Mechanistic Insight into Reversible Core Structural Changes of

Aug 24, 2017 - Mechanistic Insight into Reversible Core Structural Changes of Dinuclear μ-Hydroxoruthenium(II) Complexes with a 2,8-Di-2-pyridyl-1,9 ...
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Mechanistic Insight into Reversible Core Structural Changes of Dinuclear μ‑Hydroxoruthenium(II) Complexes with a 2,8-Di-2-pyridyl1,9,10-anthyridine Backbone Prior to Water Oxidation Catalysis Masanari Hirahara,*,† Sho Nagai,‡ Kosuke Takahashi,‡ Shunsuke Watabe,‡ Taisei Sato,‡ Kenji Saito,‡ Tatsuto Yui,‡ Yasushi Umemura,† and Masayuki Yagi*,‡ †

Department of Applied Chemistry, National Defense Academy of Japan, Hashirimizu 1-10-20, Yokosuka, Kanagawa 239-8686, Japan Department of Materials Science and Technology, Faculty of Engineering, Niigata University, 8050 Ikarashi-2, Niigata 950-2181, Japan

‡

S Supporting Information *

ABSTRACT: proximal,proximal-(p,p)-[RuII2 (tpy) 2 LXY] n+ (tpy = 2,2′;6′,2″-terpyridine, L = 5-phenyl-2,8-di-2-pyridyl1,9,10-anthyridine, and X and Y = other coordination sites) yields the structurally and functionally unusual RuII(μOH)RuII core, which is capable of catalyzing water oxidation with key water insertion to the core (Inorg. Chem. 2015, 54, 7627). Herein, we studied a sequence of bridging-ligand substitution among p,p-[Ru2(tpy)2L(μ-Cl)]3+ (Ru2(μ-Cl)), p,p-[Ru2(tpy)2L(μ-OH)]3+ (Ru2(μ-OH)), p,p-[Ru2(tpy)2L(OH)(OH2)]3+ (Ru2(OH)(OH2)), and p,p-[Ru2(tpy)2L(OH)2]2+ (Ru2(OH)2) in aqueous solution. Ru2(μ-Cl) converted slowly (10−4 s−1) to Ru2(μ-OH), and further Ru2(μ-OH) converted very slowly (10−6 s−1) to Ru2(OH)(OH2) by the insertion of water to reach equilibrium at pH 8.5−12.3. On the basis of density functional theory (DFT) calculations, Ru2(OH)(OH2) was predicted to be thermodynamically stable by 13.3 kJ mol−1 in water compared to Ru2(μ-OH) because of the specially stabilized core structure by multiple hydrogen-bonding interactions involving aquo, hydroxo, and L backbone ligands. The observed rate from Ru2(μ-OH) to Ru2(OH)2 by the insertion of an OH− ion increased linearly with an increase in the OH− concentration from 10 to 100 mM. The water insertion to the core is very slow (∼10−6 s−1) in aqueous solution at pH 8.5−12.3, whereas the insertion of OH− ions is accelerated (10−5−10−4 s−1) above pH 13.4 by 2 orders of magnitude. The kinetic data including activation parameters suggest that the associative mechanism for the insertion of water to the RuII(μ-OH)RuII core of Ru2(μ-OH) at pH 8.5−12.3 alters the interchange mechanism for the insertion of an OH− ion to the core above pH 13.4 because of relatively stronger nucleophilic attack of OH− ions. The hypothesized p,p-[Ru2(tpy)2L(μ-OH2)]4+ and p,p-[Ru2(tpy)2L(OH2)2]4+ formed by protonation from Ru2(μ-OH) and Ru2(OH)(OH2) were predicted to be unstable by 71.3 and 112.4 kJ mol−1 compared to Ru2(μ-OH) and Ru2(OH)(OH2), respectively. The reverse reactions of Ru2(μ-OH), Ru2(OH)(OH2), and Ru2(OH)2 to Ru2(μ-Cl) below pH 5 could be caused by lowering the core charge by protonation of the μOH− or OH− ligand.

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INTRODUCTION Polypyridylruthenium(II) complexes have been studied extensively because of their stability in a wide pH range and redox1 and photoredox2 properties, which invokes application of the complexes to promising catalyses,3 photocatalyses,4 and bioactivities5 in chemical and biological systems. The ligand substitution reactions of ruthenium(II) complexes need to be studied alongside redox and photoredox reactions because catalyses and photocatalyses by ruthenium complexes involve essential ligand substitution for substrate uptake. Dinuclear ruthenium(II) complexes have been reported to attain unique catalyses that are uncommon for mononuclear complexes.3a,d,6 Nevertheless, the bridging-ligand substitutions of dinuclear ruthenium(II) complexes6c,7 are considerably less studied than © 2017 American Chemical Society

the ligand substitutions of mononuclear ruthenium(II) complexes8 although the bridging sites are frequently utilized as an active site for the unique catalyses.3c,d,9 The understanding of bridging-ligand substitutions of dinuclear ruthenium(II) complexes is an important subject, particularly for the complexes affording unique catalyses. We have recently reported the synthesis of dinuclear ruthenium(II) complexes, p,p-[Ru2(tpy)2LXY]n+ (see Scheme 1), anchored by a backbone ligand L.10 This complex is able to afford the structurally unusual RuII(μ-OH)RuII core bridged by a hydroxo ion between RuII centers under weakly basic Received: April 17, 2017 Published: August 24, 2017 10235

DOI: 10.1021/acs.inorgchem.7b00978 Inorg. Chem. 2017, 56, 10235−10246

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Inorganic Chemistry Scheme 1. A Series of Dinuclear Ruthenium Complexes, p,p[RuII2(tpy)2LXY]n+

for both complexes. Only an example of the RuII(μ-OH)RuII core characterized by X-ray crystallographic analysis had been given by a [Ru2(tpy)2(μ-pdz-dc)(μ-OH)]+ complex (pdz-dc2− = pyridazine-3,6-dicarboxylate dianion) until Ru2(μ-OH), as far as we know.6c However, because isolation of this complex was restricted under strictly inert gas conditions, the reactions of the core including ligand substitutions remain little investigated. Xray crystallographically characterized Ru2(μ-OH)10 is a godgiven material to understand the properties and reactions of the RuII(μ-OH)RuII core, which has not been studied because of the lack of a stably realized core. Herein we first investigate the structural change of the RuII(μ-OH)RuII core with the bridgingligand substitution. We report the mechanism and influencing factors of the bridging-ligand substitutions to gain a better understanding of the characteristic features of the core.

conditions to provide Ru2(μ-OH), which was characterized by X-ray crystallographic analysis.10 We have further demonstrated that a key water insertion to the RuII(μ-OH)RuII core of Ru2(μOH) is necessary to yield catalytic water oxidation by intramolecular O−O bond formation between inserted water and the hydroxo ligand in a higher oxidation state of Ru2(OH)(OH2).10 So far, there have been only a few reports related to the RuII(μ-OH)RuII core. Formation of the RuII(μ-OH)RuII core was first suggested by Llobet et al. on the electrochemical reduction of [RuIII2(tpy)2(picolinate)2(μ-O)]2+ at ca. −0.2 V vs Ag/AgCl and pH >11 in an aqueous solution, although the formed RuII(μ-OH)RuII species is decomposed for several minutes.11 This was also suggested by the electrochemical reduction of [RuIII2(μ-O)(μ-OAc)2(bpy)2(L′)2]2+ [OAc = CH3CO2−, bpy = 2,2′-bipyridine, and L′ = (C5H4N)CH2NHC(O)(CH2)4CH(CH2)2SS] adsorbed on the Au(111) electrode at −0.12 V versus Ag/AgCl and pH >5 in an aqueous electrolyte solution.12 However, the spectroscopic evidence of these RuII(μ-OH)RuII cores has not been reported

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EXPERIMENTAL SECTION

Materials. The dinuclear ruthenium complexes [Ru2(μ-Cl)]Cl3, [Ru2(μ-OH)](NO3)3, and [Ru2(OH)(OH2)](NO3)3 were synthesized as previously described.10 Deuterated solvents were purchased from JUNSEI Chem. Co. All other reagents of the purest grade were purchased and used without further purification. Measurements. 1H NMR spectra over 296 K were recorded on a Varian 700 MHz or a Bruker 500 MHz spectrometer, and those at 283 K were recorded on a JEOL 500 MHz spectrometer. 1H NMR spectra were referenced using tetramethylsilane in organic solvents or sodium 3-(trimethylsilyl)propionate-2,2,3,3-d4 in D2O as an internal standard. The pD (−log [D+]) value in D2O was calculated using the following equation: pD = pH + 0.4, where pH is the measured value by a pH meter (Horiba F-73).13 At higher pD above 13, the sample solutions were prepared from D2O-containing 3-(trimethylsilyl)propionate2,2,3,3-d4 and a standard 1.0 M NaOH solution to avoid the measurement error by the pH meter, and the pD values were calculated from eq 1

Figure 1. 1H NMR spectral change of the Ru2(μ-Cl) solution (2.0 mM) in a 0.1 M phosphate buffer (pD 10.7) at room temperature. The resonances due to Ru2(μ-Cl), Ru2(μ-OH), and Ru2(OH)(OH2) are represented by black squares, green circles, and red triangles, respectively. 10236

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Figure 2. Kinetic profiles of the ruthenium complex fractions during bridging-ligand substitution among Ru2(μ-Cl) (black squares), Ru2(μ-OH) (green circles), and Ru2(OH)(OH2) (red triangles) in a 0.1 M phosphate buffer at room temperature: (A) pD 8.5; (B) pD 10.7; (C) 50 mM NaOD (pD 13.6). The total concentration of the ruthenium species is 2.0 mM. The time axes are broken at 0.5 days to see the profiles in both short and long time scales. (D) Plots of k2obs (red circles) and k3obs (blue triangles) versus NaOD concentration in strongly basic conditions (more than 30 mM NaOD).

⎛ − K + K 2 + 4K c b b b pD = log⎜⎜ 2 K D2 O ⎝

⎞ ⎟ ⎟ ⎠

Ka

Ru 2H ⇌ Ru 2− + H+ K a ref

(1)

AH HooooI A− + H+

where Kb, c, and KD2O are the base dissociation constant of NaOH (pKb = −log Kb = −0.814), the total concentration of NaOH, and the ionization constant of D2O (pKD2O = 14.915). Electrospray ionization mass spectrometry (ESI MS) spectra were measured with a Waters/ Micromass ZQ 4000 spectrometer under the following conditions: complex concentration, 5−50 μM; cone voltage, 20 V; capillary voltage, 3.5 kV. UV−visible absorption spectra were measured in a quartz cell with 1 cm of a light pass length using UV−visible spectrometers (Shimadzu Multispec-1500 and Hitachi U-3310). To ensure a constant pH during the reactions monitored, a 0.1 M phosphate buffer and a mixed buffer (20 mM boric acid, 10 mM KH2PO4, and 5 mM citric acid) were used for 1H NMR and UV− visible absorption spectral measurement, respectively. For UV−visible absorption spectral measurement, 10 mM KCl was added in the 22 μM [Ru2(μ-Cl)]Cl3 solution to ensure the pseudo-first-order conditions with respect to the formed Ru2(μ-OH) for the reaction between Ru2(μ-OH) and Cl− ions to form Ru2(μ-Cl). Density functional theory (DFT) calculations were performed using the Gaussian 09 package of programs.16 Molecular structures were fully optimized using the B3LYP method, which uses hybrid Becke’s threeparameter exchange functional17 with the correlation energy functional of Lee, Yang, and Parr.18 Calculations were performed using the standard double-ζ-type LanL2DZ basis set implemented in Gaussian 09. The pKa (−log Ka) values of dinuclear ruthenium(II) complexes (Ru2H; eq 2) were calculated from a comparison with the pKaref value of the reference acid species (HA; eq 3) according to the method by Solis and Hammes-Schiffer19 based on DFT calculations.

−

(2) (3)

−

Ru 2H + A ⇌ Ru 2 + AH

(4)

pKa is expressed by eq 5:

pK a =

ΔGr° + pK aref ln(10)RT

(5)

where ΔGr° is the free-energy change of the proton-exchange reaction between Ru2H and HA (eq 4) calculated by the PCM method (solvent = water). R and T are the gas constant and absolute temperature, respectively. Acetic acid (pKaref = 4.76) was employed as the reference acid species in the present study. For a comparison of the free energies of p,p-[Ru2(tpy)2LXY]n+, similarly calculated free energies of water (−200642.0 kJ mol−1), acetic acid (−601370.2 kJ mol−1), acetate ion (−600128.4 kJ mol−1), and Cl− (−39729.7 kJ mol−1) were used. The free energy of H+ (−1241.8 kJ mol−1) was calculated from those of acetic acid and acetate ion.

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RESULTS AND DISCUSSION Overall Bridging-Ligand Substitution Reactions As Studied by a 1H NMR Spectral Technique. The 1H NMR spectra showed that Ru2(μ-Cl) is stable in an acidic D2O solution (pD 5 indicated similar reactions, but the different kinetic profiles of the respective species are provided, as shown in Figure 2. The kinetic profiles were analyzed using a sequential reversible reaction model according to eq 6where k1obs, k−1obs, k2obs, and

Ru2(OH)(OH2) is formed as the initial stage (after 5 min) from Ru2(μ-Cl) prior to the formation of Ru2(μ-OH) under the 50 mM NaOD (pD 13.6) conditions (see the red triangles in Figure 2C). However, the kinetic data were reasonably analyzed by adding the direct and irreversible conversion from Ru2(μ-Cl) to Ru2(OH)(OH2) (eq 7) into eq 6: k3obs

OH−, − Cl−, H 2O

Ru 2(μ‐Cl) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ru 2(OH)(OH 2)

(7)

−1

,where k3obs (s ) is the observed rate constant for direct conversion from Ru2(μ-Cl) to Ru2(OH)(OH2). The observed rate constants given by kinetic analysis are listed in Table 1. For the first step from Ru2(μ-Cl) to Ru2(μ-OH), k1obs was almost independent of pD in the range of (1.5−2.8) × 10−4 s−1 from pD 8.5 to 14.2 (200 mM NaOD). A water molecule (not an OH− ion) could be inserted into the Ru center of Ru2(μ-Cl), followed by deprotonation of the inserted water to form Ru2(μOH). Otherwise, the dissociative mechanism with respect to a μ-Cl bridge could be possible to explain the pD-independent k1obs. (However, this is unlikely considering the discussion below on the activation parameters.) k−1obs decreased with a pD increase below pD 12.3, suggesting that the back-reaction from Ru2(μ-OH) to Ru2(μ-Cl) is caused by protonation of the μOH bridge on Ru2(μ-OH) followed by displacement of the formed μ-OH2 by a Cl− ion to form Ru2(μ-Cl). This is supported by the DFT calculation results (vide infra) that the hypothesized p,p-[Ru2(tpy)2L(μ-OH2)]4+ formed by protonation from Ru2(μ-OH) was predicted to be unstable compared to Ru2(μ-OH) and Ru2(μ-Cl). For the second step from Ru2(μ-OH) to Ru2(OH)(OH2), the k2obs values [(0.57−1.8) × 10−6 s−1] are lower than the k1obs values by 2 orders of magnitude and are roughly constant in the range of pD 8.5 to 12.3. The pD-independent k2obs is consistent with the insertion of a water molecule into the Ru center of Ru2(μ-OH) to form Ru2(OH)(OH2). However, k2obs increased linearly above pD 13.4 (30 mM NaOD) to be 1.1 × 10−4 s−1 at

k−2obs (s−1) are the observed rate constants for the forward and backward reactions of the first (Ru2(μ-Cl) to Ru2(μ-OH)) and second (Ru2(μ−OH) to Ru2(OH)(OH2)) steps, respectively. The sequential reversible reaction model was well-fitted to the experimental data below pD 12.3 (Figure 2A,B) but less-fitted to the data above pD 13.4. This is attributed to the fact that

Table 1. Observed Rate Constants of Ligand Substitution Reactions of Ru2(μ-Cl) (2.0 mM) in a 0.1 M Phosphate Buffer Solutions at Room Temperature

a The pD values were based on the measured pH, and the kinetic data were measured in a 0.1 M phosphate buffer. bThe pD values were calculated from the NaOD concentration, and the kinetic data were measured in mixed solutions of an aqueous 1.0 M NaOH solution and D2O (see the Experimental Section). cNo additional Cl− ion source; 4 equiv of Cl− ions (one μ-Cl bridge and three counterions for [Ru2(μ-Cl)]Cl3) versus Ru2(μ-OH) exist in the solution as the Cl− ion source for the k−1 reaction. dk2 and k3 of the second-order rate constants were given as k2 = 5.4 × 10−4 M−1 s−1 and k3 = 3.4 × 10−4 M−1 s−1 from the slopes of plots of k2obs and k3obs versus [NaOD] in Figure 2D.

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Table 2. Thermodynamic Parameters for k1obs, k2obs, and k3obs at 298 K in Weakly Basic (pD 10.7) and Strongly Basic Conditions (50 mM NaOD, pD 13.6)

pD 14.2 (200 mM NaOD; Figure 2D), showing that the second step is pD-dependent and the first order with respect to the OD− ion concertation under strongly basic conditions. This suggests that the second step occurs by nucleophilic attack of an OD− ion to the Ru center of Ru2(μ-OH) to form Ru2(OH)2. (The acid dissociation constant of the aquo ligand on Ru2(OH)(OH2) will be provided below.) k2 of the secondorder rate constant for the second step was given as k2 = 5.4 × 10−4 M−1 s−1 from the linear slope according to k2obs = k2[NaOD]. k−2obs was also pD-independent at pD 8.5−10.7. The calculated equilibrium constant [K2 = k2obs/k−2obs = 3.6− 13.8 (logK2 = 0.56−1.1) at pD 8.5−10.7] is roughly consistent with the value (log K2 = 0.45) given from the equilibrium concentration ratio, considering errors for long-time measurement. Above pD 12.3, because k−2obs can be negligible for increased k2obs, an irreversible reaction model was assumed for kinetic analysis of the second step. For direct conversion from Ru2(μ-Cl) to Ru2(OH)(OH2) (eq 7), k3obs increased linearly from 2.8 × 10−5 to 8.8 × 10−5 s−1 with an OD− concentration increase from 30 to 200 mM (from pD 13.4 to 14.2) in the strongly basic conditions (Figure 2D and Table 1), showing that direct conversion is also pDdependent and first-order with respect to the OD− concentration above pD 13.4. This suggests that direct conversion also occurs by the nucleophilic attack of an OD− ion to the Ru center of Ru2(μ-Cl) to form Ru2(OH)2. However, the plots of k3obs versus the OD− concentration gave a significant intercept in spite of the fact that direct conversion was not observed below pD 12.3. This shows that some sort of pD-independent process for direct conversion is involved under strongly basic conditions above pD 13.4. The linear relationship between k3obs and the OD− concentration in Figure 2D gave k3 = 3.4 × 10−4 M−1 s−1 and k3′ = 2.0 × 10−5 s−1 according to k3obs = k3[NaOD] + k3′, where k3 and k3′ are the second-order rate constant of direct conversion (eq 7) and the first-order rate constant of the pD-independent process for direct conversion, respectively. The k3′ value (2.0 × 10−5 s−1) is close to k2obs = 2.1 × 10−5 s−1 at pD 13.4 for water insertion into Ru2(μ-OH). The pDindependent process could be explained by the insertion of two water molecules into Ru2(μ-Cl) followed by the immediate deprotonation of the inserted water ligands to form Ru2(OH)2. Temperature Dependence of Bridging-Ligand Substitution Reactions. To provide insight into the reaction mechanism, we investigated a temperature dependence of the bridging-ligand substitution reactions in the weakly (pD 10.7) and strongly (50 mM NaOD, pD 13.6) basic solutions. The observed rate constants at various temperatures were given by kinetic analysis as described above (Table S1). The Eyring plots and activation parameters at 298 K for k1obs, k2obs, and k3obs are summarized in Figure 5 and Table 2, respectively. The Eyring plots of k1obs are nearly identical between pD 10.7 (black squares) and 13.6 (red squares) (Figure 5), consistent with the pD-independent k1obs under the pD conditions employed (Table 1). This clearly indicates that the conversion mechanism from Ru2(μ-Cl) to Ru2(μ-OH) is unchanged under the wide pD range employed. The activation entropy values (ΔS⧧ = −44 and −34 J K−1 mol−1, respectively) for k1obs are negative at both pD 10.7 and 13.6, suggesting that the conversion from Ru2(μCl) to Ru2(μ-OH) occurs by an associative mechanism with respect to the inserted water (not an OH− ion), followed by deprotonation of the inserted water, rather than the dissociative mechanism with respect to a μ-Cl bridge. Nevertheless, the activation enthalpy values (ΔH⧧ = 80 and 83 kJ mol−1) were

k1obs ⧧

conditions pD 10.7 50 mM NaOD (pD 13.6)

⧧

ΔG /kJ mol−1

ΔH /kJ mol−1

ΔS⧧ /J K−1 mol−1

−TΔS⧧ /kJ mol−1

93 ± 3 93 ± 7

80 ± 2 83 ± 5

−44 ± 8 −34 ± 16

13 ± 2 10 ± 5

k2obs ⧧

conditions pD 10.7 50 mM NaOD (pD 13.6)

⧧

ΔG /kJ mol−1

ΔH /kJ mol−1

ΔS⧧ /J K−1 mol−1

−TΔS⧧ /kJ mol−1

108 ± 9 97 ± 6

74 ± 6 98 ± 4

−113 ± 20 5.0 ± 14

34 ± 6 −1.5 ± 4.1

ΔG⧧ /kJ mol−1

ΔH⧧ /kJ mol−1

ΔS⧧ /J K−1 mol−1

−TΔS⧧ /kJ mol−1

97 ± 5

71 ± 4

−87 ± 12

26 ± 4

k3obs conditions 50 mM NaOD (pD 13.6)

larger than values of −TΔS⧧ (13 and 10 kJ mol−1) at both pD 10.7 and 13.6, meaning that this conversion is enthalpycontrolled. On the other hand, the Eyring plots for k2obs were quite different between pD 10.7 (black circles) and 13.6 (red circles) in Figure 5, which is in agreement with the mechanism change of the second step from the pD-independent k2obs at pD 8.5− 12.3 to the pD-dependent k2obs above pD 13.4 (vide supra). At pD 10.7, the ΔS⧧ value (−113 J K−1 mol−1) is significantly negative, suggesting that the conversion of Ru2(μ-OH) to Ru2(OH)(OH2) occurs by an associative mechanism with respect to an inserted water molecule. The associative mechanism is consistent with the very slow reaction (10−6 s−1) of the weak nucleophilic attack of the water molecule to the Ru center. In this case, the contribution of the ΔS⧧ factor (−TΔS⧧ = 34 kJ mol−1) to ΔG⧧ (108 kJ mol−1) is considerable compared to the ΔH⧧ factor (74 kJ mol−1). The ΔS⧧ value (5.0 J K−1 mol−1) at pD 13.6 is small, suggesting that the conversion of Ru2(μ-OH) to Ru2(OH)2 occurs rather by the interchange mechanism with faster insertion of an OD− ion to the Ru center due to relatively strong nucleophilic attack of the inserted OD− ion. Thus, the associative mechanism for the insertion of water to the RuII(μ-OH)RuII core of Ru2(μ-OH) at pH 8.5−12.3 alters the interchange mechanism for the insertion of OD− to the core above pH 13.4. The Eyring plots for k3obs at pD 13.6 also gave a linear relationship, and the significantly negative ΔS⧧ (−87 J K−1 mol−1) suggests that the pDdependent direct conversion from Ru2(μ-Cl) to Ru2(OH)2 occurs by the associative mechanism with respect to the inserted OD− ion. Speciation Diagram of a Series of p,p-[RuII2(tpy)2LXY]n+ Complexes. To explore the possibility of deprotonation of the aquo ligand on Ru2(OH)(OH2), the completed reaction solution at pD 14.0 was titrated with sulfuric acid. The resonance at 6.70 ppm shifted to 6.76 ppm (for Ru2(OH)(OH2)) by 0.06 ppm with a pD decrease to be constant below pD 11, as shown in Figure 3A. The change in the titration was completely reversible. This result suggests that deprotonation of the aquo ligand on Ru2(OH)(OH2) occurs to form Ru2(OH)2 at pD 14 (eq 8). 10239

DOI: 10.1021/acs.inorgchem.7b00978 Inorg. Chem. 2017, 56, 10235−10246

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Figure 4. Speciation diagram of a series of ruthenium complexes in a 0.1 M phosphate buffer at room temperature. The solutions were prepared from Ru2(μ-Cl) (2.0 mM) and then heated at 70 °C overnight prior to the measurements in order to accelerate the ligand substitution reactions.

Figure 5. Eyring plots of k1obs (squares), k2obs (circles), and k3obs (triangles) of the bridging-ligand substitution reactions of Ru2-Cl in a 0.1 M phosphate buffer solution (pD 10.7; black) and in a 50 mM NaOD solution (pD 13.6; red). The concentration of Ru2-Cl is 2.0 mM.

Figure 3. (A) 1H NMR spectral change of the solution in pD titration from pD 13.8 (80 mM NaOD) to 8.4 with H2SO4 (2.5 mol L−1) after the completion of bridging-ligand substitution reactions. The total concentration of the ruthenium species is 2.0 mM. The resonances at 6.70 and 6.76 ppm (assigned to five positions of the pyridine moieties on L) due to Ru2(OH)2 and Ru2(OH)(OH2) are indicated by red and blue lines, respectively. (B) pD dependence of the chemical shift of five positions of the pyridine moieties on L. The data at pD 13.4−14.2 were taken in 30−200 mM NaOD solutions. Ka

Ru 2(OH)(OH 2) ⇌ Ru 2(OH)2 + H+

The speciation diagram of a series of complex species was calculated from 1H NMR data after reaching equilibrium at various pD values (Figure 4). Ru2(μ-Cl) exists predominantly in the acidic solution (pD < 5). Ru2(OH)(OH2) is dominant in equilibrium with Ru2(μ-OH) (∼10%) in the neutral and weakly basic conditions (pD 5−12). In the strongly basic conditions, Ru2(OH)(OH2) coexists with Ru2(OH)2 because of the equilibrium in eq 8 with pKa = 12.7. The scheme of the overall bridging-ligand substitution reactions is summarized in Scheme 2. At pD 6−11, Ru2(μ-OH) is slowly formed from Ru2(μ-Cl) [k1obs = (2.4−2.8) × 10−4 s−1; Table 1] as a “kinetic product” by the substitution of μ-Cl with μ-OH (by water insertion followed by deprotonation) and then converts very slowly to Ru2(OH)(OH2) [k2obs = (0.57−1.8) × 10−6 s−1; Table 1] as a “thermodynamic product” with insertion of water into the Ru−OH bond of Ru2(μ-OH). Under the strongly basic conditions (above pH 13), Ru2(OH)2 can be formed rapidly from Ru2(μ-OH) (2.1 × 10−5−1.1 × 10−4 s−1; Table 1) by the insertion of an OH− ion into the Ru−OH bond or from Ru2(μ-Cl) [(2.8−8.8) × 10−5 s−1; Table 1] by the substitution of μ-Cl with two OH− ions. The irreversible conversion from Ru2(μ-OH) to Ru2(OH)2 under the strongly basic conditions is in contrast to the reversible conversion from Ru2(μ-OH) to Ru2(OH)(OH2) involving equilibrium under the neutral and

(8)

The plots of the observed chemical shifts (δobs, ppm) versus pD in Figure 3B were analyzed by eq 9 based on the Henderson− Hasselbalch equation: δobs = δ −

Δδ 1 + 10n(pKa − pD)

(9)

,where n, pKa, δ, and Δδ are the Hill coefficient, the acid dissociation constant of the aquo ligand of Ru2(OH)(OH2), the chemical shift (ppm) of Ru2(OH)(OH2), and the difference of the chemical shift (ppm) between Ru2(OH)(OH2) and Ru2(OH)2, respectively. Equation 9 was well-fitted to the experimental δobs data, supporting the equilibrium in eq 8. The best fitting was given using n = 1.03 ± 0.06, pKa = 12.7 ± 0.03, δ = 6.76 ± 0.001 ppm, and Δδ = 0.068 ± 0.001 ppm. The pKa value (12.7) is much higher compared to the pKa values (9−11) of most ruthenium(II) aquo complexes.3f,20 This will be discussed later in the theoretical investigation. 10240

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Inorganic Chemistry Scheme 2. Reversible Bridging-Ligand Substitution Reactions in an Aqueous Solution among Ru2(μ-Cl), Ru2(μ-OH), Ru2(OH)(OH2), and Ru2(OH)2

K1

weakly basic conditions at room temperature. However, protonation of one of the OH− ligands on Ru2(OH)2 to form Ru2(OH)(OH2) allows Ru2(OH)2 to be returned very slowly to Ru2(μ-OH) via Ru2(OH)(OH2) (Scheme 2). Because it comes back to acidic conditions (pD