JOURNAL OF RAMAN SPECTROSCOPYJ. Raman Spectrosc. 2004; 35: 1001–1005Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jrs.1249

Spectator ligand effects on the vibrational spectraof heteroleptic complexes of ruthenium with bipyrazine

Jing Wu and James R. Kincaid∗

Chemistry Department, Marquette University, Milwaukee, Wisconsin 53233, USA

Received 8 December 2004; Accepted 11 June 2004

Resonance Raman (RR) and transient resonance Raman (TR2) spectra were acquired for a series ofheteroleptic complexes of ruthenium, i.e. RuL2(bpz)2+, where bpz is 2,2′-bipyrazine and L is 2,2′-bipyridine(bpy) or an alkylated 2,2′-bipyridine [i.e. diazafluorene (daf) or 4,4′-dimethyl-5,5′-diethyl-2,2′-bipyridine(dmdeb)]. Resonance Raman spectra acquired at different excitation wavelengths for the ground-statecomplexes reveal selective enhancement of modes associated with the coordinated bpz or the spectatorligands, as expected. The TR2 spectra, acquired with excitation at 355 nm, confirm selective population ofa bpz-localized 3MLCT excited state for each complex. Both the ground-state RR spectra and the excitedstate TR2 data indicate that only slight shifts are observed in a few modes as the donor strengths of thespectator ligands are varied. Such data for a systematically manipulated set of complexes, acquired herefor the first time, imply that both the RR and TR2 spectral parameters are reliably characteristic for a givenligand, varying only slightly as the nature of other ligands in the complex is changed. Copyright 2004John Wiley & Sons, Ltd.

KEYWORDS: resonance Raman; transient resonance Raman; ruthenium(II) complexes; bipyrazine

INTRODUCTION

As the interest and activity in the study of rutheniumcomplexes of polypyridines and related ligands continues tointensify, because of their inherent fundamental interest andtheir potential utility in solar energy conversion schemes,1 – 14

it becomes increasingly important to establish effectivemethods and strategies to probe the nature of complexes.The utility of combined resonance Raman (RR) and transientresonance Raman (TR2) studies as a method of fingerprintingthe 3MLCT states of these types of complexes has bynow become well accepted.15 – 22 In such applications, theRR and TR2 spectra of a given heteroleptic complex (e.g.RuL2L2C) are acquired. In all cases so far studied (seeRefs 15–22 for examples), the TR2 spectrum of the excitedstate consists of a set of bands associated with an anionradical form of the ligand wherein the excited state electrondensity is localized, along with one (or two) sets of bandsascribable to coordinated neutral ligands. Thus, in the TR2

spectrum of Ru(bpy)2�bpz�2C, where bpy D 2,20-bipyridineand bpz D 2,20-bipyrazine, a set of bands is observed which

ŁCorrespondence to: James R. Kincaid, Chemistry Department,Marquette University, Milwaukee, Wisconsin 53233, USA.E-mail: james.kincaid@mu.eduContract/grant sponsor: Department of Energy, Basic EnergyScience Division; Contract/grant number: ER 13619.

are unambiguously assignable to (bpz�ž) along with a set ofbands clearly ascribable to the two coordinated bpy ligands,documenting the proper formulation of the excited state as[Ru(III)(bpy)2�bpz�ž�2C]Ł.

Although studies such as that outlined above provideunambiguous documentation for the selective population ofparticular 3MLCT excited states, until now the sensitivityof the excited state vibrational signature bands to subtlealterations in charge distributions has not been carefullyinvestigated. One interesting and important issue that hasnot yet been systematically addressed is the extent to whichthe vibrational wavenumbers observed in TR2 spectra canbe used to document slight changes in electron density inthe anion radical fragment that arise from variations ofthe spectator ligands; an approach used to interrogate the��CO� modes in the excited states of carbonyl complexes.23

For example, in the series of complexes RuL2�bpz�2C, wherevarious polypyridine ligands, L, are selected so as to ensurea bpz-localized 3MLCT state, how sensitive are the (bpz�ž)modes to the �- and �-donor properties of the ligands,L? In this work, this issue was effectively addressed byconducting a thorough RR and TR2 study of the series ofcomplexes RuL2�bpz�2C, wherein the donor strength of the Lligand is systematically varied by utilizing alkyl-substitutedderivatives of 2,20-bipyridine (bpy), i.e. diazafluorene (daf)and 4,40-dimethyl-5,50diethyl-2,20-bipyridine (dmdeb).

Copyright 2004 John Wiley & Sons, Ltd.

1002 J. Wu and J. R. Kincaid

EXPERIMENTAL

MaterialsRuCl3Ð3H2O was purchased from Aldrich Chemical (Mil-waukee, WI, USA). The ligands bpy and bpz were alsopurchased from Aldrich Chemical and dmdeb was availablefrom previous studies. All of them were sublimed prior touse. The ligand daf was prepared in two steps. First, 4,5-daf-9-one (dafo) was synthesized by permanganate ion oxidationof 1,10-phenanthroline (phen) in basic solution. In the secondstep, dafo was reduced by a Wolf–Kishner reduction. Theprocedures for both steps have been described by Cherryand co-workers.24,25

Preparation of compoundsRu(L)2Cl2The Ru(L)2Cl2 (L D daf, bpy, bpz, dmdeb) used in thisstudy were prepared from RuCl3Ð3H2O by the generalprocedure for preparation of Ru(L)2Cl2 (L D ˛-diimineligand) complex.17 The purity of the product was checkedby thin-layer chromatography and electronic emissionspectroscopy.

A quantity of RuCl3Ð3H2O, 2 equiv. of the appropriateligand and LiCl were refluxed in DMF (Aldrich) (25 ml ofDMF per 0.5 g of RuCl3Ð3H2O). After refluxing for 3–4 h,¾75% of the DMF was distilled off and acetone was added.The resulting solution was cooled at �5 °C overnight. Theresulting black precipitate was collected by filtration andwashed with cold water. The resulting precipitate, Ru(L)2Cl2,was then allowed to air dry.

Ru(L)2(L0) Cl2The Ru(L)2�L

0�Cl2 (L D daf, bpy, bpz or dmdeb) used inthis study were prepared from RuCl3Ð3H2O by the generalprocedure for preparation of Ru(L)2�L

0�Cl2 (L D ˛-diimineligand) complex.24,25 A quantity of the appropriate bis-complex was heated at reflux in aqueous ethanol (1 : 1) for1 h. A solution containing 1.5 equiv. of the L0 ligand bpz wasdissolved in the minimum amount of ethanol and addedslowly dropwise to the refluxing bis-complex. Heating wascontinued for 3–4 h. To precipitate the tris-complex as achloride salt, 50% of the solvent was distilled off and severaldrops of a saturated solution of LiCl were added. The solutionwas then cooled in an ice-bath and dry acetone or anhydrousdiethyl ether was added to force the precipitation of thechloride salts. The resulting precipitate was then collected byfiltration and chromatographed on Sephadex LH-20 using95% ethanol as the eluent.

[Ru(bpz)3]Cl2[Ru(bpz)3]Cl2 was prepared from Ru(DMSO)4Cl2 by fol-lowing the general procedure for preparation of Ru(L)3

(L D ˛-diimine ligand) complexes.26,27 Typically, 0.100 gof Ru(DMSO)4Cl2 and 3 equiv. of ˛-diimine ligand wererefluxed in water (7 ml) and ¾1 ml of ethanol for 12–20 h.

The solvent was then evaporated under a stream of nitrogen.The resulting dark-red solid was dissolved in 95% ethanoland directly chromatographed on Sephadex LH-20 as above.

Electronic absorption spectraElectronic absorption spectra were obtained with a Hewlett-Packard Model 8452A diode-array spectrometer using a 1 cmquartz cuvette. Spectra were obtained in the absorbancemode.

Resonance Raman spectraRR spectra were acquired using a Spex Model 1269 spec-trometer equipped with a Princeton Instruments ICCD-576UV-enhanced detector and notch filters (Kaiser Optical Sys-tems, Ann Arbor, MI, USA), or a conventional Raman spec-trometer, namely a Spex Model 1403 double monochromatorequipped with a Hamamastsu R928 PMT and computer soft-ware (SpectraMax/32 Windows software from InstrumentsSA).

The excitation sources used were 350.9 and 406 nmradiation from a Coherent Model Innova 100-K3 kryptonion laser, 441.5 nm radiation from a Liconix Model 4240NBHe—Cd laser and 488.0 nm radiation from a Spectra-PhysicsModel 2025-05 argon ion laser.

Excited-state Raman spectra were obtained with use aSpex Model 1269 spectrometer equipped with a PrincetonInstruments ICCD-576 UV-enhanced detector, FG-100 pulsegenerator, a LeCroy 9450A Dual 300 MHz oscilloscopeand Model 356 notch filters (Kaiser Optical Systems). Theharmonic (354.7 nm) of a Quanta-Ray (Spectra-Physics) GCR-11 Nd : YAG laser (operated at 20 Hz) was used as anexcitation source. The samples were placed in rotating NMRtubes (5 mm i.d.) to avoid localized heating by the laser beam.The scattered light was collected with 135° backscatteringgeometry and a conventional two-lens collection system.28

RESULTS AND DISCUSSION

Electronic absorption spectraThe structures of the ligands used in the current workare shown in Scheme 1. The UV–visible absorption spectraacquired for all the heteroleptic complexes, RuL2�bpz�2C, atroom temperature in H2O are shown in Fig. 1. Correspondingdata for the homoleptic complex Ru(bpz)3

2C is includedfor comparison. With increasing donor strength of the‘spectator’ ligand, the two absorption bands become furtherseparated. The energies of these MLCT transitions dependon the relative energies of the d-orbitals and the lowestlying �Ł-orbitals of the ligands. While it is expected thatthe bpz orbitals are indeed lower than those of all otherspectator ligands, as will be seen, such information is directlyconfirmable by considering the RR data.

Resonance Raman spectraThe RR spectra of the heteroleptic complexes Ru L2�bpz�2C

and Ru(bpz)32C obtained at different excitation wavelengths

Copyright 2004 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2004; 35: 1001–1005

Spectator ligand effects on spectra of Ru–bipyrazine complexes 1003

N

N

N

N

N

N

N

N CH2

NN

Et

Me Me

Et

NN

dafbpz bpy Me2, Et2-bpy

increasing donor strength

Scheme 1

395

496

406

484

446

350 400 450 500 550 600

Ab

sorp

tion

Ru(bpz)32+

Ru(daf)2(bpz)2+

Ru(bpy)2(bpz)2+

Ru(Me2,Et2-bpy)2(bpz)2+

412

456

415

Wavelength (nm)

Figure 1. Absorption spectra of Ru(bpz)3Cl2, Ru(daf)2(bpz)Cl2,Ru(bpy)2(bpz)Cl2 and Ru(dmdeb)2(bpz)Cl2 in water.

(406 nm and 442 or 488 nm) are shown in Figs 2 and 3.From inspection of these two figures, it is obvious that thesame set of modes are enhanced in all three complexes inFig. 3, whereas different sets of bands are enhanced in Fig. 2.The observed bands of the three complexes obtained with406 nm excitation are those coming from the corresponding‘spectator’ ligands, while the bands enhanced with 488 nmexcitation are those associated with a common ligand, i.e.bpz. This is confirmed by comparing the spectra with thoseobtained for the homoleptic complex Ru(bpz)3

2C, as shownon the top trace.

These results reveal the nature of the electronic transi-tions, i.e. the high energy band near 400 nm is associatedwith the Ru(II)–L transition, whereas the low-energy com-ponent is attributed to the Ru(II)–bipyrazine charge-transfertransition, as expected on the basis of the energy orderingor the �Ł-orbitals. What is also clear is that the vibrationalmodes of the ligand common to all these complexes (i.e. thebpz ligand) shift only slightly as the donor strength of thespectator ligand is varied. Considering only the complexes

982

1054

1152

1187

1273

1345 14

07

1502

1545 16

15

982 10

36

1177

1279 13

20

1493

1565

1607

982

1031

1158

1192

1230

1274

1441 15

11

1594

900 1000 1100 1200 1300 1400 1500 1600

Wavenumber / cm-1

Ru(daf)2(bpz)2+

Ru(bpy)2(bpz)2+

Ru(Me2,Et2-bpy)2(bpz)2+

@ 406nm

@ 406nm

@ 406nm

s

s

s

Inte

nsity

Figure 2. RR spectra of Ru(daf)2(bpz)Cl2, Ru(bpy)2(bpz)Cl2 andRu(dmdeb)2(bpz)Cl2 in 0.25 M Na2SO4 solution, excited at406 nm (s indicates peak from internal standard SO4

2�).

containing bpy-type spectator ligands, the largest variationsare only a few cm�1. However, if the homoleptic complex isconsidered also, where the ‘spectator’ ligands are both bpzligands, significant shifts are noted for the mode designatedω5, its value shifting from 1517 cm�1 to as low as 1510 cm�1

in Fig. 3. The difference in donor strength between bpz anddaf is smaller than that between daf and dmdeb (pKa values:bpz D 7.1, daf ³7.6, dmdeb ³9.2),29,30 so that the variationobserved for the tris-bipyrazine complex is most reasonablyattributable to the increased �-acidity of the bpz spectatorligands relative to the other spectator ligands.29,30

TR2 spectraFor all of the bpz-containing complexes studied, the lowestenergy 3MLCT states are expected to be selectively local-ized on the bipyrazine ligand. The TR2 spectra of these fourcomplexes are shown in Fig. 4. The spectrum of the homolep-tic tris-ligated complex Ru(bpz)3

2C exhibits a set of bands

Copyright 2004 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2004; 35: 1001–1005

1004 J. Wu and J. R. Kincaid

980

1048 11

6211

87

1268 13

44

1407

1478

1507

1596

980

1029 1163

1190

1269

1345

1403

1483

1510

1594

980

1033 11

6311

88

1269 13

44

1405

1479

1509

1594

1000 1100 1200 1300 1400 1500 1600

Wavenumber / cm-1

Ru(bpz)32+ @ 442nm

Ru(daf)2(bpz)2+ @ 488nm

Ru(bpy)2(bpz)2+ @ 488nm

Ru(Me2,Et2-bpy)2 (bpz)2+ @ 488nm15

17

1410

134712

77

1194

1164

980

1486

1596

s

s

s

s

Inte

nsi

ty

Figure 3. RR spectra of Ru(daf)2(bpz)Cl2, Ru(bpy)2(bpz)Cl2,Ru(dmdeb)2(bpz)Cl2 and Ru(bpz)3Cl2 in 0.25 M Na2SO4

solution. (s indicates peak from internal standard SO42�).

attributable to coordinated bpz and another set, labeledω0

i, ascribable to a coordinated bpz�ž ligand, i.e. as shownpreviously,17 the TR2 spectrum reveals the proper formula-tion of this excited-state species as [Ru(III)(bpz)2�bpz�ž�]2C.As reported,17 there are no contributions to this spectrum bythe ground-state complex. The spectra of the Ru(bpy)2bpz2C

and Ru(dmdeb)2bpz2C complexes are consistent with thoseexpected for the bpz-localized 3MLCT states, showing modescharacteristic of a bpz�ž, with no indication of bands ascrib-able to a neutral bpz ligand; note that this also confirms thefact that there are no contributions to these spectra from thecorresponding ground states, which then would exhibit thestrong bpz modes such as the 1510 cm�1 band. However,the TR2 spectrum of the Ru(daf)2bpz2C complex is mainlyattributable to the ground-state species, with most of thebands being similar to those of neutral daf and bpz coor-dinated ligands. Only a few bands ascribable to bpz�ž areweakly enhanced. Apparently, given the short lifetime ofthis complex, a very small population of 3MLCT state isgenerated in the laser beam.

The observed bpz�ž modes for all of the complexes aresummarized in Table 1. It is seen that the most sensitivemodes are ω0

4, which shifts from its value of 1538 cm�1

for the tris-homoleptic complex to a high of 1544 cm�1 forcomplexes containing spectator ligands with stronger ligandfields, and ω0

5, which appears to be slightly more sensitive,although its precise frequency is difficult to identify owingto its overlap with neighboring modes from the neutralspectator ligands bpy and dmdeb or the ω6 mode of bpz inthe case of Ru(bpz)3

2C.

981

1039

1076

1148

1178

*127

1

1322

1366

1383

*143

0

1499

1544

1567 16

09

981

1034

1057

1160

1230

1273

1345

1442

1514

1596

981

1055

*1147

1161

* 12

23

*127

4 1347

*1430

1489

1538

1593

981

0149

* 12

25

* 12

71

1323

1409

1501

1544

*142

8

1613

900 1000 1100 1200 1300 1400 1500 1600

Wavenumber / cm-1

Ru(bpz)32+ @354nm TR2

Ru(daf)2(bpz)2+ @354nm TR2

Ru(bpy)2(bpz)2+ @354nm TR2

Ru((Me2,Et2-bpy)2(bpz)2+ @354nmTR2

1153

*122

6

1535

* 1

430

*122

3

1061 15

18

**

s

s

s

s

ω4ω′4

ω5

ω6

ω′6

ω8ω′8 & ω9

ω′9

ω10

ω11

ω′10

1047

ω12

Inte

nsity

Figure 4. TR2 spectra of Ru(bpz)3Cl2, Ru(daf)2(bpz)Cl2,Ru(bpy)2(bpz)Cl2 and Ru(dmdeb)2(bpz)Cl2 in 0.25 M Na2SO4

solution, excited at 354 nm (s indicates peak from internalstandard SO4

2� and asterisk indicates peak from bpz�ž).

Table 1. Wavenumbers (cm�1) of (bpz�ž) radical in differentcomplexes

Ru(bpz)3

2CRu(daf)2

(bpz)2CRu(bpy)2

(bpz)2CRu(dmdeb)2

(bpz)2C

ω04 1538 1535 1544 1544

ω05 1498 — 1499 1501

ω06 1430 1430 1430 1428

ω08 1274 1273 1271 1271

ω09 1223 1223 1226 1226

ω013 — 1034 1039 1049

CONCLUSION

The RR spectra of this series of heteroleptic complexes intheir ground electronic states present a straightforwardinterpretation in terms of localized MLCT transitions toparticular ligands, with transitions to the bpz ligandoccurring at substantially lower energies than those to thebpy and substituted bpy ligands. The ω5 bpz mode exhibitssome sensitivity to the nature of the spectator ligand, but thesensitivity is slight and seems to be more responsive to �-acidity rather than �-donor strength. This modest sensitivityof bpz modes to the identity and nature of the spectator

Copyright 2004 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2004; 35: 1001–1005

Spectator ligand effects on spectra of Ru–bipyrazine complexes 1005

ligands extends to the 3MLCT excited state, where most ofthe modes observed in the TR2 spectrum show fairly smallchanges, with ω0

4 shifting by only 6–9 cm�1 and ω05 by perhaps

as much as 9–10 cm�1, although it is difficult to locate itsprecise wavenumber.

AcknowledgmentThis work was supported by a grant from the Department of Energy,Basic Energy Science Division (ER 13619).

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Copyright 2004 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2004; 35: 1001–1005