Energy-Storage Applications for a pH Gradient between Two

May 11, 2017 - Energy-Storage Applications for a pH Gradient between Two Benzimidazole-Ligated Ruthenium Complexes That Engage in Proton-Coupled Elect...
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Energy-Storage Applications for a pH Gradient between Two Benzimidazole-Ligated Ruthenium Complexes That Engage in Proton-Coupled Electron-Transfer Reactions in Solution Daisuke Motoyama,† Kai Yoshikawa,† Hiroaki Ozawa,† Makoto Tadokoro,‡ and Masa-aki Haga*,† †

Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan ‡ Department of Chemistry, Faculty of Science, Tokyo University of Science, Kagurazaka 1-3, Shinjuku-ku, Tokyo 162-8601, Japan S Supporting Information *

ABSTRACT: The judicious selection of pairs of benzimidazole-ligated ruthenium complexes allowed the construction of a rechargeable proton-coupled electron-transfer (PCET)-type redox battery. A series of ruthenium(II) and -(III) complexes were synthesized that contain substituted benzimidazoles that engage in PCET reactions. The formation of intramolecular Ru−C cyclometalation bonds stabilized the resulting ruthenium(III) complexes, in which pKa values of the imino N−H protons on the benzimidazoles are usually lower than those for the corresponding ruthenium(II) complexes. As a proof-of-concept study for a solution redox battery based on such PCET reactions, the charging/discharging cycles of several pairs of ruthenium complexes were examined by chronopotentiometry in an H-type device with half-cells separated by a Nafion membrane in unbuffered CH3CN/H2O (1/1, v/v) containing 0.1 M NaCl. During the charging/discharging cycles, the pH value of the solution gradually changed accompanied by a change of the open-circuit potential (OCP). The changes for the OCP and pH value of the solution in the anodic and cathodic half-cells were in good agreement with the predicted values from the Pourbaix diagrams for the pairs of ruthenium complexes used. Accordingly, the careful selection of pairs of ruthenium complexes with a sufficient potential gradient and a suitably large pKa difference is crucial: the charge generated between the two ruthenium complexes changes the OCP and the pH difference between the two cells in an unbuffered solution, given that the PCET reactions occur at both electrodes and that discharging leads to the original state. Because the electric energy is stored as a pH gradient between the half-cells, new possibilities for PCET-type rocking-chair redox batteries arise.

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INTRODUCTION Protons are strongly charged particles that are easily transferred between Brønsted acids and bases. Various organic and inorganic compounds act as Brønsted acids via the dissociation of proton(s), while compounds that accept protons represent Brønsted bases. Such proton transfers occurs often and is usually accompanied by the transfer of electrons. Proton transfer plays an important role in numerous chemical and biological processes.1−4 In photosynthetic membranes, for example, the flow of electrons is coupled by the flow of protons across the membrane; i.e., it contributes to the homeostasis of the proton gradient. In biological systems, the proton gradients that result from the redox reactions across the membranes are a fundamental driving force for the production of energy, i.e., the synthesis of adenosine triphosphate (ATP). In a biological environment, which is usually rich in water, protons represent good charge carriers to generate a potential field, and protoncoupled electron-transfer (PCET) reactions are ubiquitous in biologically relevant organic and inorganic compounds.5,6 For organic compounds, PCET reactions of the quinone/hydro© 2017 American Chemical Society

quinone couple have been used as key elements with different redox potentials for proton rocking-chair redox capacitors.7,8 The PCET behavior is described by the Pourbaix diagrams, which represent the plots of the electrode potential as a function of the pH value. In the Pourbaix diagrams, the potential usually increases upon decreasing pH values according to (m/n)pH (V), wherein n and m refer to the number of electrons and protons involved in the PCET reactions, respectively. In redox-active transition-metal complexes, the transfer of electrons is also coupled to the transfer of protons. Many metal complexes that contain proton-responsive ligands such as H2O,2,9 amides,10 triazoles,11 and imidazoles12,13 are able to tune the redox potentials associated with the protonation/deprotonation of ligands. Moreover, protons may act as an external stimulus to change molecular conformations, chemical energy, or electronic coupling.6,14 We have previously reported PCET reactions of redox-active benzimidazole-ligated Received: February 25, 2017 Published: May 11, 2017 6419

DOI: 10.1021/acs.inorgchem.7b00518 Inorg. Chem. 2017, 56, 6419−6428

Article

Inorganic Chemistry ruthenium complexes in solution and on solid surfaces.13,15−18 From the tridentate 2,6-bis(benzimidazol-2-yl)pyridine (H2bimp) ligand, we obtained mono- and dinuclear ruthenium complexes with various ancillary ligands, in which the imino NH groups of H2bimp act as dibasic acids in solution and in which the strength of the acidity depends on the oxidation state of the Ru center. For example, [Ru(H2bimp)2] behaves as a tetrabasic acid and exhibits pKa values of 6.1, 7.8, 9.1, and 10.7 in CH3CN/Britton−Robinson (BR) buffer (1/1, v/v) for the RuII state.13 Upon oxidation of this complex to the RuIII state, the pKa values decreased to 2.5, 3.2, 5.1, and 6.2, indicating a drastic increase of the acidity of the complex upon oxidation. The NH proton of the benzimidazole can also be used for the sensing and recognition of various anions (e.g., F−, AcO−, or H2PO4−) via hydrogen-bonding interactions. For example, the UV−vis spectrum and oxidation potential of [Ru(bpy)2(L)]2+ [L = 2-(2-pyridyl)benzimidazole] are affected by hydrogenbonding interactions with N-heterocycles such as pyridine.4,19 Similarly, the addition of water to a CH3CN solution of [Ru(BiBzImH2)3]2+ (BiBzImH2 = 2,2′-bibenzimidazole) induces a cathodic shift of the RuII/III oxidation potential due to hydrogen-bonding interactions between the N−H bonds on the BiBzImH2 ligands and water.20 Baitalik and co-workers reported that a series of heteroleptic ruthenium complexes of the type [Ru(H2bimb)(tpy-R)]+ (e.g., R = phenyl, pmethylphenyl, 2-naphthyl, or 9-anthryl) exhibited luminescence in addition to a reversible RuII/III couple, both of which are sensitive to the addition of anions.21 Because metal-centered redox potentials of benzimidazole-coordinated complexes are influenced by the protonation/deprotonation of ligands, the pH value of the solution can be used to not only tune the total charge of the complexes but also the potential gradient between two different complexes in half-cells that are separated by a membrane. In other words, starting from a neutral unbuffered solution at pH = 7, the oxidation of metal complexes that engage in PCET reactions induces a decrease of the pH value. Conversely, a reduction leads to an increase of the pH value of the solution, which depends on the pKa values of the metal complexes and the potential determined by (m/n)pH (V). Recently, we have synthesized cyclometalated ruthenium complexes that contain bis(benzimidazolyl)benzene (H2bimb) ligands.22 The pKa values for these RuII-H2bimb complexes are expected to increase relative to those of the RuII-H2bimp complexes. The combination of pairs of ruthenium complexes, e.g., Ru(H2bimp)/Ru(H2bimb), should therefore represent suitable prospects for applications in redox battery cells. Figure 1 shows pairs of PCET complexes P and Q, which are separated by a proton-exchange membrane. At pH = 7, the compartments contain the oxidized and reduced forms, respectively. Galvanostatically charging this cell generates a potential difference between the two compartments, which, in turn, creates a pH gradient between the two compartments. In this type of redox cell, the energy is stored in the form of both a voltage and a pH difference. During the discharge, the two complexes revert to their original states and to pH = 7 (Figure 1). Similar types of redox cells, which convert energy from a proton gradient to electricity, have initially been reported by Tanaka and co-workers.23 They used the acid−base equilibria [M(H2O)] ⇌ [M(OH)]− ⇌ [M(O)]2− in combination with noninnocent 3,5-di-tert-butyl-1,2-benzoquinone (dbq) in two cells containing [M(H2O)(tpy)(dbq)], which were separated by an anion-exchange membrane. In this case, the addition of

Figure 1. Pourbaix diagram for a solution redox battery based on a pair of ruthenium complexes that engage in PCET reactions. Complexes P and Q each occupy one of the half-cells in an unbuffered aqueous solution (pH = 7). Upon charging, P is oxidized under the concomitant release of proton(s), while Q is reduced under the simultaneous capture of proton(s). Thus, electric energy is stored as a pH gradient between the half-cells.

base to one of the half-cells induced a potential difference between the two cells, which led to the generation of a current as a result of the potential gradient. However, it was not possible to monitor the change of the pH value directly because of the nonaqueous solvent (acetone) employed. Recently, research in the area of energy storage has focused on redox-flow cells because they can convert electrical energy into chemical energy and store it in order to buffer electricity generated by unstable and intermittent renewable energy sources.24−27 A rechargeable redox-flow battery based on [Ru(bpy)3]2+ in a nonaqueous organic electrolyte has been reported, in which the reduction and oxidation reactions of the single [Ru(bpy)3]2+ complex occurred during the charging in each compartment of the H-type half-cells.28 Vanadium redox batteries employ the VII/VIII and VIV/VV redox couples and associated PCET reactions in order to store energy in the electrolytes of both half-cells. In the present paper, pairs of ruthenium complexes that engage in PCET reactions were electrochemically connected to afford for the first time a redox cell based on concurrent electron−proton movement induced by PCET reactions on these ruthenium complexes. As a proof-of-concept study, the charge/discharge performances of these cells were recorded.

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RESULTS AND DISCUSSION Synthesis and pKa Values of the Ruthenium Complexes Employed in This Study. Preparation of the Ruthenium Complexes. A series of heteroleptic tridentate ruthenium complexes of the type [Ru(L1)(L2)]n+ were prepared by the reaction of [Ru(L1)(CH3CN)Cl2] with L2 in ethylene glycol under microwave irradiation [L1 = 2,6bis(benzimidazol-2-yl)pyridine (H2bimp), 2,6-bis(1-methylbenzimidazol-2-yl)pyridine (Mebimp), or 2,2′:6′,2″-terpyridine (tpy); L2 = 2,6-bis(benzimidazol-2-yl)benzene (H2bimb), Mebimp, or 2,6-bis(1-methylbenzimidazol-2-yl)benzene (Mebimb)]. In the abbreviations for the ligands, dissociable imino NH protons are, henceforth, denominated as “H”, e.g., in “H2bimp”. The ruthenium complexes [Ru(tpy)(H2bimp)]2+ (4; Scheme 1), [Ru(Mebimp)2]2+ (6), and [Ru(Mebimp)(Mebimb)]2+ (7) were synthesized according to literature procedures. In the case of cyclometalated ruthenium complexes that contain Ru−C bonds, such as [Ru(H2bimb)(Mebimp)]2+ (2), [Ru(H2bimp)(Mebimb)]2+ (3), [Ru(tpy)(H2bimb)]2+ (5), 6420

DOI: 10.1021/acs.inorgchem.7b00518 Inorg. Chem. 2017, 56, 6419−6428

Article

Inorganic Chemistry

Scheme 1. Chemical Structures of the Benzimidazoles Used in This Study and the Corresponding Ruthenium Complexes (1−7)

and [Ru(Mebimp)(Mebimb)]2+ (7), the Ru center adopts the formal oxidation state III. For measurement of the 1H NMR spectra, the addition of hydrazine hydrate was required in order to obtain sharp signals because the d5 electron configuration of Ru III leads to paramagnetic complexes. All ruthenium complexes were characterized by 1H NMR spectroscopy, electrospray ionization (ESI) or time-of-flight (TOF) mass spectrometry (MS), and elemental analysis. X-ray Crystal Structure of [Ru(Mebimp)(H2bimb)][PF6]2 (3). Suitable single crystals for an X-ray diffraction analysis were obtained from the recrystallization of 3 from acetonitrile/ether, and the molecular structure of 3 is shown in Figure 2. In the Figure 3. Dependence of the UV−vis spectra of 3 in CH3CN/BR buffer on the pH value (pH = 2.2−9.0).

Upon an increase of the pH value of the solution from pH = 2 to 9, the metal-to-ligand charge-transfer (MLCT) band (λabs = 413 nm) and the ligand-to-metal charge-transfer (LMCT) band (λabs = 521 nm) were bathochromically shifted in a stepwise manner as a result of the two successive proton-transfer equilibria shown in eq 1. The UV−vis spectral data are collected in Table 1.

Figure 2. ORTEP diagram for 3 with atomic displacement parameters set at 50% probability. H atoms and anions are omitted for clarity. Selected bond lengths (Å): Ru−C52 = 1.975(5), Ru−N7 = 2.050(4), Ru−N11 = 2.065(4), Ru−N12 = 2.071(4), Ru−N13 = 2.086(4).

On the basis of this spectral change, it was possible to calculate the pKa values of 3 for the proton-transfer equilibria in the RuIII state (pKa1 = 3.50; pKa2 = 5.83) from the nonlinear regression analysis of the spectral titration curve (Figure 3). The spectrophotometric titrations of the other ruthenium complexes are shown in the Supporting Information (Figures S1−S4). From the titration curves, the pKa values of the other ruthenium complexes (1, 2, 4, and 5) for the RuII or the RuIII states were determined and are summarized in Table 2 (values in parentheses). Upon deprotonation of the ruthenium(II) complexes 1and 4, a red shift of the lower-energy bands was observed. Deprotonation induces the rise of highest occupied molecular orbital (HOMO) energies, considering the large negative shift of the RuII/III oxidation potentials. Concurrently, the rising lowest unoccupied molecular orbital (LUMO) energies based on the ligand π* orbitals took place. The larger energy rise in the HOMO energy leads to the smaller energy gap between HOMO and LUMO orbitals, resulting in a red shift of the

crystal, 3 exhibits a distorted octahedral geometry, in which the Ru center is surrounded by five N atoms (two from the benzimidazolyl group, two from the 1-methylbenzimidazolyl group, and one from the pyridyl group) and one C atom (from cyclometalation). The Ru−C52 bond length [1.975(5) Å] induces an elongation of the Ru−N13 bond length [2.086(4) Å]. The other Ru−N bond lengths (∼2.05−2.07 Å) fall within the range of typical RuIII−N bonds.29 The ruthenium complexes that contain imino NH proton(s) showed a strong dependence of their UV−vis spectra and cyclic voltammograms on the pH value of the solution. Spectrophotometric pH Titrations. As a typical example, the pH dependence of the UV−vis spectra of [Ru(Mebimp)(H2bimb)]n+ (3) in a CH3CN/BR buffer is shown in Figure 3. 6421

DOI: 10.1021/acs.inorgchem.7b00518 Inorg. Chem. 2017, 56, 6419−6428

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Inorganic Chemistry Table 1. UV−vis Spectral Data for Protonated Ruthenium Complexes 1−5 and Their Dideprotonated Ones in CH3CNa λmax/nm (ε/103 M−1 cm−1)

complexb 1 [1-2H+] 2 [2-2H+] 3 [3-2H+] 4 [4-2H+] 5 [5-2H+]

259 (40.3), 312 (43.9), 340 (52.5), 266 (38.9), 295 (45.9), 357 (55.9), 297 (48.5), 340 (33.6), 409 (15.2), 294 (44.2), 309 (45.1), 403 (14.7), (3.01) 297 (48.5), 341 (33.0), 393 (14.1), (1.67) 299 (45.5), 343 (34.6), 413 (11.4), 242 (46.7), 270 (31.2), 280 (25.5), (39.3), 407 (5.00), 477 (13.3) 275 (29.1), 315 (36.8), 357 (35.5), (1.39), 728 (1.42) 295 (53.1), 397 (13.9), 532 (2.03), 280 (48.3), 295 (47.7), 310 (49.2), (3.76), 638 (3.54), 902 (3.86)

354 514 520 445

(67.6), 486 (17.0) (11.4) (3.99), 713 (1.74) (4.79), 530 (4.35), 801

413 (17.6), 521 (2.98), 700 444 (11.9), 578 (44.7) 313 (52.7), 331 (35.2), 348

Figure 4. Cyclic voltammograms of 1 at pH = 2.8, 3.8, 4.9, 5.8, 6.9, and 9.9 in CH3CN/buffer (1/1, v/v) on a glassy carbon electrode at a scan rate of 100 mV s−1.

416 (9.79), 515 (7.77), 658 810 (2.10) 392 (14.9), 447 (6.23), 526

a

Measured by the addition of HCl and NH3 for the fully protonated and dideprotonated forms. b[X-2H+] stands for the deprotonated complex of X, where X indicates the complex number.

MLCT band on the absorption spectra. The preliminary timedependent density functional theory (TD-DFT) calculations support this explanation, which is out of the scope in this paper. Dependence of Cyclic Voltammograms and Pourbaix Diagrams on the pH Value. Figure 4 shows the pH dependence of the cyclic voltammograms of [Ru(Mebimp)(H2bimp)]n+ (1) in CH3CN/BR buffer (1/1, v/v). A reversible one-electron RuII/III oxidation process was observed at pH = 2.8−10.9. The plots for the half-wave potential (E1/2) as a function of the pH values (Pourbaix diagram; Figure 5) were analyzed by a nonlinear regression method under consideration of the proton−electron equilibria of 1 described in Scheme 2. The results of this analysis delivered the pKa values for 1 [RuII, pKa11 = 6.31 and pKa12 = 7.94; RuIII, pKa21 < 2 and pKa2 = 3.60], which suggested a drastic decrease of the pKa values upon oxidation of the RuII center. Similar differences between the pKa values of RuII and RuIII were also observed for the other ruthenium complexes (Table 2). On the other hand, [Ru(Mebimp)(H2bimb)]n+ (2) in CH3CN/BR buffer (1/1, v/v) revealed a reversible oneelectron RuIII/II reduction at −0.14 V (pH = 7), which was confirmed by the cathodic current response in a rotating-disk voltammetry experiment (Figure S5 in the Supporting Information). With an increase in the pH value of the solution, the reduction potential was shifted in the negative direction. The Pourbaix diagram for 2 is shown in Figure 6.

Figure 5. Half-wave potential (E1/2) as a function of the pH value (Pourbaix diagram) for 1 in CH3CN/BR buffer (1/1, v/v): (1) [RuII(Mebimp)(H2bimp)]2+; (2) [RuII(Mebimp)(Hbimp)]+; (3) [RuII(Mebimp)(bimp)]0; (4) [RuIII(Mebimpy)(Hbimp)]2+; (5) [RuIII(Mebimp)(bimp)]+. The measured points are shown as dots, together with the simulation curve obtained from eqs 5 and 6

Scheme 2. Scheme for the Electron−Proton Equilibria of 1

The acid dissociation constants were obtained from simulations [RuIII, pKa21 = 6.46 and pKa22 = 9.15; RuII, pKa11 = 10.91 and pKa12 > 12]. Cyclometalation in the ruthenium complexes with benzimidazole NH groups led to higher pKa

Table 2. pKa Values, Redox Potentials, and BDFEs for 1−5 in CH3CN/BR Buffera RuII state

RuIII state

complex

pKa11

pKa12

pKa21

pKa22

E1 (V vs Fc+/ Fc)

E2calc (V vs Fc+/ Fc)

E3 (V vs Fc+/ Fc)

BDFE1 (kcal mol−1)

BDFE2 (kcal mol−1)

1 2 3 4 5

6.31 (6.40) 10.91 8.73 6.64 (6.43) 11.13

7.94 (7.95) >12 [15.32] 10.90 8.12 (7.96) >12 [13.27]