Proton-Coupled Electron Transfer and Substituent Effects in Catechol

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Proton-Coupled Electron Transfer and Substituent Effects in CatecholBased Deep Eutectic Solvents: Gross and Fine Tuning of Redox Activity Parker J. Smith, and John C. Goeltz J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b10169 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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The Journal of Physical Chemistry

Proton-Coupled Electron Transfer and Substituent Effects in Catechol-Based Deep Eutectic Solvents: Gross and Fine Tuning of Redox Activity Parker J. Smith and John C. Goeltz* School of Natural Sciences, California State University, Monterey Bay, 100 Campus Center, Seaside CA. 93955

ABSTRACT: The 1,2-diol moiety in a variety of substituted catechols allows formation of room temperature ionic melts in a 2:1 ratio with choline chloride or choline dihydrogen citrate. These deep eutectic solvents were 4.3-6.6 M in redox active catechols. Substituents on 3- and 4substituted catechols shift both E° and pKa such that Hammett parameters predict the observed Ep for oxidation in square wave voltammetry. The proton acceptor for the proton-coupled oxidation shifts the observed Ep more strongly than the substituents within the substituents and acceptors reported here. The shift is predicted well by the pKa of the conjugate acid of the proton acceptor, i.e., water in aqueous solutions or chloride or dihydrogen citrate in the DESs in this study. Together, the substituent and the proton acceptor allow gross and fine tuning of the oxidation

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potential for catechol over 750 mV, the first demonstration of control of the thermodynamics of proton-coupled electron transfer in deep eutectic solvents. Changing the substituents on the HBD affords fine control in tens of mV, while changing the base strength of the anion of the organic salt affords gross control across hundreds of mV. INTRODUCTION Deep eutectic solvents (DES) are low-melting ionic mixtures of growing interest for applications that include energy storage, metal plating, solubilization of biomolecules, organic synthesis, and gas separations.1–6 They are typically two- or three-component systems with each component selected from metal halides, aquated metal halides, hydrogen bond donors, and quaternary ammoniums or similar organic salts.7 Electrochemistry and more recently pH have been studied in DESs,8,9 but proton-coupled electron transfer (PCET) has not yet been explored explicitly or systematically in these media. PCET is used as a framework for better understanding biological electron transfer chains, catalytic mechanisms, hydrogen bond strength, and electron transfer mechanisms.10–18 Type III DESs are mixtures of a HBD and an organic salt such as choline chloride.7 Recently our laboratory reported the first DES composed of a reversible redox active salt and a redox innocent HBD.19 Here we report two ways to tune the peak oxidation potential (Eox) in a DES composed of a redox innocent salt and a redox active HBD. For irreversible electrochemical processes such as those we describe here, this can be framed either as one end of the solvent window for the solvent/electrolyte or as the potential where the solvent itself can absorb excess energy in an electrochemical process.


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Catechol was selected as the HBD because it has a 1,2-diol functionality that is structurally similar to ethylene glycol and because many substituted catechols are readily available; however unlike ethylene glycol, catechol has an aromatic backbone that supports oxidation to semiquinones and quinones in certain environments.20 Several substituted catechols (Figure 1) with differing electron donating and withdrawing functionalities were selected to explore substituent effects on the electrochemical properties of the DES. Choline was selected as the organic cation, as it is to date the best studied in DESs. To elucidate the effect of the anion, choline chloride and choline dihydrogen citrate were used.

Figure 1. Catechol variants used as HBDs. From left to right, in order from electron withdrawing to electron donating: 3-fluorocatechol (FCat), catechol (Cat), 4-methylcatechol (4MeCat), 3methylcatechol (3MeCat), and 3-hydroxycatechol (Gal; also known as pyrogallol).

METHODS Each DES sample was prepared by mixing the components at 70 °C overnight. Freshly prepared samples were used for all voltammetry measurements and for freezing point determinations. The molar ratio of substituted catechol to quaternary ammonium salt was systematically varied and the freezing point of the resulting solutions was measured to determine the eutectic ratio. Small


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amounts of water changed the freezing point of the eutectic mixture substantially; to mitigate this, freezing points were measured on freshly prepared samples and the choline chloride was stored in a vacuum desiccator prior to use. Cyclic voltammetry (CV) and square wave voltammetry (SQW) were performed on each DES with an 11 µm diameter glassy carbon microelectrode with the samples heated to 70 °C in a sand bath. The bath, the samples, and the electrodes were allowed to stabilize before electrochemical sampling such that the temperature varied by no more than 2 °C throughout a given experiment. The Ag/AgCl reference electrode used was frequently compared with an unused Ag/AgCl reference in a 1 M KCl solution to confirm that the reference used in the DES was not changing or becoming clogged. The reference electrode was preheated to 70 °C before being inserted in the sample to prevent temperature changes from forcing water into the sample. Before each CV, the glassy carbon microelectrode was rinsed with 1.0 M NaOH, and polished with 0.05 micron polishing powder. After collection, the CV data were simulated using DigiElch. The pH values for each mixture were measured at 70 °C under a blanket of argon by measuring the voltage of a compact reversible hydrogen electrode (RHE) vs. a Ag/AgCl reference electrode with a high impedance instrument (pH meter or a potentiostat, with equivalent results). The pKa values for the substituted catechols were measured by titration with 1.0 M NaOH in water while being deoxygenated continuously with argon. RESULTS We combined various ratios of catechol:choline chloride (Cat:ChCl), and catechol:choline dihydrogen citrate (Cat:ChDHC), and found the eutectic point to be at 2:1 Cat:ChCl or Cat:ChDHC (Figure 2), the same ratio reported as the eutectic point for the ethylene glycol and


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urea DESs with choline chloride.7,21 The 2:1 mixtures of each substituted catechol:salt were liquid at room temperature, except that of 3-methylcatechol:choline chloride. The 2:1 ratio led to a catechol concentration of 6.55 M, which is ~2 M more concentrated than a saturated solution of catechol in water achieved in our laboratory at 18 °C.

Figure 2. The freezing point of Cat:ChCl mixtures as a function of the percent of catechol by mole. We observed a well-defined but chemically irreversible oxidation peak for each DES (Figure 3 and S2-10). Low concentrations of catechol in water exhibited a quasi-reversible oxidation, but a saturated solution of catechol underwent an irreversible oxidation (S2-S4). Thus, the


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irreversibility of the oxidation in these DESs may at this time be attributed either to the high concentration of catechol or to its being a structural constituent of the solvent. Savéant et al. reported an elegant electrochemical study of phenol oxidation in buffered and unbuffered aqueous solutions.18 They were able to preclude fouling of the electrode by oxidized phenolic species by maintaining the phenol concentrations below 1 mM and model the voltammetric response with a concerted proton-coupled electron transfer mechanism. The DESs under study here are by their nature 4.3-6.6 M in catechol, and we are thus unable to adapt this exact methodology.


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Figure 3. CVs of 2:1 Cat:ChCl DES at varied scan rates vs. a Ag/AgCl reference electrode. The inset is the linear plot of ip vs ν1/2 consistent with a freely diffusing species. We were able to fit voltammograms for each DES in the present report to 1 e- processes with an irreversible decomposition mechanism by digital simulation (i.e., in the simplest case an EC mechanism), though we are for the time being unable to distinguish between a concerted PCET and a stepwise electron and proton transfer mechanism. The voltammetric responses for the choline chloride-based DESs were fit well to freely diffusing species, while the choline dihydrogen citrate DES voltammetry was better fit to a response that included an adsorbed redox species, consistent with the observed dependencies of the peak currents on scan rates (i.e., linear behavior for ip vs. ν1/2 for the ChCl DESs and ip vs ν for ChDHC DESs). Simulations and parameters are available in the supporting information. We elected to simulate the cyclic voltammetry to support the narrowed range of possible mechanisms as there are reports that suggest that kinetic information may be lost in the square wave experiments for analogous chemically irreversible reactions.22–24 The square wave voltammograms illustrate the trends in peak potentials (see supporting information), and because of the reduced influence of kinetic effects they allow an analysis of bond dissociation energies that is more closely analogous to that of Bordwell et al.,17 vide infra. The electron donating substituents methyl and hydroxyl shifted the potential more negative and the electron withdrawing substituent, flouro, shifted the potential more positive (Table 1). By changing the substituent attached to the catechol, the oxidation potential for the electroactive solvent shifted by up to 250 mV shifts in potentials that are typical for these substituents.25,26 Table 1. Properties of deep eutectic solvents in this study


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HBD

HBA

Concentration BDE in H2O or Eox vs. Hammett ∆BDE vs. H2O RHE of HBD (M) Parameterd (Kcal mol-1)a (V)c

pyrogallol

ChDHC

4.58

-6.8

0.65

-0.37

4-Methylcatechol

ChDHC

4.28

-7.5

0.73

-0.17

3-Methylcatechol

ChDHC

4.38

-5.5

0.85

-0.069

Catechol

ChDHC

4.51

-4.5

0.84

0

3-Flourocatechol

ChDHC

4.53

-3.5

0.90

0.062

pyrogallol

ChCl

6.27

5.7

1.22

-0.37

4-Methylcatechol

ChCl

5.77

6.2

1.33

-0.17

3-Methylcatechol

ChCl

5.83

6.3

1.36

-0.069

Catechol

ChCl

6.55

7.3

1.39

0

3-Flourocatechol

ChCl

6.31

8.0

1.42

0.062

pyrogallol

Water

0.025

82.9b

0.99

-0.37

4-Methylcatechol

Water

0.025

84.6b

1.04

-0.17

3-Methylcatechol

Water

0.025

85.6b

1.06

-0.069

Catechol

Water

0.025

86.3b

1.07

0

3-Flourocatechol

Water

0.025

85.0b

1.09

0.062

a

∆BDE calculated according to the equation proposed by Bordwell.17 b The water data is BDEs calculated according to Bordwell.17 c Eox values are taken from the SWV of each sample. d Hammett parameters were taken from a review by Taft.27

DISCUSSION In type III DESs (HBD, organic cation, anion), the anion of the solvent may be referred to as a hydrogen bond acceptor (HBA) for the HBD because that is its structural function from the fluid DES perspective.21 From the PCET perspective however, the HBA is functioning as hydrogen


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atom acceptor as the HBD is oxidized. For simplicity of discussion, we use the HBA/HBD notation throughout this manuscript, as analogous roles are maintained. While the catechol substituent plays a modest role in determining the oxidation potential, we found that the HBA provides greater opportunity for tuning the oxidation potential within a range of easily accessible options. There was up to a 600 mV shift in oxidation potential of a given catechol in comparing chloride and dihydrogencitrate (DHC) DESs. Adapting analyses of Bordwell et al.,17 who studied bond dissociation energies for a range of molecules, we extracted the catechol oxidation peak potentials and plotted them versus the Hammett parameter for each catechol substituent (Figure 4). When grouped by HBA (red lines in Figure 4) we find that the oxidation potentials are linearly correlated with the Hammett parameter. In HBAs that are weaker bases (i.e., chloride), the catechols oxidize at higher potentials, indicating poorer stabilization of the oxidized products that include H-atom transferred to the base. Dihydrogen citrate is a stronger base and may also offer more potential hydrogen bonding and H-atom acceptor sites, stabilizing the products and lowering the oxidation potential.


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Figure 4. Plot of the Eox of each substituted catechol with each HBA vs the Hammett parameter. Eox is the peak potential from the positive square wave voltammogram, taken in analogy to the work of Bordwell et al.17 The relationship between the HBD and the HBA’s ability to accept protons is more clearly seen in Figure 5, where the oxidation peak is plotted as a function of the pKa of the conjugate acid of the HBA (pKa(HBA)), e.g., citric acid for the ChDHC DESs. The pKa of hydronium is taken to be 1.77 here; while an argument can be made that the pKa of hydronium should be 0.00,28 we here use the more commonly accepted value without concern for the overall conclusions of the present body of work. The higher pKa(HBA) of DHC means it is a better proton or hydrogen atom


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acceptor than Cl- or water; thus the observed oxidation potential is lowered. Conversely, the lower pKa(HBA) of HCl makes Cl- a weaker base and weaker proton or hydrogen atom acceptor than DHC or water, destabilizing the products of proton-coupled electron transfer and raising the observed oxidation potential.

Figure 5. Plot of the Eox of each substituted catechol versus the pKa of the conjugate acid of the HBA in which the peak potential was measured. We used the Eox of each substituted catechol in water to calculate the ∆BDE using the equation of Bordwell et al.17 as a function of the change in HBA (see supporting information for example calculation). As the pKa(HBA) increases, the net energy of breaking the O-H bond in the catechol to proceed towards the products decreases. This relationship is then predictive of the Eox, and


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thus Bordwell’s BDFE equation (Eq 1) can be rewritten as a function of the pKa of the HBA (Eq 2) by substituting the empirically derived equation for the relationship between the Eox and the pKa(HBA) from the linear fit in Figure 5 in place of the Eox, Eqn. 2, below. (1) = 1.37 + 23.06 (−) + (2) = 1.37 () + 23.06( ∗ () + ) + To use equation 2, the oxidation potential of a new HBD is measured across varied environments to find the empirical relationship between the pKa(HBA) and the Eox; once that relationship is determined, pKa(HBA) can predict the Eox in other environments. Critically, this can be used to predict the optimal HBA when designing a redox active DES or the high-potential solvent window of a DES intended to be redox innocent. CONCLUSION This work has demonstrated that the oxidation potential of redox active DESs can be tuned in two ways: firstly by changing the substituents on the HBD, and secondly by changing the pKa of the conjugate acid of the HBA. Changing the substituents affords fine control and allows the Eox to be tuned by 20-250 mV, while changing the pKa of the HBA affords gross control and allows the Eox to be tuned up to 600 mV by choice of common anions. Fine control of the oxidation potential over a large range can be used to tune the solvent window when studying the electrochemical properties of solvated compounds, and it can be used to design a solvent with optimal electrochemical properties for absorption of excess energy or for staying out of harm’s way in an electrochemical process.


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ASSOCIATED CONTENT The following files are available free of charge. Further experimental details; sample calculations of BDEs and ∆BDEs, sample cyclic voltammograms and square wave voltammograms, and digital simulations of voltammograms and simulation mechanistic parameters (PDF) AUTHOR INFORMATION Corresponding Author *John C. Goeltz [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes We are grateful to an anonymous reviewer who directed us to relevant literature on the strengths and limitations of square wave voltammetry and in doing so strengthened this report. ACKNOWLEDGMENT Parker Smith gratefully acknowledges support from the Undergraduate Opportunities Center (UROC) at California State University, Monterey Bay and the U.S. Department of Education (#P031C160221: Hispanic-Serving Institutions STEM Program to develop and carry out activities to improve and expand the institution’s capacity to serve Hispanic and other lowincome students), and the National Science Foundation (NSF) under grant #HRD-1302873 and the Chancellor’s Office of the California State University. ABBREVIATIONS


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DES, deep eutectic solvent; ChCl, choline chloride; ChDHC, choline dihydrogen citrate; Cat, catechol; FCat, 3-fluorocatechol; 3MeCat, 3-methylcatechol; 4MeCat, 4-methylcatechol; Gal, 3hydroxycatechol or pyrogallol; HBD, hydrogen bond donor; HBA, hydrogen bond acceptor; BDE, bond dissociation energy; RHE, reversible hydrogen electrode; CV, cyclic voltammogram or cyclic voltammetry; SWV, square wave voltammetry REFERENCES (1)

(2) (3)

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(14) Anson, C. W. . G., Soumya; Hammes-Schiffer, Sharon; Stahl, Shannon S. Co(salophen)Catalyzed Aerobic Oxidation of P-Hydroquinone: Mechanism and Implications for Aerobic Oxidation Catalysis. J. Am. Chem. Soc. 2016, 138. (15) Ghosh, S.; Soudackov, A. V.; Hammes-Schiffer, S. Electrochemical Electron Transfer and Proton-Coupled Electron Transfer: Effects of Double Layer and Ionic Environment on Solvent Reorganization Energies. J. Chem. Theory Comput. 2016, 12, 2917. (16) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Thermochemistry of Proton-Coupled Electron Transfer Reagents and Its Implications. Chem. Rev. 2010, 110, 6961–7001. (17) Bordwell, F. G.; Cheng, J. P.; Harrelson, J. A. Homolytic Bond Dissociation Energies in Solution from Equilibrium Acidity and Electrochemical Data. J. Am. Chem. Soc. 1988, 110, 1229–1231. (18) Costentin, C.; Louault, C.; Robert, M.; Savéant, J.-M. The Electrochemical Approach to Concerted Proton—electron Transfers in the Oxidation of Phenols in Water. Proc. Natl. Acad. Sci. 2009, 106, 18143–18148. (19) Goeltz, J. C.; Matsushima, L. N. Metal-Free Redox Active Deep Eutectic Solvents. Chem Commun 2017. (20) Ryan, M. D.; Yueh, A.; Chen, W.-Y. The Electrochemical Oxidation of Substituted Catechols. J. Electrochem. Soc. 1980, 127, 1489–1495. (21) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V. Novel Solvent Properties of Choline Chloride/Urea mixturesElectronic Supplementary Information (ESI) Available: Spectroscopic Data. See Http://Www.rsc.org/Suppdata/Cc/b2/b210714g/. Chem. Commun. 2003, 1, 70–71. (22) Molina, A.; Gonzalez, J.; Laborda, E.; Compton, R. G. Mass Transport at Electrodes of Arbitrary Geometry. Reversible Charge Transfer Reactions in Square Wave Voltammetry. Russ. J. Electrochem. 2012, 48, 600–609. (23) Garay, F.; Lovrić, M. Quasi-Reversible EC Reactions at Spherical Microelectrodes Analysed by Square-Wave Voltammetry. J. Electroanal. Chem. 2002, 527, 85–92. (24) Whelan, D. P.; O’Dea, J. J.; Osteryoung, J.; Aoki, K. Square Wave Voltammetry at Small Disk Electrodes: Theory and Experiment. J. Electroanal. Chem. Interfacial Electrochem. 1986, 202, 23–36. (25) Geiger, W. E. Organometallic Electrochemistry: Origins, Development, and Future. Organometallics 2007, 26, 5738–5765. (26) Connelly, N. G.; Geiger, W. E. Chemical Redox Agents for Organometallic Chemistry. Chem. Rev. 1996, 96, 877–910. (27) Hansch, C.; Leo, A.; Taft, R. W. A Survey of Hammett Substituent Constants and Resonance and Field Parameters. Chem. Rev. 1991, 91, 165–195. (28) Chaplin, M. Water ionization, the ionic product (Kw) of water and pH http://www1.lsbu.ac.uk/water/water_dissociation.html (accessed Oct 9, 2017).


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Table of Contents Graphic


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Figure 1. Catechol variants used as HBDs. From left to right, in order from electron withdrawing to electron donating: 3-fluorocatechol (FCat), catechol (Cat), 4-methylcatechol (4MeCat), 3-methylcatechol (3MeCat), and 3-hydroxycatechol (Gal; also known as pyrogallol). 434x91mm (96 x 96 DPI)



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Figure 2. The freezing point of Cat:ChCl mixtures as a function of the percent of catechol by mole. 281x217mm (149 x 149 DPI)


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Figure 3. CVs of 2:1 Cat:ChCl DES at varied scan rates vs. a Ag/AgCl reference electrode. The inset is the linear plot of ip vs v1/2 consistent with a freely diffusing species. 281x217mm (149 x 149 DPI)



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Figure 4. Plot of the Eox of each substituted catechol with each HBA vs the Hammett parameter. Eox is the peak potential from the positive square wave voltammogram, taken in analogy to the work of Bordwell et al.17 281x217mm (149 x 149 DPI)


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Figure 5. Plot of the Eox of each substituted catechol versus the pKa of the conjugate acid of the HBA in which the peak potential was measured. 279x215mm (150 x 150 DPI)



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TOC Graphic 83x44mm (149 x 149 DPI)


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Proton-Coupled Electron Transfer and Substituent Effects in Catechol

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