Metal−Metal Bonding and Structures of Metal−String Complexes

ARTICLE pubs.acs.org/JPCC

Metal-Metal Bonding and Structures of Metal-String Complexes: Tripyridyldiamido Pentanickel and Pentacobalt from IR, Raman, and Surface-Enhanced Raman Scattering Spectra Yu-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-enhanced Raman scattering (SERS) spectra of metal-string complexes Ni5(tpda)4X2 and Co5(tpda)4X2 (tpda = tripyridyldiamido, X = Cl-, NCS-) and free ligand tripyridyldiamine (H2 tpda) to determine their vibrational wavenumbers and the strength of the metal-metal bonds. For SERS measurements, these complexes were adsorbed on silver or gold nanoparticles in aqueous solution to eliminate the constraint of a crystal lattice and to maintain the complexes in thermal equilibrium. The spectra of SERS and Raman modes show insignificant deviation in spectral features and band positions. We observe a single breathing band of pyridyl in Co5(tpda)4 X2 , indicating the existence of only the symmetric form, whereas split pyridyl lines are observed for Ni5(tpda)4X2 and assigned to arise because of a varied environment of coordination: square planar for the inner nickels and square pyramidal for the outer nickels in the complexes. From our analysis of the vibrational normal modes, we assign lines at 257/266 and 302/313 cm-1 to Ni5, at 287/284 and 355/360 cm-1 to Co5 symmetric stretching 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 the prediction from molecular-orbital theory.

1. INTRODUCTION Metal-string complexes with polypyridylamine ligands coordinated helically to linear metal ions have been shown to exhibit great electric conductivities1,2 and to have a great potential to function as molecular wires. The smallest complex of this kind is M3(dpa)4X2 (M = Ni,3-8 Co,9-19 Cr,20-23 etc., dpa = di(2-pyridyl)amido). The structures of these complexes are determined mostly through X-ray diffraction of crystalline form. These results show that some complexes have isomeric structures; for instance, tricobalt complexes exist with both symmetric and unsymmetric metal-metal bonding, whereas complex r 2011 American Chemical Society

Ni3(dpa)4Cl2 has exclusively symmetric Ni-Ni bonding. Clerac et al. showed that the infrared spectrum of complex Co3(dpa)4Cl2 displays split pyridyl lines.20 Lai et al. reported both IR and Raman spectra of complexes Co3(dpa)4Cl2 and Ni3(dpa)4Cl2 in their solid forms; they showed a split in-plane deformation line for the pyridyl ring at ∼1000 cm-1 in Raman spectra for Co3(dpa)4Cl2 and a single line for Ni3(dpa)4Cl2.24 Received: October 28, 2010 Revised: December 19, 2010 Published: January 10, 2011 2454

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

These authors explained the split lines to be due to the existence of unsymmetric and symmetric structures, which was confirmed with measurements of X-ray diffraction. Lai et al. also assigned the vibrational wavenumbers of the Ni3 stretching and Co3 asymmetric stretching modes of these complexes to show the strength of the metal-metal bonds.24 Under oxidation, the bond lengths between metal atoms alter; this effect provides a further way to confirm the assignments of vibrational wavenumbers for the stretching of the metal-metal bond.25 According to these analyses, the wavenumbers for metal-metal vibrational modes agree with a trend predicted with simple molecular-orbital theory in that the Ni-Ni bond is weaker than for Co-Co in these metal-string complexes. For Co3(dpa)4Cl2, Rohmer et al. showed a shallow groundstate surface with respect to distortion of the Co3 framework based on the results of calculations;26,27 they thus averred that the two structures characterized with X-ray diffraction cannot be considered to be bond-stretch isomers. Pantazis and McGrady suggested that the three states comprise a doublet ground state 2 A with a minimal energy at a symmetric geometry, a quartet state 4 B with unsymmetric equilibrium geometry, and another doublet state 2B with a broad and smooth energy surface crossing the former two states.28 At high temperatures, symmetric and unsymmetric forms arise from the population of 2A and 4B states. Decreasing the temperature results in a spin crossing to 2B. Environmental effects, such as temperature, cosolvent in a crystal and axial ligands, can alter the shape and the energy of the potential surface. Co3 metal-string complexes thus have varied forms but are not structural isomers. In this group, we identified the symmetric and unsymmetric structures of Cr3(dpa)4Cl2 by means of their IR and SERS spectra.29 For Cr3(dpa)4(NCS)2, only the symmetric form exists under ambient conditions. A line at 570 cm-1 in the SERS spectra of Cr3(dpa)4Cl2 appeared on heating the solution and is assigned to the Cr-Cr quadruple-bond stretching mode of the u-form. The line at 346 cm-1 is assigned to the Cr3 asymmetric stretching mode of the s-form. Hence, for both trichromium complexes the structure in the ground state is the s-form. From magnetic measurements on trichromium complexes, no spin crossing is observed.20 A series of linear pentanuclear metal-string complexes, for instance, Ni5(tpda)4X2 and Co5(tpda)4X2, H2tpda = tripyridyldiamine, X = Cl-, CN-, N3-, and NCS-, etc., were

synthesized.30,31 These metal-ion lines are coordinated helically with pentadentate nitrogen chelating ligands. The chemical structures of those for X = Cl- and SCN-, with relevant bond distances, are displayed in Scheme 1. In Ni5 complexes, the crystal data show symmetric structures with both Ni-Ni distances: outer 2.369 and inner 2.300 Å in Ni5(tpda)4(NCS)2 larger than those of 2.276 and 2.231 Å, respectively, in Co5(tpda)4(NCS)2. For nickel complexes, the three inner Ni ions all have four-coordinated, square-planar conformations, when neglecting the Ni-Ni bonds. All Ni-N distances to tpda2- of 1.89-1.90 Å are small, consistent with the typical distance found in the low-spin (S = 0) square-planar Ni(II) configuration system. The terminal Ni(II) ions have a square-pyramidal geometry, and the mean Ni-N distances 2.10 Å are consistent with a high-spin Ni(II) (S = 1) configuration. These assignments are confirmed with the results of magnetic measurements and X-ray near-edge absorption spectra.30 According to X-ray crystal data, only a symmetric structure exists in the pentacobalt complexes, unlike the tricobalt complexes. A simple theoretical calculation (extended H€uckel molecular-orbital method) shows that Co510þ comprises five σ and 10 π bonding and antibonding and five nonbonding orbitals. With 35 electrons in these orbitals, there are in total 2.5 net bonded, paired electrons. Hence, for each Co-Co, the bond order is near 0.5. For Ni510þ, 40 electrons fill all bonding and antibonding orbitals, resulting in no net bonding between Ni-Ni. The Ni-Ni bond is consequently expected to be weaker than the Co-Co bond in the pentanuclear complexes. In the present work, we studied the structures and strengths of metal-metal bonds in metal-string complexes using Raman and infrared spectra for pentanuclear complexes Co5(tpda)4X2 and Ni5(tpda)4X2 (X = Cl- and SCN-). To eliminate the constraint of a crystal lattice, we bound these complexes on either silver or gold nanoparticles in aqueous solution to search for any existence of structural isomers in the equilibrium condition. We assign the vibrational wavenumbers for the bands concerning metal-metal bonding to resolve the bond strength and to examine the bonding model derived from theory.

2. EXPERIMENTS The pentanuclear metal-string complexes and tripyridyldiamine, H2tpda, were synthesized according to methods described 2455

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Figure 2. Raman spectra (150-1650 cm-1) of Ni5(tpda)4Cl2 and H2tpda in solid form, obtained with excitation at 632.8 nm and some assignments. The symbol ν denotes a stretching mode, δ and γ in-plane bending and out-of-plane bending of pyridyl; and Δ and Γ pyridyl ring or ring-ring in plane and out-of-plane twisting modes, respectively.

Figure 1. Raman spectra (150-1650 cm-1) of Co5(tpda)4(NCS)2 in solid form, recorded at excitation wavelengths of 473, 532, 632.8, and 1064 nm.

elsewhere.30-33 These complexes were extracted with CH2Cl2 and purified on recrystallization from solution in CH2Cl2 near 23 °C. Deep purple pentanickel and dark brown pentacobalt crystals 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-infrared region 400-4000 cm-1 on another infrared interferometer (Perkin-Elmer SpectrumOne B). Solid samples were mixed with CsI at a ratio of 1:1-1:2 for the small-wavenumber range to obtain sufficient absorbance. The Raman spectra were recorded in a backscattering geometry to achieve a superior ratio of signal-to-noise; 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 red light at 632.8 nm served as the excitation source. Diode-pumped Nd:YAG lasers (Photop Suwtech, Inc.) provided green light at 532 nm and blue light at 473 nm, individually. 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 adding a few drops of ethanol solution containing the dissolved metal 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 silver nitrate with sodium citrate; these greenish-yellow particles display plasmon absorption with maximum at 420 nm. The integration period was typically about 30 s for a solid sample and 1 s for SERS 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 was 300 mW. To avoid self-absorption for Raman measurements on solid samples, the complex was mixed with KBr at a ratio of roughly 1:10. Quantum-chemical calculations based on density-functional theory (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* was employed for H2tpda and metal complexes, separately, to achieve reliable results. All calculations were performed using the GAUSSIAN 03 program.34

3. RESULTS AND DISCUSSION 3.1. Spectral Analysis for Lines above 500 cm-1. Each metal complex has four pentadentate ligands tdpa2- of which the vibrational lines overlap under the experimental conditions and thus become undifferentiated. Because these metal complexes have high symmetry, symmetric form D4 group and unsymmetric C4, their IR and Raman spectra are, however, to some extent complementary. In C4, all symmetry species are both Raman- and IR-active except that modes with b symmetry are only Ramanactive. In D4, vibrational modes with symmetry b1 and b2 appear only in Raman scattering and with a2 in IR absorption. Hence, similar positions observed in IR and Raman spectra can serve to distinguish 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 mostly involve motions of metal atoms; the spectral intensities are enhanced with 532 nm excitation. From the visible absorption spectrum, the Co5 complexes have a narrow absorption band centered at 525 nm, and this feature hence corresponds to a d-d transition. For the Ni5 complexes, the spectral intensities of metal-related bands are enhanced at an excitation wavelength of 632 nm. The corresponding visible band for Ni5 complexes is centered at 580 nm. The Raman spectra of Ni5 at these excitation wavelengths are shown in the Supporting Information. The variation in the spectral intensity from resonance enhancement provides additional information pertinent to assigning the vibrational modes. Figure 2 displays Raman spectra of Ni5(tdpa)4Cl2 and H2tpda in the wavenumber region 150-1650 cm-1. In general, the 2456

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Figure 3. SERS spectrum of Co5(tpda)4Cl2 on silver nanoparticles with 632.8 nm excitation.

spectral features in IR and Raman 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 of pyridyl, dipyridylamine (Hdpa),29 and the spectrum calculated with the DFT method. This molecule is nonplanar with symmetry C2; all vibrational modes are both Raman- and IR-active. The calculated line positions agree satisfactorily with the experimental data. The positions of measured and calculated lines and their assignments for H2tpda are listed in the Supporting Information. The breathing modes have lines at 985 and 990 cm-1 for the center (C-ring) and terminal (T-ring) pyridyl, respectively. The calculated results are based on an isolated structure, whereas H2tpda in the solid form has intermolecular hydrogen bonding. Some lines in Raman spectra are split: for example, N-H in-plane bending 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 inplane deformation of the pyridyl ring shows a single line for the Co5 complex at 1020 cm-1 but is split to 1010 and 1027 cm-1 in both Ni5 complexes, as shown in Figure 2. Similarly, the line for the out-of-plane motion of the terminal pyridyl ring mixing with in-plane pyridyl ring deformation in H2tpda is assigned to be about 672 cm-1 and is red-shifted to 642 cm-1 in Co5 but is split to 643 and 632 cm-1 in Ni5. Similar splitting in both places appears in the SERS spectra of Ni5, which were recorded in aqueous solution; the spectra are shown in the Supporting Information. In the unsymmetric form of Co3(dpa)4Cl2, the split pyridyl lines are explained to reflect varied Co-Co bonding resulting in two Co-N bonds of distinct distance and a small deviation in the ring-breathing frequency.24 Accordingly, we attribute the deviation to result from nickels of two types; one with square-planar and the other with square-pyramidal coordination;resulting in varied distances of Ni-N bonds and pyridyl vibrational wavenumbers. In the trinickel complexes, two coordination modes are also present, but the distances of Ni-N (pyridyl) bonds are comparable; no deviation in wavenumbers of pyridyl vibrations under current experimental resolution is hence observed. No splitting is observed for the pentacobalt complexes in both solid and aqueous nanoparticle solution, as shown in Figure 3, to imply existence of an unsymmetric structure. This result in fact agrees with results from the structure determination for crystals. 3.2. Spectral Analysis for Lines below 500 cm-1. Figures 4 and 5 show Raman spectra in the low-wavenumber region of H2tpda and pentanuclear complexes with axial ligand chloride and isothiocyanate, respectively; the IR spectra of Ni5(tpda)4Cl2

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

Figure 5. Raman spectra of H2tpda and Ni5(tpda)4(NCS)2 excited at 632.8 nm and of Co5(tpda)4(NCS)2 excited at 532 nm in crystal form mixed with KBr (ratio 1:10) to avoid self-absorption. The line positions and the assignments for the metal-related modes are displayed. The ligand and metal-related modes are correlated with dashed and solid lines, respectively.

and Co5(tpda)4Cl2 are depicted in Figures 6 and 7, respectively. In this region, the metal-related vibrational modes occur, plus those for the deformation between pyridyl rings in ligand tpda2-. First, we compare the spectra of the metal-string complexes with H2tpda to assign the lines involving mainly ligand motion; the vibrational wavenumbers of these modes are expected to remain least altered for varied metal and axial ligands. Second, the metal-related line intensity is enhanced in resonance Raman spectra. Hence, from a comparison of spectra in both positions and intensities shown in Figures 4-7, we assign lines at 271 and 395 cm-1 in Ni5(tpda)4Cl2 and at 392 cm-1 in Co5(tpda)4Cl2 to the pyridyl-pyridyl, ring-ring in-plane deformation mode and lines at 471 and 526 cm-1 in Ni5(tpda)4Cl2 and at 451 and 524 cm-1 in Co5(tpda)4Cl2 separately to center and terminal pyridyl out-of-plane deformation modes. Their corresponding IR lines are observed at similar positions. 2457

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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 Deviation in Parentheses) from Calculated and Experimental Data for Ni5(tpda)4Cl2 and Co5(tpda)4Cl2 Ni5(tpda)4Cl2 DFT method

UB3LYP

basis set

6-31G*

6-31G*

total electron spin S

2

1/2

-5175.35637756

-5053.97354340

total energy (Hartree) exptl

a

Co5(tpda)4Cl2

a

UB3LYP

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)

1.95050

Crystal data from ref 30. b Crystal data from ref 31.

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

metal complexes with D4 symmetry, the symmetric stretching in linear pentametal ions is Raman-active, and the asymmetric stretching is IR-active. The spin states for these metal complexes are recognized based on the results from magnetic measurements to be S = 1/2 for Co532 and S = 0 or 2 for the Ni5. Near 300 K, S = 2 of Ni5 has a greater population than the singlet state according to the Boltzmann distribution. Our experimental data show no variation in vibrational frequencies to differentiate these two states. Table 1 lists some bond distances related to metals from the optimized geometries obtained from the calculations using the method B3LYP/6-31G*. The optimized geometries show on average ∼0.01 Å in the metal-metal bond distances and ∼0.02 Å for M-N greater than those from the X-ray crystal data. Table 2 depicts the displacement vectors of the normal modes for the metal symmetric and asymmetric stretching. 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 vibrational motions involve displacements of many atoms, but we distinguish them based on the substantial displacements of metal atoms. The descriptions of modes are shown in Table 2 with the calculated wavenumbers scaled by 0.96 for these six stretching modes. 2458

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The Journal of Physical Chemistry C Table 2. Vibrational Motions and Wavenumbers/cm-1 Involving Metals for Ni5(tpda)4Cl2 and Co5(tpda)4Cl2

The lines involving M-Cl symmetric and asymmetric stretching are readily identified on comparing the spectra of two axial ligands; they are assigned to 216 and 206 cm-1 and 197 and 188 cm-1 for the Ni5 and Co5 chlorides, respectively. For the isothiocyanate complexes, the M-N(CS) symmetric stretching is assigned to a line at 224 and 208 cm-1 for Ni5 and Co5, respectively; the Ni-N(CS) asymmetric stretch is assigned to an IR line at 208 cm-1, but the wavenumber for Co-N(CS) is smaller than our detection limit. The remaining unassigned bands in the Co5 spectra are blue-shifted relative to those of Ni5. In addition, for the metal modes we expect that the wavenumbers are altered significantly when replacing cobalt by nickel because of the varied bond strength. The M-N stretching and N-M-N bending likely vary to a small extent. On this basis, we assign the Raman lines at 257 and 302 cm-1 and 266 and 313 cm-1

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to Ni5 symmetric (inner and outer metal-metal) stretching modes and the IR lines observed at 255 and 297 cm-1 and 267 and 305 cm-1 to Ni5 asymmetric stretching modes (outer metal-metal and M-M as shown in Table 2) for Ni5(tpda)4Cl2 and Ni5(tpda)4(NCS)2, respectively. The Raman lines at 287 and 355 cm-1 and 284 and 360 cm-1 are assigned to Co5 symmetric stretching modes and IR lines at 319 and 391 cm-1 and 323 and 392 cm-1 to asymmetric 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. According to the assignments for trinuclear complexes, in most cases, the spectral intensity for the bending mode is weaker than that for the M-N stretching mode. The Raman lines at 234, 334, and 419/ 425 cm-1 are accordingly assigned to the Ni-N stretching mode of Ni5(tpda)4Cl2 and lines at 235, 337, and 410/427 cm-1 for Ni5(tpda)4(NCS)2. The Co-N stretching modes are assigned for Raman lines at 227, 335, and 421 cm-1 and at 227, 334, and 421 cm-1 for Co5(tpda)4Cl2 and Co5(tpda)4(NCS)2, respectively. These assignments indicate unequal M-N bonding. From the delocalized charge distribution in tpda2-, the electron density next to the metal is in the order M-Namido > central M-Npy > terminal M-Npy, and the M-N distances are in the reverse order for pentanuclear complexes. Hence, three distinct M-N stretching wavenumbers are observed in pentanuclear complexes.15 The amido N with the greatest M-N stretching vibrational wavenumber has the strongest coordination to the metal. The N-Ni-N and Ni3 bending modes are assigned to lines at 498 and 285 cm-1, respectively, and N-Co-N bending is assigned to 489 cm-1 and Co3 bending to 335 cm-1; the line for the Co3 bending mode is blended with the line for the Co-N stretching mode. Overall, most observed bands are assigned; the list of line positions and assignments in the low-wavenumber region appears in Table 3. In Figures 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 values agree satisfactorily with the experimental data, but the calculated positions and intensities for other metal-related modes are unsatisfactory for making all assignments simply based on the theory. The results of DFT calculations nevertheless provide approximate wavenumbers and intensities of vibrational modes that are informative in assigning the spectra. From comparison of the spectral intensity for metal-string compounds of varied lengths, the spectral lines for metal-metal bonding modes in trinuclear complexes are much weaker than those for M-N, whereas the lines for M510þ stretching modes in pentanuclear complexes are intense despite there being in total 20 M-N bonds. A greater polarizability for these M510þ stretching vibration 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 crystal lattice; this method is convenient to determine quantitatively the isomers exiting in an equilibrium condition. The deviation in the line positions between the obtained Raman and the SERS spectra is at most 10 cm-1, indicating that no major structural variation is present when bonded to the metal surface in the solution phase. In the low-wavenumber region, the spectral intensities are enhanced in SERS spectra. A SERS spectrum of Co5(tpda)4Cl2 is shown in Figure 3 with a single pyridyl line at 636 and 1022 cm-1. For both Ni5 and Co5 complexes, we hence conclude that no structural isomer is detected near 295 K, unlike the case of 2459

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Table 3. Raman, Infrared, and SERS Line Wavenumbers/cm-1 and Assignments for Pentanickel and Pentacobalt Complexes Ni5(tpda)4Cl2 Raman

IR

165

165

187

Ni5(tpda)4(NCS)2

SERS

Raman

167

178

IR

SERS

Co5(tpda)4Cl2 Raman

SERS

Raman

IR

SERS

Γ(ring-ring)

164 183

Δ(ring-ring)

183

206

188 215

197

M-Cl asym. str. 201

M-Cl sym. str.

208 224 234

233

228 240

235

257

241

234

227

263

266

266

M-N (axial) sym. str.

227

229

M-N str. M3 bending

265

287

Γ(ring)

267

319

280

299

266

263

268

290

284

289 323

302

297

297

300

335

313

310

355

297 334

305 339

337

359

335

340

396

394

360

335

337

345

M3 bending

366

M-M sym. str. M-M asym. str.

392 343

344

345

392

407

391

420

425

433

427

433

421

434

428

466

463

458

499

496

486

497

464

451

499

489

486 522

523

526

390

499

421

524

527

395

451

494

489

Γ(C-ring) 463

499

498

486 524

525

528

a

M-N str. M-N str. N-M-N bending

458 452

Δ(ring-ring) Γ(T-ring)

425 430

487 520

360 414

424

434 462

393

412

410

466

M-N str. M3/N-M-N bending M3/N-M-N bending

397

421

525

344

359

359

419

498

343

382 389

408

471

Δ(ring-ring)

391

356

382 393

M-M sym. str. M-M asym. str. Δ(ring-ring)

285

526

213

272 284

498

232

208 239

255

395

M-N (axial) asym. str. 219

261

271

assignmenta Ni-Ni-Cl bending

185

216

IR

Co5(tpda)4(NCS)2

Γ(C-ring) N-M-N bending N-M-N bending

524

Γ(T-ring)

The 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-pyridyl ring-ring in-plane and out-of-plane twisting modes, respectively.

tricobalt chloride for which a low-lying quartet state exists and spin crossover can occur at low temperature such that an unsymmetric form exhibits. According to our results and those from magnetic measurements, the quartet state for Co5 complexes might lie at greater energy. In Ni complexes, nickels bond to pyridyls differently to show split bands in their breathing modes. However, the Ni5 complexes only exit the symmetric structure. From the assigned bands, the strength of the Co-Co bond is greater than for Ni-Ni, in agreement with prediction based on simple extended-H€uckel molecular-orbital theory. The bonding between nickels is nearly absent in Ni5 complexes, hence the assigned vibrational wavenumbers for Ni-Ni stretching seem large. This can be because these modes also involve the metal ligand motions.

4. CONCLUSION Using Raman, IR, and SERS spectra, we found that both Co5(tpda)4X2 and Ni5(tpda)4X2 (X = Cl- and NCS-) have only symmetric metal-metal bonding near 295 K, distinct from Co3(dpa)4Cl2. In Ni5(tpda)4X2, the inner and outer Ni have dissimilar coordination to yield varied breathing wavenumbers

for the pyridyl rings in ligands. From the analysis of vibrational normal modes and comparison with spectra for four pentanuclear complexes, the Ni5 symmetric stretching mode in complex Cl/NCS is assigned to 257/266 and 302/313 cm-1 and asymmetric stretching to 255/267 and 297/305 cm-1 and the Co5 symmetric stretching to 287/284 and 355/360 cm-1 and asymmetric stretching to 319/323 and 391/392 cm-1. Overall, Co-Co has a stronger bond than Ni-Ni in the pentanuclear complexes; there is hence expected to be greater electric conductivity in Co5 than in Ni5. The structure with symmetric Co-Co bond lengths is expected to possess greater conductivity than with unsymmetric isomers. Theoretical methods, for instance, DFT and others, might be more applicable and are required to search for unsymmetic structures and an explanation of the absence of an unsymmetric form of these pentanuclear metal-string complexes.

’ ASSOCIATED CONTENT

bS

Supporting Information. Raman spectra at excitation wavelengths of 532, 473, 632, and 1064 nm and SERS spectra

2460

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The Journal of Physical Chemistry C of Ni5(tpda)4Cl2 and Co5(tpda)4Cl2 and a list of line positions and assignments for H2tpda. This material is available free of charge 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 provided support for this research, and the National Center for HighPerformance Computing, Taiwan, provided computing facilities. ’ REFERENCES

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