species5, and with experiments using Bu4NPF6 in CH2Cl2 that do not lead to isoporphyrin species.

Redox processes in the solution of Ni(II) complex with salen type ligand and in the polymer films

The investigations of oxidation processes of (±)-trans-N,N′-bis(3,5-di-methylsalicylidene)-1,2-cyclohexanediaminenickel(II)) [Ni(salcn(2Me))] and ligands with Me substituents attached in o- (H2salcn(Me)) or o- and p-positions (H2salcn(2Me)) in CH2Cl2 were performed by CV, exhaustive electrolysis, UV VIS NIR and IR methods. H2salcn(Me) and H2salcn(2Me) are irreversibly oxidized to phenoxyl radicals and bis-phenoxyl radicals. The hydrogen bonds stabilize both forms of the oxidized ligands. [Ni(salcn(2Me))] is oxidized in three steps. The overall process involves ligand and partially nickel ion. In the I step, [Ni(salcn(2Me))] is oxidized to phenoxyl radical complex, which is well stabilized by the nickel ion. In the II step the oxidation is realized by forming bis-phenoxyl radicals and phenoxonium cations complexes.The films derived from salen complexes deposited on the Pt electrode surface (poly[Ni(salcn)], poly[Ni(salcn(Me))], poly[Ni(salcn(Bu))]) were obtained as a result of anodic electropolymerization processes. On the basis of FTIR ATR spectroscopy and CV experiments, the relationship of the polymer films structures, charge transport and the potential range of the applied electrosynthesis was shown.

W@Au Nanostructures Modifying Carbon as Materials for Hydrogen Peroxide Electrogeneration

In this work, materials based on core-shell W@Au type structures were found to have promise for use as electrocatalysts on the in-situ production of H2O2 by means of the oxygen reduction reaction (ORR). We describe herein the synthesis and characterization of these materials and then present a study of electrocatalytic activity towards ORR by the electrogeneration of H2O2 employing these materials supported on Vulcan XC-72R carbon corresponding to 1 and 2wt% loading. The use of W@Au/C materials led to higher activity compared to pure carbon and commercial Pt/C, and the optimal load is 1%, which presented the highest ring current for the ORR using the rotating ring-disk electrode technique. Exhaustive electrolysis using a W@Au/C 1% gas diffusion electrode (GDE) was employed to verify the real amount of H2O2 electrogenerated comparing with a Vulcan XC-72R GDE. We verified that the W@Au/C 1% material is able to generate 50% more H2O2 than carbon. These results can be explained based on synergistic interactions presented by the W@Au/C 1% material and also by both conductivity and hydrophilicity differences provided by the nanostructures supported on carbon.

In situ electrochemical synthesis of Ni(I) complexes with aminomethylphosphines as intermediates for hydrogen evolution

Nickel (I) complexes of 1,5-diaza-3,7-diphosphacyclooctane PPh2NR2 ligands were obtained from precursor Ni(II) by exhaustive electrolysis of [Ni(PPh2NR2)2]2+ precursors at the Ni(II/I) couple potential and characterized by several techniques. Paramagnetic Ni(I) complexes were investigated by cyclic voltammetry, spectroelectrochemistry UV/Vis and ESR methods at room temperature and in a frozen solution. Catalytic behavior of Ni(I) complex in different ligand environment for hydrogen evolution from acid depends from the ligand nature. The two possible catalytic cycles were proposed depending on the nitrogen substituents in 1,5-diaza-3,7-diphosphacyclooctane.

Electrochemical Behaviour of Nitrobenzene, Nitrosobenzene, Azobenzene, and Azoxybenzene on Hg, Pt, Cu, and Ni Electrodes in Aprotic Medium

AbstractThe electrochemical behaviour of nitrobenzene and other members of the nitrobenzene family (nitrosobenzene, azoxybenzene, azobenzene) has been investigated in DMF/TEAP by cyclic voltammetry, polarography, and controlled potential electrolysis at mercury, platinum, copper, and nickel cathodes. At the four electrodes, nitrobenzene and the other members of the family are reduced to aniline via hydrazobenzene upon exhaustive electrolysis at −2.35 V vs. Ag/AgNO3 (0,05 M). The electrochemical reversibility of the nitrobenzene/nitrobenzene radical anion and of the azoxybenzene/azoxybenzene radical anion couples is decreased by the presence of oxides at the surface of copper and nickel. The presence of oxides has little influence on the reversibility of the nitrosobenzene/nitrosobenzene radical anion and azobenzene/azobenzene radical anion couples.

Amperometric flow-injection determination of the anthelmintic drugs ivermectin and levamisole using electrochemically pretreated boron-doped diamond electrodes

Sensitive, simple, low-cost, and rapid amperometric flow-injection methods were developed for determination of ivermectin (IVM) and levamisole (LVM) in anthelmintic pharmaceutical drugs and urine samples. These determinations were carried out with a cathodically (for IVM) or anodically (for LVM) pretreated boron-doped diamond electrode using aqueous 0.5molL−1 H2SO4 (with and without addition of 30% V/V of ethanol) as supporting electrolyte. The amperometric detection was performed at an applied electrode potential of E=1.40V, for IVM, and E=1.90V, for LVM, vs. Ag/AgCl (3.0molL−1 KCl). The respective analytical curves presented good linearity in the investigated concentration ranges (0.60–50μmolL−1, for IVM, and 0.010–5.0μmolL−1, for LVM) and the detection limits were 0.30μmolL−1 for IVM, and 1nmolL−1, for LVM. Investigation of possible interferents showed no significant interference with the here-proposed methods; thus, they were used to determine IVM and LVM in pharmaceutical formulations as well as in urine samples, with results that showed good agreement with comparative methods. Furthermore, a mechanism for LVM electrooxidation was proposed using exhaustive potentiostatic electrolysis and NMR measurements.

Electrochemical Reduction of CO2 by M(CO)4(diimine) Complexes (M=Mo, W): Catalytic Activity Improved by 2,2′‐Dipyridylamine

AbstractTetracarbonyl complexes of low‐valent Group VI transition metals (Mo and W), containing diimine bidentate ligands, namely W(CO)4(4,6‐diphenyl‐2,2′‐bipyridine) (1), W(CO)4(6‐(2,6‐dimethoxyphenyl)‐4‐phenyl‐2,2′‐bipyridine) (2), Mo(CO)4(2,2′‐dipyridylamine) (3), and W(CO)4(2,2′‐dipyridylamine) (4), were synthesized and tested as homogeneous catalysts for the electrochemical reduction of CO2 in nonaqueous media. Cyclic voltammetry performed under a CO2 atmosphere, revealed that these complexes have significant catalytic activity in acetonitrile, and gas chromatographic measurements together with exhaustive electrolysis showed that CO is the major reduction product. Mechanistic insights were obtained by IR‐spectroelectrochemical measurements. The substantially different electrocatalytic performances obtained for the two classes of catalysts are compared and discussed.

Electrochemical and Spectroelectrochemical Study of A4 and A2B2 Pyrene Dendronized Porphyrins

The electrochemical behaviour of two free-base pyrene dendronized porphyrins, Por-(Py2G1)2, Por-(Py2G1)4, and their zinc metallated forms, Zn-Por-(Py2G1)2 and Zn-Por-(Py2G1)4, has been studied by cyclic voltammetry in CH2Cl2. The investigated compounds exhibit several oxidation processes. The study of five model molecules, equivalent to the nuclei and branches of the dendrimer, allowed the assignment of the different oxidation processes. The first oxidation, centered on the porphyrin ring, was also studied by in situ UV-vis spectroelectrochemistry in CH2Cl2 and THF for all the dendrimers. The electrooxidation in the thin layer spectroelectrochemical cell of the two free-base dendritic molecules shows spectral changes that suggest the formation of porphyrin diprotonated species, both in THF and CH2Cl2. Even though the electrochemical and spectroscopic characteristics of the solutions after exhaustive electrolysis in THF were consistent with the formation of these protonated species; in CH2Cl2, a paramagnetic species was obtained, and no further evidence of the formation of diprotonated species could be found. Two-electrons per molecule were consumed during the electrolysis in THF and only one-electron in CH2Cl2, the starting porphyrin was not regenerated by counterelectrolysis in any case. It was demonstrated that irradiation of the porphyrin itself accounts for the observed evolution of the UV–vis spectra in CH2Cl2. For the Zn dendronized porphyrins, the oxidation was a clean one-electron transfer, giving rise to the radical cation, subsequent reductive electrolysis regenerates the initial product.

Microconcentric ring electrode/injector assembly for sensitive voltammetric analysis in single droplets of ultrasmall volumes.

This paper describes the construction of a microring electrode concentric to an inner injection capillary for voltammetric determination of trace analytes in nanoliter- to picoliter-sized droplets. The gold microring is sandwiched between a pulled fused-silica capillary and borosilicate glass tubing. Compared to polymer-coated microring electrodes, the glass-encapsulated electrode is more robust and does not swell in organic solvents. Consequently, the microring electrode is suitable for voltammetric studies of redox-active species and their accompanying ion transfers between two immiscible solvents. Droplets of variable sizes can be conveniently dispensed from front-loaded sample plugs into an immiscible liquid, greatly simplifying the experimental procedure and facilitating analysis of samples of limited availability. The size of the microring and the volume of the droplet deduced from well-defined voltammograms correlate well with those estimated from their geometric dimensions. The thin-layer cell behavior can be attained with well-defined voltammetric peaks and small capacitive current. Exhaustive electrolysis in single droplets can be accomplished in short times (e.g., ∼85 s in a droplet of 1.42 nL at a microring of 11.4 μm in radius). Anodic stripping voltammetry of Ag deposited onto the microring electrode resulted in a detection limit of 0.13 fmol (14 fg) of Ag(+). The microring electrode/injector assembly can be polished repeatedly and is versatile for various applications (e.g., sample plugs can also be back-loaded via a rotary injection valve and an HPLC pump for flow injection analysis).

Electrochemical and sonoelectrochemical processes applied to amaranth dye degradation

Amaranth dye is an organic compound largely used in the food and beverage industries with potential toxicity effects on humans. It can be found as a pollutant species in aquatic environments and has been classified as an endocrine disruptor. This study describes amaranth degradation upon ultrasonication associated with an electrochemical system that uses a boron-doped diamond anode BDD, defined as a sonoelectrochemical process. Ninety-minute electrolyses were performed using current densities in the 10–50mAcm−2 range, and the concentration decay, pH, energy and current efficiencies, as well as the discoloration rate were evaluated. The amaranth concentration decayed as a function of electrolysis time and the reactions obeyed pseudo first-order kinetics, with an apparent constant rate between 10−1 and 10−3min−1. The electrochemical and sonoelectrochemical processes at 35mAcm−2 yielded TOC removal values between 92.1% and 95.1% respectively, after 90min. Current efficiency values obtained for both processes were 18.2% and 23.6%. Exhaustive 5h electrolysis was performed and the degradation products were identified by HPLC–MS. A mechanism for the degradation of amaranth was proposed based on an analysis of the aromatic and aliphatic intermediates.

Introducing a new azoaromatic pincer ligand. Isolation and characterization of redox events in its ferrous complexes.

The isolation and complete characterization of a new bis-azoaromatic ligand, 2,6-bis(phenylazo)pyridine (L), are described, and its coordination to iron(II) is reported. A pseudo-trigonal-bipyramidal mixed-ligand complex of iron(II), FeLCl2 (1), and a homoleptic octahedral iron complex, mer-[Fe(L)2]ClO4 [2]ClO4, have been synthesized from L and FeCl2 or hydrated Fe(ClO4)2, respectively, in boiling methanol. Determination of the X-ray crystallographic structure together with magnetic data (≈ 5.06 μB) and Mössbauer analysis of 1 established a high-spin Fe(II) complex ligated by one neutral 2,6-bis(phenylazo)pyridine ligand. The X-ray crystallographic structure (showing dN-N > 1.30 Å), Mössbauer data, and magnetic susceptibility measurements (≈ 1.65 μB) as well as a nearly isotropic EPR signal with only a small metal contribution at g = 1.968, on the other hand, suggest a low-spin Fe(II) complex with a one-electron-reduced radical ligand coordination in [2]ClO4. The ligand and the metal complexes have well-behaved redox properties, with the ligand(s) functioning as the redox-active site(s) responsible for redox events. The uncoordinated ligand, L, displays a reversible one-electron wave at -1.07 V and a quasi-reversible wave at -1.39 V. The partially reduced ligand L(•-) shows a single-line EPR spectrum at g = 2.001, signifying that L(•-) is a free radical. While complex 1 shows a reversible reduction at -0.08 V and an irreversible cathodic response at -0.98 V, the bis-chelate [2]ClO4 undergoes a reversible one-electron oxidation at 0.54 V and three successive reversible one-electron reductions at -0.18, -0.88, and -1.2 V, all occurring at the ligands without affecting the metal ion oxidation state. The electronic structures of the parent monocationic complex [2](+) and its oxidized and reduced forms, generated by exhaustive electrolyses, have been characterized by using a host of spectroscopic techniques and density functional theory (DFT). It is found that the 2,6-bis(phenylazo)pyridine ligand (L) is truly redox noninnocent and is capable of coordinating transition-metal centers in its neutral ([L](0)), monoanionic monoradical ([L(•)](-)), and dianionic diradical ([L(••)](2-)) forms.