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Cooperative Electrocatalytic O2 Reduction Involving Co(salophen) with p-Hydroquinone as an Electron-Proton Transfer Mediator Colin W. Anson, and Shannon S. Stahl J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11362 • Publication Date (Web): 03 Dec 2017 Downloaded from http://pubs.acs.org on December 3, 2017
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Journal of the American Chemical Society
Cooperative Electrocatalytic O2 Reduction Involving Co(salophen) with p-Hydroquinone as an Electron-Proton Transfer Mediator Colin W. Anson, Shannon S. Stahl* Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, United States Supporting Information Placeholder ABSTRACT: The molecular cobalt complex, Co(salophen), and para-hydroquinone (H2Q) serve as effective cocatalysts for the electrochemical reduction of O2 to water. Mechanistic studies reveal redox cooperativity between Co(salophen) and H2Q. H2Q serves as an electron-proton transfer mediator (EPTM) that enables electrochemical O2 reduction at higher potentials and with faster rates than is observed with Co(salophen) alone. Replacement of H2Q with the higherpotential EPTM, 2-chloro-H2Q, allows for faster O2 reduction rates at higher applied potential. These results demonstrate a unique strategy to achieve improved performance with molecular electrocatalyst systems. Catalytic reduction of O2 is crucial to many important processes, including industrial oxidation reactions,1 fuel cells,2 and biological metabolism.3 In Nature, mitochondrial respiration features a redox cascade involving multiple enzymes and small-molecule mediators. The latter includes ubiquinone (UQ), which shuttles electrons and protons to cytochrome c oxidase (Complex IV), where O2 is reduced to H2O (Scheme 1A). Analogous redox cascades have been implemented in homogeneous catalysis for aerobic oxidation of organic molecules.4,5 These catalyst systems are often composed of metal complexes capable of binding O2, in combination with p-benzoquinone (BQ) or related mediators (Scheme 1B). The metal complexes (LnM) commonly used in these reactions consist of Co- or Fe-based macrocycles, which closely resemble complexes that have been studied as molecular electrocatalysts for O2 reduction.6 The precedents in Schemes 1A and 1B prompted us to consider whether a mediator capable of transferring both protons and electrons, such as p-hydroquinone (H2Q), could improve the efficiency of electrochemical O2 reduction by macrocyclic metal complexes (Scheme 1C). The results described herein validate this concept, showing that redox cooperativity between Co(salophen) and H2Q enables O2 reduction at higher potentials and with faster rates than is observed with either individual catalyst partner. Seminal work by Bäckvall and coworkers has led to the widespread use of Co(salophen) and p-hydroquinone (H2Q) as cocatalysts in Pd-catalyzed aerobic oxidation reactions.5 Although this concept has been employed in many synthetic applications,4 the O2 reduction pathway with these cocatalysts has received little attention. In the context, we recently investigated the mechanism of Co(salophen)-catalyzed aerobic oxidation of H2Q.7 The results provided evidence for direct reaction between H2Q and Co/O2 intermediates, including hydrogen-atom transfer and proton-coupled electron transfer as key mechanistic steps. Moreover, the reaction leads to
Scheme 1. O2 Reduction Mediated by Quinones (red) and Macrocyclic Metal Complexes (blue). A) Electron-transport chain in mitochondrial respiration H+
H+
H+ Cyt c
UQ I
IV
III H2UQ
II NADH NAD+ + H+ FADH2
1/ 2
O
FAD
O2
His
H 2O
MeO
Me
R'
UQ:
IV: H
MeO O CH3 ubiquinone
N N Fe N N
Me O
6-10
His
Cu
His
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Me R
heme A
B) Electron-proton transfer mediators in aerobic oxidations OAc
H 2O Pd0
BQ
[LnM]red
H 2Q
[LnM]ox
1/ 2
O2
AcO
PdII
+ 2 HOAc
C) EPTMs in electrocatalytic O2 reduction 2H+
BQ
O
BQ: H 2O
O
[LnM]red
–
1/ 2
O2
[LnM]: N
N Co
H 2Q
[LnM]ox
O
O
Co(salophen)
complete four-electron reduction of O2 to H2O, avoiding the formation of H2O2, which is commonly observed in O2 reduction reactions catalyzed by mononuclear Co complexes.6 These results provided a starting point for the present study, designed to test the ability of electron-proton transfer mediators (EPTMs) to improve electrocatalytic O2 reduction. Electrochemical studies were initiated to probe Co(salophen)/H2Q-catalyzed O2 reduction. MeOH and dimethylformamide (DMF), the most effective solvents identified in the aerobic oxidation of H2Q,7 were used to analyze the redox properties of Co(salophen) and BQ by cyclic voltammetry. Cyclic voltammograms (CVs) were recorded in the presence of various acid sources that could support electrochemical reduction of O2. Stronger acids, such as CF3CO2H and [HDMF][OTf], led to ligand protonation and demetalation of Co(salophen), while weak acids, such as trifluoroethanol in MeOH, lacked sufficient acidity to generate H2Q upon electrochemical reduction of BQ. The best match of solvent and acid sources proved to be DMF and AcOH,8 and
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Inset
CoIII/II
Co(salophen) under N2 under O2 no Co(salophen)
B) BQ under N2 under O2
C) Co(salophen) + BQ under N2 under O2
Current density (-mA/cm2)!
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Co(salophen) + H2Q!
Co(salophen)!
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Figure 2. Controlled-potential electrolysis traces, probing the ability of Co(salophen), H2Q, and Co(salophen)/H2Q to mediate O2 reduction at an applied potential of –760 mV vs. Fc+/Fc. Conditions: 1 mM Co(salophen), 5 mM H2Q, 0.3 M AcOH, 0.1 M Bu4NPF6, 1 atm O2, 40 mL DMF. Working electrode: reticulated vitreous carbon. Potential: –760 mV vs. Fc+/0. Chemistry in bulk solution 2 L nCoII + 2 AcOH + 1/2 O2 H 2Q + 1/2 O2
2 L nCoIII(OAc) + H 2O
cat. [LnCoII ]
(1)
BQ + H 2O
(2)
H 2Q + 2 AcO–
(3)
L nCored
(4)
Chemistry at the electrode
Potential (V vs. Fc+/0)
primary reaction:
Figure 1. Cyclic voltammograms of: Co(salophen) (A), BQ (B), and Co(salophen)/BQ (C) under N2 (black trace) and O2 (red trace). Conditions: 1.0 mM Co(salophen), 10 mM BQ, 0.3 M AcOH, 0.1 M Bu4NPF6, 10 mL DMF. Working electrode: glassy carbon. Scan rate = 10 mV/s.
CVs of Co(salophen) and BQ, obtained under N2 and under O2 in DMF with 0.3 M AcOH, are shown in Figure 1. Under N2, Co(salophen) exhibits a quasireversible wave for the CoIII/II redox process (all potentials versus Fc+/0) (see inset of Figure 1A). Under O2, the CV of Co(salophen) exhibits a catalytic wave, with an onset negative of the CoIII/II potential (Figure 1A, red trace). CVs of BQ show poor reversibility, as is commonly observed for quinones in organic solvents (Figure 1B),9 but identical CVs were observed under N2 and O2, indicating that the reduced quinone does not react with O2 on the CV time scale. The CV of a solution of both Co(salophen) and BQ under O2 reveals an increase in current at the wave corresponding to BQ reduction (Figure 1C, red vs. black trace). While this increase is higher than the sum of the currents observed independently from Co(salophen)/O2 and from BQ under O2 (see Figure S1 in the Supporting Information), the difference is not sufficiently large to use CV as a means to probe the synergistic behavior between Co(salophen) and BQ. Controlled-potential electrolysis (CPE) studies, however, proved to be effective to assess the synergy between the cocatalysts. Electrolysis was performed at –760 mV, near the peak of the benzoquinone reduction feature (Figure 2). Analysis of Co(salophen) alone in DMF (1 mM, 1 atm O2, 0.3 M AcOH) achieves a stable steady-state current density after 120 min (black trace), whereas H2Q alone shows negligible current under the same conditions (blue trace). The combination of Co(salophen) and H2Q leads to a significantly higher steadystate current density (red trace), relative to the Co-only electrolysis. The data in Figure 2 is rationalized by the combination of chemical reactions taking place in bulk solution and at the electrode (eqs 1–4). In the absence of H2Q, CoII(salophen) will
–
BQ + 2 AcOH + 2 e – secondary reaction: L nCoox + e –
–
L nCoox = CoIII(X)(salophen), CoIII(OO•)(salophen), etc.
undergo autoxidation to CoIII(OAc)(salophen) in bulk solution,7 and the steady-state electrolysis current will reflect the reduction of oxidized Co species that diffuse from bulk solution to the electrode, together with any Co-catalyzed reduction of O2 that takes place directly at the electrode (e.g., via proton-coupled reduction of Co/O2 adducts). These processes are considered together in eq 4. The decrease in current at the beginning of the electrolysis is consistent with a transition from fully-oxidized Co(salophen) species, present at the beginning of the reaction, to a mixture of oxidized and reduced Co species present at steady-state. Virtually no current is observed with H2Q alone (Figure 2, blue trace), showing that background oxidation of H2Q by O2 is very slow even on the longer time scale of bulk electrolysis. Electrolysis of a mixture of Co(salophen) and H2Q exhibits the same initial current density observed with Co(salophen) alone, but the current then rises to a higher steady-state level (Figure 2, red trace). This result arises from efficient O2 reduction Co(salophen)/H2Q in bulk solution (eq 2), leading to a buildup of BQ that can undergo reduction at the electrode (eq 3) and account for the higher steady-state current density. The influence of the H2Q mediator was probed further by varying the amount of H2Q included in the electrolysis solution (0–20 mM; Figure 3A). In all cases, the steady-state current density is higher than that observed with the Co(salophen)-only electrolysis. The relative increase in current density (i.e., the difference between the current density in the presence and absence of mediator) exhibits a saturation dependence on [H2Q] (Figure 3B, black circles). The redox state of the mediator during steady-state electrolysis was probed by analyzing aliquots of the electrolysis solutions by 1 H NMR spectroscopy (conducted for [H2Q] = 1, 5, and 20 mM). Both H2Q and BQ were evident in the spectra (cf. Figure S2 and Table S1), and the steady-state concentrations of BQ correlate closely with the steady-state rates observed
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Figure 3. A) Controlled-potential electrolysis traces with various (0-20 mM) [H2Q] in the presence of Co(salophen). Conditions: 1.0 mM Co(salophen), 0-20 mM H2Q, 0.3 M AcOH, 0.1 M Bu4NPF6, 1 atm O2, 40 mL DMF. Working electrode: reticulated vitreous carbon. Potential: –0.76 V vs. Fc+/0. B) Increase in current density (relative to the steady-state current density with Co-only) vs. [H2Q] (black trace) and steady-state [BQ] vs. initial [H2Q] (red points) detected by 1H NMR.
from the reaction mixtures (red squares, Figure 3B). This correlation is consistent with the increase in current arising from electrochemical reduction of BQ generated via Co(salophen)-catalyzed aerobic oxidation of H2Q in bulk solution (eq 3). The modular cocatalyst system used in this O2 reduction strategy allows the hydroquinone mediator to be modified independently from the Co(salophen) catalyst,10 and use of a higher-potential quinone could provide the basis for effective O2 reduction at a higher potential. 2-Chlorobenzoquinone (2ClBQ) was selected as a quinone that has a more positive reduction potential, but is still capable of undergoing aerobic oxidation catalyzed by Co(salophen). The CV of 2-ClBQ in DMF with 0.3 M AcOH reveals a peak reduction potential at -0.70 V, which is 90 mV higher than that of BQ (Figure 4A). Controlled potential electrolysis tests were then compared for electrochemical O2 reduction catalyzed by Co(salophen) in the presence of H2Q, 2-ClH2Q, and in the absence of a hydroquinone mediator at an applied potential of –670 mV. The highest sustained current density is observed with 2-ClH2Q, as shown in Figure 4B. These results may be rationalized by considering the rates of Co(salophen)catalyzed oxidation of hydroquinone in bulk solution (cf. eq 2) relative to the rates of reduction of the quinone at the electrode (cf. eq 3). Specifically, Co(salophen)-catalyzed aerobic oxidations of H2Q and 2-ClH2Q exhibit very similar rates, as determined by gas-uptake kinetic experiments (Figure S3). Meanwhile, 2-ClBQ undergoes electrochemical reduction with a rate that is nearly 75% faster than the reduction of BQ under the reaction conditions (Figure S4), presumably arising from the greater driving force for reduction of 2-ClBQ. Use of hydroquinone as an electron-proton transfer mediator in combination with Co(salophen) not only increases the O2 reduction rate (cf. Figures 2 and 4B) but also increases
Potential (V vs. Fc+/0)
B) CPE with H2Q Derivatives
Current density (-mA/cm2)
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2-ClH2Q
H2Q
no mediator
Time (minutes)
Figure 4. Comparison of data for BQ/H2Q and 2-ClBQ/2-ClH2Q for the electrochemical reduction of O2 mediated by Co(salophen) and a hydroquinone cocatalyst. (A) CVs of benzoquinone (BQ; red trace) and 2-chlorobenzoquinone (2-ClBQ; blue trace). Conditions: 10 mM quinone, 0.3 M AcOH, 0.1 M Bu4NPF6, DMF, N2. Glassy carbon working electrode. Scan rate = 10 mV/s. (B) Controlled-potential electrolysis traces with no mediator (black trace), H2Q (red trace), and 2-ClH2Q (blue trace). Conditions: 1 mM Co(salophen), 5 mM H2Q derivative, 0.3 M AcOH, 0.1 M Bu4NPF6, 40 mL DMF, 1 atm O2. Working electrode: RVC. Potential = –670 mV vs. Fc+/0.
the reaction selectivity. Mononuclear cobalt complexes typically mediate two-electron reduction of O2 to H2O2.6 Similar behavior is evident here for the reactions mediated by Co(salophen) in the absence of H2Q. Use of a Ti-peroxo colorimetric assay11 to quantify H2O2 in the electrolysis reaction mixture reveals 40% selectivity for H2O2. This value represents a lower limit for the H2O2 yield, however, due to background disproportionation of H2O2 mediated by the Co(salophen) on the timescale of the electrolysis reaction.7 An independent experiment conducted with decamethylferrocene as a chemical reductant, which enables more-rapid reduction of O2, led to a 91% selectivity for H2O2 (see Supporting Information for details). This value approaches the nearquantitative formation of H2O2 recently observed with a series of related N2O2-ligated Co-based catalysts under different conditions.6g These observations contrast Co(salophen)catalyzed reduction of O2 with H2Q (eq 2), which exclusively generates H2O.7,12 The results outlined above demonstrate a new cooperative catalytic strategy for electrocatalytic reduction of O2, whereby a mononuclear cobalt complex benefits from partnership with a hydroquinone cocatalyst. The improved rates and selectivity achieved with this cocatalyst system arise from the direct involvement of hydroquinone in the mechanism of O2 reduction. Recent experimental and computational studies indicate that hydroquinone reacts with Co-superoxide via hydrogen-atom transfer (HAT), followed by proton-coupled electron transfer from semihydroquinone to a CoIIIhydroperoxide species (Figure 5).7,13 Subsequent 2 H+/2 e– transfer by a second equivalent of H2Q leades to rapid formation of H2O, rather than release of H2O2 as the product of O2 reduction. This reaction pathway exhibits an intriguing relationship to the O2 reduction pathway in cytochrome c oxidase, wherein a modified tyrosine residue undergoes HAT to an activated oxygen species.14 The cooperative catalyst system described here also may be compared to other strategies that have been explored to improve O2 reduction with molecular catalysts. Prominent examples include the use of cofacial Co-macrocycles15,16 and the incorporation of proton relays into mononuclear
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O O H
HAT
O O O O Co N N
O H
H O O O O Co N N
PCET
Figure 5. Key hydrogen-atom transfer and proton-coupled electron transfer steps in H2Q-mediated reduction of O2 by Co(salophen).
metalloporphyrin catalysts.17 Electron-proton transfer mediators (EPTMs),such as H2Q, represent a linear combination of these systems, incorporating both redox equivalents and proton sources needed to promote O2 reduction. The modular nature of such cooperative systems and the ability to tune the EPTM properties, evident here and in other recent examples,18 present unique opportunities to improve the performance and utility of molecular electrocatalysts.
ASSOCIATED CONTENT Supporting Information
Full experimental procedures, electrochemical data, NMR aliquot data, gas uptake kinetic measurements, H2O2 selectivity analysis. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
[email protected] ORCID Colin W. Anson: 0000-0002-8514-3865 Shannon S. Stahl: 0000-0002-9000-7665
ACKNOWLEDGMENT This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. NMR spectroscopy facilities were supported by a gift from the Paul J. and Margaret M. Bender Fund.
REFERENCES (1) Liquid Phase Aerobic Oxidation Catalysis; Stahl, S. S., Alsters, P. L., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2016. (2) For recent reviews on the electrocatalytic oxygen reduction reaction, see: (a) Nie, Y.; Li, L.; Wei, Z. Chem. Soc. Rev. 2015, 44, 2168. (b) Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R. Chem. Rev. 2016, 116, 3594. (3) Babcock, G. T.; Wikström, M. Nature 1992, 356, 301. (4) For a review, see: Piera, J.; Bäckvall, J.-E. Angew. Chem. Int. Ed. 2008, 47, 3506. (5) For early precedents introducing this concept, see: (a) Bäckvall, J.-E.; Hopkins, R. B.; Grennberg, H.; Mader, M. M.; Awasthi, A. K. J. Am. Chem. Soc. 1990, 112, 5160. (b) Bäckvall, J.-E.; Chowdhury, R. L.; Karlsson, U. J. Chem. Soc., Chem. Commun. 1991, 473. (6) For reviews and leading references, see: (a) Wiesener, K.; Ohms, D.; Neumann, V.; Franke, R. Mater. Chem. Phys. 1989, 22, 457. (b) Vasudevan, P.; Santosh; Mann, N.; Tyagi, S. Transition Met. Chem. 1990, 15, 81. (c) Anson, F. C.; Shi, C.; Steiger, B. Acc. Chem. Res. 1997, 30, 437. (d) Masa, J.; Ozoemena, K.; Schuhmann, W.; Zagal, J. H. J. Porphyrins Phthalocyanines 2012, 16, 761. (e) Liu, Y.; Yue, X.;
Li, K.; Qiao, J.; Wilkinson, D. P.; Zhang, J. Coord. Chem. Rev. 2016, 315, 153. (f) Zhang, W.; Lai, W.; Cao, R. Chem. Rev. 2017, 117, 3717. (g) Wang, Y.-H.; Pegis, M. L.; Mayer, J. M.; Stahl, S. S. J. Am. Chem. Soc. 2017, 139, 16458. (7) Anson, C. W.; Ghosh, S.; Hammes-Schiffer, S.; Stahl, S. S. J. Am. Chem. Soc. 2016, 138, 4186. (8) The combination of AcOH in MeOH led to Co(salophen) decomposition. (9) (a) Eggins, B. R.; Chambers, J. Q. J. Electrochem. Soc. 1970, 117, 186. (b) Aguilar-Martinez, M.; Macías-Ruvalcaba, N. A.; BautistaMartínez, J. A.; Gómez, M.; González, F. J.; González, I. Curr. Org. Chem. 2004, 8, 1721. (c) Guin, P. S.; Das, S.; Mandal, P. C. Int. J. Electrochem. 2011, Article ID 816202. (10) For leading references on quinones with different redox properties, see: (a) Clark, W. M. Oxidation-Reduction Potentials of Organic Systems; The Williams & Wilkins Company: Baltimore, 1960. (b) Evans, D. H. Carbonyl Compounds. In Encylopedia of Electrochemistry; Bard, A. J., E.; Marcel Dekker, Inc.: New York, 1978; Chapter XII-1, pp 198. (c) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961. (d) Er, S.; Suh, C.; Marshak, M. P.; Aspuru-Guzik, A. Chem. Sci. 2015, 6, 885. (e) Huynh, M. T.; Anson, C. W.; Cavell, A. C.; Stahl, S. S.; Hammes-Schiffer, S. J. Am. Chem. Soc. 2016, 138, 15903. (f) Son, E. J.; Kim, J. H.; Kim, K.; Park, C. B. J. Mater. Chem. A. 2016, 4, 11179. (11) Lee, Y.; Park, G. Y.; Lucas, H. R.; Vajda, P. L.; Kamaraj, K.; Vance, M. A.; Milligan, A. E.; Woertink, J. S.; Siegler, M. A.; Sarjeant, A. A. N.; Zakharov, L. V.; Rheingold, A. L.; Solomon, E. I.; Karlin, K. D. Inorg. Chem. 2009, 48, 11297. (12) Complete deconvolution of the contribution of O2-reduction selectivity to the current density is challenging for this system because typical assays for analysis of H2O2 are complicated by the presence of H2Q and BQ. Formation of H2O rather than H2O2, however, would lead to no more than a two-fold current enhancement. (13) For other fundamental studies of hydroquinone-mediated O2 reduction, see (a) Abel, E. W.; Pratt, J. M.; Whelan, R.; Wilkinson, P. J. J. Am. Chem. Soc. 1974, 96, 7119. (b) Simándi, L. I.; Barna, T.; Argay, G.; Simándi, T. L. Inorg. Chem. 1995, 34, 6337. (c) Simándi, T. M.; May, Z.; Szigyártó, I. C.; Simándi, L. I. Dalton Trans. 2005, 365. (d) Henthorn, J. T.; Lin, S.; Agapie, T. J. Am. Chem. Soc. 2015, 137, 1458. (e) Horak, K. T.; Agapie, T. J. Am. Chem. Soc. 2016, 138, 3443. (14) For leading references, see the following and references cited therein: Schaefer, A. W.; Kieber-Emmons, M. T.; Adam, S. M.; Karlin, K. D.; Solomon, E. I. J. Am. Chem. Soc. 2017, 139, 7958. (15) For reviews, see ref. 6f and the following: Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E. Angew. Chem. Int. Ed. 1994, 33, 1537. (16) For leading references, see: (a) Collman, J. P.; Marrocco, M.; Denisevich, P. J. Electroanal. Chem. 1979, 101, 117. (b) Collman, J. P.; Denisevich, P.; Konai, Y.; Marrocco, M.; Koval, C.; Anson, F. C. J. Am. Chem. Soc. 1980, 102, 6027. (c) Liu, H. Y.; Weaver, M. J.; Wang, C.-B.; Chang, C. K. J. Electroanal. Chem. 1983, 145, 439. (d) Chang, C. J.; Loh, Z.-H.; Shi, C.; Anson, F. C.; Nocera, D. G. J. Am. Chem. Soc. 2004, 126, 10013. (e) Fukuzumi, S.; Okamoto, K.; Gros, C. P.; Guilard, R. J. Am. Chem. Soc. 2004, 126, 10441. (f) Kadish, K. M.; Frémond, L.; Ou, Z.; Shao, J.; Shi, C.; Anson, F. C.; Burdet, F.; Gros, C. P.; Barbe, J.-M.; Guilard, R. J. Am. Chem. Soc. 2005, 127, 5625. (17) (a) McGuire, R., Jr.; Dogutan, D. K.; Teets, T. S.; Suntivich, J.; Shao-Horn, Y.; Nocera, D. G. Chem. Sci. 2010, 1, 411. (b) Dogutan, D. K.; Stoian, S. A.; McGuire, R., Jr.; Schwalbe, M.; Teets, T. S.; Nocera, D. G. J. Am. Chem. Soc. 2011, 133, 131. (c) Carver, C. T.; Matson, B. D.; Mayer, J. M. J. Am. Chem. Soc. 2012, 134, 5444. (d) Sinha, S.; Aaron, M. S.; Blagojevic, J.; Warren, J. J. Chem. Eur. J. 2015, 21, 18072. (18) (a) Gerken, J. B.; Stahl, S. S. ACS Cent. Sci. 2015, 1, 234. (b) Badalyan, A.; Stahl, S. S. Nature, 2016, 535, 406.
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