Toward Improved Catholyte Materials for Redox Flow Batteries: What

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Towards Improved Catholyte Materials for Redox Flow Batteries: What Controls Chemical Stability of Persistent Radical Cations? Jingjing Zhang, Ilya A. Shkrob, Rajeev S. Assary, Siu On Tung, Benjamin Silcox, Larry A Curtiss, Levi T. Thompson, and Lu Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08281 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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The Journal of Physical Chemistry

Towards Improved Catholyte Materials for Redox Flow Batteries: What Controls Chemical Stability of Persistent Radical Cations? Jingjing Zhang, 1,2 Ilya A. Shkrob*, 1.2 Rajeev S. Assary, 1,3 Siu on Tung, 1,4 Benjamin Silcox, 1,4 Larry A. Curtiss, 1,3 Levi Thompson, 1,4 and Lu Zhang 1,2 1

Joint Center for Energy Storage Research, Argonne National Laboratory, Argonne, IL 60439,

USA 2

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass

Avenue, Argonne, IL 60439-4837, USA 3

Materials Science Division; Argonne National Laboratory, Argonne, IL 60439, USA

4

Department of Chemical Engineering, University of Michigan, 2300 Hayward St, Ann Arbor,

MI 48109, USA * Corresponding author: Ilya A. Shkrob, [email protected], Tel. 630-2529516

ABSTRACT Catholyte materials are used to store positive charge in energized fluids circulating through redox flow batteries (RFBs) for electric grid and vehicle applications. Energy-rich radical cations (RCs) are being considered for use as catholyte materials, but to be practically relevant, these RCs (that are typically unstable, reactive species) need to have long lifetimes in liquid electrolytes under the ambient conditions. Only few families of such energetic RCs possess stabilities that are suitable for their use in RFBs; currently, the derivatives of 1,4dialkoxybenzene look the most promising. In this study, we examine factors that define the chemical and electrochemical stabilities for RCs in this family. To this end, we engineered rigid bis-annulated molecules that by design avoid the two main degradation pathways for such RCs, viz. their deprotonation and radical addition. The decay of the resulting RCs are due to the single remaining reaction: O-dealkylation. We establish the mechanism for this reaction and examine factors controlling its rate. In particular, we demonstrate that this reaction is initiated by the nucleophile attack of the counter anion on the RC partner. The reaction proceeds through the formation of the aroxyl radicals whose secondary reactions yield the corresponding quinones. 1 ACS Paragon Plus Environment


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The O-dealkylation accelerates considerably when the corresponding quinone has poor solubility in the electrolyte, and the rate depends strongly on the solvent polarity. Our mechanistic insights suggest new ways of improving the RC catholytes through molecular engineering and electrolyte optimization.

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INTRODUCTION Redox flow batteries (RFB) are electric energy storage systems in which charge carriers are stored in liquid electrolytes.

1, 2

These energized fluids flow through an electrochemical cell;

consequently, energy storage and power production become decoupled. Such RFBs can satisfy flexible needs, including the electric grid energy storage on a massive scale. 3-10 Crucial for these grid storage applications is the energy density of the battery fluids, which depends on (i) the concentration of charge carriers in the electrolyte and (ii) the cell operation voltage. 10 In aqueous systems, this voltage is limited to < 1.5 V due to water electrolysis. 11 The electrochemical window can become significantly widened in polar, aprotic organic solvents and ionic liquids.

12, 13

To take full advantage of such nonaqueous systems, the redox-active species

need to be at the extremes of the redox scale as defined by the electrochemical window of the solvent. 11, 12, 14 Naturally, such energy-dense species tend to be reactive and short-lived, whereas in the RFBs the charge carriers need to stay stable for days or even weeks of operation.

15

The

two requirements, the high energy density and chemical stability, are difficult to reconcile, yet both need to be satisfied. Organic radicals, ions, and transition metal complexes have been considered as candidates for charge carriers in RFBs. In this study, we limit our consideration to redox active organic molecules (ROMs). The latter can be classified as anolytes and catholytes depending on their use for storage of negative charges (as radical anions) or positive charges (as radical cations, RCs). From the perspective of physical organic chemistry, finding catholyte ROMs requires us to address the following questions: What RCs with the highest redox potentials have the greatest chemical and electrochemical stability? What factors control the longevity of persistent RCs in liquid electrolytes? Before proceeding further, we stress that “stability” and “persistence” are poorly defined notions that do not exist outside of the specific context.

16

What a spectroscopist might call a

“stable” species is not the same as a synthetic chemist will, and a battery scientist could apply still different criteria. Most of the organic RCs decay within microseconds and milliseconds, so it is natural for a spectroscopist to consider all exceptions to this general rule as “stable” RCs. However, a synthetic organic chemist can only use RCs that last longer than they react with the target molecules; the natural time scale for “persistence” becomes minutes and hours (e.g., 17-20). Consequently, out of the already small subset of the “stable” RCs, a still smaller subset of very stable RCs is selected, and those exceptions are called “stable” or “persistent”.

21

For energy 3

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storage in RFBs or overcharge protection in the conventional lithium-ion batteries, the stability requirements are still more demanding. 22, 23 With such escalating requirements, one increasingly enters into the realm of the exceptional, rare species that are exceedingly stable by nearly every standard – and still they may be lacking in their performance as the catholyte ROMs. For the general RCs, an educated guess can be made regarding the causes for their rapid decay. For the exceptionally stable RCs, elucidating this cause becomes a difficult task, as such species are stable precisely due to their ability to avoid the common RC reactions. Perhaps for this reason the studies of such species have lagged behind. For research described in this paper, we tackle these causes for the largest class of durable, energetic RCs: the 1,4-dialkoxybenzene derivatives (Scheme 1).

24-27

These compounds are closely related to the persistent RCs of the

Wurster’s blue family; unlike these diaminoarylenes, the dialkyloxyarylenes have high redox potentials (> 4 V vs. Li/Li+) and excellent stability in the charged state,justifying their use as the catholyte ROMs. The progenitor of the family, compound 1 (Scheme 1), does not yield stable RCs,

28-30

although it avoids the main parasitic reaction of RCs, which is their deprotonation. Instead, 1+● decays via the radical addition in the arene ring of the parent molecule.

29-32

Recognition of this

reaction suggested a simple way to improve RC stability: crowding the arene ring with bulky substituting groups, such as the tert-butyl group (structures 2-3 in Scheme 1), to sterically hinder the radical addition.

33, 34

This proved to be a successful strategy, and compound 2 became the

standard in the field. 35-37 Various derivatives of 2 have been suggested for overcharge protection in the Li+ batteries and the use in RFBs.

31, 38

However, such tert-butylated ROMs are

suboptimal for their use in the flow cells, where high solubility and high specific charge density are required: 2 has high molecular weight (i.e., low specific charge density) and it is poorly soluble in polar electrolytes. The solubility problem can be remedied by replacing the methoxy groups in 2 with the ester groups (designated U and V in Scheme 1). Some of these ROMs (3-6 in Scheme 1) are highly soluble in the common electrolytes, but all of them are heavier than 2. A natural question is whether smaller, lighter “protective” groups in the arene ring can be used to inhibit the undesired ring-addition radical reactions (of RCs with their parent molecules). It is also noteworthy that RCs with the bulky groups R’ (Scheme 1), such as –XMe3 (X = C, Si) can lose them as carbocations; such reactions are less likely for less substituted groups as the corresponding carbocations are less stable. In ref.

39

we examined a series of methyl substituted 4


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compounds 7-12 (Scheme 1). Compounds 8 and 9 yielded persistent RCs, but these RCs were less stable than 2+●. For 9+●, a ring-to-ring addition was observed, while for 8+●, a branch-to-ring addition through 2,5-methyl groups was observed. Surprisingly, more extensively methyl substituted compounds 11 and 12, which should have avoided such radical addition reactions yielded short-lived RCs. U=

V=

(1) R1,2=Me, R'=H (2) R1,2=Me, R'=t-Bu (3) R1,2=U, R'=t-Bu (4) R1=Me, R2=U, R'=t-Bu (5) R1=Me, R2=V, R'=t-Bu (6) R1,2=V, R'=t-Bu

(7) X=Y=Z=H (8) Z=Me, X=Y=H (9) X=Me, Y=Z=H (10) Y=Me, X=Z=H (11) X=Y=Me, Z=H (12) X=Y=Z=Me

(R1) X=CH2, R=Me (R2) X=C2H4, R=Me (R3) X=CH2, R=U (R4) X=C2H4, R=U

Scheme 1. Chemical Structures for (1-12) dialkoxybenzene derivatives and (R1-R4) Kochi’s derivatives. Logically it follows that there is yet another parasitic reaction that takes over for RC decay when other reaction routes are inhibited. Product analyses in ref.

39

indicated that there was

indeed such a reaction that resulted in the symmetric loss of the alkyl groups in the RCs (the Odealkylation), yielding the corresponding quinone. Neither the mechanism of this reaction nor the factors controlling its efficiency were elucidated. Assary et al. speculated 40 that the first step of this reaction could be the formation of the corresponding aroxyl radical RO-Ar-O● and the transfer of the carbocation to a nucleophile (RO-Ar-OR)+● + Nu- → RO-Ar-O● + RNu

(1)

The released aroxyl radicals can eliminate alkyl radicals and/or disproportionate in the bulk, yielding the corresponding quinone; 41 these are well-known reactions for such radicals, 5 ACS Paragon Plus Environment


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2 RO-Ar-O● → RO-Ar-OR + O=Ar=O

(2a)

RO-Ar-O● → R● + O=Ar=O

(2b)

No direct evidence for reaction 1 was obtained, however, and the nature of the nucleophile remained unclear (as several electrolyte components can potentially serve this role). However, there was strong indirect evidence in favor of this mechanism that manifested itself in the structural effects we observed. Hypothetical reaction 1 implies that RC symmetry should play an important role in controlling the efficiency of the O-dealkylation. 42 Indeed, the latter should depend on the excess positive charge in the alkoxy groups that weakens the Cα-O bonds in these groups. If the excess positive charge in the RC is shared equally between the two alkoxy groups (which occurs when this RC has central or axial symmetry), the O-dealkylation would be slower as compared to the situation when this positive charge is preferentially localized in only one of these alkoxy groups. In their energetically preferred conformations, the two alkoxy groups in the neutral and charged 2,5-disubstituted dialkoxybenzenes shown in Scheme 1 rotate into the plane of the aromatic ring.

39, 40

For a neutral o,o’-disubstituted molecule, the alkoxy group rotates into the

plane perpendicular to the plane of the arene ring to minimize steric hindrance. In the RC, this rotation breaks the conjugation between the O 2p orbitals in the alkoxy group and the π-system of the arene ring, resulting in a steep energy penalty; however, rotating of this 1-alkoxy group into the plane of the aromatic ring also incurs penalty due to steric repulsion; these two effects oppose each other. For 11+●, one of the methoxy groups remains in the perpendicular (mid-) plane, whereas another methoxy group rotates into the plane of the arene ring. As a result, the spin and charge in this RC are unequally divided between the two methoxy groups, and the methoxy group having a greater positive charge rapidly O-demethylates. For 12+●, this scenario appears impossible due to central symmetry in the neutral molecule, but our computational examination in ref.

39

(see

Figure S23 therein) indicated that 12+● spontaneously breaks this symmetry, and once again the two methoxy groups become inequivalent. This symmetry breaking is possible due to rotation of the methyl groups in the 2,3,5,6-positions and close balance of the two opposing factors mentioned above. The increased substitution in the arene ring inhibits radical addition but promotes the O-dealkylation via this symmetry breaking. 6 ACS Paragon Plus Environment

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This examination suggested to us that crowding of the arene ring could improve RC stability only if the resulting structure remains rigid when positively charged, so this spontaneous reduction of the symmetry does not occur. With these considerations in mind we examined the literature and found two examples of molecular structures (compounds R1 and R2 in Scheme 1) conforming to our paradigm. These molecules were developed for the synthetic use of their RCs by Kochi, Rathore and their coworkers in the late 1990s 43-47 and annulated molecules of this family are still actively studied 48, 49

We broadly refer to these compounds as “Kochi’s molecules” in honor of the great Jay Kazuo

Kochi (1927-2008) in whose laboratory many of such RCs were discovered and introduced for preparative synthesis. These rigid, bis-annulated molecules are known to yield persistent RCs in dichloromethane solutions. By oxidation of R1 and R2 with SbCl5, solid hexacloroantimonate (SbCl6-) salts of these RCs can be obtained, in which these species are stable indefinitely at room temperature. By single-crystal X-ray crystallography 43 it was established that in these RCs the two methoxy groups occupy the plane of the arene ring, and the central symmetry of these RCs is almost perfect. Thus, these Kochi’s compounds combine many of the features desired in a catholyte ROM. Early in this inquiry, however, we realized that R1 and R2 were unsuited for their use in the flow cells on the account of poor solubility of the parent compounds in the typical electrolytes and the insufficient stability of their RCs in these same electrolytes. Below we demonstrate that both of these problems can be addressed in one stroke through the synthesis of R3 and R4 shown in Scheme 1. In this report, we examine the relevant properties of RCs of R1-R4 and elucidate the reaction mechanisms for their decay. We show that the most stable RCs of this class very slowly decay through the O-dealkylation; no evidence for the radical addition, deprotonation, or carbocation elimination was found. We establish the mechanism for this one residual decay reaction and demonstrate that it involves an anion as the nucleophile. To save room, supporting tables and figures have been placed in the Supporting Information (SI). When referenced in the text, these materials have the designator "S", as in Figure S1.

METHODS Synthetic procedures, 1H and

13

C nuclear magnetic resonance (NMR), mass spectroscopy,

and crystallographic data for R3 and R4 are given in ref.

50

. R1 and R2 and the 7



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hexachloroantimonate RC salts of R1-R4 (S●+ SbCl6-) were obtained using the methods reported in ref.

43

Unless specified otherwise, all other materials were obtained from Aldrich in their

purest available form and used without further purification. Cyclic voltammetry (CV) measurements were conducted in three-electrode glass cells at 25 o

C using CHI660D poteniostat with iR compensation. A Pt wire working electrode (2 mm

diameter), a Li metal pseudo-reference electrode, and a Li metal counter electrode were used. The carbonate electrolyte used in this study (“Gen2”) was a 3:7 w/w mixture of ethylene carbonate and ethyl methyl carbonate containing 1.2 M LiPF6 as supporting salt. All solvents were dried over molecular sieves before use. For measurements in dichloromethane and acetonitrile, 0.5 M tetra(n-butyl)ammonium hexafluorophosphate (NBu4 PF6) was used as supporting salt in a Pt/Pt cell equipped with an Ag/AgTFSI reference electrode. For gas chromatography-mass spectrometry (GC-MS) analyses, 1 µL liquid sample was loaded on an HP-5MS (bore 0.25 µm, length 30 m) column using an Agilent Technologies Model 7890B chromatograph equipped with a Model 5977 mass detector. The typical program included a 20 min holdup at 50 oC, followed by 20 oC/min ramp to 250 oC followed by another 20 min holdup. For high-performance liquid chromatography (HPLC), isocratic elution of the reaction mixture (5 µL aliquot) at 0.25 mL/min on Supelco LCPAH column (length 25 mm, bore 4.6 mm, particle size 5 µm); the absorbance was detected using a photodiode array detector using a ThermoScientific Accela suit. For gas chromatography analyses of these electrolytes, 0.5 mL of the solution was mixed with 0.5 mL benzene, and the salt was removed using extraction with two 10 mL portions of water. The organic phase was washed with water and dried. 1H NMR spectra were obtained using an Avance DMX 500 MHz or an Avance III HD 300 MHz spectrometers (Bruker), with benzene serving as the internal standard.

Continuous-wave Electron Paramagnetic Resonance (EPR) spectroscopy was used to characterize the RCs in situ and study their decay kinetics at room temperature. To this end, 100 µL aliquots of chemically or electrochemically oxidized liquid samples were placed in the borosilicate glass capillaries and these capillaries were then placed in glass tubes equipped with greaseless Teflon piston seal (Wilmad-LabGlass model 734-LPV-7). The first-derivative EPR spectra were collected at room temperature at 2 mW using a Bruker ESP300E X-band spectrometer operating at ~9.45 GHz. The typical modulation field was 8 ACS Paragon Plus Environment

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0.1-0.2 G at 100 kHz. The EPR spectra were analyzed using WinSim program suite (v. 0.98). For kinetic analyses, the first-derivative EPR spectra were recorded at fixed time intervals, centered, background corrected and doubly integrated. For chemical oxidation, sulfuric acid or PIFA (PhI(OC(O)CF3)2) was used.

51

The latter oxidizer reacts with

aromatic molecules (S), yielding the RC-trifluoroacetate pairs (S+● CF3CO2-) and eliminating iodobenzene (PhI); the loss of PIFA and the formation of iodobenzene can be quantified by 1H NMR. For electrochemical oxidation, a glass H-cell equipped with a Li anode, carbon cathode and lithium pseudo-reference electrode was used; fritted glass separated two 5 mL chambers, and the fluids were stirred using magnetic bars. The oxidation was carried at the charge rate of 1-5C to 100% nominal state-of-charge. At the end of this charging cycle, liquid samples were collected from the working electrode chamber. Galvanostatic cycling tests were carried out at 1C in the same H-cell. At the end of the test, the fluids were collected and analyzed. B3LYP density functional theory (DFT) functional Gaussian 03

54

52, 53

and 6-31+G(d,p) basis set from

were used to obtain energetics and estimate the reaction barriers for reaction 1.

We also used this method to calculate magnetic properties of RCs in Scheme 1. Solvation free energies were computed using SMD solvation model

40

by performing single point energy

evaluations in acetonitrile or water as dielectric medium. The oxidation potentials (EOx vs. Li/Li+) were estimated from the Gibbs free energy change (∆GOx, eV ) at 298 K in the solution for ionization of the species of interest, using the following equation: =

∆

− 1.24 ,

where F is the Faraday constant (in eV). The second term is a correction to convert the free energy changes to the oxidation potential, see refs. 55, 56 The change in energy of electrons when going from vacuum to non-aqueous solution (V0) is assumed to be zero, following ref. 40 Further details regarding the computation of the redox potential can be found elsewhere.

55, 57-61

The

solvation energy contributions were added to the gas phase enthalpies to approximate enthalpies in solution according to Hsoln = Hgas + ∆Gsolv. Consistent with our previous study, 40 the reaction barriers presented here are apparent enthalpy barriers, estimated as the difference between the computed enthalpy of the transition state (TS) structure (H†) and the sum of enthalpies of reactants in solution at 298 K. We used the solution phase enthalpy approximation as the main free energy contributions (GCDS: cavitation, dispersion, and solvent structure terms) cancel out 9 ACS Paragon Plus Environment


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(or are negligible) when computing the apparent barriers. Therefore, the dominant solvation contributions are electronic, nuclear, and polarization terms (GENP) which are included in the computation of solution enthalpies. A similar approach was used in refs. 40, 62

RESULTS AND DISUSSION Structural analyses and energetics. According to our DFT calculations, both R1 and R3 and their radical cations, R1+● and R3+●, have central symmetry (Ci) in their lowest energy conformation (Figure 1), while R2 and R4 and their RCs prefer the axial symmetry (C2). A brief summary of the computed properties is given in Table 1. In Figure 2, we plot the conformation energy as a function of the torsion angle Θ between the arene plane and the O-Cα bond in the RO- group (with all other degrees of freedom optimized). Symmetry-preserving rotation of these RO- groups was considered. The optimum angles Θ for the parent molecules and RCs are given in Table 1. For R3 and R4, we obtained Θ of 94o and 91o, respectively, which is close to the angles observed in their single crystals,

50

that is ~ 93o (Figure S1 in SI). The bond lengths and bond angles in the annuli were also similar to the ones obtained in the X-ray analyses, validating the use of the computational method. As seen from the plots in Figure 2, the thermal energy of 25 meV (300 K) would correspond to ±15o deviation from the mid-plane (“the acceptance cone”), so the RO- group is locked mid-plane due to the steepness of the rotational barrier. The remarkable property of these RCs is that the steric hindrance from the arene substituting groups is just enough to allow the RO- groups to rotate into the plane of the arene ring. For R1+● and R2+●, this was already observed by Rathore et al. 43 in the crystalline salts. However, steric hindrance from the annuli considerably reduces the rotation barrier for small torsion angles, so the acceptance cones for R1+● and R3+● are twice wider than for their parent molecules, extending out to ±30o. That is, the RO- groups are more “floppy” in these RCs compared to the neutral molecules. For gas phase R2 and R4 and their RCs (Figures S6 and S7 in SI), two symmetric conformations are possible, one with the inversion symmetry and one with the axial symmetry. Both of these conformations are broken symmetry Jahn-Teller active C2h species.

63

The C2

conformations are just a few meV lower in energy than the Ci ones. In both of these conformations, the rotational barriers for R2 and R4 become shallow, and the acceptance cones 10 ACS Paragon Plus Environment

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extend further than for R1 and R3, reaching ±45o. The increased steric hindrance causes nonzero Θ in the C2 conformations: this angle becomes ~11o for R2+● and ~25o for R4+● (Table 1). Thus, these RCs are even more flexible than R1+● and R3+●; i.e., they are less “locked” into their minimum-energy conformations compared to their parent molecules (see Figure S2 in SI). Since greater flexibility means that the RC stochastically attain conformations in which the excess positive charge in the RO- groups becomes unevenly shared between them, R1 and R3 should be more stable than R2 and R4. It will be seen that this prediction is supported experimentally. Table 1 gives the differences between the Mulliken atomic charge in the oxygen atoms of the two RO- groups in the RCs vs. neutral molecules. In our previous studies, 39, 42 we found that a greater positive charge reduces the energy barrier for the O-dealkytion. As seen from Table 1, for R1, R2, and R3 this parameter is approximately the same, whereas for R4 it is much greater, suggesting lower stability for R4+● to the O-dealkylation. For R1+● and R3+●, the structural flexibility is less of a concern. As shown in Figure S3 in SI, within the acceptance cone, the difference in the excess positive charges on the oxygen atoms is small, and the lowest energy Θ≈0ο conformation is a good representation of the entire ensemble. For R2+● and R4+●, this is not the case. The estimates of the oxidation potentials for all four Kochi’s molecules corresponded well with the estimates obtained using CV measurements (ref. 50).

Cyclic voltammetry. CV provides a useful assessment of the short-term (< 1 s) stabilities of the RCs. While a redox species that reversibly oxidizes on this short time scale may not be suitable for charge storage, a system that is irreversible on the CV time scale obviously cannot be used for the charge storage. Rathore et al. 43 reported CVs for R1 and R2 in dichloromethane; however, they did not examine these molecules in the typical battery electrolytes. For R1, the CVs indicate excellent oxidative reversibility not only in the dichloromethane, but also in organic carbonates and acetonitrile (not shown). For R2, the CVs demonstrated good reversibility in the carbonate solvent, but poor reversibility in acetonitrile (Figure S4 in SI). For R3 and R4, good reversibility was observed in all three of the solvents (Figure S5 in SI). These CV data suggest that R1, R3 and R4 yield RCs that are relatively stable, whereas R2+● is unstable in acetonitrile. Using EPR (see below) we showed that the stability of R2+● in the carbonate solvent was also relatively poor. 11 ACS Paragon Plus Environment


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EPR spectroscopy: radical signatures. Aiming to compare our DFT computations with the experimental data, we calculated the proton hyperfine coupling constants (hfcc’s) for the four Kochi’s RCs in Scheme 1 (Table S1). These constants can be determined experimentally by analyzing solution EPR spectra. It is seen that for these RCs, the hfcc’s for protons other than Hα in the RO- groups are quite small. In the annuli, only a few of the protons are strongly coupled (with their hfcc’s > 0.5 G): due to hyperconjugation, only certain axial or equatorial protons have favorable orientations relative to the π orbitals to acquire significant H 1s densities. The computationally estimated hfcc’s correspond well with the ones determined in our EPR experiments (Table 2). Figure 3 demonstrates the EPR spectra for R1 and R2 oxidized by (i) concentrated aqueous sulfuric acid (70 wt%) and (ii) PIFA in the dichloromethane. For R3 in the sulfuric acid, the EPR spectrum was close to the one published by Rathore et al.

43

for a solution of R1+●

SbCl6- in the dichloromethane. The hfcc pattern indicates that the geometry of R1+● corresponds closely to the C2h symmetry. While the EPR spectrum observed in the dichloromethane looks different, it can be simulated using the same parameters; the apparent difference originates through greater line broadening observed in the sulfuric acid. For R2, the EPR spectrum in dichloromethane was poorly resolved; nevertheless, the elucidated hfcc parameters qualitatively correspond to the ones estimated for R2+● SbCl6- in ref. 43

(see Table 2). In contrast, the radical species observed in the concentrated sulfuric acid was

different. The hfcc’s for this radical differ both from the computed hfcc and the set obtained for R2+● SbCl6-, suggesting that this radical is not R2+●. By comparing the hfcc pattern of this radical with several candidate species derived from R2 we found (Table 2) that it is an aroxyl radical RO-Ar-O● (see reaction 1). For the 2,5-disubstituted RCs in Scheme 1, the aroxyl radicals would be unstable. The 2,6-disubstituted aroxyl radicals can be very stable,

64, 65 41

especially under the acidic conditions, so observing such a radical by EPR would be possible provided it were generated in the reaction mixture. Figure 4 shows the EPR spectra obtained by chemical oxidation of R3 and R4 by (i) the sulfuric acid and (ii) PIFA in dichloromethane. The hfcc patterns for oxidized R3 corresponded to the one determined for R3+● SbCl6- solutions and RCs generated by electrochemical oxidation of R3, so we are confident that in all three cases the same RC was observed. The EPR spectrum 12 ACS Paragon Plus Environment

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of R3 in the concentrated sulfuric acid was clearly different: there was a set of additional resonance lines (indicated with the open circles in Figure 4a) that overlapped the signal from the RC. We believe that this secondary radical is also the corresponding aroxyl radical (that was observed for R2 under the same conditions). In contrast only RC was observed in (electro)chemical oxidation of R4 (which was also the case for R1). Thus, our EPR experiments yielded an unexpected dividend: not only have we confirmed the deduced RC structure in solution, but we also observed the postulated intermediate of reaction 1.

EPR spectroscopy: decay kinetics for RC. To observe the decay kinetics of RCs in liquid electrolytes, three methods were used: (i) galvanostatic charging of a parent compound in a Li/C cell containing carbonate solvent, (ii) dissolution of R3+● SbCl6- salt, and (iii) chemical oxidation with PIFA. The oxidized R1 and R2 have been referred to as stable or persistent RCs, 17, 18, 20, 43, 44, 47, 66-70 however, according to our EPR measurements, the life time of R2+● generated by oxidation of R2 by PIFA in dichloromethane was < 5 min; in the acetonitrile this life time was so short that R2+● decayed during CV sweeps (Figure S4 in SI). For R1+● in dichloromethane, the half-life was significantly longer, ~ 45 min (Figure S6 in SI), but even this life time is too short to use this ROM for longterm charge storage, and in the practically relevant electrolytes the decay of R1+● was still faster. A most significant difference between R3 and R4 from R1 and R3 is the much slower decay kinetics for their RCs. In Figure S7 in SI we show the kinetics for decay of the EPR signal for chemical oxidation of R3 and R4 by PIFA in dichloromethane (using method iii). These kinetics have “flat” tops as there was significant stoichiometric excess of PIFA; still, it is clear from these plots that the life time of these RCs exceeds 10 h. Using method ii, better quality data were obtained shown in Figure 5 (to conduct NMR analyses, deuterated solvents were used). No EPR signals from species other than R3+● were observed at any time (Figures 5a and 5b) For CD2Cl2, the observed kinetics are close to second-order decay with half life time, t1/2, of 138 h (Figure 5c). For acetonitrile, the kinetics significantly deviate from the second order; a better fit was obtained using a biexponential dependence with t1/2’s of 1.4 h and 24 h, accounting for 34% and 66% of the total decay, respectively (Figure 5c). For the carbonate electrolyte, methods i and ii can be compared directly at the same concentration of RC (Figure 6). The decay kinetics of 13 ACS Paragon Plus Environment


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R3+● (method ii) are close to second order with t1/2 of 13 h, and for method i we obtained t1/2 of 17 h. This difference is within the uncertainties of the experiment. When the electrochemical oxidation was carried out for R4+● in Gen2 (Figure S8 in SI), rapid decay kinetics with t1/2 of 1.4 h were observed. In acetonitrile, R4+● decays so rapidly, that R4 cannot be electrochemically converted to R4+●; using chemical oxidation, we estimated a life time < 10 min. Thus, despite the excellent stability of R4+● in dichloromethane (Figure S7 in SI), this RC is too unstable to be used for charge storage in the flow cells.

Chemical oxidation: product analyses. For 2,5-disubstituted 1,4-dialkoxybenzenes in Scheme 1, optical spectroscopy was a useful tool for identifying reaction products.

39

This approach can be used for the Kochi’s

molecules (Figure S9 in SI), and the absorption bands of the RC in the visible can be tracked over time. However, the absorption bands of the quinone (Q) and the parent compound strongly overlap (Figure S10 in SI), so in situ 1H NMR proved to be a more suitable analytical method. To initiate the reactions of RC, the parent compounds were oxidized by PIFA in CD2Cl2 and the analyte solution was placed in the resonator of the NMR spectrometer, where the products were monitored every 15 min for several days. The results for PIFA oxidation of R3 are summarized in Figure 7. Three reaction products were observed: the quinone (O=Ar=O) and two RX derivatives, where group X has no proton resonances. We labeled these two products P1 and P2. Product P1 was initially formed in 2 mol/mol stoichiometric ratio with the quinone (in Figure 7a we halved the yields for P1 and P2 to facilitate comparison), whereas product P2 was derived through the subsequent transformation of P1. Table S2 in SI summarizes 13C and 1H chemical shifts for these two products. Chemical shifts for P2 correspond to the cellosolve, ROH (Figure 8). The NMR spectrum of P1 reveals

13

C=O and

13

C

13

CF3 resonances, suggesting that P1 is the

trifluoroacetate ester, ROC(O)CF3 (Figure 8). This ester was also observed in GC-MS chromatograms shown in Figure S11 in SI along with trifluoroacetic acid, CF3CO2H, which is (like ROH) the product of its hydrolysis. We conclude that the oxidation of R3 by PIFA followed by the O-dealkylation of R3+● yields the quinone, PhI, and ROC(O)CF3, as shown in Figure 8. Over a longer period of time, the ester slowly hydrolyzes to CF3CO2H and ROH. We have puzzled, where the water for this 14 ACS Paragon Plus Environment

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hydrolysis originated from. The culprit was the oxidation agent: PIFA was a monohydrate. Our attempts to remove this water by heat treatment failed: either the water remained in the compound, or PIFA decomposed. Changing the approach, we added the water intentionally, by pre-saturating the CD2Cl2 with D2O. Due to rapid hydrolysis of P1 in this water-logged solution, little ester was observed, while the cellosolve (P2) prevailed. This experiment concludes our demonstration that R3+● reacted via reaction 1 with the trifluoroacetate serving as the nucleophile (Figure 8). Similar results were obtained for oxidation of R4 in dichloromethane (Figures S12 and S13 in SI). No significant differences in the rates of the O-dealkylation for R3 and R4 were found, which is also suggested by the similarity of the EPR kinetics shown in Figure S7 in SI. However, when the same oxidation was carried out for R4 in acetonitrile, the reaction completed in a matter of minutes, and yellow needles separated from the solution (see the inset in Figure S12 in SI). These needles were the quinone crystals. As the reader may recall, R2+● also showed poor stability in acetonitrile, (Figure S4 in SI) and the same quinone crystals were observed when R2 was chemically oxidized in acetonitrile. It follows that the quinone obtained by oxidation of R2 and R4 has extremely low solubility in acetonitrile, so the precipitation shifts the chemical equilibria in Figure 8 towards the O-dealkylation. In dichloromethane and carbonate solvents, the quinone solubility is greater, and R2+● and R4+● are more stable. While the ester substitution (Scheme 1) improves the solubility of R4 compared to R2, the quinone product for these two molecules is the same, and this makes both of the RCs short lived in the polar solvent. Similar chemical oxidation experiments were performed with R1 and R2 in dichloromethane (Figures S14 and S15 in SI). Methanol and the quinones were observed in the corresponding NMR spectra and chromatograms. No other products derived from the parent compounds other than the quinones were observed. Apparently, the O-demethylation was the only reaction of these RCs. Experiments with R3+● SbCl6-. The reaction mechanism in Figure 8 implies that the nature of the counter anion can have significant effect on RC stability. Using R3+● SbCl6- allowed us to probe the kinetics of RC decay in CD2Cl2 and CD3CN without the chemical oxidizer present in the solution.



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In dichloromethane, the kinetics for the quinone formation were extremely slow (in agreement with the EPR results shown in Figures 5a and 5c), and the reaction remained incomplete after 150 h. Two MeOC2H4X products were observed, P1 and P2 (Figure S16 in SI). The sum yield of these two products equaled the quinone yield stoichiometrically (2:1 mol/mol). The chemical shifts of P2 (given in panel b of Figure S16 in SI) are close to the reported for 3bromoethyl methyl ether, and we identify P2 as MeOC2H4Cl. Product P1 has unusual chemical shifts suggesting the formation of the C-Sb bond. This is a minor product; the main reaction appears to be (RO-Ar-OR)+● SbCl6- → RO-Ar-O● + RCl + SbCl5

(1a)

Had the ion pair on the left side been generated electrochemically (so both the neutral and RC species were present in this solution), the released SbCl5 would oxidize R3, yielding SbCl3 and another R3+● SbCl6- pair that enters reaction 1a. The occurrence of reaction 1a suggests that the RCs can react with “stable” anions used as supporting salts. Decomposition of such anions yields reactive fragments that can contribute to the degradation of RCs and their parent molecules. Our DFT calculations (see below) suggest that while oxidation of R3 by SbCl5 is strongly exergonic (by -17.9 kcal/mol), reaction 1a would be endergonic by 16.7 kcal/mol, which explains its relatively slow rate. For R3 in acetonitrile (Figure S17 in SI) the kinetics were shorter (also in agreement with the EPR data shown in Figures 5b and 5c). Once again the quinone was formed and there were two MeOC2H4X products, P1 and P2. P1 is the chloride that was observed from the earliest observation times, whereas product P2 was slowly formed as P1 decayed. Together P1 and P2 stoichiometrically balance the quinone. Product P2 is different from product P1 in CD2Cl2; we believe that it is a product of RCl solvolysis.

Energetics. To supplement our experimental findings, reaction energetics were examined using DFT calculations. The computed results are given in Table 1, and a schematic illustrating the activation (∆H†) and reaction (∆Hrxn) enthalpies are shown in Figure 9. This figure also shows an example of the transition state structure (more of these structures are shown in Figure S18 in SI). 16 ACS Paragon Plus Environment

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The computed reaction enthalpies indicate that the O-dealkylation of R2, R3 and R4 involving the trifluoroacetate in acetonitrile would be a weakly exergonic (almost thermoneutral) reaction. For the trifluoroacetate, the reaction barriers are < 21 kcal/mol, and R1+● and R3+● are less reactive than R2+● and R4+●. For the carbonate solvent serving as the nucleophile in reaction 1, these reaction barriers are > 25 kcal/mol and the reactions are strongly endergonic (Table 1), suggesting a much slower O-dealkylation for isolated RCs.

Bulk electrolysis. While experiments for the oxidized ROMs are illuminating, electrochemical cycling can be different, as the RCs are both generated and discharged near the electrode, and additional parasitic reactions are possible. Thus we sought to establish whether the O-dealkylation was also the prevalent reaction of the RC under such conditions. Galvanostatic cycling was carried out for 5 mM solutions of R3 or R4 in Gen2 at 1C rate. For R4, all capacity was lost during the first (2 h) charging cycle, which agrees with our EPR observations suggesting a short lifetime for R4+●. For R3, the capacity was lost over a much longer period of time (up to 115 cycles) with the average rate of 3% per cycle. At the end of the test, the cell fluids were collected and analyzed (the HPLC chromatogram for R3 is shown in Figure S19(a) in SI). Chromatographic peaks for the quinone (28 mol% yield) and unreacted R3 are observed. In addition to these two compounds, there was a species whose optical spectrum was similar to R3 (see Figure S19(b) in SI). In the gas chromatograms, in addition to R3 and the quinone, three other products were observed in a very low yield (< 5%) with the mass peaks at 300.2, 314.2 and 328.2 (the latter was present in < 0.5%). These mass peaks correspond to R3 in which one of the RO- groups is shortened to the hydroxyl, methoxy and ethoxy groups, respectively, suggesting the occurrence of C-O bond cleavage in the ester groups. One of these products is the progenitor of the minor 15.6 min peak seen in the chromatogram in Figure S19(a) in SI. No such products were observed in our chemical oxidation experiments. We had the cycling test for R3 several times varying the experimental conditions, and in each case obtained ~30 mol% quinone yield at the end of the run. The minor products have been observed only in one of these runs (it was the longest run). We believe that these minor products originate from secondary reactions that occur due to cross over between the H-cell compartments (which are separated only by a porous glass separator). This cross over is endemic in such 17 ACS Paragon Plus Environment


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experiments, as these separators cannot block molecular diffusion occurring over a long period of time. Our conclusion, therefore, is that the O-dealkylation of RCs controls not only their chemical stability but also their electrochemical stability. CONCLUSION Radical cations (RCs) are the energy-rich charged species that can be used as catholyte materials in nonaqueous RFBs providing they are sufficiently stable in liquid electrolytes. In this study we inquired as to what reactions control the chemical and electrochemical stability of the most persistent energetic RCs in organic solutions. While we cannot answer this question in general, we answer it for the practically-important 1,4-dialkoxybenzene derivatives shown in Scheme 1. For sterically “protected” ROMs, the RCs avoid the deprotonation and ring-addition reactions, but they can still undergo the O-dealkylation. To study the latter reaction, we synthesized molecules R1-R4, in which the ring-addition and carbocation elimination reactions are entirely suppressed; these molecules are also sufficiently rigid, so that their RCs do not break symmetry, resulting in the rapid O-dealkylation. All of these compounds yielded relatively stable RCs (at least, under some conditions), but R3+● proved to be exceptionally stable in the common electrolytes. In dichloromethane, R3+● has the lifetime > 120 h, which is the current record for the 1,4-dialkoxybenzene RCs. Our experiments suggest that the O-dealkylation is the main decay channel for these RCs, which makes these RCs particularly suitable to study this one remaining reaction. Our studies validate the mechanism given by reactions 1-3 (see also Figure 8). We observed the postulated aroxyl intermediates of reaction 1 and identified the nucleophile as the anion partner of the RCs. In no case did we observe the direct nucleophile attack by the solvent on the RC. Many of the reaction products are unstable and can react further, so the resulting chemistry was complicated. Regarding the degree of the control over the O-dealkylation, choosing the right electrolyte is no less important than optimizing the ROM structure. Poor solubility of the resulting quinone in polar solvents can dramatically accelerate the O-delakylation by shifting the reaction equilibria towards the products. However, even in the low-concentration regime, the solvent polarity and nucleophilicity have significant effects on RC stability, which tends to decrease in the order of the increasing polarity. Currently, acetonitrile based electrolytes are the preferred choice by the RFB community (from the engineering, economic, and environmental 18 ACS Paragon Plus Environment

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standpoints); unfortunately, this solvent is also the most problematic from the standpoint of RC chemistry. Carbonate solvents cover the middle ground, and in ref. 50 we used R3 in a carbonate solvent to demonstrate the hybrid flow cell capable of 160-cycle operation; this is the current record for such a device. We believe that with the mechanistic factors limiting the cell performance presently understood, further optimization of the 1,4-dialkoxybenzene ROMs is possible, as this optimization becomes a straightforward exercise in controlling a single chemical reaction that it particularly amenable to in silico studies. 71, 72

ASSOCIATED CONTENT Supporting Information: A PDF file containing the list of abbreviations and additional figures and tables, including the computed magnetic, energetic, and geometric parameters, EPR, NMR and mass spectra, NMR kinetics and HPLC chromatograms. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS This work was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.



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Figure captions.

Figure 1. Optimized structures for the gas phase R3 and R3+●. In the neutral molecule, the ether chains are in the mid-plane, while in the RC they rotate into the arene plane.

Figure 2. Conformational energy for neutral molecules R1 to R4 (lines) and their RCs (symbols) plotted vs. the torsion angle Θ between the Cα-O bond in the RO- groups and the plane of the arene ring (with all other degrees of freedom optimized). The central symmetry was imposed on R1 and R3 in panel a and the axial symmetry was imposed on R2 and R4 in panel b. The two RO- groups rotate synchronously to preserve this symmetry. The dashed line indicates the thermal energy at 300 K (~25 meV).

Figure 3. First-derivative X-band EPR spectra of 5 mM R1 and R2 oxidized in 70 wt% aqueous sulfuric acid (blue traces) and anhydrous dichloromethane containing 5 mol.eq PIFA (red traces).

Figure 4. Like Figure 3 for compounds R3 and R4. For R3 in the sulfuric acid, the EPR spectrum of R3+● overlaps with the resonance lines from another radical (indicated with the open circles) that we identify with the corresponding aroxyl radical RO-Ar-O●.

Figure 5. Time evolution of EPR signals from 5 mM R3+● SbCl6- in (a) dichloromethane-d2 and (b) acetonitrile-d3. The delay times are indicated in the plot. (c) Normalized decay kinetics of the doubly integrated EPR signals from the same solutions (see the legend). The decay curves are fit by two-exponential dependences with the time constants given in the plot. For dichloromethaned2, second order decay kinetics also gives a good fit.


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Figure 6. Time evolution of EPR signals and the normalized decay kinetics of R3+● in Gen2 (a carbonate based electrolyte). The radical cation was generated by electrolysis of 5 mM R3 in a Li/C cell (100% oxidation of the material) or by dissolving 5 mM R3+● SbCl6- in this electrolyte. In both cases, the decay kinetics corresponds to the second order decay of RC, and the time constants of this decay are similar.

Figure 7. In situ 1H NMR kinetics observed after combining 10 mM R3 with 3 mol.eq PIFA in CD2Cl2. The formation of the quinone (Q) and product P1 in 1:2 mol/mol yield are observed in the beginning (the yields of P1 and P2 in the inset plot are scaled down by a factor of two). Over time, P1 converts to P2. The resonance lines from the reaction mixture are shown in the two panels and compared to the reference NMR spectra from R3 and Q. We have identified P2 as the cellosolve and P1 as the cellosolve trifluoroacetate.

Figure 8. Chemical oxidation of the bis annulated compounds (S) in Scheme 1 by PIFA. The reaction sequence begins with the formation of the S+● CF3CO2- pair in which one of the RO- groups undergoes reaction 1 to form ROC(O)CF3 (P1) that is subsequently hydrolyzed to ROH (P2). The two aroxyl radicals disproportionate (reaction 2) to yield the parent compound and the quinone (Q).

Figure 9. A schematic representation of the energy profile along the reaction coordinate for the Odemethylation of R2+● to the trifluoroacetate anion that serves as the methyl-receiving nucleophile. Also shown is the computed transition state (TS) geometry (with the oxygen atoms in red, carbons and hydrogen atoms in grey, and fluorine atoms in green; the distances are given in Å). The corresponding TS and reaction enthalpies are indicated in the scheme with the arrows.



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Table 1. Computed Properties for Bis-annulated Compounds R1 – R4.

Species

R

R1

Me

Sym metry Ci

R2

Me

C2

R2

Me

Ci

R3

MeO C2H4 MeO C2H4 MeO C2H4

Ci

R4 R4

Θ,

C2 Ci

o a,b

-

Excess Charge in Oxygens, at.u. d 0.026

Oxidation Potential. V vs. Li/Li+ 3.84

0

0.017

6.6 (4.6) -

0.014

∆E, meV

101.4 (0.23) 91.7 (11.1) 91.5 (0.10) 93.8 (0.33) 91.7 (25.2) 91.4 (29.5)

c

∆H†,

∆Hrxn,

∆H†,

∆Hrxn,

e

e

f

f

29.8

14.3

20.5

0.4

3.98

29.1

10.9

17.6

-2.9

0.019

3.92

26.8

14.5

20.0

-0.1

0

0.114

4.24

25.0

7.1

19.7

-7.6

10 (8.9)

0.128

a) Italics are for the neutral form, see Scheme 1; b) the torsion angle between O-Cα bond in the RO- group and the plane of the arene ring; c) relative electronic energy of the geometry optimized centrosymmetric vs. axisymmetric structure; d) difference in the Mulliken atomic charges for oxygen atoms in the RO- groups of the RC and parent molecules; e) reaction 1 with the propylene carbonate; f) reaction 1 with the trifluoroacetate. The enthalpies are given in kcal/mol,


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Table 2. Experimental EPR Parameters for Selected Radicals. Radical species R1+●

Liquid medium H2SO4

(g-2) x104 d 41

R1+●

PIFA, CH2Cl2 H2SO4

41

PIFA, CH2Cl2 CH2Cl2 e H2SO4 PIFA, CH2Cl2 CH2Cl2

43

R3+● SbCl6- a

MeCN

52

R3+● SbCl6- a,b

Gen2

44

R4+●

H2SO4

40

R4+●

PIFA, CH2Cl2 Gen2

45

R2+● ? R2-phenoxyl R2+● R2+● SbCl6R3+● R3+●

a

R3+● SbCl6-

a

R4+●

39

40 46 46

48

Isotropic hfcc, G c 6H 2.206, 2H 1.818 2H 1.261, 4H 0.278 6H 3.261, 2H 2.103 2H 1.210, 4H 0.382 2H 2.73, 8H 0.68 2H 1.41, 2H 1.55; 8H 0.5 f ~6H 3.20 6H 4.02; 4H 0.8 2H 3.911, 2H 3.502 2H 1.697, 2H 1.218 2H 3.755, 2H 3.482 2H 1.550, 2H 1.430 2H 4.094, 2H 3.614 2H 1.604, 2H 1.247 (2H 0.268?) 2H 4.092, 2H 3.624 2H 1.656, 2H 1.265 4H 3.214, 4H 0.786 4H 0.677 4H 2.494, 4H 0.727 4H 0.680 4H 2.629, 4H 0.719 4H 0.555

a) solutions of solid ionic compound; b) the same parameters were obtained by electrochemical oxidation of R3 in Gen2; c) g-factor for RC; d) the sign of the hyperfine coupling constant (hfcc) cannot be determined by EPR, 1 G (Gauss) = 10-4 T; in some cases the spectral resolution was too low to estimate all of the proton hfcc’s; e) ref. 43; f) DFT calculation for geometry optimized aroxyl radical using the B3LYP/6-31+G(d,p) method.



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Figure 1. Optimized structures for the gas phase R3 and R3+●. In the neutral molecule, the ether chains are in the mid-plane, while in the RC they rotate into the arene plane.


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Figure 2. Conformational energy for neutral molecules R1 to R4 (lines) and their RCs (symbols) plotted vs. the torsion angle Θ between the Cα-O bond in the RO- groups and the plane of the arene ring (with all other degrees of freedom optimized). The central symmetry was imposed on R1 and R3 in panel a and the axial symmetry was imposed on R2 and R4 in panel b. The two RO- groups rotate synchronously to preserve this symmetry. The dashed line indicates the thermal energy at 300 K (~25 meV). 25 ACS Paragon Plus Environment


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Figure 3. First-derivative X-band EPR spectra of 5 mM R1 and R2 oxidized in 70 wt% aqueous sulfuric acid (blue traces) and anhydrous dichloromethane containing 5 mol.eq PIFA (red traces).


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Figure 4. Like Figure 3 for compounds R3 and R4. For R3 in the sulfuric acid, the EPR spectrum of R3+● overlaps with the resonance lines from another radical (indicated with the open circles) that we identify with the corresponding aroxyl radical RO-Ar-O●.



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Figure 5. Time evolution of EPR signals from 5 mM R3+● SbCl6- in (a) dichloromethane-d2 and (b) acetonitrile-d3. The delay times are indicated in the plot. (c) Normalized decay kinetics of the doubly integrated EPR signals from the same solutions (see the legend). The decay curves are fit by two-exponential dependences with the time constants given in the plot. For dichloromethaned2, second order decay kinetics also gives a good fit.


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EPR signal

t, min 0 4 13 38 65

(a) -10

1.0

0 offset field

10G

el. ox. of R3

(b)

+•

R3 SbCl6

I(t)/I0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2nd order t1/2 = 17 h

0.5

2nd order t1/2 = 13 h

0.0 0

25

50

75

100

time, h

Figure 6. Time evolution of EPR signals and the normalized decay kinetics of R3+● in Gen2 (a carbonate based electrolyte). The radical cation was generated by electrolysis of 5 mM R3 in a Li/C cell (100% oxidation of the material) or by dissolving 5 mM R3+● SbCl6- in this electrolyte. In both cases, the decay kinetics corresponds to the second order decay of RC, and the time constants of this decay are similar.



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Figure 7. In situ 1H NMR kinetics observed after combining 10 mM R3 with 3 mol.eq PIFA in CD2Cl2. The formation of the quinone (Q) and product P1 in 1:2 mol/mol yield are observed in the beginning (the yields of P1 and P2 in the inset plot are scaled down by a factor of two). Over time, P1 converts to P2. The resonance lines from the reaction mixture are shown in the two panels and compared to the reference NMR spectra from R3 and Q. We have identified P2 as the cellosolve and P1 as the cellosolve trifluoroacetate.


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2

PIFA

- PhI

-

P2 water

P1

disproportionation

Q

Figure 8. Chemical oxidation of the bis annulated compounds (S) in Scheme 1 by PIFA. The reaction sequence begins with the formation of the S+● CF3CO2- pair in which one of the ROgroups undergoes reaction 1 to form ROC(O)CF3 (P1) that is subsequently hydrolyzed to ROH (P2). The two aroxyl radicals disproportionate (reaction 2) to yield the parent compound and the quinone (Q).



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Figure 9. A schematic representation of the energy profile along the reaction coordinate for the O-demethylation of R2+● to the trifluoroacetate anion that serves as the methyl-receiving nucleophile. Also shown is the computed transition state (TS) geometry (with the oxygen atoms in red, carbons and hydrogen atoms in grey, and fluorine atoms in green; the distances are given in Å). The corresponding TS and reaction enthalpies are indicated in the scheme with the arrows.


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ToC graphic.


Toward Improved Catholyte Materials for Redox Flow Batteries: What

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