PROCESSES FOR THE SYNTHESIS OF SUBSTITUTED 1,4-NAPHTOQUINONES

Abstract

TITLE: METHODS FOR SYNTHESIZING SUBSTITUTED 1,4-NAPHTOQUINONES The present invention relates to a method for the electrosynthesis of substituted 1,4-naphthoquinones in an electrolytic cell, as well as a method for synthesizing substituted 1,4-naphthoquinones in a flow fuel cell. The present invention also relates to the use of said substituted 1,4-naphthoquinones as a redox molecule in the negolyte of a redox flow battery. Finally, the present invention relates to a redox flow battery comprising a posolyte including a redox molecule and a negolyte including at least one substituted 1,4-naphthoquinone obtained by one of the methods according to the invention.

Description

Title of the invention: PROCESSES FOR THE SYNTHESIS OF SUBSTITUTED 1,4-NAPHTOQUINONES FIELD OF THE INVENTION

[0001] The present invention relates to a method for the electrosynthesis of substituted 1,4-naphthoquinones in an electrolytic cell, as well as to a method for the synthesis of substituted 1,4-naphthoquinones in a flow fuel cell. The present invention also relates to the use of said substituted 1,4-naphthoquinones as a redox molecule in the negolyte of a redox flow battery. Finally, the present invention relates to a redox flow battery comprising a posolyte comprising a redox molecule and a negolyte comprising at least one substituted 1,4-naphthoquinone obtained by one of the methods according to the invention. STATE OF THE ART

[0002] Recently, redox flow batteries have experienced rapid development. The use of organic redox molecules is favored, in particular the use of molecules belonging to the quinone family. Within this family, anthraquinones seem particularly interesting for the following reasons: - the size of their basic skeleton is large enough to limit the crossover phenomenon through the separator when the battery is in operation, - the structural diversity of anthraquinones is very important, and - their commercial availability is satisfactory.

[0003] 1,4-Naphthoquinones are much less widespread as organic-type redox molecules in redox flow batteries. Currently, juglone, lawsone and plumbagin represent the majority of commercially available 1,4-naphthoquinones. However, their use as redox molecules in the negolyte of a flow battery is limited due to their high cost, low solubility in aqueous solution and / or the existence of a significant crossover.

[0004] The addition of a carbon chain carrying at least one solubilizing group of the carboxylate type on the skeleton of the 1,4-naphthoquinones makes it possible to increase their solubility in aqueous solution in a basic medium while reducing the crossover phenomenon via the increase in steric hindrance and the increase in negative charges. A known route for the synthesis of these substituted 1,4-naphthoquinones corresponds to the oxidation of a hydroxylated anthraquinone in the presence of potassium ferricyanide (Scholl et al., Berichte der Deutschen Chemischen Gesellschaft (A and B Sériés), Vol. 52, 1919, pages 1142-1160 and Scholl et al. Berichte der Deutschen Chemischen Ge-sellschaft (A and B Sériés), Vol. 56, 1923, 2548-2555). However, the isolation of the substituted 1,4-naphthoquinone thus obtained is problematic, particularly given the presence of ferricyanide in the final solution.

[0005] There is therefore a real need to develop one or more alternative synthesis routes for 1,4-naphthoquinones substituted by a carbon chain carrying at least one solubilizing group of carboxylate type, in which the final product is not mixed with an oxidant and in which the isolation of the substituted 1,4-naphthoquinone obtained is not obligatory before its use in a redox flow battery.

[0006] Two processes for the synthesis of 1,4-naphthoquinones substituted by a carbon chain carrying at least one solubilizing group of carboxylate type have now been discovered: the first is an electrosynthesis process in an electrolytic cell and the second is a process for the synthesis of said substituted 1,4-naphthoquinones in a flow fuel cell. These two processes comprise a reaction step of oxidation, in a basic medium, of a particular anthraquinone. In the case of the electrosynthesis process in an electrolytic cell, the anthraquinone is oxidized in contact with the anode of the electrolytic cell, which makes it possible to obtain the substituted 1,4-naphthoquinone directly in the electrolysis solution and simply mixed with the solutes present in said solution.In the case of the synthesis process in a flow fuel cell, the chemical oxidant and the anthraquinone are in separate tanks, thus avoiding their mixing and therefore avoiding difficult separation of the oxidant from the substituted 1,4-naphthoquinone obtained at the end of the process.

[0007] Furthermore, it has now also been discovered that the electrolysis solution obtained from the electrosynthesis process in an electrolytic cell can be used directly as a negolyte in a redox flow battery, without the need to isolate and / or purify the substituted 1,4-naphthoquinone thus obtained. Similarly, the fuel cell negolyte obtained from the synthesis process in a flow fuel cell can also be used directly as a negolyte in a redox flow battery, and advantageously the fuel cell posolyte obtained from the same process can also be used directly as a posolyte in a redox flow battery.

[0008] Such a redox flow battery comprising at least one substituted 1,4-naphthoquinone obtained according to one of the synthesis methods of the invention has electrochemical performances equivalent to or even superior to a redox flow battery in which the negolyte comprises the corresponding hydroxylated anthraquinone as redox molecule. SUMMARY

[0009] The present invention relates to a process for the electrosynthesis of a compound of formula (II) or formula (III) or a union thereof: [Chem 1] in which R1 and R4, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, or a nitro; R2 and R3, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, a nitro, or a hydroxyl; R6 represents a straight or branched C1-C6 alkyl chain, hydrogen, halogen, sulfonate, carboxylic acid, ketone, ester, ether oxide, thiol, thioalkyl, amine, nitro, or hydroxyl; R8 or R9 represents a linear or branched C1 to C6 alkyl chain, hydrogen, halogen, sulfonate, carboxylic acid, ketone, ester, ether oxide, thiol, thioalkyl, amine, or nitro; represents a single bond which can be on either side of the double bond; in an electrolytic cell comprising an electrolysis solution, an anode and a cathode; said method comprising a step of electrochemical oxidation at the anode of the electrolytic cell of a compound of formula (I): [Chem 3] (I), in which RB R2, R3, R4 and R6 are as defined above, R5 and R7, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, a nitro, or a hydroxyl; preferably, at least one of R5 and R7 represents a hydroxyl; said compound of formula (I) being included in the electrolysis solution whose pH varies between 10 and 14.

[0010] The present invention also relates to a process for the synthesis of a compound of formula (II) or formula (III) or an anion thereof: [Chem 1] Ri O in which R1 and R4, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, or a nitro; R2 and R3, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, a nitro, or a hydroxyl; R6 represents a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether oxide, a thiol, a thioalkyl, an amine, a nitro, or a hydroxyl; R8 or R9 represents a linear or branched alkyl chain, C1 to C6, hydrogen, halogen, sulfonate, carboxylic acid, ketone, ester, ether oxide, thiol, thioalkyl, amine, or nitro; represents a single bond which can be on either side of the double bond; in a flow fuel cell comprising a negolyte, a posolyte, an anode and a cathode; said method comprising a step of electrochemical oxidation at the anode of the flow fuel cell of a compound of formula (I): [Chem 3] in which RB R2, R3, R4 and R6 are as defined above, R5 and R7, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, a nitro, or a hydroxyl; preferably, at least one of R5 and R7 represents a hydroxyl; said compound of formula (I) being included in the negolyte whose pH varies between 10 and 14, the posolyte comprising an oxidant.

[0011] Advantageously, only one of R5 and R7 represents a hydroxyl and preferably the other of R5 and R7 represents a hydrogen.

[0012] Advantageously, R6 represents a hydrogen and / or R1 to R4 are identical and represent a hydrogen, preferably R6 represents a hydrogen and R1 to R4 are identical and represent a hydrogen.

[0013] Advantageously, in the synthesis process of the present invention, the difference in redox potential between the oxidant and the compound of formula (I) is between 50 and 600 mV, preferably between 100 and 400 mV and more preferably it is equal to 200 mV.

[0014] Advantageously, in the synthesis process of the present invention, the oxidation potential of the compound of formula (I) is between - 0.2 and 0.2 V vs Ag / AgCl.

[0015] Advantageously, the electrosynthesis process and / or the synthesis process of the present invention further comprises an additional step of isolating the compound of formula (II) or of formula (III), or an anion thereof, preferably the additional isolation step comprises the following successive steps: a) acidification of the electrolysis solution of the electrolytic cell obtained at the end of the electrosynthesis process of the present invention or of the negolyte of the flow fuel cell obtained at the end of the synthesis process of the invention, until a precipitate is obtained, to obtain an acidified electrolyte, and b) filtration of the acidified electrolyte from step a); or a step of concentrating the electrolysis solution of the electrolytic cell obtained at the end of the electrosynthesis method of the invention or the negolyte of the flow fuel cell obtained at the end of the synthesis method of the present invention.

[0016] The present invention further relates to the use of a compound of formula (II) or formula (III), or an anion thereof, obtained by one of the methods of the invention, as a redox molecule in the negolyte of a redox flow battery.

[0017] The present invention also relates to the use of the electrolysis solution of the electrolytic cell obtained at the end of the electrosynthesis method of the invention or of the negolyte of the flow fuel cell obtained at the end of the synthesis method of the present invention as a negolyte in a redox flow battery, preferably the posolyte of the flow fuel cell obtained at the end of the synthesis method can also be used as a posolyte in the redox flow battery.

[0018] The present invention also relates to a redox flow battery comprising: (a) a posolyte comprising a redox molecule; and (b) a negolyte comprising: • at least one compound of formula (II) or formula (III), or an anion thereof, obtained by one of the processes of the invention, or • the electrolysis solution of the electrolytic cell obtained at the end of the electrosynthesis process of the present invention, or • the flow fuel cell negolyte obtained at the end of the synthesis process of the invention. DESCRIPTION OF FIGURES

[0019] [Fig. 1] represents a diagram of a redox flow battery.

[0020] [Fig.2] represents an exploded diagram of a percolation electrolytic cell. DEFINITIONS

[0021] In the present invention, the terms below are defined as follows:

[0022] “Carboxylic acid” means the COOH group.

[0023] “Weak acid” means an acid whose pKa in water is greater than 0 and less than or equal to 14.

[0024] “Strong acid” means an acid whose pKa in water is less than or equal to 0.

[0025] “Amine” means the group NH2, NHR or NRR', in which R and R', identical or different, independently represent a linear or branched alkyl chain, C1 to C6.

[0026] “Redox flow battery” means a type of battery (see [Fig.l]) comprising a first electrode (1) (at the negative pole of the battery) and a second electrode (2) (at the positive pole of the battery) separated by a separator (3); and further comprising two electrolyte solutions: the negolyte (4) (at the negative pole of the battery) and the posolyte (5) (at the positive pole of the battery). Redox flow batteries are characterized in that energy is stored in the two electrolyte solutions, these solutions being stored in separate reservoirs and being pumped through the battery. The negative pole of the battery is assigned to the redox molecule with the lowest redox potential. The positive pole of the battery is assigned to the redox molecule with the highest redox potential. During battery charging, the first electrode (1) is the cathode and the second electrode (2) is the anode. During battery discharging, the first electrode (1) is the anode and the second electrode (2) is the cathode.

[0027] “Electrolytic cell” means an electrochemical cell in which the supply of electrical energy produces electrolysis. An electrolytic cell comprises at least one electrolysis compartment comprising an electrolysis solution and a working electrode. The electrolytic cell further comprises at least one counter electrode and optionally a reference electrode. The electrolytic cell may be in a galvanostatic or potentiostatic mode. The galvanostatic mode allows a current to be imposed between the working electrode and the at least one counter electrode. The reference electrode is then absent. The potentiostatic mode allows a potential to be imposed on the working electrode. The reference electrode is then necessary and the potential is imposed using a potentiostat. The electrolytic cell may be a batch or percolation electrolytic cell. If the electrolytic cell is a percolation electrolytic cell (see [Fig.2]), it then comprises at least two compartments: the electrolysis compartment comprising the working electrode (6) which is necessarily porous, and a compartment comprising the counter-electrode (7) and located on one of the sides of the electrolysis compartment. A semi-permeable membrane (8), cationic or anionic, separates the electrolysis compartment from the compartment comprising the counter-electrode (7). The percolation electrolytic cell may further comprise a reference electrode (9) in the potentiostatic mode, as indicated above. In all cases, the current flows between the working electrode (6) and the counter-electrode (7). The electrolysis solution enters from the bottom of the electrolysis compartment, is brought into contact with the working electrode (6) and exits through the . top of the electrolysis cell (see arrows in [Fig.2]). The compartment containing the counter electrode (7) is supplied with an electrolyte solution of the same pH as the electrolysis solution.

[0028] “Ketone” means the group C(O)R, in which R represents a chain linear or branched alkyl, C1 to C6.

[0029] “Alkyl chain” means a hydrocarbon chain which may be saturated or unsaturated and which may optionally be interrupted by one or more heteroatoms such as O, S or N and / or be optionally substituted by at least one group such as for example a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an amine, a thiol, a thio-alkyl, a nitro, a hydroxyl or an ether-oxide. In the present invention, the alkyl chain comprises from 1 to 6 carbon atoms (C1 to C6).

[0030] “Understand” or “includes” should be interpreted in an open and inclusive, but not limited to the compounds and / or features that follow this term.

[0031] “Electrolysis” means the chemical transformation of a substrate under the effect of electrical energy.

[0032] “Electrosynthesis” means a reaction step allowing the transformation from a substrate into a product at the working electrode of an electrolytic cell, said transformation occurring with an exchange of electrons (oxidation or reduction). According to the present invention, the electrosynthesis process is preferably carried out by imposing a voltage (potentiostatic mode of the electrolytic cell).

[0033] “Between X and Y” means the interval of values ​​between X and Y, the limits X and Y being included in the said interval.

[0034] “Ester” denotes the group C(O)OR, in which R represents a chain linear or branched alkyl, C1 to C6.

[0035] “Ether-oxide” denotes the OR group, in which R represents a chain linear or branched alkyl, C1 to C6.

[0036] “Halogen” means fluoro, chloro, bromo or iodo.

[0037] “Hydroxyl” means the OH group.

[0038] “Redox molecule” means a substance capable of capturing and / or releasing at least an electron. A redox molecule used in the negolyte or posolyte of a redox flow battery is necessarily reversible. Thus, for example, when charging the redox flow battery, the electrode present at the negative pole is the cathode and the redox molecule present in the negolyte is an oxidant capable of being transformed into a reducing agent by capturing one or more electrons (reduction). Subsequently, when discharging the redox flow battery, the electrode present at the negative pole is the anode and the reducing agent formed during charging in the negolyte is capable of giving up one or more electrons to form the oxidant again (oxidation). Redox molecules are defined by their redox potential and are presented in the form of a redox couple, that is to say a couple consisting of an oxidant and a reducer.

[0039] “Negolyte” means the electrolyte solution located at the negative pole of a battery redox flow or a flow fuel cell. In the case of a redox flow battery, the negolyte comprises the redox molecule with the lowest redox potential.

[0040] “Nitro” designates the NO2 group.

[0041] “Oxidation” means a chemical reaction by which electrons are removed from an atom or a molecule.

[0042] “Fuel cell” means an electrochemical cell which makes it possible to convert spontaneously converts chemical substance (a fuel and an oxidizer) into electrical energy.

[0043] “Flow fuel cell” means a fuel cell comprising a first electrode (at the negative pole of the battery) which is an anode and a second electrode (at the positive pole of the battery) which is a cathode, these two electrodes being separated by a separator. The flow fuel cell further comprises two electrolyte solutions: that at the anode (at the negative pole of the battery), called negolyte, and that at the cathode (at the positive pole of the battery), called posolyte. For example, the flow fuel cell can be a redox flow battery used in battery mode (i.e. used in discharge mode). In this case, the electrolyte solutions (the negolyte and the posolyte) are stored in tanks and pumped through the redox flow battery.

[0044] “Posolyte” means the electrolyte solution located at the positive pole of a battery redox flow or a flow fuel cell. In the case of a redox flow battery, the posolyte comprises the redox molecule with the highest redox potential.

[0045] “Oxidation potential” means the potential at which a redox molecule undergoes a or more oxidation steps. In the context of the present invention, the oxidation potential of the compound of formula (I) designates the potential at which said compound of formula (I) is oxidized to a compound of formula (II) or formula (III).

[0046] “Redox potential” means a quantity expressed in volts and applied to couples redox potential to predict the reactivity of chemical species with each other. This potential is always expressed relative to a reference electrode, for example, relative to the silver chloride electrode (Ag / AgCl). The redox potential can be measured experimentally with a voltmeter or calculated using the activities of the chemical species present in solution that participate in redox reactions using the Nernst equation.

[0047] “Reduction” means a chemical reaction by which electrons are supplied to an atom or a molecule.

[0048] “Aqueous solution” means a mixture of a compound in small quantity (the solute) in water, the latter being in the majority quantity. The water content is greater than or equal to 20% by weight, preferably greater than or equal to 40% by weight, and even more preferably greater than or equal to 80% by weight, relative to the total weight of the solution.

[0049] “Electrolysis solution” means the electrolyte solution included in the com electrolysis compartment of an electrolytic cell.

[0050] “Electrolyte solution” means a conductive aqueous solution. Thus, the solute is at least one salt capable of dissociating into ions in water (e.g. Na+OH , K+0H, Na+Cl or 2Na+SO42 ).

[0051] “Sulfonate” designates the SO3 group.

[0052] “Room temperature” means 20-25°C.

[0053] “Thiol” denotes the SH group.

[0054] “Thio-alkyl” denotes the SR group, in which R represents a chain linear or branched alkyl, C1 to C6. DETAILED DESCRIPTION

[0055] Electrosynthesis process and process for synthesizing a compound of formula (II) or formula (IIP) or an anion thereof

[0056] The present invention relates firstly to a process for the electrosynthesis of a compound of formula (II) or of formula (III) or of an anion thereof: [Chem 1] in which R1 and R4, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, or a nitro; R2 and R3, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, ketone, ester, ether oxide, thiol, thioalkyl, amine, nitro, or hydroxyl; R6 represents a straight or branched C1-C6 alkyl chain, hydrogen, halogen, sulfonate, carboxylic acid, ketone, ester, ether oxide, thiol, thioalkyl, amine, nitro, or hydroxyl; R8 or R9 represents a linear or branched C1 to C6 alkyl chain, hydrogen, halogen, sulfonate, carboxylic acid, ketone, ester, ether oxide, thiol, thioalkyl, amine, or nitro; represents a single bond which can be on either side of the double bond; in an electrolytic cell comprising an electrolysis solution, an anode and a cathode; said method comprising a step of electrochemical oxidation at the anode of the electrolytic cell of a compound of formula (I): [Chem 3] R4 O R5 in which Rb R2, R3, R4 and R6are as defined above, R5 and R7, identical or different, represent independently of one another a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, a nitro, or a hydroxyl; preferably, at least one of R5 and R7 represents a hydroxyl; said compound of formula (I) being included in the electrolysis solution whose pH varies between 10 and 14, preferably between 12 and 14 and more preferably the pH is equal to 14.

[0057] In other words, according to this method, the electrochemical oxidation of the compound of formula (I) into a compound of formula (II) or formula (III), or into an anion thereof, is carried out in the electrolysis solution, in contact with the working electrode of the electrolysis compartment, said working electrode being an anode. An electrochemical reduction takes place in contact with the counter-electrode, which is a cathode. The electrochemical oxidation at the anode of the electrolytic cell and the electrochemical reduction at the cathode of the electrolytic cell are carried out by means of an electric current. The pH of the electrolysis solution being basic, the compound of formula (II) or of formula (III) is in anionic form in the electrolysis solution. This electrosynthesis process advantageously makes it possible to obtain the substituted 1,4-naphthoquinone directly in the electrolysis solution and simply mixed with the solutes present in said solution.

[0058] Advantageously, the electrolytic cell is in a potentiostatic mode. Thus, the electrolytic cell further comprises a reference electrode. This mode advantageously makes it possible to control the potential at the working electrode and therefore the selectivity of the reaction. The potential applied to the working electrode is advantageously between - 0.3 and 0.3 V vs Ag / AgCl, preferably between - 0.2 and 0.2 V vs Ag / AgCl, and more preferably the potential applied to the working electrode is equal to - 0.05 V vs Ag / AgCl.

[0059] Advantageously, the step of electrochemical oxidation at the anode of the electrolytic cell of a compound of formula (I) is carried out at room temperature.

[0060] Advantageously, the electrochemical reduction taking place at the cathode of the electrolytic cell is chosen from the reduction of water (into dihydrogen), the reduction of CO2, the reduction of an anthraquinone and the reduction of a naphthoquinone.

[0061] Advantageously, the concentration of compound of formula (I) in the electrolysis solution is between 0.01 and 0.8 mol / L, preferably between 0.03 and 0.6 mol / L, more preferably between 0.05 and 0.1 mol / L and even better the concentration of compound of formula (I) in the electrolysis solution is equal to 0.05 mol / L.

[0062] Advantageously, the pH of the electrolysis solution is between 12 and 14, preferably equal to 14, and the concentration of compound of formula (I) in the electrolysis solution is advantageously between 0.01 and 0.8 mol / L, preferably between 0.03 and 0.6 mol / L and more preferably the concentration of compound of formula (I) in the electrolysis solution is equal to 0.05 mol / L.

[0063] Advantageously, argon bubbling is maintained in the electrolysis solution.

[0064] The present invention also relates to a process for the synthesis of a compound of formula (II) or formula (III) or an anion thereof: [Chem 1] [Chem 2] R1 0 in which R1 and R4, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, or a nitro; R2 and R3, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, a nitro, or a hydroxyl; R6 represents a straight or branched C1-C6 alkyl chain, hydrogen, halogen, sulfonate, carboxylic acid, ketone, ester, ether oxide, thiol, thioalkyl, amine, nitro, or hydroxyl; R8 or R9 represents a linear or branched alkyl chain, C1 to C6, hydrogen, halogen, sulfonate, carboxylic acid, ketone, ester, ether oxide, thiol, thioalkyl, amine, or nitro; represents a single bond which can be on either side of the double bond; in a flow fuel cell comprising a negolyte, a posolyte, an anode and a cathode; said method comprising a step of electrochemical oxidation at the anode of the flow fuel cell of a compound of formula (I): [Chem 3] R4 O R5 in which Rb R2, R3, R4 and R6 are as defined above, R5 and R7, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, a nitro, or a hydroxyl; preferably, at least one of R5 and R7 represents a hydroxyl; said compound of formula (I) being included in the negolyte whose pH varies between 10 and 14, preferably between 12 and 14, and more preferably the pH is equal to 14, the posolyte comprising an oxidant.

[0065] In other words, according to this method, the electrochemical oxidation of the compound of formula (I) into the compound of formula (II) or formula (III), or into an anion thereof, is carried out in the negolyte, in contact with the anode of the flow fuel cell. An electrochemical reduction of the oxidant is carried out in the posolyte, in contact with the cathode of the flow fuel cell. The electrochemical oxidation at the anode of the flow fuel cell and the electrochemical reduction at the cathode of the flow fuel cell take place spontaneously and these chemical reactions allow the flow fuel cell to produce electrical energy which is usable, which makes it possible to lower the cost of synthesis of the compound of formula (II) or formula (III), or of an anion thereof. Since the pH of the fuel cell electrolyte is basic, the compound of formula (II) or formula (III) is in anionic form in this electrolyte solution at the anode.The synthesis of the compound of formula (II) or formula (III), or an anion thereof, in a flow fuel cell advantageously makes it possible to obtain said compound of formula (II) or formula (III) in a pole distinct from the pole comprising the oxidant, which makes it possible to avoid their mixing and therefore to avoid the step of separating these two compounds, this step which can be problematic.

[0066] Advantageously, the step of electrochemical oxidation at the anode of the flow fuel cell of a compound of formula (I) is carried out at room temperature.

[0067] Advantageously, the concentration of compound of formula (I) in the negolyte of the flow fuel cell is between 0.01 and 0.8 mol / L, preferably between 0.03 and 0.6 mol / L, more preferably between 0.06 and 0.3 mol / L and even better the concentration of compound of formula (I) in the negolyte of the flow fuel cell is equal to 0.1 mol / L.

[0068] Advantageously, the weight ratio between the content of the oxidant in the posolyte of the flow fuel cell and the content of the compound of formula (I) in the negolyte of the flow fuel cell is greater than or equal to 1, preferably varies from 2 to 10 and more preferably from 4 to 8.

[0069] Advantageously, the volume of the posolyte of the flow fuel cell is greater than the volume of the negolyte of the flow fuel cell. In this way, the potential of the redox system of the oxidant is preferably blocked.

[0070] Advantageously, the separator of the flow fuel cell is a cation exchange membrane. Mention may be made, for example, of the Nafion 50 pm cation exchange membrane.

[0071] Advantageously, the difference in redox potential between the oxidant and the compound of formula (I) is between 50 and 600 mV, preferably between 100 and 400 mV and more preferably the difference in redox potential between the oxidant and the compound of formula (I) is equal to 200 mV.

[0072] Advantageously, the oxidation potential of the compound of formula (I) is between - 0.2 and 0.2 V vs Ag / AgCl.

[0073] The electrosynthesis process in an electrolytic cell according to the invention and the synthesis process in a flow fuel cell according to the invention may optionally further comprise an additional step of isolating the compound of formula (II) or of formula (III), or an anion thereof.

[0074] In a first embodiment, the optional additional step of isolating the compound of formula (II) or formula (III), or an anion thereof, comprises a step of acidifying the electrolysis solution of the electrolytic cell obtained at the end of the electrosynthesis method according to the invention or the negolyte of the flow fuel cell obtained at the end of the synthesis method according to the invention, until a precipitate is obtained, to obtain an acidified electrolyte. This acidification step makes it possible to protonate the anion of the compound of formula (II) or formula (III) obtained by oxidation of the compound of formula (I). The acidified electrolyte obtained therefore comprises a heterogeneous dispersed phase consisting of an acidic aqueous solution and a precipitate corresponding to the compound of formula (II) or formula (III) in a non-ionized molecular form.Preferably, the acid used during the acidification step is a strong acid, such as hydrochloric acid, with a concentration greater than or equal to 102 mol / L, or a weak acid whose pKa is less than or equal to 4, such as citric acid.

[0075] According to this first embodiment, the optional additional isolation step may optionally further comprise a step of filtration of the acidified electrolyte obtained at the end of the acidification step. This step of filtration of the acidified electrolyte makes it possible to separate and isolate the compound of formula (II) or of formula (III) from the acidic aqueous solution obtained. At the end of this filtration step, the compound of formula (II) or of formula (III) is then found in the filter residue. All methods of filtration of a solution known to those skilled in the art may be used.

[0076] The electrosynthesis process in an electrolytic cell according to the invention and the synthesis process in a flow fuel cell according to the invention may optionally further comprise an additional step of purifying said compound of formula (II) or formula (III), after the optional filtration step. This purification step makes it possible to improve the purity of the compound of formula (II) or formula (III) obtained in the filter residue at the end of the optional filtration step of the acidified electrolyte. All methods of purifying a solid known to those skilled in the art may be used. Preferably, the purification of the compound of formula (II) or formula (III) may be carried out by solubilization in a volatile organic solvent and filtration through silica.

[0077] In a second embodiment, the optional additional step of isolating the compound of formula (II) or of formula (III), or of an anion thereof, comprises a step of concentrating the electrolysis solution of the electrolytic cell obtained at the end of the electrosynthesis method according to the invention or of the negolyte of the flow fuel cell obtained at the end of the synthesis method according to the invention, to obtain the anion of the compound of formula (II) or of formula (III) mixed with the at least one salt present as a solute in the electrolysis solution of the electrolytic cell or in the negolyte of the flow fuel cell.

[0078] The methods according to the invention may optionally further comprise, after the step of electrochemical oxidation of the compound of formula (I), the following successive additional steps: a. transfer of the electrolysis solution of the electrolytic cell obtained at the end of the electrosynthesis method according to the invention, or of the negolyte of the flow fuel cell obtained at the end of the synthesis method according to the invention, into the negolyte of a redox flow battery which comprises a posolyte comprising a redox molecule; and b. use of the redox flow battery.

[0079] Advantageously, the posolyte of the redox flow battery (said posolyte comprising a redox molecule) corresponds to the posolyte of the flow fuel cell obtained at the end of the synthesis method according to the invention.

[0080] Compounds of formulas (I), (II) and (IIP

[0081] The methods according to the invention comprise a step of electrochemical oxidation of a compound of formula (I) as defined previously.

[0082] Advantageously, the compound of formula (I) is chosen among hydroxyanthraquinone: [Chem 4] O alizarin: [Chem 5] quinizarin: [Chem 6] O OH • More preferably, the compound of formula (I) is chosen from alizarin and quinizarin.

[0083] Advantageously, at least one of R5 and R7 represents a hydroxyl.

[0084] Advantageously, R6 represents a hydrogen and / or R1 to R4 are identical and represent a hydrogen and preferably, R6 represents a hydrogen and R1 to R4 are identical and represent a hydrogen.

[0085] In one embodiment, only one of R5 and R7 represents a hydroxyl. In other words, in this embodiment, the compound of formula (I) is selected from the compounds of the following formulas (Ia) and (Ib): [Chem 7] in which Rb R2, R3, R4 and R6 are as defined above, R5 represents a linear or branched C1 to C6 alkyl chain, hydrogen, halogen, sulfonate, carboxylic acid, ketone, ester, ether oxide, thiol, thioalkyl, amine, or nitro; And [Chem 8] wherein RB R2, R3, R4 and R6 are as defined above, R7 represents a linear or branched alkyl chain, C1 to C6, hydrogen, halogen, sulfonate, carboxylic acid, ketone, ester, ether oxide, thiol, thioalkyl, amine, or nitro.

[0086] Preferably, R5 represents hydrogen in formula (Ia) and R7 represents hydrogen in formula (Ib). More preferably, in formulas (Ia) and (Ib), R6 represents hydrogen and / or R1 to R4 are identical and represent hydrogen. Even more preferably, R6 represents hydrogen and R1 to R4 are identical and represent hydrogen. In other words, even more preferably, the compound of formula (Ia) is alizarin, and the compound of formula (Ib) is quinizarin.

[0087] Advantageously, when the compound of formula (I) is chosen from the compounds of formula (Ia), R8 is equivalent to R5. In other words, when the compound of formula (I) is chosen from the compounds of formula (Ia), it is preferably oxidized to the compound of formula (II), or to an anion thereof.

[0088] Advantageously, when the compound of formula (I) is chosen from the compounds of formula (Ib), R9 is equivalent to R7. In other words, when the compound of formula (I) is chosen from the compounds of formula (Ib), it is preferably oxidized to the compound of formula (III), or to an anion thereof.

[0089] When the compound of formula (I) is alizarin, the compound obtained by the processes according to the invention is 4-(3-hydroxy-1,4-dioxo-1,4-dihydronaphthalen-2-yl)-4-oxobut-2-enoic acid: [Chem 9]

[0090] When the compound of formula (I) is quinizarin, the compound obtained by the processes according to the invention is the acid 4-(3-hydroxy-1,4-dioxo-1,2,3,4-tetrahydronaphthalen-2-yl)penta-2,4-dienoic: [Chem 10]

[0091] Use

[0092] The present invention also relates to the use of a compound of formula (II) or formula (III), or an anion thereof, obtained at the end of the processes according to the present invention, as a redox molecule in the negolyte of a redox flow battery.

[0093] The present invention further relates to the use of the electrolysis solution of the electrolytic cell obtained at the end of the electrosynthesis process of a compound of formula (II) or formula (III), or an anion thereof, as a negolyte in a redox flow battery. In other words, at the end of the electrochemical oxidation step, the electrolysis solution which comprises the compound of formula (II) or formula (III) or an anion thereof can be used directly, without isolation of the compound of formula (II) or formula (III), or its anion, as a negolyte in a redox flow battery.Thus, the electrolysis solution of the electrolytic cell obtained at the end of the electrosynthesis process according to the present invention can be used directly as a negolyte of a redox flow battery, without needing to go through a step of isolation and / or purification of the compound of formula (II) or of formula (III), or of an anion thereof.

[0094] The present invention further relates to the use of the flow fuel cell negolyte obtained from the process for synthesizing a compound of formula (II) or formula (III), or an anion thereof, in a flow fuel cell, as a negolyte in a redox flow battery. In other words, after the electrochemical oxidation step, the flow fuel cell negolyte, which comprises the compound of formula (II) or formula (III), or an anion thereof, can be used directly, without isolation of the compound of formula (II) or formula (III), or its anion, as a negolyte in a redox flow battery.Thus, the negolyte of the flow fuel cell obtained at the end of the synthesis process according to the present invention can be used directly as a negolyte of a redox flow battery, without needing to go through a step of isolation and / or purification of the compound of formula (II) or of formula (III), or of an anion thereof.

[0095] Advantageously, the posolyte of the flow fuel cell obtained at the end of the process for synthesizing a compound of formula (II) or of formula (III), or an anion thereof, in a flow fuel cell, can be used as posolyte in the redox flow battery. In other words, after the electrochemical oxidation step, the negolyte and posolyte from the flow fuel cell can be used directly, without isolation and / or purification steps, as negolyte and posolyte respectively in the same redox flow battery. Thus, the preparation of a posolyte is avoided and the preparation time of the redox flow battery, the financial impact and the ecological impact are reduced.

[0096] The present invention also relates to the use of a compound of formula (II) or formula (III), or an anion thereof, obtained by the methods according to the present invention, as an active molecule in the medical field. In other words, the present invention also relates to a compound of formula (II) or formula (III), or an anion thereof, obtained according to the methods of the invention for its use as a medicament.

[0097] Redox flow battery

[0098] The present invention further relates to a redox flow battery comprising: a. a posolyte comprising a redox molecule; and b. a negolyte comprising: • at least one compound of formula (II) or formula (III), or an anion thereof, obtained by one of the processes according to the invention; or • the electrolysis solution of the electrolytic cell obtained at the end of the electrosynthesis process; or • the flow fuel cell negolyte obtained at the end of the synthesis process in a flow fuel cell.

[0099] A redox flow battery according to the present invention has electrochemical performances equivalent to or even superior to conventional redox flow batteries in which the negolyte comprises the anthraquinone corresponding to the compound of formula (II) or formula (III).

[0100] Advantageously, when the at least one compound of formula (II) or of formula (III), or an anion thereof, is obtained by the synthesis method in a flow fuel cell according to the present invention, the negolyte of the redox flow battery comprises the negolyte of the flow fuel cell obtained at the end of the step of electrochemical oxidation of the compound of formula (I) and the posolyte of the redox flow battery comprises the posolyte of the flow fuel cell obtained at the end of the step of electrochemical oxidation of the compound of formula (I). PREFERRED EMBODIMENTS

[0101] A first preferred subject of the present invention is a process for the electrosynthesis of 4-(3-hydroxy-1,4-dioxo-1,4-dihydronaphthalen-2-yl)-4-oxobut-2-enoic acid, or an anion thereof, in an electrolytic cell comprising an electrolysis solution, an anode and a cathode; said process comprising a step electrochemical oxidation at the anode of the electrolytic cell of alizarin, the latter being included in the electrolysis solution whose pH varies between 10 and 14; the method further optionally comprising an isolation step.

[0102] A second preferred subject of the present invention is a process for the electrosynthesis of 4-(3-hydroxy-1,4-dioxo-1,2,3,4-tetrahydronaphthalen-2-yl)penta-2,4-dienoic acid, or an anion thereof, in an electrolytic cell comprising an electrolysis solution, an anode and a cathode; said process comprising a step of electrochemical oxidation at the anode of the electrolytic cell of quinazirine, the latter being included in the electrolysis solution whose pH varies between 10 and 14; the process further optionally comprising an isolation step.

[0103] A third preferred subject of the present invention is a process for the synthesis of 4-(3-hydroxy-1,4-dioxo-1,4-dihydronaphthalen-2-yl)-4-oxobut-2-enoic acid, or an anion thereof, in a flow fuel cell comprising a negolyte, a posolyte, an anode and a cathode; said process comprising a step of electrochemical oxidation at the anode of the flow fuel cell of alizarin; the latter being included in the negolyte whose pH varies between 10 and 14; the posolyte comprising an oxidant; the process further optionally comprising an isolation step.

[0104] A fourth preferred object of the present invention is a process for the synthesis of 4-(3-hydroxy-1,4-dioxo-1,2,3,4-tetrahydronaphthalen-2-yl)penta-2,4-dienoic acid, or an anion thereof, in a flow fuel cell comprising a negolyte, a posolyte, an anode and a cathode; said process comprising a step of electrochemical oxidation at the anode of the flow fuel cell of qui-nizarin; the latter being included in the negolyte whose pH varies between 10 and 14; the posolyte comprising an oxidant; the process further optionally comprising an isolation step. EXAMPLES

[0105] The present invention will be better understood by reading the following examples which illustrate the invention in a non-limiting manner.

[0106] Example 1: Electrosynthesis of acid 4-(3-hvdroxv-L4-dioxo-L4-dihvdronaphthalen-2-vl)-4-oxobut-2-enoic

[0107] Materials and methods

[0108] A percolation electrolytic cell comprising a graphite felt (surface area of ​​about 1 cm2) as a working electrode (Le., as anode) is used at room temperature. The electrolysis solution (volume = 70 mL) has a pH equal to 14 ([KOH] = 1 mol / L) and comprises alizarin (concentration = 0.05 mol / L). The flow rate of the electrolysis solution and the electrolyte solution of the compartment comprising the counter electrode (i.e., the cathode) is 20 mL / min. The electrochemical reaction taking place at the cathode is the reduction of water to dihydrogen. The potential applied to the working electrode is - 0.05 V vs Ag / AgCl. During electrolysis, argon bubbling is maintained in the electrolysis solution and in the electrolyte solution of the compartment comprising the cathode. After 50 hours of electrolysis, the final product is isolated as follows: the electrolysis solution is recovered and acidified with dilute HCl until a precipitate is obtained. Said solution is then filtered on a frit and the solid product obtained by filtration is dried using a vacuum ramp at room temperature. The dried solid obtained is purified by solubilization in ethanol, filtration on silica and evaporation of the ethanol from the filtrate.

[0109] Results

[0110] The final product obtained is the acid 4-(3-hydroxy-1,4-dioxo-1,4-dihydronaphthalen-2-yl)-4-oxobut-2-enoic acid and the yield before isolation, determined by analytical electrochemistry (rotating electrode), is 50-60%. [Chem 11] O 4-(3-hydroxy-l,4-dioxo-l,4-dihydronaphthalen-2-yl)-4-oxobut-2-enoic acid (C14H8O6). NMR Hz, 4J = 1.2 Hz, H14), 7.72 (1H, td, 3J = 7.4 Hz, 4J = 1.2 Hz, H12), 7.69 (1H, d, Jtrans = 16 Hz, H3), 7.61 (1H, td, 3J = 7.4 Hz, 4J = 1.2 Hz, H13). 13C NMR (MeOD-d4, 268 K, 500 MHz): ô (ppm): 201.47 (C2), 186.75 (C7), 184.67 (CIO), 176.41 (C6), 175.68 (Cl), 146.64 (C4), 137.76 (C9), 136.26 (C12), 134.15 (C8), 133.48 (C13), 128.17 (Cil), 127.91 (C14), 120.98 (C3), 116.52 (C5). ESI-MS: m / z = 271 [MH] for Ci4H8O6 Fragmentation: m / z = 227.045 [MH] for Ci3H7O4 m / z = 199.050 [MH] for Ci2H7O3 m / z = 171.045 [MH] for CnH7O2 m / z = 143.050 [MH] for Ci0H7O. [YES] Example 2: Synthesis of acid 4-f 3-hydroxy-1.4-dioxo-1.4-dihydronaphthalen-2-yl)-4-oxobut-2-enoic

[0112] Materials and methods

[0113] A flow fuel cell is used at room temperature. The electrodes used are graphite felts 4 mm thick and 25 cm2 in surface area. The negolyte of said cell (volume = 250 mL) has a pH equal to 14 ([KOH] = 1 mol / L) and includes alizarin (concentration = 0.1 mol / L). The cell posolyte (volume = 1000 mL) has a pH between 13 and 14 ([KOH] = 0.3 mol / L) and includes potassium iron ricyanide (concentration = 0.2 mol / L). The flow rate of both electrolyte solutions, at the anode and cathode, is 100 mL / min. Oxidation begins once the negative pole of the cell and the positive pole of the cell are connected together. The separator used is a 50 pm Nafion cation exchange membrane. The current displayed at start-up is around 2.3 A. The potential difference between the anode and the cathode of the flow fuel cell is maintained at 200 mV. Oxidation is stopped when the current intensity reaches the value of 35 mA. The electrical capacity then obtained is 4.1 Ah. An aliquot of 1 mL of the negolyte is then taken and acidified with diluted HCl until a precipitate is obtained.Said aliquot is then filtered on a frit and the solid product obtained by filtration is dried using a vacuum ramp at room temperature. The dried solid obtained is purified by solubilization in ethanol, filtration on silica and evaporation of the ethanol from the filtrate.

[0114] Results

[0115] The final product obtained is the acid 4-(3-hydroxy-1,4-dioxo-1,4-dihydronaphthalen-2-yl)-4-oxobut-2-enoic acid and the final yield, determined by 'H NMR spectroscopy in the presence of an internal standard, is 82%.

[0116] Example 3: Use of the negolyte from the flow fuel cell of Example 2 as a negolyte in a redox flow battery

[0117] Materials and methods

[0118] A redox flow battery is used at room temperature. A sample of the negolyte from the flow fuel cell of Example 2, obtained at the end of the alizarin oxidation step, is used without purification as a negolyte (volume = 25 mL) in the redox flow battery. A sample of the posolyte from the flow fuel cell of Example 2, obtained at the end of the alizarin oxidation step, is used without purification as a posolyte (volume = 25 mL) in the redox flow battery. The flow rate of the negolyte and posolyte from the redox flow battery is 50 mL / min. The battery is cycled and the number of charge-discharge cycles performed is 730.

[0119] Results

[0120] The coulombic efficiency of the battery is 99.86%, which means that the delivered charge is almost completely restored and that side reactions are negligible. The energy efficiency is 86.43%, which is comparable to the values ​​obtained with anthraquinones as a redox molecule. The capacity drop per cycle is 0.030%, which corresponds to a capacity retention of 99.970% per cycle which is higher than the capacity retention of alizarin (with Ed&harge = 0.6 V) which is equal to 99.921% per cycle.

[0121] Thus, a redox flow battery which comprises a negolyte comprising a compound of formula (II) or formula (III), or an anion thereof, has electrochemical performances equivalent to or even superior to conventional redox flow batteries in which the negolyte comprises an anthraquinone.

Claims

Claims

1. A process for the electrosynthesis of a compound of formula (II) or of formula (III) or a union thereof: [Chem 1] in which R1 and R4, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, or a nitro; R2 and R3, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, a nitro, or a hydroxyl; R6 represents a straight or branched C1-C6 alkyl chain, hydrogen, halogen, sulfonate, carboxylic acid, ketone, ester, ether oxide, thiol, thioalkyl, amine, nitro, or hydroxyl; R8 or R9 represents a linear or branched C1 to C6 alkyl chain, hydrogen, halogen, sulfonate, carboxylic acid, ketone, ester, ether oxide, thiol, thioalkyl, amine, or nitro; represents a single bond which can be on either side of the double bond; in an electrolytic cell comprising an electrolysis solution, an anode and a cathode; said method comprising a step of electrochemical oxidation at the anode of the electrolytic cell of a compound of formula (I): [Chem 3]

2. in which R2, R3, R4 and R6 are as defined above, R5 and R7, identical or different, represent independently of one another a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, a nitro, or a hydroxyl; preferably, at least one of R5 and R7 represents a hydroxyl; said compound of formula (I) being included in the electrolysis solution whose pH varies between 10 and 14. A process for the synthesis of a compound of formula (II) or formula (III) or an anion thereof: [Chem 1] in which R1 and R4, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, or a nitro; R2 and R3, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, a nitro, or a hydroxyl; R6 represents a straight or branched C1-C6 alkyl chain, hydrogen, halogen, sulfonate, carboxylic acid, ketone, ester, ether oxide, thiol, thioalkyl, amine, nitro, or hydroxyl; R8 or R9 represents a linear or branched alkyl chain, C1 to C6, hydrogen, halogen, sulfonate, carboxylic acid, ketone, ester, ether oxide, thiol, thioalkyl, amine, or nitro; represents a single bond which can be on either side of the double bond; in a flow fuel cell comprising a negolyte, a posolyte, an anode and a cathode; said method comprising a step of electrochemical oxidation at the anode of the flow fuel cell of a compound of formula (I): [Chem 3] in which RB R2, R3, R4 and R6 are as defined above, R5 and R7, identical or different, represent independently of each other a linear or branched alkyl chain, C1 to C6, a hydrogen, a halogen, a sulfonate, a carboxylic acid, a ketone, an ester, an ether-oxide, a thiol, a thio-alkyl, an amine, a nitro, or a hydroxyl; preferably, at least one of R5 and R7 represents a hydroxyl; said compound of formula (I) being included in the negolyte whose pH varies between 10 and 14, the posolyte comprising an oxidant.

3. A method according to claim 1 or claim 2, characterized in that that only one of R5 and R7 represents a hydroxyl and preferably the other of R5 and R7 represents a hydrogen.

4. Method according to any one of claims 1 to 3, characterized in that R6 represents hydrogen and / or R1 to R4 are identical and represent hydrogen, preferably R6 represents hydrogen and R1 to R4 are identical and represent hydrogen.

5. Method according to any one of claims 2 to 4, characterized in that the difference in redox potential between the oxidant and the compound of formula (I) is between 50 and 600 mV, preferably between 100 and 400 mV and more preferably it is equal to 200 mV.

6. Method according to any one of claims 2 to 5, characterized in that the oxidation potential of the compound of formula (I) is between - 0.2 and 0.2 V vs Ag / AgCl.

7. Method according to any one of the preceding claims, characterized in that it further comprises an additional step of isolating the compound of formula (II) or of formula (III), or an anion thereof, preferably the additional isolation step comprises the following successive steps: a) acidification of the electrolysis solution of the electrolytic cell obtained at the end of the method according to any one of claims 1, 3 or 4 or of the negolyte of the flow fuel cell obtained at the end of the method according to any one of claims 2 to 6 until a precipitate is obtained, to obtain an acidified electrolyte, and b) filtration of the acidified electrolyte of step a);or a step of concentrating the electrolysis solution of the electrolytic cell obtained at the end of the method according to any one of claims 1, 3 or 4 or the negolyte of the flow fuel cell obtained at the end of the method according to any one of claims 2 A / x;

8. d O. Use of a compound of formula (II) or formula (III), or an anion thereof, obtained by the process according to any one of claims 1 to 7, as a redox molecule in the negolyte of a redox flow battery.

9. Use of the electrolysis solution of the electrolytic cell obtained at the end of the method according to any one of claims 1, 3 or 4 or of the negolyte of the flow fuel cell obtained at the end of the method according to any one of claims 2 to 6 as negolyte in a redox flow battery, preferably the posolyte of the flow fuel cell obtained at the end of the method according to any one of claims 2 to 6 can also be used as a posolyte in the redox flow battery.

10. A redox flow battery comprising: (a) a posolyte comprising a redox molecule; and (b) a negolyte comprising: • at least one compound of formula (II) or formula (III), or an anion thereof, obtained by the process according to any one of claims 1 to 7, or • the electrolysis solution of the electrolytic cell obtained at the end of the process according to any one of claims 1, 3 or 4, or • the flow fuel cell negolyte obtained at the end of the process according to any one of claims 2 to 6.

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