WO2018160618A1

WO2018160618A1 - High-and low-potential, water-soluble, robust quinones

Info

Publication number: WO2018160618A1
Application number: PCT/US2018/020086
Authority: WO
Inventors: Shannon S. Stahl; James B. Gerken; Colin W. ANSON; Sung-Eun Suh; Alexiouis G. STAMOULIS
Original assignee: Wisconsin Alumni Research Foundation
Priority date: 2017-02-28
Filing date: 2018-02-28
Publication date: 2018-09-07

Abstract

Substituted hydroquinones, 1,4-quinones, catechols, 1,2-quinones, anthraquinones, and anthrahydroquinones are disclosed herein. The substituted hydroquinones and catechols have the formula: while the substituted 1,4-quinones or 1,2-have the corresponding oxidized structure (1,4- benzoquinones and 1,2-benzoquinones). One or more of R1, R2, R3 and R4 include a sulfonate moiety, a sulfonimide moiety, or a phosphonate moiety, and any of R1, R2, R3 and R4 that do not include one of these moieties include an alkyl, a cycloalkyl, a thioether, a sulfoxide, a sulfone, a haloalkyl, a halogen, a nitrile, an imide, a pyrazole, or combinations thereof. The substituted anthraquinones have the formula: while the substituted anthrahydroquinones have the corresponding reduced structure. One or more of R1-R8 have a sulfonate or phosphate tethered to the ring by a thi other, amine, or ether including one or more alkyl groups. Any of R1-R8 that do not contain one of these moieties include an alkyl, a cycloalkyl, a thiother, a sulfoxide, a sulfone, a haloalkyl, a halogen, a hydroxyl, an alkoxyl, an ether, an amine, or hydrogen The substituted hydroquinones, 1,4-quinones, catechols, 1,2-quinones, anthraquinones, or anthrahydroquinones are soluble in water, stable in aqueous acid solutions, and have useful reduction potentials in the oxidized form. Accordingly, they can be used as redox mediators in emerging technologies, such as in mediated fuel cells or organic-mediator flow batteries.

Description

HIGH- AND LOW-POTENTIAL, WATER-SOLUBLE, ROBUST QUINONES CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisional Application No.62/464,441 filed on 2/28/2017, which is incorporated by reference herein in its entirety. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

[0002] This invention was made with government support under DE-AC05-76RL01830 awarded by the US Department of Energy. The government has certain rights in the invention. BACKGROUND OF THE INVENTION

[0003] The quinone/hydroquinone redox couple is used in many different technologies and has been extensively studied. In U.S. Patent Publication No.2015/0263371, which is incorporated by reference herein in its entirety, we disclosed using the quinone/hydroquinone redox couple as a charge transfer mediator to facilitate more efficient electrocatalytic oxygen reduction in electrochemical cells. However, in the context of emerging electrochemical cell technologies, such as organic mediator flow batteries and mediated fuel cells, the available quinones are inadequate.

[0004] In the context of such technologies, effective redox mediators must have a reduction potential close to the thermodynamic potential for reduction of oxygen to water, high solubility in water, and stability in acidic aqueous solutions under the conditions used in such applications. Although the unsubstituted hydroquinone/1,4-benzoquinone redox couple has a sufficiently high reduction potential in the oxidized form, it has relatively low solubility in water and is unstable in acid solution.

[0005] Hydroquinone can be sulfonated to yield useful compounds, such as the commercially available potassium hydroquinone monosulfonate. More vigorous sulfonation conditions give rise to the 2,5- and 2,6-disulfonated isomers.¹ These sulfonate salts have high water solubility compared to the parent hydroquinone, and the solubility of the acid is even higher (see Figure 1). Aerobic or electrochemical oxidation of these compounds produces the corresponding para-quinone. Sulfonation of catechol gives the 3,5-disulfonate, which can be oxidized to an ortho-quinone. These quinones have been proposed as redox-active species in flow batteries.² [0006] We undertook experiments on these quinones to determine their suitability for use in a mediated fuel cell. To our dismay, the mono- and di-substituted quinones described above all proved to be unstable in aqueous acid, even at low temperature (see Scheme 1). The condensation of the mono-sulfonated quinone is presumed to be analogous to previously studied decomposition reactions of benzoquinone and toluquinone.³ The presence of two sulfonate groups prevents polymerization, but addition of water still takes place, even in 1 M H2SO4. Although the resulting dihydroxyquinone disulfonates are stable in solution, their reduction potentials are too low to be useful in a flow battery or fuel cell. The addition of water has been since confirmed for the disulfonated ortho-quinone in a recent paper by Yang, et al.⁴

[0007] Scheme 1: Quinone decomposition pathways that prevent reversible cycling. The quinone products of the bottom two reactions have unusably low reduction potentials.

[0008] Accordingly, there is a need in the art for highly substituted hydroquinones/quinones with substituents that result in a quinone reduction potential at least as high as the reduction potential of unsubstituted benzoquinone, while also having the greater stability exhibited by low-potential polysubstituted quinones. Water soluble substituted hydroquinones/quinones having such properties could function as improved redox mediators in electrochemical cells, particularly to facilitate oxygen reduction in mediated fuel cells or in organic-mediator flow batteries. SUMMARY OF THE INVENTION

[0009] We disclose herein highly substituted hydroquinones/quinones with useful reduction potentials that are also water soluble and stable in acid solution.

[0010] In a first aspect, the disclosure encompasses a number of substituted 1,4- hydroquinones and substituted 1,4-quinones. The substituted hydroquinones have the chemical formula:

where (a) one, two, three, or all four of R¹, R², R³ and R⁴ include a sulfonate or sulfonimide moiety, and each R¹, R², R³ and R⁴ that does not include a sulfonate or sulfonimide moiety is independently an alkyl, a cycloalkyl, a thioether, a sulfoxide, a sulfone, a haloalkyl, a halogen, a nitrile, an imide, a phosphonate, a pyrazole, or combinations thereof; or (b) one, two, three, or all four of R¹, R², R³ and R⁴ include a phosphonate moiety, and each R¹, R², R³ and R⁴ that does not include a phosphonate moiety is independently an alkyl, a cycloalkyl, a thioether, a sulfoxide, a sulfone, a haloalkyl, a halogen, a nitrile, an imide, a pyrazole, or combinations thereof. This first aspect also includes the oxidized forms (the substituted 1,4- benzoquinone forms) of the described substituted hydroquinones.

[0011] In some embodiments, each R¹, R², R³ and R⁴ that includes a sulfonate moiety is (a) a sulfonate moiety directly bound to the hydroquinone or quinone ring, or (b) includes both a sulfonate moiety and a linking group that links the sulfonate moiety to the hydroquinone or quinone ring.

[0012] In some embodiments, each R¹, R², R³ and R⁴ that includes a phosphonate moiety is (a) a phosphonate moiety directly bound to the hydroquinone or quinone ring, or (b) includes both a phosphonate moiety and a linking group that links the phosphonate moiety to the hydroquinone or quinone ring. [0013] In some embodiments, all four of R¹, R², R³ and R⁴ cannot be sulfonate directly bound to the hydroquinone or quinone ring.

[0014] In some embodiments, all four of R¹, R², R³ and R⁴ cannot be phosphonate directly bound to the hydroquinone or quinone ring.

[0015] In some embodiments, if R², R³ and R⁴ are all–Cl, then R¹ cannot be sulfonate directly bound to the hydroquinone or quinone ring.

[0016] In some embodiment, if R² and R⁴ are both–CH3, then R¹ and R³ cannot both be sulfonate directly bound to the hydroquinone or quinone ring.

[0017] In some embodiments, each linking group may include an ester, an amide, a sulfonamide, an imide, a sulfone, a sulfoxide, a thioether, a ketone, one or more alkyl chains, an aromatic ring, a pyrazole, or combinations thereof.

[0018] In some embodiments, R¹ and R², R³ and R⁴, or both R¹/R² and R³/R⁴ each include a single linking group attached to the hydroquinone or quinone ring at the two designated positions.

[0019] In some embodiments, one or more of the linking groups are selected from the imide - CONYCO-, where Y is -CH2CH2- or a benzene ring; the amide -CONHY, where Y is -CH₂CH₂- or a benzene ring; pyrazole; or dimethylpyrazole.

[0020] In some embodiments, R¹ and R², R³ and R⁴, or both R¹/R² and R³/R⁴ each include a single sulfonimide attached to the hydroquinone or quinone ring at the two designated positions.

[0021] In some embodiments, multiple substituted hydroquinone or quinone structures as described above are covalently linked together to form a substituted bicyclic, tricyclic or polycyclic hydroquinone or quinone. In some such embodiments, the substituted

hydroquinone or quinone includes 2, 3, 4, 5 or 6 covalently linked hydroquinone or quinone structures as described above.

[0022] In some embodiments, the multiple substituted hydroquinone or quinone structures are covalently linked through one or more of the R¹, R², R³ or R⁴ that do not comprise a sulfonate moiety, sulfonimide moiety, or phosphonate moiety. In some such embodiments, the multiple substituted hydroquinone or quinone structures are covalently linked through one or more alkyls. In some such embodiments, one or more of the alkyls through which the multiple substituted hydroquinone or quinone structures are covalently linked are selected from -CH₂- (methanediyl) or -CH(-)₂ (methanetriyl). In some such embodiments, the -CH₂- covalently links two different substituted hydroquinone or quinone structures, or the -CH(-)2 covalently links three different hydroquinone or quinone structures. [0023] In some embodiments, one or more of the R¹, R², R³ or R⁴ that include a sulfonate moiety, sulfonimide moiety, or phosphonate moiety are independently selected from the imide -CONRCO- attached at R¹ and R² or R³ and R⁴, where R is

the amide -CONHR, wherein R is

attached at R¹ and R² or R³ and R⁴.

[0024] In some embodiments, one or more of the R¹, R², R³ or R⁴ that do not include a sulfonate moiety, a sulfonimide moiety, or a phosphonate moiety are independently selected from -Cl, -CH3, -CH2-, -CF3, -CN, -CH(-)2 , and attached at R¹ and R² or R³ and R⁴. In some such embodiments, the -C

H₂- links two

different substituted hydroquinone or quinone structures, or the -CH(-)2 links three different hydroquinone or quinone structures.

[0025] In some embodiments, the substituted hydroquinone or quinone is more soluble in water than the corresponding unsubstituted hydroquinone or quinone, is stable in 1 M H2SO4, and has a reduction potential in the oxidized form that is equal to or greater than the reduction potential of the corresponding unsubstituted quinone (1,4-benzoquinone).

[0026] In some embodiments, the substituted hydroquinone or quinone is one of the compounds shown in Figures 2-5 or a salt, acid form, reduced form, or oxidized form of any of these compounds.

[0027] In some embodiments, the substituted hydroquinone is one of the following compounds, or a salt, acid form, reduced form, or oxidized form of any of these compounds:

[0028] In a secon aspect, t e sc osure encompasses a num er o su st tute 1,2- hydroquinones and substituted 1,2-quinones. The substituted hydroquinones have the chemical formula:

w ere a one, wo, three, or all four of R¹, R², R³ and R⁴ include a sulfonate or sulfonimide moiety, and each R¹, R², R³ and R⁴ that does not include a sulfonate or sulfonimide moiety is independently an alkyl, a cycloalkyl, a thioether, a sulfoxide, a sulfone, a haloalkyl, a halogen, a nitrile, an imide, a phosphonate, a pyrazole, or combinations thereof; or (b) one, two, three, or all four of R¹, R², R³ and R⁴ include a phosphonate moiety, and each R¹, R², R³ and R⁴ that does not include a phosphonate moiety is independently an alkyl, a cycloalkyl, a thioether, a sulfoxide, a sulfone, a haloalkyl, a halogen, a nitrile, an imide, a pyrazole, or combinations thereof. This second aspect also includes the oxidized forms (the substituted 1,2-benzoquinone forms) of the described substituted hydroquinones.

[0029] In some embodiments, each R¹, R², R³ and R⁴ that includes a sulfonate moiety is (a) a sulfonate moiety directly bound to the hydroquinone or quinone ring, or (b) includes both a sulfonate moiety and a linking group that links the sulfonate moiety to the hydroquinone or quinone ring.

[0030] In some embodiments, each R¹, R², R³ and R⁴ that includes a phosphonate moiety is (a) a phosphonate moiety directly bound to the hydroquinone or quinone ring, or (b) includes both a phosphonate moiety and a linking group that links the phosphonate moiety to the hydroquinone or quinone ring.

[0031] In some embodiments, each linking group may include an ester, an amide, a sulfonamide, an imide, a sulfone, a sulfoxide, a thioether, a ketone, one or more alkyl chains, an aromatic ring, a pyrazole, or combinations thereof.

[0032] In some embodiments, R¹ and R², R³ and R⁴, or both R¹/R² and R³/R⁴ each include a single linking group attached to the hydroquinone or quinone ring at the two designated positions.

[0033] In some embodiments, one or more of the linking groups are selected from the imide - CONYCO-, where Y is -CH2CH2- or a benzene ring; the amide -CONHY, where Y is -CH₂CH₂- or a benzene ring; pyrazole; or dimethylpyrazole.

[0034] In some embodiments, R¹ and R², R³ and R⁴, or both R¹/R² and R³/R⁴ each include a single sulfonimide attached to the hydroquinone or quinone ring at the two designated positions.

[0035] In some embodiments, multiple substituted hydroquinone or quinone structures as described above are covalently linked together to form a substituted bicyclic, tricyclic or polycyclic hydroquinone or quinone. In some such embodiments, the substituted

[0036] In some embodiments, the multiple substituted hydroquinone or quinone structures are covalently linked through one or more of the R¹, R², R³ or R⁴ that do not comprise a sulfonate moiety, sulfonimide moiety, or phosphonate moiety. In some such embodiments, the multiple substituted hydroquinone or quinone structures are covalently linked through one or more alkyls. In some such embodiments, one or more of the alkyls through which the multiple substituted hydroquinone or quinone structures are covalently linked are selected from -CH₂- (methanediyl) or -CH(-)₂ (methanetriyl). In some such embodiments, the -CH₂- covalently links two different substituted hydroquinone or quinone structures, or the -CH(-)2 covalently links three different hydroquinone or quinone structures.

[0037] In some embodiments, one or more of the R¹, R², R³ or R⁴ that include a sulfonate moiety, sulfonimide moiety, or phosphonate moiety are independently selected from the imide -CONRCO- attached at R¹ and R² or R³ and R⁴, where R is

; attached at R¹ and R² or R³ and R⁴.

[0038] In some embodiments, one or more of the R¹, R², R³ or R⁴ that do not include a sulfonate moiety, a sulfonimide moiety, or a phosphonate moiety are independently selected from -Cl, -CH3, -CH2-, -CF3, -CN, -CH(-)2, , and attached at R¹ and R² or R³ and R⁴. In some such embodiments, the -CH₂- links two different substituted hydroquinone or quinone structures, or the -CH(-)2 links three different hydroquinone or quinone structures.

[0039] In some embodiments, the substituted hydroquinone or quinone is more soluble in water than the corresponding unsubstituted hydroquinone or quinone, is stable in 1 M H2SO4, and has a reduction potential in the oxidized form that is equal to or greater than the reduction potential of the corresponding unsubstituted quinone (1,4-benzoquinone).

[0040] In some embodiments, the substituted hydroquinone or quinone is one of the compounds shown in Figure 6 or a salt, acid form, reduced form, or oxidized form of any of these compounds.

[0041] .In some embodiments, the substituted hydroquinone is one of the following compounds, or a salt, acid form, reduced form, or oxidized form of any of these compounds:

[0042] In a third aspect, the disclosure encompasses a number of substituted 9,10- anthrahydroquinones and substituted 9,10-anthraquinones. The substituted anthraquinones have the chemical formula:

,

w ere a at east one o R¹ - R⁸ includes both a sulfonate moiety and a linking group that links the sulfonate moiety to the anthraquinone ring and each R¹ - R⁸ that does not include a sulfonate moiety is independently an alkyl, a cycloalkyl, a thioether, a hydroxy, an amino, or hydrogen, or combinations thereof; or (b) at least one of R¹ - R⁸ includes both a phosphonate moiety and a linking group that links the phosphonate moiety to the anthraquinone ring and each R¹ - R⁸ that does not include a phosphonate moiety is independently an alkyl, a cycloalkyl, a thioether, a hydroxy, an amino, or hydrogen, or combinations thereof. This third aspect also includes the reduced forms (the substituted 9,10-anthrahydroquinone forms) of the described substituted anthraquinones. [0043] In some embodiments, each linking group may include an ether, an amine, a sulfonamide, an imide, a sulfone, a sulfoxide, a thioether, a ketone, one or more alkyl chains, or combinations thereof.

[0044] In some embodiments, R¹ and R², R³ and R⁴, R⁵ and R⁶, R⁷ and R⁸, R² and R³, R⁶ and R⁷, or multiple non-overlapping combinations of the above pairs each include a single linking group attached to the anthraquinone ring at the two designated positions.

[0045] In some embodiments, the substituted anthraquinone is one of the following compounds, or a salt, acid form, reduced form, or oxidized form of any of these compounds:

.

n a ourt aspect, te scosure encompasses a systemncu ng a liquid electrolyte solution in contact with an electrode, wherein the electrolyte solution includes a substituted hydroquinone,1,4-quinone, catechol, 1,2-quinone, anthraquinone or anthrahydroquinone as described above dissolved therein. [0047] In some embodiments, the electrolyte solution is an aqueous solution. In some embodiments, the electrolyte solution includes an organic solvent.

[0048] In some embodiments, the electrolyte solution further includes oxygen.

[0049] In a fifth aspect, the disclosure encompasses an electrochemical cell that includes the system described above in ionic communication with an anodic half-cell.

[0050] In some embodiments, the disclosure encompasses an electrochemical cell that includes the system described above in ionic communication with a cathodic half-cell.

[0051] In some embodiments, the cell is a fuel cell or a flow battery.

[0052] In a sixth aspect, the disclosure encompasses a method of producing electricity. The method includes the steps of contacting the anodic half-cell of the cell described above with a fuel, and contacting the system of the cell described above with oxygen, whereby the fuel is oxidized, oxygen is reduced, and electricity is produced.

[0053] In a seventh aspect, the disclosure encompasses a substituted hydroquinone, 1,4- quinone, catechol, 1,2-quinone, anthraquinone, or anthrahydroquinone as described above for use as a redox mediator in a fuel cell or in a flow battery.

[0054] In an eighth aspect, the disclosure encompasses a method of making a substituted hydroquinone, 1,4-quinone, catechol, 1,2-quinone, anthraquinone, or anthrahydroquinone as described above. Such methods are described in detail in the examples below.

[0055] The following descriptions are of certain exemplary embodiments, and should not be considered limiting. The full scope of the invention is defined by the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 illustrates a previously reported hydroquinone sulfonate synthesis, along with reported reactant, intermediate, and reactant solubilities in water.

[0057] Figure 2 shows the chemical structures of 28 exemplary substituted hydroquinones (compounds 1, 3, 6, 11a-d, 12a-d, 16-18, 21-30, 35a-d).

[0058] Figure 3 shows the chemical structure of an additional 21 exemplary substituted hydroquinones and quinones (compounds 36, 39-43, 44a-d, 45a-d, 46-47, 48a-d, 49).

[0059] Figure 4 shows the chemical structure of an additional 17 exemplary substituted hydroquinones and quinones containing thioether-linked sulfonates (compounds 52-54, 57- 70).

[0060] Figure 5 shows the aliphatic region of the 1H and the full 13C NMR of compound 52 and MESNA, mercaptoethanesulfonate, Na salt. [0061] Figure 6 shows the chemical structure of an additional 11 exemplary substituted hydroquinones and quinones (compounds 73-77, 79, 85-86, 88, 90, 92).

[0062] Figure 7 shows the chemical structure of 12 exemplary substituted 1,2-hydroquinones and 1,2-quinones (compounds 93 - 104).

[0063] Figure 8 shows the chemical structure of 12 exemplary substituted 9,10- anthrahydroquinones and 9,10-anthraquinones (compounds 105 - 116).

[0064] Figure 9 shows the 1H and 13C NMR spectrum of compound 109.

[0065] Figure 10 shows the chemical structure of 12 exemplary substituted 9,10- anthrahydroquinones and 9,10-anthraquinones (compounds 117 - 128).

[0066] Figure 11 shows the chemical structure of 12 exemplary substituted 9,10- anthrahydroquinones and 9,10-anthraquinones (compounds 129 - 140). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

I. IN GENERAL

[0067] This disclosure is not limited to the particular methodology, protocols, materials, and reagents described, as these may vary. Furthermore, the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention which will be limited only by the pending claims.

[0068] As used herein and in the appended claims, the singular forms“a”,“an”, and“the” include plural reference unless the context clearly dictates otherwise. As well, the terms“a” (or“an”),“one or more” and“at least one” can be used interchangeably herein. The terms “comprising”,“including”, and“having” can be used interchangeably.

[0069] Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the methods and materials of several embodiments now described. All publications and patents specifically mentioned herein are incorporated by reference in their entirety for all purposes. II. THE INVENTION

[0070] We have developed highly substituted hydroquinones/1,4-quinones and catechols/1,2- quinones, as well as substituted anthraquinones/anthrahydroquinones that are water soluble and stable in acid solution. In order to confer water solubility, we constrained the quinones to have at least one sulfonate, sulfonimide or phosphonate group. [0071] There are sparse reports of 1,4-quinones that satisfy these requirements, but reproducing that literature has been difficult (e.g., compounds 1 and 3 in Figure 2).⁵ Some quinones bearing these motifs have been described in the computational literature, but not with any method for their synthesis or with a method of using them in a mediated fuel cell.⁶

[0072] In some embodiments, the disclosed hydroquinones/quinones and

anthraquinones/anthrahydroquinones are substituted with one or greater groups that each consist of or comprise a sulfonate moiety.

[0073] The term "sulfonate" or“sulfonate moiety” as used herein refers to a substituent having the general structure:

[0074] , as well as the corresponding salts, acid (terminating with–OH instead

of–O-), and esters. R is either the hydroquinone/quinone ring (i.e., the sulfonate is directly bound to the hydroquinone or quine ring) or a linking group that links the sulfonate moiety to the hydroquinone/1,4-quinone, catechol/1,2-quinone or anthraquinone/anthrahydroquinone ring.

[0075] Each linking group independently includes an ester, an amide, a sulfonamide, an imide, a sulfone, a sulfoxide, a thioether, a ketone, one or more alkyl chains, an aromatic ring, a pyrazole, or any combination thereof. A non-limiting example of an aromatic ring that could be included in the linking group is a benzene ring. Non-limiting examples of the one or more alkyl chains that could be included in the linking group include unbranched C1 or C2 alkyl chains, and unbranched, branched or cyclic C3, C4, C5, C6, C7, C8, C9 or C10 alkyl chains. Multiple alkyl chains or aromatic rings may be included in the linking group, separated by one or more of the other listed groups. Furthermore, if one or more of the alkyl chains is branched, one or more of the groups that comprises a sulfonate moiety may include two or more sulfonate moieties.

[0076] In some embodiments, the disclosed hydroquinones/1,4-quinones, catechol/1,2- quinone and anthraquinones/anthrahydroquinones are substituted with one or greater groups that each consist of or comprise a phosphonate moiety.

[0077] The term "phosphonate" or“phosphonate moiety” as used herein refers to a substituent having the general formula R-PO₃ ^2-, as well as the corresponding salts, acids (R- PO3H- and R-PO3H2), and esters. R is either the hydroquinone/quinone ring (i.e., the phosphonate is directly bound to the hydroquinone or quinone ring) or a linking group that links the phosphonate moiety to the hydroquinone/1,4-quinone, catechol/1,2-quinone, or anthraquinone/anthrahydroquinone ring.

[0078] Each linking group independently includes an ester, an amide, a sulfonamide, an imide, a sulfone, a sulfoxide, a thioether, a ketone, one or more alkyl chains, an aromatic ring, a pyrazole, or any combination thereof. A non-limiting example of an aromatic ring that could be included in the linking group is a benzene ring. Non-limiting examples of the one or more alkyl chains that could be included in the linking group include unbranched C1 or C2 alkyl chains, and unbranched, branched or cyclic C₃, C₄, C₅, C₆, C₇, C₈, C₉ or C₁₀ alkyl chains. Multiple alkyl chains or aromatic rings may be included in the linking group, separated by one or more of the other listed groups. Furthermore, if one or more of the alkyl chains is branched, one or more of the groups that comprises a phosphonate moiety may include two or more phosphonate moieties.

[0079] In some embodiments, the disclosed hydroquinones/quinones are substituted with one, two, three or four groups that each consist of or comprise a sulfonimide moiety.

[0080] The term“sulfonimide moiety” as used herein refers to a substituent having the general formula R-SO2NHSO2-R’. R and R’ can be the hydroquinone/quinone ring (i.e., the sulfonimide is directly bound in two places to the hydroquinone or quine ring(e.g., at R¹/R² or at R³/R⁴) or linking groups that links the sulfonimide moiety to the hydroquinone/quinone ring.

[0081] Each linking group independently includes an ester, an amide, a sulfonamide, an imide, a sulfone, a sulfoxide, a thioether, a ketone, one or more alkyl chains, an aromatic ring, a pyrazole, or any combination thereof. A non-limiting examples of an aromatic ring that could be included in the linking group is a benzene ring. Non-limiting examples of the one or more alkyl chains that could be included in the linking group include unbranched C1 or C₂ alkyl chains, and unbranched, branched or cyclic C₃, C₄, C₅, C₆, C₇, C₈, C₉ or C₁₀ alkyl chains. Multiple alkyl chains or aromatic rings may be included in the linking group, separated by one or more of the other listed groups.

[0082] In some embodiments, each of the quinone/hydroquinone R¹, R², R³ or R⁴ that do not include a sulfonate moiety, phosphonate moiety, or sulfonimide moiety is an alkyl, a cycloalkyl, a thioether, a sulfoxide, a sulfone, a haloalkyl, a halogen, a nitrile, an imide, a pyrazole, or a combination of these. Non-limiting examples of the alkyls that could be included include unbranched C₁ or C₂ alkyl chains, and unbranched, branched or cyclic C₃, C4, C5, C6, C7, C8, C9 or C10 alkyl chains. [0083] The disclosed compounds are capable of transferring protons and/or electrons by acid/base and/or oxidation/reduction reactions, have useful reduction potentials, are water soluble, and are stable under acid conditions. Accordingly, the disclosed compounds may be used as redox-active species in a variety of applications. In a non-limiting example, the disclosed compounds may be used to facilitate the reduction of oxygen in cathode half-cells, particularly in the context of emerging technologies such as in mediated fuel cells or organic mediator flow batteries.

[0084] The use of hydroquinones/quinones as redox mediators to facilitate the reduction of oxygen in mediated fuel cells is described in, e.g., U.S. Patent Publication No.2015/0263371, which is incorporated by reference herein in its entirety.

[0085] A flow battery is a rechargeable fuel cell in which an electrolyte solution containing one or more dissolved redox-active mediators flows through the electrochemical cell.

Additional electrolyte is solution is stored externally, generally in tanks, and is usually pumped through the cell (or cells) of the battery, although gravity feed systems are also known. Flow batteries can be rapidly "recharged" by replacing the electrolyte liquid, while simultaneously recovering the spent material for processing and reuse.

[0086] Further details regarding specific embodiments and syntheses thereof are provided in the following examples. These specific embodiments do not in any way limit the scope of the disclosure.

III. EXAMPLES– SYNTHESES OF EXEMPLARY COMPOUNDS

Example 1– Synthesis of Compound 1 of Figure 2

[0087] The synthesis of compound 1 of Figure 2 as its PPh4 salt in one step from p-chloranil (2) has been described (see Scheme 2 below).⁷ The reported yield is low, but no attempts at optimization were reported. In our hands, the protocol in the literature is irreproducible across multiple attempts and variations of the reaction conditions.

[0088] Scheme 2: Proposed synthesis of compound 1 of Figure 2.

[0089] In the examples that follow, we provide schemes for synthesizing a number of other exemplary substituted hydroquinones/quinones that could be used to facilitate oxygen reduction at the cathode of an electrochemical cell. The following examples are offered for illustrative purposes only, and are not intended to limit the scope of the invention in any way. Indeed, various modifications of the disclosed method in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and the following examples and fall within the scope of the appended claims. Example 2– Synthesis of Compound 3 of Figure 2

[0090] The synthesis of compound 3 is outlined in Scheme 3 below. From p-chloranil, 2, there is a literature-precedented exhaustive phosphonation.^8,9 The resulting quinone 4 is hydrogenated to the hydroquinone 5, which was the object of the previously reported synthesis.

[0091] Scheme 3: Proposed synthesis of compound 3 of Figure 2.

[0092] Based on the previous literature, the expected yield was 30 % and the product M.P. of 154.5—155 °C, its crystal structure has been reported.⁸ In our hands, the yield of 4 is closer to 3.5 % and the process disclosed by Reetz is irreproducible.

[0093] Out of many variations of the reaction conditions, the only method that gave isolable amounts of 4 is the following.2.48 g of chloranil were dispersed in 10 ml of toluene and heated under nitrogen to 90 °C. Over 2.25 hours, 4 ml of tri-isopropyl phosphite were added dropwise and then allowed to react for 1.25 hours at 90 °C. The resulting mixture was then concentrated under vacuum at temperatures up to 80 °C. The residue was dispersed in 50 ml of heptane, chilled to 0 °C, filtered, and rinsed with 10 ml of heptane. The resulting solids were dissolved in 20 ml of acetonitrile, filtered, and rinsed with 5 ml of acetonitrile. The filtrate was rotovapped to give an orange solid. This solid was crystallized from 5 ml of ethyl acetate and filtered to remove unreacted chloranil which was rinsed with 2 ml of ethyl acetate. The combined filtrates were mixed with 7 ml of heptane and concentrated by heating. On cooling, 0.267 g of an orange solid formed with a sharp melting point of 150 °C with an additional 35 mg being obtained from the filtrate. Efforts to improve this yield by varying the reaction conditions are in progress.

[0094] Hydrogenation of 4 has been claimed to give 5 in high yield.⁸ Take 11.46 g (15 mmol) of 13 dissolved in 33 ml ethanol. Place the solution with a Pd/C catalyst in a hydrogen- purged hydrogenation apparatus. The hydrogenator is then placed under 60 psi of hydrogen for a period of two hours at 35—40 °C. The hydrogen gas is then released and the reaction mixture filtered to remove the palladium. Water is added to the remaining alcoholic solution causing fluorescent pale green crystals to precipitate. These crystals are then further recrystallized from an ethanol/water solution to give p-hydroquinone tetrakis (di-isopropyl phosphonate), 5. Expected yield: 84 %. [0095] Either 4 or 5 should be able to be hydrolyzed to 3 as shown in Scheme 3. Hydrolysis of diphosphonic esters has been the subject of considerable previous research towards various pharmaceuticals, and the mild protocols devised there should be effective in yielding the tetraphosphonic acid if the method shown is too harsh.¹⁰ Example 3– Synthesis of Compound 6 of Figure 2

[0096] Synthesis of compound 6 is unprecedented, however the even more sterically- congested benzenehexasulfonic acid has been reported.¹¹ Applying similar copper-catalyzed conditions to 1,4-diacetoxytetrachlorobenzene or tetrachlorohydroquinone with other protecting groups appended may result in a similarly profound dechlorosulfonation (Scheme 4).¹² Deprotection of the hydroquinone should be facile. Preliminary experiments using chloranil directly have been performed and suggest that CuCl may be a better catalyst precursor.

[0097] Scheme 4: Proposed synthesis of compound 6 of Figure 2.

n a erna ve rou e nvo ves e reac on o c oran w og yco c acid to form the tetrathia-substituted hydroquinone 9 (reduction happens in-situ, see Scheme 5).¹³ Other aryl thioglycolates have been oxidized to the corresponding sulfonic acids by refluxing nitric acid.¹⁴ We prophesize that treatment of 9 with refluxing nitric acid will produce compound 10 (the oxidized version of 6) by a similar reaction. [0099] Scheme 5: Proposed synthesis of compound 10 of Figure 2 via thiol adduct 9.

Example 4– Synthesis of Compounds 11a-d and 12a-d of Figure 2

[00100] Solubilized dihydroxypyromellitimide and dichlorophthalimide mediators.

[00101] There are routes to dihydroxypyromellitimides described in the polymer literature as model compounds.¹⁵ The synthesis of dichlorodihydroxyphthalimide is also precedented.¹⁶ These reactions could be adapted to incorporate substituents with solubilizing groups to yield the heretofore-unknown hydroquinones 11a-d and 12a-d illustrated in Figure 2.

[00102] Compound 11a-d synthesis.

[00103] The synthesis of compounds 11a-d proceed from dihydroxypyromellitic anhydride 13, which can be accessed via a number of routes.¹⁷ One such route is shown in Scheme 6, where the final product was an analogue of 11 where R = 4-butylphenyl.^18,19 A similar condensation of this anhydride with a salt of taurine or 2-aminoethylphosphonic acid should give the desired compounds 11a and 11b.²⁰ Condensation with 4- aminophenylsulfonic acid or 4-aminophenylphosphonic acid should give 11c and 11d respectively. An alternative route would produce the quinone form, 14a-d, starting from durene.^21,22,23,24 [00104] Scheme 6: Proposed syntheses of compounds 11a-d and 14a-d of Figure 2.

[ ompoun a- syn es s.

[00106] The synthesis of compounds 12a and 12b begins from the reduced form of DDQ. Analogous substituted phthalimides have also been made from 2,3- dicyanohydroquinone and dibromodicyanohydroquinone.²⁵ A similar treatment of tetracyano hydroquinone could be an alternative route to 11a-d. Gabriel reaction of 15 or its di-acetate (also reported in Ref.21) with 2-chloroethylsulfonate or 2-chloroethylphosphonate should then yield 15a and 15b respectively. Synthesis of 15c and 15d would require first forming the anhydride in an analogous pathway to synthesis of 11, or a catalyzed amination of the appropriate arene acid.

[00107] Scheme 7: Proposed synthesis of compound 12 of Figure 2.

Example 5– Synthesis of Compounds 16-18 of Figure 2

[00108] Ring-methylated hydroquinone phosphonic acids.

[00109] The proposed synthesis of compounds 16-18 of Figure 2 relies on insight into clues hidden in the literature.²⁶ In DMSO, diethyl phosphite will add to benzoquinone twice to produce hydroquinone bis-phosphonate and hydroquinone in equal amounts. These authors also demonstrated efficient mono-phosphonation of 2,5-dialkyl-quinones in wet toluene under otherwise similar conditions. While trimethylquinone was not tried as a substrate by Han and co-workers, our experiments have shown that it yields an ester of 18 and that it is necessary to let the reaction run for a longer time than previously reported. Combining the conditions of the first set of experiments with the substrates of the second set of conditions should yield di-alkylhydroquinone-diphosphonate esters. Our preliminary experiments towards synthesis of 16 and 17 suggest that the reaction conditions will need to be re- optimized in ways that were not initially obvious. Hydrolysis of the esters should then give 16-18.

[00110] Compound 16 synthesis.

[00111] Our experiments have shown that starting material 19 is recovered unchanged after 18 hours under the conditions shown in Scheme 8. However, variations on these conditions are expected to lead to reaction.

[00112] Scheme 8: Proposed synthesis of compound 16 of Figure 2.

– - [00113] Trifluoromethyl and nitrile quinone phosphate derivatives.

[00114] Using Han's route to hydroquinone bis-phosphonate,²⁶ one can make further substituted derivatives of this compound. For instance, exhaustive free-radical

trifluoromethylation of the different isomers would yield esters of 21 and 22 (Figure 2).²⁷ Alternatively, one could start from the appropriate trifluoromethylated quinones and phosphonate them.

[00115] Trifluoromethylation of methyl-protected bis-iodo hydroquinones is an established reaction,²⁸ and benzyl protected structures would presumably react similarly (Scheme 9). Starting from an isomeric aryl iodide will place the trifluoromethyl groups in the correct position to eventually yield compound 21 instead of 22.

[00116] Scheme 9: Proposed synthesis of compound 22 of Figure 2.

[00117] Cyano- and chloro-cyano- hydroquinone phosphonates.

[00118] Cyanide ion or HCN readily adds to many quinones as proposed in Scheme 10. Mono-trifluoromethylation of 2,5-bisphosphonate ester-hydroquinone, addition of cyanide, and hydrolysis would give 23 (Figure 2). Similar oxidation/cyanation reactions of the appropriate phosphonates would yield esters of 24–26 (Figure 2).

[00119] Scheme 10. Addition of cyanide to form structure 23 in Figure 2.

[00120] In 1998, Abdou and co-workers described a method to react 2,3-dichloro-5,6- dicyanoquinone (DDQ) with tri-isopropyl phosphite to yield the di-isopropyl ester of 28 (Figure 2).²⁹ By using a different stoichiometry and other conditions, esters of 27 (Figure 2) should also be achievable.

[00121] Trifluoromethylated hydroquinone disulfonates. [00122] If the trifluoromethylation methods used for the synthesis of compounds 21 and 22 (Figure 2) are employed on hydroquinone disulfonates, 29 and 30 (Figure 2) are the expected products. Example 7– Synthesis of Compounds 35a-d of Figure 2

[00123] A densely functionalized quinone, 34, has been prepared in only two steps from diethyl succinate.^30,31,32 Scheme 11 shows how this could produce the desired compounds 35a-d (Figure 2). If the cyanide-catalyzed aminolysis to install polar functionality is unsuccessful, more conventional hydrolysis/coupling sequences could be used on the hydroquinone diester.^33,34

[00124] Scheme 11: Proposed synthesis of compounds 35a-d of Figure 2.

Example 8– Synthesis of Compound 36 of Figure 3 and Related Pyrazole-containing Compounds

[00125] Pyrazole- and pyrazolesulfonate- substituted hydroquinones.

[00126] In this example, we disclose the synthesis of compound 36 and related compounds, which are labeled as compounds 38-40 in Scheme 12 below. Compound 38 is the oxidized version of 36.

[00127] Compounds 38-40 of Scheme 12 share a common reagent and synthesis, differing in the substrate to which it is employed. Pyrazoles with various substituents (4-Cl, 4-NO2, 3,5-dimethyl, etc.) have been shown to add to quinones to give pyrazole-substituted hydroquinones.^35,36,37 This reaction has been demonstrated with chloranil and pyrazole itself to give the tetrasubstituted quinone product.³⁶ Pyrazole has been sulfonated at its 4-position to give pyrazole-4-sulfonic acid, 37.³⁷ In combination, these reactions should give 38-40 as shown in Scheme 12.

[00128] Comparisons of literature procedures suggests that compound 36 could also be accessed by sulfonation of the unsulfonated precursor. Analogues of 36 or 38 with one to three sulfonate groups appended may also be of interest if they are formed.

[00129] Scheme 12: Observations from the literature and proposed synthesis of compounds 38-40.

[00130] Following similar procedures, pyrazoles could be installed on quinones containing phosphonates or sulfonates, reduction of which would result in structures 41-43 in Figure 3. Example 9– Synthesis of Compounds 44a-d and 45a-d of Figure 3

[00131] 4,5-disubstituted dihydroxyphthalimides (Scheme 13).

[00132] The bis-silyl enolate of succinic anhydride is capable of performing as a diene in Diels-Alder reactions, including with N-substituted maleimides to give 3,6- dihydroxyphthalimides.³⁸ With appropriately substituted succinic anhydrides and maleimides, this reaction would yield compounds 44a-d and 45a-d (Figure 3).

[00133] Scheme 13: Observations from the literature and proposed synthesis of compounds 44a-d and 45a-d.

Example 10– Synthesis of Compound 46 of Figure 3

[00134] Bicyclic hydroquinone (Scheme 14).

[00135] An analogue to the carboximide group is the bis-sulfonimide group.³⁹ In addition to being more resistant to hydrolysis, the more electron-withdrawing sulfonyl groups make the N-H bond more acidic. As an example, benzene-1,2-bis-sulfonimide is as acidic as HCl. Thus, quinones substituted with this group should be water-soluble anions. Quinones can also act as Diels-Alder dienophiles, such as cyclohexadiene, and produce tricyclic products. Through formation of a bis-sulfonylimine quinone and its reaction with

cyclohexadiene and subsequent reduction, compound 46 could be produced (Figure 3). This Diels-Alder/reduction sequence could also be used to produce tricyclic analogues of 44a-d (Figure 3).

[00136] Scheme 14: Observations from the literature and proposed synthesis of compound 46.

Example 11– Synthesis of Compounds 47, 48a-d, 49 of Figure 3

[00137] Triptycene-triquinone and pillar[6]arene quinone mediators. [00138] If a substituted anthracene is used as the diene in a Diels-Alder reaction with a quinone, a triptycene results.⁴⁰ Use of this reaction with appropriately-functionalized reactants would result in 47, 48a-d, or analogues thereof (Figure 3). It is advantageous to have a redox mediator that delivers multiple electrons, since this will lead to higher currents, other things being equal. These species, having three quinone moieties per molecule would act as six-electron oxidants or reductants.

[00139] A further increase in the number of quinone units is possible in pillar[n]arene macrocycles such as 49 (Figure 3). A dibromo analogue of 49 has been previously synthesized,⁴¹ and could serve as a starting point for derivatization of the molecule. Example 12 - Synthesis of quinones with thioether-linked sulfonates and derivatives from Figure 4

[00140] Thiols will readily add to quinones. The resulting thioether linkage can be used to tether a solubilizing group, such as a sulfonate. Some examples of quinones containing thioether-linked sulfonates are given in Figure 4 and discussed here. Reaction of 2-mercaptoethanesulfonate with chloranil (2) results in the substitution of chlorine by sulfur to eventually give the desired fully-substituted product, 50, in a mixture with a disulfide- linked byproduct, 51. In contrast, dichlorodicyanoquinone gives the disulfide exclusively (Scheme 15).

[00141] Scheme 15. Observations from treating Cl-containing quinones with mercaptoethanesulfonate.

,

with a suspension of benzoquinone in water (Scheme 16).0.655 g of sodium

mercaptoethanesulfonate in 10 ml of water with 60 microliters of acetic acid was treated with 0.438 g of benzoquinone and allowed to stir at room temperature for 45 minutes. The resulting mixture was filtered and the filtrate was extracted with three 5 ml portions of ethyl acetate. The aqueous phase was then diluted with 30 ml of ethanol and heated to clarify it. On cooling, crystals formed that gave ¹H and ¹³C NMR spectra in D₂O in accord with tetra- thioethylsulfonato hydroquinone, 52 (Figure 5). This hydroquinone shows a reversible redox couple in aqueous 1 M H₂SO₄ of 0.63 V vs. NHE. Experiments suggest that slow addition of the thiol to portions of the quinone will improve the yield of the desired product.

[00143] Scheme 16. Synthesis of 52 from benzoquinone.

[00144] Experiments similar to those discussed above also show that 2,6- dimethylbenzoquinone will react to give a tetrasubstituted product, 53 (Scheme 17).

[00145] Scheme 17. Addition of mercaptoethanesulfonate to 2,6-

[00146] This method is not unique to mercaptoethanesulfonate, having been adapted from procedures used with 3-mercaptopropionate and other thiols. For example, structure 54 in Figure 4 can be produced.

[00147] The inclusion of electron-withdrawing trifluoromethyl acetyl (–C(O)CF₃) substituents on a hydroquinone containing thioether-linked sulfonates is shown in Scheme 18. Synthesis of starting material 55 and 56 has been reported by Sevenard et. al.⁴² Introduction of mercaptoalkylsulfonates in a similar manner as discussed above should give the products 57 and 58. [00148] Scheme 18. Literature synthesis and proposed thioalkyl-linked sulfonation of trifluoromethylacetyl-containing quinones.

[00149] Thiother-linked sulfonates should also be installable on the bis-CF₃ hydroquinone 31. Adaption of the mercaptoalkylsulfonate installation procedure should give 59 in Scheme 19.

[00150] Scheme 19. Proposed synthesis of structure 59 in Figure 4.

e prop es ze a verse o s w an w ou an on c unc onal groups can be appended to quinones to produce tetrasubstituted quinones in a similar process, including illustrative examples 60-69 in Figure 4. [00152] Thioethers can be readily oxidized by many methods to sulfoxides and sulfones.⁴³ Using one of these techniques on a thioether-quinone such as one of the ones shown above will produce a sulfoxide-quinone or sulfone-quinone. These quinones will have higher reduction potentials than the parent thioether and may have other properties that further enhance their usefulness. An alternative route to synthesize sulfone-quinones that may be preferred in some cases would be the reaction of chloranil, benzoquinone, or another quinone with the salt of appropriate sulfinic acid to form the desired sulfone such as structure 70 shown in Scheme 20.

[00153] Scheme 20. Proposed synthesis of 70 in Figure 4.

Example 13 - Synthesis of quinones with imide-linked sulfonates and phosphonates

[00154] The reaction of chloranil, 2, and potassium phthalimidate gives a tetra-imido quinone 71 (Figure 6) with relatively robust C-N bonds from the central ring that resist displacement by such strong nucleophiles as hydrazine, shown in Scheme 21.⁴⁴ The positioning of the imide rings perpendicular to the quinone ring may protect the quinone from attack. By using imides with pendant phosphonate or sulfonate groups, the imide becomes a linking group to attach the anionic group while simultaneously protecting the quinone from further attack. Use of sulfophthalimide 72 instead of the parent phthalimide should produce structure 73 (Scheme 21) from Figure 6.

[00155] Scheme 21. Literature route and proposed synthesis to structures 71 and 73 from Figure 6.

[00156] Alternatively, an attached maleimide could be sulfonated, such as proposed to form structure 74, shown in Scheme 22.

[00157] Scheme 22. Proposed synthesis of structure 74.

ng imides and other groups or solubilizing groups in addition to imides is shown in Figure 6, structures 75- 77. Example 14– Alternative Sulfonation and Sulfomethylation Methods [00159] In this example, we disclose alternative methods of sulfonation and sulfomethylation that could be used to produce the quinones or hydroquinones. In Scheme 23, we disclose a method of attaching a methylsulfonate to an unsubstiuted hydroquinone. This method can be extended to substituted hydroquinones as well.⁴⁵

[00160] Scheme 23: Sulfomethylation of hydroquinone.

[00161] In Scheme 24, we disclose a method of attaching a two methylsulfonates to a ketone-substituted hydroquinone.⁴⁵ This method can be extended to more substituted hydroquinones as well.“PG” represents a protecting group, which are well-known in the art.

[00162] Scheme 24: Sulfomethylation of ketone-substituted hydroquinone.

[00163] In Scheme 25, we disclose a method of attaching a sulfonate to either end of the double bond of a propenyl-substituted hydroquinone.⁴⁶ This method can also be extended to more substituted hydroquinones as well.“PG” represents a protecting group, which are well-known in the art.

[00164] Scheme 25: Sulfonation of propenyl-substituted hydroquinone.

[ ser es o a y - or uoroa y - e ere su ona e-con a n ng ydroquinones could be accessed via the common 1,4-ditrifluoromethyl -2,5-diiodo-3,6-dimethoxybenzene intermediate 78. Two routes to this proposed intermediate are shown in Scheme 26. The first route involves treatment of 1,4-dimethoxybenzene with n-BuLi and TMSCl to form the silylated product,⁴⁷ treatment with Togni's reagent to install two CF₃ groups, and treatment with ICl to install the two iodine substituents on 78. The second route involves direct iodination of the 1,4-ditrifluoromethyl-2,5-dimethoxybenzene⁴⁸ to form 78. [00166] Scheme 26. Two proposed synthetic routes to useful precursor 78.

[00167] From 78, various tethered sulfonates are proposed be introduced onto the hydroquinone core, as shown in Scheme 27. Treatment with n-Buli and 1,3-propanesultone followed by deprotection could access 79 in Scheme 27. Suzuki coupling of 78 with boronic acid 80 or 81 should access structures 82 and 83. Treatment of 82 with acetic anhydride and sulfuric acid would form the bis-sultone structure 84. Treatment with KF and 18-crown-6 should open the sultones,⁴⁹ and methyl deprotection should yield the hydroquinone structure 85. Similar procedures from 83 should yield the hydroquinone structure 86. Conversely, treatment of 78 with CF3CO2Na under Cu conditions should yield the tris-CF3 structure 87. Suzuki coupling with 81, sultone formation, ring opening and methyl deprotection as above should yield 88. Ullman-type coupling of 78 with the poly-fluorinated sulfonic acid 89⁵⁰ followed by methyl deprotection and hydrolysis could achieve the polyfluorinated hydroquinone structure 90. A similar procedure, using 91⁵¹ as the Ullman coupling partner, could achieve the simpler hydroquinone structure 92.

[00168] Scheme 27. Proposed synthesis of a fluorine-containing hydroquinone structures 79 85 86 88 90 and 92 of Fi ure 6.

[00169] Example 15– Synthesis of 1,2-hydroquinone/1-2,quinone structures.

[00170] In general, 1,2-quinones have higher reduction potentials than the correspondingly substituted 1,4-quinones. Thus, accessing these structures would be an efficient strategy to achieve quinones with increased redox potentials. Similar sulfonate tethered by thioethers, as used in structure 52 or 54, should give access to higher potential, water-soluble, robust 1,2-hydroquinone structures. Several 1,2-hydroquinone with thioether substituents have been synthesized,⁵² though these thioether substituents do not confer water solubility (in fact, thiol addition causes precipitation of the formed structure) and are limited to single or double substitution Preliminary synthetic efforts towards 93 yield a compound with a redox potential of 0.752 V vs. NHE, approximately 100 mV higher than the correspond structures 52 or 54. Using similar procedures as described above for 1,4- hydroquinone/1,4-quinones, 1,2-hydroquinone/1,2-quinone structures 93– 104 in Figure 7 should be accessible.

[00171] Example 16– Synthesis of anthraquinones with tethered sulfonates and phosphates.

[00172] Water-soluble anthraquinone structures have been utilized as the anodic mediator in aqueous flow batteries (see for instance U.S. Patent Application 2016/0043423). These structures typically include sulfonate substituents connected directly on the ring. Such compounds have been reported to undergo desulfonation, which leads to precipitation of the structures.⁵³ For this reason, alternate anthraquinone mediators without sulfonates directly connected to the ring are required. To address this concern, quinone 109 with sulfonate substituents attached to the ring via a thioether tether, was synthesized. Anthraquinones with pendant thioethers have been previously synthesized from the corresponding chlorinated anthraquinones,⁵⁴ but not with tethered sulfonates, which would provide increased water solubility, a desired trait.

[00173] To 3.956 g of sodium 3-mercaptopropanesulfonate dispersed in 50 mL of nitrogen-purged N-methyl pyrrolidinone were added 10.5 mL of 2 M aqueous NaOH. Once mixed, 2.766 g of 1,8-dichloroanthraquinone were added (Scheme 28). The suspension was heated under nitrogen to 90 °C for six days then allowed to cool.50 mL of water containing 1 mL of acetic acid was added to the cooled solution, which was further diluted with 25 mL water. The solution was extracted with four 50 mL portions of methylene chloride, and the aqueous phase was evaporated. The residue was dissolved with 35 mL of water and 60 mL of methanol was added. The mixture was heated until clear, and left to slowly cool. The resulting crystals were filtered, rinsed twice with 15 mL of 2:1 methanol/water, three times with 15 mL of methanol, and dried to give 4.326 g of material, for an overall yield of 77%. 1H and 13C NMR spectra were obtained (Figure 9). Cyclic voltammetry of this compound in 1 M H2SO4 reveal a reduction potential of approximately 85 mV vs. NHE.

[00174] Scheme 28. Synthesis of structure 109 from Figure 8.

[00175] It is expected that related structures, such as structures 105-106 and 108-116 in Figure 8, structures 117-128 in Figure 10, and structures 129-140 in Figure 11 could be accessed via similar methods. [00176] There can be a considerable degree of mixing and matching of functional groups in all of the above frameworks, and the above examples are not exhaustive. While a number of embodiments of the present invention have been described above, the present invention is not limited to just these disclosed examples. There are other modifications that are meant to be within the scope of the invention, which is defined by the appended claims. Cited References:

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Claims

We claim: 1. A substituted hydroquinone having the formula:

, wherein:

(a) one, two, three, or all four of R¹, R², R³ and R⁴ comprise a sulfonate or sulfonimide moiety, and each R¹, R², R³ and R⁴ that does not comprise a sulfonate or sulfonimide moiety is independently selected from the group consisting of an alkyl, a cycloalkyl, a thioether, a sulfoxide, a sulfone, a haloalkyl, a halogen, a nitrile, an imide, a phosphonate, a pyrazole, and combinations thereof; or

(b) one, two, three, or all four of R¹, R², R³ and R⁴ comprise a phosphonate moiety, and each R¹, R², R³ and R⁴ that does not comprise a phosphonate moiety is independently selected from the group consisting of an alkyl, a cycloalkyl, a thoether, a sulfoxide, a sulfone, a haloalkyl, a halogen, a nitrile, an imide, pyrazole, and combinations thereof; or

the oxidized form (the substituted 1,4-benzoquinone form) of said substituted hydroquinone.

2. The substituted hydroquinone or quinone of claim 1, wherein: each R¹, R², R³ and R⁴ that comprises a sulfonate moiety independently either (a) consists of sulfonate directly bound to the hydroquinone or quinone ring, or (b) consists of sulfonate and a linking group that links the sulfonate moiety to the hydroquinone or quinone ring; or

each R¹, R², R³ and R⁴ that comprises a phosphonate moiety independently either (a) consists of phosphonate directly bound to the hydroquinone or quinone ring, or (b) consists of phosphonate and a linking group that links the phosphonate moiety to the hydroquinone or quinone ring.

3. The substituted hydroquinone or quinone of claim 2, wherein:

(a) all four of R¹, R², R³ and R⁴ do not consist of sulfonate directly bound to the hydroquinone or quinone ring;

(b) all four of R¹, R², R³ and R⁴ do not consist of phosphonate directly bound to the hydroquinone or quinone ring;

(c) if R², R³ and R⁴ are all–Cl, then R¹ does not consist of sulfonate directly bound to the hydroquinone or quinone ring; and

(d) if R² and R⁴ are both–CH3, then R¹ and R³ do not both consist of sulfonate directly bound to the hydroquinone or quinone ring.

4. The substituted hydroquinone or quinone of claim 2 or claim 3, wherein each linking group independently comprises an ester, an amide, a sulfonamide, an imide, a sulfone, a sulfoxide, a thioether, a ketone, one or more alkyl chains, an aromatic ring, a pyrazole, or combinations thereof.

5. The substituted hydroquinone or quinone of any of claims 2-4, wherein R¹ and R², R³ and R⁴, or both R¹/R² and R³/R⁴ each comprise a single linking group attached to the hydroquinone or quinone ring at the two designated positions.

6. The substituted hydroquinone or quinone of any of claims 2-5, wherein one or more of the linking groups is independently selected from the group consisting of the imide -CONYCO-, where Y is -CH2CH2- or a benzene ring; the amide -CONHY, where Y is -CH₂CH₂- or a benzene ring; pyrazole; dimethylpyrazole.

7. The substituted hydroquinone or quinone of claim 1, wherein R¹ and R², R³ and R⁴, or both R¹/R² and R³/R⁴ each comprise a single sulfonimide attached to the hydroquinone or quinone ring at the two designated positions.

8. The substituted hydroquinone or quinone of any of claims 1-7, wherein multiple substituted hydroquinone or quinone structures according to claim 1 are covalently linked together to form a substituted bicyclic, tricyclic or polycyclic hydroquinone or quinone.

9. The substituted hydroquinone or quinone of claim 8, comprising 2, 3, 4, 5 or 6 covalently linked hydroquinone or quinone structures according to claim 1.

10. The substituted hydroquinone or quinone of claim 8 or claim 9, wherein the multiple substituted hydroquinone or quinone structures are covalently linked through one or more of the R¹, R², R³ or R⁴ that do not comprise a sulfonate moiety, sulfonimide moiety, or phosphonate moiety.

11. The substituted hydroquinone or quinone of claim 10, wherein the multiple substituted hydroquinone or quinone structures are covalently linked through one or more alkyls.

12. The substituted hydroquinone or quinone of claim 10, wherein one or more of the alkyls through which the multiple substituted hydroquinone or quinone structures are covalently linked are -CH₂- or -CH(-)₂.

13. The substituted hydroquinone or quinone of claim 12, wherein the -CH2- covalently links two different substituted hydroquinone or quinone structures, or wherein the -CH(-)2 covalently links three different hydroquinone or quinone structures.

14. The substituted hydroquinone or quinone of any of claims 1-13, wherein one or more of the R¹, R², R³ or R⁴ comprising a sulfonate moiety, sulfonimide moiety, or phosphonate moiety are selected from the group consisting of: the imide -CONRCO- attached at R¹ and R² or R³ and R⁴, wherein R is

,

attached at R¹ and R² or R³ and R⁴.

15. The substituted hydroquinone or quinone of any of claims 1-14, wherein one or more of the R¹, R², R³ or R⁴ that do not comprise a sulfonate moiety, a sulfonimide moiety, or a phosphonate moiety are independently selected from the group consisting of -Cl, -CH3, - CH2-, -CF3, -CN, -CH(-)2,

, and attached at R¹ and R² or R³ and R⁴.

16. The substituted hydroquinone or quinone of claim 15, wherein the -CH2- links two different substituted hydroquinone or quinone structures, or wherein the -CH(-)₂ links three different hydroquinone or quinone structures.

17. The substituted hydroquinone or quinone of any of claims 1-16, wherein the substituted hydroquinone or quinone is more soluble in water than the corresponding unsubstituted hydroquinone or quinone, is stable in 1 M H2SO4, and has a reduction potential in the oxidized form that is equal to or greater than the reduction potential of the

corresponding unsubstituted quinone (1,4-benzoquinone).

18. The substituted hydroquinone or quinone of any of claims 1-16, wherein the substituted hydroquinone or quinone is more soluble in water than the corresponding unsubstituted hydroquinone or quinone, and is stable in 1 M H₂SO₄.

19. The substituted hydroquinone or quinone of claim 1, wherein the substituted hydroquinone or quinone is selected from the group consisting of:

,

,

r

and salts, acids, reduced forms, and oxidized forms of any of the

orego ng.

20. The substituted hydroquinone or quinone of claim 1, wherein the substituted hydroquinone or quinone is selected from the group consisting of:

and s

a s, ac s, reuce orms, an ox ze orms o any o e oregong.

21. A substituted catechol having the formula:

, wherein:

the oxidized form (the substituted 1,2-benzoquinone form) of said substituted catechol.

22. The substituted catechol or 1,2-quinone of claim 21, wherein:

each R¹, R², R³ and R⁴ that comprises a sulfonate moiety independently either (a) consists of sulfonate directly bound to the hydroquinone or quinone ring, or (b) consists of sulfonate and a linking group that links the sulfonate moiety to the hydroquinone or quinone ring; or

23. The substituted catechol or 1,2-quinone of claim 21 or claim 22, wherein each linking group independently comprises an ester, an amide, a sulfonamide, an imide, a sulfone, a sulfoxide, a thioether, a ketone, one or more alkyl chains, an aromatic ring, a pyrazole, or combinations thereof.

24. The substituted catechol or 1,2-quinone of any of claims 21-23, wherein R¹ and R², R³ and R⁴, or both R¹/R² and R³/R⁴ each comprise a single linking group attached to the hydroquinone or quinone ring at the two designated positions.

25. The substituted catechol or 1,2-quinone of any of claims 22-24, wherein one or more of the linking groups is independently selected from the group consisting of the imide -CONYCO-, where Y is -CH₂CH₂- or a benzene ring; the amide -CONHY, where Y is -CH2CH2- or a benzene ring; pyrazole; dimethylpyrazole.

26. The substituted catechol or 1,2-quinone of any of claims 21-25, wherein multiple substituted hydroquinone or quinone structures according to claim 20 are covalently linked together to form a substituted bicyclic, tricyclic or polycyclic hydroquinone or quinone.

27. The substituted catechol or 1,2-quinone of claim 26, comprising 2, 3, 4, 5 or 6 covalently linked catechol or 1,2-quinone structures according to claim 20.

28. The substituted catechol or 1,2-quinone of claim 26 or claim 27, wherein the multiple substituted hydroquinone or quinone structures are covalently linked through one or more of the R¹, R², R³ or R⁴ that do not comprise a sulfonate moiety or phosphonate moiety.

29. The substituted catechol or 1,2-quinone of claim 28, wherein the multiple substituted hydroquinone or quinone structures are covalently linked through one or more alkyls.

30. The substituted hydroquinone or quinone of claim 28, wherein one or more of the alkyls through which the multiple substituted hydroquinone or quinone structures are covalently linked are -CH₂- or -CH(-)₂.

31. The substituted catechol or 1,2-quinone of claim 30, wherein the -CH2- covalently links two different substituted hydroquinone or quinone structures, or wherein the -CH(-)2 covalently links three different hydroquinone or quinone structures.

32. The substituted catechol or 1,2-quinone of any of claims 21-31, wherein one or more of the R¹, R², R³ or R⁴ that do not comprise a sulfonate moiety or a phosphonate moiety are independently selected from the group consisting of -Cl, -CH3, -CH2-, -CF3, -CN, -CH(-)2, , and attached at R¹ and R² or R³ and R⁴.

33. The substituted catechol or 1,2-quinone of claim 32, wherein the -CH2- links two different substituted hydroquinone or quinone structures, or wherein the -CH(-)₂ links three different hydroquinone or quinone structures.

34. The substituted catechol or 1,2-quinone of any of claims 21-33, wherein the substituted catechol or 1,2-quinone is more soluble in water than the corresponding unsubstituted hydroquinone or quinone and is stable in 1 M H2SO4.

35. The substituted catechol or 1,2- of claim 21, wherein the substituted catechol or 1,2- quinone is selected from the group consisting of:

and salts, acids, and oxidized forms of any of the foregoing.

36. A substituted anthraquinone having the formula:

, wherein at least one of R¹-R⁸ comprises a sulfonate that is connected to the ring via a linking group further comprising a thioether and one or more alkyl chains, or the reduced form of said substituted anthraquinone.

37. The anthraquinolne of claim 36, where each of R¹-R⁸ that does not bear a sulfonate linked via a thioether is independently selected from the group consisting of an alkyl, a cycloalkyl, a thiother, a sulfoxide, a sulfone, a haloalkyl, a halogen, a hydroxyl, an alkoxyl, an ether, an amine, or hydrogen.

38. The anthraquinone of claim 36, wherein the substituted anthraquinone is

;

39. A substituted anthraquinone having the formula:

wherein at least one of R¹-R⁸ comprises a phosphonate that is connected to the ring via a linking group further comprising a thioether and one or more alkyl chains, or the reduced form of said substituted anthraquinone.

40. The anthraquinolne of claim 39, where each of R¹-R⁸ that does not bear a phosphonate linked via a thioether is independently selected from the group consisting of an alkyl, a cycloalkyl, a thiother, a sulfoxide, a sulfone, a haloalkyl, a halogen, a hydroxyl, an alkoxyl, an ether, an amine, or hydrogen.

41. The anthraquinone of claim 39, wherein the substituted anthraquinone is selected from the group consisting of:

42. A substituted anthraquinone having the formula:

wherein at least one of R¹-R⁸ comprises a sulfonate that is connected to the ring via a linking group further comprising an ether and one or more alkyl chains, or the reduced form of said substituted anthraquinone.

43. The anthraquinolne of claim 42, where each of R¹-R⁸ that does not bear a sulfonate linked via an ether is independently selected from the group consisting of an alkyl, a cycloalkyl, a thiother, a sulfoxide, a sulfone, a haloalkyl, a halogen, a hydroxyl, an alkoxyl, an ether, an amine, or hydrogen.

44. The anthraquinone of claim 42, wherein the substituted anthraquinone is

selected from the group consisting of: ;

45. A substituted anthraquinone having the formula:

wherein:

(a) at least one of R¹-R⁸ comprises a sulfonate that is connected to the ring via a linking group further comprising an amine and one or more alkyl chains, or

(b) at least one of R¹-R⁸ comprises a phosphonate that is connected to the ring via a linking group further comprising either an ether or an amine and one or more alkyl chains, or the reduced form of said substituted anthraquinone, or

the reduced form of said substituted anthraquinone.

46. The anthraquinolne of claim 45, where each of R¹-R⁸ that does not bear a sulfonate linked via an amine, a phosphonate linked via an ether, or a phosphonate linked via an amine, is independently selected from the group consisting of an alkyl, a cycloalkyl, a thiother, a sulfoxide, a sulfone, a haloalkyl, a halogen, a hydroxyl, an alkoxyl, an ether, an amine, or hydrogen.

47. A system comprising a liquid electrolyte solution in contact with an electrode, wherein the electrolyte solution comprises a substituted hydroquinone or quinone according to any of claims 1-18 dissolved therein.

48. A system comprising a liquid electrolyte solution in contact with an electrode, wherein the electrolyte solution comprises a substituted hydroquinone or quinone according to any of claims 19-46 dissolved therein.

49. The system of claims 47, wherein the electrolyte solution is an aqueous solution.

50. The system of claims 48, wherein the electrolyte solution is an aqueous solution.

51. The system of claim 47, wherein the electrolyte solution comprises an organic solvent.

52. The system of claim 48, wherein the electrolyte solution comprises an organic solvent.

53. The system of any of claims 47, 49, or 51, wherein the electrolyte solution further comprises oxygen.

54. The system of any of claims 48, 50, or 52, wherein the electrolyte solution further comprises oxygen.

55. A cell comprising the system of any of claims 47, 49, 51, or 53 in ionic communication with an anodic half-cell.

56. A cell comprising the system of any of claims 48, 50, 52, or 54 in ionic communication with an anodic half-cell.

57. A cell comprising the system of any of claims 48, 50, 52, 54, or 56 in ionic communication with a cathodic half-cell.

58. The cell of claim 55, wherein the cell is a fuel cell or a flow battery.

59. The cell of claims 56-57, wherein the cell is a fuel cell or a flow battery.

60. A method of producing electricity, comprising contacting the anodic half-cell of the cell of claim 58 with a fuel, and contacting the system of the cell of claim 57 with oxygen, whereby the fuel is oxidized, oxygen is reduced, and electricity is produced.

61. A method of producing electricity, comprising contacting the anodic half-cell of the cell of claim 59 with a fuel, and contacting the system of the cell of claim 58 with oxygen, whereby the fuel is oxidized, oxygen is reduced, and electricity is produced.

62. A substituted hydroquinone or quinone according to any of claims 1-18 for use as a redox mediator in a fuel cell or in a flow battery.

63. A substituted hydroquinone or quinone according to any of claims 19-46 for use as a redox mediator in a fuel cell or in a flow battery.

64. A method of making a substituted hydroquinone or quinone according to any of claims 1-18, comprising the steps disclosed in the examples herein.

65. A method of making a substituted hydroquinone or quinone according to any of claims 19-46, comprising the steps disclosed in the examples herein.