Bonding between Chromium Atoms in Metal-String Complexes from Raman Spectra and Surface-Enhanced Raman Scattering: Vibrational Frequency of the Chromium Quadruple Bond

Published: June 28, 2011

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

ARTICLE

pubs.acs.org/JPCC

Bonding between Chromium Atoms in Metal-String Complexes fromRaman Spectra and Surface-Enhanced Raman Scattering: VibrationalFrequency of the Chromium Quadruple BondYu-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

’ INTRODUCTION

Hurley and Robinson1 synthesized the first metal-string com-plex, Ni3(dpa)4Cl2 (dpa

� = di(2-pyridyl)amido anion). Later,Aduldecha and Hathaway2 determined that its structure con-tained collinear metal ions and helically coordinated dipyridyla-mine ligands. Longermetal-string complexes with a similar structurewere subsequently synthesized;3�11 these complexes exhibit uniqueelectric and magnetic properties presaging a prospective function asmolecular wires or switches.12,13 The structures of these extended

metal atom chain complexes have been characterized mostlythrough X-ray diffraction of their crystalline forms. Some complexesexhibit isomeric structures: for instance, tricobalt complexes existwith both symmetric and unsymmetric metal�metal bonding,whereas the Ni3(dpa)4Cl2 complex has exclusively symmetric

Received: April 1, 2011Revised: June 9, 2011

ABSTRACT: By measuring the vibrational wavenumbers of theirstretching modes in Raman and surface-enhanced Raman scattering(SERS) spectra, we investigated the strength of the Cr�Cr bonds inmetal-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 spectralpositions, indicating no major structural variation between the solidand solution forms. For SERS measurements, these complexes werebound to silver or gold nanoparticles in aqueous solution to eliminatethe constraint of a crystal lattice and to maintain the complexes in thermal equilibrium; this method is convenient to identify thestable structure. We identified both penta- and heptachromium complexes in both symmetric (s) and unsymmetric (u) forms. Forpentachromium complexes, our data agree with the results obtained from structural determination of the crystalline form, but for theheptachromium complex, this experimental evidence is the first for the existence of the u-form structure. From our analysis of thevibrational normal modes, we assign the band at 280 cm�1 to the Cr�Cr symmetric stretching mode of the s-form pentachromiumcomplex. According to comparisons of SERS spectra obtained at either high temperatures or under oxidizing conditions, we assign570 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 ofinteraction with the metal nanoparticles. The metal-string complexes with a linear arrangement of metal ions have an increasedabsorption coefficient in the visible spectra and, consequently, an increased resonance Raman intensity for the metal�metalstretching modes, yielding information about the strength of chromium�chromium multiple bonding.

13920 dx.doi.org/10.1021/jp203065v |J. Phys. Chem. C 2011, 115, 13919–13926

The Journal of Physical Chemistry C ARTICLE

Ni�Ni bonding.14,15 The symmetric and unsymmetric metal�metal bonding in the trinuclear complexes was confirmed fromvibrational analysis based on infrared, Raman, and surface-enhanced 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-formhas bond distances of Cr�Cr ∼ 2.36 Å, and the u-form has abond distance of 2.061 Å,18 nearly quadruple bonding betweentwo chromium ions and a high spin Cr2+ (electron spin, S = 2).Berry et al.19 concluded that a Cr3(dpa)4X2 complex would havea u-form for a weak σ donor ligand X. Using density functionaltheory (DFT), Benard and co-workers calculated that, without aconstraint of a crystalline lattice, Cr3(dpa)4Cl2 exists in an s-formin the ground state.20,21 Hsiao et al. identified the s- andu-structures of Cr3(dpa)4Cl2 based on analysis on their infraredand SERS spectra.22 For Cr3(dpa)4(NCS)2, in which NCS

� is astrong σ donor ligand, only the s-form exists under ambientconditions. These authors assigned a band at 570 cm�1 appear-ing on heating the solution in their SERS spectra of Cr3(dpa)4Cl2to the Cr�Cr quadruple-bond-stretching mode of the u-form;the band at 346 cm�1 was assigned to theCr3 asymmetric stretchingmode of the s-form. Both u- and s-forms for trichromium chloridecomplexes are hence thermally interconvertible in aqueoussolution when the complex is bound to a metal nanoparticle.According to the experimental data, the s-form has the lesserenergy. The magnetic measurements indicate no spin crossing intrichromium complexes.15

For pentanuclear complexes of Ni5(tpda)4X2 and Co5(tpda)4-X2 (tpda

2� = tripyridyldiamido dianion; X = Cl�, NCS�), theirRaman and SERS spectra indicate only the s-form for bothmolecules at room temperature.23 For SERS measurements,

these complexes were bound to silver or gold nanoparticles inaqueous solution to eliminate the constraint of a crystal lattice.These results contrast with the existence of two structures oftrinuclear complexes but agree with the crystalline forms ofpentanuclear 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 betweenmetals are listed in Table 1. In the s-form, the bond distances ofthe outer metal�metal bonds are 2.285 Å, slightly greater than2.246 Å for the inner bonds. For the u-form, the lengths of theCr�Cr bonds alternate: 1.921�2.072 Å indicates a nearlyquadruple bond, but 2.497�2.573 Å indicates nearly nonbond-ing. Limited by the structure of ligands, the metal�metaldistances can be constrained. Another spectral technique isrequired to identify these isomeric structures especially for largesizes of complexes. Raman and SERS spectra provide the bestmeans to resolve this problem and the metal�metal bondingstrength in these structures.

Da Re et al. applied Raman spectra and DFT calculations toelucidate the bonding character of extraordinarily short metal�metal bonds.24 For instance, for the Cr2(dmp)4 (dmp = 2,6-dimethyoxyphenyl) 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-ligandmodes showed isotopic shifts and enhanced intensity in theresonance Raman spectra. These authors proposed mixing of theCr�Cr quadruple-bond motion with the chromium-ligandmotions, unlike the case of the stretching motion of theMo�Moquadruple bond. Themolybdenum stretchingmode is decoupledfrom the metal�ligand modes such that the vibrational wave-number for this stretching mode is well-determined. Thesemetal-string complexes possess unique structures that allow anassessment of the strength of Cr�Cr quadruple bonding. In ouranalyses of vibrations for trinuclear and pentanuclear complexes,we found that the bands for Ni5

10+ and Co510+ stretching modes

in pentanuclear complexes are more intense than those intrinuclear complexes. An enhanced absorption of d�d transi-tions in the visible range for the pentanuclear complexes arisesfrom the linear arrangement of metal ions, which might yield aresonant enhancement of Raman intensity for the metal-relatedbands, for instance, the stretchingmode of the quadruple bond ofchromium. These results are essential in spectrally characterizingmetal�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 purifiedon recrystallization from solution in CH2Cl2 near 23 �C.

Scheme 1. Chemical Structures of (a) s- and (b) u-Cr5-(tpda)4X2 and (c) s- and (d) u-Cr7(teptra)4X2

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

M5 Outer M�M Inner M�M M(1)�N M(2)�N M(3)�N M(4)�N M(5)�N

Cr5�Cla 2.2849(1) 2.2405(8) 2.119(3) 2.021(3) 2.050(3) 2.021(3) 2.119(3)

Cr5�NCSa 2.285(2) 2.246(1) 2.086(3) 2.019(5) 2.055(3) 2.019(5) 2.086(3)

Cr5�NCSb 2.072(3)/2.497(3) 2.573(4)/1.921(4) 2.101(6) 2.029(9) 2.062(6) 2.035(9) 2.100(6)aData from ref 6. bData from ref 8.



The chromium(II) acetate complex, Cr2(OAC)4(H2O)2, wasobtained commercially and used without further purification.The oxidized molecules [Cr3(dpa)4Cl2]PF6 were synthesized onadding the same equivalent proportion of AgPF6 to the Cr3complex to attain the dark brown solid. The solid sample wasthen added to a nanoparticle solution to record the SERS spectra.To attain the SERS spectra, trichromium complexes in solid formwere added directly to the aqueous nanoparticle solution, but forpenta- and heptachromium complexes, ethanol was used todissolve the complexes before addition to the nanoparticlesolution. The concentrations of Cr5 and Cr7 solutions mixedwith nanoparticles were 10�5�10�6 M. Oxidized penta- andheptachromium complexes were synthesized by adding CuCl2and [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 monochro-mator (length, 0.6 m; grating with 600 grooves/mm). A He�Nelaser operated with a red light at 632.8 nm served as the excitationsource. 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 at15 mW at each excitation wavelength. The scattered signalpassing through a notch filter and monochromator was recordedwith a thermoelectrically cooled charge-coupled device (CCD)detector. Samples for SERS measurements were prepared onaddition of ethanol solution (a few drops) containing thedissolved metal complexes to an aqueous solution of silvernanoparticles (diameter = 50�70 nm) or gold nanoparticles(diameter ∼ 30 nm). The silver nanopartcles were prepared onreduction of silver nitrate with sodium citrate; these greenish-yellow particles display plasmon absorption with a maximum at414�420 nm. The integration period was typically about 30 s fora solid sample and 1 s for SERS; 100 scans were averaged. An FT-Raman spectrometer (Bruker) was used to record the Ramanspectra with the near-IR excitation wavelength at 1064 nm; thelaser power was 300 mW. To avoid self-absorption for Ramanmeasurements on dark solid samples, the complex was mixedwith KBr at a ratio of roughly 1:10.Computational Details. Quantum chemical calculations

based on density functional theory (DFT) were performed toobtain optimized geometries, vibrational wavenumbers, and Ra-man intensities. The initial geometries for the input to DFTcalculations were obtained from the crystal structures. The lackof an imaginary wavenumber assured a stable structure. MethodsB3LPY, BLYP, and BPW91 with basis set LANL2DZ for Cr and6-31G* for other atoms in Cr3 and Cr5 metal-string complexeswere employed. All calculations were performedwith theGaussian03 program.25

’RESULTS AND DISCUSSION

Spectra andAnalysis of Cr5(tpda)4X2.The Raman spectra ofCr5(tpda)4(NCS)2 and Cr5(tpda)4Cl2 in solid form were re-corded at an excitation wavelength of 1064 nm. At wavelengthslower than 1064 nm, the pentachromium complexes readilydecomposed even at a fairly low laser power. The Raman spectraof Cr5 complexes, with those of Co5 and Ni5 and free ligandH2tpda for comparison, are shown in Figure 1. In the high-wavenumber region, these three complexes display similar fea-tures; bands are assigned to pyridyl vibrational modes.23 Modesrelated to metals and deformation between pyridyl rings lie in the

low-wavenumber region. The Raman spectra of chloride com-plexes appear in Figure 1, and those for isothiocyanate complexesare available in the Supporting Information. Spectra of complexeswith varied axial ligands, isothiocyanate versus chloride, show nodistinct variation in band positions and intensities except for theaxial ligand related bands. The spectra of Ni5 and Co5 complexeswere recorded with excitation wavelengths of 632 and 532 nm,respectively, and display resonant enhancement of the metal-related modes because these wavelengths are near visible d�dtransition bands of those complexes. This condition assistsassignment of the wavenumbers of the vibrational modes relatedto metals, especially for complicated systems, such as the penta-nuclear complexes. Huang et al. assigned the vibrational modes inthe Raman spectra for Ni5 and Co5 complexes andH2tpda from acomparison of spectra for varied axial ligands and metals andspectra calculated with DFT.23

The SERS spectra recorded at an excitation wavelength of632 nm for Cr5(tpda)4(NCS)2 are displayed in Figure 2. Oncomparison of SERS and Raman spectra, similar to those in Co5and Ni5 complexes, the SERS spectra show variations of bandwavenumbers less than 10 cm�1 and exhibit enhanced spectralintensity of the metal-related modes. The Raman spectra of bothCr5 complexes recorded for solid samples with a 1064 nmexcitation show features with split bands in the low-wavenumberregion. For instance, the bands of the Co5 complex at 421, 451,and 489 cm�1 assigned to Co�N stretching, tpda2� out-of-planebending, and N�Co�N bending, respectively, are split to403/413, 442/453, and 479/488 cm�1 in Cr5. Such split bandsare observed for both metal-related modes and ligand modes,according to which we infer the existence of both u- and s-forms.

Figure 1. Raman spectra of M5(tpda)4Cl2 and H2tpda in solid form,recorded at 296 K with excitation wavelengths of 632.8, 1064, and532 nm, separately, as indicated. The band wavenumbers and assign-ments for the metal-related modes are displayed. Correlations betweenligand and metal-related modes are indicated with dotted and solid lines,respectively.



Although DFT methods B3LYP, BLYP, and BPW91 wereused in calculating the Raman spectra, the positions and inten-sities agree unsatisfactorily with the experimental data. To assignthese vibrational bands in the small-wavenumber region, first, weassumed that the vibrational modes related to tpda2� are leastaltered for varied metal and axial ligands. Second, the metal-relatedmodesmight be enhanced through resonance or via SERSbased on the observation obtained previously.23 Third, thewavenumbers of M�N stretching modes varied less than thatfor the M�Mmode on replacing metal ions. From a comparisonof spectral intensities, positions, and shapes obtained for threemetals, two axial ligands, and varied excitation wavelengths, wemade the assignments for the Cr5 complexes. Considering themetal�metal stretching modes only, including two N or Clatoms in axial ligands along the metal line, we have seven atoms,resulting in six vibrational stretching modes; among them, threeare IR-active and the other three are Raman-active. They includeM�N(CS) or M�Cl symmetric and asymmetric stretching andinner and outer M�M symmetric and asymmetric stretchingmodes.We accordingly assigned split bands at 193 (but red shiftedto 184 cm�1 in SERS)/209 cm�1 to Cr�N(CS) symmetricstretching 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

to the Cr�Cr symmetric stretching mode of the s-form. Whenthe sulfur becomes bound to a silver or gold surface, the C�Nbond is strengthened from decreased π back-bonding; theCr�N(CS) bond is consequently weakened and the Cr�Crbond strengthened. This mechanism explains their spectral shiftsin SERS spectra. The wavenumber for the third metal�metalstretching mode is expected to be near 360 cm�1, but no band inthat 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 andhave wavenumbers similar to those of pentacobalt and penta-nickel complexes.23

We assigned split bands at 641, 633, and 621 cm�1 to mixedvibrations of in-plane and out-of-plane deformations of terminaland center pyridyl rings. The N�H out-of-plane mode at about600 cm�1 for H2tpda splits into four lines because of intermo-lecular hydrogen bonding. These bands disappear on coordina-tion to metals. With 1064 nm excitation, we observed no bandassignable to the stretchingmode for the Cr�Cr quadruple bondof the u-form, although, fromour calculations, we expect ameasurableRaman intensity for this band. The observationmight result froma weak signal for this vibrational mode similar to the finding forthe Cr2(dmp)4 complex.24

In SERS spectra with excitation at 632 nm, as shown in Figure 2for Cr5(tpda)4(NCS)2, the split bands at 405/417 cm

�1 assignedto Cr�N(tpda) stretching became a broad band with a maximumat 410 cm�1 and enhanced intensity. The tpda2� bands at 440/452 and 477/486 cm�1 in Raman spectra appear to becomesingle bands at 458 and 481 cm�1, separately. The intensity of theband for the s-form Cr�Cr stretching mode is greatly enhanced.On comparison with band positions of Co5 and Ni5 complexesthat exist in only the s-form, for the three pairs of vibrationalbands mentioned above, the band with the greater wavenumberis assigned to the s-form. According to their relative intensities,the dominant species in solution is hence the s-form. Weobserved a weak band at 568 cm�1 that appeared as a shoulderon the band for the pyridyl twisting mode at 620�640 cm�1. Thebands at 344/357 cm�1 for the Cr�N(tpda) bond-stretchingmodes vanished, but a single band at 365 cm�1 appeared.Because metal-related modes exhibit enhanced intensity inSERS, some vibrational bands attributed to the u-form appeared,although it has less population. The band at 568 cm�1 is assignedto the stretching mode of the Cr�Cr quadruple bond and the365-cm�1 band to the Cr�N(tpda) stretching mode of theu-form. Table 2 lists the observed wavenumbers of bands in theRaman and SERS spectra of Cr5(tpda)4X2 and their assignmentsfor 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. Thisspectrum shows that bands at both 568 and 365 cm�1 increase inintensity. The broadCr�Nstretching band at 410 cm�1 splits againto show a second band at 405 cm�1; these two bands correspond tothe split bands at 417/405 cm�1 in the Raman spectrum excited at1064 nm. A similar behavior was observed when reaction withoxidant CuCl2 converted to [Cr5(tpda)4(NCS)2]

+, as shown inFigure 2; the oxidized complex is known to have exclusivelythe 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; itsSERS spectrum is depicted in Figure 3. This oxidized trichromium

Figure 2. Raman and SERS spectra of Cr5(tpda)4(NCS)2. The Ramanspectrum of a solid form was recorded with excitation at 1064 nm, andthe SERS spectra for the complex on gold nanoparticles in aqueoussolution was recorded with excitation at 632.8 nm and temperatures of296 and 363 K, separately. The top trace is a SERS spectrum for oxidizedCr5(tpda)4(NCS)2 prepared from reaction with oxidant CuCl2.



complex has an unsymmetric structure. Under conditions ofeither high temperatures or oxidation, the unsymmetric formemerges. Nevertheless, the Cr3(dpa)4(NCS)2 complex showsonly the s-form even on heating to nearly 90 �C; no band near570 cm�1 was observed. As predicted by Berry et al.,18 nounsymmetric structure is expected for a complex with the strongσ donor ligand NCS� for trichromium complexes. Lin et al.used a scanning tunneling microscope to measure electrontransfer in these metal-string complexes;11 they proposed that

the pentachromium isothiocyanate complex became convertedfrom the s- to the u-form after oxidation because of decreasedconductivity to manipulate the rate of electron transfer. Accord-ing to our data, the Cr5(tpda)4(NCS)2 complex converted tothe u-form at high temperatures despite NCS� being a strongσ donor ligand. The structure of the Cr5 complex in the u-form isreported from X-ray diffraction of the crystal. All experimentaldata show conclusively that both u- and s-forms exist and that thes-form is more stable. This basis yields a definite assignment ofthe 570 cm�1 band to the stretching mode of the Cr�Crquadruple bond. The broad nature of this band might resultfrom 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 thestretching mode of the Cr�N(tpda) bond of the u-form. Theenhanced intensity of this mode in SERS indicates coupling tothe motion involving metals similar to the case of dichromiumcomplexes as Da Re et al. discussed.24

SERS Spectra of the Cr7 Complex. Figure 4 displays SERSspectra of Cr7(teptra)4(NCS)2 (teptra

3� = tetrapyridyltriamineanion), oxidant [FeCp2]

+, and oxidized Cr7 complex and theRaman spectrum of ligand H3teptra. From the X-ray diffractiondata for this complex in the crystalline form, Cr�Cr bond distancesare symmetrically distributed in the range of 2.211�2.291 Å,which is a delocalized arrangement, as shown in Scheme 1c. Theu-form (a localized arrangement, shown in Scheme 1d) cannot,however, be excluded, even though the s-form is expected to bemore stable. Comparison of these spectra shows the SERSspectra to have lines mostly attributed to ligand modes. Withexcitation at 632.8 nm, the metal-related lines are weak becauseof being off resonance. The absorption spectrum of the Cr7complex shows a much smaller absorption coefficient at thiswavelength than that of the Cr5 complex. Their absorptionspectra are displayed in Figure 5. Two weak bands at 554 and571 cm�1 appear, and no corresponding bands are found in thespectra of the oxidant [FeCp2]

+ and the ligand H3teptra. Thesetwo bands become intense in the spectra of the oxidized form,which are thus accordingly assigned to stretching of the Cr�Crquadruple bond of the u-form. For a sample near 300 K, the lowspectral intensity indicates a small proportion of the complex tobe 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�Crquadruple bond, we recorded the Raman spectra of chromium-(II) acetate, Cr2(OAC)4(H2O)2. The spectra for the complex inthe solid form, excited at 473, 532, and 632.8 nm, appear inFigure 6. Limited by the poor Raman signal, the duration ofacquisition of each spectrum was about 50�60 min. We haddifficulty obtaining a sufficient SERS signal for this complex; theSERS spectrum is hence unavailable. The calculated spectrumwith method UB3LYP and basis set LANL2DZ for Cr and6-311G** for other atoms is shown in Figure 6 for comparison.We obtained an optimized geometry with a Cr�Cr bond distanceof 1.835 Å and aCr�Obond distance of 2.014 and 2.011 Å; thesedistances agree (deviation < 0.01 Å) with those determined fromX-ray data. The displacement vectors for vibrational modesinvolve mainly Cr�Cr stretching and O�Cr�Cr�O twisting;their wavenumbers scaled by 0.97 are listed in Table 3. Compar-ing the calculated and the observed and the spectrum of OAC�,we assigned bands at 548 and 337 cm�1 to stretching ofthe Cr�Cr quadruple bond and twisting of O�Cr�Cr�O,separately. Both bands show low intensity even though that at

Figure 3. SERS spectrum of [Cr3(dpa)4Cl2]+PF6

� on gold nanoparti-cles recorded at 296 K and with excitation at 632.8 nm.

Table 2. Wavenumber/cm�1 of Bands in Raman and SERSSpectra for Cr5(tpda)4Cl2 and Cr5(tpda)4(NCS)2 andAssignments of Vibrational Modes

Cr5(tpda)4Cl2 Cr5(tpda)4(NCS)2

Raman SERS-Ag Raman SERS-Au assignmenta

145 145 Γ(ring�ring)

166 Cr�Cr�Cl bending

191 187 193 184 Cr�N(axial) str.

202 208 209 208

222 224 Γ(C-ring)

243 253 247 249 Cr�N str.

263 262 259 262

280 297 284 295 s-Cr-Cr str.

315 321 315 318 Δ(C-ring)

344 344 Cr�N str.

357 357

360 365 u-Cr-Cr/Cr-N str./Γ(ring)

403 413 405 410 Cr�N str.

413 417

440 459 440 458 Γ(C-ring)

453 452

479 485 477 481 N�Cr�N bending

488 486

524 522 523 522 Γ(T-ring)

570 u-Cr-Cr str.

621 627 619 625 Γ(C-ring)/Δ(T-ring)

633 631 633 Γ(C-ring)/Δ(T-ring)

642 644 640 642 Γ(C-ring)/Δ(T-ring)a Symbols Δ and Γ denote pyridyl ring in-plane and out-of-planetwisting modes, respectively.



548 cm�1 is resonantly enhanced at excitation wavelengths of532 and 473 nm; the complex has a visible d�d transitioncentered at 476 nm (absorption coefficient near 4000 cm�1

M�1). According to the calculated displacement of the vibra-tional motion for the mode at 338 cm�1, the Cr-ligand twist iscoupled to the motion of the Cr�Cr bond. A wavenumber forthe Cr�Cr stretching mode less than those for chromium metal-string complexes is attributed to coordination of axial ligandH2Othat weakens the bonding between chromium ions.

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 silvernanoparticles.

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

Figure 6. Raman spectra ofCr2(OAC)4(H2O)2 in solid form recorded at296 K and with excitation wavelengths of 473, 532, and 632 nm,separately. The lowest trace is the calculated Raman spectrum usingmethodUB3LYP/6-311G**with vibrational wavenumbers scaled by 0.97.



’CONCLUSION

From analysis of the vibrational bands in their Raman andSERS spectra, we identified metal-string pentachromium andheptachromium complexes in their s- and u-forms. We assignedthe vibrational wavenumber at 280 cm�1 to the symmetricstretching mode of Cr�Cr in the s-form for the pentachromiumcomplex. The vibrational wavenumber for the stretching motionof the Cr�Cr quadruple bond is 570 cm�1 for tri- and penta-chromium complexes and 554/571 cm�1 for heptachromiumcomplexes. These three metal-string complexes have Cr�Crquadruple bonding with approximately equal strength, accordingto the experimental results. The vibrational wavenumber is548 cm�1 for the same mode in Cr2(OAC)4(H2O)2 that has aslightly weakened Cr�Cr quadruple bond.

The intensity of the stretching mode of the Cr�Cr quadruplebond is enhanced in SERS spectra, possibly because the vibra-tional motions involving metals interact with metal nanoparti-cles. The metal-string complexes with metal ions arrangedlinearly display a large absorption coefficient for the d�d bandin the visible range. For the Cr3 and Cr5 complexes at 632.8 nm,the absorption coefficient is 5000�7000 cm�1 M�1 greater thanthat of chromium acetate. This condition might enable resonantenhancement of the Raman intensity for the metal-relatedmodes, especially the metal�metal stretching band. For theCr7 complex, the d�d bands may red shift to the near-IR range.Weak enhancement in spectral intensity is observed for themetal-related modes. The experimental data show that thepentachromium complex in both s- and u-structures exists intheir crystalline forms. In solution phase when the complex isbound to the metal surface of nanoparticles, no major structuralvariation is observed, according to the recorded Raman lines. Weassumed the complex to be in thermal equilibrium. The s-form isaccordingly present in a large proportion. The u-form is ther-mally accessible via the s-form, indicating that the s-form is theground state. For heptachromium complexes, the u-form isidentified in SERS spectra and exists in addition to the morestable s-form. X-ray diffraction is unable to distinguish these twostructures for such large metal-string complexes. The heptachro-mium complexes of pyrazine-modulated oligo-R-pyridylamidoligands show localized structures consisting of three quadrupleCr�Cr bonds and a single terminal Cr(II) atom, according to theresults of X-ray diffraction.26 With varied ligands, these com-plexes show variation on the bonding of metal ions and multipleoxidation forms. Further study on these complexes shouldprovide more information on the bonding character of metal

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

’ASSOCIATED CONTENT

bS Supporting Information. Raman spectra ofCr5(tpda)4Cl2,Cr5(tpda)4(NCS)2, Co5(tpda)4(NCS)2, Ni5(tpda)4(NCS)2, andH2tpda in solid form and SERS spectra of Cr3(dpa)4(NCS)2 insilver nanoparticle aqueous solution. This material is availablefree of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

’ACKNOWLEDGMENT

The National Science Council of the Republic of Chinaprovided support for this research, and the National Center forHigh-Performance Computing, Taiwan, provided computingfacilities.

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Table 3. Vibrational Normal Modes for the Cr�CrStretching and O�Cr�Cr�O Twisting Modes forCr2(OAc)4(H2O)2 Using Method UB3LYP and BasisSet LANL2DZ for Cr and 6-311G** for the Other Atoms



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Bonding between Chromium Atoms in Metal-String Complexes from Raman Spectra and Surface-Enhanced Raman Scattering: Vibrational Frequency of the Chromium Quadruple Bond