Electrocatalytic Water Oxidation by a Water-Soluble Copper(II

Article Cite This: Inorg. Chem. 2017, 56, 13368-13375

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Electrocatalytic Water Oxidation by a Water-Soluble Copper(II) Complex with a Copper-Bound Carbonate Group Acting as a Potential Proton Shuttle Fangfang Chen,†,§ Ni Wang,†,§ Haitao Lei,† Dingyi Guo,† Hongfei Liu,† Zongyao Zhang,‡ Wei Zhang,† Wenzhen Lai,*,‡ and Rui Cao*,†,‡

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Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, China ‡ Department of Chemistry, Renmin University of China, Beijing 100872, China S Supporting Information *

ABSTRACT: Water-soluble copper(II) complexes of the dianionic tridentate pincer ligand N,N′-2,6-dimethylphenyl-2,6-pyridinedicarboxamidate (L) are catalysts for water oxidation. In [L-CuII-DMF] (1, DMF = dimethylformamide) and [L-CuII-OAc]− (2, OAc = acetate), ligand L binds CuII through three N atoms, which define an equatorial plane. The fourth coordination site of the equatorial plane is occupied by DMF in 1 and by OAc− in 2. These two complexes can electrocatalyze water oxidation to evolve O2 in 0.1 M pH 10 carbonate buffer. Spectroscopic, titration, and crystallographic studies show that both 1 and 2 undergo ligand exchange when they are dissolved in carbonate buffer to give [L-CuII-CO3H]− (3). Complex 3 has a similar structure as those of 1 and 2 except for having a carbonate group at the fourth equatorial position. A catalytic cycle for water oxidation by 3 is proposed based on experimental and theoretical results. The two-electron oxidized form of 3 is the catalytically active species for water oxidation. Importantly, for these two oxidation events, the calculated potential values of Ep,a = 1.01 and 1.59 V vs normal hydrogen electrode (NHE) agree well with the experimental values of Ep,a = 0.93 and 1.51 V vs NHE in pH 10 carbonate buffer. The potential difference between the two oxidation events is 0.58 V for both experimental and calculated results. With computational evidence, this Cu-bound carbonate group may act as a proton shuttle to remove protons for water activation, a key role resembling intramolecular bases as reported previously.

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INTRODUCTION Developing new and efficient catalysts for the oxygen evolution reaction (OER) attracts extensive interests because water oxidation is kinetically slow and becomes the bottleneck for water splitting, which is an appealing way toward artificial photosynthesis.1−10 Recently, molecular complexes of Mn,11−14 Fe,15−21 Co,22−29 Ni,30−34 and Cu35−41 have been identified as active OER catalysts. On the basis of these results, base anions are suggested to play several significant roles in the OER catalysis. For example, they can assist proton-coupled electron transfer (PCET) processes by removing protons from the water molecule.42−44 In addition, base anions can facilitate the nucleophilic water attack to a metal-oxo unit by accepting a proton from the attacking water molecule.30 This O−O bond formation step is typically considered as the rate-determining step during the catalytic cycle for water oxidation.45−48 Importantly, an intramolecular base group appended to the second coordination sphere of a metal center is able to more significantly improve the catalytic OER efficiency based on results from Nocera22 and from us.28,49 However, the introduction of a base group to the second coordination sphere with appropriate orientation and space to the metal © 2017 American Chemical Society

center is highly challenging from the aspects of ligand design and synthesis. Cu complexes of polyamide ligands have been shown to be efficient OER catalysts.37−39 Deprotonated amides are negatively charged σ-donating ligands that can provide strong binding affinity to metal ions and can stabilize metal ions in their high oxidation states.37−39 These factors make polyamide ligands valuable in OER catalysis, particularly in consideration that high-valent metal-oxo units are thought to be key intermediates involved in the catalytic cycle.1,3,7 The initial goal of this work is to test the Cu complex of N,N′-2,6dimethylphenyl-2,6-pyridinedicarboxamidate (L) as a catalyst for water oxidation. Complexes [L-CuII-DMF] (1) and [L-CuIIOAc]− (2) were synthesized. In both structures, ligand L binds a CuII ion through three N atoms with the fourth coordination site on the equatorial plane occupied by a DMF or an acetate group, respectively. Complexes 1 and 2 are soluble in water, and they undergo ligand exchange reaction upon the dissolution in carbonate buffer to give complex [L-CuIIReceived: August 17, 2017 Published: October 16, 2017 13368

DOI: 10.1021/acs.inorgchem.7b02125 Inorg. Chem. 2017, 56, 13368−13375

Article

Inorganic Chemistry HCO3]− (3). Complex 3 has a similar structure as those of 1 and 2 except that it bears a carbonate group at the fourth equatorial position. Considering that this Cu-bound carbonate group has the potential to act as an intramolecular base to improve the OER efficiency, we are therefore interested in investigating the effect of this carbonate group on water oxidation. Herein, we report on water-soluble CuII complexes of the dianionic tridentate pincer ligand L and their catalytic properties for water oxidation in carbonate buffer solutions. Complexes 1 and 2 convert to complex 3 upon ligand exchange, and the latter one acts as the real molecular catalyst for OER in carbonate buffer solutions. Experimental and theoretical studies show that the two-electron oxidized form of 3 is the catalytic active species for water oxidation. On the basis of computational results, this Cu-bound carbonate group is suggested to act as a potential proton shuttle to remove protons in the catalytic cycle, including steps toward the catalytically active species and its subsequent O−O bond formation. This Cu-bound carbonate group is thus likely to resemble the key role of an intramolecular base as reported previously.

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clear solution was stirred for 3 h and was filtered. Attempts to grow crystals from this filtrate suitable for single crystal X-ray diffraction studies always led to crystals that were severely twinned. As a result, to this filtrate was added an excess amount of Et4N(ClO4) (100 mg), intending to use the Et4N+ countercation for growing high-quality crystals. Slow evaporation of the resulting solution at room temperature afforded purple-blue crystals of 3 in a week (36 mg, yield 61%). Anal. Calcd for [C32H42CuN4O5]: C, 61.37; H, 6.76; N, 8.95. Found: C, 61.02; H, 6.91; N, 8.76. Caution! Sodium hydride is potentially explosive upon exposure to water and moisture and should be handled with care and in small amounts. Crystallographic Studies. Complete data sets for [L-CuII-DMF] (1, CCDC 1519812), Na[L-CuII-OAc] (2, CCDC 1519813), and (Et4N)[L-CuII-HCO3] (3, CCDC 1519814) were collected. Single crystals suitable for X-ray analysis were each coated with Paratone-N oil, suspended in a small fiber loop, and placed in a cooled gas stream at 153(2) K on a Bruker D8 VENTURE X-ray diffractometer. Diffraction intensities were measured using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Data collection, indexing, data reduction, and final unit cell refinements were carried out using APEX2.51 Absorption corrections were applied using the program SADABS.52 The structure was solved with direct methods using SHELXS53 and refined against F2 on all data by full-matrix leastsquares with SHELXL-9754 following established refinement strategies. In the single-crystal X-ray structures of 1−3, all non-hydrogen atoms were refined anisotropically. All hydrogen atoms binding to carbon were included in the model at geometrically calculated positions and refined using a riding model. The isotropic displacement parameters of all hydrogen atoms were fixed to 1.2 times the U value of the atoms they are linked to (1.5 times for methyl groups). Details of the data quality and a summary of the residual values of the refinements are given in Table S1. In the structure of 1, two solvent water molecules were co-crystallized with one molecule of 1. In the structure of 2, three solvent water molecules were co-crystallized with one molecule of 2. The CheckCIF report for 2 shows one level B alert (D-H Without Acceptor). Atom O3S is an O atom of a co-crystallized solvent water molecule. This alert might be caused by the possible positional disorder of solvent water molecules. In the structure of 3, there is a co-crystallized solvent water molecule per molecule of 3. The Et4N+ countercation was severely disordered, and thus its hydrogen atoms were not included in the refinement. The maximum/minimum residual density was due to the disorder of the Et4N+ countercation. Electrochemical Studies. Electrochemical measurements were performed using a CH Instruments (Model CHI660E Electrochemical Analyzer). Cyclic voltammogram (CV) recorded in acetonitrile (0.1 M Bu4NPF6) used a three-compartment cell possessing a 0.07 cm2 glassy carbon (GC) electrode as the working electrode, Pt wire as the auxiliary electrode, and Ag/AgNO3 as the reference electrode (BASi, 10 mM AgNO3, 0.1 M Bu4NPF6 in acetonitrile). Ferrocene was added at the end of the measurement as an internal standard. In aqueous solvents, a three-electrode system, with a GC or an indium tin oxide (ITO) working electrode, a Pt wire counter electrode, and a Ag/AgCl (KCl saturated) reference electrode, was used. Potentials versus the normal hydrogen electrode (NHE) were obtained by adding 199 mV to the potentials measured versus the Ag/AgCl reference electrode. All potentials reported in this work are referenced to NHE unless otherwise noted. Theoretical Studies. All calculations were carried out using density functional theory (DFT) as implemented in the Gaussian 09 software package.55 Optimization and frequency calculations were performed using the B3LYP-D3 in combination with the 6-31+G* basis set. The solvation using the conductor-like polarizable continuum model (CPCM) in water was considered.56 The energy was then corrected by using a larger basis set with the Wachters+f basis set for copper and 6-311++G(2dp,3df) for the remaining atoms. For redox potential calculations, the reference potential of NHE was set at −4.28 eV,57,58 and the standard free energy of proton was set at −272.2 kcal mol−1.23

EXPERIMENTAL SECTION

General Materials and Methods. Manipulations of air- and moisture-sensitive materials were performed under nitrogen gas using standard Schlenk line techniques. All reagents were purchased from commercial suppliers and used as received unless otherwise noted. Acetonitrile for electrochemical studies was distilled with calcium hydride. Tetrabutylammonium hexafluorophosphate (n-Bu4NPF6) was recrystallized from absolute ethanol. All aqueous solutions were prepared freshly with Milli-Q water. The tridentate pincer ligand N,N′2,6-dimethylphenyl-2,6-pyridinedicarboxamidate (L) was prepared according to the reported procedure.50 1H NMR spectra were acquired on a Brüker spectrometer operating at 400 MHz. UV−vis absorption spectra were measured on a Hitachi U-3310 Spectrophotometer. High-resolution mass spectra (HRMS) were acquired using a Brüker MAXIS. The morphologies of the working electrode surface before and after electrolysis were examined using a Hitachi SU8020 cold-emission field emission scanning electron microscope (FE-SEM) with an accelerating voltage of 1 kV. Energy dispersive X-ray (EDX) spectra were collected from three randomly selected areas of each sample. In addition, the materials were analyzed at several local spots to ensure their chemical homogeneity. The O2 produced during controlled potential electrolysis was determined by using an SP-6890 Gas Chromatograph. Synthesis of Complex 1. To a DMF solution (3 mL) of ligand L (75 mg, 0.20 mmol) was added NaH (10 mg, 0.40 mmol) to give a light yellow solution. Cu(OTf)2 (91 mg, 0.25 mmol) was then added, and the mixture was stirred for 12 h at room temperature to give a purple-blue solution. The resulting solution was filtered, and slow vapor diffusion of diethyl ether to the filtrate resulted in purple-blue crystals of 1 in 3 days (85 mg, yield 81%). Anal. Calcd for [C26H30CuN4O4]: C, 59.36; H, 5.75; N, 10.65. Found: C, 59.01; H, 5.98; N, 10.33. High-resolution ESI-MS for [M − DMF − H2O + Na+]+: calcd. 457.0827; found, 457.0807. Synthesis of Complex 2. To a DMF solution (3 mL) of ligand L (75 mg, 0.20 mmol) was added NaH (10 mg, 0.40 mmol) to give a light yellow solution. Cu(OAc)2·H2O (50 mg, 0.25 mmol) was then added, and the mixture was stirred for 12 h at room temperature to give a purple-blue solution. The resulting solution was filtered, and slow vapor diffusion of diethyl ether to the filtrate resulted in purpleblue crystals of 2 in 3 days (96 mg, yield 93%). Anal. Calcd for [C25H24CuN3NaO4]: C, 58.08; H, 4.68; N, 8.13. Found: C, 57.68; H, 4.91; N, 7.83. High-resolution ESI-MS for [M − Na+]−: calcd. 493.1063; found, 493.1045 (Figure S1). Synthesis of Complex 3. To 3.0 mL of a 0.1 M pH 10 sodium carbonate buffer solution was added 50 mg of complex 1 or 2. The 13369

DOI: 10.1021/acs.inorgchem.7b02125 Inorg. Chem. 2017, 56, 13368−13375

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Inorganic Chemistry Scheme 1. Synthetic Routes of Complexes 1−3

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L, the CuII ion was coordinated by a carbonate group locating at the fourth equatorial position (Figure 1). The O5 atom of this carbonate group was protonated based on (1) the long C24−O5 distance at 1.315(5) Å as compared to the short C24−O3 (1.269(6) Å) and C24−O4 (1.258(6) Å) distances and (2) the charge balance calculation showing that 3 is one negatively charged. These results give a formula of [Cu(HCO3) L]− for 3. In order to verify that the carbonate is bound to the Cu ion in solution, we determined the stoichiometry and binding constant between 1 and carbonate. As shown in Figures S3 and S4, upon addition of carbonate, the absorption at 258 nm in the UV−vis spectra of 1 increased and reached a saturation point when the concentration of carbonate equaled that of 1, suggesting a 1:1 complexation between these two species. This 1:1 binding stoichiometry was further confirmed by the Job’s method (Figure S5). Therefore, the binding constant between 1 and carbonate could be determined to be 1.67 × 105 M−1 by following UV−vis absorption changes at 258 nm using the method we reported previously61 (please see the Supporting Information for details, Figure S6). Electrochemical Water Oxidation. Because complexes 1 and 2 are not soluble in phosphate buffers, we carried out electrocatalytic studies in carbonate buffers. In typical electrocatalytic measurements in carbonate buffer solutions, we used 1 as the starting material because both 1 and 2 converted to 3 upon dissolution. Therefore, 1 can be considered as a precatalyst. The cyclic voltammogram (CV) of 1 on a glassy carbon (GC) electrode in acetonitrile (0.1 M Bu4NPF6) displayed two reversible redox events at E1/2 = 0.32 and 0.66 V (vs ferrocene, Figure 2a). For comparison, the CV of the free ligand L is depicted in Figure S7, showing no redox events in this range. CV of 1 mM 1 in 0.10 M pH 10 carbonate buffer exhibited a reversible oxidation wave with E1/2 = 0.94 V (ΔEp = 70 mV) and a pronounced catalytic wave with an onset potential of 1.29 V (Figure 2b). From now on, we will refer to 3 in the following discussions since 1 converts to 3 upon dissolution in a carbonate buffer. If we assume this catalytic wave is due to water oxidation, the OER onset overpotential can be calculated to be 650 mV. The reversible redox process is assigned to the formally CuIII/CuII redox couple. No catalytic waves were observed in CVs of a buffer-only solution or a buffer with Cu(OTf)2, indicating that the OER was catalyzed by 3. We also tried to examine the catalytic OER feature of 1 in different buffer solutions. As shown in Figure S8, the water oxidation

RESULTS AND DISCUSSION Synthesis and Characterization. In the presence of NaH, the reaction of Cu(OTf)2 and L in DMF gave purple crystalline solids of 1 (Scheme 1). The introduction of two methyl groups at the two ortho-positions of the aniline units of ligand L is aimed to prevent the formation of CuL2 species through steric effect. The structure of 1 was determined, showing a fivecoordinated Cu ion with the three N atoms of L and the O atom of DMF occupying the four equatorial positions and an aqua ligand at the axial position (Figure 1). The Cu−N

Figure 1. Thermal ellipsoid plots (50% probability) of the X-ray structures of 1−3.

distances are 2.0030(18), 1.9206(19), and 2.0144(19) Å, and the Cu−O distances are 1.9682(16) Å for the equatorial DMF group and 2.2604(18) Å for the axial aqua group. This geometry indicates a d9 CuII electronic structure, which has a significant Jahn−Teller effect.59,60 In addition, bond valence sum (BVS) calculation showed a value of 1.78, a result consistent with this CuII structure. If Cu(OAc)2 instead of Cu(OTf)2 was used in the synthesis, complex 2 was obtained (Scheme 1). In the structure of 2, the Cu ion is coordinated by ligand L through three N atoms, which define an equatorial plane (Figure 1). The acetate group, locating at the fourth equatorial position, binds the Cu ion through one O atom with a Cu−O bond length of 1.918(2) Å. A BVS value of 1.63 is also indicative of a d9 CuII structure. Importantly, complexes 1 and 2 display identical electronic absorption spectra in 0.1 M pH 10 sodium carbonate buffer solution (Figure S2; this solution is used for electrocatalytic water oxidation; please see below). This result indicates that the same new complex is likely generated from 1 and 2 upon their dissolution in carbonate buffer. Slow evaporation of the resulting solution from either 1 or 2 dissolved in a carbonate solution afforded the same purple-blue crystals of 3 in a week (Scheme 1). Crystallographic studies revealed that 3 crystallized in the monoclinic space group P21/n. In addition to ligand 13370

DOI: 10.1021/acs.inorgchem.7b02125 Inorg. Chem. 2017, 56, 13368−13375

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Inorganic Chemistry

basis of these results, the turnover frequency (TOF) of 3 for OER catalysis can be estimated to be 20.1 s−1 at 1.60 V potential (Figure 2d).30,35,37 This TOF value is comparable to those of molecular Cu catalysts recently reported in the literature (please see Table S2 for details).37,39 The stability of 3 during OER catalysis was confirmed by controlled potential electrolysis (CPE). First, the current maintained at ∼0.75 mA cm−2 in a 10 h CPE at 1.50 V (Figure 2e). The slight current drop was likely due to the pH decrease from 10.0 to 9.6 in CPE. If the pH was adjusted back to 10.0, CV of 3 after CPE showed negligible change compared to that before CPE (Figure 2f). Second, the ITO working electrode after CPE displayed no catalytic current in a bufferonly solution (Figure S12). Examination of the ITO electrode by SEM (Figure S13) and EDX (Figure S14) showed the absence of any heterogeneous Cu phases on the working electrode. Third, UV−vis spectra of the buffer solution of 3 before and after CPE were identical (Figure S15). It is necessary to note that the UV−vis spectra of 3 are almost identical in 0.10 M carbonate buffer solutions with various pH values of 9.0−10.0 (Figure S16). Fourth, the solution after CPE was inspected by Tyndall scattering analysis, showing no sign of the formation of any CuOx particles (Figure S17). In addition, the evolved O2 was analyzed by using gas chromatography, which gave a turnover number of 3.91 and a Faradaic efficiency of 95% for O2 evolution (Figure S18). Mechanism Studies. In order to get more insights into the mechanism, we carried out electrochemical measurements using different concentrations of 3 and carbonate. Figure 3a shows that the catalytic current increases linearly with the concentration of 3, a result further confirming the molecular nature of this catalysis. As shown in Figure 3b, the catalytic current also varies linearly with the concentration of carbonate. The ionic strength of the solution is maintained constant with

Figure 2. (a) CV of 1.0 mM 1 in acetonitrile (0.1 M Bu4NPF6) with ferrocene (Fc). Conditions: GC electrode, 50 mV s−1 scan rate. (b) CVs of 0.1 M pH 10 carbonate buffer with 1 or Cu(OTf)2, or without catalyst. Inset: Magnified view of the CV of 1 showing the reversible CuIII/CuII redox couple at E1/2 = 0.94 V. Conditions: GC electrode, 50 mV s−1 scan rate, 20 °C. (c) Normalized CVs (icat/ν1/2) of 1. Inset: Dependence of the oxidation peak current id of 1 at E1/2 = 0.94 V on the square root of scan rates. (d) Plot of icat/id vs ν−1/2. (e) CPE of 0.10 M pH 10 carbonate buffer at 1.50 V with or without 1. Conditions: ITO electrode, 20 °C. (f) CVs of a buffer-only solution, a freshly prepared buffer with 1, and a buffer with 1 after 10 h CPE and pH adjustment to 10. Conditions: ITO electrode, 0.1 M pH 10 carbonate buffer, 20 °C.

activity of 1 had the order of carbonate > borate > acetate. In addition, 1 was not stable during OER catalysis in borate buffer because the catalytic current increased gradually in 20 successive CV cycles (Figure S9), indicating the formation and deposition of active CuOx on the working electrode. On the contrary, the current of 1 in carbonate buffer decreased in the first six cycles and then remained relatively stable in subsequent 50 successive CV cycles (Figure S10). The initial drop of the peak current is likely due to the generation and accumulation of O2 gas bubbles on the surface of the working electrode, which can be observed during the successive CV scans. This result shows that (1) the catalytic wave is due to water oxidation to evolve O2 and (2) complex 3 is stable by functioning as an OER catalyst in carbonate buffers. With scan rates (ν) in the range of 10−50 mV s−1, the peak current of the first oxidation process id varies linearly with ν1/2 (Figure 2c, inset), while the catalytic current icat remains almost unchanged (Figure S11).62,63 The normalized catalytic current (icat/ν1/2) decreases with increasing scan rates (Figure 2c), confirming a catalytic process with a chemical rate-determining step, which is likely the O−O bond-forming step.30,37 On the

Figure 3. (a) CVs of 0.10 M pH 10 carbonate buffers with various concentrations of 1. Inset: icat measured at 1.60 V vs [1]. Conditions: GC electrode, 50 mV s−1 scan rate, 20 °C. (b) CVs of 1 mM 1 in pH 10 carbonate buffers with various concentrations of carbonates. Inset: icat measured at 1.60 V vs [carbonate]. Conditions: GC electrode, 50 mV s−1 scan rate, 20 °C. (c) DPVs of 1 in 0.1 M carbonate buffers with various pH values. (d) Dependence of the first and second oxidation peak potentials of 1 on pH from 9.0 to 11.0. 13371

DOI: 10.1021/acs.inorgchem.7b02125 Inorg. Chem. 2017, 56, 13368−13375

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Inorganic Chemistry

the first one-electron oxidation happens at an oxidation potential of 1.01 V, leading to the generation of a CuIII species, [L-CuIII-CO3H] (4). This calculated potential value agrees well with the experimental value of 0.93 V. After this oxidation, a water molecule binds to the CuIII center, giving [L-CuIII(H2O)CO3H] (5). Subsequent intramolecular proton transfer from the Cu-bound water molecule to the −CO3H group occurs very easily with an energy barrier of 5.3 kcal mol−1 to generate [LCuIII(H2CO3)-OH] (6). It is essential to note that a structural rearrangement takes place during this intramolecular proton transfer process (Figure 5a). In species 5, the −CO3H group occupies the equatorial

the addition of sodium sulfate. Inhibition effects are usually observed at high buffer concentrations for molecular catalysts.25,30 For example, a Co porphyrin OER catalyst showed an inhibition at >60 mM phosphate concentrations;25 a Ni porphyrin OER catalyst showed a similar inhibition at >50 mM phosphate concentrations.30 Under low buffer concentrations, the increase of buffer concentrations will lead to the increase of catalytic currents by buffer-anion-assisted O−O bond formation. On the other hand, under high buffer concentrations, the coordination of buffer anions on metal centers becomes prevailing, which competes with the binding and therefore activation of water on metal centers.25,30 This linear dependence can be explained using the rate law of ν = kcat[3] = kcatKa[1][carbonate], in which kcat is the catalytic rate constant for water oxidation and Ka is the association constant between 1 and carbonate. Next, we studied the pH-dependent electrochemical behavior of 3. As shown in Figure S19, complex 3 is active in 0.1 M carbonate buffers from pH 8.5 to 11.0. Significantly, differential pulse voltammetry (DPV) of 3 displayed two oxidation waves at Ep,a = 0.93 and 1.51 V in a pH 10 carbonate buffer (Figure 3c). Comparison of the CV and DPV of 3 suggests that a twoelectron oxidized form of 3 is the catalytically active species for water oxidation. In DPV measurements, the first oxidation peak shows pH independence in the pH range of 9−11, while the second oxidation peak shows a linear pH dependence in this pH range with a slope of −63 mV per pH unit, indicating a 1H+/1e− process (Figure 3d). Kinetic isotope effect (KIE) was also studied, giving a KIE value of 1.81 according to current values measured in H2O and D2O carbonate buffers under identical conditions (KIE = kcat,H2O/kcat,D2O = (icat,H2O/icat,D2O)2, Figure S20). This result is indicative of a rate-limiting O−O bond formation step, which involves the removal of protons.3,30 Computational Studies. In order to better understand the water oxidation mechanism with complex 3, DFT (B3LYP-D3) calculations were carried out. On the basis of our results, a complete catalytic cycle is proposed and presented in Figure 4 with calculated energies labeled for each step. Starting from 3,

Figure 5. Optimized structures for (a) species 6 and (b) TSO−O. For clarity, the hydrogen atoms of the pincer ligand (L) are omitted. The bond lengths are given in Å.

position while the water molecule locates at the axial position of the Cu atom. In species 6, the resulting carbonic acid molecule moves to the axial position and the equatorial position is now occupied by the hydroxyl group. The carbonic acid molecule is weakly bound to the Cu atom with a long Cu−O bond length of 2.54 Å and has a hydrogen-bonding interaction with the Cubound hydroxyl group. After releasing the carbonic acid molecule, the resulting species [L-CuIII-OH] (7) undergoes a one-electron oxidation to give an oxo radical species [L-CuIIIO•−] (8). It is necessary to note that the released carbonic acid molecule will convert to a carbonate anion in the buffer solution. This one-electron oxidation is calculated to be a concerted PCET process (for details, see Figure S21), and the calculated potential is 1.59 V under the experimental condition of pH 10 buffer solutions. These results are in good agreement with those observed in electrochemical measurements. First, this calculated potential value of 1.59 V in pH 10 carbonate buffer agrees well with the experimental value of 1.51 V. Second, this oxidation step has been determined to be a 1H+/ 1e− process in pH-dependent studies. More importantly, careful analysis of the first and second oxidation potential

Figure 4. Proposed catalytic cycle for the water oxidation mediated by complex 3. Energies are in kcal mol−1. 13372

DOI: 10.1021/acs.inorgchem.7b02125 Inorg. Chem. 2017, 56, 13368−13375

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Inorganic Chemistry

previously reported Cu-based molecular catalysts (Table S2), the effect of the carbonate group in this system could not be prevailing as we initially expected. Two possible reasons may account for this somewhat noncritical role of the carbonate group. First, this Cu-bound carbonate group is protonated under the experimental conditions. The basicity of the bicarbonate anion is small (pKa1 of H2CO3 is 3.6). The Cubound bicarbonate has even smaller basicity because of the Cu−O bonding interaction. It is known that the OER efficiency will increase with the basicity of buffer anions25 or intramolecular base groups28 due to the rate-determining PCET step (i.e., O−O bond formation). Therefore, this Cu-bound bicarbonate group is not a strong base to assist the removal of protons during the O−O bond formation. Second, as mentioned above, in pH 10 buffer solutions, the hydroxyl anions are also possible nucleophiles that attack the oxo radical group for the O−O bond formation.39,64 In this alternative mechanism, there is no need to remove protons. As a consequence, the proton shuttle role of the carbonate group may not be critical in the case of hydroxyl anions acting as the nucleophile.

values of 3 showed that the calculated separation between the first and second oxidation is 0.58 V (1.59−1.01 V), which is identical to the value observed from electrochemical measurements (1.51−0.93 V). Upon the generation of the two-electron oxidized species 8, a water molecule nucleophilically attacks the oxo radical group to form the O−O bond. This process is assisted by a HCO3− anion, which can accept a proton from the attacking water molecule to generate [L-CuII-OOH]− (9) and a carbonic acid molecule (again, it converts to a carbonate anion in the buffer solution). It can be seen from Figure 5b, at the transition state for the O−O bond formation (denoted as TSO−O), the HCO3− anion is weakly bound to the Cu atom as an axial ligand with a Cu−O bond distance of 2.38 Å. It also hydrogen bonds to the attacking water molecule to assist this proton transfer process. The calculated free energy barrier for this step is 22.7 kcal mol−1. The O−O bond formation with this free energy barrier is affordable under the experimental conditions. It should be pointed out that the hydroxyl anions presented in the pH 10 buffer solution are also possible nucleophiles that attack the oxo radical group of species 8 to form the O−O bond. In this alternative mechanism, the free energy barrier is expected to be much lower based on previous theoretical studies.39,64 After releasing the carbonic acid molecule, further oxidation undergoes at a potential of 0.03 V to produce [L-CuII-OO•−]− (10), which is also a PCET process (Figure S21). Subsequent one-electron oxidation of 10 generates [L-CuII-O2] (11) with a calculated potential value of 0.53 V. Electronic structure analysis indicated that 11 is a CuII-O2 species with the doublet CuII antiferromagnetically coupled to the triplet dioxygen. Finally, the release of O2 and the binding of HCO3− take place to give the initial complex 3, which completes the reaction cycle. As shown in Figure 4, the computational results show that the Cu-bound HCO3− group acts as a shuttle to remove protons from water molecules during the catalytic cycle for water oxidation. First, an intramolecular proton transfer from the Cu-bound water molecule of [L-CuIII(H2O)-CO3H] (5) to its −CO3H group and the concomitant structural rearrangement lead to the generation of [L-CuIII(H2CO3)-OH] (6). Subsequent release of the resulting carbonic acid molecule and further one-electron oxidation give the oxo radical species [LCuIII-O•−] (8), which is the catalytically active species for water oxidation. Second, at the water nucleophilic attack step, a Cubound HCO3− group (at the axial position) can accept a proton from the attacking water molecule. Therefore, this axial HCO3− group can be regarded as an intramolecular base to assist the O−O bond formation by removing protons. Recently, base groups appended to the second coordination sphere of a metal center are demonstrated by Nocera22 and by us28,49 to significantly improve the catalytic OER efficiency. However, the situations in these systems are different. In previous works, the base groups are covalently attached to the ligand backbone and thus can only accept protons. On the other hand, the HCO3− group can accept protons and the resulting carbonic acid molecule can dissociate from the Cu atom. As a consequence, the Cu-bound HCO3− group is more like a proton shuttle to remove protons for water oxidation. Although computational studies suggest a proton shuttle role of this Cu-bound carbonate group during water activation, the calculated free energy barrier of 22.7 kcal mol−1 for the O−O bond formation step is quite high. In consideration that the activity of complex 3 is similar to or a bit less than those of

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CONCLUSION In conclusion, a water-soluble Cu complex with a Cu-bound carbonate group was reported to be an active OER catalyst in carbonate buffers. This Cu complex can electrocatalyze water oxidation to evolve O2 in 0.1 M pH 10 carbonate buffer with an onset overpotential of 650 mV. A catalytic cycle for water oxidation is proposed based on experimental and theoretical results. The two-electron oxidized form of the Cu complex is suggested to be the catalytically active species for water oxidation. Although the onset overpotential for water oxidation was moderate, it is valuable to shed light on the potential role of the Cu-bound carbonate group during catalytic OER. With computational evidence, this carbonate group may act as an intramolecular base to assist the removal of protons for water activation, including steps toward the catalytically active species and its subsequent 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.7b02125. Figures S1−S21; Tables S1 and S2; Cartesian coordinates of the DFT-optimized structures (PDF) Accession Codes

CCDC 1519812−1519814 contain 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 [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 Authors

*E-mail: [email protected] (W.L.). *E-mail: [email protected] (R.C.). ORCID

Wenzhen Lai: 0000-0003-2830-6151 Rui Cao: 0000-0002-1821-9583 13373

DOI: 10.1021/acs.inorgchem.7b02125 Inorg. Chem. 2017, 56, 13368−13375

Article

Inorganic Chemistry Author Contributions

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§

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for support from the “Thousand Talents Program” of China, the National Natural Science Foundation of China (Grant Nos. 21101170, 21573139, 21673286, and 21773146), the Fundamental Research Funds for the Central Universities, and the Research Funds of Shaanxi Normal University.

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