Proton-Coupled Electron Transfer and Lewis Acid Recognition at Self

DOI: 10.1021/la401736f. Publication Date (Web): July 5, 2013. Copyright © 2013 American Chemical Society. *E-mail: [email protected] (M.A...
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Proton-Coupled Electron Transfer and Lewis Acid Recognition at Self-Assembled Monolayers of an Oxo-Bridged Diruthenium(III) Complex Functionalized with Two Disulfide Anchors Hua-Xin Zhang,†,§ Masaaki Abe,*,‡ Yi Zhang,† Guofang Li,† Shen Ye,† Masatoshi Osawa,*,† and Yoichi Sasaki*,†,§ †

Catalysis Research Center, Hokkaido University, Kita-ku, Sapporo 001-0021, Japan Department of Chemistry and Biochemistry, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan § Department of Chemistry, Graduate School of Science, Hokkaido University, Kita-ku, Sapporo 060-0810, Japan ‡

S Supporting Information *

ABSTRACT: A new μ-oxo-bis(μ-acetato)diruthenium(III) complex bearing two pyridyl disulfide ligands {[Ru2(μ-O)(μOAc)2(bpy)2(Lpy‑SS)2](PF6)2 (OAc = CH3CO2−, bpy = 2,2′-bipyridine, Lpy‑SS = (C5H4N)CH2NHC(O)(CH2)4CH(CH2)2SS) (1)} has been synthesized to prepare self-assembled monolayers (SAMs) on the Au(111) electrode surface. The SAMs have been characterized by contact-angle measurements, reflection−absorption surface infrared spectroscopy, cyclic voltammetry, and reductive desorption experiments. The SAMs exhibited proton-coupled electron transfer (PCET) reactions when the electrochemistry was studied in aqueous electrolyte solution (0.1 M NaClO4 with Britton−Robinson buffer to adjust the solution pH). The potential−pH plot (Pourbaix diagram) in the pH range from 1 to 12 has established that the dinuclear ruthenium moiety was involved in the interfacial PCET processes with four distinct redox states: RuIIIRuIII(μ-O), RuIIRuIII(μ-OH), RuIIRuII(μ-OH), and RuIIRuII(μ-OH2). We also demonstrated that the interfacial redox processes were modulated by the addition of Lewis acids such as BF3 or Al3+ to the electrolyte media, in which the externally added Lewis acids interacted with μO of the dinuclear moiety within the SAMs.

1. INTRODUCTION In recent years, self-assembled monolayers (SAMs) of functional transition-metal complexes have been extensively developed in an interdisciplinary field of research covering both coordination chemistry and surface chemistry.1−8 In particular, redox-active transition-metal complexes are regarded as excellent molecular modules for head groups of SAMs with controllable and switchable characters at electrochemical interfaces whose properties can be tuned by external stimuli such as light, solvent polarity, ligand exchange, and solution pH.9−14 A well-defined chemistry of Au−S linkage has been widely used to confine molecules as SAMs on Au surfaces.15−19 Those SAMs comprising functional coordination compounds organized on electrode surfaces in a well-defined manner can be ultimately utilized for molecular sensing and electronic materials.20,21 © 2013 American Chemical Society

Proton-coupled electron transfer (PCET) reactions play important roles in a wide range of chemical and biological processes. Recently, many molecular compounds displaying PCET reactions have been developed and their properties in homogeneous media extensively studied.22−25 In contrast, studies on the PCET reactions occurring within thin film materials in the form of SAMs have been limited to date.26−29 Diiron(III) complexes containing oxo and carboxylato ligands have been prepared as model compounds for metalloenzymes such as hemerythrin.30−35 We have previously introduced the biomimetic oxo-bridged diiron(III) complexes, [{Fe2(μ-O)(tpa)2}2{μ-OOC(CH2)nS−}2](ClO4)6 (tpa = trisReceived: May 7, 2013 Revised: July 1, 2013 Published: July 5, 2013 10110

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(2-pyridylmethyl)amine); n = 2 and 10), onto the Au(111) surface and studied the electrochemical behavior of the resultant SAMs.36 Despite our successful observation of PCET characteristics of these diiron(III) motifs on the surface, we did not obtain a persistent stability for the multiple potential cycling. The gradual decomposition of the SAMs is most likely due to the inherent lability of the diiron motif in its reduced state, FeIIFeIII, which was formed in situ electrochemically.36 We then turned our attention to the use of oxo-bridged diruthenium(III) analogues37−44 as molecular entities with PCET characteristics due to their thermodynamically more stable and kinetically more inert characters relative to the diiron cores, especially in their low-valent oxidation states. In our previous study, we have prepared diruthenium(III) SAMs on a Au(111) electrode at a low surface-coverage level and demonstrated their well-defined and persistently stable PCET behavior under aqueous conditions.45 The interfacial PCET processes observed for the diruthenuium(III) SAMs involved the generation of four distinct redox chromophores, Ru III Ru III (μ-O), Ru II Ru III (μ-OH), Ru II Ru II (μ-OH), and RuIIRuII(μ-OH2), depending on the solution pHs of the electrolyte media employed for cyclic voltammetry.45 An additional interest in the electrochemistry of the oxobridged diruthenium(III) complexes concerns the ability to recognize Lewis acids at the oxo site as a Lewis base.46 Our previous study has revealed that externally added Lewis acids (or metal ions) such as BF3, Al3+, and Na+ modulate the redox potentials of [Ru2(μ-O)(μ-OAc)2(bpy)2(L)2]2+ (L = 1methylimidazole or pyridine) in a homogeneous electrolyte medium (0.1 M n-Bu4NPF6−CH3CN).46 This electrochemical behavior was rationalized by acid−base interactions, in which the oxo ligand behaves as a Lewis base. Stronger base ability of the μ-O ligand is apparent as the oxidation state of the metal centers is decreased. This observation suggests that electrochemically reduced oxo-bridged diruthenium species is a feasible sensor for the detection of Lewis acids by monitoring the shift of those redox waves. On the basis of these previous results, we have thought that such stimuli-responsive, welldefined redox properties of the diruthenium(III) compounds can be extended to the development of molecular sensor devices if those molecules are confined to electrode surfaces as SAMs at a high surface coverage level. In this work, we have developed densely packed SAMs of a new diruthenium(III) compound on Au(111) and studied their PCET reactions in aqueous electrolyte media and the potential ability for Lewis acid recognition in nonaqueous electrolyte media. This contribution presents the synthesis and characterization of complex 1, [Ru2(μ-O)(μOAc)2(bpy)2(Lpy‑SS)2](PF6)2 (Lpy‑SS = (C5H4N)CH2NHC(O)(CH2)4CH(CH2)2SS), bearing two disulfide pendants Lpy‑SS at the termini of the {Ru2(μ-O)(μ-OAc)2}2+ unit, as well as the preparation of the stable SAMs on Au(111) electrodes. The chemical structure of 1 is shown in Figure 1, and the SAMs of 1 on the Au(111) surface are designated as 1/Au hereafter.

SAMs 1/Au have been characterized by cyclic voltammetry, reductive desorption experiments (0.5 M KOH aqueous solution), contact angle measurements, and surface reflection−absorption infrared spectroscopy. The PCET reactions of 1/Au were studied by cyclic voltammetry in a wide pH region (pH ∼1−12, Britton−Robinson buffer solutions) and also the Lewis acid recognition (BF3 and Al3+) in a 0.1 M n-Bu4NPF6− CH3CN solution.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization of Complex 1. Complex 1 contains two 4-pyridyl disulfide ligands, Lpy‑SS, which facilitate the formation of SAMs on Au electrode surfaces by forming Au−S bonds.47 Complex 1 was prepared by reacting [Ru2(μ-O)(μ-OAc)2(bpy)2(CH3CN)2](PF6)245 with Lpy‑SS in the mixture of CH3CN/CH3OH at room temperature for 3 days, where the weakly coordinated CH3CN ligands were substituted by externally added Lpy‑SS. The 1H NMR spectrum of 1 in CD3CN exhibited a diamagnetic feature owing to the antiferromagnetic coupling between two RuIII centers (S = 1/2) via the oxo bridge.37 Due to the molecular symmetry of 1, a single set of bpy, Lpy‑SS, and acetate CH3 signals was observed with the expected integrated intensity. The UV−vis spectrum (CH3CN) showed absorption bands at λmax = 603, 464, 345, and 287 nm. The absorption at 603 nm was assigned to the transitions in the dπ(Ru)−pπ(μ-O) molecular orbitals of the dinuclear moiety proposed in our previous studies.37,48 More intense absorption bands in the higher energy, λmax = 464 and 345 nm, were ascribed to metal-to-ligand charge transfer (MLCT) transitions in which the π* orbitals of bpy and pyridyl groups in the Lpy‑SS ligands, respectively, were involved. The band at 287 nm was ascribed to ligand-localized π−π* transitions. Electrospray ionization mass spectrometry (ESIMS) showed a strong parent envelop at m/z = 1387.3, corresponding to [M−PF6]+. Some other fragment peaks were also observed as a result of the successive dissociation of coordinated ligands (Lpy‑SS and bpy) from 1 that may occur during the ionization/detection process. 2.2. Redox Properties of Complex 1 Dissolved in Nonaqueous Aprotic Media. In nonaqueous aprotic media, oxo-bridged diruthenium(III) complexes of the formula [Ru2(μ-O)(μ-OAc)2(bpy)2L2]2+ (L denotes pyridyl-based monodentate ligands) undergo two reversible one-electron transfer processes ascribed to RuIIIRuIV(μ-O)/RuIIIRuIII(μ-O) and RuIIIRuIII(μ-O)/RuIIRuIII(μ-O) along with a single irreversible one-electron process ascribed to RuIIRuIII(μ-O) → RuIIRuII(μ-O).37 A similar redox behavior was observed for 1 in 0.1 M n-Bu4NPF6−CH3CN (E1/2 = 0.96 and −0.19 V and Epc = −1.67 V vs Ag/AgCl). The electrochemical processes of 1 in nonaqueous aprotic media are represented by eqs 1−3,37 and the redox potentials are summarized in Table 1. The cyclic voltammetry data clearly indicate that the principal feature of the redox activity of oxobridged diruthenium groups was maintained in the disulfidefunctionalized complex 1. [Ru IIIRu IV (μ‐O)(μ‐OAc)2 (bpy)2 (Lpy‐SS)2 ]3 + + e− ⇄ [Ru IIIRu III(μ‐O)(μ‐OAc)2 (bpy)2 (Lpy‐SS)2 ]2 +

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[Ru IIIRu III(μ‐O)(μ‐OAc)2 (bpy)2 (Lpy‐SS)2 ]2 + + e− ⇄ [Ru IIRu III(μ‐O)(μ‐OAc)2 (bpy)2 (Lpy‐SS)2 ]+

Figure 1. Chemical structure of 1. 10111

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Table 1. Electrochemical Data of 1 and [Ru2(μ-O)(μ-OAc)2(bpy)2(pyridine)2](PF6)2 Dissolved in 0.1 M n-Bu4NPF6−CH3CN at a Scan Rate of 100 mV s−1 E1/2a/V vs Ag/AgCl (ΔEpb/mV) complex

Ru Ru (μ-O) → Ru Ru (μ-O)

RuIIIRuIII(μ-O)/RuIIRuIII(μ-O)

RuIIIRuIV(μ-O)/RuIIIRuIII(μ-O)

1 [Ru2(μ-O)(μ-OAc)2(bpy)2(pyridine)2](PF6)2

−1.67 −1.09c

−0.19 (70) −0.39 (60)

0.96 (90) 0.95 (60)

II

III

II

II

c

E1/2 = (Epa + Epc)/2, where Epa and Epc are anodic and cathodic peak potentials, respectively. bΔEp = Epa − Epc. cEpc is presented for the irreversible wave. a

Figure 2. (a) Infrared transmission spectra of complex 1 in a KBr matrix. (b) Surface reflection−absorption infrared spectra of SAMs 1/Au in air. Symbols: py = pyridine; bpy = 2,2′-bipyridine. The spectra were integrated from 128 interferograms.

Figure 3. Cyclic voltammograms of SAMs 1/Au in contact with (a) CH3CN and (b) CH2Cl2 containing 0.1 M n-Bu4NPF6. Scan rate = 500 mV s−1.

SAMs for electron transfer studies.49 The resulting SAMs have been characterized by contact-angle measurements, surface reflection−absorption infrared spectroscopy, and cyclic voltammetry as described below. It is noted here that our previous paper concerned similar SAMs containing oxo-bridged diruthenium(III) complexes as redox-active head groups, where a “stepwise approach” included the preparation of pyridyl-terminated SAMs on Au(111) followed by surface complexation of [Ru2(μ-O)(μ-OAc)2(bpy)2(CH3CN)2]2+ via a regioselective ligand exchange.45 This protocol, however, yielded low surface coverage SAMs (2.9 × 10−11 mol cm−2). In general, an efficient coordination reaction, e.g., dense tagging of the molecular components, is hardly expected for the

[Ru IIRu III(μ‐O)(μ‐OAc)2 (bpy)2 (Lpy‐SS)2 ]+ + e− → [Ru IIRu II(μ‐O)(μ‐OAc)2 (bpy)2 (Lpy‐SS)2 ]

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2.3. Preparation and Characterizations of the Diruthenium(III) SAMs, 1/Au. To prepare the diruthenium(III) SAMs, the Au(111) electrode was soaked in a CH3CN/ CH2Cl2 solution (1:9, v/v) of 1 (0.2 mM) for 24 h, rinsed thoroughly with CH3CN and CH2Cl2 to remove any physically adsorbed materials, and finally dried by purging Ar gas. This “presynthesized approach”, in which entire molecular components are first synthesized and subsequently confined to the surface, proved efficient for preparing high-quality redox-active 10112

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Table 2. Electrochemical Data of SAMs 1/Au in Contact with Nonaqueous Aprotic Media (CH3CN and CH2Cl2) Containing 0.1 M n-Bu4NPF6 at a Scan Rate of 500 mV s−1 E1/2a/V vs Ag/AgCl (ΔEpb/mV)

a

III

III

II

III

solvent

Ru Ru (μ-O)/Ru Ru (μ-O)

RuIIIRuIV(μ-O)/RuIIIRuIII(μ-O)

CH3CN CH2Cl2

−0.08 (160) −0.15 (150)

1.07 (40) 0.94 (70)

E1/2 = (Epa + Epc)/2, where Epa and Epc are anodic and cathodic peak potentials, respectively. bΔEp = Epa − Epc.

stepwise method,49 and, as discussed below, we have found in the current study that denser packing of the diruthenium(III) components were actually achieved on Au(111) by conducting the presynthesized method with the use of 1 as a SAM-forming molecule. Figure 2 shows the transmission infrared spectra of complex 1 in a KBr matrix (Figure 2a) and the surface reflection− absorption infrared spectra of SAMs 1/Au (Figure 2b). Complex 1 shows a strong band at 1655 cm−1 which is ascribed to the amide group embedded in the Lpy‑SS ligands (Figure 2a). The bands at 1547 and 1427 cm−1 are assigned to the antisymmetric and symmetric stretching modes of COO− groups, respectively, where the frequency difference, 118 cm−1, supports the bridging mode of the acetate ligands in 1.50,51 In the higher frequency region, the NH-, CH3-, and CH2-derived bands appear. SAMs 1/Au display an absorption band assigned to the CO stretching mode of the amide group at 1650 cm−1. According to the surface selection rules on reflective metal surfaces,52 the bands of those bonds that are perfectly parallel to the surface should be absent in the IR spectrum. It is only possible to observe IR bands when bonds have a vertical fraction with respect to the surface. Therefore, the observation of the CO stretching vibration for 1/Au indicates an unparallel orientation of the amide CO group in 1 with respect to the Au(111) surface. Figure 3 depicts cyclic voltammograms of 1/Au in contact with CH3CN and CH2Cl2 containing 0.1 M n-Bu4NPF6 as the supporting electrolyte. The electrochemical data are summarized in Table 2. As shown in Figure 3, we observed two sequential redox waves for both CH3CN and CH2Cl2 media in the potential region between +1.5 and −1.0 V vs Ag/AgCl. These waves were assigned to the metal-based processes Ru I I I Ru I II (μ-O)/Ru I I Ru I I I (μ-O) and Ru I I I Ru I V (μ-O)/ RuIIIRuIII(μ-O) (Table 2) on the basis of the aforementioned assignment for freely diffusing molecules.37 Beyond −1.1 V, an irreversible extensive current flow appeared due to the occurrence of the reductive desorption of surface-confined 1 from the Au(111) surface. By integrating the charge under the voltammetric wave, the surface coverage of 1 on the Au(111) electrode surface was estimated to be approximately 6.0 × 10−11 mol cm−2. The surface coverage of complex 1 on Au(111) was further studied by the reductive desorption experiment.53 A cyclic voltammogram of 1/Au recorded in contact with an aqueous KOH medium is presented in Figure 4. A single reductive desorption peak was observed at Epc = −0.90 V. The occurrence of a single peak suggests the homogeneity of 1/ Au. The charge integration of the cathodic wave gave a surface coverage of the disulfide group of 1.2 × 10−10 mol cm−2. This value corresponds to twice of the surface coverage of the diruthenium(III) redox centers obtained from Figure 3 (6.0 × 10−11 mol cm−2), which illustrates that complex 1 is confined to the Au surface with two disulfide ends (see below).

Figure 4. Cyclic voltammogram of SAMs 1/Au in contact with an aqueous solution of 0.5 M KOH, showing the reductive desorption of 1 from the Au(111) electrode surface. Scan rate = 20 mV s−1.

Water contact-angle measurements showed that the surface of SAMs 1/Au was more hydrophobic (46 (±3)°) than our previously determined, lower surface coverage SAMs possessing identical head groups (24.6 (±3.2)°).45 According to the above-mentioned observations, a molecular orientation of 1 on the Au(111) surface is proposed in Figure 5. The single reductive desorption peak in Figure 4 suggested a uniform deposition of the molecules. On the basis of the stoichiometry of the diruthenium(III) groups/disulfide anchor groups (1/2 ratio) in 1/Au characterized by metal-based electrochemistry and the reductive desorption experiment, it is reasonable to assume that complex 1 stands on the surface with two legs and is confined to the surface by forming the Au−S bonds, though the possibility of a minor inclusion of singlelegged attachment of 1 with a dangling end is not fully excluded due to the inherent experimental uncertainty. The surfaceconfined diruthenium(III) head groups are less than their dense packing as a result, at least in part, of the dicationic electrostatic repulsions and/or the steric bulkiness of the chargecompensating PF 6 − counteranions surrounding the diruthenium(III) groups. According to the surface-selection rule of IR, the amide CO bond within the tethering ligand (Lpy‑SS) of surface-oriented 1 on Au(111) has a vertical component with respect to the surface. 2.4. Interfacial PCET Processes of the SAMs in Contact with Aqueous Solution Containing 0.1 M HClO4. Figure 6 displays the redox behavior of 1/Au in contact with aqueous solution containing 0.1 M HClO4. Under the highly acidic conditions, such as pH 1, the diruthenium(III) complexes confined to the top of the SAMs give two sequential redox processes that are ascribed to one-proton and one-electron coupled redox reactions as represented by eqs 4 and 5, as established by our previous study on low surface coverage SAMs by plotting the potential−pH diagram.45 [Ru IIIRu III(μ‐O)(μ‐OAc)2 (bpy)2 (L)2 ]2 + + H+ + e− ⇄ [Ru IIRu III(μ‐OH)(μ‐OAc)2 (bpy)2 (L)2 ]2 + 10113

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Figure 5. Proposed orientation of surface-confined 1 in SAMs 1/Au.

Figure 6. (a) Cyclic voltammograms of SAMs 1/Au in contact with an aqueous solution of 0.1 M HClO4. Scan rates = 50, 100, 200, 300, 400, and 500 mV s−1. (b) Linear relationship between the scan rate (v) and the peak current intensity of the anodic peak at E1/2 = +0.27 V.

is consistent with that obtained in the CH3CN electrolyte media (vide supra). This surface coverage corresponds to ca. 43% of the maximum coverage of densely packed monolayers of the {Ru2(μ-O)(μ-OAc)2(bpy)2}2+ motif (1.4 × 10−10 mol cm−2) based on the geometrical packing considerations, in which a structural model of the diruthenium(III) complex was assumed to have a diameter of 8.7 Å.48 The electrostatic repulsions among the dicationic units and the presence of two bulky PF6− counteranions to compensate the charge may hamper the maximum coverage of the diruthenium(III) groups on the Au(111) surface. Nevertheless, the surface coverage of 1/Au is apparently higher than that (2.9 (±0.3) × 10−11 mol cm−2) of our earlier SAMs which were prepared via the stepwise deposition procedure.45 This indicates that the onestep deposition of the diruthenium(III) complex with disulfide anchors employed in the current study is more favorable to achieve the dense molecular packing on the Au(111) surface. 2.5. Interfacial PCET Processes of the SAMs in Contact with Britton−Robinson Buffer Solutions (2 < pH < 12). We have previously reported that oxo-bridged diruthenium(III) complexes dissolved in homogeneous solutions showed PCET behavior when acids were added externally to the electrolyte solutions.54 The reactions were affected by the proton-donation ability of the acids. For example, complex [Ru2(μ-O)(μOAc)2(bpy)2(mim)2]2+ (mim =1-methylimidazole) dissolved in a 0.1 M n-Bu 4 NPF 6 −CH 3 CN solution exhibited two consecutive one-electron-reduction waves and a one-electronoxidation wave. Upon addition of the strong proton donors such as p-toluenesulfonic acid (pKa = 1.7), two reductive waves (RuIIIRuIII/RuIIRuIII and RuIIRuIII/RuIIRuII) were positively

[Ru IIRu III(μ‐OH)(μ‐OAc)2 (bpy)2 (L)2 ]2 + + H+ + e− ⇄ [Ru IIRu II(μ‐OH 2)(μ‐OAc)2 (bpy)2 (L)2 ]2 +

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As shown in Figure 6a, the SAMs exhibit two consecutive redox waves at E1/2 = +0.27 V (ΔEp = 25 mV, full width at halfmaximum (W1/2) = 98−142 mV at scan rates of 50−500 mV s−1) and E1/2 = +0.05 V (ΔEp = 17 mV, W1/2 = 93−114 mV at scan rates of 50−500 mV s−1) in the potential region between −0.15 and +0.50 V vs Ag/AgCl, which were ascribed to processes Ru III Ru III (μ-O)/Ru II Ru III (μ-OH) (eq 4) and RuIIRuIII(μ-OH)/RuIIRuII(μ-OH2) (eq 5), respectively. Freely diffusing diruthenium(III) complexes in organic solutions exhibit an additional one-electron-transfer process RuIIIRuIV(μ-O)/RuIIIRuIII(μ-O) in the more positive potential region (>+1.0 V), but this was not observed for the SAMs due to the limitation of the potential window. The surface confinement of the redox-active units was supported by the linear increase in the peak current intensity of the voltammetric wave against scan rates (50−500 mV s−1), as shown in Figure 6b. The W1/2 values for both redox waves exceed the value of 90.6 mV expected for a one-electron redox process of surfaceconfined molecules with no lateral interactions. These somewhat larger values suggest the existence of repulsive interactions between the adjacent diruthenium(III) redox centers within the SAMs.50 The electrochemical response was reproducible upon repetitive scans of the applied potential between −0.15 and +0.50 V. The surface coverage of 1 estimated from the charge of the redox wave at E1/2 = +0.27 V was 6.0 × 10−11 mol cm−2, which 10114

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Figure 7. Cyclic voltammograms of SAMs 1/Au in contact with the Britton−Robinson buffer solution containing 0.1 M NaClO4 recorded at different solution pHs: (a) 1.4 < pH < 3.1; (b) 4.7 < pH < 7.0; (c) 8.2 < pH < 12.1. Scan rate = 500 mV s−1. (d) Potential−pH diagram in the pH range 1.4−12.1.

shifted due to the protonation at the oxo bridges in the reduced states RuIIRuIII and RuIIRuII, while the oxidative wave corresponding to the RuIIIRuIV/RuIIIRuIII remained almost unchanged. In the presence of weak proton donors such as benzimidazoles (pKa = ca. 12.3), the two reductive processes merged into an apparent one-step two-electron process owing to the only protonation of the RuIIRuII state. However, the insolubility of the diruthenium(III) complexes in aqueous solutions hampered the extensive study of their PCET reactions in aqueous media. Preparation of the SAMs makes it possible to observe the PCET reactions of diruthenium(III) complexes in aqueous solutions. Indeed, in our previous experiments, the PCET reactions of the diruthenium(III) SAMs were successfully observed by cyclic voltammetry in the Britton−Robinson buffer aqueous media in a wide pH region, 2 < pH < 12.45 In acidic media (2 < pH < 6), two one-proton/one-electron coupled reactions were observed as expressed by eqs 4 and 5. In neutral media (6 < pH < 8), one-proton/one-electron coupled transfer reaction identical to eq 4 and one-electron transfer reaction exhibited by eq 6 sequentially occurred.

[Ru IIIRu III(μ‐O)(μ‐OAc)2 (bpy)2 (L)2 ]2 + + H+ + 2e− ⇄ [Ru IIRu II(μ‐OH)(μ‐OAc)2 (bpy)2 (L)2 ]+

Figure 7 shows selected cyclic voltammograms of SAMs 1/ Au recorded in contact with Britton−Robinson buffer solutions at pH from 2.0 to 12.0, where the voltammetric change is presented separately according to three pH regions. Under the acidic conditions (Figure 7a), two redox couples appear and are assigned to the RuIIIRuIII(μ-O)/RuIIRuIII(μ-OH) (eq 4) and RuIIRuIII(μ-OH)/RuIIRuII(μ-OH2) (eq 5) processes. As the pH is increased, these two redox waves shift to the negative direction. Approaching pH 7.0 (Figure 7b), the two redox waves overlap seriously into a broad wave. As the pH is further increased, two redox waves completely merge into a single sharp wave (Figure 7c). On the basis of these observations, a potential−pH diagram was prepared and is shown in Figure 7d. The least-squares fitting indicates that the slopes for processes (i), (ii), (iii), and (iv) are −60, −48, −5, and −29 mV per pH unit, respectively, which are consistent with the expected proton/electron stoichiometry for the PCET reactions shown in eqs 4−7. Accordingly, four distinct oxidation states, Ru III Ru III(μ-O), Ru IIRu III(μ-OH), Ru IIRu II(μ-OH 2), and RuIIRuII(μ-OH), are formed on the surface-confined diruthenium moiety within the potential window (0.5 to −0.5 V vs Ag/AgCl) and in the pH region (1.4−12.1) studied. The pKa for the dissociation of a single proton from the surface-confined RuIIRuII(μ-OH2) (aqua bridge) to yield RuIIRuII(μ-OH) (hydroxo bridge) was determined from Figure 7d to be 5.0 for SAMs 1/Au. In our previous experiments,45 the

[Ru IIIRu III(μ‐OH)(μ‐OAc)2 (bpy)2 (L)2 ]2 + + e− → [Ru IIIRu III(μ‐OH)(μ‐OAc)2 (bpy)2 (L)2 ]2 +

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(6)

Under alkaline conditions (8.0 < pH < 12), a one-proton/ two-electron coupled transfer process was observed as represented by eq 7. 10115

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for three redox waves are 1.08, 0.58, and 0.21 V. On further increasing the concentration of BF3 ([BF3] = 2.66 mM), three one-electron processes are observed at E1/2 = 1.14 V (ΔEp = 15 mV), 0.60 V (22 mV), and 0.37 V (52 mV). The addition of Al3+ was also found to have a similar influence on the redox potentials of 1/Au; three redox waves appear in the presence of Al3+ and are positively shifted upon increasing the concentrations of Al3+, which are shown in Figure S2 and Table S2 in the Supporting Information. It is remarkable that the potential shift was seen at the concentrations of ∼10 μM of BF3 and Al3+, indicating a high sensitivity of the diruthenium(III) SAMs in the context of electrochemical recognition of Lewis acids.

pKa of the low surface coverage diruthenium(III) SAMs prepared by stepwise deposition was 5.8, which means that the SAMs in the current study are more acidic. In general, the surface pKa of SAMs is influenced by several factors including the interfacial potential effect, the chain length of the molecules, and the microenvironment surrounding.55−58 In the current case, the following structural features can, at least in part, be raised as possible factors to determine the surface pKa of 1/Au: the length of the alkyl chains in the tether, the surface coverage of the redox groups, and the absence of the coordination-free pyridine groups directing toward the electrolyte media. 2.6. Lewis Acid Recognition. The solution-based electrochemistry of oxo-bridged diruthenium(III) complexes showed that their redox response was highly affected by externally added Lewis acids such as BF3 and this behavior was rationalized by acid−base interactions in which the oxo bridge within the dinuclear moiety acted as a Lewis base.46 It is of considerable interest in studying how these acid−base interactions are expressed and tuned at a SAM/solution interface and in exploring the potential utility of the diruthenium(III) complexes for the detection of diffusing Lewis acids in solution. In the current study, the effects of Lewis acids on the electrochemical behavior of the SAMs have been examined by cyclic voltammetry with various concentrations of BF3 or Al3+ cation. Figure 8 shows cyclic voltammograms of 1/Au recorded in the absence and presence of Lewis acids BF3·Et2O and Al3+ in a

3. CONCLUSION A new oxo-bridged diruthenium(III) complex with two disulfide ends, 1, was synthesized and confined to the Au(111) surface to form proton- and Lewis acid responsive SAMs, 1/Au. In aqueous media, the SAMs showed interfacial proton-coupled electron transfer reactions in a wide pH region. In the nonaqueous media, where Lewis acids were used as the external stimuli instead of a proton, the response of the SAMs to BF3 and Al3+ cation has been investigated by cyclic voltammetry. On addition of Lewis acids, the redox potentials of 1/Au were positively shifted as a result of the interactions between the oxo bridge and Lewis acids. The redox potentials were sensitive to the concentrations of the Lewis acids, and the detection limit was as low as the micromolar level. The result clearly indicates that the Lewis acid−base interactions can be utilized as a new additional trigger to control the interfacial redox events of coordination molecule based SAMs. This work presents a perspective that external stimuli-responsive properties of surface-confined biomimetic coordination building blocks could be applied to the design of sensor devices. 4. EXPERIMENTAL SECTION 4.1. Materials and Methods. All reagents and solvents were purchased from commercial sources and used as received unless otherwise stated. The precursor complex [Ru 2 (μ-O)(μOAc)2(bpy)2(CH3CN)2](PF6)245 and N-(4-pyridylmethyl)lipoamide (Lpy‑SS)47 were prepared as described previously. The organic solvents used for electrochemical measurements, CH3CN and CH2Cl2, were distilled over CaH2 under an Ar atmosphere prior to use. The supporting electrolyte, n-Bu4NPF6, was recrystallized from ethanol and completely dried in vacuo. Ultrapure water was prepared from a MilliQ water purification system (Yamato, WQ-500). Ultrapure N2 (99.99%) or Ar (99.95%) (Daido Hokusan) was used as an inert atmosphere. Ultraviolet and visible absorption spectra were measured with a JASCO V-560 spectrophotometer. 1H NMR spectra were obtained on an EXC-400 NMR spectrometer at 400 MHz. The infrared spectra were recorded with a Bio-Rad FTS 60A/896 FT-IR spectrometer. Complex 1 dispersed in a KBr matrix was used to measure the IR transmission spectra. Electrochemical measurements of 1 in a homogeneous solution were carried out at room temperature by using a glassy-carbon working electrode, a Pt mesh counter electrode, and an Ag/AgCl reference electrode under an Ar atmosphere. The potentials were controlled by a potentiostat (EG&G, Model 263A). The electrolyte solution was 0.1 M n-Bu4NPF6−CH3CN. ESI mass spectrometry and elemental analyses were performed at the Center for Instrumental Analysis, Hokkaido University. 4.2. Synthesis of [Ru2(μ-O)(μ-OAc)2(bpy)2(Lpy‑SS)2](PF6)2 (1). To a CH3CN solution (10 mL) of [Ru2(μ-O)(μOAc)2(bpy)2(CH3CN)2](PF6)2 (60 mg, 0.06 mmol) was added the methanolic solution (10 mL) of Lpy‑SS (35 mg, 0.12 mmol). The mixture was stirred at room temperature for 3 days. A dark blue solid was obtained after the solvent was removed, and the solid was

Figure 8. Cyclic voltammograms of SAMs 1/Au in contact with 0.1 M n-Bu4NPF6−CH3CN solution: without Lewis acids (black curve); [BF3] = 2.66 × 10−4 M (red curve); [Al(ClO4)3·9H2O] = 1.0 × 10−3 M (green curve). Scan rate = 500 mV s−1.

0.1 M n-Bu4NPF6−CH3CN solution. A series of cyclic voltammograms responding to various concentrations of BF3 (2.66 × 10−5 M to 2.66 mM) and the electrochemical data are provided in Figure S1 and Table S1 of the Supporting Information, respectively. Prior to the addition of BF3 (Figure 8, black curve), SAMs 1/ Au displays two sequential waves at E1/2 = +1.07 and −0.08 V vs Ag/AgCl, which are assigned to RuIIIRuIV(μ-O)/RuIIIRuIII(μO) and RuIIIRuIII(μ-O)/RuIIRuIII(μ-O), respectively, as already described (Table 2). On addition of BF3 at the 0.266 mM concentration (Figure 8, red curve), three redox waves appear at E1/2 = +1.12 V (ΔEp = 35 mV), +0.59 V (36 mV), and +0.31 V (200 mV), which can be assigned to metal-based processes, RuIIIRuIV/RuIIIRuIII, RuIIIRuIII/RuIIRuIII, and RuIIRuIII/RuIIRuII, respectively.46 The modulation is less effective on the most positive wave, but it becomes more significant as metal centers are reduced. At a lower concentration ([BF3] = 26.6 μM, Figure S1 and Table S1 in the Supporting Information), the potentials 10116

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redissolved in a mixture of CH3CN and H2O (10 mL, 1:1, v/v). CH3CN was removed under reduced pressure, and the resultant solution with dark blue precipitate was filtered. The resultant solid was washed thoroughly with water, methanol, and diethyl ether and dried in vacuo. Yield: 62 mg (68%). Anal. Calcd for C52H62F12N8O7P2Ru2S4: C, 40.73%; H, 4.08%; N, 7.31%. Found: C, 40.72%; H, 4.09%; N, 7.38%. ESI-MS (positive, CH3CN): m/z = 1387.3 ([M − PF6]+). IR (KBr pellet, cm−1): 1655 m (amide), 1545 m (vas(COO−)), 1427 m (vs(COO−)), 843 (PF6−). 1H NMR (CD3CN): δ 8.58 (d, 4H, py 2,6H), 8.54 (d, 4H, bpy 3,3′-H), 7.85 (t, 4H, bpy 4,4′-H), 7.72 (d, 4H, py 3,5-H), 7.20 (t, 4H, bpy 5,5′-H), 7.06 (s, br, 2H, NH), 6.05 (d, 4H, bpy 6,6′-H), 4.66 (d, 4H, py−CH2−NH−), 3.59 (m, 2H, −CH− SS−), 3.14 (m, 4H, −CH2−SS−), 2.26 (t, 4H, −CO−CH2−), 2.08 (s, 6H, acetate CH3), 1.90 (m, 4H, −CH2−), 1.68 (m, 8H, −CH2−), 1.45 (m, 4H, −CH2−). UV−vis (CH3CN) λmax/nm (ε/M−1 cm−1): 603 (23 200), 464 (5300), 345 (13 600, sh), 287 (51 400), 241 (33 300) (sh = shoulder). 4.3. Preparation of Au(111) Electrodes and SAMs. The gold disk substrate with an atomically flat (111) surface was prepared by the reported procedures.59,60 Prior to the surface modification, the gold substrate was annealed in a hydrogen flame and quenched with pure water. The surface area of the Au(111) electrode was determined by the observed charge in the cyclic voltammogram for the reduction of gold oxide in 0.1 M H2SO4 solution. SAMs were prepared by immersing the gold substrate into the CH3CN−ethanol (1:9, v/v) solution of 1 (0.2 mM) and keeping for 24 h at room temperature. The SAM-modified electrode was taken out, rinsed thoroughly with CH3CN, and dried in an Ar flow before any measurements. 4.4. Fourier Transform Infrared Reflection−Absorption Spectroscopy. FT-IR reflection−absorption spectra of the SAMs were recorded at 298 K in dry air on a Bio-Rad FTS-60A/896 spectrometer equipped with an MCT detector and a Harrick gazing angle (ca. 70°) reflection accessory.61 The samples were prepared by adsorbing 1 on a gold film evaporated on a Ti-coated glass plate. 4.5. Contact Angle Measurements. The contact angle was measured by putting a Milli-Q water droplet (11 mg) on the SAMmodified Au(111) surface and recording the image of the interface with an optical microscope (Intel Play QX3 with magni cation 60).62 The contact angles were estimated from the recorded digital images of the water droplet on the gold surface. Contact angles for each sample were measured and averaged at least at three different positions. 4.6. Cyclic Voltammetry for the SAMs. A three-electrode cell consisting of a SAM-modified Au(111) electrode, a platinum mesh, and an Ag/AgCl/KCl(saturated) electrode as the working, counter, and reference electrodes, respectively, was used for the measurements of cyclic voltammetry. The electrode potentials were controlled by a potentiostat, ALS/CHI644A. An aqueous solution of HClO4 was used as the electrolyte solution. CH3CN and CH2Cl2 containing n-Bu4NPF6 (0.1 M) were used as the electrolytes for nonaqueous systems. 4.7. Study on PCET of the SAMs. For the study on PCET reactions of 1/Au, aqueous solutions of 0.04 M Britton−Robinson buffer solutions containing 0.1 M NaClO4 were used as the electrolytes for the measurements in the pH range 2−12. The electrolyte solutions with desired pH values were prepared by mixing solution A (0.04 M CH3COOH + 0.04 M H3PO4 + 0.04 M H3BO3 + 0.1 M NaClO4) with solution B (0.2 M NaOH) in proportion.45 Prior to the measurements, the electrolyte solutions were deaerated by bubbling pure Ar gas for 30 min. During the measurements, only one face of the SAM-modified Au(111) substrate was made to touch the electrolyte solution with a meniscus. 4.8. Study on the Lewis Acid Recognition of the SAMs. The electrolyte solution was CH3CN containing 0.1 M n-Bu4NPF6. The commercially available solution of BF3 in diethyl ether (BF3·Et2O) was used as the source of BF3. A solution of Al(ClO4)3·9H2O in CH3CN was prepared and used as a source of Al3+ cation. During the measurements, the aforementioned solutions of the Lewis acids were added to the electrolyte solutions via a syringe to prepare solutions with desired concentrations of the Lewis acids.

Article

ASSOCIATED CONTENT

S Supporting Information *

Electrochemical data of 1/Au in the presence of BF3 (Table S1) and Al(ClO4)3·9H2O (Table S2). Cyclic voltammograms of 1/Au with various concentrations of BF3 (Figure S1) and Al(ClO4)3·9H2O (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.A.); osawam@cat. hokudai.ac.jp (M.O.); [email protected] (Y.S.). Tel./ fax: +81-92-802-2828 (M.A.); +81-11-706-9123 (M.O.); +8111-706-9125 (Y.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by a JSPS KAKENHI Grant (Nos. 24550143 and 25288031), a Grant-in-Aid for Scientific Research on Innovative Areas “Coordination Programming” (Area 2107, Nos. 24108701 and 24108730), and the Global COE Program, “Science for Future Molecular Systems”, from the Ministry of Education, Culture, Sports, Science and Technology of Japan. H.-X.Z. is grateful to JSPS for a postdoctoral fellowship (P05123). M.A. and S.Y. also gratefully acknowledge financial support from the Cooperative Research Program of Catalysis Research Center, Hokkaido University (No. 12A0007).

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REFERENCES

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dx.doi.org/10.1021/la401736f | Langmuir 2013, 29, 10110−10119