Reversible Two-electron Oxidation Research Articles

Thieno[3,2-b]thiophene Incorporated Redox Active 22π and 34π Core-Modified Expanded Isophlorinoids.

Incorporation of a thieno[3,2-b]thiophene into an isophlorinoid-like framework alters the aromaticity by extending the macrocyclic π-circuit. Their opto-electronic and aromatic properties significantly differ from other 22π and 34π porphyrinoids. Among the three different 34π hexaphyrins, furan based hexaphyrin adopts a non-aromatic 'figure-of-eight' conformation. Replacing all the furans either by thiophene or selenophene induces a planar conformation with aromatic characteristics. Spectro-electrochemical studies revealed reversible two-electron ring oxidation to yield their 4nπ dicationic species respectively.

Crystalline Radical Anion of a Diboratriazole and Its Conversion to a Neutral Radical Driven by a Carbene.

One-electron reduction of diboratriazole 1 with potassium graphite (KC8) generates the radical anion 1•-•K+, which undergoes a salt (KCl) elimination reaction upon addition of an N-heterocyclic carbene (NHC) to afford the neutral diboratriazole radical 3. An X-ray diffraction analysis, electron paramagnetic resonance spectroscopy, and computational studies revealed that an unpaired electron in radical species 1•-•K+ and 3 is delocalized over the π-system of the B2N3 and carbene rings. Reversible oxidation of 3 gives rise to a diboratriazole cation 4 featuring a 6π aromatic character. Moreover, treating 1•-•K+ with a half equivalent of a bis(NHC) produces a biradical species 5, in which there is little interaction between two radical moieties separated by the bis(NHC) linker, suggesting the dis-biradical property. 5 undergoes stepwise and reversible two-electron oxidation, establishing three formal oxidation states.

Open shell (4n + 2)π and closed shell 4nπ planar core-modified decaphyrins.

Planar 44π and 46π core-modified decaphyrins with ten thiophene units have been synthesized from short thiophene oligomers. They have been structurally characterized by single crystal X-ray diffraction with further support from spectroscopic analysis and quantum chemical calculations. Our analysis revealed diradicaloid characteristics for 46π species in contrast to the closed shell property of the 44π congener. Further, 44π and 46π undergo reversible two-electron chemical oxidation, as observed by spectro-electrochemical measurements.

Topological Diversity in Electrochemically Active Core-Modified Expanded Porphyrinoids.

Thiophene-based expanded porphyrinoids undergo a transition from a planar conformation to a twisted conformation upon a systematic increase in the number of thiophene units. Octaphyrin, with 40π electrons, displayed temperature-dependent interconversion between planar and nonplanar conformations in the solution state, in contrast to the rigid planar conformation in the solid state. 60π-dodecaphyrin and 70π-tetradecaphyrin have the maximum number of π-electrons for 12- and 14-heterocycle porphyrinoids, respectively. Spectro-electrochemical measurements confirmed facile reversible two-electron oxidation and the unstable radical cation intermediate in these systems.

Symmetric 32π (1.0.1.0.1.0.1) Core-Modified Heptaphyrins from Asymmetric Building Block.

An asymmetrical precursor with three thiophene subunits on condensation with a thiophene/furan diol yield a symmetrical 32π heptaphyrin. Here are the first examples of pyrrole-free antiaromatic heptaphyrin synthesized by the acid assisted condensation reaction, followed by an oxidative α-α coupling between the terminal thiophene rings. The macrocycles attains a slightly bent configuration, which undergoes reversible two-electron oxidation between a neutral 4nπ and (4n + 2)π dication state.

A Two-Electron Bispyridinylidene Anolyte for Non-Aqueous Organic Redox Flow Batteries

Bispyridinylidenes (BPYs) are promising anolyte materials for organic redox flow batteries owing to their low potential, reversible two-electron oxidation and low molecular weight; however, a recent study suggested that without appropriate substitution, these compounds are inherently unsuitable for this application owing to an apparent chemical reaction between the neutral and dicationic redox partners. It is now demonstrated that the electrolyte itself is key to their stability. In a dimethylformamide-based electrolyte, both BPY charge states (0/2+) exhibit complete compatibility, long lifetime, and excellent solubility (1.18 M, corresponding to a high capacity of 63 Ah l−1). In symmetric cell testing, capacities of up to 100% of the theoretical value and coulombic efficiencies above 98% were achieved, though cell lifetimes with cycling were less than those of the individual BPY redox partners alone in the electrolyte. Considering the tuneability of BPY properties by structural modification, these results should promote further development of this exciting and unique class of materials for energy storage.

A Bispyridinylidene Anolyte for an All-Organic Redox Flow Battery

Global warming associated with CO2 emissions caused by the excessive use of fossil fuels has become a worldwide concern. The use of renewable energy resources as an alternative power source can help reduce the amount of CO2 in air. Redox flow batteries (RFBs) have attracted a lot of attention recently as promising systems for energy storage from intermittent renewable resources and to allow integration with the power grid. Most RFBs are based on metallic active species in aqueous media, however there is a growing interest around the use of soluble organic redox couples in non-aqueous solvents to achieve higher energy density. Organic compounds with high redox potentials (catholyte) are readily available, but new organic compounds that undergo multi-electron redox processes at a low redox potential (anolyte) are needed to boost the energy density of RFBs. This presentation will outline efforts to demonstrate a new bispyridinylidene-based anolyte that undergoes a reversible two-electron oxidation (-1.69 V vs. ferrocene) and assess its applicability in a RFB. In addition to stability tests for the bispyridinylidene, an all-organic non-aqueous RFB employing 2,2,6,6-tetramethyl-1-piperidinyloxy and bispyridinylidene as cathodic and anodic active materials, respectively has been investigated.

Direct electrochemical reduction of carbon dioxide by a molybdenum-containing formate dehydrogenase

Formate dehydrogenase enzymes catalyse the reversible two-electron oxidation of formate to carbon dioxide. The class of metal-dependent formate dehydrogenases comprises prokaryotic enzymes holding redox-active centres and a catalytic site, containing either molybdenum or tungsten ion, that mediates the formate/carbon dioxide interconversion. The carbon dioxide reduction is of a particular interest, since it may be a route for its atmospheric mitigation with the simultaneous production of added-value products, as formate-derived compounds. Recently, the periplasmic formate dehydrogenase from Desulfovibrio desulfuricans, a molybdenum-containing enzyme, was proven to be an efficient enzyme for the CO2 reduction to formate. In this work, the immobilized formate dehydrogenase isolated from Desulfovibrio desulfuricans direct electrochemical behaviour was attained in the presence and absence of substrates and the formal potentials associated with the catalytic centre transitions were determined in non-turnover conditions. The enzyme catalytic activity towards carbon dioxide reduction was observed using direct electrochemical methods.

New Nickel(II) Complexes with Tetradentate Schiff Bases Containing Electron-Acceptor Substituents

Complexes [Ni(3-NO2SalEn)] and [Ni(3-NO2SaltmEn)] were synthesized and identified, and polymers based on them were obtained. It was found with the help of voltammetry that the introduction of nitro groups in the complexes shifts their redox activity in the direction of positive potentials; it was shown by in situ quartz microgravimetry and in situ absorption spectroscopy methods that fragments of the polymer complexes under study can undergo reversible two-electron oxidation at the expense of formation of polarons and bipolarons in the ligand surrounding.

Aqueous zinc-organic polymer battery with a high rate performance and long lifetime

A novel redox-active polymer based on a 9,10-di(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (exTTF) system in combination with a conjugated backbone was synthesized via rhodium (Rh)-catalyzed polymerization of 2-ethynyl(exTTF), leading to polymers with low polydispersities. Composite electrodes containing this polymer exhibited chemically reversible two-electron oxidation in aqueous media. The application of these electrodes as active cathode materials in hybrid zinc-organic batteries using an aqueous electrolyte enabled the production of air-stable charge storage systems with a theoretical capacity of 133 mAh g−1. These batteries featured high performance, charge/discharge rates of up to 120 C (30 s) and an ultra-long lifetime, of over 10 000 charge/discharge cycles (accompanied by a minor capacity loss of 14%). Finally, the polymer was compared with its nonconjugated derivative, revealing the positive influence of the conjugated backbone on the material activity owing to improved electron transfer within the polymer chain. Researchers in Germany have unlocked the secret to using water as an electrolyte in rechargeable batteries with a new conjugated polymer. Polymer batteries are flexible and lightweight, but a reliance on flammable organic solvents for electrolytes has raised concerns. Ulrich S. Schubert and colleagues synthesized a poly(acetylene)-based compound containing charge-active organosulphurs to improve battery safety while maintaining high performance. Water has a narrow range of stability under applied voltages, making it tricky to use as an electrolyte. The team's polymer, however, is capable of a reversible, two-electron oxidation reaction in this range. A prototype with the polymer cathode, a zinc anode and an aqueous electrolyte revealed the possibilities of this approach — the battery could be charged to its 1-volt potential within 30 seconds and had a lifetime exceeding 10,000 recharge cycles. The application of a conjugated polymer with 9,10-di(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (exTTF) units as cathode and zinc as anode enables an aqueous-based hybrid organic battery. This energy storage device features ultra-high rate performance of up to a full charge in 30 s and an extended lifetime up to 10 000 cycles.

Anthraquinone Derivatives in Aqueous Flow Batteries

Aqueous quinone/hydroquinone couples exhibit rapid redox kinetics, require no electrocatalyst, and are potentially very inexpensive, making them attractive candidates for large-scale energy storage devices such as flow batteries [1-3]. We evaluate four anthraquinone derivatives for use as the negolyte in aqueous flow batteries. These anthraquinones are shown to undergo reversible two-electron reduction and oxidation in acidic aqueous solution, with tunable reduction potentials. When paired with a posolyte containing bromine and hydrobromic acid, three of these quinone derivatives exhibit cell voltages above 1 V. These derivatives are evaluated for chemical stability against bromination and stability upon reduction. Two anthraquinone derivatives stable under both conditions are cycled in a flow cell, showing greater than 98% current efficiency at 0.25 A/cm2cycling current. Mechanisms for capacity loss and energy inefficiency are discussed to guide further research. Figure 1. (a) Cyclic voltammetry of four anthraquinone derivatives, showing two-electron redox events at a range of potentials. (b) Open circuit potential of flow batteries employing quinone/hydroquinone negolytes and bromine/bromide posolyte. [1] B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon and M.J. Aziz, “A metal-free organic-inorganic aqueous flow battery”, Nature 505, 195 (2014), http://dx.doi.org/10.1038/nature12909[2] B. Huskinson, M.P. Marshak, M.R. Gerhardt and M.J. Aziz, “Cycling of a quinone-bromide flow battery for large-scale electrochemical energy storage”, ECS Trans. 61, 27 (2014)[3] B. Yang, L. Hoober-Burkhardt, F. Wang, G.K. Surya Prakash, and S.R. Narayanan, J. Electrochem. Soc. 161, A1371 (2014) Figure 1

Platinum-Containing Polyoxometalates: syn- and anti-[Pt(II)2(α-PW11O39)2](10-) and Formation of the Metal-Metal-Bonded di-Pt(III) Derivatives.

The first examples of dimeric, di-Pt(II) -containing heteropolytungstates are reported. The two isomeric di-platinum(II)-containing 22-tungsto-2-phosphates [anti-Pt(II)2(α-PW11O39)2](10-) (1 a) and [syn-Pt(II)2(α-PW11O39)2](10-) (2 a) were synthesized in aqueous pH 3.5 medium using one-pot procedures. Polyanions 1 a and 2 a contain a core comprising two face-on PtO4 units, with a Pt⋅⋅⋅Pt distance of 2.9-3 Å. Both polyanions were investigated by single-crystal XRD, IR, TGA, UV/Vis, (31) P NMR, ESI-MS, CID-MS/MS, electrochemistry, and DFT. On the basis of DFT and electrochemistry, we demonstrated that the {Pt2(II)} moiety in 1 a and 2 a can undergo fully reversible two-electron oxidation to {Pt2(III)}, accompanied by formation of a single Pt-Pt bond. Hence we have discovered the novel subclass of Pt(III)-containing heteropolytungstates.

Long-Term Stability of Quinone Electrolytes in Aqueous Flow Batteries

Quinone-based flow batteries are attractive candidates for large-scale electrical energy storage, as quinones can be inexpensive, exhibit rapid redox kinetics, and require no electrocatalyst [1-3]. We present the evaluation of several quinone negative electrolyte materials for use in a flow battery with a positive electrolyte containing bromine and hydrobromic acid. Several substituted quinones are shown to undergo reversible two-electron reduction and oxidation in acidic aqueous solution, with tunable reduction potentials. Some are found to be stable in the presence of bromine, while others undergo decomposition reactions. One stable substituted quinone is currently being tested in a quinone-bromide flow battery. As of writing this abstract, the cell exhibits a peak galvanic power density above 700 mW/cm2, an average round-trip energy efficiency of 70%, and current efficiency of 95% as tested to 30 constant-current charge-discharge cycles at 0.25 A/cm2. Further cycling will be reported. [1] B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon and M.J. Aziz, “A metal-free organic-inorganic aqueous flow battery”, Nature 505, 195 (2014), http://dx.doi.org/10.1038/nature12909 [2] B. Huskinson, M.P. Marshak, M.R. Gerhardt and M.J. Aziz, “Cycling of a quinone-bromide flow battery for large-scale electrochemical energy storage”, ECS Trans. 61, 27 (2014) [3] B. Yang, L. Hoober-Burkhardt, F. Wang, G.K. Surya Prakash, and S.R. Narayanan, J. Electrochem. Soc. 161, A1371 (2014). Figure 1. a. Polarization curves and b. galvanic power density as a function of current density for a substituted quinone – bromide flow battery at three different states of charge (SoC). The peak galvanic power density exceeds 0.7 W/cm2at 90% SoC. Figure 2. Cell voltage during constant current charge-discharge cycling of a substituted quinone-bromide flow battery at ± 0.25 A/cm2. Voltage cutoffs were set at 1.3 V and 0.3 V. The time window chosen shows cycles 14-24 of a 30-cycle experiment. Figure 1

Synthesis, Redox Properties and Structural Features of Expanded Isophlorins

The expanded derivatives of isophlorin provide sufficient examples for Huckel’s 4nπ anti-aromatic systems. They vary significantly in their electrochemical properties compared to the aromatic expanded porphyrins. In comparison to the small macrocycles, the large and planar expanded isophlorins exhibit reversible two-electron oxidation. The structure of these macrocycles also undergo reversible structural transformation between planar and figure of eight conformations accompanying the redox process. The oxidized product could be identified as aromatic dication or as a polaron pair depending on the size of the macrocycle. The number of non-pyrrolic heterocyclic units is crucial to stabilize aromatic, antiaromatic or neutral radical isophlorin. A stable 25π neutral radical identified in a one-pot synthesis, exhibits reversible one electron redox to form 24π anti-aromatic cation and a 26π aromatic anion. Synthesis, experimental details and structural features of expanded isophlorins will be highlighted in this presentation.

Trinuclear complexes and coordination polymers of redox-active guanidino-functionalized aromatic (GFA) compounds with a triphenylene core.

Herein, we report on the synthesis, redox activity, and coordination chemistry of 2,3,6,7,10,11-hexakis(tetramethylguanidino)triphenylene. CV measurements indicated that the new compound could be oxidized in three separate reversible two-electron oxidation events. The HOMO and LUMO energies were estimated from the oxidation wave and the onset of absorption in the UV/vis spectrum. Our discussion also includes the related new compound 2,3,6,7,10,11-hexakis(N,N'-dimethylethyleneguanidino)triphenylene. Then trinuclear Cu(I) and Cu(II) complexes of the new triphenylene ligands were characterized, and their electronic properties are discussed. In contrast to previously studied redox-active GFA ligands, oxidation of trinuclear copper(I) iodide complexes with I2 leads to copper instead of ligand oxidation. In the tetra-coordinated Cu(II) complexes, the coordination mode is intermediate between tetrahedral and square planar. The optical properties of the complexes were studied, and low-energy electronic transitions were assigned to ligand-to-metal charge-transfer (LMCT) excitations. We then extended our analysis to trinuclear Ni(II) and Co(II) complexes. The magnetic coupling mediated through the triphenylene ligand in the trinuclear Cu(II) and Co(II) complexes was studied by SQUID magnetometry, revealing ferromagnetic coupling of the spin centers and different degrees of spin delocalization into the guanidino groups. Finally, we show that the GFA ligands could be linked to one- or two-dimensional coordination polymers and porous materials with a layer structure by reaction with silver halides.

Anisyl and ferrocenyl adducts of methylenepyran-containing β-diketone: Synthesis, spectral, structural, and redox properties

Wittig reaction between the in situ generated [2,6-diphenyl-4H-pyran-4-yl phosphorane and methyl 4-formylbenzoate afforded the methyl 4-[(2,6-diphenyl-4H-pyran-4-ylidene)methyl]benzoate precursor 2 in 75% yield. The anisyl and ferrocenyl adducts of methylenepyran-containing β-diketones 3 and 4, respectively, were prepared from the base-catalyzed Claisen condensation reaction between precursor 2 and 4-methoxyacetophenone or acetylferrocene, using t-BuO−K+ as basic initiator. Both β-diketones 3 and 4 were isolated as an orange and dark-red crystalline solids in 50% yield, respectively. The acetyl derivative 6, 1-acetyl-1′-methylenepyranyl-ferrocene, was synthesized following a classical Friedel–Crafts acylation reaction of the parent ferrocenylmethylenepyran 5, and isolated in 61% yield as a red microcrystalline powder. The condensation reaction between the electron-rich acetyl complex 6 and acetyl acetate, using lithium diisopropylamide as the active base initiator, provided the ferrocenylmethylenepyran-containing β-diketone, 1-(1,3-dioxobutyl)-1′-[(2,6-diphenyl-4H-pyran-4-ylidene)methyl] ferrocene 7, isolated in 20% yield as an orange powder. The composition and identity of the new compounds 2–4, 6 and 7 are supported by elemental analysis, FT-IR, one- and two-dimensional 1H and 13C NMR spectral data and mass spectrometry. Additionally, the molecular structures of compounds 2, 4 and 6 were ascertained by single-crystal X-ray diffraction study. FT-IR and NMR spectral data indicate that both β-diketones 3 and 4 do solely exist as their keto–enol tautomeric form in solution and in the solid state, whereas a mixture of keto–enol and β-diketone tautomers in a 73:27 spectroscopic ratio was observed for 7. In the three cases, enolization takes place exclusively away from the anisyl or ferrocenyl group with formation of a six-membered pseudo-aromatic keto–enol ring, confirmed by the crystal structure of 4. Cyclic voltammetric measurements revealed that all the compounds undergo a single one-electron oxidation, localized presumably at the methylenepyran unit. This first oxidation generates a radical cation that undergoes an intermolecular C–C bond coupling to form the corresponding dimer. Anodically shifted to the first oxidation, cyclovoltammogram of 4 exhibits one reversible two-electron oxidation while that of 6 and 7 present two successive reversible one-electron oxidations, attributed in the three cases to the ferrocenyl fragment of the dimer.

7,7,8,8‐Tetraaryl‐o‐quinodimethane Stabilized by Dibenzo Annulation: A Helical π‐Electron System That Exhibits Electrochromic and Unique Chiroptical Properties

When two benzene rings are fused to a tetraaryl-o-quinodimethane skeleton, sterically hindered helical molecules 1 acquire a high thermodynamic stability. Because the tetraarylbutadiene subunit contains electron-donating alkoxy groups, 1 undergo reversible two-electron oxidation to 2(2+), which can be isolated as deeply colored stable salts. Intramolecular transfer of the point chirality (e.g., sec-butyl) on the aryl groups to helicity induces a diastereomeric preference in dications 2 b(2+) and 2 c(2+), which represents an efficient method for enhancing circular-dichroism signals. Thus, those redox pairs can serve as new electrochiroptical response systems. X-ray analysis of dication 2(2+) revealed π-π stacking interaction of the diarylmethylium moieties, which is also present in solution. The stacking geometry is the key contributor to the chirosolvatochromic response.

Electrochemical, Computational, and Photophysical Characterization of New Luminescent Dirhenium–Pyridazine Complexes Containing Bridging OR or SR Anions

A series of [Re(2)(μ-ER)(2)(CO)(6)(μ-pydz)] complexes have been synthesized (E = S, R = C(6)H(5), 2; E = O, R = C(6)F(5), 3; C(6)H(5), 4; CH(3), and 5; H, 6), starting either from [Re(CO)(5)O(3)SCF(3)] (for 2 and 4), [Re(2)(μ-OR)(3)(CO)(6)](-) (for 3 and 5), or [Re(4)(μ(3)-OH)(4)(CO)(12)] (for 6). Single-crystal diffractometric analysis showed that the two μ-phenolato derivatives (3 and 4) possess an idealized C(2) symmetry, while the μ-benzenethiolato derivative (2) is asymmetrical, because of the different conformation adopted by the phenyl groups. A combined density functional and time-dependent density functional study of the geometry and electronic structure of the complexes showed that the lowest unoccupied molecular orbital (LUMO) and LUMO+1 are the two lowest-lying π* orbitals of pyridazine, whereas the highest occupied molecular orbitals (HOMOs) are mainly constituted by the "t(2g)" set of the Re atoms, with a strong Re-(μ-E) π* character. The absorption spectra have been satisfactorily simulated, by computing the lowest singlet excitation energies. All the complexes exhibit one reversible monoelectronic reduction centered on the pyridazine ligand (ranging from -1.35 V to -1.53 V vs Fc(+)|Fc). The benzenethiolato derivative 2 exhibits one reversible two-electron oxidation (at 0.47 V), whereas the OR derivatives show two close monoelectronic oxidation peaks (ranging from 0.85 V to 1.35 V for the first peak). The thioderivative 2 exhibits a very small electrochemical energy gap (1.9 eV, vs 2.38-2.70 eV for the OR derivatives), and it does not show any photoluminescence. The complexes containing OR ligands show from moderate to poor photoluminescence, in the range of 608-708 nm, with quantum yields decreasing (ranging from 5.5% to 0.07%) and lifetimes decreasing (ranging from 550 ns to 9 ns) (3 > 4 > 6 ≈ 5) with increasing emission wavelength. The best emitting properties, which are closely comparable to those of the dichloro complex (1), are exhibited by the pentafluorophenolato derivative (3).

Air- and Heat-Stable Planar Tri-p-quinodimethane with Distinct Biradical Characteristics

A heptacyclic carbocycle possessing three p-quinodimethane units conjugated in one plane has been synthesized and shown to exhibit distinct biradical characteristics. The molecule has a HOMO/LUMO band gap of ca. 1 eV and a S(0)-T(1) energy gap of 2.12 kcal/mol, and it absorbs and emits near-IR light at room temperature. It is air-stable under ambient light for several months and thermally stable up to 160 °C under nitrogen, and it undergoes reversible two-electron oxidation and reduction. The synthetic approach is such that a smaller and larger oligo-p-quinodimethane can be synthesized.

Electrochemical and Chemical Oxidation of Dithia-, Diselena-, Ditellura-, Selenathia-, and Tellurathiamesocycles and Stability of the Oxidized Species

The diverse electrochemical and chemical oxidations of dichalcogena-mesocycles are analyzed, broadening our understanding of the chemistry of the corresponding radical cations and dications. 1,5-Diselenocane and 1,5-ditellurocane undergo reversible two-electron oxidation with inverted potentials analogous to 1,5-dithiocane. On the other hand, 1,5-selenathiocane and 1,5-tellurathiocane undergo one-electron oxidative dimerization. The X-ray crystal structures of the Se-Se dimer of the 1,5-selenathiocane one-electron oxidized product and the monomeric two-electron oxidized product (dication) of 1,5-tellurathiocane are reported. 1,5-Dithiocanes and 1,5-diselenocanes with group 14 atoms as ring members undergo irreversible oxidation, unlike the reversible two-electron oxidation of the corresponding silicon-containing 1,5-ditellurocanes. These results demonstrate the chemical consequences of the dication stabilities Te(+)-Te(+) > Se(+)-Se(+) > S(+)-S(+), as well as Se(+)-Se(+) > Se(+)-S(+) and Te(+)-Te(+) > Te(+)-S(+).