Synthesis and electrochemical characterization of self-assembled monolayers of redox-active oxide-bridged triruthenium(III) clusters on Au(111)

www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 473 (1999) 93 – 98

Synthesis and electrochemical characterization of self-assembled monolayers of redox-active oxide-bridged triruthenium(III) clusters on Au(111) Masaaki Abe *, Toshihiro Kondo, Kohei Uosaki, Yoichi Sasaki 1 Di6ision of Chemistry, Graduate School of Science, Hokkaido Uni6ersity, Kita-ku, Sapporo 060 -0810, Japan Received 31 October 1998; received in revised form 12 May 1999; accepted 25 May 1999

Abstract Novel self-assembled monolayers of oxide and acetate-bridged triruthenium(III) complexes have been prepared on Au(111) electrode surface. Parent discrete triruthenium(III) complexes with a terminal disulfide ligand, [Ru3(m3-O)(mCH3CO2)6(L)2(C2PY)]ClO4, where C2PY = {NC5H4CH2NHC(O)(CH2)2S –}2, L = 4-methylpyridine (mpy, 4a) and 1-methylimidazole (MeIm, 4b), were isolated through multistep synthesis. Complex 4a exhibits three reversible redox waves, while complex 4b exhibits two reversible and one irreversible redox waves in (n-C4H9)4NPF6/CH3CN, all ascribed to one-electron redox of the triruthenium cluster core. In aqueous solution containing a 0.1 M supporting electrolyte (NaClO4 or Na2SO4), self-assembled monolayers of 4a and 4b on Au(111) electrodes (4a/Au and 4b/Au) exhibit a single reversible redox wave, ascribed to the Ru3(III,III,III/II,III,III) redox process for the surface-attached clusters. The redox potential and the shape of the cyclic voltammograms of Ru3 cluster modified SAMs were affected by the kind of terminal ligands and supporting electrolytes. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Triruthenium(III) cluster; Self-assembled monolayers; Reversible redox; Au(111) electrode

1. Introduction The design and construction of self-assembled monolayers (SAMs) on atomically-defined metal surfaces have been subjects of extensive research in the last decade [1–3]. Development of monolayers on Au surfaces offers access to highly-ordered molecular structures based on functionalized end groups. The majority of interest in the study of SAMs has been largely based on surface-attached organic and biological molecules as well as simple mononuclear metal complexes [4 – 13]. In order to gain further insights into surface electrochemistry, investigations on SAMs of a much wider variety of molecules should be required. In this context, we were especially interested in construction of novel SAMs based on transition-metal multinuclear com

Presented at the International Symposium on Electrochemistry of Ordered Interfaces, Sapporo, Japan, 11–12 September, 1998. * Corresponding author. Fax: +81-11-706-3447. E-mail address: [email protected] (M. Abe) 1 Also corresponding author.

plexes with well-defined redox and reactivity properties. Oxo-centered, carboxylate-bridged trinuclear ruthenium complexes of the type, [Ru3(m3-O)(mRCO2)6(L)3]n+ (RCO2− = a carboxylate anion, L=a monodentate terminal ligand) [14–33], constitute an attractive family for self-assembly study, since they are known to exhibit reversible multistep redox processes due to the strongly-coupled trimetallic centers. Advantages for the use of the triruthenium clusters as a modifier include their thermal stability in varied oxidation states, i.e. from Ru3(II,II,III) to Ru3(III,IV,IV), known synthetic methodology to modulate the redox potentials finely [18–30], the possible formation of interfacial linearly-arranged oligomers by sequential incorporation of the cluster building blocks [31], and their catalytic activities [32,33]. Only one report on SAMs based on metal cluster complexes is available at present [34]. Herein we wish to report the synthesis and electrochemical behavior of novel (m3-oxo)triruthenium(III) complexes containing a disulfide ligand and their SAMs

0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 9 ) 0 0 2 4 0 - 5

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M. Abe et al. / Journal of Electroanalytical Chemistry 473 (1999) 93–98

on Au electrode surfaces. The effects of terminal ligands of the triruthenium head groups and supporting electrolytes in aqueous solution to the redox response of SAMs are described.

2. Experimental

2.1. Materials Reagents and solvents used were of commercially available reagent-grade quality unless otherwise stated. Acetonitrile used for electrochemical measurements was distilled from CaH2 under Ar. Silica gel (Wako C–300, Wako chemicals) was used for column chromatography. Ultrapure water was obtained by using a Milli-Q water purification system (Yamato, WQ – 500). A gold disk (Tanaka Pure Metals, 99.99%; diameter: 10 mm, thickness: 3 mm) was used for a substrate. Ultrapure N2 (Daido Hokusan, 99.999%) was used for a dearation. Ultrapure N2 (99.99%) and Ar (99.95%) (Daido Hokusan) were used. The ligand C2PY, [NC5H4CH2NHC(O)(CH2)2S – ]2, was prepared according to the literature procedure [35].

2.2. Preparation of new complexes 2.2.1. [Ru3O(CH3CO2)6(THF)2(CO)] ·2THF (1) This complex was obtained by recrystallization of [Ru3O(CH3CO2)6(CO)(S)2] (S=CH3OH and/or H2O) [24,29] from THF+n-pentane at 0°C. Anal. Calc. for [Ru3O(CH3CO2)6(THF)2(CO)]·2THF: C, 35.19; H, 5.09. Found: C, 34.88; H, 5.01. 2.2.2. [Ru3O(CH3CO2)6(mpy)2(CO)] (2a) Complex 1 (305 mg, 0.308 mmol) was dissolved in an Ar-purged CH3OH (60 cm3). To this was added dropwise 4-methylpyridine (0.54 g, 5.80 mmol). The mixture was refluxed for 30 min. A deep blue solid thus appeared was collected by filtration after cooling and crystallized from CH2Cl2 +n-hexane at room temperature. A crystalline solid 2a was collected, washed with n-hexane and n-pentane and dried for 4 h in vacuo. Yield, 180 mg (65.8%). Anal. Calc. for [Ru3O(CH3CO2)6(mpy)2(CO)]·H2O: C, 33.15; H, 3.78; N, 3.09. Found: C, 33.03; H, 3.56; N, 3.25. 2.2.3. [Ru3O(CH3CO2)6(mpy)2(H2O)]ClO4 (3a) A CH3OH solution containing Br2 (ca. 0.1 M) was added dropwise to a CH2Cl2 solution (20 cm3) containing complex 2a (489 mg, 0.566 mmol) until an absorption peak at 679 nm developed. The solvent was removed and the residue was dried for 3 h in vacuo. The residue was dissolved in CH3OH (20 cm3) and the solution was stirred at room temperature for 1 h and then at 50°C for 15 min. After the solution was cooled,

NaClO4 (100 mg, 0.817 mmol) dissolved in CH3OH (3 cm3) was added. The solution was reduced to a half volume in a rotary evaporator and diethyl ether (5 cm3) was added dropwise, which was stored at 5°C overnight. A crystalline solid 3a was filtered off, washed with diethyl ether (20 cm3), and dried in air. Yield, 295 mg (52.6%). Anal. Calc. for [Ru3O(CH3CO2)6(mpy)2(H2O)]ClO4·H2O: C, 28.97; H, 3.65; N, 2.81. Found: C, 29.15; H, 3.56; N, 2.82.

2.2.4. [Ru3O(CH3CO2)6(mpy)2(C2PY)]ClO4 (4a) To a CH2Cl2 solution (15 cm3) of 3a (102 mg, 0.101 mmol) was added a CH3OH solution (5 cm3) containing the ligand C2PY (119 mg, 0.302 mmol). The mixture was stirred at room temperature for 12 h. The solvent was removed in a rotary evaporator and the blue residue was thoroughly dried by pumping. The solid was purified by column chromatography (silica gel, 10% CH3OH/CH2Cl2 as an eluent). The main fraction was collected and the solvent was removed. The residue was redissolved in CH3OH (2 cm3) containing NaClO4 (5 mg, 0.041 mmol). An addition of diethyl ether gave a blue crystalline solid 4a, which was collected, washed with water, cold CH3OH, and diethyl ether, and dried in vacuo. Yield, 40 mg (26.9%). Anal. Calc. for [Ru3O(CH3CO2)6(mpy)2(C2PY)]ClO4·NaClO4 (C42H54Cl2N6NaO23Ru3S2): C, 34.27; H, 3.70; N, 5.71. Found: C, 34.49; H, 3.77; N, 5.31. FAB MS (NBA as a matrix): m/z =1251 ([M–ClO4– ] + ). 2.2.5. [Ru3O(CH3CO2)6(MeIm)2(CO)] (2b) To a CH2Cl2 + CH3OH solution (1/1 v/v, 20 cm3) of 1 (300 mg, 0.327 mmol) was added a CH3OH solution (5 cm3) containing MeIm (108 mg, 1.315 mmol). The mixture was stirred for 12 h at room temperature and the solvent was removed in a rotary evaporator. The residue obtained was dissolved in CH2Cl2 (20 cm3). Addition of n-pentane (30 cm3) and storage at 0°C afforded a blackish blue crystalline solid 2b, which was collected, washed with n-pentane (50 cm3), and dried in vacuo. Yield, 227 mg (80.2%). Anal. Calc. for [Ru3O(CH3CO2)6(MeIm)2(CO)]·0.5THF: C, 30.63; H, 3.81; N, 6.08. Found: C, 30.68; H, 4.01; N, 6.08. 2.2.6. [Ru3O(CH3CO2)6(MeIm)2(CH3OH)]ClO4 (3b) Complex 2b (150 mg, 0.173 mmol) was dissolved in CH3OH+ CH2Cl2 solution (5:2 v/v, 70 cm3) at room temperature. To this, a CH3OH solution (5 cm3) containing AgClO4 (72 mg, 0.347 mmol) was added dropwise, and the mixture was stirred for 1 h. After the deposited white solid was removed by passing through Celite, the filtrate was evaporated to dryness in a rotary evaporator. The resultant blue solid was dissolved in CH3OH (25 cm3). Addition of diethyl ether (35 cm3) and storage of the mixture at 0°C for 12 h gave a dark blue solid 3b. Yield, 132 mg (78.5%). Anal. Calc. for

M. Abe et al. / Journal of Electroanalytical Chemistry 473 (1999) 93–98

[Ru3O(CH3CO2)6(MeIm)2(CH3OH)]ClO4: C, 26.03; H, 3.54; N, 5.78. Found: C, 25.86; H, 3.53; N, 5.83.

2.2.7. [Ru3O(CH3CO2)6(MeIm)2(C2PY)]ClO4 (4b) A mixture of 3b (50 mg, 0.052 mmol) and the ligand C2PY (55 mg, 0.141 mmol) was dissolved in CH3OH (20 cm3) and it was stirred for 21 h at room temperature. The solvent was removed in a rotary evaporator, and the resultant blue solid was purified by column chromatography (silica gel, 10% CH3OH + CH2Cl2). The main fraction (the second band) was collected and evaporated to dryness. The residue was dissolved in CH3OH (8 cm3) containing NaClO4 (4.5 mg, 0.037 mmol) and diethyl ether (18 cm3) was layered. Standing the mixture at 0°C for 3 days afforded a crystalline blue solid 4b, which was collected by filtration, washed with diethyl ether (10 cm3), and dried in vacuo. Yield, 24.6 mg (35.9%). Anal. Calc. for [Ru3O(CH3CO2)6(MeIm)2(C2PY)]ClO4 (C38H52ClN8O19Ru3S2): C, 33.47; H, 4.14; N, 8.22; Cl, 2.60; S, 4.70. Found: C, 33.50; H, 3.97; N, 8.59; Cl, 2.77; S, 4.70. FAB MS: m/z =1229 [M – (ClO4− )] + , 1065 [M – (ClO4− ) – 2(MeIm)] + .

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of the substrate touched the electrolyte solution with a meniscus. A platinum wire and an Ag AgCl NaCl (sat) were used as counter and reference electrodes, respectively. The pH of the solution was adjusted by using McIlvaine buffer solution (pH 2–8) containing 0.1 M NaClO4 as an electrolyte or HClO4 (pH 1.0). The electrolyte solution was deaerated by bubbling pure N2 gas for 30 min prior to electrochemical measurements. The electrode potential was controlled by using a potentiostat (Hokuto Denko, HA-301) and an external potential was provided by using a function generator (Hokuto Denko, HB-115). Cyclic voltammograms (CVs) were recorded on an X–Y recorder (Rika Denki, RW-11T). All the measurements were carried out at room temperature. Safety Note. Caution! While we have experienced no problems in handling perchlorate salts, these should be handled with extreme caution with small quantities due to the potential for explosion.

3. Results and discussion

2.3. Electrochemistry for discrete complexes Cyclic voltammograms and differential-pulse voltammograms of discrete triruthenium complexes were obtained with a Hokuto HA-501G potentiostat and a Hokuto HB-105 function generator equipped with a Graphtec WX2400 X-Y recorder. The working electrode, the auxiliary electrode, and a reference electrode were glassy carbon, platinum coil, and Ag AgCl NaCl (sat), respectively. All the electrochemical measurements were achieved at room temperature under Ar atmosphere in CH3CN solution containing 0.1 M tetrabutylammonium hexafluorophosphate, (n-C4H9)4NPF6, with a complex concentration of 1 mM. Under these conditions, a half-wave potential of the ferrocene/ferrocenium couple (Fc/Fc + ) was observed at +0.435 V versus Ag AgCl with the peak-topeak separation (DEp) of 60 mV.

2.4. Electrochemistry of SAMs modified gold electrodes The gold disk substrate (f =10 mm) was prepared by the same procedures reported previously [36]. Its surface was atomically flat with a (111) ordered structure [37]. The roughness factor of the surface was estimated from the charge for the reduction of gold oxide to be less than 1.2, which means that the apparent surface area is ca. 1.0 cm2. Just before the surface modification, the gold was washed with pure water, annealed in a hydrogen flame, and quenched with pure water. The SAM modified gold samples were placed in an electrode holder made from Kel-F so that only one face

3.1. Synthesis of triruthenium complexes with the disulfide ligand Our preparative strategy to construct monolayers of triruthenium complexes is based on the reactions of Au(111) surfaces with isolated discrete triruthenium(III) analogs having a disulfide ligand C2PY [35]. The ligand C2PY contains a disulfide moiety along with two pyridyl groups at the ends which can coordinate to metal center(s). The triruthenium(III) complexes with the ligand C2PY, 4a and 4b, were prepared through multiple steps (see Experimental) by analogous methods reported previously [18]. The new complexes were characterized by elemental analysis, FAB MS spectrometry, UV–vis, IR, and 1H-NMR spectroscopic methods [38], and electrochemistry (vide supra). The disulfide complexes 4a and 4b are soluble in common organic solvents and stable in solution over a month at room temperature (based on the invariable UV–vis and 1HNMR spectra of the complexes in alcoholic solvents and acetonitrile).

3.2. Electrochemistry of discrete molecules Like reported oxo-centered triruthenium complexes [18–24], 4a and 4b display three consecutive one-electron redox waves in cyclic voltammograms over the range between − 1.8 and +1.2 V versus Ag AgCl (0.1 M (n-C4H9)4NPF6 + CH3CN). The cyclic and differential-pulse voltammograms for complex 4a are shown in Fig. 1 and redox potentials for 4a and 4b are summa-

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Fig. 1. Cyclic voltammogram (CV) and differential-pulse voltammogram (DPV) of complex 4a in 0.1 M (n-C4H9)4NPF6 + CH3CN. Scan rate = 200 mV s − 1 for CV and 20 mV s − 1 for DPV.

rized in Table 1. All the waves correspond to one-electron redox processes (confirmed by the identical current intensity of each wave to one-electron redox wave of ferrocene of the same concentration) and they are ascribed to the ‘‘Ru3O’’ cluster-based redox processes, Ru3(III,III,IV/III,III,III), Ru3(III,III,III/II,III,III), and Ru3(II,III,III/II,II,III). Complex 4b also shows three redox processes in the same region but the most negative one, Ru3(II,III,III/II,II,III), appears to be irreversible. The redox potentials of two reversible waves for the MeIm-coordinated complex 4b are negatively shifted as compared to the corresponding waves of the mpy-coordinated complex 4a, reflecting the higher electron-donating ability of MeIm than mpy (pKa values: 7.3 for MeIm and 6.0 for mpy). No redox waves due to the coordinated disulfide ligand are found in this potential range.

3.3. Preparation and electrochemical response of gold SAMs Densely-packed SAMs of triruthenium(III) clusters were formed by soaking freshly prepared Au(111) subTable 1 Electrochemical data for the discrete triruthenium(III) complexes 4a and 4b in 0.1 M [(n-C4H9)4N]PF6+CH3CN Complex

4a 4b

E1/2 a/V (DEp/mV) b E(1)

E(2)

E(3)

+1.00 (70) +0.93 (70)

−0.05 (70) −0.17 (70)

−1.34 (80) −1.57 c

The E1/2 values are reported with respect to Ag AgCl. The assignments are as follows. E(1): Ru3(III,III,IV/III,III,III); E(2): Ru3(III,III,III/II,III,III); E(3): Ru3(II,III,III/II,II,III). b Peak-to-peak separation. c Cathodic peak potential for irreversible process. a

Fig. 2. CVs of the monolayers 4a/Au in a 0.1 M NaClO4 aqueous solution (a) and in a 0.1 M Na2SO4 aqueous solution (b). The solution pH is adjusted to 4.5 in both cases. Curve 1: 6 = 100 mV s − 1; 2: 6 =200 mV s − 1; 3: 6 =300 mV s − 1; 4: 6 =400 mV s − 1; 5: 6 = 500 mV s − 1. (c) Plots of anodic peak intensities (Ipa) versus scan rates (6) of CVs for the monolayers 4a/Au, based on (a) and (b).

strates into 5 cm3 of methanol +ethanol solutions (2:3, v/v) containing the appropriate disulfide complexes 4a or 4b ([complex]=1 mM) for two weeks at 20°C (vide supra). As shown in Fig. 2, parts a and b, each monolayer shows a well-defined reversible redox wave in aqueous solution containing a 0.1 M supporting electrolyte (NaClO4 or Na2SO4) over the potential range between − 0.25 and +0.25 V versus Ag AgCl. Table 2 summarizes surface electrochemical data. The monolayers 4a/ Au display a reversible one-electron redox wave at E1/2 = − 0.055 V with DEp (peak-to-peak separation)= 30 mV and W1/2 (the full width at half maxima)=165 mV in 0.1 M NaClO4 aq. (Fig. 2, part a), while in 0.1

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M Na2SO4 aq. (Fig. 2, part b) the corresponding redox occurs at the more positive potential E1/2 = +0.030 V (DEp =20 mV) with a much larger W1/2 (255 mV). On the basis of redox data obtained for the discrete complexes (Table 1), the waves observed for the surface system are assignable to the cluster-based Ru3(III,III,III/II,III,III) redox process. The surface coverages of the Ru3 head groups in 4a/Au and 4b/Au are both 5 × 1013 molecules cm − 2 (the Ru3 cluster headgroup) on the basis of the observed one-electron redox waves in cyclic voltammograms of SAMs. The significant stability of the monolayers was verified by repeated scans with no sign of either Au – S cleaveage or dissociation of cluster units from the ligand moiety on surface as long as the potential was maintained between + 0.4 and −0.5 V versus Ag AgCl. Further observation of cluster-based redox waves, i.e. Ru3(II,III,III/ II,II,III) and Ru3(III,III,IV/III,III,III), was hampered by gold oxidation and hydrogen evolution taking place in the positive and the negative potential range, respectively, together with anodic and cathodic cleaveage of Au – S bonds. Monolayers obtained through shorter immersing time (e.g., 1 week) exhibit a single redox wave but with much smaller intensity and broader shape, indicating that it takes at least 2 weeks for the formation of densely-packed SAMs 4a/Au and 4b/Au. This is a much longer time than that required for the formation of the SAMs of thiol/disulfide modifier molecules. We note here two possible reasons for the longer immersion times for the present SAMs. One is that complexes 4a and 4b contain a disulfide group as a binding group, which generally requires a longer time to form SAMs than a thiol group [1,39]. Another is that the complexes possess a bulky and mono-charged cationic head group, {Ru3(O)(CH3CO2)6(L)2} + (L = mpy or MeIm); the steric hindrance and electrostatic interactions between the molecules should be significant and may affect the SAMs formation. Table 2 Electrochemical data of Au SAMs of the triruthenium(III) complexes 4a/Au and 4b/Au in aqueous solution a SAMs

Supporting electrolyte b

E1/2 c/V

DEp d/mV

W1/2 e/mV

4a/Au

NaClO4 Na2SO4 NaClO4

−0.055 +0.030 −0.020

30 20 0

165 255 180

4b/Au

a The solution pH was adjusted to 4.5 by using a phosphate buffered solution. The data collected at a scan rate of 500 mV s−1 at room temperature are summarized. The redox wave reported here is assigned to the Ru3(III,III,III/II,III,III) couple. b The concentration of the supporting electrolyte is 0.1 M. c The E1/2 values are reported with respect to Ag AgCl. d Peak-to-peak separation. e Half-width of the current maxima.

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Fig. 3. A proposed structure of monolayers of the triruthenium(III) complexes on gold. L = mpy or MeIm. Dotted lines indicate possible hydrogen-bonding interactions between amide groups of the neighboring ligands.

The reductive desorption of Au–S bonds for 4a/Au were observed as a single relatively broad cathodic wave centered at −0.62 V without any indication of isolated domains of the C2PY ligand itself. The reductive desorption waves of Au–S bonds for self-assembled monolayers of the C2PY ligand, prepared by immersing an Au electrode into a CH3OH solution of C2PY (1 mM) for 12 h, was detected at − 0.74 V as a sharp cathodic peak under the same experimental conditions. Shown in Fig. 2, part c, is the linear relationship of anodic current peak intensities (Ipa) with scan rates (6, 100–500 mV s − 1) for 4a/Au, indicating that the redox waves come from surface-immobilized species. The reversibility and the redox potentials do not show any dependence on scan rates (6= 100 to 500 mV s − 1) and the solution pH (pH 4.5 and 7.0). Anion-dependent redox response observed for the two anions, ClO4− and SO24 − (Table 2), is analogous to the earlier reports on ferrocene-terminated SAMs [4,40] and indicative of stronger ion pairing affinity of the monopositive Ru3(III,III,III) head groups with ClO4− than SO24 − . A reversible redox wave is also obtained for 4b/Au at nearly the same potential as 4a/Au within experimental errors (Table 2). For the particular complexes examined here, the terminal-ligand effect upon redox potentials seems less significant for the SAMs (the potential difference between 4a/Au and 4b/Au is 35 mV) than that for the parent disulfide complexes 4a and 4b in CH3CN (120 mV). Presented in Fig. 3 is an idealized view of the monolayers, including possible NH···O hydrogen bonding interactions between amide groups in the neighboring ligand moieties. The 1/1 cluster/ligand stoichiometry on

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surfaces is supported by comparison of peak areas of redox waves due to the cluster core with those due to the reductive desorption of Au – S bonds [41]. Such hydrogen-bonding interactions have been found for some peptide-containing alkanethiol SAMs [42–46]. We believe that the significant stability observed for 4a/Au and 4b/Au comes not only from the substitutionally-inert nature of Ru(III) but also from the denselypacked, mixed cluster/ligand structures supported by the amide-based interchain interactions on surface.

4. Conclusions The present system offers a novel example of SAMs based on redox-active multinuclear metal complexes. A reversible one-electron redox wave ascribed to the Ru3(III,III,III/II,III,III) process was observed for surface-bound triruthenium(III) units, whose redox potentials were substantially influenced by the kind of anions present in solution. Our synthetic methodology, simply based upon coordination of a pyridyl pendant of a disulfide ligand to one of the metal centers in the clusters, can be readily extended to novel SAMs based on inorganic/organometallic clusters which are of interest for their electrochemical, spectroscopic, and reactivity properties.

Acknowledgements This work was supported by Grant-in-Aids for Scientific Research on Priority Area of ‘Electrochemistry of Ordered Interfaces’ (Nos. 09554037 and 09237106), and No. 09740483 from the Ministry of Education, Science, Sports, and Culture, Japan. MA acknowledges Hocscitec Foundation for financial support. The authors thank Dr. S. Ye for valuable discussions.

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