Pyridinium compounds with low reduction potentials and persistent radical states

Abstract

A series of pyridinium compounds having a low reduction potential and a highly persistent radical state are utilized as anolytes in redox flow, thin-film metal-organic hybrid and all-organic batteries.

Description

PYRIDINIUM COMPOUNDS WITH UOW REDUCTION POTENTIALS AND PERSISTENT RADICAL STATESCROSS-REFERENCE TO RELATED APPLICATION

[0001] The subject application claims priority to and the benefit of U.S. Provisional Patent Application No. 63 / 565,565, filed on March 15, 2024, the disclosure of which is hereby incorporated by reference in its entirety.FIELD OF THE DISCLOSURE

[0002] The subject application relates generally to the preparation of pyridinium compounds and their derivatives.BACKGROUND OF DISCLOSURE

[0003] Numerous pyridinium compounds have been explored as anolytes in redox flow batteries, but it has been difficult to achieve all of the desirable characteristics in a single system, namely a low reduction potential, high solubility, and an extremely stable radical state. Recent work (Christo S. Sevov, David P. Hickey, Monique E. Cook, Sophia G. Robinson, Shoshanna Barnett, Shelley D. Minteer, Matthew S. Sigman, and Melanie S. Sanford, Journal of the American Chemical Society 2017 139 (8), 2924-2927 DOI: 10.1021 / jacs.7b00147) has shown that predictive modeling can be used to reveal important structure / property relationships, thus generating new synthetic targets. This publication noted that certain pyridinium compounds would likely possess particularly attractive properties; however, such materials proved virtually impossible to synthesize by conventional routes.

[0004] The subject application seeks to address these issues by identifying new pyridinium compounds and associated methods for forming same.SUMMARY OF THE DISCLOSURE

[0005] The subject application provides pyridinium compounds having low reduction potentials (-1.57 to -2.00 V vs Fc / Fc+) and highly persistent radical states. The subject application also provides associated high yield, low-cost routes for synthesis of these compounds.

[0006] In certain embodiments, the pyridinium compounds are according to formula (4):

[0007] In formula (4), Ri and Rs are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, nitrile and nitro groups. Also, R2, R3 and R4 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, alkoxy, haloalkyl, aralkyl, nitro, nitrile, halide, acetyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups. Still further, Reand Rio are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, nitrile and nitro groups. In addition, R7, Rs, and R9 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, aryl, substituted aryl, aralkyl, nitro, nitrile, acetyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups. Still further, Rn and R14 are the same or different and are independently selected from an unbranched alkyl, a branched alkyl, haloalkyl, perhaloalkyl, alkoxy, aralkyl, tri alkyl am monium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups. Still further, R12 and R13 are the same or different and are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, haloalkyl, perhaloalkyl, alkoxy, aralkyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups.

[0008] In certain of these embodiments, Ri and R5 in formula (4) are hydrogen.

[0009] In certain of these embodiments, Ri, R5, R12 and R13 in formula (4) are hydrogen, and Rn and R14 in formula (4) are the same or different and are independently selected from a branched alkyl group and an unbranched alkyl group.

[0010] In certain of these embodiments, Ru and R14 in formula (4) are z-Pr and wherein at least one of Re and Rio in formula (4) is a branched alkyl group, an unbranched alkyl group or an alkoxy group.

[0011] In certain of these embodiments, Rn and R14 in formula (4) are z-Pr and wherein at least one of Re and Rio in formula (4) is a branched alkyl group or an unbranched alkyl group.

[0012] In certain of these embodiments, at least one of R2, R3 and R4 in formula (4) is a branched alkyl group, an unbranched alkyl group or an alkoxy group.

[0013] In certain embodiments, the pyridinium compounds of formula (4) are pyridinium compounds according to formula (1A):

[0014] In formula (IA), Ri and Rs are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, nitrile and nitro groups. Also, R2, R3 and R4 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, alkoxy, haloalkyl, aralkyl, nitro, nitrile, halide, acetyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups. Still further, Re and Rio are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, nitrile and nitro groups. In addition, R7, Rs, and R9 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, aryl, substituted aryl, aralkyl, nitro, nitrile, acetyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups.

[0015] In certain of these embodiments, Ri and Rs in formula (1A) are hydrogen.

[0016] In certain other embodiments, the pyridinium compounds of formula (4) are pyridinium compounds according to formula (IB):

[0017] In formula (IB), Ri and Rs are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, nitrile and nitro groups. Also, R2, R3 and R4 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, alkoxy, haloalkyl, aralkyl, nitro, nitrile, halide, acetyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups. Still further, Re and Rio are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, nitrile and nitro groups. In addition, R7, Rs, and R9 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, aryl, substituted aryl, aralkyl, nitro, nitrile, acetyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups.

[0018] In certain of these embodiments, Ri and Rs in formula (IB) are hydrogen.

[0019] In certain further embodiments, the pyridinium compounds of formula (4), which may or may not encompass one or more of formulas (1A) or (IB), are selected from pyridinium compounds (1C)-(1Q) below as follows:

[0020] In certain other embodiments, the pyridinium compounds are according to formula (2):

[0021] In formula (2), Ri and Rs are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, nitrile and nitro groups. Also, R2, R3 and R4 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, alkoxy, haloalkyl, aralkyl, nitro, nitrile, halide, acetyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups. Still further, R’ is selected from an unbranched alkyl, a branched alkyl, haloalkyl, aralkyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups.

[0022] In certain of these embodiments, Ri and Rs in formula (2) are hydrogen.

[0023] In still further embodiments, the pyridinium compounds are according to formula (3 A) and (3B):

[0024] In formulas (3 A) and (3B), “Ar” is any aryl group.

[0025] Several of these pyridinium compounds corresponding to any one or more of formulas (1A)-(1Q), (2), (3A), (3B), or (4) of the subject application are found to display excellent solubility in polar organic solvents.

[0026] In addition, the compounds according to any one or more of formulas (1A)-(1Q), (2), (3A), (3B), or (4) of the subject application undergo chemically and electrochemicallyreversible one-electron reduction at low potentials (ca. (about) -1.57 to -2.00 V vs. Fc / Fc+). Electrode kinetics are favorable in all cases. Moreover, the compounds are extremely soluble in polar organic solvents (over IM in acetonitrile for several derivatives) and display remarkably high diffusion coefficients.

[0027] Other features and advantages of the subject application will be readily appreciated, as the same becomes better understood, after reading the subsequent description taken in conjunction with the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The advantages of the subject application will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing. It is to be understood that the drawings are purely illustrative and not necessarily drawn to scale.

[0029] Figure 1 is a schematic illustration of redox flow battery according to one nonlimiting embodiment of the subject application.DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0030] The subject application provides pyridinium compounds having desired low reduction potentials (i.e., from about -1.57 to -2.00 V vs Fc / Fc+) and highly persistent radical states and associated high yield, low-cost route to synthesis of these compounds.

[0031] As used herein, the term “ca.” or “about”, as it relates to reduction potentials or percentage yields, allows for a variability of a number or range of numbers as described herein of 5%.

[0032] Still further, the description of an “R” group in any of the representative formulas may be described below with or without the term group but are intended to be equivalent. Byway of one non-limiting example, in a description below where a particular R group is being described as “... selected from methyl”, it is understood that the term “methyl” can be used interchangeably with the phrase “methyl group” and is meant to refer to a CH3 group that is positioned in the position located by R in the respective formula. The same holds true for any groups described in any one or more of formulas (1A)-(1Q), (2), (3 A), (3B), or (4) described below.

[0033] The pyridinium compounds are ideally suited for use as anolytes in redox flow batteries owing to the afore-mentioned low reduction potentials, high solubility, and highly persistent radical states.

[0034] In still further embodiments, the pyridinium compounds are according to formula (4):

[0035] In formula (4), Ri and Rs are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, nitrile and nitro groups. Also, R2, R3 and R4 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, alkoxy, haloalkyl, aralkyl, nitro, nitrile, halide, acetyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups. Still further, Re and Rio are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, nitrile and nitro groups. In addition, R7, Rs, and R9 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, aryl, substituted aryl, aralkyl, nitro, nitrile, acetyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups. Still further, Rn and R14 are the same or different and are independently selected from an unbranched alkyl, a branched alkyl, haloalkyl, perhaloalkyl, alkoxy, aralkyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups. Still further, R12 and Rn are the same or different and are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, haloalkyl, perhaloalkyl, alkoxy, aralkyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups.

[0036] In certain of these embodiments, Ri and R5 in formula (4) are hydrogen.

[0037] In certain of these embodiments, Ri, R5, R12 and R13 in formula (4) are hydrogen, and Rn and R14 in formula (4) are the same or different and are independently selected from a branched alkyl group and an unbranched alkyl group.

[0038] In certain of these embodiments, Rn and R14 in formula (4) are z'-Pr and wherein at least one of Re and Rio in formula (4) is an alkyl group or an alkoxy group.

[0039] In certain of these embodiments, Rn and R14 in formula (4) are z-Pr and wherein at least one of Re and Rio in formula (4) is an alkyl group.

[0040] In certain of these embodiments, at least one of R2, R3 and R4 in formula (4) is a branched alkyl group, an unbranched alkyl group or an alkoxy group.

[0041] In certain embodiments, the pyridinium compounds of formula (4) are pyridinium compounds according to formula (1A):

[0042] In formula (IA), Ri and R5 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, nitrile and nitro groups. Also, R2, R3 and R4 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, alkoxy, haloalkyl, aralkyl, nitro, nitrile, halide, acetyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups. Still further, Re and Rio are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, nitrile and nitro groups. In addition, R7, Rs, and R9 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl,perhaloalkyl, alkoxy, aryl, substituted aryl, aralkyl, nitro, nitrile, acetyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups.

[0043] In certain of these embodiments, Ri and R5 in formula (1A) are hydrogen.

[0044] In certain other embodiments, the pyridinium compounds of formula (4) are pyridinium compounds according to formula (IB):

[0045] In formula (IB), Ri and Rs are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, nitrile and nitro groups. Also, R2, R3 and R4 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, alkoxy, haloalkyl, aralkyl, nitro, nitrile, halide, acetyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups. Still further, Re and Rio are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, nitrile and nitro groups. In addition, R7, Rs, and R9 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl,perhaloalkyl, alkoxy, aryl, substituted aryl, aralkyl, nitro, nitrile, acetyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups.

[0046] In certain of these embodiments, Ri and R5 in formula (IB) are hydrogen.

[0047] In certain further embodiments, the pyridinium compounds of formula (4), which may or may not encompass one or more of formulas (1A) or (IB), are selected from pyridinium compounds (1C)-(1Q) below as follows:

[0048] In certain other embodiments, the pyridinium compounds are according to formula (2):

[0049] In formula (2), Ri and Rs are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, halide, haloalkyl, perhaloalkyl, alkoxy, nitrile and nitro groups. Also, R2, R3 and R4 are independently selected from hydrogen, an unbranched alkyl, a branched alkyl, alkoxy, haloalkyl, aralkyl, nitro, nitrile, halide, acetyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups. Still further, R’ is selected from an unbranched alkyl, a branched alkyl, haloalkyl, aralkyl, trialkylammonium alkyl, alkyl carboxylate, alkyl sulfonate, alkyl phosphonate, and alkyl ether groups.

[0050] In certain of these embodiments, Ri and Rs in formula (2) are hydrogen.

[0051] In still further embodiments, the pyridinium compounds are according to formula(3 A) and (3B):

[0052] In formulas (3 A) and (3B), “Ar” is any aryl group.

[0053] The subject application also discloses synthesis routes for forming some the pyridinium compounds according to formulas (1A)-(1Q), (2), (3A), (3B), or (4) described above.

[0054] As noted above, the pyridinium compounds in accordance with the subject application (including formulas (1A)-(1Q), (2), (3A), (3B), or (4) described above) are suitable for use as an anolyte in a redox flow battery.

[0055] Figure 1 illustrates a redox flow battery 10 according to an embodiment of the subject application. The redox flow battery 10 may be described as an energy storage device that utilizes redox (reduction and oxidation) reactions to generate energy, which is stored in electrolyte solutions flowing through the battery 10. During discharge of the battery 10, electrons are released during an oxidation reaction on the negative (or anode) side of the battery 10. The electrons move through an external circuit to do useful work and are thereafter accepted during a reduction reaction at the positive (or cathode) side of the battery 10. The direction of the current and the chemical reactions are reversed during charging. The energy produced by the redox flow battery 10 is often used for grid storage applications. It is to be appreciated, however, that the redox flow battery 10 can be scaled to fit the needs of any suitable application.

[0056] Turning to Figure 1, the redox flow battery 10 has an electrochemical cell 12 including a positive (cathode) side 14 and a negative (anode) side 16. The positive side 14 has a first receptacle 18 containing a charge carrying electrolyte and the oxidized form of an electroactive material (herein referred to as an active material), and the negative side 16 has a second receptacle 20 containing the charge carrying electrolyte and the reduced form of an electroactive material. The positive side 14 further includes a cathode 22, and the negative side further has an anode 24. The net charge in each receptacle is zero. Any positively charged species are balanced by a negatively charged species, and vice versa.

[0057] During charging and discharging of the redox flow battery 10, the charge-carrying electrolyte on the positive side 14 circulates from the first receptacle 18 and through the cathode 22 by a first pump 26. The charge-carrying electrolyte on the negative side 16 circulates from the second receptacle 20 and through the anode 24 by a second pump 28. The cathode 22 and the anode 24 may be electrically connected through current collectors with an external load 30.As the electrolyte passes through the cathode 22 and the anode 24, the electroactive material reacts (via redox reaction(s)) to generate energy.

[0058] The cathode 22 may be one or a pair of electrodes or an array of electrodes. The anode 24 may be one or a pair of electrodes or an array of electrodes. The cathode 22 and the anode 24 are not particularly limited and may be any known in the art. In a non-limiting example, one or more of the cathode 22 and the anode 24 is a carbon-based electrode, a metalbased electrode, and combinations thereof. Non-limiting examples of carbon-based electrodes include electrodes made or formed from porous carbon (e.g., carbon felt, carbon paper and graphite felt), carbon nanotubes, carbon nanowires, graphene, and / or the like, and / or combinations thereof. Non-limiting examples of metal-based electrodes include electrodes made or formed from gold, steel, nickel, platinum-coated gold, platinum-coated carbon, and / or the like, and / or combinations thereof. In another non-limiting example, the cathode 22 and / or the anode 24 is porous. The cathode 22 and / or anode 24 may further include additives, such as carbon black, flake graphite, and / or the like. Each of the cathode 22 and the anode 24 may be in any convenient form, including foils, plates, rods, screens, pastes, or as a composite made by forming a coating of the electrode material on a conductive current collector or other suitable support.

[0059] The charge-carrying electrolyte includes a charge-carrying medium (i.e., a solvent or gel, an electrolyte salt and one or more redox-active materials (i.e., the pyridinium compounds according to any one of formulas (1A)-(1Q), (2), (3A), (3B), or (4) and ions. The chargecarrying medium may be one or more liquids and / or gels. In addition, the charge-carrying medium may be used over a wide temperature range, for example, from about -30°C to about 70°C without freezing or boiling and is typically stable in the electrochemical window within which the cathode 22 and the anode 24 operate.

[0060] The charge-carrying medium, in certain embodiments, is present in an amount of from 10% to 100% by weight, such as from 40% to 99% by weight, such as from 60 to 99% by weight, such as from 65% to 95% by weight, or such as from 70% to 90% by weight, each based on a total weight of the charge-carrying electrolyte. All values and ranges of values within those values described above are hereby expressly contemplated in various non-limiting embodiments.

[0061] While the pyridinium compounds according to formulas (1A)-(1Q), (2), (3A), (3B), or (4) are suitable for use in redox flow batteries, as described above, potential application of these materials may extend beyond their use in redox flow batteries. Possible additional applications may include, but are not limited to, as redox catalysts, as chemical reductants (from the reduced form), as additives for conventional batteries, or in molecular electronics.

[0062] In certain exemplary embodiments, the pyridinium compound according to any one of formulas (1A)-(1Q), (2), (3A), (3B), or (4) described above can also be linked to an oxidizable moiety for subsequent used in the systems described above. Exemplary oxidizable moieties include but are not limited to ferrocene, carbazole, phenazine, phenoxazine, phenothiazine, phenothiazine-5-oxide, phenothiazine-5, 5-dioxide, benzoquinone, TEMPO, and / or cyclopropenium.

[0063] In another embodiment, the redox-active pyridinium compounds described herein may be attached to or be a component of a polymer backbone. Such redox active polymers can then be employed as the soluble anolyte in a redox flow battery, where their large size and high charge serves to decrease crossover.

[0064] In yet another embodiment, any one of pyridinium compounds according to any one of formulas (1A)-(1Q), (2), (3A), (3B), or (4) described above can also be used in metalorganic hybrid or all-organic thin fdm ( / .e., “jelly-roll”) batteries. Such batteries take advantageof established low-cost manufacturing methods such as roll-to-roll processing and additive methods but utilize redox-active organic compounds or polymers as the active material at one or both electrodes. In these batteries, the redox-active pyridinium compounds may be attached to or be a component of a polymer backbone. Alternatively, the pyridinium compounds according to any one of formulas (1A)-(1Q), (2), (3A), (3B), or (4), or polymers incorporating them may be covalently attached to an electrode such as a metal or carbon, carbon surface. In a non-limiting example, the electrode is a carbon-based electrode, a metal-based electrode, and combinations thereof. Non-limiting examples of carbon-based electrodes include electrodes made or formed from porous carbon (e.g., carbon felt, carbon paper and graphite felt), carbon nanotubes, carbon nanowires, graphene, and / or the like, and / or combinations thereof. Non-limiting examples of metal-based electrodes include electrodes made or formed from gold, steel, nickel, platinum- coated gold, platinum-coated carbon, and / or the like, and / or combinations thereof. The electrode may further include additives, such as carbon black, flake graphite, and / or the like. The electrode may be in any convenient form, including foils, plates, rods, screens, pastes, or as a composite made by forming a coating of the electrode material on a conductive current collector or other suitable support. Such batteries also contain a charge-carrying electrolyte. The chargecarrying electrolyte includes a charge-carrying medium (i.e., a solvent or gel, and an electrolyte salt). It may also include other electroactive materials, stabilizing agents, or other additives to improve battery performance and / or service life. The charge-carrying medium may be one or more liquids and / or gels. In addition, the charge-carrying medium may be used over a wide temperature range, for example, from about -30°C to about 70°C without freezing or boiling and is typically stable in the electrochemical window within which the battery operates.

[0065] See, for example, “High-Power-Density Organic Radical Batteries”, C. Friebe and U.S. Schubert, Top Curr Chem (Z) (2017) 375: 19, “Sustainable Energy Storage: Recent Trends and Developments toward Fully Organic Batteries”, C. Friebe, A. Lex-Balducci, and U.S. Schubert, ChemSusChem (2019), 12, 4093- 4115, or “Organic Batteries Based on Just Redox Polymers”, N. Goujon, N. Casada, N. Patil, R. Marcilia, and D. Mecerreyes, Prog. Polmer Sci (2021), 122, 101449.

[0066] The pyridinium compounds according to any one of formulas (1A)-(1Q), (2), (3 A), (3B), or (4) of the subject disclosure undergo chemically and electrochemically reversible one-electron reduction at low potentials (ca. (about) -1.57 to -2.00 vs. Fc / Fc+). Electrode kinetics are favorable in all cases. Moreover, the pyridinium compounds according to any one of formulas (1A)-(1Q), (2), (3A), (3B), or (4) of the subject application are extremely soluble in polar organic solvents (over IM in acetonitrile for several derivatives) and display remarkably high diffusion coefficients.

[0067] Portions of the subject application can be exemplified or otherwise prepared and characterized as described in the Examples below.EXAMPLESPreparation of 2.3.5.6-tetrainethyl-4 / / -pyran-4-one

[0068] 200g (2.04 mol) polyphosphoric acid (PPA) and 125ml (2.0 mol) acetic acid were added to a IL round bottom flask. After 45 minutes of manual shaking, the mixture was homogenous. 10.0g 3-pentanone (0.116 mol) was added and the resultant mixture was heated to 90 °C with stirring. A reflux condenser was attached, and the mixture was stirred for 20 hours. 500ml of a water / ice mixture was added and the mixture was stirred for an additional 30 minutes. The reaction mixture was extracted with ethyl acetate (3x300ml). The ethyl acetate layer was washed with 10% KOH (750ml) until the ethyl acetate layer was no longer acidic as determined by a pH paper. The ethyl acetate layer was then washed with saturated NaCl (200ml) and dried over MgSO4. Solvent was removed under vacuum and 5.7g of a dark oil was obtained. The dark oil was chromatographed on silica using 0-30% ethyl acetate in hexanes to get 2.6g (14.8% yield) of an orange solid. 1H NMR (nuclear magnetic resonance) confirmed the desired product.Preparation of 4-phenyl-2,3,5,6-tetramethylpyryIium tetrafluoroborate

[0069] An oven dried 50 milliliter round bottom flask was charged with a magnetic stir bar and 2,3,5,6-tetramethyl-477-pyran-4-one (0.5g, 0.0033 mol) was dissolved in dry tetrahydrofuran (THF, 10ml) while under nitrogen. At room temperature, phenyllithium as a 1.9M solution in dibutylether (2.1ml, 0.0039 mol) was added to the solution dropwise via syringe. The resultant solution was stirred at room temperature for 3 hours. Boron trifluoride diethyl etherate (1.22ml, 0.0099 mol) was added to the crude solution via syringe to yield a tanprecipitate which was filtered and washed with diethyl ether. A quantitative yield of product was realized. MS (mass spectrometry) and 1H NMR confirm desired product.Preparation of N-(2,4,6 trimethylphenyl)-4-phenyl-2,3,5,6-tetramethylpyridinium tetrafluoroborate

[0070] In a 100 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-phenyl-2,3,5,6-tetramethylpyrylium tetrafluoroborate (0.50g, 0.0017 mol) was dissolved in ethanol (20ml). 2,4,6 trimethylaniline (0.27g, 0.002 mol) was added, and the mixture was heated to reflux for 3 hours while under nitrogen. The solution was cooled to room temperature and solvent was removed on a rotary evaporator to yield a brown paste. The mixture was diluted with diethyl ether, and a precipitate formed was isolated by filtration, washed with diethyl ether, and dried under vacuum to afford a tan solid (0.57g, 0.0013 mol) as the product in82% yield. MS and 1H NMR confirm desired product.Preparation of 4-(p-tolyl)-2,3,5,6-tetramethylpyrylium tetrafluoroborate

[0071] An oven dried 50 milliliter round bottom flask was charged with a magnetic stir bar and / i-bromotoluene (0.68g, 0.0039 mol) was dissolved in dry diethyl ether while under nitrogen. At 0 °C / / -BuLi as a 2.5M solution in hexane (1.58ml, 0.0039 mol) was added to the solution dropwise via a syringe. The resultant solution was stirred at 0 °C for 45 minutes. 2, 3,5,6- tetramethyl-4H-pyran-4-one (0.5g, 0.0033 mol) was dissolved in dry tetrahydrofuran (THF, 10ml) and added dropwise. The reaction was warmed to room temperature and stirred for 2 hours. Boron trifluoride diethyl etherate (1.22ml, 0.0099 mol) was added to the crude solution via syringe to yield a dark precipitate which was filtered and washed with diethyl ether to afford a tan solid (0.75g, 0.0024mol) as the product in a 72% yield. MS and 1H NMR confirm desired product.Preparation of N-(2,4,6 trimethylphenyl)-4-(p-tolyl)— 2,3,5, 6-tetramethylpyridinium tetrafluoroborate

[0072] In a 50 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-( / ?-tolyl)-2,3,5,6-tetramethylpyrylium tetrafluoroborate (0.35g, 0.0011 mol) was dissolved in ethanol (15ml). 2,4,6 trimethylaniline (0.27g, 0.0013 mol) was added, and the mixture was heated to reflux 3 hours while under nitrogen. The solution was cooled to room temperature and solvent was removed on a rotary evaporator to yield a brown oil. The mixture was diluted with diethyl ether, and a precipitate formed that was isolated by fdtration, washedwith diethyl ether and dried under vacuum to afford a tan solid (0.39g, 0.0009 mol) as the product in 82% yield. MS and 1H NMR confirm desired product.Preparation of 4-(4-methoxyphenyl)-2,3,5,6-tetramethylpyrylium tetrafluoroborate

[0073] An oven dried 50 milliliter round bottom flask was charged with a magnetic stir bar and / ?-bromoanisole (0.69g, 0.0039 mol) was dissolved in dry diethyl ether while under nitrogen. At 0 °C n-BuLi as a 2.5M solution in hexane (1.58ml, 0.0039 mol) was added to the solution dropwise via syringe. The resultant solution was allowed stirred at 0 °C for 45 minutes. 2,3,5,6-tetramethyl-4Z / -pyran-4-one (0.5g, 0.0033 mol) was dissolved in dry tetrahydrofuran (THF, 10ml) and added dropwise. The reaction mixture warmed to room temperature and stirred for 2 hours. Boron trifluoride diethyl etherate (1.22ml, 0.0099 mol) was added to the crude solution via syringe to yield a dark oil. MS confirmed the desired product was present. 1H NMR confirmed the desired product was present, but impure. Yield near quantitative.Preparation of N-(2,4,6 trimethylphenyl)-4-(4-methoxyphenyl)— 2,3,5,6- tetramethylpyridinium tetrafluoroborate

[0074] In a 100 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-(4-methoxyphenyl)-2,3,5,6-tetramethylpyrylium tetrafluoroborate (1.0g, 0.0030 mol) was dissolved in ethanol (40ml). 2,4,6 trimethylaniline (0.49g, 0.0036 mol) was added, and the mixture was heated to reflux 2 hours while under nitrogen. The solution was cooled to room temperature and stirred overnight. Solvent was then removed on a rotary evaporator to yield a brown oil. The mixture was chromatographed on silica to get a 10% yield. MS and 1H NMR confirmed the desired product.Preparation of 4-(2,4-dimethylphenyl)-2,3,5,6-tetramethylpyrylium tetrafluoroborate

[0075] An oven dried 50 milliliter round bottom flask was charged with a magnetic stir bar and 2,4-dimethylbromobenzene (0.73g, 0.0039 mol) was dissolved in dry diethyl ether whileunder nitrogen. At 0 °C n-BuLi as a 2.5M solution in hexane (1 ,58ml, 0.0039 mol) was added to the solution dropwise via syringe. The resultant solution was allowed stirred at 0 °C for 45 minutes. 2,3,5,6-tetramethyl-4 / 7-pyran-4-one (0.5g, 0.0033 mol) was dissolved in dry tetrahydrofuran (THF, 10ml) and added dropwise. The reaction was warmed to room temperature and stirred for 2 hours. The reaction was then heated to reflux for 1.5 hours and cooled to room temperature. Boron trifluoride diethyl etherate (1.22ml, 0.0099 mol) was added to the crude solution via syringe to yield a dark oil. MS confirmed the desired product was present. 1H NMR confirmed the desired product was present, but impure, 0.66g, 61% crude yield.Preparation of N-(2,4,6 trimethylphenyl)-4-(2,4-dimethylphenyl)— 2,3,5,6- tetramethylpyridinium tetrafluoroborate

[0076] In a 100 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-(2,4-dimethylphenyl)-2,3,5,6-tetramethylpyrylium tetrafluoroborate (0.66g, 0.0020 mol) was dissolved in ethanol (40ml). 2,4,6 trimethylaniline (0.32g, 0.0024 mol) was added, and the mixture was heated to reflux for 4 hours while under nitrogen. The solution was cooled to room temperature and stirred overnight. Solvent was then removed on a rotary evaporator to yield a brown oil. The mixture was chromatographed on silica to get a 5% yield. MS and 1H NMR confirmed the desired product.Preparation of N-(butyl)-4-(^-tolyl)--2,3>5,6-tetramethylpyridinium tetrafluoroborate

[0077] In a 50 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-(p-tolyl)-2,3,5,6-tetramethylpyrylium tetrafluoroborate (0.35g, 0.0011 mol) was dissolved in ethanol (15ml). 1-butlyamine (0.097g, 0.0013 mol) was added, and the mixture was heated to reflux for 3 hours while under nitrogen. The solution was cooled to room temperature and solvent was removed on a rotary evaporator to yield a golden oil. The oil was chromatographed on silica to get 0.30g pale yellow oil, 73% yield. MS and 1H NMR confirmed the desired product.

[0078] Each of the structures formed as examples above was evaluated for reduction potential (vs. Fc / Fc+) with the results shown in Table 1 below:Table 1Preparation of 2,6-diethyl-y-pyrone

[0079] In a 500 milliliter round bottom flask, propionic anhydride (170 milliliters, 1.3 moles) and sulfuric acid (2 milliliters, 0.03 moles) were added. Acetone-1, 3-dicarboxylic acid (60 grams, 1.0 moles) was added rapidly, and the flask was placed in a 100 °C oil bath for 30 minutes with occasional swirling. The solution was rapidly cooled in an ice-salt bath and a dark red solid formed. The solid was added to a 1000 milliliter Erlenmeyer flask with 500 milliliters of cold DI water, slurried, and filtered. The solid was added to a 1000 milliliter Erlenmeyer flask with 400 milliliters of cold DI water, slurried and filtered. The product, 4,6-dihydroxy-3,5- dipropionyl-2-pyrone (36.5 grams), was air dried overnight.

[0080] To a 1000 milliliter Erlenmeyer flask charged with a stir bar, 250 milliliters of hot DI water and 4,6-dihydroxy-3,5-dipropionyl-2-pyrone (36.5 grams, 0.153 moles) was added and stirred to suspend. Aqueous sodium carbonate (160.5 milliliters, 0.16 moles) was added slowly. Carbon dioxide (CO2) was evolved. The solution was stirred at 100 °C for 30 minutes and then at 85 °C for 80 minutes. The solution was cooled to room temperature and 30% aqueous acetic acid was added until no more CO2 evolved. A white-yellow solid precipitated. The solid was filtered and washed with DI water. The product, 6-ethyl-4-hydroxy-3-propionyl-2-pyrone (23 grams), was air dried overnight.

[0081] In a 250 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 6-ethyl-4-hydroxy-3-propionyl-2-pyrone (23 grams, 0.11 moles) and concentrated hydrochloric acid (HCL, 70 milliliter) were added and then mixture was brought to reflux at a refluxing temperature for 4 hours. The solution was cooled in an ice bath. Solid sodium carbonate was added to the mixture until the solution was a neutral pH. The neutral solution was added to a separatory funnel with dichloromethane to extract the solution. The solvent was stripped to afford a brown oil (16.18 grams) as the product. Column chromatography on silica gel was used to purify the product.Preparation of 4-(p-tolyl)-2,6-diethylpyrylium tetrafluoroborate

[0082] An oven dried 250 milliliter round bottom flask was charged with a magnetic stir bar and 2, 6-diethyl-y -pyrone (2.0 grams, 0.013 moles) was dissolved in tetrahydrofuran (THF, 40 milliliters) while under nitrogen. The solution was cooled via an ice bath and p- tolylmagnesium bromide as a 1 molar solution in THF (13.1 milliliters, 0.013 moles) was added to the solution dropwise via syringe. The resultant crude solution was allowed to warm to room temperature for over an hour. Boron trifluoride diethyl etherate (BF4, 4.84 milliliters, 0.039 moles) was added to the crude solution via syringe to yield a tan precipitate which was fdtered and washed with diethyl ether. After recrystallizing in acetonitrile and diethyl ether, the product was isolated as a tan solid (2.3 grams, 0.007 moles) in 56% yield.Preparation of N-phenyl-4-(^-tolyl)-2,6-diethylpyridinium tetrafluoroborate

[0083] In a 100 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-(p-tolyl)-2,6-diethylpyrylium tetrafluoroborate (0.45 grams, 0.0014 moles) was dissolved in ethanol (40 milliliters). Aniline (0.15 grams, 0.0016 moles) was added, and the mixture was refluxed at a refluxing temperature for 3 hours while under nitrogen. The solutionwas cooled to room temperature and a precipitate formed. The mixture was diluted with diethyl ether, and the precipitate was isolated by filtration and dried under vacuum to afford a light pink solid (0.44 grams, 0.0011 moles) as the product in 81% yield.Preparation of N-(2,6-xyxyl)-4-(p-tolyl)-2,6-diethylpyridinium tetrafluoroborate

[0084] In a 100 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-(p-tolyl)-2,6-diethylpyrylium tetrafluoroborate (0.45 grams, 0.0014 moles) was dissolved in ethanol (40 milliliters). 2,6-dimethylaniline (0.19 grams, 0.0016 moles) was added, and the mixture was refluxed at a refluxing temperature for 3 hours while under nitrogen. The solution was cooled to room temperature and the solvent was stripped. Diethyl ether was added to the flask and a solid was isolated by filtration and dried under vacuum to afford an off-white solid (0.42 grams, 0.0010 moles) as the product in 74% yield.Preparation of N-(mesityl)-4-(p-tolyl)-2,6-diethylpyridinium tetrafluoroborate

[0085] In a 100 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-(p-tolyl)-2,6-diethylpyrylium tetrafluoroborate (0.45 grams, 0.0014 moles) was dissolved in ethanol (40 milliliters). 2,4,6-trimethylaniline (0.21 grams, 0.0016 moles) was added, and the mixture was refluxed at a refluxing temperature for 3 hours while under nitrogen. The solution was cooled to room temperature and the solvent was stripped. Diethyl ether was added to the flask and a solid was isolated by filtration and dried under vacuum to afford a light tan solid (0.49 grams, 0.0010 moles) as the product in 81% yield.Preparation of N-(2,6-diethyl)-4-(p-tolyl)-2,6-diethylpyridinium tetrafluoroborate

[0086] In a 100 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-(p-tolyl)-2,6-diethylpyiylium tetrafluoroborate (0.45 grams, 0.0014 moles) was dissolved in ethanol (40 milliliters). 2,6-diethylaniline (0.24 grams, 0.0016 moles) was added, and the mixture was refluxed at a refluxing temperature for 3 hours while under nitrogen. The solution was cooled to room temperature and the solvent was stripped. Diethyl ether was added to the flask and a solid was isolated by filtration and dried under vacuum to afford a light green solid (0.48 grams, 0.0010 moles) as the product in 78% yield.Preparation of N-(2,6-diethyl-4-methyl)-4-(p-tolyl)-2,6-diethylpyridinium tetrafluoroborate

[0087] In a 100 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-(p-tolyl)-2,6-diethylpyrylium tetrafluoroborate (0.35 grams, 0.0011 moles) was dissolved in ethanol (40 milliliters). 2,6-diethyl-4-methylaniline (0.21 grams, 0.0013 moles) was added, and the mixture was refluxed at a refluxing temperature for 3 hours while under nitrogen. The solution was cooled to room temperature and the solvent was stripped. Diethyl ether was added to the flask and a solid was isolated by filtration and dried under vacuum to afford an off- white solid (0.40 grams, 0.0008 moles) as the product in 81% yield.Preparation of 4-phenyl-2,6-diethylpyrylium tetrafluoroborate

[0088] An oven dried 250 milliliter round bottom flask was charged with a magnetic stir bar and 2,6-diethyl-y-pyrone (1.4 grams, 0.0091 moles) was dissolved in diethyl ether (30 mb) while under nitrogen. Phenyllithium as a 1.9 molar solution in diethyl ether (5.1 milliliters, 0.0101 moles) was added to the solution dropwise via syringe over 7 minutes. A precipitate formed and the resultant crude mixture was stirred for 2 hours. Boron trifluoride diethyl ether(BF4, 3.43 milliliters, 0.0273 moles) was added to the crude mixture via syringe to afford an orange precipitate which was filtered and washed with diethyl ether. The product was isolated as an orange solid (3.4 grams, 0.011 moles) in greater than 100% yield.Preparation of 2,6-Diethyl-l,4-diphenylpyridinium tetrafluoroborate

[0089] In a 100 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-phenyl-2,6-diethylpyrylium tetrafluoroborate (0.65 grams, 0.0021 moles) was dissolved in ethanol (40 milliliters). Aniline (0.22 grams, 0.0023 moles) was added, and the mixture was refluxed at a refluxing temperature for 4.5 hours while under nitrogen. The solution was cooled to room temperature overnight and diluted with diethyl ether to precipitate the product. The product was isolated by filtration and dried under vacuum to afford a white crystalline solid (0.408 grams, 0.0010 moles) as the product in 52% yield.Preparation of N-(2,6-xyIyl)-4-phenyl-2,6-diethylpyridinium tetrafluoroborate

[0090] In a 100 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-phenyl-2,6-diethylpyrylium tetrafluoroborate (0.65 grams, 0.0021 moles) was dissolved in ethanol (40 milliliters). 2,6-dimethylniline (0.27 grams, 0.0023 moles) was added, and the mixture was refluxed at a refluxing temperature for 4.5 hours while under nitrogen. The solution was cooled to room temperature overnight and diluted with diethyl ether to precipitate the product. The solid was isolated by filtration and dried under vacuum to afford a white powder (0.47 grams, 0.0011 moles) as the product in 56% yield.Preparation of N-(mesityl)-4-phenyl-2,6-diethylpyridinium tetrafluoroborate

[0091] In a 100 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-phenyl-2,6-diethylpyrylium tetrafluoroborate (0.65 grams, 0.0021 moles) was dissolved in ethanol (40 milliliters). 2,4,6-trimethylniline (0.31 grams, 0.0023 moles) was added, and the mixture was refluxed at a refluxing temperature for 4 hours while under nitrogen. The solution was cooled to room temperature overnight and diluted with diethyl ether to precipitate the product. The solid was isolated by filtration and dried under vacuum to afford a white powder (0.39 grams, 0.0009 moles) as the product in 45% yield.Preparation of N-(2,6-diethyl)-4-phenyl-2,6-diethylpyridinium tetrafluoroborate

[0092] In a 100 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-phenyl-2,6-diethylpyrylium tetrafluoroborate (0.65 grams, 0.0021 moles) was dissolved in ethanol (40 milliliters). 2,6-diethylaniline (0.34 grams, 0.0023 moles) was added, and the mixture was refluxed at a refluxing temperature for 4 hours while under nitrogen. The solution was cooled to room temperature overnight and diluted with diethyl ether to precipitate product. The solid was isolated by filtration and dried under vacuum to afford a white crystalline solid (0.42 grams, 0.0009 moles) as the product in 47% yield.Preparation of N-(2,6-diethyl-4-methyl)-4-phenyl-2,6-diethylpyridinium tetrafluoroborate

[0093] In a 100 milliliter round bottom flask equipped with a magnetic stir bar and condenser, 4-phenyl-2,6-diethylpyiylium tetrafluoroborate (0.65 grams, 0.0021 moles) was dissolved in ethanol (40 milliliters). 2,6-diethyl-4-methylaniline (0.37 grams, 0.0023 moles) was added, and the mixture was refluxed at a refluxing temperature for 4 hours while under nitrogen. The solution was cooled to room temperature overnight. Diethyl ether was added to the flask and the product formed as crystals in the bottom of the flask overnight.

[0094] Many modifications and variations of the subject application are possible in light of the above teachings. Therefore, the subject application may be practiced other than as specifically described.

Claims

WHAT IS CLAIMED IS1. A pyridinium compound according to formula (4):wherein:Ri and R5 are independently selected from hydrogen, an unbranched alkyl group, a branched alkyl group, a halide group, a haloalkyl group, a perhaloalkyl group, an alkoxy group, a nitrile group and a nitro group;R2, R3 and R4 are independently selected from hydrogen, an unbranched alkyl group, a branched alkyl group, an alkoxy group, a haloalkyl group, an aralkyl group, a nitro group, a nitrile group, a halide group, an acetyl group, a trialkylammonium alkyl group, an alkyl carboxylate group, an alkyl sulfonate group, an alkyl phosphonate group, and an alkyl ether group;Rs and Rio are independently selected from hydrogen, an unbranched alkyl group, a branched alkyl group, a halide group, a haloalkyl group, a perhaloalkyl group, an alkoxy group, a nitrile group and a nitro group;R7, Rs, and R9 are independently selected from hydrogen, an unbranched alkyl group, a branched alkyl group, a halide group, a haloalkyl group, a perhaloalkyl group, an alkoxy group, an aryl group, a substituted aryl group, an aralkyl group, a nitro group, a nitrile group, an acetyl group, a trialkylammonium alkyl group, an alkyl carboxylate group, an alkyl sulfonate group, an alkyl phosphonate group, and a alkyl ether group;R11 and R14 are the same or different and are independently selected from an unbranched alkyl group, a branched alkyl group, a haloalkyl group, a perhaloalkyl group, an alkoxy group, an aralkyl group, a trialkylammonium alkyl group, an alkyl carboxylate group, an alkyl sulfonate group, an alkyl phosphonate group, and an alkyl ether group; andR12 and R13 are the same or different and are independently selected from hydrogen, an unbranched alkyl group, a branched alkyl group, a haloalkyl group, a perhaloalkyl group, an alkoxy group, an aralkyl group, a trialkylammonium alkyl group, an alkyl carboxylate group, an alkyl sulfonate group, an alkyl phosphonate group, and an alkyl ether group.

2. The pyridinium compound of claim 1, wherein:Ri, R5, R12 and R13 in formula (4) are hydrogen; andR11 and R14 in formula (4) are the same or different and are independently selected from a branched alkyl group and an unbranched alkyl group.

3. The pyridinium compound of claim 1, wherein Ru and Ru in formula (4) are z-Pr and wherein at least one of Re and Rio in formula (4) is an unbranched alkyl group or a branched alkyl group or an alkoxy group.

4. The pyridinium compound of claim 2, wherein Ru and RM in formula (4) are z-Pr and wherein at least one of Re and Rio in formula (4) is an unbranched alkyl group or a branched alkyl group or an alkoxy group.

5. The pyridinium compound of claim 3, wherein at least one of Re and Rio in formula (4) is an unbranched alkyl group or a branched alkyl group.

6. The pyridinium compound of claim 4, wherein at least one of Re and Rio in formula (4) is an unbranched alkyl group or a branched alkyl group.

7. The pyridinium compound of claim 1, at least one of R2, R3 and R4 in formula (4) is a branched alkyl group, an unbranched alkyl group or an alkoxy group.

8. The pyridinium compound of claim 2, at least one of R2, R3 and R4 in formula (4) is a branched alkyl group, an unbranched alkyl group or an alkoxy group.

9. The pyridinium compound of claim 3, at least one of R2, R3 and R4 in formula (4) is a branched alkyl group, an unbranched alkyl group or an alkoxy group.

10. The pyridinium compound of claim 4, at least one of R2, R3 and R4 in formula (4) is a branched alkyl group, an unbranched alkyl group or an alkoxy group.

11. The pyridinium compound of claim 5, at least one of R2, R3 and R4 in formula (4) is a branched alkyl group, an unbranched alkyl group or an alkoxy group.

12. The pyridinium compound of claim 6, at least one of R2, R3 and R4 in formula (4) is a branched alkyl group, an unbranched alkyl group or an alkoxy group.

13. The pyridinium compound of claim 1, wherein the pyridinium compound according to formula (4) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

14. The pyridinium compound according to claim 1 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine-5, 5-dioxide, a benzoquinone, and a cyclopropenium.

15. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 1.

16. The pyridinium compound of claim 2, wherein the pyridinium compound according to formula (4) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

17. The pyridinium compound according to claim 2 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine- 5, 5 -di oxi de, a benzoquinone, and a cyclopropenium.

18. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 2.

19. The pyridinium compound of claim 3, wherein the pyridinium compound according to formula (4) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

20. The pyridinium compound according to claim 3 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine-5, 5-dioxide, a benzoquinone, and a cyclopropenium.

21. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 3.

22. The pyridinium compound of claim 4, wherein the pyridinium compound according to formula (4) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

23. The pyridinium compound according to claim 4 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine- 5, 5 -di oxi de, a benzoquinone, and a cyclopropenium.

24. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 4.

25. The pyridinium compound of claim 5, wherein the pyridinium compound according to formula (4) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

26. The pyridinium compound according to claim 5 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine- 5, 5 -di oxi de, a benzoquinone, and a cyclopropenium.

27. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 5.

28. The pyridinium compound of claim 6, wherein the pyridinium compound according to formula (4) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

29. The pyridinium compound according to claim 6 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine- 5, 5 -di oxi de, a benzoquinone, and a cyclopropenium.

30. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 6.

31. The pyridinium compound of claim 7, wherein the pyridinium compound according to formula (4) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

32. The pyridinium compound according to claim 7 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine-5, 5-dioxide, a benzoquinone, and a cyclopropenium.

33. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 7.

34. The pyridinium compound of claim 8, wherein the pyridinium compound according to formula (4) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

35. The pyridinium compound according to claim 8 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine-5, 5-dioxide, a benzoquinone, and a cyclopropenium.

36. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 8.

37. The pyridinium compound of claim 9 wherein the pyridinium compound according to formula (4) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

38. The pyridinium compound according to claim 9 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine- 5, 5 -di oxi de, a benzoquinone, and a cyclopropenium.

39. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 9.

40. The pyridinium compound of claim 10, wherein the pyridinium compound according to formula (4) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

41. The pyridinium compound according to claim 10 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine-5, 5-dioxide, a benzoquinone, and a cyclopropenium.

42. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 10.

43. The pyridinium compound of claim 11, wherein the pyridinium compound according to formula (4) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

44. The pyridinium compound according to claim 11 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine- 5, 5 -di oxi de, a benzoquinone, and a cyclopropenium.

45. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 11.

46. The pyridinium compound according to claim 1, wherein the pyridinium compound of formula (4) comprises a pyridinium compound according to formula (1A):wherein:R1 and R5 are independently selected from hydrogen, an unbranched alkyl group, a branched alkyl group, a halide group, a haloalkyl group, a perhaloalkyl group, an alkoxy group, a nitrile group and a nitro group;R2, R3 and R4 are independently selected from hydrogen, an unbranched alkyl group, a branched alkyl group, an alkoxy group, a haloalkyl group, an aralkyl group, a nitro group, a nitrile group, a halide group, an acetyl group, a trialkylammonium alkyl group, an alkyl carboxylate group, an alkyl sulfonate group, an alkyl phosphonate group, and an alkyl ether group;Rs and Rio are independently selected from hydrogen, an unbranched alkyl group, a branched alkyl group, a halide group, a haloalkyl group, a perhaloalkyl group, an alkoxy group, a nitrile group and a nitro group; andR7, R8, and R9 are independently selected from hydrogen, an unbranched alkyl group, a branched alkyl group, a halide group, a haloalkyl group, a perhaloalkyl group, analkoxy group, an aryl group, a substituted aryl group, an aralkyl group, a nitro group, a nitrile group, an acetyl group, a trialkylammonium alkyl group, an alkyl carboxylate group, an alkyl sulfonate group, an alkyl phosphonate group, and an alkyl ether group.

47. The pyridinium compound of claim 46, wherein Ri and Rs in formula (1A) are hydrogen.

48. The pyridinium compound of claim 46, wherein the pyridinium compound according to formula (4) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

49. The pyridinium compound according to claim 46 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5-oxide, a phenothiazine-5, 5-dioxide, a benzoquinone, and a cyclopropenium.

50. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 46.

51. The pyridinium compound of claim 47, wherein the pyridinium compound according to formula (4) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

52. The pyridinium compound according to claim 47 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine-5, 5-dioxide, a benzoquinone, and a cyclopropenium.

53. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 47.

54. The pyridinium compound according to claim 1, wherein the pyridinium compound of formula (4) comprises a pyridinium compound according to formula (IB):wherein:R1 and Rs are independently selected from hydrogen, an unbranched alkyl group, a branched alkyl group, a halide group, a haloalkyl group, a perhaloalkyl group, an alkoxy group, a nitrile group and a nitro group;R2, R3 and R4 are independently selected from hydrogen, an unbranched alkyl group, a branched alkyl group, an alkoxy group, a haloalkyl group, an aralkyl group, a nitro group, a nitrile group, a halide group, an acetyl group, a trialkylammonium alkyl group, an alkyl carboxylate group, an alkyl sulfonate group, an alkyl phosphonate group, and an alkyl ether group;Re and Rio are independently selected from hydrogen, an unbranched alkyl group, a branched alkyl group, a halide group, a haloalkyl group, a perhaloalkyl group, an alkoxy group, a nitrile group and a nitro group; andR7, R8, and R9 are independently selected from hydrogen, an unbranched alkyl group, a branched alkyl group, a halide group, a haloalkyl group, a perhaloalkyl group, an alkoxy group, an aryl group, a substituted aryl group, an aralkyl group, a nitro group, a nitrile group, an acetyl group, a trialkylammonium alkyl group, an alkyl carboxylate group, an alkyl sulfonate group, an alkyl phosphonate group, and a alkyl ether group.

55. The pyridinium compound of claim 54, wherein Ri and Rs in formula (IB) are hydrogen.

56. The pyridinium compound of claim 54, wherein the pyridinium compound according to formula (IB) has a reduction potential from -1.57 to -2.00V vs Fc / Fc .

57. The pyridinium compound according to claim 54 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine-5, 5-dioxide, a benzoquinone, and a cyclopropenium.

58. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 54.

59. The pyridinium compound of claim 55, wherein the pyridinium compound according to formula (4) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

60. The pyridinium compound according to claim 55 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine-5, 5-dioxide, a benzoquinone, and a cyclopropenium.

61. A redox flow battery comprising:(I) a cathode;(II) an anode; and(III) a charge-carrying electrolyte comprising the pyridinium compound according to claim 55.

62. The pyridinium compound according to claim 1 , wherein the pyridinium compound according to formula (4) comprises a pyridinium compound selected from the group consisting of:

63. The pyridinium compound of claim 62, wherein the pyridinium compound according to any one of formulas (1C) to (IQ) has a reduction potential from -1.57 to -2.00V vs Fc / Fc+.

64. The pyridinium compound according to claim 62 linked to an oxidizable moiety selected from a ferrocene, a carbazole, a phenazine, a phenoxazine, a phenothiazine, a phenothiazine-5 -oxide, a phenothiazine- 5, 5 -di oxi de, a benzoquinone, and a cyclopropenium.

65. A redox flow battery comprising:(I) a cathode;(II) an anode; and(Ill) a charge-carrying electrolyte comprising the pyridinium compound according to claim 62.

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