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Metal?Metal Bonding and Structures of Metal String Complexes Cr3(dpa)4Cl2, Cr3(dpa)4(NCS)2, and [Cr3(dpa)4Cl2](PF6) from IR, Raman, and Surface-Enhanced Raman Spectra

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Metal-Metal Bonding and Structures of Metal String Complexes Cr

3

(dpa)

4

Cl

2

,

Cr

3

(dpa)

4

(NCS)

2

, and [Cr

3

(dpa)

4

Cl

2

](PF

6

) from IR, Raman, and

Surface-Enhanced Raman Spectra

Chung-Jen Hsiao,† Szu-Hsueh Lai,I-Chia Chen,*,†Wen-Zhen Wang, and Shie-Ming Peng

Department of Chemistry, National Tsing Hua UniVersity, 101 Kuang Fu Road Section 2,

Hsinchu, Taiwan 30013, Republic of China, and Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan 10617, Republic of China

ReceiVed: September 12, 2008; ReVised Manuscript ReceiVed: October 17, 2008

We recorded infrared, Raman, and surface-enhanced Raman scattering (SERS) spectra of metal-string complexes Cr3(dpa)4X2(dpa ) di(2-pyridyl)amido, X ) Cl, NCS) and [Cr3(dpa)4Cl2](PF6) and dipyridylamine (Hdpa) to determine their vibrational frequencies and to study their structures. For the SERS measurements these complexes were adsorbed on silver nanoparticles in aqueous solution to eliminate the constraints of a crystal lattice. From the results of analysis of the vibrational normal modes we assign the infrared band at 346 cm-1to the Cr3asymmetric stretching vibration of the symmetric form and the Raman line at 570 cm-1 to the Cr-Cr stretching mode for the unsymmetric form of Cr3(dpa)4Cl2. Complex Cr3(dpa)4Cl2exhibits both symmetric (s-) and unsymmetric (u-) forms in solution but Cr3(dpa)4(NCS)2only the s-form. The structures for both complexes in their ground states have the s-form. The oxidized complex [Cr3(dpa)4Cl2](PF6) has only a u-form for which the Cr-Cr stretching mode is assigned to the band at 570 cm-1. From the variation with temperature from 23 to 60 °C of the intensity of this line, we obtained the proportion of the u-form Cr3(dpa)4Cl2; the enthalpy change is thus obtained to be∆H ) 46.2 ( 3.3 kJ mol-1and the entropy change is∆S ) 138 ( 10.3 J K-1mol-1for the reaction u-Cr3(dpa)4Cl2Ts-Cr3(dpa)4Cl2. From the spectral intensities and band frequencies in SERS spectra, Hdpa is expected to adsorb on a silver nanoparticle with the amido nitrogen and pyridyl rings tilted from the silver surface, whereas the trichromium complex with the chromium ion line is orthogonal to the silver surface normal in aqueous silver solution.

Introduction

Trimetal complexes M3(dpa)4X2(M ) Cu,1,2Ni,3-8Co,9-20

Cr,21-26etc., dpa ) di(2-pyridyl)amido) are the simplest metal

string complexes with polypyridylamine ligands coordinated helically to linear metal ions. These complexes with unique electric and magnetic properties have prospective applications as nanoscale molecular wires. Among those complexes tricobalt and trichromium complexes have attracted the most attention because according to their X-ray crystal structures they exhibit structures with both symmetric and unsymmetric metal-metal bonding. For unsymmetric (u-) Co3(dpa)4Cl2 with unequal

Co-Co bond distances, Clérac et al. showed that the infrared spectra of this complex displays split pyridyl lines.14Lai et al.

reported both IR and Raman spectra of complexes Co3(dpa)4Cl2

and Ni3(dpa)4Cl2in solid form; they showed split Raman lines

of the in-plane deformation of the pyridyl ring for

u-Co3(dpa)4Cl2and a single line for Ni3(dpa)4Cl2that exists only

as a symmetric structure;27 they also assigned the vibrational

frequencies of the Ni3stretching and Co3asymmetric stretching

modes of these complexes that agree with a trend predicted with simple molecular-orbital theory.

From the crystal structure of Cr3(dpa)4Cl2, Cotton et al.

determined that the complex has a symmetric Cr-Cr bond of length 2.36 Å;21 after study of other trichromium complexes

they suggested that complex Cr3(dpa)4XY would have an u-form

with unequal Cr-Cr bond distances when X * Y,26for the X

) Y the s-form exists only for ligands that act as strong σ donor, for example X ) CN, CCPh, etc. Hence, for Cr3(dpa)4Cl2in

which Cl is a weakσ donor ligand, unsymmetric structure is

expected. Using density-functional theory, Rohmer and Benard found that, without a constraint of a crystal lattice, Cr3(dpa)4Cl2

exists in a symmetric form in the ground state.28,29Measurements

of the effective magnetic moment for a trichromium complex indicate spin S ) 2 that remains constant for temperatures to 350 K.23Rohmer and Bénard proposed an unsymmetric structure

to exist also as a quintet state but at greater energy.29

The bond order predicted from simple extended Huckel theory for symmetric trichromium ions Cr36+is 1.5 and for the u-form

it is the quadruple bonding for the bonding pair and no bonding for the other pair. In terms of electric conductance they may present different properties because of distinct bonding character. Compared with the other trimetal complexes Ni3and Co3, the s-form of Cr3has the best electric conductance.30The motivation

of our work is to identify structures of s- and u-forms of Cr3(dpa)4Cl2and Cr3(dpa)4(NCS)2, using vibrational

spectros-copy. These complexes have axial ligand Cl and NCS with weak and strong σ donating ability, respectively. In addition the

metal-metal bonding strength is undetermined experimentally for trichromium metal string complexes. The oxidized form [Cr3(dpa)4Cl2]PF6that has been shown to exist as the u-form

exclusively is also studied for comparison. We employed surface-enhanced Raman scattering (SERS) spectroscopy to record the vibrational spectra for molecules in solution phase to remove the crystal lattice constraint. Presumably this provides * To whom correspondence should be addressed. E-mail: icchen@

mx.nthu.edu.tw.

National Tsing Hua University.National Taiwan University.

10.1021/jp8081326 CCC: $40.75 2008 American Chemical Society Published on Web 11/24/2008

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the information of molecular structure under thermal equilib-rium. Vibrational spectra of dipyridylamine, Hdpa, are also investigated to assist in the vibrational normal-mode analysis for metal string complexes. The structures of Cr3(dpa)4Cl2,

Cr3(dpa)4(NCS)2, and the oxidized form [Cr3(dpa)4Cl2] +

for reference, from X-ray crystal data,26,31 are displayed in

Scheme 1.

Experimental Section

The solid metal complexes and dipyridylamine, Hdpa, were synthesized according to methods described elsewhere.5,9,22,32

Compound Cr3(dpa)4X2was purified by recrystallization from

a solution of CH2Cl2at room temperature. IR absorption spectra

in the far-infrared region 150-650 cm-1were recorded at the NTHU Instrument Center (Bomem FTIR spectrometer). A solid sample was mixed with CsI at a ratio 1:1-2 for the low-wavenumber range to obtain sufficient absorbance. The Raman spectra were recorded in a backscattering geometry to improve the 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 (wavelength 632.8 nm) served as the excitation source; the laser power at the sample was set at 15 mW. The scattered signal passing through a notch filter was recorded with a thermoelectrically cooled CCD detector. Samples for SERS measurements were prepared on adding complexes to an aqueous solution of silver nanoparticles (diameter 65-75 nm). The silver nanoparticles were prepared by reduction of silver nitrate with sodium citrate. The greenish-yellow particles display plasmon absorption with a maximum at 420 nm. The integration period was typically about 30 s for a solid sample and 1 s for SERS, and was averaged for 100 scans. All spectra were recorded under room temperature except as indicated. To avoid self-absorption for Raman measurements on solid samples, the complex was mixed with KBr at a ratio of 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 B3LYP method with basis set 6-31G* was employed for Hdpa to achieve reliable results. All calculations were performed with the GAUSSIAN 03 program.33

Results and Discussion

Spectra and Analysis for Hdpa. Hdpa, C10H9N3 with

molecular symmetry group C2, has 60 vibrational normal modes;

all are both IR and Raman active. Among those each pyridyl has 24 vibrational normal modes, and two pyridyls account for 48 modes. The other 12 vibrational modes are comprised of six for motions of C-N(H)-C, N-H out-of-plane bending, N-H in-plane bending, N-H stretching, C-N-C bending, symmetric, and asymmetric stretching and six for motions between pyridyl rings, one ring-ring stretching, two in-plane deformation, and three out-of-plane deformation. Figure 1 displays the IR, Raman, and SERS spectra of Hdpa in the wavenumber range 150-1650 cm-1. The line positions in the IR and Raman spectra agree within 2-3 cm-1; small deviations result from measurement uncertainties.

Full analyses of vibrational normal modes were made based on comparison of the observed spectra, results calculated by using the DFT method and the reported vibrational frequencies of pyridine. Table 1 lists the observed and the calculated line positions and assignments of vibrational motions. In general the vibrational frequencies of the pyridyl moiety agree with the calculated positions, but because the calculated results were based on an isolated structure whereas Hdpa in a solid form and has hydrogen bonding between two molecules, we expect some line splitting and frequency shifts; for example, lines at 989 and 995 cm-1are observed whereas the calculated results show a single line at 975 cm-1. The assignments for modes other than those of pyridyl are as follows: a line at 593 cm-1is assigned to the N-H out-of-plane bending, 1540 cm-1to N-H in-plane bending, 1254 and 1353 cm-1to C-N-C symmetric and asymmetric stretching, respectively, and 247 cm-1 to C-N-C bending. Vibrational modes for pyridyl-pyridyl ring-ring motion are expected to lie at low frequencies; a line at 207 cm-1is assigned to ring-ring wagging, 142 cm-1 to scissoring, 321 cm-1 to stretching, and 335 cm-1 to rocking. The remaining two out-of-plane ring-ring motions have low frequencies beyond our detection limit.

The SERS spectrum recorded in aqueous solution differs from the Raman spectrum of a solid sample in spectral intensity, line

SCHEME 1: Chemical Structures of (a) Symmetric and (b) Unsymmetric Cr3(dpa)4Cl2, (c) Cr3(dpa)4(NCS)2, and

(d) [Cr3(dpa)4Cl2](PF6) and Their Bond Lengths (Å)

taken from References 28 and 31, Respectively

Figure 1. IR and Raman spectra (150-1650 cm-1) of Hdpa in a solid form and the SERS spectrum of Hdpa on silver nanoparticles in aqueous solution, recorded at excitation wavelength 632.8 nm. ν denotes a stretching mode,γ and δ in-plane bending and out-of-plane bending, and∆ and Γ pyridyl ring in-plane and out-of-plane twisting modes, respectively.

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width, and line positions for some vibrations. The SERS spectral lines are broad with a full width at 15-20 cm-1 at half-maximum (fwhm) whereas it is at 6-8 cm-1 for the solid sample. The lines of N-H in-plane and out-of-plane bending disappear in SERS indicating that Hdpa became deprotonated and adsorbed onto the silver nanoparticle with the amido nitrogen in this weakly basic solution. For lines at large wavenumbers, the shifts are typically less than 5 cm-1from the corresponding Raman lines except that lines at 1602 and 1567 cm-1are shifted and split to 1597, 1588 cm-1and 1555, 1545 cm-1, respectively. A line at low wavenumber appearing in the normal Raman spectrum at 248 cm-1that is assigned to C-N-C bending is shifted to 262 cm-1, indicating a stiffer

C-N-C bonding after deprotonation. Lines at 323 and 414 cm-1that are assigned as pyridyl-pyridyl ring-ring and pyridyl ring motions, respectively, are also shifted to 363 and 424 cm-1. This condition implies rigid pyridyl rings when bonded to a silver nanoparticle.

The line assigned to the ring breathing mode is split for the solid sample but became broader in silver solution with no shift; similar results were observed for the other pyridyl modes implying no direct π interaction of pyridyl with the silver

surface.34,35The spectral intensities of lines, for example, at 363,

424, 668, 1057, 1311, and 1429 cm-1assigned to vibrational modes with a symmetry, are relatively enhanced. According to the surface selection rules for Raman intensity,36modes with

TABLE 1: Infrared, Raman, SERS, and Calculated Line Positions (cm-1), Relative Intensity, and Assignments for Hdpa

experimental calculated

IR Raman SERS B3LYP/6-31G*

υ I υ I υ υ× 0.97 symmetry assignment and mode descriptiona

41 B butterfly,Γ(ring-ring)

50 A ring-ring twisting,Γ(ring-ring)

142 m 147 m 111 A ring-ring scissoring,Γ(ring-ring)

207 w 208 m 211 205 A ring-ring wagging,Γ(ring-ring)

247 m 248 vw 262 215 B C-N-C bending

321 m 323 m

363 314 A ring-ring stretching,ν(ring-ring)

335 m 332 w 316 B ring-ring rocking,Γ(ring-ring)

403 m 403 vw 408 406 B pyridyl 16a,Γ(ring)

413 m 414 w 424 413 A pyridyl 16a,Γ(ring)

509 vw 509 w 514 498 B pyridyl 16b,Γ(ring)

525 m 527 vw 530 505 A pyridyl 16b,Γ(ring)

595 m 593 w 579 B N-H out-of-plane bending

619 m 620 603 A pyridyl 6a,∆(ring)

625 w 612 B pyridyl 6a,∆(ring)

639 vw 635 w 629 617 B pyridyl 6b,∆(ring) 670 w 675 w 668 665 A pyridyl 6b,∆(ring) 731 s 723 B pyridyl 4,γ(CH) 737 vw 744 738 A pyridyl 4,γ(CH) 762 vs 758 B pyridyl 11,γ(CH) 765 m 766 761 A pyridyl 11,γ(CH) 836 vw 836 m 837 808 A pyridyl 1, breathing 854 w 855 vw 877 843 B pyridyl 10a,γ(CH) 869 vw 869 vw 864 A pyridyl 10a,γ(CH) 912 w 912 vw 905 905 B pyridyl 1, breathing 949 vw 951 vw 949 B pyridyl 17a,γ(CH) 961 vw 961 vw 950 A pyridyl 17a,γ(CH) 969 B pyridyl 5,γ(CH) 990 m 989 vs 991 pyridyl 5,γ(CH) 997 w 995 s 975 A pyridyl 5,γ(CH)

1034 B pyridyl 18a,δ(CH) + ν(ring)

1054 w 1054 s 1057 1039 A pyridyl 18a,δ(CH) + ν(ring)

1094 w 1092 m 1078 B pyridyl 18b,δ¨(CH) + ν(ring) 1105 vw 1106 m 1104 1093 A pyridyl 18b,δ(CH) + ν(ring) 1142 s 1151 m 1139 B pyridyl 15,δ(CH) + ν(ring) 1164 vw 1163 w 1166 1149 A pyridyl 15,δ(CH) + ν(ring) 1236 w 1241 m 1237 1210 B pyridyl 14,δ(CH) + ν(ring) 1255 vw 1254 s 1266 1242 A C-N-C symmetric stretching 1275 vw 1278 vs 1273 B pyridyl 3,δ(CH) + ν(ring) 1285 vw 1274 A pyridyl 14,δ(CH) + ν(ring) 1315 s 1312 m 1311 1307 A pyridyl 3,δ(CH) + ν(ring) 1351 s 1353 m 1316 B C-N-C asymmetric stretching 1420 m 1420 m 1429 1407 B pyridyl 19b,ν(ring) 1439 vs 1435 m 1425 B pyridyl 19b,ν(ring)

1466 s 1463 m 1470 1433 A pyridyl 19a,ν(ring)

1484 s 1484 vw 1457 A pyridyl 19a,ν(ring)

1532 s 1540 m 1512 B N-H in-plane bending

1567 s 1545,1555 1563 A pyridyl 8a,ν(ring)

1568 m 1566 B pyridyl 8a,ν(ring)

1596 s 1588 1576 B pyridyl 8b,ν(ring)

1615 s 1602 vs 1597 1592 A pyridyl 8b,ν(ring)

aWilson notation for vibrational modes of pyridyl is used. Symbols are defined in Figure 1.

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vibrational motion vertical to the surface are intense but those with parallel motion are weak. When the molecule lies vertical to the silver surface, the molecular C2axis is orthogonal to the

surface; modes with vertical vibrational motion belong to the a symmetry class. Furthermore, lines for C-N-C symmetric and antisymmetric stretching modes are not observed in SERS because these motions are parallel to the surface. According to the spectral intensity and band positions, the molecule is thus expected to lie vertically with a tilted angle from the surface. Bands with broad features might result from pyridyl rings twisted at various angles to the amido moiety in solution, being released from constraints of the crystal lattice.

Spectra and Analysis for Cr3Complexes. Each trichromium complex has four dpa ligands of which the vibrational lines overlap under the experimental conditions, and thus become undifferentiated, but because these metal complexes have high symmetry their IR and Raman spectra are to some extent

complementary. Complexes u-Cr3(dpa)4Cl2and Cr3(dpa)4(NCS)2

belong to symmetry group C4in solid form because the axial

ligand NCS of the latter complex is tilted from the Cr3+6ion

line whereas s-Cr3(dpa)4Cl2has symmetry group D4. In C4all

symmetry species are both Raman and IR active except that modes with b symmetry are only Raman active. In D4vibrational

modes with symmetry b1and b2appear only in Raman, and a2

in IR. Figures 2-5 display the IR and SERS spectra of Cr3(dpa)4Cl2, Cr3(dpa)4(NCS)2, and [Cr3(dpa)4Cl2](PF6),

respec-tively. As the complexes in a solid form decomposed under laser radiation over a protracted period, Raman spectra were unattainable. Table 2 lists the observed line positions and mode assignments. The SERS and IR spectra of Cr complexes exhibit features generally similar to those of Hdpa in the range 410-1600 cm-1, in which most pyridyl vibrations are located. The SERS lines are narrower than those of Hdpa indicating that the twisting angle of pyridyl rings is essentially fixed upon TABLE 2: Infrared and SERS Line Positions (cm-1) and Assignments for Cr3(dpa)4Cl2, Cr3(dpa)4(NCS)2, and

[Cr3(dpa)4Cl2]+(PF6)

-Cr3(dpa)4(NCS)2 Cr3(dpa)4Cl2 [Cr3(dpa)4Cl2]+PF6

-SERS IR SERS IR SERS IR assignment and mode descriptiona

147 154 157 Γ(ring-ring) 163 173 Cr-N bending 184 200 Cr-Cl stretching 211 213 212 211 Γ(ring-ring) + Cr-N bending 249 251 250 251 241 Cr-N stretching 265 265 265 C-N-C bending + Cr-N stretching 275 275 Cr-Cr-Cr bending 300 304 303 305 303 298, 303 Cr-N stretching 334 330 326 328 Cr-N stretching 347 346 Cr-Cr-Cr asymmetric stretching 365 361 365 362 364 363 ∆(ring-ring) 398 386 400 388 400 397 Cr-N stretching 429 422 429 419 424 425 pyridyl 16a 446 435 446 435 445 438 pyridyl 16a 489 490 SCN bending 516 519 517 518 pyridyl 16b 536 539 538 539 pyridyl 16b 570 570 Cr-Cr stretching 628 629 623 629 pyridyl 6a 648 647 649 646 666 654 pyridyl 6b 693 692 675 691 pyridyl 6b 723 740,750 737,749 742, 751 pyridyl 4 764 766 770 pyridyl 11 799 C-S stretching 860 861 863 pyridyl 10a 882 879 881 pyridyl 10a 924 918 924 919 924 917 pyridyl 1 953 962 974 968 pyridyl 17a 996 979 989 990 pyridyl 17a 1019 1018 1020 1017 1019 1021 pyridyl 5 + pyridyl 12 1058 1055 1058 1055 1058 1057 pyridyl 18a 1114 1112 1116 1111 1114 1113 pyridyl 18b 1157 1158 1162 1154,1167 1157 1158 pyridyl 15 1244 1243 1246 pyridyl 14 1255 1268 1254 1269 1256 C-N-C symmetric stretching 1272 1273 1272 pyridyl 3 1281 1284 1284 pyridyl 14 1311 1313 1311 1313 1311 1313 pyridyl 3 1372 1370 1371 1363 1372 1367 C-N-C asymmetric stretching 1429 1428 1429 1431 1429 1428 pyridyl 19b 1445 1445 1445 pyridyl 19b 1465 1465 1466 1468 1465 1466 pyridyl 19a 1478 1480 1478 pyridyl 19a 1559 1549 1559 1548 1559 1550 pyridyl 8a 1598 1597 1599 1596 1598 1598 pyridyl 8b 1611 1608 1614 1607 1611 1608 pyridyl 8b a

Wilson notation for vibrational modes of pyridyl is used. Symbols are defined in Figure 1.

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bonding to Cr36+. Because of deprotonation upon coordination,

bands for the N-H out-of-plane bending mode at 595 cm-1 and the in-plane bending mode at 1540 cm-1for Hdpa disappear in trichromium complexes. In-plane deformation modes of pyridyl at 668 and 991 cm-1 in Hdpa are shifted to 693 and 1019 cm-1,respectively, in Cr3(dpa)4(NCS)2.

As reported previously, the IR lines split at 737, 749 cm-1 and 1153, 1167 cm-1and the Raman lines at 1010 and 1020 cm-1are found for Co3(dpa)4Cl2in a u-form.28Similar to the

Co3complex, the pyridyl lines of u-Cr3(dpa)4Cl2 are split to

737, 749 cm-1and 1154, 1167 cm-1in IR but there is a single Raman line at 1020 cm-1. The oxidized form [Cr3(dpa)4Cl2](PF6)

that has unsymmetric Cr-Cr bond lengths exhibits only a single line at 1158 cm-1in the IR spectrum but split lines at 742 and 751 cm-1. In the SERS spectrum (Figures 4 and 5) the broad line at 570 cm-1indicates the existence of a Cr-Cr quadruple bond. Similarly the SERS spectra of Cr3(dpa)4Cl2display a line

at 570 cm-1at increased temperatures, as shown in Figures 3 and 5. According to these experimental results we assign that complex Cr3(dpa)4Cl2exhibits both s- and u- forms at elevated

temperatures and that the s-form is the ground state structure. The IR spectrum of Cr3(dpa)4(NCS)2has lines split at 740 and

750 cm-1but a single line at 1158 cm-1; in the Raman spectrum there is a single line at 1019 cm-1, but no line corresponding to the Cr-Cr quadruple bond mode appears at temperatures even up to 60°C. Accordingly we assign that only an s-form exists for this complex.

According to the crystal data, the bond distances within the pyridyl ring for two pyridyls are similar for trichromium complexes in both s- and u-forms, and even for the oxidized Cr3complex. The small splitting in pyridyl vibrational

frequen-cies might hence result from a small variation in bonding to separate Cr atoms. For instance, the bond lengths Co-N(pyridyl) in u-Co3(dpa)4Cl2deviate about 0.16 Å, but Cr-N(pyridyl) in

[Cr3(dpa)4Cl2](PF6) deviates only 0.015 Å producing nearly

similar pyridyl vibrational wavenumbers for the latter complex. In u-Co3(dpa)4Cl2the isolated, high-spin CoIIhas one electron

in the Co-N antibonding molecular ortibal, yielding long metal-nitrogen distance around this atom. With three electrons less than Co, CrIIhas no electron to populate this orbital. Hence

no large deviation in Cr-N bond distance is expected and consequently less splitting in pyridyl vibrations in Cr3complexes.

We assign the broad band at 570 cm-1to the stretching mode of the Cr-Cr quadruple-bond both in the u-form of Cr3(dpa)4Cl2

and in [Cr3(dpa)4Cl2](PF6); this vibrational mode is inactive in

IR spectra. This wavenumber agrees with the value reported for other metal complexes with a Cr-Cr quadruple bond.37The

IR lines of [Cr3(dpa)4Cl2](PF6) at 559 and 841 cm-1are assigned

to vibrational modes of PF6- by comparison with spectra of

AgPF6.

In the region of low frequencies in which metal-metal, metal-ligand, and ligand pyridyl-pyridyl, ring-ring out-of-plane vibrations are located, these modes are the most sensitive to reveal the strength of metal-metal bonding. We assign the IR lines at 429, 365, and 265 cm-1to ligand modes of pyridyl, pyridyl-pyridyl, ring-ring out-of-plane, and C-N-C bending vibrations, respectively, for Cr3(dpa)4(NCS)2 because these

frequencies are expected to vary least on altering the metal or axial ligand.27Compared with those in Hdpa, these modes are

blue-shifted. From a comparison with assigned spectra of

Figure 2. IR spectrum (dotted line) for the solid form and SERS

spectra of u-Cr3(dpa)4Cl2on silver nanoparticles in aqueous solution

at temperatures 296 (solid line) and 333 K (dashed line) (150-1250 cm-1). Symbols are defined in the caption of Figure 1.

Figure 3. IR spectrum of the solid form and SERS spectra of Cr3(dpa)4(NCS)2on silver nanoparticles in aqueous solution (150-1650

and 2000-2200 cm-1). Symbols are defined in the caption of Figure 1.

Figure 4. IR spectrum of the solid form and SERS spectra of [Cr3(dpa)4Cl2](PF6) on silver nanoparticles in aqueous solution (150-1650

cm-1). Symbols are defined in the caption of Figure 1.

Figure 5. SERS spectra of Cr3(dpa)4Cl2at various temperatures and

of [Cr3(dpa)4Cl2](PF6) at 296 K. Insert: Plot of -R ln K vs 1/T.

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Co3(dpa)4Cl2and Co3(dpa)4(NCS)2we assign the SERS lines

of Cr3(dpa)4Cl2 at 400, 303, 251 cm-1, and IR lines at 388,

305, and 250 cm-1to Cr-N stretching modes.27The remaining

IR line at 346 cm-1is assigned to the Cr3asymmetric stretching

of the s-form and 275 cm-1to Cr-Cr-Cr bending because they disappear for the oxidized trichromium complex. The line at 275 cm-1 is also assigned in part to C-N-C bending. For Cr3(dpa)4(NCS)2the corresponding assignments are SERS lines

at 398, 300, 249 cm-1, and IR lines at 386 and 304 cm-1to Cr-N stretching. Lines at 490 cm-1in IR and 489 cm-1 in Raman spectra are assigned to axial-ligand NCS bending.

The SERS spectra of Cr3(dpa)4(NCS)2and Cr3(dpa)4Cl2have

similarly enhanced intensity in the low wavenumber region and near 800 cm-1. For the formal complex the C-N stretch (in NCS) appears in IR at 2028 cm-1 in the solid form but is increased to 2113 cm-1in silver solution; a single line in SERS indicates that both S atoms are bonded to the silver surface under the experimental conditions. The blue-shifted band position of the C-N stretch is due to the less nonbonding electron of the nitrogen atom to antibonding in C-N from bonding to silver. Positions of lines for other modes of both complexes agree within 5 cm-1. The structures are hence similar for these two complexes in an aqueous solution of silver nanoparticles. In SERS the Cr-N lines are more intense than the ligand lines, relative to those in Raman spectra of Co3and Ni3complexes;27

moreover no line is assigned to the Cr3symmetric stretching

mode. These results imply that the metal ion line in the complex lies nearly parallel to the silver surface so that the vibrational motions orthogonal to the metal ion line and silver surface, for example, Cr-N stretching modes, are enhanced according to the surface selection rules whereas motions parallel to the metal ion line, for example, C-N-C stretching and Cr3symmetric

stretching, are either fairly weak or not observed. Because of poor solubility of this complex in an aqueous solution, a low coverage of the complex on the silver surface is expected. The experimental data imply that, under these conditions, the complex might adsorb on a silver surface with the line of metal ions parallel to the surface and both sulfur or chlorine atoms bonded or adsorbed to silver atoms.

Conversion of u- and s-Form of Cr3(dpa)4Cl2. As shown in Figure 5 relative to [Cr3(dpa)4Cl2](PF6) that exists in only a u-form, the intensity of the line at 570 cm-1 for the Cr-Cr quadruple bond of Cr3(dpa)4Cl2is weak at 296 K, indicating a

small proportion of the u-form. The intensity of this line remains unaltered upon exposure to red light from the laser over 1 h. When the solution is heated to 60°C, the increased intensity of a line at 570 cm-1indicates conversion of the s-form to the

u-form. For a temperature exceeding 80 °C, the complex decomposed, but the intensity of the line at 570 cm-1showed conversion of the reaction to be nearly complete at 60°C. We used the spectral intensity of [Cr3(dpa)4Cl2](PF6) as an external

standard to calibrate the intensity of the mode for the Cr-Cr quadruple bond of u-Cr3(dpa)4Cl2. We first normalized the

intensity of lines at low wavenumber and for the region∼800 cm-1, as shown in Figure 5. The intensity of a line of [Cr3(dpa)4Cl2](PF6) at 570 cm-1 is assumed to be that for

Cr3(dpa)4Cl2 100% in the u-form. The ratio of the measured

intensities of the line at 570 cm-1 for Cr3(dpa)4Cl2 and

[Cr3(dpa)4Cl2](PF6) yielded the proportion of the unsymmetric

form. The equilibrium constant K for the reaction u-form T

s-form, equal to the ratio [u-form]/[s-form] of concentrations,

is thus obtained from the variation of spectral intensity with temperature. From the relation∆G ) -RT ln K ) ∆H - T∆S, the reaction enthalpy∆H ) 46.2 ( 3.3 kJ mol-1and entropy

∆S ) 138 ( 10.3 J K-1mol-1are obtained; the linear plot is shown in the insert of Figure 5.

Rohmer and Bénard29calculated the energy of conversion of

Cr3(dpa)4Cl2to be 42.3 kJ mol-1for the distorted conformation

that has a quadruple bond for Cr-Cr and ferromagnetic coupling between the center Cr and the nonbonded Cr, with respect to the symmetric ground state. These two structures exist with the same spin multiplicity and are thermally accessible according to experimental data, implying formation of bond-stretch isomers. Further theoretical calculations might provide informa-tion about the correlainforma-tion of these two structures on the ground state surface. Exposed to light for 1 h and for temperatures up to 60 °C, complex Cr3(dpa)4(NCS)2 remains as a symmetric

form, indicating a large barrier for conversion to the u-form.

Conclusion

Symmetric and unsymmetric structures of Cr3(dpa)4Cl2are

distinguished by means of their IR and SERS spectra. For Cr3(dpa)4(NCS)2only the symmetric form exists under ambient

conditions. We obtained the enthalpy and entropy differences between the s- and u-Cr3(dpa)4Cl2forms to be 46.2 kJ mol-1

and 138 J K-1 mol-1, respectively. For both trichromium complexes the structure of the ground state is the s-form. A line at 570 cm-1is assigned to the Cr-Cr stretching mode of the u-form, and a line at 346 cm-1 to the Cr3 asymmetric

stretching mode of the s-form. According to the spectral intensity and frequency shifts in the SERS spectra, a molecule of Hdpa is likely adsorbed on a silver nanoparticle with the amido nitrogen and pyridyl rings tilted from the silver surface whereas the trimetal complexes with a metal ion line parallel to the silver surface under low coverage condition.

Acknowledgment. National Science Council, Taiwan,

pro-vided financial support and the National Center for High-Performance Computing provided computing facilities.

References and Notes

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數據

Figure 1. IR and Raman spectra (150-1650 cm -1 ) of Hdpa in a solid form and the SERS spectrum of Hdpa on silver nanoparticles in aqueous solution, recorded at excitation wavelength 632.8 nm
Figure 3. IR spectrum of the solid form and SERS spectra of Cr 3 (dpa) 4 (NCS) 2 on silver nanoparticles in aqueous solution (150-1650 and 2000-2200 cm -1 )

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