Bonding between Chromium Atoms in Metal-String Complexes from

ARTICLE pubs.acs.org/JPCC

Bonding between Chromium Atoms in Metal-String Complexes from Raman Spectra and Surface-Enhanced Raman Scattering: Vibrational Frequency of the Chromium Quadruple Bond Yu-Min Huang, Huei-Ru Tsai, Szu-Hsueh Lai, Sheng Jui Lee, and I-Chia Chen* Department of Chemistry, National Tsing Hua University, Kuang Fu Road, Hsinchu, Taiwan 30013, Republic of China

Cheng Liang Huang Department of Applied Chemistry, National Chiayi University, No. 300 Syuefu Rd., Chiayi, Taiwan 60004, Republic of China

Shie-Ming Peng Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan 10617, Republic of China

Wen-Zhen Wang School of Chemistry and Chemical Engineering, Xi’an Shiyou University, No. 18 Second Dianzi Road, Xi’an, Shaanxi Province, People's Republic of China 710065

bS Supporting Information ABSTRACT: By measuring the vibrational wavenumbers of their stretching modes in Raman and surface-enhanced Raman scattering (SERS) spectra, we investigated the strength of the Cr Cr bonds in metal-string complexes Cr5(tpda)4X2 and Cr7(teptra)4(NCS)2 (tpda = tripyridyldiamido; teptra = tetrapyridyltriamido; X = Cl , NCS ). The bands in SERS and Raman differ insignificantly in spectral positions, indicating no major structural variation between the solid and solution forms. For SERS measurements, these complexes were bound to silver or gold nanoparticles in aqueous solution to eliminate the constraint of a crystal lattice and to maintain the complexes in thermal equilibrium; this method is convenient to identify the stable structure. We identified both penta- and heptachromium complexes in both symmetric (s) and unsymmetric (u) forms. For pentachromium complexes, our data agree with the results obtained from structural determination of the crystalline form, but for the heptachromium complex, this experimental evidence is the first for the existence of the u-form structure. From our analysis of the vibrational normal modes, we assign the band at 280 cm 1 to the Cr Cr symmetric stretching mode of the s-form pentachromium complex. According to comparisons of SERS spectra obtained at either high temperatures or under oxidizing conditions, we assign 570 cm 1 to the stretching mode of the Cr Cr quadruple bond in the u-form for the pentachromium complex and 554/571 cm 1 analogously for the heptachromium complex. The bands for metal-related modes in SERS spectra might be enhanced because of interaction with the metal nanoparticles. The metal-string complexes with a linear arrangement of metal ions have an increased absorption coefficient in the visible spectra and, consequently, an increased resonance Raman intensity for the metal metal stretching modes, yielding information about the strength of chromium chromium multiple bonding.

’ INTRODUCTION Hurley and Robinson1 synthesized the first metal-string complex, Ni3(dpa)4Cl2 (dpa = di(2-pyridyl)amido anion). Later, Aduldecha and Hathaway2 determined that its structure contained collinear metal ions and helically coordinated dipyridylamine ligands. Longer metal-string complexes with a similar structure were subsequently synthesized;3 11 these complexes exhibit unique electric and magnetic properties presaging a prospective function as molecular wires or switches.12,13 The structures of these extended r 2011 American Chemical Society

metal atom chain complexes have been characterized mostly through X-ray diffraction of their crystalline forms. Some complexes exhibit isomeric structures: for instance, tricobalt complexes exist with both symmetric and unsymmetric metal metal bonding, whereas the Ni3(dpa)4Cl2 complex has exclusively symmetric Received: April 1, 2011 Revised: June 9, 2011 Published: June 28, 2011 13919

dx.doi.org/10.1021/jp203065v | J. Phys. Chem. C 2011, 115, 13919–13926

The Journal of Physical Chemistry C

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Ni Ni bonding.14,15 The symmetric and unsymmetric metal metal bonding in the trinuclear complexes was confirmed from vibrational analysis based on infrared, Raman, and surfaceenhanced Raman scattering (SERS) spectra.16,17 For Cr3(dpa)4Cl2, both symmetric (s-) and unsymmetric (u-) structures were determined through X-ray diffraction. The s-form has bond distances of Cr Cr ∼ 2.36 Å, and the u-form has a bond distance of 2.061 Å,18 nearly quadruple bonding between two chromium ions and a high spin Cr2+ (electron spin, S = 2). Berry et al.19 concluded that a Cr3(dpa)4X2 complex would have a u-form for a weak σ donor ligand X. Using density functional theory (DFT), Benard and co-workers calculated that, without a constraint of a crystalline lattice, Cr3(dpa)4Cl2 exists in an s-form in the ground state.20,21 Hsiao et al. identified the s- and u-structures of Cr3(dpa)4Cl2 based on analysis on their infrared and SERS spectra.22 For Cr3(dpa)4(NCS)2, in which NCS is a strong σ donor ligand, only the s-form exists under ambient conditions. These authors assigned a band at 570 cm 1 appearing on heating the solution in their SERS spectra of Cr3(dpa)4Cl2 to the Cr Cr quadruple-bond-stretching mode of the u-form; the band at 346 cm 1 was assigned to the Cr3 asymmetric stretching mode of the s-form. Both u- and s-forms for trichromium chloride complexes are hence thermally interconvertible in aqueous solution when the complex is bound to a metal nanoparticle. According to the experimental data, the s-form has the lesser energy. The magnetic measurements indicate no spin crossing in trichromium complexes.15 For pentanuclear complexes of Ni5(tpda)4X2 and Co5(tpda)4X2 (tpda2 = tripyridyldiamido dianion; X = Cl , NCS ), their Raman and SERS spectra indicate only the s-form for both molecules at room temperature.23 For SERS measurements, Scheme 1. Chemical Structures of (a) s- and (b) u-Cr5(tpda)4X2 and (c) s- and (d) u-Cr7(teptra)4X2

these complexes were bound to silver or gold nanoparticles in aqueous solution to eliminate the constraint of a crystal lattice. These results contrast with the existence of two structures of trinuclear complexes but agree with the crystalline forms of pentanuclear complexes from X-ray diffraction data. Distinct from those findings for Ni5 and Co5 complexes, Cr5(tpda)4X2, according to X-ray crystal data, has both forms,6,8 as displayed in Scheme 1. The relevant bond distances between metals are listed in Table 1. In the s-form, the bond distances of the outer metal metal bonds are 2.285 Å, slightly greater than 2.246 Å for the inner bonds. For the u-form, the lengths of the Cr Cr bonds alternate: 1.921 2.072 Å indicates a nearly quadruple bond, but 2.497 2.573 Å indicates nearly nonbonding. Limited by the structure of ligands, the metal metal distances can be constrained. Another spectral technique is required to identify these isomeric structures especially for large sizes of complexes. Raman and SERS spectra provide the best means to resolve this problem and the metal metal bonding strength in these structures. Da Re et al. applied Raman spectra and DFT calculations to elucidate the bonding character of extraordinarily short metal metal bonds.24 For instance, for the Cr2(dmp)4 (dmp = 2,6dimethyoxyphenyl) complex, they found no assignable metal metal stretching band near 650 700 cm 1 as predicted by DFT; instead, bands at 345, 363, and 387 cm 1 related to Cr-ligand modes showed isotopic shifts and enhanced intensity in the resonance Raman spectra. These authors proposed mixing of the Cr Cr quadruple-bond motion with the chromium-ligand motions, unlike the case of the stretching motion of the Mo Mo quadruple bond. The molybdenum stretching mode is decoupled from the metal ligand modes such that the vibrational wavenumber for this stretching mode is well-determined. These metal-string complexes possess unique structures that allow an assessment of the strength of Cr Cr quadruple bonding. In our analyses of vibrations for trinuclear and pentanuclear complexes, we found that the bands for Ni510+ and Co510+ stretching modes in pentanuclear complexes are more intense than those in trinuclear complexes. An enhanced absorption of d d transitions in the visible range for the pentanuclear complexes arises from the linear arrangement of metal ions, which might yield a resonant enhancement of Raman intensity for the metal-related bands, for instance, the stretching mode of the quadruple bond of chromium. These results are essential in spectrally characterizing metal metal multiple bonding.

’ EXPERIMENTS Samples. All metal-string complexes, dipyridylamine (Hdpa), tripyridyldiamine (H2tpda), and tetrapyridyltriamine (H3teptra), were synthesized according to methods described elsewhere.3 9 The metal complexes were extracted with CH2Cl2 and purified on recrystallization from solution in CH2 Cl 2 near 23 °C.

Table 1. Bond Distances (Å, with One Standard Deviation in Parentheses) in Cr5(tpda)4Cl2 and Cr5(tpda)4(NCS)2 Complexes from X-ray Data M5

Outer M M a

a

Inner M M

M(1) N

M(2) N

M(3) N

M(4) N

M(5) N

Cr5 Cl

2.2849(1)

2.2405(8)

2.119(3)

2.021(3)

2.050(3)

2.021(3)

2.119(3)

Cr5 NCSa Cr5 NCSb

2.285(2) 2.072(3)/2.497(3)

2.246(1) 2.573(4)/1.921(4)

2.086(3) 2.101(6)

2.019(5) 2.029(9)

2.055(3) 2.062(6)

2.019(5) 2.035(9)

2.086(3) 2.100(6)

Data from ref 6. b Data from ref 8. 13920

dx.doi.org/10.1021/jp203065v |J. Phys. Chem. C 2011, 115, 13919–13926

The Journal of Physical Chemistry C The chromium(II) acetate complex, Cr2(OAC)4(H2O)2, was obtained commercially and used without further purification. The oxidized molecules [Cr3(dpa)4Cl2]PF6 were synthesized on adding the same equivalent proportion of AgPF6 to the Cr3 complex to attain the dark brown solid. The solid sample was then added to a nanoparticle solution to record the SERS spectra. To attain the SERS spectra, trichromium complexes in solid form were added directly to the aqueous nanoparticle solution, but for penta- and heptachromium complexes, ethanol was used to dissolve the complexes before addition to the nanoparticle solution. The concentrations of Cr5 and Cr7 solutions mixed with nanoparticles were 10 5 10 6 M. Oxidized penta- and heptachromium complexes were synthesized by adding CuCl2 and [FeCp2]PF6, respectively. Raman Spectra. To achieve a superior ratio of signal to noise, the Raman spectra were recorded in a back-scattering geometry; the spectral resolution, 3 cm 1, was limited by the monochromator (length, 0.6 m; grating with 600 grooves/mm). A He Ne laser operated with a red light at 632.8 nm served as the excitation source. Diode-pumped Nd:YAG lasers (Photop Suwtech, Inc.) provided a green light at 532 nm and a blue light at 473 nm, separately. The laser power at the sample region was set at 15 mW at each excitation wavelength. The scattered signal passing through a notch filter and monochromator was recorded with a thermoelectrically cooled charge-coupled device (CCD) detector. Samples for SERS measurements were prepared on addition of ethanol solution (a few drops) containing the dissolved metal complexes to an aqueous solution of silver nanoparticles (diameter = 50 70 nm) or gold nanoparticles (diameter ∼ 30 nm). The silver nanopartcles were prepared on reduction of silver nitrate with sodium citrate; these greenishyellow particles display plasmon absorption with a maximum at 414 420 nm. The integration period was typically about 30 s for a solid sample and 1 s for SERS; 100 scans were averaged. An FTRaman spectrometer (Bruker) was used to record the Raman spectra with the near-IR excitation wavelength at 1064 nm; the laser power was 300 mW. To avoid self-absorption for Raman measurements on dark solid samples, the complex was mixed with KBr at a ratio of roughly 1:10. Computational Details. Quantum chemical calculations based on density functional theory (DFT) were performed to obtain optimized geometries, vibrational wavenumbers, and Raman intensities. The initial geometries for the input to DFT calculations were obtained from the crystal structures. The lack of an imaginary wavenumber assured a stable structure. Methods B3LPY, BLYP, and BPW91 with basis set LANL2DZ for Cr and 6-31G* for other atoms in Cr3 and Cr5 metal-string complexes were employed. All calculations were performed with the Gaussian 03 program.25

’ RESULTS AND DISCUSSION Spectra and Analysis of Cr5(tpda)4X2. The Raman spectra of Cr5(tpda)4(NCS)2 and Cr5(tpda)4Cl2 in solid form were recorded at an excitation wavelength of 1064 nm. At wavelengths lower than 1064 nm, the pentachromium complexes readily decomposed even at a fairly low laser power. The Raman spectra of Cr5 complexes, with those of Co5 and Ni5 and free ligand H2tpda for comparison, are shown in Figure 1. In the highwavenumber region, these three complexes display similar features; bands are assigned to pyridyl vibrational modes.23 Modes related to metals and deformation between pyridyl rings lie in the

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Figure 1. Raman spectra of M5(tpda)4Cl2 and H2tpda in solid form, recorded at 296 K with excitation wavelengths of 632.8, 1064, and 532 nm, separately, as indicated. The band wavenumbers and assignments for the metal-related modes are displayed. Correlations between ligand and metal-related modes are indicated with dotted and solid lines, respectively.

low-wavenumber region. The Raman spectra of chloride complexes appear in Figure 1, and those for isothiocyanate complexes are available in the Supporting Information. Spectra of complexes with varied axial ligands, isothiocyanate versus chloride, show no distinct variation in band positions and intensities except for the axial ligand related bands. The spectra of Ni5 and Co5 complexes were recorded with excitation wavelengths of 632 and 532 nm, respectively, and display resonant enhancement of the metalrelated modes because these wavelengths are near visible d d transition bands of those complexes. This condition assists assignment of the wavenumbers of the vibrational modes related to metals, especially for complicated systems, such as the pentanuclear complexes. Huang et al. assigned the vibrational modes in the Raman spectra for Ni5 and Co5 complexes and H2tpda from a comparison of spectra for varied axial ligands and metals and spectra calculated with DFT.23 The SERS spectra recorded at an excitation wavelength of 632 nm for Cr5(tpda)4(NCS)2 are displayed in Figure 2. On comparison of SERS and Raman spectra, similar to those in Co5 and Ni5 complexes, the SERS spectra show variations of band wavenumbers less than 10 cm 1 and exhibit enhanced spectral intensity of the metal-related modes. The Raman spectra of both Cr5 complexes recorded for solid samples with a 1064 nm excitation show features with split bands in the low-wavenumber region. For instance, the bands of the Co5 complex at 421, 451, and 489 cm 1 assigned to Co N stretching, tpda2 out-of-plane bending, and N Co N bending, respectively, are split to 403/413, 442/453, and 479/488 cm 1 in Cr5. Such split bands are observed for both metal-related modes and ligand modes, according to which we infer the existence of both u- and s-forms. 13921



Figure 2. Raman and SERS spectra of Cr5(tpda)4(NCS)2. The Raman spectrum of a solid form was recorded with excitation at 1064 nm, and the SERS spectra for the complex on gold nanoparticles in aqueous solution was recorded with excitation at 632.8 nm and temperatures of 296 and 363 K, separately. The top trace is a SERS spectrum for oxidized Cr5(tpda)4(NCS)2 prepared from reaction with oxidant CuCl2.

Although DFT methods B3LYP, BLYP, and BPW91 were used in calculating the Raman spectra, the positions and intensities agree unsatisfactorily with the experimental data. To assign these vibrational bands in the small-wavenumber region, first, we assumed that the vibrational modes related to tpda2 are least altered for varied metal and axial ligands. Second, the metalrelated modes might be enhanced through resonance or via SERS based on the observation obtained previously.23 Third, the wavenumbers of M N stretching modes varied less than that for the M M mode on replacing metal ions. From a comparison of spectral intensities, positions, and shapes obtained for three metals, two axial ligands, and varied excitation wavelengths, we made the assignments for the Cr5 complexes. Considering the metal metal stretching modes only, including two N or Cl atoms in axial ligands along the metal line, we have seven atoms, resulting in six vibrational stretching modes; among them, three are IR-active and the other three are Raman-active. They include M N(CS) or M Cl symmetric and asymmetric stretching and inner and outer M M symmetric and asymmetric stretching modes. We accordingly assigned split bands at 193 (but red shifted to 184 cm 1 in SERS)/209 cm 1 to Cr N(CS) symmetric stretching and at 191/202 cm 1 to Cr Cl symmetric stretching. The band at 280 cm 1, but shifted to 297 cm 1 in SERS, is assigned

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to the Cr Cr symmetric stretching mode of the s-form. When the sulfur becomes bound to a silver or gold surface, the C N bond is strengthened from decreased π back-bonding; the Cr N(CS) bond is consequently weakened and the Cr Cr bond strengthened. This mechanism explains their spectral shifts in SERS spectra. The wavenumber for the third metal metal stretching mode is expected to be near 360 cm 1, but no band in that region corresponding to this mode is definitely assigned. Split bands at 243/263, shifted to 253/262 in SERS, and 344/ 357 cm 1 for the chloride complex are both assigned to a Cr N(tpda) stretching mode because they are intense in SERS and have wavenumbers similar to those of pentacobalt and pentanickel complexes.23 We assigned split bands at 641, 633, and 621 cm 1 to mixed vibrations of in-plane and out-of-plane deformations of terminal and center pyridyl rings. The N H out-of-plane mode at about 600 cm 1 for H2tpda splits into four lines because of intermolecular hydrogen bonding. These bands disappear on coordination to metals. With 1064 nm excitation, we observed no band assignable to the stretching mode for the Cr Cr quadruple bond of the u-form, although, from our calculations, we expect a measurable Raman intensity for this band. The observation might result from a weak signal for this vibrational mode similar to the finding for the Cr2(dmp)4 complex.24 In SERS spectra with excitation at 632 nm, as shown in Figure 2 for Cr5(tpda)4(NCS)2, the split bands at 405/417 cm 1 assigned to Cr N(tpda) stretching became a broad band with a maximum at 410 cm 1 and enhanced intensity. The tpda2 bands at 440/ 452 and 477/486 cm 1 in Raman spectra appear to become single bands at 458 and 481 cm 1, separately. The intensity of the band for the s-form Cr Cr stretching mode is greatly enhanced. On comparison with band positions of Co5 and Ni5 complexes that exist in only the s-form, for the three pairs of vibrational bands mentioned above, the band with the greater wavenumber is assigned to the s-form. According to their relative intensities, the dominant species in solution is hence the s-form. We observed a weak band at 568 cm 1 that appeared as a shoulder on the band for the pyridyl twisting mode at 620 640 cm 1. The bands at 344/357 cm 1 for the Cr N(tpda) bond-stretching modes vanished, but a single band at 365 cm 1 appeared. Because metal-related modes exhibit enhanced intensity in SERS, some vibrational bands attributed to the u-form appeared, although it has less population. The band at 568 cm 1 is assigned to the stretching mode of the Cr Cr quadruple bond and the 365-cm 1 band to the Cr N(tpda) stretching mode of the u-form. Table 2 lists the observed wavenumbers of bands in the Raman and SERS spectra of Cr5(tpda)4X2 and their assignments for most bands in the wavenumber range of 145 650 cm 1. Conversion between s- and u-Forms in the Cr5 Complex. Figure 2 shows the SERS spectrum recorded near 363 K. This spectrum shows that bands at both 568 and 365 cm 1 increase in intensity. The broad Cr N stretching band at 410 cm 1 splits again to show a second band at 405 cm 1; these two bands correspond to the split bands at 417/405 cm 1 in the Raman spectrum excited at 1064 nm. A similar behavior was observed when reaction with oxidant CuCl2 converted to [Cr5(tpda)4(NCS)2]+, as shown in Figure 2; the oxidized complex is known to have exclusively the u-form.7 Previous results for the trinuclear complex Cr3(dpa)4Cl2 show an intense, but also broad, band at 570 cm 1 when the sample solution was heated. The oxidized form [Cr3(dpa)4Cl2]PF6 has a band at the same wavenumber; its SERS spectrum is depicted in Figure 3. This oxidized trichromium 13922



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Table 2. Wavenumber/cm 1 of Bands in Raman and SERS Spectra for Cr5(tpda)4Cl2 and Cr5(tpda)4(NCS)2 and Assignments of Vibrational Modes Cr5(tpda)4Cl2 Raman

SERS-Ag

145

Cr5(tpda)4(NCS)2 Raman

SERS-Au

assignmenta Γ(ring ring)

145

166

Cr Cr Cl bending

191

187

193

184

202

208

209

208

222 243

253

224 247

249

263

262

259

262

280

297

284

295

s-Cr-Cr str.

315

321

315

318

Δ(C-ring)

344

344

357

357

Cr N(axial) str. Γ(C-ring) Cr N str.

Cr N str. 365

u-Cr-Cr/Cr-N str./Γ(ring)

403 413

413

405 417

410

Cr N str.

440

459

440

458

Γ(C-ring)

481

N Cr N bending

360

453 479

452 485

488 524

477 486

522

523

522

Γ(T-ring)

570

u-Cr-Cr str.

621 633

627

619 631

625 633

Γ(C-ring)/Δ(T-ring) Γ(C-ring)/Δ(T-ring)

642

644

640

642

Γ(C-ring)/Δ(T-ring)

Symbols Δ and Γ denote pyridyl ring in-plane and out-of-plane twisting modes, respectively.

a

Figure 3. SERS spectrum of [Cr3(dpa)4Cl2]+PF6 on gold nanoparticles recorded at 296 K and with excitation at 632.8 nm.

complex has an unsymmetric structure. Under conditions of either high temperatures or oxidation, the unsymmetric form emerges. Nevertheless, the Cr3(dpa)4(NCS)2 complex shows only the s-form even on heating to nearly 90 °C; no band near 570 cm 1 was observed. As predicted by Berry et al.,18 no unsymmetric structure is expected for a complex with the strong σ donor ligand NCS for trichromium complexes. Lin et al. used a scanning tunneling microscope to measure electron transfer in these metal-string complexes;11 they proposed that

the pentachromium isothiocyanate complex became converted from the s- to the u-form after oxidation because of decreased conductivity to manipulate the rate of electron transfer. According to our data, the Cr5(tpda)4(NCS)2 complex converted to the u-form at high temperatures despite NCS being a strong σ donor ligand. The structure of the Cr5 complex in the u-form is reported from X-ray diffraction of the crystal. All experimental data show conclusively that both u- and s-forms exist and that the s-form is more stable. This basis yields a definite assignment of the 570 cm 1 band to the stretching mode of the Cr Cr quadruple bond. The broad nature of this band might result from interaction of metal ions with the metal nanoparticle. After heating and an oxidative reaction, the intensity of the band at 365 cm 1 increased; this band is assigned to the stretching mode of the Cr N(tpda) bond of the u-form. The enhanced intensity of this mode in SERS indicates coupling to the motion involving metals similar to the case of dichromium complexes as Da Re et al. discussed.24 SERS Spectra of the Cr7 Complex. Figure 4 displays SERS spectra of Cr7(teptra)4(NCS)2 (teptra3 = tetrapyridyltriamine anion), oxidant [FeCp2]+, and oxidized Cr7 complex and the Raman spectrum of ligand H3teptra. From the X-ray diffraction data for this complex in the crystalline form, Cr Cr bond distances are symmetrically distributed in the range of 2.211 2.291 Å, which is a delocalized arrangement, as shown in Scheme 1c. The u-form (a localized arrangement, shown in Scheme 1d) cannot, however, be excluded, even though the s-form is expected to be more stable. Comparison of these spectra shows the SERS spectra to have lines mostly attributed to ligand modes. With excitation at 632.8 nm, the metal-related lines are weak because of being off resonance. The absorption spectrum of the Cr7 complex shows a much smaller absorption coefficient at this wavelength than that of the Cr5 complex. Their absorption spectra are displayed in Figure 5. Two weak bands at 554 and 571 cm 1 appear, and no corresponding bands are found in the spectra of the oxidant [FeCp2]+ and the ligand H3teptra. These two bands become intense in the spectra of the oxidized form, which are thus accordingly assigned to stretching of the Cr Cr quadruple bond of the u-form. For a sample near 300 K, the low spectral intensity indicates a small proportion of the complex to be present as the u-form. Quadruple Bonding in Cr2(OAC)4(H2O)2. To compare the vibrational motion for the stretching mode of the Cr Cr quadruple bond, we recorded the Raman spectra of chromium(II) acetate, Cr2(OAC)4(H2O)2. The spectra for the complex in the solid form, excited at 473, 532, and 632.8 nm, appear in Figure 6. Limited by the poor Raman signal, the duration of acquisition of each spectrum was about 50 60 min. We had difficulty obtaining a sufficient SERS signal for this complex; the SERS spectrum is hence unavailable. The calculated spectrum with method UB3LYP and basis set LANL2DZ for Cr and 6-311G** for other atoms is shown in Figure 6 for comparison. We obtained an optimized geometry with a Cr Cr bond distance of 1.835 Å and a Cr O bond distance of 2.014 and 2.011 Å; these distances agree (deviation < 0.01 Å) with those determined from X-ray data. The displacement vectors for vibrational modes involve mainly Cr Cr stretching and O Cr Cr O twisting; their wavenumbers scaled by 0.97 are listed in Table 3. Comparing the calculated and the observed and the spectrum of OAC , we assigned bands at 548 and 337 cm 1 to stretching of the Cr Cr quadruple bond and twisting of O Cr Cr O, separately. Both bands show low intensity even though that at 13923



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Figure 4. SERS spectra of oxidant Fe(Cp)2+PF6 , oxidized Cr7(teptra)4(NCS)2, and Cr7(teptra)4(NCS)2 and the Raman spectrum of H3teptra recorded at 296 K and with excitation at 632.8 nm. Complexes were dissolved in ethanol solution, then added to the aqueous phase containing silver nanoparticles.

Figure 6. Raman spectra of Cr2(OAC)4(H2O)2 in solid form recorded at 296 K and with excitation wavelengths of 473, 532, and 632 nm, separately. The lowest trace is the calculated Raman spectrum using method UB3LYP/6-311G** with vibrational wavenumbers scaled by 0.97.

Figure 5. Absorption spectra of Cr5(tpda)4(NCS)2 and Cr7(teptra)4(NCS)2 at 296 K and in the solvent CH2Cl2.

548 cm 1 is resonantly enhanced at excitation wavelengths of 532 and 473 nm; the complex has a visible d d transition centered at 476 nm (absorption coefficient near 4000 cm 1 M 1). According to the calculated displacement of the vibrational motion for the mode at 338 cm 1, the Cr-ligand twist is coupled to the motion of the Cr Cr bond. A wavenumber for the Cr Cr stretching mode less than those for chromium metalstring complexes is attributed to coordination of axial ligand H2O that weakens the bonding between chromium ions. 13924


The Journal of Physical Chemistry C Table 3. Vibrational Normal Modes for the Cr Cr Stretching and O Cr Cr O Twisting Modes for Cr2(OAc)4(H2O)2 Using Method UB3LYP and Basis Set LANL2DZ for Cr and 6-311G** for the Other Atoms

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atom chains. Conventional DFT calculations method B3LYP, BLYP and BPW91 yield vibrational wavenumbers that deviate from the experimental values; other methods are required to obtain accurate structures and vibrational wavenumbers.

’ ASSOCIATED CONTENT

bS

Supporting Information. Raman spectra of Cr5(tpda)4Cl2, Cr5(tpda)4(NCS)2, Co5(tpda)4(NCS)2, Ni5(tpda)4(NCS)2, and H2tpda in solid form and SERS spectra of Cr3(dpa)4(NCS)2 in silver nanoparticle aqueous solution. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

’ CONCLUSION From analysis of the vibrational bands in their Raman and SERS spectra, we identified metal-string pentachromium and heptachromium complexes in their s- and u-forms. We assigned the vibrational wavenumber at 280 cm 1 to the symmetric stretching mode of Cr Cr in the s-form for the pentachromium complex. The vibrational wavenumber for the stretching motion of the Cr Cr quadruple bond is 570 cm 1 for tri- and pentachromium complexes and 554/571 cm 1 for heptachromium complexes. These three metal-string complexes have Cr Cr quadruple bonding with approximately equal strength, according to the experimental results. The vibrational wavenumber is 548 cm 1 for the same mode in Cr2(OAC)4(H2O)2 that has a slightly weakened Cr Cr quadruple bond. The intensity of the stretching mode of the Cr Cr quadruple bond is enhanced in SERS spectra, possibly because the vibrational motions involving metals interact with metal nanoparticles. The metal-string complexes with metal ions arranged linearly display a large absorption coefficient for the d d band in the visible range. For the Cr3 and Cr5 complexes at 632.8 nm, the absorption coefficient is 5000 7000 cm 1 M 1 greater than that of chromium acetate. This condition might enable resonant enhancement of the Raman intensity for the metal-related modes, especially the metal metal stretching band. For the Cr7 complex, the d d bands may red shift to the near-IR range. Weak enhancement in spectral intensity is observed for the metal-related modes. The experimental data show that the pentachromium complex in both s- and u-structures exists in their crystalline forms. In solution phase when the complex is bound to the metal surface of nanoparticles, no major structural variation is observed, according to the recorded Raman lines. We assumed the complex to be in thermal equilibrium. The s-form is accordingly present in a large proportion. The u-form is thermally accessible via the s-form, indicating that the s-form is the ground state. For heptachromium complexes, the u-form is identified in SERS spectra and exists in addition to the more stable s-form. X-ray diffraction is unable to distinguish these two structures for such large metal-string complexes. The heptachromium complexes of pyrazine-modulated oligo-R-pyridylamido ligands show localized structures consisting of three quadruple Cr Cr bonds and a single terminal Cr(II) atom, according to the results of X-ray diffraction.26 With varied ligands, these complexes show variation on the bonding of metal ions and multiple oxidation forms. Further study on these complexes should provide more information on the bonding character of metal

*E-mail: [email protected].

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Bonding between Chromium Atoms in Metal-String Complexes from

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