Determine the NiâNi Bonding Strength in Metal-String Complexes

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Determine the Ni–Ni Bonding Strength in MetalString Complexes Using Head-to-Head Nanorods and Electrochemical Surface Enhanced Raman Spectroscopy Bo-Han Wu, Li-Yen Hung, Jheng-Yang Chung, Shie-Ming Peng, and I-Chia Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00717 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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The Journal of Physical Chemistry

Determine the Ni–Ni Bonding Strength in Metal-String Complexes Using Head-to-Head Nanorods and Electrochemical Surface Enhanced Raman Spectroscopy Bo-Han Wu,a Li-Yen Hung,a Jheng-Yang Chung,a Shie-Ming Peng,b I-Chia Chena* a

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan, Republic of China Department of Chemistry, National Taiwan University, Taipei, 106, Taiwan, Republic of China Supporting Information Placeholder

b

ABSTRACT: We report the bonding strength of nickel ions in trinickel extended metal atom chains (EMACs) and dinickel complexes using Raman, surface enhanced Raman scattering (SERS) and electrochemical SERS (ECSERS). Use of redox ability of gold and silver nanoparticles during plasmonic excitation, the bonding strength and the valence state of metal ions can be determined. For dinickel complexes, we assign the Raman band at 322 cm-1 to Ni2+–Ni3+ stretch in [Ni2(TPG)4BF4]− (TPG = N,N',N''triphenylguanidinate, [Ni2]5+) and 327 cm-1 for Ni2+–Ni1+stretch of [Ni2]3+ moieties in Ni5(camnpda)4. For trinickel EMACs, no band is assigned to Ni3 symmetric stretch νNi3 sym in the neutral form Ni3(dpa)4X2 (dpa = dipyridyl amido, X = NCS, Cl). In the reduced form, the ECSERS curves display the band at 242 cm-1 which also appeared at gold nanoparticle SERS measurement, assigned to νNi3 sym for [Ni3]5+ core. The trinickel complexes were reduced by gold nanosphere and this νNi3 sym band is further enhanced with SERS measurements when gold nanorods were used and the trinickel EMACs served as bridging compounds on both ends. Increase in the applied voltage in ECSERS to +1.3 V, complexes were oxidized and one additional band at 351 cm-1 appeared. This new band is assigned to νNi3 sym of [Ni3]7+ in [Ni3(dpa)4X2]+. Great vibrational frequency indicates that one electron from the metal σ* orbital instead of ligand was removed leading to a three metal center bond. Distinct from the vibrational band wavenumber obtained in dinickel complexes, we confirm that [Ni3]5+,7+ have delocalized electronic structures.

INTRODUCTION Trinuclear metal string complexes are the simplest extended metal atom chains (EMACs).1-5 One kind of EMACs, the metal ion chain is helically coordinated by four polypyridyl amido ligands; some examples are displayed in Scheme 1. The first metal string complex Ni3(dpa)4Cl2 (dpa = dipyridyl amido) was synthesized and the structure was then determined by Aduldecha and Hathaway.4 The bond lengths between two adjacent nickels in this trinickel complex are quite long dNi-Ni = 2.42–2.44 Å. Berry et al.6 reported the structure of the oxidized complex and proposed that for the neutral form, the d electrons in nickel ions are localized to have nearly isolated square-planar Ni2+ in the center and two five coordinated highspin Ni2+ ions on the terminal positions with antiferromagnetic interaction. For the oxidized form [Ni3(dpa)4]3+(PF6−)3 without axial ligands, the structure determined via x-ray crystal diffraction displays shorter nickel bond distances dNi-Ni yielding a delocalized three-center metal bonding. Kiehl et al. used density functional theory (DFT) to calculate the geometry of the trinikel complexes and obtained correct geometrical change in Ni---Ni separation between the neutral with axial ligands and the oxidized trinickel without axial ligands species.7 They suggested that there is no net change in the Ni-Ni bond order of the two forms, both [Ni3]6+

and [Ni3]7+ complexes have a net bond order of 1/2, and the major change in dNi-Ni is due primarily to the concomitant removal of axial ligands upon one-electron oxidation. Lack of experimental evidence on the bond distance of [Ni3]7+ complex with ligand, the Ni-Ni bonding strength of these trinickel species remains unclear. Lai et al. studied the vibrational structures of trinuclear metal string complexes using infrared and Raman spectroscopy.8 The measured bands attributed to polypyridyl ligand agree with the calculated positions using DFT/B3LYP. However, the calculated spectral positions and features for metal related modes disagree with the experimental observations. Nevertheless, they assigned observed bands to the vibrational normal modes related to metal ions and metal ligand motions. The weak vibrational band at 242 cm–1 was assigned to the Ni3 symmetric stretch. Because of complication in this molecular system further experiments are required to confirm this assignment to elucidate the bonding strength between Ni ions and the electronic structures. Surface enhanced Raman scattering (SERS) is a sensitive technique to obtain the vibrational structures of molecules. This provides the bonding information when molecules are attached or bonded to the surface of nanoparticles in solution and without crystal lattice constraint. However, metal nanoparticles inject hot electrons/holes to nearby molecules at

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plasmonic excitation.9 For samples with low redox potential can undergo reduction/oxidation reaction during SERS measurements. On the basis of these properties, Wu et al. determined the valence states of [Ru2] moiety in the heterodiruthe-

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nium EMACs.10 Nevertheless, the SERS measurements provide the bonding strength, which is essential for further realizing the character of metal string complexes.

Scheme 1 Structure of (1) Ni3(dpa)4Cl2, (2) Ni3(dpa)4NCS2 (3) ∆-[Ni5((+)camnpda)4] and (4) Ni2(TPG)4

(1)

(2)

(3)

(4)

Electrochemical SERS (ECSERS) is a technique that Raman signal can be obtained in situ for molecules absorbing on an electrode containing nanostrucutres.11-13 Using the electrochemical setup, the absorption during the redox processes can be recorded in situ to assist in assigning the electronic structures of the molecules.14 Ying et al. reported that metallaynes with sulfide moieties can serve as connectors for gold nanoparticle assemblies.15 Besides, the gold nanorods (AuNRs) can self-assemble into head-to-head structures by molecules with anchoring groups.16 Here, we utilize the isothiocyanate axial ligands in Ni3(dpa)4(NCS)2 as the bridges to connect AuNRs. The vibrational modes with motion normal to the metal surface is preferentially enhanced. If the complexes with the metal ion lines are along the longitudinal direction the Ni ion stretching mode would gain more intensity. This provides a means to identify the Ni ion mode. Vice versa, the bridging structure between rods is defined here for any further applications in molecular electronics. In order to differentiate delocalized vs. localized Ni bonding (for example, three metal center [Ni3]7+ vs. Ni2+/[Ni2]5+ mixed valence in [Ni3]7+ core), dinickel complex Ni2(TPG)4 (TPG: N,N',N''-triphenylguanidinate, [Ni2]4+) and ∆-[Ni5(()camnpda)4] (structures in Scheme 1) are employed in the present work to compare the metal-metal bonding strength.17,18 On the basis of the reported magnetic properties and crystal structures, EMAC ∆-[Ni5((-)camnpda)4] is assigned to have two localized [Ni2]3+ moieties in the metal ion chain. We investigate the Ni-Ni bonding strength in these Ni2 and Ni3 compounds using SERS and ECSERS and determine the valence states based on these data.

EXPERIMENTS

Samples. Complex Ni3(dpa)4Cl2, Ni3(dpa)4(NCS)2, ∆[Ni5((-)camnpda)4] were synthesized following the methods described before.4,18 Ni2(TPG)4 and [Ni2(TPG)4]BF4 were synthesized following method by Berry et al.17 Silver nitrate (AgNO3), dichloromethane, acetonitrile, ascorbic acid and tetrabutylammonium perchlorate (TBAP) were purchased from Alfa Aesar. Gold(III) chloride trihydrate (HAuCl4•3H2O), sodium citrate, cetyltrimethylammonium bromide (CTAB), cetyltrimethylammonium chloride (CTAC), sodium borohydride (NaBH4), and ethanol were from Sigma-Aldrich. Au wire (0.3 mm diameter, 99.95%) was from Leesan, Tainan, Taiwan. All chemicals were used as received. Pure deionized water (MilliQ Millipore, 18.2 MΩ/cm) was used in all preparations. SERS Sample Preparation. SERS sample solution was prepared by mixing powder of Ni3(dpa)4Cl2 or Ni3(dpa)4(NCS)2 and gold nanosphere (AuNS) solution in a quartz cell. Here, we prepared two AuNS solutions; one was synthesized by citrate reduction,19 AuNS(citrate). In brief, 500 µL 0.029 M HAuCl4 aqueous was added in 49.5 ml water. The mixture was heated until 70 °C, subsequently, adding 0.1 g sodium citrate and stirred until the color changed from light yellow to bright red. The other AuNS was stabilized via surfactant CTAB, AuNS(CTAB). This solution was synthesized by seed growth method. For seed solution, 86 µl 0.029 M HAuCl4(aq) and 0.282 g CTAB were added into 7.75 ml water, subsequently, plugged into iced 600 µl 0.1 M NaBH4(aq) under stirring and then placed 20 min. The growth step was carried out by adding 100 µL of 0.029 M HAuCl4(aq) and 0.15 g CTAB in 10 mL water, followed by addition of 50 uL of 0.1 M ascorbic acid aqueous solution and shaking until clear. Consequently, 1 ml of seed solution was added to the solution and aged overnight. The diameters of AuNS(citrate) and AuNS(CTAB) are 44 ± 4 and 91 ± 9 nm, respectively.


RESULTS Raman and gold SERS of Ni3(dpa)4(NCS)2. The Raman on crystals and SERS spectra of Ni3(dpa)4(NCS)2 are showed in Figure. 1. The vibrational bands were assigned previously.8 Briefly, the vibration modes of ligands lie in 350-1650 cm-1 and the metal-ligand and metal-metal bands in 200-500 cm-1. In the Raman spectra, the bands at 392/292/265 cm-1 are assigned to Ni–N stretching modes νNi–N and 222 cm-1 to the N– Ni–N bending δN–Ni–N (Figure. 1e). In AuNS(citrate) SERS, all band positions are similar as those in the normal Raman curves except one band appeared at 242 cm-1 (Figure. 1d). This band disappeared in AgNS SERS spectra. The reduction potential of Ni3(dpa)4Cl2 was reported to be −0.95 V.14 We infer that the reduction potential of the complex with axial ligand NCS is similar. As the oxidation potential of AuNS is +1.53 V,21 the Ni3 complex was reduced by AuNS in this case. Hence, the band at 242 cm-1 unobserved at normal Raman on neat crystals, but on AuNS SERS, is reassigned to Ni3 symmetric stretching νNi3 sym of [Ni3]5+ in [Ni3(dpa)4NCS2]–. The oxidation potential of AgNS is +0.61 V and cannot reduce the Ni3 complex.22 The other bands are within a few wavenumbers to those of the neutral form, hence the structure of dipyridyl anions remain unaltered during oxidation. Reduction mainly occurred on metals.

222

691

242

*

265

(a)

(AuNR)n(CTAC)[Ni3]5+ 517

290

536

*

Γ (ring-ring) C-N-C bend +N-Ni-N bend +Ni-N str.

6+ (AuNR)n(CTAB) [Ni3]

(b) 6+ AuNS(CTAB) [Ni3]

(c) [Ni3]5+

5+ AuNS(citrate) [Ni3]

Ni3 sym. str.

(d) 222

(e)

265 Ni-N str. 292

Ni-N str.

Head-to-head AuNR. The AuNR was prepared according to the seeded growth method by Vigderman and Zubarev.20 For seed solution, the preparation was followed above mentioned. The growth step was carried out by adding 138 µL of 0.029 M HAuCl4(aq), 15 µL of 0.01 M AgNO3(aq) and 0.346 g CTAB in 10 mL water, followed by addition of 60 uL of 0.1 M ascorbic acid aqueous solution and shaking until clear. Consequently, 40 µl of seed solution was added to the solution and aged overnight. The synthesized AuNR size (12.1 ± 0.7) × (29.4 ± 2.7) nm aspect ratio 2.4 ± 0.2 is covered with surfactant CTAB, called AuNR(CTAB). We replaced with CTAC through ion exchange method to form AuNR(CTAC). In brief, purified AuNR(CTAB) upon centrifugation and redistributed by CTAC aqueous solution. This procedure was repeated twice. Self-assembly method is adopted the Kumar and Thomas’s study.16 Solvent exchange of AuNR into water/acetonitrile solution removed partial surfactants, so the sample molecules can anchor to the head sides of AuNR. Here, few mg of Ni3(dpa)4(NCS)2 was added into AuNR water/acetonitrile solution during stirring, and the absorption and Raman spectrum were recorded after short time periods. SERS active gold working electrode. The SERS active gold working electrode was made by immersion of AuNS (diameter ∼50 nm) and metal string molecules on an Au wire. A gold wire was immersed in the AuNP solution mixed with Ni3(dpa)4Cl2(alc) or Ni3(dpa)4(NCS)2. Subsequently, the electrode was dried then assembled into the electrochemical cell for in situ ECSERS measurements. Characterization. The ECSERS is consisted of a potentiostat (CH Instruments, Inc.), spectroelectrochemical cell (ALS Co., Ltd) and a Raman spectrometer. The cell was equipped with a Pt counter electrode (ALS Co., Ltd) and the SERS active gold working electrode. All electrode potentials were referred to the non-aqueous Ag/Ag+ reference electrode (CH Instruments, Inc.) in measurements. The 0.1 M TBAP dichloromethane solution mixed with very few amount Ni3(dpa)4Cl2 or Ni3(dpa)4NCS2 powder served as the electrolyte. In order to confirm no molecules decomposed during the ECSERS in situ measurements, we recorded the ECSERS three times and checked the identical spectra before applied potentials. In this work, all voltages are reported reference versus standard hydrogen electrode (SHE). The solid Raman, SERS and ECSERS spectra were recorded in a back-scattering geometry Raman spectroscopy. The He-Ne laser (Lasos) through a laser line filter (Thorlabs) operated with a red light at 632.8 nm as the excitation light source, reflected by the 632.8 nm edge filter (Semrock). A 10x objective (ZEISS) focused the laser beam into a size of less than 1 µm and the power at the sample region was set at ~10 mW for solid sample and less than 1 mW for SERS/ECSERS. The scattered signal passing through an edge filter, a lens, an optical fiber, and a monochromator (length, 0.5 m; grating 600 grooves/mm) was recorded with a liquid nitrogen-cooled charge-coupled device detector (Horiba). The spectral resolution was maintained at 3 cm-1. The integration period per scan was typically about 30 s and was averaged for 5-10 scans for a sample. For blue light resonance Raman detection, the harmonic output of a YAG laser with 473 nm served as the excitation light source. The alignment of Raman was followed the above setup, and the 473 nm edge filter (Semrock) was used.

Γ (ring)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


∆ (ring-ring)

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392

Powder [Ni3]

6+

∆(ring)

Γ(ring)

430

358

200

300

400

500

600

700

Raman shift (cm-1) Figure 1. SERS Spectra of (a) AuNR(CTAC), (b) AuNR(CTAB), (c) AuNS(CTAB) and (d) AuNS(citrate) Ni3(dpa)4NCS2 in solution and (e) Raman spectrum of Ni3(dpa)4NCS2 in solid crystals.


The Journal of Physical Chemistry The asterisk denotes bands from acetonitrile. Some of band assignments, positions and the [Ni3] core are as indicated.

Besides, the bands 517 and 536 cm–1 assigned to out-ofplane pyridyl ring bending have varied intensities from those in powder forms. For the reduced form, the 517 cm–1 band, which is wagging of two pyridyl rings, has more intensity than the twisting. Similar change in intensity also displays for bands at 1255 and 1277 cm–1, the C-N-C symmetric stretching and pyridyl ring stretching, respectively (Figure. S1). Consequently, we observe no band corresponding νNi3 sym in [Ni3]6+ core. Gold nanorod self-assembly. In order to assure the assignment on the Ni bonding, AuNRs were used. Figure. 2 shows the absorbance of AuNR with portions of Ni3(dap)4(NCS)2 solution added at various time periods. The

AuNR has two absorption features peaked at 514 and 639 nm, attributed to the transverse and longitudinal mode, respectively. When adding Ni3(dap)4(NCS)2, the intensity of the 639 nm band decreased and one new band at 729 nm increased with time increased. The red shift of the longitudinal mode indicates extension of the plamonic mode and bridging of complex at the ends of the AuNRs by isothiocyanates. The transmission electron microscopy (TEM) image, as shown in Figure. 2, illustrates the self-assembled (AuNR)n into a long chain structure. From the gap size, the Ni ion chain is roughly parallel to the longitudinal direction of rods. From the TEM images, the contact length between two rods is near 4 nm, hence contact surface area is roughly 16 nm2. The diameter of a metal string molecule is about 1 nm. We estimated the number of bridging metal string complexes between two rods is ∼10. This yields an enhancement in the vibrational modes with net motion along the longitudinal direction.

Absorbance 500

600

700

800

900

Wavelength (nm)

223

No volt +0.64 V +0.74 V +0.84 V +0.94 V +1.04 V

Ni3 sym str. 351

*

200

(c)

250

300 350 400 Raman shift (cm-1)

450

200

500

(d)

4000

zero V +1.05 V +1.15 V +1.25 V +1.29 V +1.35 V +1.39 V

3000 2500 2000

300 350 400 Raman shift (cm-1)

450

zero V +0.04 V -0.16 V -0.36 V -0.46 V -0.66 V -1.06 V

2500

1500

500

3000

ε (M-1cm-1)

3500

250

+1.34 V +1.39 V +1.43 V +1.49 V

391

266 (269)

(b)

391

*

240

(a)

265 285

223

Figure 2. Left: Absorption curves of AuNR without (black line) and with Ni3(dpa)4(NCS)2 at various periods (time interval ~15 s). Center: TEM image of AuNR(CTAB) with Ni3(dpa)4(NCS)2. The scale bar is 10 nm. Right: schematic representation of AuNRs linked by Ni3(dpa)4(NCS)2.

ε (M-1cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 9

2000

1500

1000 1000 500 500 0

400

0

500

600

700

800

400

Wavelengh (nm)

500

600

700

800

Wavelengh (nm)


Figure 3. ECSERS curves of Ni3(dpa)4(NCS)2 at (a) +0.64 – +1.04 V, (b) +1.34 – +1.49 V. Asterisk denotes band from dichloromethane. (c) Absorption curves of Ni3(dpa)4(NCS)2 without applied voltage and at +0.74 – +1.39 V and (d) without applied voltage and at +0.14 – −1.06 V in 0.1 M TBAP/DCM.

Berry et al. used AgBF4 to oxidize Ni2(TPG)4 to form [Ni2(TPG)4]BF4 and reported their structures.17 The measured Raman spectra in the wavenumber region 200–450 cm−1 of Ni2(TPG)4, [Ni2(TPG)4]BF4 and ligand HTPG in solid form are shown in Figure. 5. One new intense band appearing at 322 cm-1 for the one-electron-oxidized complex is then assigned to νNi–Ni. The [Ni2]4+ has a full filled electron configuration (σ)2(π)4(δ)2(δ*)2(π*)4(σ*)2 and is expected to be nonbonding between the nickel ions. If one electron is removed from the σ* orbital instead of ligand the Ni–Ni bond order is 0.5. We performed the DFT calculations method B3LYP trying to obtain the vibrational structure. However, deviation of the calculated from the experimental Raman spectra is too large to assure the assignments. Even for HTPG, the deviation between theory and experiment remains substantial. Nevertheless, in Figure. 5, we display the assignment for the low wavenumber bands on comparing these spectra and those made for the metal string complexes.

7+

[Ni3] Ni3 sym. str.

225 268

Γ (ring)

(a)

Ni3(dpa)4Cl2-AuNS/DCM +AgPF6

C-N-C bend +Ni-N str.

355

*

438 [Ni3]5+ Ni3 sym. str.

*

391

(b) 200

Ni3(dpa)4Cl2-AuNS/DCM Ni-N str.

As shown in Figure. 1, the SERS spectra on (AuNR)n(CTAB) display similar features as those in powder Raman spectra, but on (AuNR)n(CTAC), the intense band at 242 cm–1 appeared. As reported by Tanabe et al. nanoparticles can oxidize nearby molecules, but the oxidation can be prevented via surface modification of carboxyl-terminated alkanethiols.23 Surfactants CTAB covered nanoparticles to prevent hot electrons ejected from metal surface while CTAC with cation Cl– covered the surface loosely as suggested by Meena et al.24 Dense Br- layer on AuNP surface can serve as a passivation to prevent hot electron reacting with metal string complexes. The intense 242 cm-1 band is further confirmed to be νNi3 sym. ECSERS of Ni3(dpa)4(NCS)2. Figure. 3 shows the ECSERS curves recorded at anodic volts +0.64 – +1.04 V and +1.34 – +1.49 V for Ni3(dpa)4(NCS)2 using SERS active gold (dia. ∼50 nm) working electrode. With this electrode, the 240 cm-1 band appeared when no voltage was applied then decreased in intensity from +0.64 V in Figure. 3a. After subsequently increasing the applied voltage, samples were gradually oxidized and one new band appeared at 351 cm−1 starting at +1.3 V. The corresponding visible absorption curves using regular platinum electrode were measured as shown in Figure. 3c and 3d; the cyclic voltammogram is shown in the Supporting Information. Both Figure 3c and 3d have similar signalto-noise ratio; we used FFT filtering method to filter out the fixed frequency noise to determine the peak positions and Figure 3c displays a better effect after treatment. A new absorption band centered ∼630 nm appeared starting at +1.3 V; this applied voltage was sufficient to attain the one-electron oxidized species. Hence, the electronic band is attributed to [Ni3(dpa)4(NCS)2]+ and the Raman band at 351 cm−1 is then assigned to νNi3 sym of [Ni3]7+ core. When the voltage is tuned to -0.46 V, the visible band at 522 nm (no volt) is shifted to 505 nm attributed to the reduced species [Ni3(dpa)4(NCS)2]−. Overall, the vibrational band at 240 cm−1 is assigned to νNi3 sym of the reduced form and 351 cm−1 to the oxidized form. Changing the axial ligand to chloride we obtain similar results (See the Supporting Information). Hence, this vibrational band νNi3 sym is unaltered with varied axial ligand. Ni3(dpa)4(PF6)3. We used SERS method to record the spectra of Ni3(dpa)4(PF6)3. First, the gold working electrode with Ni3(dpa)4Cl2 was immersed in dichloromethane then SERS curves were recorded in situ when AgPF6/DCM solution was added, as shown in Figure. 4. The 242 cm-1 band gradually disappeared and one new band appeared at 350 cm-1 first then shifted to 355 cm-1. Complex was first oxidized to [Ni3(dpa)4Cl2]+ then to form [Ni3(dpa)4]3+. The band at 355 cm-1 is thus assigned to νNi3 sym of [Ni3]7+ without axial ligand. According to the calculated results of Kiehl et al.,7 dNi-Ni becomes significantly shorter because of no sigma donor axial ligand involved. Nevertheless, νNi3 sym in [Ni3(dpa)4]3+ is only blue shifted 5 cm-1 from that in [Ni3(dpa)4Cl2]+. Ni2(TPG)4 & [Ni2(TPG)4]BF4. The Ni2 complex is a lantern-type compound which has four guanidinate ligands and bears [Ni2]4+ core without axial ligand for the neutral form.

N-Ni-N bend +Γ (ring-ring)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


221 242 267

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300

400

500

600

Raman shift (cm-1) Figure 4. Au SERS spectra of Ni3(dpa)4Cl2 in CH2Cl2 (dichloromethane, DCM) (a) with and (b) without AgPF6. Asterisk denotes band from CH2Cl2.



(a) Ni2(TPG)4

412

Dinickels. Berry et al.17 explained that the dNi-Ni in the oxidized form [Ni2(TPG)4]BF4 is 0.098 Å shorter than that in the neutral, caused by removal of one electron from the σ* orbital. The molecular orbital diagrams for the dinickel cores are shown in Figure. 7. Our experimental finding νNi−Ni is 322 cm-1, a relative strong Ni–Ni bonding further confirming this assignment.

383

303

268

213

DISCUSSION

∆(ring)

Ni-N stretching mixed ring

[Ni2]5+

412

298

(a) ∆-[Ni5((-)campda)4]- 633 nm

300

327

410

377

332 350

400

361 377

250

υ(Ni-Ni) [Ni2]3+

255 267

200

295

257

(c) HTPG

266

232

322

383

υ(Ni-Ni)

(b) [Ni2(TPG)4]BF4

214

450

Raman shift (cm-1) 350

400

977

655

300

377

267

Δ-[Ni5((-)camnpda)4]. The other complex containing the [Ni2] core is Ni5((-)camnpda)4. Yu et al. synthesized the chiral EMAC pentanickel complexes, ∆-[Ni5((-)camnpda)4], the chiral ligand, camnpda, containing a naphthyridine and a camphorsulfonyl group (Scheme 1) leading to the chirality of helical structure.18 This complex was assigned to consist of two [Ni2]3+ (Ni+/Ni2+) on the terminals and one Ni2+ d8 square planar in the center. The absorption bands at 412 and 648 nm were assigned to the ligand and intervalence band in the [Ni2]3+ moieties, respectively. The Raman spectra of ∆-[Ni5(()camnpda)4] in solid crystals were recorded at two excitation wavelengths 473 and 632 nm (Figure. 6). The 473 nm excitation provides the electronic resonance Raman enhancement in ligand and 632 nm in nickel and related bands. As expected, the ligand bands were greatly enhanced at 473 nm excitation. An intense band at 327 cm-1 with 633 nm excitation appeared, is thus assigned to be νNi–Ni for the [Ni2]3+. The nearby weak bands 377, 361, 267, and 255 cm–1 are assigned to νNi–N. Only one sharp νNi–Ni band implies identical [Ni2]3+ moieties in this complex. The observed Raman and SERS band positions for nickel complexes are summarized in Table 1.

361

250

Figure 5. Raman spectra of (a) Ni2(TPG)4, (b) [Ni2(TPG)4]BF4, and (c) HTPG in solid form at 633 nm excitation. 255

977

(b) ∆-[Ni5((-)campda)4]- 473 nm

642 682

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 9

200 400 600 800 1000 1200 1400 1600 1800

Raman shift (cm-1)

Figure 6. Raman spectra of ∆-[Ni5((-)camnpda)4] in solid form at (a) 633 nm and (b) 473 nm excitation. Insert in (a) displays the four νNi–N bands.

Table 1. List of Raman and SERS Band Positions and Valence States of [Ni2] and [Ni3] Moieties dNi-Ni Åa

dNi-Ni cal. Åb

Absorption λ (nm) (ε x103 M-1cm-1, transition)

2.280

-

843 (0.2, π*→σ*), 648 (6.9, π*→M-L σ*), 412 (47), 335 (40)a

[Ni2]3+

∆-[Ni5((-)camnpda)4]

νNi−Ni str. cm-1 327

[Ni2]4+

Ni2(TPG)4

-

2.4280

-

621 (0.2, π*→M-L σ*), 495 (1.3), 394 (2.5)

Ni2(TPG)4(BF4)

322

2.3296

-

711 (0.9, π*→σ*), 587 (1.1, π*→M-L σ*), 453 (1.7)

[Ni2]

5+

νNi3 sym., cm-1


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The Journal of Physical Chemistry [Ni3]5+ [Ni3]

6+

[Ni3]

7+

[Ni3]7+

[Ni3(dpa)4X2]−, Ni3(dpa)4Cl2 [Ni3(dpa)4X2] [Ni3(dpa)4]3+

+

242

-

-

505 (2.4, π*→Mo-L σ*)

-

2.4386

2.42-2.45

522 (2, π*→Mo-L σ*)

350

-

2.36-2.38

634 (3.5, π*→Mo-L σ*), 1065 (0.6, π*→σ*)a

355

2.2851

2.29

580 (π*→Mo-L σ*), 1045 (π*→σ*)a

a.

X = Cl, NCS; ∆-[Ni5((-)camnpda)4] ref. 18; Ni2(TPG)4, Ni2(TPG)4BF4 ref. 17; Ni3(dpa)4Cl2 ref. 3; [Ni3(dpa)4]3+(PF6)3 ref. 6.

b.

DFT GGA/BP86 functional ref. 7.

Figurer 7. Molecular orbital diagram of [Ni2]n+ dinickel moiety.

Figure 8. Molecular orbital diagram of [Ni3]5+,7+ for delocalized electronic structure and [Ni3]6+ for localized electronic structure. Mo and Mi denote for outer and inner metal, respectively.

The visible absorption of [Ni2(TPG)4]+ shows a weak broad band centered at 587 nm with a shoulder at 711 nm in the near infrared (NIR) region. In Ni2(TPG)4 this visible band is at 621 nm. Time-dependent DFT (TD-DFT) was used to study these electronic transitions. All calculations were performed using the GAUSSIAN Package.25 We employed the density fitting basis set W06 containing split valence and triple zeta basis

set.26 Using method UBP86/def2TZVP/W06, we obtain the vertical transition at 1.79 eV (691 nm) mainly involving π*→M-L σ*, oscillator strength f = 0.0037 in Ni2(TPG)4 in agreement with the experimental finding. However, for [Ni2(TPG)4]+ the calculation results display that the highest occupied molecular orbital (HOMO) is a nonbonding orbital mostly attributed to ligands TPG. Similar results were obtained by Berry et al. using method B3LYP on a simplified Ni2 complex cation.17 The calculated outcomes using these methods failed in obtaining correct energies of MOs for [Ni2(TPG)4]+. Nevertheless, based on these calculations and our resonance Raman data, the 587 nm band in [Ni2(TPG)4]+ and 621 nm band in [Ni2(TPG)4] are assigned to metal π*→ML σ*. The shoulder peak 721 nm in [Ni2(TPG)4]+ is assigned to [Ni2]5+ π*→σ*. Similar assignments were made by Berry et al. for the near IR 887 nm band of [Ni2(DAniF)4]BF4 (DAniF = N,N’-di-p-anisylformamidinate).17 Yu et al. reported total six unpaired electrons, with spin S1 = S2 = 3/2 on the terminal [Ni2]3+ in ∆-[Ni5((-)camnpda)4]. Hence, [Ni2]3+ is a delocalized mixed valence Ni+ and Ni2+ that three unpaired electrons occupy the M-L σ* and σ* orbitals.18 This results in three electrons two metal center bond (Figure. 6) and short Ni–Ni.27 The vibrational wavenumber for νNi−Ni is 327 cm-1, a stronger bond than in [Ni2]5+; this may be explained that the extra two electrons filling the M-L σ* orbitals (Figure.6), intrinsically antiorbitals in complex, resulting in weakening Ni-ligand bonding resulting in strengthening the Ni-Ni. The Ni d orbital involving in M-L σ* can form weak δ bonding enforcing the metal bonding strength. Slightly greater vibrational frequency νNi−Ni in [Ni2]3+ moiety is consistent with a relatively shorter dNi–Ni obtained from x-ray crystal structure. This also shows that the short bond length is not because of being constrained by naphthyridine. Trinickels. For trinickel complexes, the new bands at 350 cm-1 and 355 cm-1 are attributed to the nickel bond symmetric stretching in [Ni3]7+ with and without axial ligands, respectively. Berry et al.6 reported that the Ni-Ni bond distances in [Ni3(dpa)4]3+(PF6−)3 are 2.2851 and 2.2885 Å about equal and remarkable short. They proposed that is caused by removal of one electron from the σ* orbital and leading to three center (3c) delocalized bond. Based on the obtained vibrational wavenumbers of νNi–Ni, the localized electronic structure Ni3+/Ni2+---Ni2+ can be excluded. Kiehl et al.7 reported the calculated dNi-Ni in [Ni3(dpa)4Cl2]+ and [Ni3(dpa)4]3+ to be 2.36 and 2.29 Å, respectively and proposed that the long dNi-Ni is caused by the axial ligand. The small difference between the observed band positions 350 and 355 cm-1 cannot account for the large variation in bond distance. Instead, the great wavenumbers indicate strong three center (3c) delocalized Ni bonding for both complexes and minor effect from the axial ligand for [Ni3]7+ core. The Cl and NCS are both sigma donor ligands,


The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and the back bonding effect only slightly weakens the Ni-Ni bonding. Both the results of AuNR SERS and ECSERS revealed that the band at 242 cm-1 only appeared in the reduced form and is enhanced further in bridging (AuNR)n. Thus, this band is assigned to νNi3 sym of [Ni3]5+. Similarly, based on the observed data, the localized electronic structure Ni+/Ni2+ (= [Ni2]3+)--- Ni2+ is nonexistent in the reduced form. The bond strength is particularly weak, hence, the extra electron is likely to fill the Mo-L σ* orbital to weaken the Ni-ligand bonding and increasing the metal-metal bonding. The Ni d orbital involving the Mo-L σ* might form delocalized δ bonding to exhibit weak bonding strength. No band in [Ni3]6+ can be attributed to νNi3 sym indicating no bonding between nickels thus in agreement with the proposed localized electronic structure for the neutral form. The diagrams of Ni3 MOs are shown in Figure. 8. Limited by the computational capacity, method B3LYP/ LanL2DZ(Ni), 3-21G was used to calculate the energies of MOs in [Ni3(dpa)4Cl2]1−,0,1+ and TD-DFT to obtain the vertical transitions.28-31 Similar to Ni2 complexes, DFT yields the ligand nonbonding orbital as HOMO to display long Ni-Ni bond lengths 2.391 Å in [Ni3]7+. In Ni3(dpa)4Cl2, the vertical transition at 2.2 eV (563 nm) for mainly metal π*→ Mo-L σ* and with oscillator strength f = 0.0011 is obtained. Thus, the band at 505, 522, and 630 nm for [Ni3]n+ n = 5, 6, and 7, respectively are assigned to metal π*→ Mo-L σ* (assuming that the [Ni3]6+ has a similar molecular orbital structure for this orbital assignment here.) This indicates the energy of π* is lifted more in [Ni3]7+. As the bonding becomes strengthened with the charge of metal core, π* would be elevated. Similar to Ni2 complexes, the near infrared band in neutral species at 1065/1045 nm in [Ni3]7+ is assigned to metal π*→σ*. In the cases of dinickel and trinickel complexes studied at the present work, the normal mode assigned to Ni-Ni or Ni3 stretch represents the Ni stretching motion quite well. This can be understood from Fig 6 the Raman spectra of Ni5(camnpda)4 recorded at 633 nm excitation from resonance enhancement on Ni2 d-d transition. The Ni-Ni stretch is significantly enhanced and the other nearby modes only show very little intensity, indicating less mixing of those motions in this mode. In this situation, the measured band frequency can directly imply the bonding strength in Ni ions. The shorter Ni-Ni bond length implies stronger bonding, greater stretching frequency. Our data show a nice agreement between the measured Ni stretching wavenumber and bond distance for all complexes studied here. Based on the wavenumbers obtained, for example, 242 cm-1 assigned to [Ni3]5+ the bond order is less than 1/2..

CONCLUSION Combining the techniques ECSERS, SERS, and normal Raman provides a special means not just to investigate the vibrational structures and bonding strength also to determine the valence states of organometallic complexes. One of example employed at the present work is the trinickel complex system. An extra band at 242 cm-1 only appearing during SERS measurements using gold nanoparticles on trinickel complexes, is assigned to νNi3 sym in [Ni3]5+. Complexes underwent reduction during plasmonic excitation. When the complexes were placed between gold nanorods, the intensity of this band is further enhanced because the arrangement of metal ion line

Page 8 of 9

is along the longitudinal direction of rods. We also measured νNi3 sym = 351 and 355 cm-1 in [Ni3]7+ core with/without axial ligands. By comparing with the observed vibrational wavenumbers for dinickel, these data indicate strong 3c nickel bonding. Conclusively, both the reduced and oxidized forms of trinickel EMACs possess delocalized metal bonding and ought to be superior single molecular conductor than the neutral form. Unlike [Ni3]6+ having localized electronic structures, the electron configuration of [Ni3]5+ core is 2 4 6 4 4 2 2 (σ) (π) (δ) (πnb) (π*) (σnb) (σ*) (Mo-L σ*)1 and [Ni3]7+ (σ)2(π)4(δ)6(πnb)4(π*)4(σnb)2(σ*)1. Our data reveal that electron was removed or added mainly on nickel ions instead of ligands in either dinickel or trinickel cases. DFT with method B3LYP and UB86/w06 failed to predict correct HOMOs, consequently, wrong geometries and Raman spectra. Other method might be considered on this type of complexes. Our method using SERS and ECSERS might be able to be applied extensively to determine the valence states and bonding strength of compounds with relatively low redox potentials.

ASSOCIATED CONTENT Supporting Information Raman and SERS curves, CV, absorption curves of complexes, SEM image of AuNRs. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * [email protected].

ACKNOWLEDGMENT We are grateful Chuan-Sheng Huang for synthesizing Ni2(TPG)4, for National Tsing Hua University under project “Frontier Research Center on Fundamental and Applied Sciences of Matter” and Ministry of Science and Technology of Republic of China for support of this research.

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Determine the NiâNi Bonding Strength in Metal-String Complexes

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