Metal−Metal Bonding and Structures of Metal−String Complexes: Tripyridyldiamido Pentanickel and Pentacobalt from IR, Raman, and Surface-Enhanced Raman Scattering Spectra

Published: January 10, 2011

r 2011 American Chemical Society 2454 dx.doi.org/10.1021/jp110311t | J. Phys. Chem. C 2011, 115, 2454–2461

ARTICLE

pubs.acs.org/JPCC

Metal-Metal Bonding and Structures of Metal-String Complexes:Tripyridyldiamido Pentanickel and Pentacobalt from IR, Raman, andSurface-Enhanced Raman Scattering SpectraYu-Min Huang, 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 Road, 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,P. R. China 710065

bS Supporting Information

ABSTRACT: We recorded infrared, Raman, and surface-enhancedRaman scattering (SERS) spectra of metal-string complexes Ni5-(tpda)4X2 and Co5(tpda)4X2 (tpda = tripyridyldiamido, X = Cl-,NCS-) and free ligand tripyridyldiamine (H2tpda) to determinetheir vibrational wavenumbers and the strength of the metal-metalbonds. For SERS measurements, these complexes were adsorbed onsilver or gold nanoparticles in aqueous solution to eliminate theconstraint of a crystal lattice and to maintain the complexes inthermal equilibrium. The spectra of SERS and Raman modes showinsignificant deviation in spectral features and band positions. Weobserve a single breathing band of pyridyl in Co5(tpda)4X2, indicating the existence of only the symmetric form, whereas splitpyridyl lines are observed for Ni5(tpda)4X2 and assigned to arise because of a varied environment of coordination: squareplanar for the inner nickels and square pyramidal for the outer nickels in the complexes. From our analysis of the vibrationalnormal modes, we assign lines at 257/266 and 302/313 cm-1 to Ni5, at 287/284 and 355/360 cm-1 to Co5 symmetricstretching modes, and at 255/267 and 297/305 cm-1 and 319/323 and 391/392 cm-1 to Ni5 and Co5 asymmetric stretching,respectively, for complex with axial ligand Cl/NCS. The bonding in Ni-Ni is weaker than for Co-Co, consistent with theprediction from molecular-orbital theory.

1. INTRODUCTION

Metal-string complexes with polypyridylamine ligands co-ordinated helically to linear metal ions have been shown toexhibit great electric conductivities1,2 and to have a great pot-ential to function as molecular wires. The smallest complex ofthis kind isM3(dpa)4X2 (M=Ni,3-8 Co,9-19 Cr,20-23 etc., dpa =di(2-pyridyl)amido). The structures of these complexes aredetermined mostly through X-ray diffraction of crystalline form.These results show that some complexes have isomeric struc-tures; for instance, tricobalt complexes exist with both symmetricand unsymmetric metal-metal bonding, whereas complex

Ni3(dpa)4Cl2 has exclusively symmetric Ni-Ni bonding. Cleracet al. showed that the infrared spectrum of complexCo3(dpa)4Cl2 displays split pyridyl lines.

20 Lai et al. reportedboth IR and Raman spectra of complexes Co3(dpa)4Cl2 andNi3(dpa)4Cl2 in their solid forms; they showed a split in-planedeformation line for the pyridyl ring at ∼1000 cm-1 in Ramanspectra for Co3(dpa)4Cl2 and a single line for Ni3(dpa)4Cl2.

24

Received: October 28, 2010Revised: December 19, 2010

2455 dx.doi.org/10.1021/jp110311t |J. Phys. Chem. C 2011, 115, 2454–2461

The Journal of Physical Chemistry C ARTICLE

These authors explained the split lines to be due to the existenceof unsymmetric and symmetric structures, which was confirmedwith measurements of X-ray diffraction. Lai et al. also assignedthe vibrational wavenumbers of the Ni3 stretching and Co3asymmetric stretching modes of these complexes to show thestrength of the metal-metal bonds.24 Under oxidation, thebond lengths between metal atoms alter; this effect provides afurther way to confirm the assignments of vibrational wave-numbers for the stretching of the metal-metal bond.25 Accord-ing to these analyses, the wavenumbers for metal-metalvibrational modes agree with a trend predicted with simplemolecular-orbital theory in that the Ni-Ni bond is weaker thanfor Co-Co in these metal-string complexes.

For Co3(dpa)4Cl2, Rohmer et al. showed a shallow ground-state surface with respect to distortion of the Co3 frameworkbased on the results of calculations;26,27 they thus averred that thetwo structures characterized with X-ray diffraction cannot beconsidered to be bond-stretch isomers. Pantazis and McGradysuggested that the three states comprise a doublet ground state2Awith aminimal energy at a symmetric geometry, a quartet state4B with unsymmetric equilibrium geometry, and another doubletstate 2B with a broad and smooth energy surface crossing theformer two states.28 At high temperatures, symmetric andunsymmetric forms arise from the population of 2A and 4Bstates. Decreasing the temperature results in a spin crossing to 2B.Environmental effects, such as temperature, cosolvent in a crystaland axial ligands, can alter the shape and the energy of thepotential surface. Co3 metal-string complexes thus have variedforms but are not structural isomers.

In this group, we identified the symmetric and unsymmetricstructures of Cr3(dpa)4Cl2 by means of their IR and SERSspectra.29 For Cr3(dpa)4(NCS)2, only the symmetric form existsunder ambient conditions. A line at 570 cm-1 in the SERSspectra of Cr3(dpa)4Cl2 appeared on heating the solution and isassigned to the Cr-Cr quadruple-bond stretching mode of theu-form. The line at 346 cm-1 is assigned to the Cr3 asymmetricstretching mode of the s-form. Hence, for both trichromiumcomplexes the structure in the ground state is the s-form. Frommagnetic measurements on trichromium complexes, no spincrossing is observed.20

A series of linear pentanuclear metal-string complexes, forinstance, Ni5(tpda)4X2 and Co5(tpda)4X2, H2tpda = tripyridyl-diamine, X = Cl-, CN-, N3

-, and NCS-, etc., were

synthesized.30,31 These metal-ion lines are coordinated helicallywith pentadentate nitrogen chelating ligands. The chemicalstructures of those for X = Cl- and SCN-, with relevant bonddistances, are displayed in Scheme 1. In Ni5 complexes, thecrystal data show symmetric structures with both Ni-Ni dis-tances: outer 2.369 and inner 2.300 Å in Ni5(tpda)4(NCS)2larger than those of 2.276 and 2.231 Å, respectively, in Co5-(tpda)4(NCS)2. For nickel complexes, the three inner Ni ions allhave four-coordinated, square-planar conformations, when ne-glecting the Ni-Ni bonds. All Ni-N distances to tpda2- of1.89-1.90 Å are small, consistent with the typical distance foundin the low-spin (S = 0) square-planar Ni(II) configurationsystem. The terminal Ni(II) ions have a square-pyramidalgeometry, and the mean Ni-N distances 2.10 Å are consistentwith a high-spin Ni(II) (S = 1) configuration. These assignmentsare confirmed with the results of magnetic measurements andX-ray near-edge absorption spectra.30

According to X-ray crystal data, only a symmetric structureexists in the pentacobalt complexes, unlike the tricobalt com-plexes. A simple theoretical calculation (extended H€uckelmolecular-orbital method) shows that Co5

10þ comprises five σ and10 π bonding and antibonding and five nonbonding orbitals.With 35 electrons in these orbitals, there are in total 2.5 netbonded, paired electrons. Hence, for each Co-Co, the bondorder is near 0.5. For Ni5

10þ, 40 electrons fill all bonding andantibonding orbitals, resulting in no net bonding betweenNi-Ni. The Ni-Ni bond is consequently expected to beweaker than the Co-Co bond in the pentanuclear complexes.

In the present work, we studied the structures and strengths ofmetal-metal bonds in metal-string complexes using Ramanand infrared spectra for pentanuclear complexes Co5(tpda)4X2

and Ni5(tpda)4X2 (X = Cl- and SCN-). To eliminate theconstraint of a crystal lattice, we bound these complexes oneither silver or gold nanoparticles in aqueous solution to searchfor any existence of structural isomers in the equilibrium condi-tion. We assign the vibrational wavenumbers for the bandsconcerning metal-metal bonding to resolve the bond strengthand to examine the bonding model derived from theory.

2. EXPERIMENTS

The pentanuclear metal-string complexes and tripyridyldia-mine, H2tpda, were synthesized according to methods described

Scheme 1. Chemical Structures of Ni5(tpda)4Cl2, Ni5(tpda)4(NCS)2, Co5(tpda)4Cl2, and Co5(tpda)4(NCS)2 and Their BondLengths/Å Taken from Refs 30 and 31, Respectively



elsewhere.30-33 These complexes were extracted with CH2Cl2and purified on recrystallization from solution in CH2Cl2 near23 �C. Deep purple pentanickel and dark brown pentacobaltcrystals were obtained.

IR absorption spectra in the far-infrared region 150-650 cm-1

were recorded with an infrared spectrometer (Bomem,NTHU Instrument Center) and the near and mid-infraredregion 400-4000 cm-1 on another infrared interferometer(Perkin-Elmer SpectrumOne B). Solid samples were mixed withCsI at a ratio of 1:1-1:2 for the small-wavenumber range toobtain sufficient absorbance. The Raman spectra were recordedin a backscattering geometry to achieve a superior ratio ofsignal-to-noise; the spectral resolution, 3 cm-1, was limited bythe monochromator (length 0.6 m, grating with 600 grooves/mm).A He-Ne laser operated with red light at 632.8 nm served as theexcitation source. Diode-pumped Nd:YAG lasers (Photop Suw-tech, Inc.) provided green light at 532 nm and blue light at473 nm, individually. The laser power at the sample region was setat 15 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 onadding a few drops of ethanol solution containing the dissolvedmetal complexes to an aqueous solution of silver nanoparticles(diameter 50-70 nm) or gold nanoparticles (diameter∼30 nm).The silver nanoparticles were prepared on reduction of silvernitrate with sodium citrate; these greenish-yellow particles displayplasmon absorption with maximum at 420 nm. The integra-tion period was typically about 30 s for a solid sample and 1 s forSERS and averaged over 100 scans. An FT-Raman spectrometer(Bruker) was used to record the Raman spectra with near-IR

excitation wavelength at 1064 nm; the laser power was300 mW. To avoid self-absorption for Raman measurementson solid samples, the complex was mixed with KBr at a ratio ofroughly 1:10.

Quantum-chemical calculations based on density-functionaltheory (DFT) were performed to obtain optimized geometries,vibrational wavenumbers, and both Raman and IR intensities.The B3LPY method with basis set 6-311þþG** and 6-31G* wasemployed for H2tpda andmetal complexes, separately, to achievereliable results. All calculations were performed using theGAUSSIAN 03 program.34

3. RESULTS AND DISCUSSION

3.1. Spectral Analysis for Lines above 500 cm-1. Eachmetal complex has four pentadentate ligands tdpa2- of which thevibrational lines overlap under the experimental conditions andthus become undifferentiated. Because these metal complexeshave high symmetry, symmetric formD4 group and unsymmetricC4, their IR and Raman spectra are, however, to some extentcomplementary. InC4, all symmetry species are both Raman- andIR-active except that modes with b symmetry are only Raman-active. In D4, vibrational modes with symmetry b1 and b2 appearonly in Raman scattering and with a2 in IR absorption. Hence,similar positions observed in IR and Raman spectra can serve todistinguish the symmetry species.Figure 1 displays Raman spectra of Co5(dpa)4(NCS)2 in solid

form with excitation wavelengths of 473, 532, 632, and 1064 nm.In the low-wavenumber region, the vibrational lines mostlyinvolve motions of metal atoms; the spectral intensities areenhanced with 532 nm excitation. From the visible absorptionspectrum, the Co5 complexes have a narrow absorption bandcentered at 525 nm, and this feature hence corresponds to a d-dtransition. For the Ni5 complexes, the spectral intensities ofmetal-related bands are enhanced at an excitation wavelength of632 nm. The corresponding visible band for Ni5 complexes iscentered at 580 nm. The Raman spectra of Ni5 at these excitationwavelengths are shown in the Supporting Information. Thevariation in the spectral intensity from resonance enhancementprovides additional information pertinent to assigning the vibra-tional modes.Figure 2 displays Raman spectra of Ni5(tdpa)4Cl2 and H2tpda

in the wavenumber region 150-1650 cm-1. In general, the

Figure 1. Raman spectra (150-1650 cm-1) of Co5(tpda)4(NCS)2 insolid form, recorded at excitation wavelengths of 473, 532, 632.8, and1064 nm.

Figure 2. Raman spectra (150-1650 cm-1) of Ni5(tpda)4Cl2 andH2tpda in solid form, obtained with excitation at 632.8 nm and someassignments. The symbol ν denotes a stretching mode, δ and γ in-planebending and out-of-plane bending of pyridyl; andΔ and Γ pyridyl ring orring-ring in plane and out-of-plane twisting modes, respectively.



spectral features in IR andRaman spectra in the range 400-1600 cm-1,in which most pyridyl vibrations are located, are similar.H2tpda spectral lines are assignable from comparison with those ofpyridyl, dipyridylamine (Hdpa),29 and the spectrum calculatedwith the DFTmethod. This molecule is nonplanar with symmetryC2; all vibrational modes are both Raman- and IR-active. Thecalculated line positions agree satisfactorily with the experimentaldata. The positions of measured and calculated lines and theirassignments for H2tpda are listed in the Supporting Inform-ation. The breathing modes have lines at 985 and 990 cm-1 forthe center (C-ring) and terminal (T-ring) pyridyl, respectively.The calculated results are based on an isolated structure, whereasH2tpda in the solid form has intermolecular hydrogen bonding.Some lines in Raman spectra are split: for example, N-H in-planebending and N-H out-of-plane mode in regions ∼600 and∼1550 cm-1, separately, each of them split into four lines.These N-H bending modes for H2tpda disappear in metal

complexes because of deprotonation upon coordination. The in-plane deformation of the pyridyl ring shows a single line for theCo5 complex at 1020 cm

-1 but is split to 1010 and 1027 cm-1 inboth Ni5 complexes, as shown in Figure 2. Similarly, the line forthe out-of-plane motion of the terminal pyridyl ring mixing within-plane pyridyl ring deformation in H2tpda is assigned to beabout 672 cm-1 and is red-shifted to 642 cm-1 in Co5 but is splitto 643 and 632 cm-1 in Ni5. Similar splitting in both placesappears in the SERS spectra of Ni5, which were recorded inaqueous solution; the spectra are shown in the SupportingInformation. In the unsymmetric form of Co3(dpa)4Cl2, thesplit pyridyl lines are explained to reflect varied Co-Co bondingresulting in two Co-N bonds of distinct distance and a smalldeviation in the ring-breathing frequency.24 Accordingly, weattribute the deviation to result from nickels of two types;one with square-planar and the other with square-pyramidalcoordination;resulting in varied distances of Ni-N bonds andpyridyl vibrational wavenumbers. In the trinickel complexes, twocoordination modes are also present, but the distances of Ni-N(pyridyl) bonds are comparable; no deviation in wavenumbers ofpyridyl vibrations under current experimental resolution is henceobserved. No splitting is observed for the pentacobalt complexesin both solid and aqueous nanoparticle solution, as shown inFigure 3, to imply existence of an unsymmetric structure. Thisresult in fact agrees with results from the structure determinationfor crystals.3.2. Spectral Analysis for Lines below 500 cm-1. Figures 4

and 5 show Raman spectra in the low-wavenumber region ofH2tpda and pentanuclear complexes with axial ligand chlorideand isothiocyanate, respectively; the IR spectra of Ni5(tpda)4Cl2

and Co5(tpda)4Cl2 are depicted in Figures 6 and 7, respectively.In this region, the metal-related vibrational modes occur, plusthose for the deformation between pyridyl rings in ligand tpda2-.First, we compare the spectra of the metal-string complexeswith H2tpda to assign the lines involving mainly ligand motion;the vibrational wavenumbers of these modes are expected toremain least altered for variedmetal and axial ligands. Second, themetal-related line intensity is enhanced in resonance Ramanspectra. Hence, from a comparison of spectra in both positionsand intensities shown in Figures 4-7, we assign lines at 271 and395 cm-1 in Ni5(tpda)4Cl2 and at 392 cm

-1 in Co5(tpda)4Cl2 tothe pyridyl-pyridyl, ring-ring in-plane deformation mode andlines at 471 and 526 cm-1 in Ni5(tpda)4Cl2 and at 451 and524 cm-1 in Co5(tpda)4Cl2 separately to center and terminalpyridyl out-of-plane deformationmodes. Their corresponding IRlines are observed at similar positions.

Figure 3. SERS spectrum of Co5(tpda)4Cl2 on silver nanoparticles with632.8 nm excitation.

Figure 4. Raman spectra of H2tpda and Ni5(tpda)4Cl2 in solid formexcited at 632.8 nm and of Co5(tpda)4Cl2 excited at 532 nm in theregion 150-550 cm-1. The line positions and the assignments for themetal-related modes are displayed. The ligand and metal-related modesare correlated with dashed and solid lines, respectively.

Figure 5. Raman spectra of H2tpda and Ni5(tpda)4(NCS)2 excited at632.8 nm and of Co5(tpda)4(NCS)2 excited at 532 nm in crystal formmixed with KBr (ratio 1:10) to avoid self-absorption. The line positionsand the assignments for the metal-related modes are displayed. Theligand and metal-related modes are correlated with dashed and solidlines, respectively.



Because of the complicated nature of metal vibrational mo-tions, we performed quantum-chemical calculations to obtain thevibrational motions of normal modes and to estimate thevibrational wavenumbers for the metal-related modes. For our

metal complexes withD4 symmetry, the symmetric stretching inlinear pentametal ions is Raman-active, and the asymmetricstretching is IR-active. The spin states for these metal complexesare recognized based on the results from magnetic measure-ments to be S= 1/2 forCo5

32 and S= 0 or 2 for theNi5. Near 300K,S = 2 of Ni5 has a greater population than the singlet stateaccording to the Boltzmann distribution. Our experimental datashow no variation in vibrational frequencies to differentiatethese two states. Table 1 lists some bond distances related tometals from the optimized geometries obtained from thecalculations using the method B3LYP/6-31G*. The optimizedgeometries show on average∼0.01 Å in the metal-metal bonddistances and ∼0.02 Å for M-N greater than those from theX-ray crystal data. Table 2 depicts the displacement vectors ofthe normal modes for the metal symmetric and asymmetricstretching. In complexes, including two chlorides (or nitrogen),seven atoms have six stretching modes along the metal ion line,among them three Raman- and three IR-active. All vibrationalmotions involve displacements of many atoms, but we distin-guish them based on the substantial displacements of metalatoms. The descriptions of modes are shown in Table 2 with thecalculated wavenumbers scaled by 0.96 for these six stretchingmodes.

Figure 6. IR (upper) and Raman (lower) spectra of Ni5(tpda)4Cl2 in the region 150-550 cm-1. Symbols are defined in the caption of Figure 2.

Figure 7. IR (upper) and Raman (lower) spectra of Co5(tpda)4Cl2 in the region 150-550 cm-1. Symbols are defined in the caption of Figure 2.

Table 1. Geometries (Bond Distance/Å, One Standard De-viation in Parentheses) from Calculated and ExperimentalData for Ni5(tpda)4Cl2 and Co5(tpda)4Cl2

Ni5(tpda)4Cl2 Co5(tpda)4Cl2

DFT method UB3LYP UB3LYP

basis set 6-31G* 6-31G*

total electron spin S 2 1/2

total energy (Hartree) -5175.35637756 -5053.97354340

exptla calcd exptlb calcd

M-M(inner) 2.305(1) 2.31634 2.234(4) 2.24818

M-M(outer) 2.385(2) 2.41516 2.281(4) 2.29319

M(1)-N 2.111(9) 2.15398 1.983(5) 2.01572

M(2)-N 1.90(2) 1.93410 1.914(4) 1.93184

M(3)-N 1.904(8) 1.92740 1.930(4) 1.95050aCrystal data from ref 30. bCrystal data from ref 31.



The lines involvingM-Cl symmetric and asymmetric stretch-ing are readily identified on comparing the spectra of two axialligands; they are assigned to 216 and 206 cm-1 and 197 and188 cm-1 for the Ni5 and Co5 chlorides, respectively. For theisothiocyanate complexes, the M-N(CS) symmetric stretchingis assigned to a line at 224 and 208 cm-1 for Ni5 and Co5,respectively; the Ni-N(CS) asymmetric stretch is assigned to anIR line at 208 cm-1, but the wavenumber for Co-N(CS) issmaller than our detection limit. The remaining unassignedbands in the Co5 spectra are blue-shifted relative to those ofNi5. In addition, for the metal modes we expect that the wave-numbers are altered significantly when replacing cobalt by nickelbecause of the varied bond strength. The M-N stretching andN-M-N bending likely vary to a small extent. On this basis, weassign theRaman lines at 257 and 302 cm-1 and 266 and 313 cm-1

to Ni5 symmetric (inner and outer metal-metal) stretchingmodes and the IR lines observed at 255 and 297 cm-1 and 267 and305 cm-1 to Ni5 asymmetric stretchingmodes (outer metal-metalandM-Mas shown in Table 2) forNi5(tpda)4Cl2 andNi5(tpda)4-(NCS)2, respectively. The Raman lines at 287 and 355 cm-1 and284 and 360 cm-1 are assigned to Co5 symmetric stretching modesand IR lines at 319 and 391 cm-1 and 323 and 392 cm-1 toasymmetric stretching modes for Co5(tpda)4Cl2 and Co5(tpda)4-(NCS)2, respectively.After assignment of the metal-stretching modes, the remaining

lines are M-N stretching and N-M-N bending modes. Accord-ing to the assignments for trinuclear complexes, in most cases, thespectral intensity for the bending mode is weaker than that for theM-N stretching mode. The Raman lines at 234, 334, and 419/425 cm-1 are accordingly assigned to the Ni-N stretching modeof Ni5(tpda)4Cl2 and lines at 235, 337, and 410/427 cm-1 forNi5(tpda)4(NCS)2. The Co-N stretching modes are assigned forRaman lines at 227, 335, and 421 cm-1 and at 227, 334, and421 cm-1 for Co5(tpda)4Cl2 andCo5(tpda)4(NCS)2, respectively.These assignments indicate unequal M-N bonding. From thedelocalized charge distribution in tpda2-, the electron density nextto themetal is in the orderM-Namido > centralM-Npy > terminalM-Npy, and the M-N distances are in the reverse order forpentanuclear complexes. Hence, three distinct M-N stretchingwavenumbers are observed in pentanuclear complexes.15 Theamido N with the greatest M-N stretching vibrational wavenum-ber has the strongest coordination to themetal. TheN-Ni-NandNi3 bending modes are assigned to lines at 498 and 285 cm-1,respectively, and N-Co-N bending is assigned to 489 cm-1 andCo3 bending to 335 cm-1; the line for the Co3 bending mode isblendedwith the line for theCo-Nstretchingmode.Overall, mostobserved bands are assigned; the list of line positions and assign-ments in the low-wavenumber region appears in Table 3. InFigures 4-7 we show these assignments and their correlations.From comparison of the calculated and the observed positions

for the metal-metal stretching modes, the calculated valuesagree satisfactorily with the experimental data, but the calculatedpositions and intensities for other metal-related modes areunsatisfactory for making all assignments simply based on thetheory. The results of DFT calculations nevertheless provideapproximate wavenumbers and intensities of vibrational modesthat are informative in assigning the spectra. From comparison ofthe spectral intensity for metal-string compounds of variedlengths, the spectral lines for metal-metal bonding modes intrinuclear complexes are much weaker than those for M-N,whereas the lines for M5

10þ stretching modes in pentanuclearcomplexes are intense despite there being in total 20 M-Nbonds. A greater polarizability for these M5

10þ stretching vibra-tion modes might arise from the linear arrangement of metals,yielding a more polarizable electron density in these complexes.Using the SERS technique, we recorded vibrational spectra of

the complex with great sensitivity and no constraint of a crystallattice; this method is convenient to determine quantitatively theisomers exiting in an equilibrium condition. The deviation in theline positions between the obtained Raman and the SERS spectrais at most 10 cm-1, indicating that nomajor structural variation ispresent when bonded to the metal surface in the solution phase.In the low-wavenumber region, the spectral intensities are enhancedin SERS spectra. A SERS spectrum of Co5(tpda)4Cl2 is shownin Figure 3 with a single pyridyl line at 636 and 1022 cm-1.For both Ni5 and Co5 complexes, we hence conclude that nostructural isomer is detected near 295 K, unlike the case of

Table 2. Vibrational Motions and Wavenumbers/cm-1

Involving Metals for Ni5(tpda)4Cl2 and Co5(tpda)4Cl2



tricobalt chloride for which a low-lying quartet state exists andspin crossover can occur at low temperature such that anunsymmetric form exhibits. According to our results and thosefrom magnetic measurements, the quartet state for Co5 com-plexes might lie at greater energy. In Ni complexes, nickels bondto pyridyls differently to show split bands in their breathingmodes. However, the Ni5 complexes only exit the symmetricstructure. From the assigned bands, the strength of the Co-Cobond is greater than for Ni-Ni, in agreement with predictionbased on simple extended-H€uckel molecular-orbital theory. Thebonding between nickels is nearly absent in Ni5 complexes,hence the assigned vibrational wavenumbers for Ni-Ni stretch-ing seem large. This can be because these modes also involve themetal ligand motions.

4. CONCLUSION

Using Raman, IR, and SERS spectra, we found that bothCo5(tpda)4X2 andNi5(tpda)4X2 (X =Cl- andNCS-) have onlysymmetric metal-metal bonding near 295 K, distinct fromCo3(dpa)4Cl2. In Ni5(tpda)4X2, the inner and outer Ni havedissimilar coordination to yield varied breathing wavenumbers

for the pyridyl rings in ligands. From the analysis of vibrationalnormal modes and comparison with spectra for four penta-nuclear complexes, the Ni5 symmetric stretching mode in com-plex Cl/NCS is assigned to 257/266 and 302/313 cm-1 andasymmetric stretching to 255/267 and 297/305 cm-1 and theCo5 symmetric stretching to 287/284 and 355/360 cm-1 andasymmetric stretching to 319/323 and 391/392 cm-1. Overall,Co-Co has a stronger bond than Ni-Ni in the pentanuclearcomplexes; there is hence expected to be greater electric con-ductivity in Co5 than in Ni5. The structure with symmetricCo-Co bond lengths is expected to possess greater conductivitythan with unsymmetric isomers. Theoretical methods, for in-stance, DFT and others, might be more applicable and arerequired to search for unsymmetic structures and an explanationof the absence of an unsymmetric form of these pentanuclearmetal-string complexes.

’ASSOCIATED CONTENT

bS Supporting Information. Raman spectra at excitationwavelengths of 532, 473, 632, and 1064 nm and SERS spectra

Table 3. Raman, Infrared, and SERS Line Wavenumbers/cm-1 and Assignments for Pentanickel and Pentacobalt Complexes

Ni5(tpda)4Cl2 Ni5(tpda)4(NCS)2 Co5(tpda)4Cl2 Co5(tpda)4(NCS)2

Raman IR SERS Raman IR SERS Raman IR SERS Raman IR SERS assignmenta

165 165 Ni-Ni-Cl bending

167 178 164 Γ(ring-ring)

187 185 183 183 Δ(ring-ring)

206 188 M-Cl asym. str.

216 215 197 201 M-Cl sym. str.

208 M-N (axial) asym. str.

224 219 208 213 M-N (axial) sym. str.

234 233 228 235 241 234 227 232 227 229 M-N str.

240 239 M3 bending

261 263 266 265 266 263 268 Γ(ring)

257 266 287 290 284 289 M-M sym. str.

255 267 319 323 M-M asym. str.

271 272 Δ(ring-ring)

284 280 299 Δ(ring-ring)

285 297 300 335 335 343 344 337 345 M3 bending

302 297 313 310 355 359 360 366 M-M sym. str.

297 305 391 392 M-M asym. str.

334 339 337 340 335 343 344 345 M-N str.

359 356 359 M3/N-M-N bending

382 382 M3/N-M-N bending

395 393 389 396 394 397 392 391 393 390 360 395 Δ(ring-ring)

408 407 412 414 Γ(T-ring)

419 421 410 420 421 424 421 425 M-N str.

425 433 427 433 M-N str.

434 434 428 430 N-M-N bending

466 463 458 458 Γ(C-ring)

471 466 462 464 451 452 451 463 Γ(C-ring)

498 498 499 496 497 499 489 499 494 489 499 498 N-M-N bending

486 486 487 486 N-M-N bending

526 525 522 523 526 520 524 527 524 525 528 524 Γ(T-ring)aThe symbol ν denotes a stretching mode, and δ and γ denote in-plane bending and out-of-plane bending of pyridyl and Δ and Γ pyridyl-pyridylring-ring in-plane and out-of-plane twisting modes, respectively.



of Ni5(tpda)4Cl2 and Co5(tpda)4Cl2 and a list of line positionsand assignments for H2tpda. This material is available free ofcharge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

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

’ACKNOWLEDGMENT

The National Science Council of Republic of China providedsupport for this research, and the National Center for High-Performance Computing, Taiwan, provided computing facilities.

’REFERENCES

(1) Lin, S.-Y.; Chen, I.-W. P.; Chen, C.-h.; Hsieh, M.-H.; Yeh, C.-Y.;Lin, T.-W.; Chen, Y.-H.; Peng, S.-M. J. Phys. Chem. B 2004, 108, 959.(2) Chen, I.-W. P.; Fu,M.-D.; Tseng,W.-H.; Yu, J.-Y.;Wu, S.-H.; Ku,

C.-J.; Chen, C.-h.; Peng, S.-M. Angew. Chem., Int. Ed. 2006, 45, 5814.(3) Hurley, T. J.; Robinson, M. A. Inorg. Chem. 1968, 7, 33.(4) Aduldecha, S.; Hathaway, B. J. Chem. Soc., Dalton Trans. 1991,

993.(5) Clerac, R.; Cotton, F. A.; Dunbar, K. R.; Murillo, C. A.; Pascual,

I.; Wang, X. Inorg. Chem. 1999, 38, 2655.(6) Berry, J. F.; Cotton, F. A.; Daniels, L. M.; Murillo, C. A. J. Am.

Chem. Soc. 2002, 124, 3212.(7) Berry, J. F.; Cotton, F. A.; Lu, T.; Murillo, C. A.; Wang, X. Inorg.

Chem. 2003, 42, 3595.(8) Berry, J. F.; Cotton, F. A.; Daniels, L.M.;Murillo, C. A.;Wang, X.

Inorg. Chem. 2003, 42, 2418.(9) Bera, J. K.; Dunbar, K. R. Angew. Chem., Int. Ed. Engl. 2002, 41,

4453 and references therein.(10) Yang, E.-C.; Cheng, M.-C.; Tsai, M.-S.; Peng, S.-M. J. Chem.

Soc., Chem. Commun. 1994, 2377.(11) Cotton, F. A.; Daniels, L. M.; Jordan, G. T., IV; Murillo, C. A.

J. Am. Chem. Soc. 1997, 119, 10377.(12) Cotton, F. A.; Murillo, C. A.; Wang, X. Inorg. Chem. 1999, 38,

6294.(13) Clerac, R.; Cotton, F. A.; Daniels, L. M.; Dunbar, K. R.; Lu, T.;

Murillo, C. A.; Wang, X. J. Am. Chem. Soc. 2000, 122, 2272.(14) Clerac, R.; Cotton, F. A.; Daniels, L. M.; Dunbar, K. R.;

Kirschbaum, K.; Murillo, C. A.; Pinkerton, A. A.; Schultz, A. J.; Wang,X. J. Am. Chem. Soc. 2000, 122, 6226.(15) Clerac, R.; Cotton, F. A.; Dunbar, K. R.; Lu, T.; Murillo, C. A.;

Wang, X. Inorg. Chem. 2000, 39, 3065.(16) Clerac, R.; Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Wang,

X. Inorg. Chem. 2001, 40, 1256.(17) Clerac, R.; Cotton, F. A.; Jeffery, S. P.; Murillo, C. A.; Wang, X.

Inorg. Chem. 2001, 40, 1265.(18) Clerac, R.; Cotton, F. A.; Jeffery, S. P.; Murillo, C. A.; Wang, X.

J. Chem. Soc., Dalton Trans. 2001, 386.(19) Berry, J. F.; Cotton, F. A.; Daniels, L. M.; Dunbar, K. R.; Lu, t.;

Murillo, C. A.; Wang, X. J. Am. Chem. Soc. 2000, 122, 2272.(20) Clerac, R.; Cotton, F. A.; Daniels, L. M.; Dunbar, K. R.; Murillo,

C. A.; Pascual, I. Inorg. Chem. 2000, 39, 748.(21) Clerac, R.; Cotton, F. A.; Daniels, L. M.; Dunbar, K. R.; Murillo,

C. A.; Pascual, I. Inorg. Chem. 2000, 39, 752.(22) Clerac, R.; Cotton, F. A.; Daniels, L. M.; Dunbar, K. R.; Murillo,

C. A.; Zhou, H. C. Inorg. Chem. 2000, 39, 3414.(23) Berry, J. F.; Cotton, F. A.; Lu, T.; Murillo, C. A.; Roberts, B. K.;

Wang, X. J. Am. Chem. Soc. 2004, 126, 7082.(24) Lai, S.-H.; Hsiao, C.-J.; Ling, J.-W.; Wang, W.-Z.; Peng, S. M.;

Chen, I.-C. Chem. Phys. Lett. 2008, 456, 181.(25) Lai, S.-H.; Hsiao, C.-J.; Huang, Y.-M.; Chen, I.-C.;Wang,W.-Z.;

Peng, S. M.; Raman Spectrosco, J. J. Raman Spectrosc. 2010, 41, 1404.

(26) Rohmer, M.-M.; B�enard, M. J. Am. Chem. Soc. 1998, 120, 9372–9373.

(27) Rohmer, M.-M.; Strich, A.; B�enard, M.; Malrieu, J.-P. J. Am.Chem. Soc. 2001, 123, 9126–9134.

(28) Pantazis, D. A.; McGrady, J. E. J. Am. Chem. Soc. 2006, 128,4128.

(29) Hsiao, C.-J.; Lai, S.-H.; Chen, I.-C.; Wang, W.-Z.; Peng, S.-M.J. Phys. Chem. A 2008, 112, 13528.

(30) Shieh, S.-J.; Chou, C.-C.; Lee, G.-H.; Wang, C.-C.; Peng, S.-M.Angew. Chem., Int. Ed. Engl. 1997, 36, 56.

(31) Wang, C.-C; Lo, W.-C.; Chou, C.-C.; Lee, G.-H.; Chen, J.-M.;Peng, S.-M. Inorg. Chem. 1998, 37, 4059.

(32) Yeh, C.-Y.; Chou, C.-H.; Pan, K.-C.; Wang, C.-C.; Lee, G.-H.;Sua, Y. O.; Peng, S.-M. J. Chem. Soc., Dalton Trans. 2002, 2670.

(33) Berry, J. F.; Cotton, F. A.; Fewox, C. S.; Lu, T.; Murillo, C. A.;Wang, X. Dalton Trans. 2004, 2297.

(34) Frisch, M. J. et al. GAUSSIAN 03, Gaussian, Inc.: Wallingford,CT, 2004.

Documents

Metal−Metal Bonding and Structures of Metal−String Complexes: Tripyridyldiamido Pentanickel and Pentacobalt from IR, Raman, and Surface-Enhanced Raman Scattering Spectra