Determination of the Valence State of Diruthenium Moiety Using

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Determination of the Valence State of Diruthenium Moiety Using Redox Reactions and Surface-Enhanced Raman Scattering: Application in Heterometal Extended Metal-Atom Chain Diruthenium Nickel Complexes Bo-Han Wu,† Jyun-You Lin,† Kuan-Yi Ho,† Min-Jie Huang,† Shao-An Hua,‡ Ming-Chuan Cheng,§ Yaw-Wen Yang,∥ Shie-Ming Peng,‡,§ Chun-hsien Chen,*,‡ and I-Chia Chen*,† †

Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30013, Republic of China Department of Chemistry, National Taiwan University. Taipei, Taiwan 10617, Republic of China § Institute of Chemistry, Academia Sinica, Taipei, Taiwan 11529, United States ∥ National Synchrotron Radiation Research Center, Hsinchu, Taiwan 30076, Republic of China ‡

S Supporting Information *

ABSTRACT: Redox reaction was photoinduced on metal nanoparticle (NP) with diruthenium complexes absorbing on the surface. Surface enhanced Raman scattering (SERS) was employed to detect the vibrational wavenumber of the Ru−Ru stretching mode νRu−Ru,str. During reaction, the local environment remains roughly unaltered; the variation of bonding strength of Ru−Ru related to its valence state can be measured. The heterometal extended metal atom chain (EMAC) complexes diruthenium nickel dipyridylamide [Ru2Ni(dpa)4Cl2]0,1+ (dpa = dipyridylamide) display the Raman νRu−Ru,str at 327 (broad) and 333 cm−1 in solid form for the neutral and oxidized complexes, respectively, but red-shift to 312 cm−1 on gold NP substrate and to 328 cm−1 on AgNP. According to the oxidation potentials from voltammogram, and the Raman shifts obtained for the model complex Ru2(OAc)4Cl we assign the diruthenium core with charge +5/+4 and +6, respectively for the neutral and oxidized diruthenium nickel dipyridylamide in solid form. On substrate AuNP the hot electrons produced to fill in the antibonding orbital, the Ru24+ π*4 core with weak Ru−Ru bonding was formed and on AgNP the oxidized complex was reduced to neutral form. For pentanuclear EMAC [Ni−Ru2-Ni2(tpda)4(NCS)2] (tpda = tripyridyldiamide) νRu−Ru,str is at 337 cm−1 and SERS band peaked at 328 cm−1. We assign the Ru26+ core in solid and reduction by NPs to form Ru25+.

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INTRODUCTION The extended metal atom chains (EMACs) with polypyridylamine ligands coordinated helically have attracted much attention because of their great electric conductivity, magnetic properties, and potential applications as molecular wires.1,2 Heterometal EMACs complexes were synthesized, for example, [Ru2M(dpa)4Cl2] (Hdpa = dipyridylamine, M = Cu, Ni),3 penta-core [NiRu2Ni2(tpda)4(NCS)2] (H2tpda= tripyridyldiamine),4 and FePtFe(dpa)4Cl2, the Ma-Mb-Ma type complexes.5 For the character and current development in both homo- and heterometal EMACs, refer to the review paper by Hua et al.5 The structures of heteroruthenium nickel complexes [Ru2Ni(dpa)4Cl2] 1 and [NiRu2Ni2(tpda)4(NCS)2] 2 are shown in Scheme 1. Substituted metals in the metal ion chains yield varied electric and magnetic properties; for example, complex 2

exhibits the unusual negative differential resistance property in the I−V curves.4 Replacing the nickel atoms in pentanickel EMACs with diruthenium ion improves the electric conductivity. Heterometal EMACs with mixed-metal framework are inclined to have a crystallographic disorder. During the refinement process, these disordered ions are arbitrarily fixed to the same position to attain the crystal structures. Because of the uncertain bond length between metals and complexity of this system, the assignment of charge number of individual metal ions is a problematic task. In 1, the near-IR (NIR) band at 880 nm in the absorption curve is attributed to an electronic transition associated with the Ru2 moiety that is generally assigned, by Miskowski et al.6 and Ren et al.,7,8 to δ−δ* transition mixed with a partial character of the transition between metal centers and ligands.6−8 Besides, the assignment of mixed-valent structure Ru25+ (RuII−RuIII) in 1 also agrees well with the magnetic moment measured.4 In 2, the NIR band at 868 nm has an extinction coefficient 3600 M−1 cm−1 and the results of a magnetic moment measurement, and hence it is

Scheme 1. Structures of 1 and 2

Received: August 18, 2016

© XXXX American Chemical Society

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DOI: 10.1021/acs.jpcc.6b08351 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

sample region was set at ∼10 (solid samples) and ∼1 mW (SERS samples). The scattered signal passing through an edge filter, optical fiber, and monochromator (length, 0.5 m; grating 600 grooves/mm) was recorded with a liquid-nitrogen-cooled charge-coupled device (CCD) detector. IR absorption spectra in the far-infrared region 150−500 cm−1 were recorded with an infrared spectrometer (Bruker, NTHU Instrument Center). Solid samples and dry NPs with EMACs were mixed with CsI at a ratio of 1:30 to 1:50 for the low-wavenumber range to obtain sufficient absorbance. Experiments of XPS were measured using the wide-range spherical grating monochromator beamline of NSRRC (BL24A1, the National Synchrotron Radiation Research Center, Hsinchu, Taiwan). The photon energy of 620 eV was utilized to acquire Ru 3d spectra with the energy resolution of ∼0.3 eV. Thin-film samples were prepared by drop-casting EMAC-containing solution onto a silicon wafer without treatment to remove negative oxide. No charging problem was noticed during the course of measurements. The binding energy was calibrated with reference to the bulk Si 2p3/2 core level at 99.3 eV. Measurements of Ni 2p of XPS were recorded for thin film samples of complex 1 and 2.

assigned to bear a Ru25+ moiety. Although the synchrotronbased X-ray photoelectron spectroscopy (XPS) data display a single XPS peak centered 281.2 eV with narrow width 1.2 eV in the Ru 3d5/2 region, the width and shape of the peak allow no further deconvolution, indicating indistinguishable charge numbers for the two Ru ions.4 This is somehow inconsistent with the mixed-valence state assignment. Hence, determination on the valence state of the diruthenium core requires more investigation in addition to the spectroscopic techniques used. For diruthenium complexes, Ru24+ is expected to have a weaker Ru−Ru bonding than that of Ru25+ because the former has an electron configuration σ2π4δ2(δ*,π*)4, an additional electron in the antibonding orbital.9 The Raman shift of the Ru−Ru stretching band νRu−Ru,str should reveal the bonding strength. However, the νRu−Ru,str appears to be sensitive to ligand and local environments. Miskowski et al. reported that the Ru−Ru bond length in [Ru(carboxylate)4]+ lies in the ranges of 2.267 to 2.286 Å, and the bond order is 2.5.6 They reported the vibrational wavenumber of νRu−Ru,str is in the range of 321−336 cm−1 for complexes in solid form. Chisholm et al. reported that the νRu−Ru,str in Ru2(O2CR)4 (Ru24+ core) lies in the range 332−347 cm−1.10 Interaction with solvent molecule might be encompassed in these complexes to display these diverse results. Hence, elucidation the bond strength of Ru−Ru using Raman spectroscopy requires some extra amendment. Coinage nanoparticles (NPs) are known to exert catalytic effects. Under radiation with plasmon resonance light, hot electrons and holes are, respectively, generated above and below the Fermi level of the metal NPs.11−16 This results in redox reactions with the absorbing molecules. Alternatively, Tanabe et al. suggested to use 3-mercaptopropionic acid (3MPA) to cover the surface of metal NPs to prevent the hot electrons and holes from reacting with the adsorbing complexes.17 In the present work we use the coinage NP in combination with surface-enhanced Raman scattering (SERS) technique to study the redox reactions of ruthenium metal string complexes. For these complexes, redox reactions occur only on the metal ions. In this way, the local environment essentially remains unperturbed and the bonding strength as well as the valence state of diruthenium moiety can be determined from the Raman frequency.

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RESULTS AND DISCUSSION Ru2(OAc)4Cl. We used complex Ru2(OAc)4Cl 3 (Ru25+ core) as a model compound. Figure 1 shows its Raman spectra

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EXPERIMENTAL SECTION EMAC samples were prepared as the methods described elsewhere.3,4 Diruthenium complexes Ru2(OAc)4Cl and [Ru2(OAc)3bpnp]PF6 were synthesized and purified to obtain the crystal forms. We prepared the AuNPs with citrate reduction of HAuCl4 to yield gold NPs diameter ∼50 nm.18 AgNPs (∼55 nm) were prepared with citrate reduction of AgNO3 of method Lee and Meisel.19 In 5 mL of Au or Ag NP solution, 50 μL of 3-mercaptopropionic (3-MPA, 12.5 mM) was added with stirring. Then, the mixture solution was stirred for 3 h and centrifuged for 15 min (3000 rpm) to yield the thiol-protected NPs. These protected NPs were put on a clean glass surface, and a few drops of EMACs in ethanol were added; then, they were allowed to dry for SERS measurements. Samples with unprotected NPs in aqueous solution were used for SERS measurements. The Raman spectra were recorded in a backscattering geometry employing an objective lens (10×). The He−Ne laser operated with a red light at 632.8 nm served as the excitation light source for AuNP substrates. Diodepumped Nd:YAG lasers (Photop Suwtech) provided the green light at 532 nm for AgNP substrates. The laser power at the

Figure 1. Raman and SERS curves of [Ru2(OAc)4Cl] 3 in crystal form (a) and SERS on AgNP (b), thiol-protected AgNP with 532 nm excitation (c), AuNPs (d), and thiol-protected AuNPs with 632 nm excitation (e).

in solid form and SERS spectra when complexes were attached on either AuNP or AgNP substrates. The Raman band at 370 cm−1 is assigned to the Ru−O stretching mode νRu−O,str and the intense band at 327 cm−1 is assigned to νRu−Ru,str. In SERS, the position of the νRu−O,str band is within ±1 cm−1 from the normal Raman bands, but the νRu−Ru,str red-shifts to 321 cm−1 on AuNPs and blue-shifts to 337 cm−1 on AgNPs. With thiol B

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The Journal of Physical Chemistry C protection, this band is back to the normal Raman position ∼328 cm−1. The cyclic voltammetry of AuNPs and AgNPs yields the oxidation potential ∼1.33 and ∼0.41 V, respectively (vs EAg/AgCl).20,21 Complex 3 has an oxidation potential of ∼0.5 V and a reduction potential between 0.0 and −0.8 V versus Ag/ AgCl estimated from diruthenium complexes with similar structures.22−27 Accordingly, 3 can be reduced by AuNPs and the hot electron filled in the antibonding orbital weakening the Ru−Ru bonding to form Ru24+, whereas on AgNPs an oxidation reaction occurred and Ru26+ core was formed with a strong Ru−Ru bonding, bond order = 3. Consequently, we attained νRu−Ru,str ≈ 321, 327, and 337 cm−1 for complex with Ru2n+ core n = 4, 5, and 6, respectively. [Ru2(OAc)3bpnp]PF6. To further confirm our assignment, we obtained the Raman band νRu−Ru,str = ∼320 cm−1 (spectrum shown in Figure 2) in [Ru2(OAc)3bpnp]PF6 4 (bpnp =2,7-

Figure 3. Raman and SERS spectra of Ru2Ni(dpa)4Cl2 1 and [Ru2Ni(dpa)4Cl2](PF6) 1′ in crystal form (a,f) and SERS on substrate AgNps (b,g), thiol-protected AgNPs (c,h), AuNPs (d,i), and thiolprotected AuNPs (e,j).

the same position 312 cm−1 on AuNPs and moves to 328 cm−1 on AgNPs. Again, both bands are at the normal Raman positions with thiol-protect NPs. Both intensity and band positions of ligand dpa in the large wavenumber region vary only slightly on metal NPs. On the basis of the guideline obtained from the model compound 3, the diruthenium moiety in 1 in solid is assigned to bear mixed Ru24+ and Ru25+ forms with population ratio roughly 2 for our sample. The Ni XPS displays two 2p3/2 peaks at 855.8 and 857.8 eV for 1 (Figure S4), assigned to Ni1+ and Ni2+, respectively with a greater portion for the later species. This agrees with the present finding for the ion chain (Ru− Ru−Ni)6+. For 1′ in solid form, the feature of the νRu−Ru,str 333 cm−1 band is sharp and the Raman shift lies between the wavenumbers of Ru25+ and Ru26+. With AgNPs, 1 was unreacted but 1′ was reduced to 1 with Ru25+ core, position 328 cm−1. Accordingly, the valence state of 1′ in solid should be 6+. The electron configuration of Ru25+ is (δ*π*)3. Taking one electron from π* or δ* would yield Ru26+ π*δ* or π*2. Hence, 1′ possibly has a Ru26+ core π*2 with weaker Ru−Ru bonding strength, whereas complex 3 has π*δ* when it was oxidized by AgNPs. On AuNPs, both 1 and 1′ were reduced to produce Ru24+ but with weaker Ru−Ru bond strength than the expected 320 cm−1. Ru24+ has an electron configuration of π*3δ*1, π*2δ*2, or π*4. Cotton et al. reported that the Ru2[(pCH3C6H4)NNN(p-CH3C6H4)]4 and Ru2[(p-CH3C6H4)NC(H)N(p-CH3C6H4)]4 have Ru24+ π*4 with long Ru−Ru bond length 2.4 Å.30,31 Similarly, complexes with ligand RNNNR type tend to have energy of E(δ*) > E(π*);30,31 the reduced forms of 1 and 1′ are consequently assigned to have Ru24+ π*4. The spectra of Raman and SERS of [Ru2Ni(dpa)4(NCS)2] 1a are shown in Figure S5. Axial ligand isothiocyanide NCS has strong σ-donating ability, slightly weakening the Ru−Ru bond to yield νRu−Ru,str = 325 cm−1 in solid-state Raman spectra. Similar to 1, this band is a broad but more symmetric shape, indicating more Ru25+ than Ru24+ form existing. The band position red-shifts to 311 cm−1 on AuNP substrate and no variation with AgNP in the SERS measurements. Similarly, 1a

Figure 2. Raman and SERS curves of [Ru2(OAc)3bpnp]PF6 4 in crystal form; SERS on AgNPs and AuNPs.

bis(2-pyridyl)-1,8-naphthyridine) with Ru24+ core-electron configuration δ*2π*2 or π*3δ*1.28 Nevertheless, complex 4 is harder to be either reduced or oxidized by NPs. We have the position of the νRu−Ru,str band virtually unaltered in SERS measurements. Binamira-Soriaga et al. reported that 4 has Ru− Ru bond length 2.28 Å and is expected to have two unpaired electron from the magnetic measurements.28 It possesses a high one-electron oxidation state 1.34 V, confirming our finding. Hence, this complex cannot serve as a model compound for determination of the valence states of Ru2 using NPs. [Ru2Ni(dpa)4Cl2]0,1+. The Raman and SERS spectra of 1 and its oxidized form [Ru2Ni(dpa)4Cl2]+(PF6)− 1′ are shown in Figure 3 (full spectra in Figure S2). The infrared curves of 1 in comparison with the Raman curves are displayed in the Supporting Information, Figure S3. The detailed assignments of vibrational bands are reported by Chiu et al.29 Comparing these data, the spectra of 1 and 1′ obtained under the same conditions are alike, but those obtained in solid form and in SERS for the same complex are dissimilar. The Raman band νRu−Ru,str for 1 and 1′ is the intense peak at 327 and 333 cm−1, respectively. This band exhibits the most intensity similar to that in 3. For 1′ this band displays sharp features but is broad for 1. After deconvolution of this broad peak into two bands with fixed peak positions 320 and 327 cm−1, we attain an area ratio 2. In SERS, the νRu−Ru,str of 1 is unaltered on AgNP substrate but red-shifts to 312 cm−1 with sharp feature on substrate AuNP. The IR band of 1 on the surface of AuNP also appears at the same position. For 1′ the νRu−Ru,str also shifts to C

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Figure 4. Raman (a,d) and SERS (b,e) AuNP and (c,f) AgNP curves of NiRu2Ni2(tpda)4(NCS)2 2 and [NiRu2Ni2(tpda)4(NCS)2 ][PF6] 2′. Some vibrational mode assignments are given. Δ(ring−ring) denotes the in-plane twist of pyridyl rings.

Table 1. List of Band Position νRu−Ru‑str and Valence State of Ru2 and Ni Obtained in Solid and on AuNP and AgNP Substrates complex 1 1′ 1a 2 2′ 3 4

solid form 327 333 325 337 335 327 320

AuNP substrate

(broad) Ru24+ Ni2+/Ru25+ Ni1+ Ru26+ (π*2) Ni1+ (broad) Ru25+Ni1+/Ru24+ Ni2+ Ru26+ Ni34+ Ru26+ Ni35+ Ru25+ Ru24+

312 312 311 328 329 321

with Ru25+ core underwent reduction by AuNP to yield Ru24+ π*4. [NiRu2Ni2(tpda)4(NCS)2]0,1+. The Raman and SERS spectra of 2 and its oxidized form [NiRu2Ni2(tpda)4(NCS)2](PF6) 2′ are shown in Figure 4. Bands of metal-related modes display great intensities. Similar to those in 1 and 1′, the spectra obtained under the same conditions are alike for 2 and 2′, but spectra obtained in solid form and in SERS for the same complex are dissimilar. The band at 337/335 cm−1 for 2/2′ in solid is assigned to νRu−Ru,str and that at 311 cm−1 is assigned to Ni−Ni stretching νNi−Ni,str; the assignment of Ni−Ni stretch is based the results of Ni5(tpda)4(NCS)2.32 In SERS with AuNP/ AgNP substrate, the νRu−Ru,str of both complexes red-shifts to ∼328 cm−1, indicating that it is undergoing reduction to Ru25+. The band of in-plane deformation Δ(ring−ring) becomes intense and red-shifted. In solid form, the νRu−Ru,str band with great wavenumber is then assigned to Ru26+ core for both complexes. This yields the total valence state for nickel ions [Ni34+]/[Ni35+] in 2/2′. Both complexes display νNi−Ni,str band at 311 cm−1 in the solid form, indicating existing a similar Ni2 core. In SERS, this band is unassigned because it is either too weak or red-shifted to overlap with the metal-N bands.

Ru24+ (π*4) Ru24+ (π*4) Ru24+ (π*4) Ru25+ Ru25+ Ru24+

AgNP substrate 328 328 324 328 328 337

Ru25+ Ru25+ Ru25+ Ru25+ Ru25+ Ru26+

tentatively assigned to Ni2+ and Ni1+, respectively, with more Ni2+ existing. The XPS Ru 3d spectrum displays single peak, 3d5/2 281.1 eV for 2, indicating indistinguishable Ru ions in better agreement with current assignment. In fact, for the obtained XPS Ru 3d data of the EMACs in the present work, only complex 1a displays two split peaks Ru 3d 280.9/281.9 eV and single XPS peak at 280.9 eV with narrow line width for 1. First, the terminal Ru ion in 1a is possibly affected by the strong σ-donating axial ligand NCS to deviate from the center Ru ion to display unequivalent electronic structures. Second, complex 1 has a smaller portion of mixed valence state Ru25+ core that tends to smear out spectral structure and results a single peak. Besides, Raman data display more mixed valence state form for 1a. Hence, only the XPS Ru 3d of 1a displays split peaks. All of the neutral and oxidized trinuclear and pentanuclear EMACs have Ru25+ or Ru26+ and hence possess NIR bands in their absorption spectra, as shown in the Supporting Information (Figure S7). Overall, the local environment of Ru2 moiety is similar for complexes 1 and 2. Comparing the data obtained for other diruthenium complexes, we would expect that the Raman shift equally reveals the bonding strength of Ru−Ru in 2. Deducing the values of magnetic moment purely from the spin state might estimate less than measured. In summary, using SERS spectroscopy combined with the redox reactivity of NPs provides a means to detect the bonding strength of Ru−Ru and the correlated valence state of the diruthenium moiety. The charge of the Ru2 core and its stretching wavenumbers of heterometal EMACs are assigned, and those values are summarized in Table 1. In most cases, AuNPs with great oxidation potential tend to reduce the

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DISCUSSION AND SUMMARY The magnetic moment (μeff) of 2 is reported to be 5.42 μB at 300 K.4 As deduced from the spin state for the previously assigned metal ion structure, Ni+−Ru25+−Ni2+−Ni2+ yields μeff of 5.1 μB close to the measured value. A smaller μeff = 3.5 μB would be obtained for the proposed valence state [Ru26+][Ni34+]. The Ni 2p3/2 XPS displays complicated spectrum a broad peak at 857.4 eV and a shoulder 855.4 eV and are D

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Carbon-rich Organometallic Compounds and Distance-dependent Electronic Coupling Therein. J. Am. Chem. Soc. 2003, 125, 10057− 10065. (9) Angaridis, P. Chapter 9 Ruthenium Compounds. Multiple Bonds between Metal Atoms, 3rd ed.; Cotton, F. A., Murillo, C. A., Walton, R. A., Eds.; Springer, 2005. (10) Chisholm, M. H.; Christou, G.; Folting, K.; Huffman, J. C.; James, C. A.; Samuels, J. A.; Wesemann, J. L.; Woodruff, W. H. Solution Studies of Ru2(O2CR)4n+ Complexes (n = 0 and 1 and O2CR = Octanoate, Crotanate, Dimethylacrylate, Benzoate and pToluate) and Solid-State Structures of Ru2(O2C-p-tolyl)4(THF)2, [Ru2(O2C-p-tolyl)4(THF)2]+[BF4]-, and Ru2(O2C-p-tolyl)4(CH3CN)2: Investigations of the Axial Ligation on the Ru2 Core. Inorg. Chem. 1996, 35, 3643−3658. (11) Mukherjee, S.; Libisch, F.; Large, N.; Neumann, O.; Brown, L. V.; Cheng, J.; Lassiter, J. B.; Carter, E. A.; Nordlander, P.; Halas, N. J. Plasmon-induced Dissociation of H2 on Au. Nano Lett. 2013, 13, 240−247. (12) Wang, Y.; Ji, W.; Sui, H.; Kitahama, Y.; Ruan, W.; Ozaki, Y.; Zhao, B. Exploring the Effect of Intermolecular H-Bonding: A Study on Charge-Transfer Contribution to Surface-Enhanced Raman Scattering of p-Mercaptobenzoic Acid. J. Phys. Chem. C 2014, 118, 10191−10197. (13) Kim, K.; Kim, K. L.; Lee, H. B.; Shin, K. S. Similarity and Dissimilarity in Surface-Enhanced Raman Scattering of 4-Aminobenzenethiol, 4,4′-Dimercaptoazobenzene, and 4,4′-Dimercaptohydrazobenzene on Ag. J. Phys. Chem. C 2012, 116, 11635−11642. (14) Thrall, E. S.; Steinberg, A. P.; Wu, X.; Brus, L. E. The Role of Photon Energy and Semiconductor Substrate in the PlasmonMediated Photooxidation of Citrate by Silver Nanoparticles. J. Phys. Chem. C 2013, 117, 26238−26247. (15) Kim, K.; Lee, S. H.; Choi, J.-Y.; Shin, K. S. Fe3+ to Fe2+ Conversion by Plasmonically Generated Hot Electrons from Ag Nanoparticles: Surface-Enhanced Raman Scattering Evidence. J. Phys. Chem. C 2014, 118, 3359−3365. (16) Wee, T.-L.; Schmidt, L. C.; Scaiano, J. C. Photooxidation of 9Anthraldehyde Catalyzed by Gold Nanoparticles: Solution and Single Nanoparticle Studies Using Fluorescence Lifetime Imaging. J. Phys. Chem. C 2012, 116, 24373−24379. (17) Tanabe, I.; Egashira, M.; Suzuki, T.; Goto, T.; Ozaki, Y. Prevention of Photooxidation of Deoxymyoglobin and Reduced Cytochrome c during Enhanced Raman Measurements: SERRS with Thiol-Protected Ag Nanoparticles and a TERS Technique. J. Phys. Chem. C 2014, 118, 10329−10334. (18) Long, N. N.; Vu, L. V.; Kiem, C. D.; Doanh, S. C.; Nguyet, C. T.; Hang, P. T.; Thien, N. D.; Quynh, L. M. Synthesis and Optical Properties of Colloidal Gold Nanoparticles. J. Phys.: Conf. Ser. 2009, 187, 012026. (19) Lee, P. C.; Meisel, D. Adsorption and Surface-enhanced Raman of Dyes on Silver and Gold Sols. J. Phys. Chem. 1982, 86, 3391−3395. (20) Hezard, T.; Fajerwerg, K.; Evrard, D.; Collière, V.; Behra, P.; Gros, P. Gold Nanoparticles Electrodeposited on Glassy Carbon Using Cyclic Voltammetry: Application to Hg(II) Trace Analysis. J. Electroanal. Chem. 2012, 664, 46−52. (21) Giovanni, M.; Pumera, M. Size Dependant Electrochemical Behavior of Silver Nanoparticles with Sizes of 10, 20, 40, 80 and 107 nm. Electroanalysis 2012, 24, 615−617. (22) McCarthy, H. J.; Tocher, D. A. Synthetic, structural and electrochemical investigations into the compound [Ru2(μ-O2CCH3)(μ-HNC5H3NCH3)3Cl]. Polyhedron 1992, 11, 13−20. (23) Bear, J. L.; Han, B.; Huang, S.; Kadish, K. M. Effect of Axial Ligands on the Oxidation State, Structure, and Electronic Configuration of Diruthenium Complexes. Synthesis and Characterization of Ru2(dpf)4Cl, Ru2(dpf)4(C⋮CC6H5), Ru2(dpf)4(C⋮CC6H5)2, and Ru2(dpf)4(CN)2 (dpf = N,N‘-Diphenylformamidinate). Inorg. Chem. 1996, 35, 3012−3021. (24) Cooke, M. W.; Murphy, C. A.; Cameron, T. S.; Beck, E. J.; Vamvounis, G.; Aquino, M. A. S. Synthesis and Crystal Structures of

diruthenium moiety. Employment of the redox activity of metal NP provides a unique way to vary the valence state of molecule but keeps the local environment relatively unaltered. We propose that the νRu−Ru,str is roughly equal to 312, 320, 327, 333, and 337 cm−1 for Ru2n+ core, n = 4 π*4, 4 (π*δ*)4, 5 (π*δ*)3, 6 π*2, and 6 π*1δ*1, respectively, in diruthenium complexes with similar ligand and structure as EMACs. The valence states of Ni ions in EMACs remain problematic. We report νNi−Ni,str for the Ni−Ni stretch in pentanuclear EMACs. However, more study is required to correlate the bonding strength with valence state of the [Ni2] core.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b08351. UV−vis absorption, XPS, and Raman spectra of complexes. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*I.-C.C.: E-mail: [email protected]; *C.-h.C.: E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the support of National Tsing Hua University, under project “Frontier Research Center on Fundamental and Applied Sciences of Matter”, and the Ministry of Science and Technology of Republic of China.

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DOI: 10.1021/acs.jpcc.6b08351 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b08351 J. Phys. Chem. C XXXX, XXX, XXX−XXX