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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Electrocatalytic Water Oxidation by an Unsymmetrical Di-Copper Complex Qin-Qin Hu, Xiao-Jun Su, and Ming-Tian Zhang* Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing, China

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S Supporting Information *

inserted into the Cu(III)−O−Cu(III) accompanied by intramolecular proton transfer (Scheme 1c), rather than the classical

ABSTRACT: An unsymmetrical di-copper complex, ([Cu2(TPMAN)(μ-OH)(H2O)]3+, was prepared and used for electrocatalytic water oxidation in neutral conditions. This complex is a stable and efficient homogeneous catalyst during the electrocatalytic oxygen evolution process (kcat = 0.78 s−1) with 780 mV onset overpotential in 0.1 M phosphate buffer (pH 7.0). The water oxidation mechanism of the unsymmetrical catalyst [Cu2(TPMAN)(μ-OH)]3+ exhibits different behaviors than that of [Cu2(BPMAN)(μ-OH)]3+, such as two redox steps with different pH dependences, a significant kinetic isotope effect, and buffer concentration dependence. All these changes were ascribed to the open site on the Cu center that is formed by removal of the hemilabile pyridyl site, which acts as an intramolecular proton acceptor to assist the O−O bond formation step.

Scheme 1. Schematic Representation of the O−O Bond Formation Pathways for Artificial Water Oxidation Catalysts (WOCs)

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lternative energy sources to fossil fuel are essential for sustainable development.1 Solar energy is considered to be an ideal alternative energy source because of its sufficient reserves. Inspired by natural photosynthesis, artificial systems for solar-powered fuel production, such as making hydrogen by water splitting, has attracted widespread attention.2,3 Particular efforts have been devoted to the development of robust and efficient molecular catalysts for H2O oxidation.4,5 Unfortunately, the efficiency of the reported catalysts has not reached a sufficient level for application in commercial cells for solar energy conversion.6−26 Therefore, further work is required to gain a better understanding of the oxygen evolution process, which, in turn, will shed light on the rational design of catalysts. Recently, we reported a bio-inspired di-Cu catalyst, [Cu2(μOH)(BPMAN)]3+ (Figure 1, BMPAN = 2,7-[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine) for electrocatalytic water oxidation at pH 7.0.27,28 Interestingly, the O−O bond formation for this di-Cu catalyst proceeds through a bimetallic cooperative pathway in which a Cu-bonded OH nucleophilic

O−O bond formation pathways (WNA or I2M, Scheme 1a,b). A similar O−O bond formation mechanism was also proposed for the oxygen evolving complex (OEC) of photosystem II.29,30 The benefit of the bimetallic cooperative O−O bond formation pathway is that the oxidation equivalent is distributed on two metal sites rather than a single site and, thus, avoids an extremely high oxidation state such as Cu(IV)O, which has been extensively proposed as a key intermediate for Cu-based water oxidation catalysis.11 According to the catalytic cycle reported for the symmetric diCu catalyst, [Cu2(BPMAN)(μ-OH)]3+, a pyridyl site acts as a hemilabile site to provide an empty site for the coordination of a water molecule.27 Furthermore, this water molecule acts as another O atom source and participates in the rate-limiting step of the O−O bond formation (Scheme 1c). Building on the above-mentioned mechanism, we became interested in the unsymmetrical di-Cu catalyst with an open coordination site for the water molecule. Herein, we report an unsymmetrical di-Cu complex ([Cu2(TPMAN)(μ-OH)(H2O)]3+, Figure 1, TPMAN = 2-[bis(2-pyridylmethyl)aminomethyl]-7-[N-methyl-N-(2pyridyl-methyl)aminomethyl]-1,8-naphthyridine) for catalytic water oxidation at pH 7.0 in a phosphate buffer solution. This unsymmetrical di-Cu catalyst ([Cu 2 (TPMAN)(μ-OH)(H2O)]3+) exhibits excellent catalytic efficiency toward electrocatalytic water oxidation.

Figure 1. Structure of our previous di-Cu catalyst ([Cu2(μ-OH)(BPMAN)]3+, left) and the unsymmetrical di-Cu catalyst ([Cu2(μOH)(TPMAN)]3+, right) investigated in this work. © XXXX American Chemical Society

Received: April 29, 2018

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DOI: 10.1021/acs.inorgchem.8b01173 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry The dinucleating ligand TPMAN was designed to possess 3and 4-coordinate sites on the two hands, which is one pyridyl less than the BPMAN ligand. It was prepared as shown in Scheme S1 (the details are reported in the SI). Starting from 1,8dichloromethyl-naphthyridine (1), TPMAN was prepared in 14% overall yield by two successive substitution reactions. The 1 H NMR and HRMS spectra of TPMAN confirmed its unsymmetrical character and showed a characteristic resonance corresponding to the methyl group at 2.34 (3H) ppm; resonances corresponding to five methylene groups at 4.10 (s, 2H), 4.00 (s, 2H), 3.91 (s, 4H), and 3.81 (s, 2H) ppm; and m/z = 498.2381. The addition of two equivalents of Cu(II) precursors to the TPMAN ligand led to the formation of the corresponding unsymmetrical di-Cu complex (Scheme S1). The complex, [Cu2(μ-OH)(TPMAN)(H2O)]3+, was recrystallized from a mixture of CH3CN/Et2O and characterized by HRMS (m/z = 206.0361 ([Cu2(TPMAN)(μ-OH)]3+, z = 3) and m/z = 383.5371 ([Cu2(TPMAN)(μ-OH)(OTf−)]2+, z = 2; Figure S4). Several unsuccessful attempts to crystallize the cationic unsymmetrical di-Cu complex, [Cu 2 (TPMAN)(μ-OH)(H2O)]3+, gave very poor quality crystals prone to melting, likely owing to catching of moisture by the hydrogen bond. When the −OH bridge was replaced by μ-Cl, a blue crystal of this μ-Cl analogue was obtained by diffusion of ether into a CH3CN solution of the complex. The X-ray crystal structure (Figure 2) showed that this complex contained a

Figure 3. (a) CV of [Cu2(TPMAN)(μ-OH)]3+ (scan rate = 100 mV/ s). Inset: CV of the Cu2II,II/Cu2I,II couple during a negative scan. (b) CV of [Cu2(TPMAN)(μ-OH)]3+ with a different scan rate (1 mM catalyst, 0.1 M phosphate buffer (pH 7.0), scan rates = 5−500 mV/s).

decreased (Figure 3b), indicating that a catalytic process contributed to the oxidation peak. Interestingly, bubbles formed on the electrode surface when the potential scanning was above E = 1.6 V vs NHE, indicating that this could be a catalytic water oxidation process. The oxygen evolution was further confirmed by using controlled-potential electrolysis at 1.87 V vs NHE with a planar ITO electrode (1.5 cm2) with 1.0 mM [Cu2(TPMAN)(μOH)]3+ complex in 0.1 M phosphate buffer (pH 7.0). The oxygen formed in the solution was measured using a calibrated Ocean Optics FOXY probe (Figure 4). The formation of

Figure 4. (a) Oxygen evolution during the bulk electrolysis without (black line) and with the [Cu2(TPMAN)(μ-OH)]3+ catalyst (1 mM) (red line), as measured with a fluorescence probe. (b) The correlation of the current with the concentration of [Cu2(TPMAN)(μ-OH)]3+. The electrolysis was performed in 0.1 M phosphate buffer (pH 7.0) at E = 1.87 V vs NHE with a BDD electrode (0.07 cm2).

Figure 2. ORTEP view (thermal ellipsoids set at 50% probability) of the cation of [Cu2(TPMAN)(μ-Cl)]3+. Selected bond lengths [Å] and angles [deg]: Cu(1)−Cl, 2.259(6); Cu(1)−N(1), 1.98475); Cu(1)− N(2), 2.039(2); Cu(1)−N(3), 1.992(8); Cu(1)−N(4), 2.8105(4); Cu(2)−Cl, 2.804(9); Cu(2)−O, 1.979(6); Cu(2)−N(5), 2.020(5); Cu(2)−N(6), 2.020(6); Cu(2)−N(7), 1.978(7); Cu1−Cl−Cu2, 108.3(9). The details about this structure were listed in Table S1.

background O2 without the copper catalyst was quite small. After the long-term electrolysis of the unsymmetrical catalyst at 1.87 V vs NHE for 2 h, the catalytic current remained at 0.2 mA/ cm2 (Figure S6), indicating the catalyst has acceptable stability during the catalytic process. Furthermore, the ITO electrode used for the bulky electrolysis exhibited no catalytic response in a fresh blank phosphate buffer solution. SEM and XPS (Figures S7 and S8) both showed that no copper oxide formed on the electrode surface during the long-time electrolysis. Furthermore, the UV−vis spectra exhibit little change before and after the bulk electrolysis (Figure S9). All these evidences indicate that, as in the case of the [Cu2(BPMAN)(μ-OH)]3+ catalyst, a homogeneous catalytic process was present for the unsymmetrical [Cu2(TPMAN)(μ-OH)]3+ under the used conditions. At negative potential scanning, a quasi-reversible wave is observed with Epc = −0.42 V vs NHE and Epa = −0.33 V vs NHE (scan rate = 80 mV/s, with a peak to peak splitting of ΔEp = 90 mV). The peak currents vary linearly with the square root of the scan rate, indicating that this wave is a diffusion-controlled

[Cu2(TPMAN)(μ-Cl)]3+ cation, of which the Cu1 atom was coordinated by four N atoms and one Cl atom in tetragonal pyramid geometry, and the Cu2 atom was also in the bottomface-center of a tetragonal pyramid with three N atoms, one Cl atom, and a water molecule on the vertices. Furthermore, around each copper atom, the coordination spheres had close to octahedral geometries. Figure 3a shows a typical cyclic voltammetry (CV) graph of [Cu2(TPMAN)(μ-OH)]3+ (1 mM) in 0.1 M phosphate buffer (PBs, pH 7.0) using a boron-doped diamond (BDD) electrode as a working electrode and a saturated calomel electrode as a reference electrode (SCE, 0.244 V vs NHE). In contrast to the CV of the blank solution (black line), [Cu2(TPMAN)(μOH)]3+ displayed an irreversible oxidation wave at E = 1.6 V vs NHE (onset potential), and the current reached 400 μA. The normalized peak current (i/v1/2) increases as the scan rate is B

DOI: 10.1021/acs.inorgchem.8b01173 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry process. Taken together, including the comparison with the redox properties of [Cu2(BPMAN)(μ-OH)]3+, this redox wave could be ascribed to the diffusion-controlled reduction of CuII,II to CuI,II, and the electrochemical behavior obeyed eq 1a (Figure S10).31−34 id = 0.496a1/2 FA[Cu 2](nFvDCu /RT)1/2

Scheme 2. Proposed Catalytic Mechanism for Unsymmetrical Catalyst [Cu2(TPMAN)(μ-OH)]3+

(1a)

The catalytic wave current exhibits a good linear correlation with the catalyst concentration in the CV measurements (Figure S11); therefore, the catalytic constants could be expressed as eq 1b, in which ncat = 4 is the electrochemical stoichiometry for water oxidation. icat = ncat FA[Cu 2]DCu1/2kcat1/2

(1b)

The steady-state catalytic current, icat (Figure S12), is proportional to [Cat], as shown in Figure 4b, which is also consistent with eq 1b. The parameter kcat in the case of a 0.1 M phosphate buffer solution at pH 7.0 was determined to be 0.78 s −1 on the basis of eq 1b. Despite the similar electrochemical behavior of the unsymmetrical catalyst [Cu2(TPMAN)(μ-OH)]3+ with [Cu2(BPMAN)(μ-OH)]3+, there are also new features in this unsymmetrical case. First, differential pulse voltammetry (Figure S13) showed two overlapping peaks of the catalytic wave; the first peak at 1.75 V vs NHE is independent of the pH value, whereas the second peak is dependent on the pH value, with a slope of −64 mV/pH unit, which is consistent with a proton-coupled electron transfer oxidation (Figure S14). The observed splitting in the oxidation peaks is ascribed to the effect of the unsymmetrical structure in [Cu2(TPMAN)(μ-OH)]3+. Second, the kinetic isotope effect (KIE) of the catalyst was evaluated through CV measurements with different catalyst concentrations in H2O and D2O (Figure S15). According to the equation kcat,H2O/kcat,D2O = (slopecat,H2O/slopecat,D2O)2, the KIE value was calculated to be 2.04. This value indicates that the H atom is involved in the rate-limiting step in the water oxidation. This is different with the catalyst [Cu2(BPMAN)(μ-OH)]3+ (KIE = 1). Finally, the phosphate buffer solution was observed to play a dual role in the water oxidation cycle. As shown in Figure S16, the catalytic current is enhanced with increasing phosphate concentration below 20 mM, which suggests that phosphate could act as a proton acceptor to facilitate the catalysis in the absence of the intramolecular base (the hemilabile pyridyl group). On the basis of the results discussed above as well as those for [Cu2(BPMAN)(μ-OH)]3+, a plausible mechanism was proposed, as shown in Scheme 2. Complex 1 could be oxidized to complex 2 by a single electron oxidation. Complex 2 is further oxidized through a PCET process to afford complex 3. As depicted in Scheme 2, the basic form of the buffer (i.e., B) is considered to play a role as a proton acceptor to promote the proton-coupled O−O bond formation between the CuIII−OH and μ-O units, in which the proton transfers from OH to B and the O−O bond formation concomitantly proceeds in a concerted fashion. The resulting Int1 is then further oxidized to afford Int2, i.e., a Cu2II,II superoxide. The superoxide-bridged Cu2 complex is oxidized to release the oxygen and closes the catalytic cycle. This mechanism is similar to the previously reported mechanism of [Cu2(BPMAN)(μ-OH)]3+. The key feature of this unsymmetrical [Cu 2 (TPMAN)(μ-OH)] 3+ complex is the lack of an intramolecular proton acceptor. Therefore, the buffer concentration dependent catalytic

behavior observed for the present system indicates that buffer is required to assist the proton transfer required to drive the catalytic process.24 In summary, we introduced an open site on the copper atom by using an unsymmetrical ligand possessing 3- and 4coordinate sites on the two hands. The catalytic performance toward water oxidation has been carefully investigated. Although the kcat value is similar to the [Cu2(BPMAN)(μ-OH)]3+ catalyst, the unsymmetrical catalyst [Cu2(TPMAN)(μ-OH)]3+ exhibits some different behavior such as a significant kinetic isotope effect and buffer-assisted proton transfer in the catalytic process, indicating that the hemilabile pyridyl group in [Cu2(BPMAN)(μ-OH)]3+ could be an intramolecular proton acceptor to assist O−O bond formation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01173. Full experimental details, 1H NMR, 13C NMR, HR-MS spectra for all newly reported compounds; UV−vis spectrum and all electrochemical data (PDF) Accession Codes

CCDC 1584076 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ming-Tian Zhang: 0000-0001-8808-9641 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the support from the National Natural Science Foundation of China (NSFC Grant No. 21572113, 21661132006), Tsinghua University Initiative Scientific Research Program (Grant No. 20151080404). C

DOI: 10.1021/acs.inorgchem.8b01173 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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(24) Li, T.-T.; Zheng, Y.-Q. Electrocatalytic water oxidation using a chair-like tetranuclear copper(II) complex in a neutral aqueous solution. Dalton Trans. 2016, 45, 12685. (25) Shen, J.; Wang, M.; Gao, J.; Han, H.; Liu, H.; Sun, L. Improvement of Electrochemical Water Oxidation by Fine-Tuning the Structure of Tetradentate N4 Ligands of Molecular Copper Catalysts. ChemSusChem 2017, 10, 4581. (26) Kafentzi, M.; Papadakis, R.; Gennarini, F.; Kochem, A.; Iranzo, O.; Le Mest, Y.; Le Poul, N.; Tron, T.; Faure, B.; Simaan, A. J.; Réglier, M. Electrochemical Water Oxidation and Stereoselective Oxygen Atom Transfer Mediated by a Copper Complex. Chem. - Eur. J. 2018, 24, 5213. (27) Su, X.-J.; Gao, M.; Jiao, L.; Liao, R.-Z.; Siegbahn, P. E. M.; Cheng, J.-P.; Zhang, M.-T. Electrocatalytic Water Oxidation by a Dinuclear Copper Complex in a Neutral Aqueous Solution. Angew. Chem., Int. Ed. 2015, 54, 4909. (28) Su, X.-J.; Zheng, C.; Hu, Q.-Q.; Du, H.-Y.; Liao, R.-Z.; Zhang, M.-T. Bimetallic cooperative effect on O−O bond formation: copper polypyridyl complexes as water oxidation catalyst. Dalton Trans. 2018, 47, 8670. (29) Cox, N.; Retegan, M.; Neese, F.; Pantazis, D. A.; Boussac, A.; Lubitz, W. Electronic structure of the oxygen-evolving complex in photosystem II prior to O-O bond formation. Science 2014, 345, 804. (30) Suga, M.; Akita, F.; Sugahara, M.; Kubo, M.; Nakajima, Y.; Nakane, T.; Yamashita, K.; Umena, Y.; Nakabayashi, M.; Yamane, T.; Nakano, T.; Suzuki, M.; Masuda, T.; Inoue, S.; Kimura, T.; Nomura, T.; Yonekura, S.; Yu, L.-J.; Sakamoto, T.; Motomura, T.; Chen, J.-H.; Kato, Y.; Noguchi, T.; Tono, K.; Joti, Y.; Kameshima, T.; Hatsui, T.; Nango, E.; Tanaka, R.; Naitow, H.; Matsuura, Y.; Yamashita, A.; Yamamoto, M.; Nureki, O.; Yabashi, M.; Ishikawa, T.; Iwata, S.; Shen, J.-R. Lightinduced structural changes and the site of OO bond formation in PSII caught by XFEL. Nature 2017, 543, 131. (31) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; p 235. (32) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; p 501. (33) Compton, R. G.; Banks, C. E. Understanding Voltammetry, 2nd ed.; Imperial College Press: London, 2011; p 121. (34) Chen, Z.; Chen, C.; Weinberg, D.; Kang, P.; Concepcion, J. J.; Harrison, D. P.; Brookhart, M. S.; Meyer, T. J. Electrocatalytic reduction of CO2 to CO by polypyridyl ruthenium complexes. Chem. Commun. 2011, 47, 12607.

REFERENCES

(1) Lewis, N. S.; Nocera, D. G. Powering the planet: Chemical challenges in solar energy utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729. (2) Parent, A. R.; Sakai, K. Progress in Base-Metal Water Oxidation Catalysis. ChemSusChem 2014, 7 (8), 2070. (3) Kärkäs, M. D.; Verho, O.; Johnston, E. V.; Åkermark, B. Artificial Photosynthesis: Molecular Systems for Catalytic Water Oxidation. Chem. Rev. 2014, 114, 11863. (4) Kärkäs, M. D.; Åkermark, B. Water oxidation using earth-abundant transition metal catalysts: opportunities and challenges. Dalton Trans. 2016, 45, 14421. (5) Blakemore, J. D.; Crabtree, R. H.; Brudvig, G. W. Molecular Catalysts for Water Oxidation. Chem. Rev. 2015, 115, 12974. (6) Liu, F.; Concepcion, J. J.; Jurss, J. W.; Cardolaccia, T.; Templeton, J. L.; Meyer, T. J. Mechanisms of Water Oxidation from the Blue Dimer to Photosystem II. Inorg. Chem. 2008, 47, 1727. (7) Eisenberg, R.; Gray, H. B. Preface on Making Oxygen. Inorg. Chem. 2008, 47, 1697. (8) Hurst, J. K. In Pursuit of Water Oxidation Catalysts for Solar Fuel Production. Science 2010, 328, 315. (9) Cox, N.; Pantazis, D. A.; Neese, F.; Lubitz, W. Biological Water Oxidation. Acc. Chem. Res. 2013, 46, 1588. (10) Barnett, S. M.; Goldberg, K. I.; Mayer, J. M. A soluble copper− bipyridine water-oxidation electrocatalyst. Nat. Chem. 2012, 4, 498. (11) Zhang, M.-T.; Chen, Z.; Kang, P.; Meyer, T. J. Electrocatalytic Water Oxidation with a Copper(II) Polypeptide Complex. J. Am. Chem. Soc. 2013, 135, 2048. (12) Chen, Z.; Kang, P.; Zhang, M. − T.; Stoner, B. R.; Meyer, T. J. Cu(II)/Cu(0) electrocatalyzed CO2 and H2O splitting. Energy Environ. Sci. 2013, 6, 813. (13) Coggins, M. K.; Zhang, M. − T.; Chen, Z.; Song, N.; Meyer, T. J. Preface on Making Oxygen Single-Site Copper(II) Water Oxidation Electrocatalysis: Rate Enhancements with HPO42‑ as a Proton Acceptor at pH 8. Angew. Chem., Int. Ed. 2014, 53, 12226. (14) Lu, C.; Du, J.; Su, X. − J.; Zhang, M. − T.; Xu, X.; Meyer, T. J.; Chen, Z. Cu(II) Aliphatic Diamine Complexes for Both Heterogeneous and Homogeneous Water Oxidation Catalysis in Basic and Neutral Solutions. ACS Catal. 2016, 6, 77. (15) Zhu, L.; Du, J.; Zuo, S.; Chen, Z. Cs(I) Cation Enhanced Cu(II) Catalysis of Water Oxidation. Inorg. Chem. 2016, 55, 7135. (16) Garrido-Barros, P.; Funes-Ardoiz, I.; Drouet, S.; Benet-Buchholz, J.; Maseras, F.; Llobet, A. Redox Non-innocent Ligand Controls Water Oxidation Overpotential in a New Family of Mononuclear Cu-Based Efficient Catalysts. J. Am. Chem. Soc. 2015, 137, 6758. (17) Garrido-Barros, P.; Gimbert-Suriñach, C.; Moonshiram, D.; Picón, A.; Monge, P.; Batista, V. S.; Llobet, A. Electronic πDelocalization Boosts Catalytic Water Oxidation by Cu(II) Molecular Catalysts Heterogenized on Graphene Sheets. J. Am. Chem. Soc. 2017, 139, 12907. (18) Huang, H. − H.; Wang, J. − W.; Sahoo, P.; Zhong, D.-C.; Lu, T.B. Electrocatalytic water oxidation by Cu(II) ions in a neutral borate buffer solution. Chem. Commun. 2017, 53, 9324. (19) Yu, F.; Li, F.; Hu, J.; Bai, L.; Zhu, Y.; Sun, L. Electrocatalytic water oxidation by a macrocyclic Cu(II) complex in neutral phosphate buffer. Chem. Commun. 2016, 52, 10377. (20) Shen, J.; Wang, M.; Zhang, P.; Jiang, J.; Sun, L. Electrocatalytic water oxidation by copper(II) complexes containing a tetra- or pentadentate amine-pyridine ligand. Chem. Commun. 2017, 53, 4374. (21) Koepke, S. J.; Light, K. M.; VanNatta, P. E.; Wiley, K. M.; KieberEmmons, M. T. Electrocatalytic Water Oxidation by a Homogeneous Copper Catalyst Disfavors Single-Site Mechanisms. J. Am. Chem. Soc. 2017, 139, 8586. (22) Wang, J.; Huang, H.; Lu, T. Homogeneous Electrocatalytic Water Oxidation by a Rigid Macrocyclic Copper(II) Complex. Chin. J. Chem. 2017, 35, 586. (23) Czioska, S.; Wang, J.; Teng, X.; Zuo, S.; Xie, S.; Chen, Z. Hierarchically structured multi-shell nanotube arrays by self-assembly for efficient water oxidation. Nanoscale 2018, 10, 2887. D

DOI: 10.1021/acs.inorgchem.8b01173 Inorg. Chem. XXXX, XXX, XXX−XXX