Multielectron Redox Reactions Research Articles

Room Temperature, Hybrid Sodium-Based Flow Batteries with Multi-Electron Transfer Redox Reactions.

We introduce a new concept of hybrid Na-based flow batteries (HNFBs) with a molten Na alloy anode in conjunction with a flowing catholyte separated by a solid Na-ion exchange membrane for grid-scale energy storage. Such HNFBs can operate at ambient temperature, allow catholytes to have multiple electron transfer redox reactions per active ion, offer wide selection of catholyte chemistries with multiple active ions to couple with the highly negative Na alloy anode, and enable the use of both aqueous and non-aqueous catholytes. Further, the molten Na alloy anode permits the decoupled design of power and energy since a large volume of the molten Na alloy can be used with a limited ion-exchange membrane size. In this proof-of-concept study, the feasibility of multi-electron transfer redox reactions per active ion and multiple active ions for catholytes has been demonstrated. The critical barriers to mature this new HNFBs have also been explored.

High Capcity, High Voltage Hybrid Sodium-Based Flow Batteries with Aqueous Catholyte

We introduce a new concept of hybrid Na-based flow batteries (HNFBs) with sodium-based stationary anode in conjunction with a flowing catholyte separated by a solid Na-ion exchange membrane for grid-scale energy storage [1]. Such HNFBs can operate at ambient temperature; allow catholytes to have multiple electron transfer redox reactions per active ion; offer wide selection of catholyte chemistries with multiple active ions to couple with the highly negative Na anode. Further, the Na anode gives high voltage (> 3 V) for this hybrid flow battery. In this work, the feasibility of using the aqueous catholyte in HNFBs to utilize multi-electron transfer redox reactions per active ion and multiple active ions [2] for catholytes has been evaluated and demonstrated. The catholyte chemistry can be the same as or similar to that of traditional RFBs. For example, the reactions in both the anolyte and catholyte in all vanadium RFBs can potentially be used in the catholyte of HNFBs, as shown by reactions (1) to (3) below. Cathode: VO2+ + H2O ↔ VO2 + + e- Eo = +1.0 V vs. SHE (1) V3+ + H2O ↔ VO2+ + 2H+ + e- Eo = +0.34 V vs. SHE (2) V2+ ↔ V3+ + e- Eo = -0.26 V vs. SHE (3) Anode: Na ↔ Na+ + e− Eo = -2.7 V vs.SHE (4) Based on these redox reactions of the catholyte and assuming that the catholyte contains 2.5M active V ions, one can obtain a theoretical specific energy of 483.7 Wh/kg, which is 18 times the specific energy provided by conventional all vanadium RFBs (~25 Wh/kg) [3]. The feasibility of using the aqueous catholyte in HNFBs is evaluated using VOSO4 (V4+) solution. The CV test (Figure 1a) shows the stable electrochemical potential window for the aqueous catholyte solution and the redox potential of V ions. Addition of BiCl3 in V aqueous electrolytes has been found an effective approach to improve the reversibility of V ion redox reactions [4]. The A, B, C redox peaks are attributed to the V4+/V5+, V3+/V4+, and V2+/V3+ redox reaction couples, respectively, while the strong D peaks are generated by the redox reaction of Bi3+/Bi0. Figure 1a also shows that the O2 evolution reaction (OER) and H2 evolution reaction (HER). Based on that and further considering the overpotential in the cell, the practical threshold can be 2.2 – 4.0 V vs. Na+/Na. The charge/discharge behavior of the HNFB with aqueous catholyte and a flat β”-Al2O3 membrane are examined and shown in Figure 1b. Three plateaus can be observed on the 1st cycle of both discharge and charge curves, corresponding to the redox reactions of V(IV)/V(III), Bi3+(III)/Bi(0), and V(III)/V(II) as labelled in the figure. Therefore, for the first time the concept of multiple electron transfer redox reactions per active ion in aqueous electrolytes (2 redox per V active ion) has been validated. Moreover, the result shows multiple electron transfer obtained by two active species, i.e. V and Bi ions. It should be noted the low capacity for each redox reaction shown in Figure 1b is mainly attributed to the insufficient mass transportation in the catholyte due to weak stirring and thick, porous graphite felt electrode making the mass transportation inside the felt slow and difficult. The investigations on improving the capacity are undergoing. Reference [1] L. Shaw, J. Shamie, United States Patent, in application, 2014. [2] W. Wang, L. Li, Z. Nie, B. Chen, Q. Luo, Y. Shao, X. Wei, F. Chen, G.-G. Xia, Z. Yang, Journal of Power Sources 2012, 216, 99. [3] W. Wang, Q. Luo, B. Li, X. Wei, L. Li, Z. Yang, Adv. Funct. Mater. 2013, 23, 970. [4] B. Li, M. Gu, Z. Nie, Y. Shao, Q. Luo, X. Wei, X. Li, J. Xiao, C. Wang, V. Sprenkle, W. Wang, Nano Lett. 2013, 13, 1330. Figure 1. (a) CV curves of aqueous electrolytes with and without BiCl3 in half cells at a scan rate of 10 mV/s in which a graphite felt, Ag/AgCl, and Pt wire were used as the working, reference, and counter electrodes, respectively. (b) The charge/discharge profile of a full cell with the 0.01M VOSO4 -0.05M Na2SO4 -1.5M HCl -0.002M BiCl3aqueous solution as the catholyte. Figure 1

Polyoxometalates for Non-Aqueous Redox Flow Battery Applications

Scalable energy storage technologies are needed to enable integration of high levels of integration of intermittent energy sources such as wind and solar into the grid. Redox Flow Batteries (RFBs) designed to store energy in chemical form in the electrolyte can help to address the energy storage problem. However, most traditional RFBs are aqueous systems for which the cell voltage is limited by water splitting. Non-aqueous RFBs could permit higher cell voltages by removing this limitation. Polyoxometalates (POMs), with versatile, tunable properties, undergo multi-electron redox reactions which may meet the requirements of high performance Redox Flow Batteries. Research conducted to date has focused on the electrochemical characteristics of different combinations of Keggin-type polyoxometalates (XMmO40 n-). Metals substituted into the Keggin framework play a key role in altering the redox properties of polyoxometalates. Based on the result, the effects of 1A or 2A counter cations on cell potentials were fairly minor. And Keggin ions containing different central heteroatoms (phosphorous, arsenic and silicon) were compared. While the charge on the Keggin ion does affect the telative positions of redox couples in the CV spectrum, the number of redox couples is not affected. Last mixed addenda POMs were also compared. The mechanism of addenda element effects on POM redox properties is still unclear, but the mixed addenda elements play an important role in determining the redox properties. Investigation of the mix addenda POMs is the focus of current efforts. This research aims to select suitable polyoxometalate species that can be used for non-aqueous Redox Flow Batteries. Preliminary results have shown high coulombic efficiency (~90%), reflecting the reversibility of polyoxometalate redox reactions.

Dinuclear Nickel Complexes in Five States of Oxidation Using a Redox-Active Ligand

Redox-active nitrogen donor ligands have exhibited broad utility in stabilizing transition metal complexes in unusual formal oxidation states and enabling multielectron redox reactions. In this report, we extend these principles to dinuclear complexes using a naphthyridine-diimine (NDI) framework. Treatment of ((i-Pr)NDI) with Ni(COD)2 (2.0 equiv) yields a Ni(I)-Ni(I) complex in which the two metal centers form a single bond and the ((i-Pr)NDI) ligand is doubly reduced. A homologous series of ((i-Pr)NDI)Ni2 complexes in five oxidation states were synthesized and structurally characterized. Across this series, the ligand ranges from a neutral state in the most oxidized member to a dianionic state in the most reduced. The interplay between metal- and ligand-centered redox activity is interrogated using a variety of experimental techniques in combination with density functional theory models.

Improvement in Cycle-Stability of Naphthoquinone-Based Rechargeable Organic Batteries By Polymerization

Using redox active organic materials, which contain no scarce metal resources, as a positive-electrode active material instead of conventionally used rare-metal oxides can be a solution to the resource problem of the current rechargeable lithium batteries. We have focused our attention on the redox active quinone-based materials which can lead to a high discharge capacity due to their multi-electron redox reactions and found that certain low-molecular-weight crystalline quinone derivatives exhibit high discharge capacities [1]. However, their discharge capacities tend to decrease upon cycling resulting from the loss of the active materials from the electrodes by the dissolution of the quinone molecules in the electrolyte solutions. To suppress the dissolution level of the quinone skeleton, we examined the polymerization of a quinone skeleton. In this paper, polymeric compounds were synthesized from 5,8-dihydroxy-1,4-naphthoquinone (DHNQ) (Fig. 1a) and their cycle-life performances were compared to that of the monomer.Polymers including the DHNQ skeleton (PDNQs) were synthesized by the condensation reaction between DHNQ and formaldehyde in an acidic media condition at the temperatures of 90, 100, and 150 oC. The obtained materials were characterized by the 1H-NMR, FT-IR spectroscopies, and the gel permeation chromatography (GPC). The positive-electrode was prepared by applying a composite sheet composed of the powder of the organic active material, acetylene black, and polytetrafluoroethylene in the weight ratio of 4:5:1 to a mesh-type aluminum current collector. The prepared positive-electrode was evaluated by making a coin-type half cell.The solubility of the quinone-based materials in the ordinary organic solvents was successfully decreased by polymerization. Fig. 1b compares the cycle-life performance of the prepared materials (PDNQs) to that of monomer (DHNQ). The electrodes using DHNQ showed a discharge capacity of 110 mAh g−1 in the first cycle. The obtained value is less than one fourth of the theoretical value based on the assumption of the four-electron redox reaction per molecule. The discharge capacity decreased rapidly upon cycling. On the other hand, the electrodes using PDNQs showed larger capacities than that of monomer at the first cycle. The obtained capacities are about 240-290 mAh g−1, and the utilization ratio of these polymeric materials were improved to 45 to 55 %. Furthermore, the stabilities were better than that of the monomer.There is a report about the charge/discharge behavior of a naphthoquinone polymer which was synthesized in a different way [2]. The previously reported polymer only showed a discharge capacity of about 60 mAh g−1 during cycling although its cycle-life was fair. On the other hand, our samples in this study showed relatively large capacities. We also performed another polymerization on the DHNQ moiety with sulfur. While the cycle-stability of the sulfur-containing polymer was also good, the capacity was not large enough during initial cycles. These results indicate that polymerization can be effective to improve the cycle-life performance of naphthoquinone-based active materials; however the synthesis method has a profound effect on their discharge capacities. Further optimization in the synthesis condition is considered to improve the discharge property.[1] M. Yao et al., J. Power Sources, 195 (2010) 8336.[2] A. Kassam et al., Electrochem. Solid-State Lett., 14, (2011) A22.

Electrochemical Performance of Quinone-Based Positive Electrodes for Lithium Secondary Battery

Quinone-based organic materials have reversal multielectron redox reactions. Cations in electrolytes are withdrawn or released by reduced quinone-based materials. Lithium ions being adopted, quinone-based materials are applied as an active material for lithium secondary battery. Due to their multielectron redox reactions, organic materials have an energy density even equal to inorganic materials. Inorganic materials must keep their crystal structure during cell operation, relating to reversibility of charge-discharge capacity, and the lithium conducting path in their crystal structure limit the mobility of lithium. On the other hand, organic materials do not need to keep their crystal structure owing to redox reaction within a molecule. Their electrochemical performances are not controlled by the lithium mobility in the lithium conducting path as in the case of inorganic materials. In addition, organic materials are relatively low-cost due to no metal elements and can be easily designed to fabricate their organic structure. However, their high solubility to electrolytes gives a large capacity loss, and their insulating properties lead directly to poor electron conduction during their redox reaction. These properties limit the rate performance and utilization ratio of organic materials. In this study, several organic materials, anthraquinone (AQ), 9,10-phenanthrenequinone (PQ), 1,10-phenanthroline-5,6-dione (N2PQ), and so on, were supported as much as possible on the surface of activated carbon (Maxsorb®) with nano-sized pores as a current collector for compensating their insulating properties. The organic materials-supported activated carbon was covered by a lithium conductive polymer (polyethyleneoxide, PEO) in order to restrict a dissolution of organic materials to electrolytes. The effect of organic structures on the electrochemical performance as positive electrodes was discussed. Discharge curves of positive electrodes using AQ, PQ, and N2PQ at 0.2 C show that two clear plateaus are detected for PQ (2.7 and 2.3 V) and N2PQ (2.9 and 2.5 V), respectively, and one plateau is detected for AQ (2.1 V). When two ketone groups in PQ and N2PQ are adjacent, radical anions reduced from neutral molecules through one electron redox reaction could exist stably by coordinating a lithium ion for a bridge formation. The coexistence regions of neutral molecules-radical anions and of radical anions-divalent anions result in two plateaus in the charge-discharge test of PQ and N2PQ. Redox potentials of PQ are higher than those of AQ because 1,2-diketone compound (PQ) receives a repulsion of two dipoles from each ketone, leading to a destabilization of neutral molecules and to easily reduction at the higher potential than 1,4-diketone compound (AQ). Heterocyclic compound (N2PQ) offers higher redox potential than PQ. In the field of complex chemistry, N2PQ behaves as a ligand for chelation. A resonance form of reduced N2PQ gives a negative charge to nitrogen atoms, and lithium ions are possibly withdrawn not only at oxygen atoms but also at nitrogen atoms. This coordinating ability of nitrogen atoms stabilizes the reduced form of N2PQ more than that of non-heterocyclic compound (PQ), reflecting higher redox potentials of N2PQ. Low polarization and high redox potentials in the charge-discharge test for N2PQ gives a high energy density of about 600 mWh g-1 when an electric double layer capacitance obtained from the activated carbon was removed. This value is comparable to inorganic active materials. A charge-discharge curve for N2PQ at 1.0 C shows two plateaus at 2.8 and 2.4 V. Discharge capacity in the 5th cycle at 0.2 and 1.0 C was 226 and 194 mAh g-1, respectively, and their utilization ratios of N2PQ (theoretical discharge capacity: 255 mAh g-1) were 89 and 76 %. The redox reaction of N2PQ through stable radical anion occurs smoothly to offer highly discharge capacity and rate performance.

ChemInform Abstract: Synthesis and Electrochemical Properties of Ferrocene Dimers and Trimers Bridged by an Oligothiophene Spacer

AbstractReview: 30 refs.

A New High-Energy Cathode for a Na-Ion Battery with Ultrahigh Stability

Large-scale electric energy storage is a key enabler for the use of renewable energy. Recently, the room-temperature Na-ion battery has been rehighlighted as an alternative low-cost technology for this application. However, significant challenges such as energy density and long-term stability must be addressed. Herein, we introduce a novel cathode material, Na1.5VPO4.8F0.7, for Na-ion batteries. This new material provides an energy density of ~600 Wh kg(-1), the highest value among cathodes, originating from both the multielectron redox reaction (1.2 e(-) per formula unit) and the high potential (~3.8 V vs Na(+)/Na) of the tailored vanadium redox couple (V(3.8+)/V(5+)). Furthermore, an outstanding cycle life (~95% capacity retention for 100 cycles and ~84% for extended 500 cycles) could be achieved, which we attribute to the small volume change (2.9%) upon cycling, the smallest volume change among known Na intercalation cathodes. The open crystal framework with two-dimensional Na diffusional pathways leads to low activation barriers for Na diffusion, enabling excellent rate capability. We believe that this new material can bring the low-cost room-temperature Na-ion battery a step closer to a sustainable large-scale energy storage system.

Titanyl phthalocyanines: Electrochemical and spectroelectrochemical characterizations and electrochemical metal ion sensor applications of Langmuir films

Electrochemical and spectroelectrochemical characterizations of titanyl phthalocyanines, which were peripheral (α-TiOPc) and nonperipheral (β-TiOPc) tetra and octa substituted (octa-TiOPc) with 3,4-(methylendioxy)-phenoxy moieties were performed to determine their possible applications in different electrochemical technologies. Voltammetric and in situ spectroelectrochemical measurements illustrate that position and number of 3,4-(methylendioxy)-phenoxy substituents affect the redox behavior of the complexes. While α-TiOPc gives up to four reduction processes, β-TiOPc and octa-TiOPc gives up to three reduction processes. While octa-TiOPc gives metal–metal–ring–ring reductions, α-TiOPc and β-TiOPc complexes give metal–ring–ring–metal reduction mechanism. Solvent of the media also alter the electrochemical behavior of the complexes. In situ electrocolorimetric measurements shows distinct color differences among the electrogenerated anionic and cationic redox species, which indicates their possible applications in display technologies. Multiple and reversible, diffusion controlled and multi-electron redox reaction indicate possible usage of the complexes especially in electrosensing applications. The complexes sense Ag+ and Pd2+ ions in the solution and in the solid states.

Organic polymer material with a multi-electron process redox reaction: towards ultra-high reversible lithium storage capacity

Sulfur chain grafted poly(pyridinopyridine) (SPPY) presents an over five-electron process redox reaction, a high reversible capacity of up to 1750 mAh g−1-SPPY and excellent cycling stability. Combined with its facile and eco-efficient synthesis, SPPY is a potential green material for ultra-high capacity of reversible lithium storage and opens a new widow to seek ultra-high capacity materials for lithium ion batteries.

Multi-electron redox reaction of an organic radical cathode induced by a mesopore carbon network with nitroxide polymers

An organic radical based composite cathode comprised of poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA)-Ketjenblack was developed by a simple solvent-less electrode fabrication method. The composite cathode demonstrated a two-electron redox reaction of PTMA that is from an aminoxy anion (n-type) via a radical to an oxoammonium cation (p-type) with the corresponding redox potential at 2.8-3.1 V and 3.5-3.7 V vs. Li/Li(+) when evaluated in lithium half cells. Moreover, the PTMA-Ketjenblack composite electrode exhibits fast electrode reaction kinetics and an enhanced solid electrolyte interface by cyclic voltammetry and electrochemical impedance spectroscopy measurements. These improved electrochemical properties contribute to increased capacity (300 mA h g(-1)), a high rate (50% capacity retention after 100 C rate excursions) and a long cycle life in the cell performance evaluations. Morphological and compositional characterization indicates a unique mesopore network of Ketjenblack with the PTMA matrix, which highly facilitates the interaction between the conductive media and the radical species, resulting in the performance enhancement of the PTMA-Ketjenblack composite cathode.

Synthesis and electrochemical properties of ferrocene dimers and trimers bridged by an oligothiophene spacer

We describe the design and synthesis of ferrocene dimers and trimers bridged by an oligothiophene spacer as both cathodic and anodic multiple redox-active organic–organometallic hybrid molecules. The electrochemical properties of all compounds were examined by cyclic voltammetry (CV). The voltammograms showed different multi-step, multi-electron redox reactions derived from multi-ferrocene fragments and the organic spacer. The results indicate that a number of thiophene rings in the spacer can be attuned to the redox potentials and the electron-transfer processes.

Crystalline polycyclic quinone derivatives as organic positive-electrode materials for use in rechargeable lithium batteries

Abstract The performance of 9,10-anthraquinone (AQ), and 5,7,12,14-pentacenetetrone (PT) as active materials for rechargeable lithium batteries was investigated. Positive-electrodes in which AQ and PT were incorporated showed initial discharge capacities of greater than 200 mAh/g (AQ or PT) . The obtained discharge capacities suggest that a multi-electron redox reaction takes place in each derivative. The discharge capacity of the positive-electrode with AQ rapidly decreased during the charge/discharge cycles; however, the positive-electrode with PT showed a relatively good cycle-life performance; it maintained about 80% of the initial capacity even after 100 cycles.

The Role of One- and Two-Electron Transfer Reactions in Forming Thermodynamically Unstable Intermediates as Barriers in Multi-Electron Redox Reactions

In the aquatic geochemical literature, a redox half-reaction is normally written for a multi-electron process (n > 2); e.g., sulfide oxidation to sulfate. When coupling two multi-electron half-reactions, thermodynamic calculations indicate possible reactivity, and the coupled half-reactions are considered favorable even when there is a known barrier to reactivity. Thermodynamic calculations should be done for one or two-electron transfer steps and then compared with known reactivity to determine the rate controlling step in a reaction pathway. Here, thermodynamic calculations are presented for selected reactions for compounds of C, O, N, S, Fe, Mn and Cu. Calculations predict reactivity barriers and agree with one previous analysis showing the first step in reducing O2 to O2− with Fe2+ and Mn2+ is rate limiting. Similar problems occur for the first electron transfer step in these metals reducing NO3−, but if reactive oxygen species form or if two-electron transfer steps with O atom transfer occur, reactivity becomes favorable. H2S and NH4+ oxidation in a one-electron transfer step by O2 is also not favorable unless activation of oxygen can occur. H2S oxidation by Cu2+, Fe(III) and Mn(III, IV) phases in two-electron transfer steps is favorable but not in one-electron steps indicating that (nano)particles with bands of orbitals are needed to accept two electrons from H2S. NH4+ oxidation by Fe(III) and Mn(III, IV) phases is generally not favorable for both one- and two-electron transfer steps, but their reaction with hydroxylamine and hydrazine to form N2O and N2, respectively, is favorable. The anammox reaction using hydroxylamine via nitrite reduction is the most favorable for NH4+ oxidation. Other chemical processes including photosynthesis and chemosynthesis are considered for these element–element transformations.

Poly[3,4-(ethylenedithio)thiophene]: High specific capacity cathode active material for lithium rechargeable batteries

Poly[3,4-(ethylenedithio)thiophene] (PEDTT) has been synthesized by oxidative-coupling polymerization of 3,4-(ethylenedithio)thiophene (EDTT) in the absence of solvent at ambient conditions. The resulting polymer has been characterized by FT-IR, XRD, TGA, UV–vis and solution NMR analyses. In addition, PEDTT has been evaluated as the cathode active material for rechargeable lithium batteries. The charge–discharge tests are carried out at room temperature. PEDTT shows discharge specific capacity above 425 mAh g −1. It is tentatively proposed that electrode reaction involves the formation of thioether cation, which imparts multi-electron redox reaction, high discharge specific capacity, high charge voltage and low discharge voltage.

Identification and Electroanalytical Characterization of Redox Transitions in Solid‐State Keggin type Phosphomolybdic Acid

AbstractThe tetragonal single crystal, H3PMo12O40⋅29H2O, is considered here as a model system to describe multielectron redox reactions of Keggin type phosphododecamolybdate. Electrochemical experiments have been performed in solid‐state, i.e. in the absence of contact with liquid electrolyte phase, and they have utilized a gold wire ultramicrodisk (diameter: 10 μm) working electrode, a silver disk quasireference electrode, and a glassy carbon counter electrode within all solid cell. The results of voltammetric measurements are explained and verified against the simulated data. The reversible redox reactions of phosphododecamolybdic acid (H3PMo12O40⋅29H2O) single crystal, that appear at most positive potentials, can be understood in terms of two overlapping one‐electron redox processes (separated by only ≈50 mV), rather than a single two‐electron redox transition, followed by another one‐electron process. Comparison is made to the conventional electrochemical behavior of phosphododecamolybdate in solution (0.5 mol dm−3 H2SO4) as well as to the characteristic of its adsorbate on the electrode surface. The following parameters have been determined from the combination of potential step experiments performed in two limiting (radial and linear) diffusional regimes: the concentration of heteropolymolybdate redox centers, 1.4 mol dm−3, and the apparent diffusion coefficient for charge propagation, 3.6×10−7 cm2 s−1. The result supports the concept of the kinetic control of charge transport by electron hopping (self‐exchange) between mixed‐valence (MoVI/MoV) sites.

Density Functional Theory Study of Redox Pairs. 1. Dinuclear Iron Complexes That Undergo Multielectron Redox Reactions Accompanied by a Reversible Structural Change

The single two-electron reduction for the Fe−Fe bonded dinuclear complexes Fe2(CO)6(μ2-PR2)2 (R = CH3, 1-CH3; R = CF3, 1-CF3) is studied by electronic structure calculations based on density functional theory (DFT) methods. Several theoretical models are evaluated, including gas-phase models and models that include solvation (COSMO model) and/or countercations. The experimentally observed cleavage of the Fe−Fe bond upon addition of electrons is reproduced in all calculations. The different theoretical models are evaluated by calculating the energy of the disproportionation reaction 2A- → A + A2- using the energies of the complexes [1-R]0, [1-R]-, and [1-R]2-. As expected, gas-phase calculations poorly model the experimental redox behavior, and the inclusion of solvation or countercations is necessary to correctly predict that the disproportionation reaction is energetically downhill. The distribution of the added electrons over the molecules and the charge distribution as a function of alkali metal countercation (Li+, Na+, K+) are evaluated using the Hirshfeld charge analysis scheme. A qualitative correlation is found between the HOMO/LUMO energies of the redox species and the calculated redox potentials.

Induced Fit in a Molecular Pac-Man

CHEMISTRY Many fundamental oxidation-reduction (“redox”) reactions require the transfer of several electrons. In biological systems, such multielectron redox reactions are often catalyzed by enzymes that contain multiple metal atoms. Chemists are devising biomimetic synthetic systems to understand the underlying mechanisms and to develop new catalysts. For example, molecules consisting of two porphyrin units linked by a spacer—cofacial bisporphyrins—can catalyze reactions such as the direct reduction of oxygen to water. J. P. Collman suggested in the mid-1980s that some cofacial bisporphyrins may show a “Pac-Man”-like flexibility that would allow their binding pocket to accommodate reaction intermediates during catalysis in a manner similar to “induced fit” in enzyme catalysis. Now, Deng et al. have synthesized cofacial bisporphyrins linked by a rigid dibenzofuran pillar and show that the molecules can open and close their binding pockets with a “bite” of more than 4 angstroms in the presence of suitable ligands. The authors have recently used the biscobalt(II) derivative of the dibenzofuran platform to catalyze the electrochemical four-electron reduction of oxygen to water.— JU J. Am. Chem. Soc. 122 , 410 (2000).

Chalcogen atom transfer reactions. Kinetics of terminal bonded sulfur atom transfer between main group metal centers

The first comprehensive investigation of S atom transfer between two different metal centers is presented using the reaction of the terminal chalcogen atom bearing complex [CyNC(tBu)NCy]2SnS with [CyNC(tBu)NCy]2GeII; this multielectron redox reaction between GeII and SnIV proceeds via a second-order process with an inner sphere mechanism involving a μ-S intermediate as the proposed pathway; results of S and Se atom transfer between [CyNC(tBu)NCy]2Ge and PPh3 are also presented.

Molecular Catalysts for Multielectron Redox Reactions of Small Molecules: The “Cofacial Metallodiporphyrin” Approach

AbstractThe role of metalloenzymes in important biological transformations has attracted increasing attention over the past several decades. Of the many chemical transformations mediated by enzymes, few are as challenging as multielectron redox reactions. Recent studies have revealed a partial structural and mechanistic description of these redox‐active metalloenzymes, but there is much still to be learned regarding the mechanisms of substrate transformation. Due to the complexity of the metalloenzyme systems, simplified model systems are employed to mimic structural or functional features of the enzyme. In multielectron redox enzymes, several metals are probably in‐volved in both substrate binding and the subsequent redox reactions. Thus, functional mimics of multielectron redox enzymes might also need two or more metal centers to be efficacious. The roles of multiple metal centers are to (1) increase the substrate's affinity for the catalyst, (2) increase the rate of electron transfer to the bound substrate, (3) increase the reactivity of the bound substrate, and (4) inhibit deleterious side reactions. Deter‐mining the importance of each factor may help in the development of these catalysts. Cofacial metallodiporphyrins, because of the control they provide over the geometric and electronic properties of the synthetic reaction center, are ideal bimetallic model complexes. The knowledge gained from model studies will help in understanding the mechanisms of metalloenzymes and can be used to design new homogeneous catalysts to effect multielectron transformations.