A new family of ruthenium(II) polypyridine complexes bearing 5-aryltetrazolate ligands as systems for electrochemiluminescent devices

Inorg. Chem. 2006, 45, 695−709

A New Family of Ruthenium(II) Polypyridine Complexes Bearing

5-Aryltetrazolate Ligands as Systems for Electrochemiluminescent

Devices

Stefano Stagni,^†Antonio Palazzi,*^,†Stefano Zacchini,^†Barbara Ballarin,^†Carlo Bruno,^‡

Massimo Marcaccio,*^,‡Francesco Paolucci,^‡Magda Monari,^‡Maurizio Carano,^§and Allen J. Bard*^,§

Dipartimento di Chimica Fisica ed Inorganica, UniVersita` di Bologna, Viale Risorgimento 4,

I-40136 Bologna, Italy, Dipartimento di Chimica “G. Ciamician”, UniVersita` di Bologna,

Via Selmi 2, I-40126 Bologna, Italy, and Department of Chemistry & Biochemistry,

The UniVersity of Texas at Austin, Austin, Texas 78712

Received August 31, 2005

A new family of mono- and dinuclear ruthenium polypyridyl complexes containing 5-aryltetrazolate ligands such as

the deprotonated form of 4-(1H-tetrazol-5-yl)benzonitrile (4-TBNH) and bis(1H-tetrazol-5-yl)benzene (BTBH₂) have

been synthesized and thoroughly characterized. The reactivity of the mononuclear species toward different

electrophiles such as H⁺and CH₃⁺has been investigated, and the effects of the resulting regioselective electrophilic

attacks on the electronic and structural properties of the tetrazolate ligand have been studied by NMR (¹H, ¹³C)

spectroscopy and X-ray crystal structures. Absorption and emission spectroscopy, together with an electrochemical

and UV−vis−NIR spectroelectrochemical investigation of the uncoordinated ligand and complexes, has been

performed, highlighting a rather good luminescence efficiency and a poor bridge-mediated electronic communication

between the metal centers of the dinuclear complexes. The electrogenerated chemiluminescence (ECL) of the

dinuclear species has been explored, and for one of these, an exceptionally high ECL efficiency has been observed,

comparable to that of [Ru(bpy)₃]²⁺, which is considered a standard in ECL studies.

Introduction

idyl complexes have been widely used as ECL-lumino-

phores.⁶Since ligand design plays a crucial role in deter-

mining and eventually improving both the light-emitting and

the electron transfer performances of such systems,⁷a large

number of neutral⁸and anionic⁹“actor” ligands have been

Polypyridine derivatives of Ru(II) represent one of the

most extensively studied class of coordination compounds.

Interest in similar chemically stable systems stems from their

peculiar electrochemical and photophysical properties,¹which

have favored the application of these molecules in several

research fields such as conversion of solar energy,²fabrica-

tion of molecular devices,³DNA intercalation,⁴and protein

binding.⁵Furthermore, mono- and dinuclear Ru(II) polypyr-

(2) (a) Puntoriero, F.; Serroni, S.; Galletta, M.; Juris, A.; Licciardello,

A.; Chiorboli, C.; Campagna, S.; Scandola, F. ChemPhysChem 2005,

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* To whom correspondence should be addressed. E-mail: palazzi@

ms.fci.unibo.it (A.P.), ajbard@mail.utexas.edu (A.J.B.), massimo.marcaccio@

unibo.it (M.M.).

^†Dipartimento di Chimica Fisica ed Inorganica, Universita` di Bologna.

^‡Dipartimento di Chimica “G. Ciamician”, Universita` di Bologna.

^§Department of Chemistry & Biochemistry, The University of Texas at

Austin.

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10.1021/ic0514905 CCC: $33.50

Published on Web 12/16/2005

Inorganic Chemistry, Vol. 45, No. 2, 2006 695

Stagni et al.

included in the coordination spheres of Ru(II) polypyridyl-

metal fragments. In this context, five-membered N-heterocycle-

based ligands containing imidazolyl,¹⁰triazolyl,¹¹or pyra-

zolyl¹²moieties have been employed, but surprisingly, only

a few examples of ruthenium¹³or osmium¹⁴polypyridyl

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and characterization of some of the first examples of mono-

and dinuclear Fe(II) organometallic complexes containing

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showed by such tetrazolate ligands as well as the possibility

to reversibly modulate their electronic and structural proper-

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that these compounds are good candidates for incorporation

into Ru(II) polypyridine complexes. Here we report studies

of the synthesis, characterization, and evaluation of the main

structural and electronic properties of new mono- and

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Experimental Section

Materials. Solvents were dried and distilled under nitrogen prior

to use. Unless otherwise stated, chemicals were obtained com-

mercially (e.g. Aldrich) and used without any further purification.

Ru(tpy)Cl₃¹⁸and [Ru(tpy)(bpy)Cl][PF₆]¹⁹were prepared according

to literature procedures. The ligands 4-(1H-tetrazol-5-yl)benzonitrile

(4-TBNH) and 1,4-bis(1H-tetrazol-5-yl)benzene (BTBH₂) were

prepared in high yields (80% for 4-TBNH; over 95% for BTBH₂)

according to the first established method involving 1,3 dipolar

cycloaddition of azide anion (N₃^-) on the appropriate aromatic

nitriles.²⁰

Warning! Tetrazole derivatives are used as components for

explosive mixtures.^15cIn this laboratory, the reactions described

here were run on only a few grams and no problems were

encountered. However, great caution should be exercised when

handling or heating compounds of this type.

The formation of the anions [4-TBN]^-and [BTB]^2-was achieved

by addition of equimolar amounts (2 equiv in the case of BTBH₂)

of triethylamine to a suspension of the neutral 5-substituted

tetrazoles in acetone (5 mL). The resulting colorless or pale yellow

solutions were used without any further purification. Throughout

this paper, the percentage yields of the product complexes are

referred to the molar quantity of the starting [Ru(tpy)(bpy)Cl][PF₆].

Synthesis of the Mononuclear Complex [Ru(4-TBN)][PF₆].

[Ru(tpy)(bpy)Cl][PF₆] (0.500 g, 0.74 mmol) was dissolved in

deaerated acetone (15 mL) in a 100 mL round-bottom flask

protected from light. A slight excess (1.1 equiv) of AgPF₆was

added, and the mixture was stirred with reflux for 4 h. The reaction

mixture was filtered through a Celite pad, and the filtrate was added

dropwise to an acetone (10 mL) solution of the tetrazolate ligand

[4-TBN]^-(0.8 mmol). Once the addition was complete, the deep

red solution was stirred at reflux temperature overnight. The mixture

was then cooled to rt (room temperature), concentrated to about

20 mL, added to 10 mL of an aqueous solution containing ca. 0.5

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2003, 669, 135. (c) Palazzi, A.; Stagni, S.; Bordoni, S.; Monari, M.;

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1404.

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34, 821.

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37, 3610. (b) Demadis, K. D.; El-Samanody, E.-S.; Meyer, T. J.; White,

P. S. Inorg. Chem. 1998, 37, 838.

(15) The rapid development of tetrazole-based compounds is mostly

associated with the wide scale employment of these heterocycles in

several areas of research connected to medicinal science; see: (a)

Fu¨rmeier, S.; Metzger, J. O. Eur. J. Org. Chem. 2003, 885. (b) Herr,

R. J. Bioorg. Med. Chem. 2002, 10, 3379. (c) Butler, R. N. In

ComprehensiVe Heterocyclic Chemistry II; Storr, R. C., Ed.; Tetrazoles,

Vol.4; Pergamon Press: Oxford, U.K., 1996; pp 621-678 and

references therein. (d) Adamantini, A.; Beleggia, R.; Fringuelli, F.;

Pizzo, F.; Vaccaro, L. J. Org. Chem. 2004, 69, 2896. (e) Demko, Z.

P.; Sharpless, K. B. J. Org. Chem. 2001, 66, 7945. (f) Koguro, K.;

Oga, T.; Mitsui, S.; Orita, R. Synthesis 1998, 910.

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80, 3908.

696 Inorganic Chemistry, Vol. 45, No. 2, 2006

Ruthenium(II) Polypyridine Complexes

Chart 1. Ligands, Complexes, and Acronyms Used in This Work

g of NH₄PF₆, and extracted with dichloromethane (3 × 20 mL)

until the aqueous phase became colorless. The organic layers were

dried over MgSO₄, and the solvent was removed in vacuo. The

red mixture was redissolved in a minimal quantity of dry acetone

and added to a copious amount of diethyl ether, causing the

precipitation of a crude product which was collected by suction

filtration and purified by alumina filled column chromatography

with acetone/toluene mixtures as the eluent. Three bands were eluted

in the following order: a purple band identified as the starting [Ru-

(tpy)(bpy)Cl][PF₆] (acetone/toluene 1/1 (v/v) as the eluent); the

target mononuclear complex as a deep red fraction (acetone/toluene

1.5/1); the final brown band consisting of small amount of the

dinuclear species [Ru(4-TBN)Ru][PF₆]₃eluted with pure acetone.

The fractions containing the mononuclear compound were evapo-

rated to dryness, affording [Ru(4-TBN)][PF₆] (0.360 g, 60%) as a

deep red microcrystalline powder. ESI-MS: m/z 661, [M - PF₆^-]⁺.

Suitable crystals for X-rays analysis were obtained by the slow

diffusion of diethyl ether into a dichloromethane solution of the

complex, which crystallized as [Ru(4-TBN)][PF₆]‚2CH₂Cl₂. Anal.

Calcd for C₃₅H₂₇Cl₄F₆N₁₀PRu‚2CH₂Cl₂(M_r) 975.51): C, 43.09;

H, 2.80; N, 14.36. Found: C, 42.95; H, 2.92; N, 14.25.

(Me)TBN)][PF₆]_1.56[SO₃CF₃]_0.44showing the presence of residual

triflate anion. Anal. Calcd for C_34.44H₂₆F_10.68N₁₀O_1.33P_1.56RuS_0.44

(M_r) 967.48): C, 42.75; H, 2.71; N, 14.48. Found: C, 42.88; H,

2.84; N, 14.05.

Protonation Reaction. The deep red solution obtained by

dissolving under argon atmosphere a 0.100 g aliquot of the starting

complex [Ru(4-TBN)][PF₆] (0.12 mmol) in dichloromethane (10

mL) was cooled to -50 °C. Then a slight excess (1.1 equiv) of

HOSO₂CF₃(1 mL, 0.136 M in dichloromethane, 0.136 mmol) was

added dropwise. After 30 min, the reaction mixture was allowed

to warm to rt and stirred for additional 6 h. The solvent was the

removed in vacuo, affording a red oily residue which was

redissolved in acetonitrile (5 mL), and added to a copious amount

of diethyl ether, causing the precipitation of a microcrystalline

powder. The mixture was maintained under argon atmosphere and

filtered through a glass frit, affording a deep orange solid which

analyzed well as [Ru(4-(H)TBN)][SO₃CF₃]₂(0.085 g, 90%). ESI-

MS: m/z 331, [M - 2SO₃CF₃]

^{- 2+}. Suitable crystals for X-ray

determination were obtained by the slow diffusion of diethyl ether

into an acetonitrile solution of the protonated complex, which

crystallized as [Ru(4-(H)TBN)][SO₃CF₃]₂‚CH₃CN‚H₂O. Anal.

Calcd for C₃₇H₂₉F₆N₁₁O₇RuS₂(M_r) 1018.90): C, 43.62; H, 2.87;

N, 15.12. Found: C, 44.00; H, 2.98; N, 15.75.

Preparation of the Dinuclear Complexes [Ru(4-TBN)Ru]-

[PF₆]₃and [Ru(BTB)Ru][PF₆]₂. The reactive species obtained from

the Ag(I)-mediated chloride extraction of [Ru(tpy)(bpy)Cl][PF₆]

(0.250 g, 0.37 mmol) was dissolved in 15 mL of deareated acetone

and successively combined with an acetone solution (5 mL)

containing 0.5 equiv (calculated with respect to the starting [Ru-

(tpy)(bpy)Cl][PF₆]) of the appropriate tetrazolate ligand. Then the

deep red mixture was stirred at reflux temperature for 15 h, and a

workup procedure analogous to that reported above for the

mononuclear compound afforded a crude product which was

purified by alumina filled column chromatography. The dinuclear

Methylation Reaction. [Ru(4-TBN)][PF₆] (0.100 g, 0.12 mmol)

was dissolved in dichloromethane (10 mL) under argon atmosphere,

and the resulting deep red solution was cooled to -50 °C; then

CH₃OSO₂CF₃(1 mL, 0.124 M in dichloromethane, 0.124 mmol)

was added dropwise. After 30 min, the red mixture was allowed to

warm to rt and stirred for additional 6 h. Evaporation of the solvent

afforded a red oily residue which was dissolved into a minimal

amount of acetonitrile and added to 10 mL of an aqueous solution

containing ca. 0.5 g of NH₄PF₆. The resulting mixture was extracted

with dichloromethane (3 × 20 mL) until the aqueous phase became

colorless. The organic layers were dried over MgSO₄and the solvent

was removed in vacuo, affording a red solid which was identified

as the methylated product [Ru(4-(Me)TBN)][PF₆]₂(0.090 g, 78%).

ESI-MS: m/z 338, [M - 2PF₆

]

^{- 2+}. Suitable crystals for X-rays

complex [Ru(4-TBN)Ru][PF₆]₃(0.110 g, 37%) was eluted as the

- 3+

analysis were obtained by the slow diffusion of diethyl ether into

final product with pure acetone. ESI-MS: m/z 384, [M - 3PF₆] .

an acetonitrile solution the complex, which crystallized as [Ru(4-

Anal. Calcd for C₅₈H₄₂N₁₅F₁₈P₃Ru₂: C, 43.92; H, 2.67; N, 13.25.

Inorganic Chemistry, Vol. 45, No. 2, 2006 697

Stagni et al.

Found: C, 44.20; H, 2.85; N, 13.55. An analogous procedure was

followed for the preparation of the symmetrically bridged species

[Ru(BTB)Ru][PF₆]₂. In this latter case, the alumina filled column

chromatography of the crude product afforded the target dinuclear

complex [Ru(BTB)Ru][PF₆]₂(0.150 g, 27%) as a deep red-brown

fraction by elution with a 2/1 (v/v) acetone/toluene mixture. ESI-

MS: m/z: 597, [M - 2PF₆]²⁺. Anal. Calcd for C₅₈H₄₂N₁₈F₁₂P₂-

Ru₂: C, 46.97; H, 2.85; N, 17.00. Found: C, 47.20; H, 2.95; N,

17.30.

model 3091 digital oscilloscope, and the data were transferred to a

personal computer by the program Antigona.²⁵The minimization

of the uncompensated resistance effect in the voltammetric mea-

surements was achieved by the positive-feedback circuit of the

potentiostat. Digital simulation of the cyclic voltammetric curves

was carried out either by Antigona or DigiSim 3.0. The spectro-

electrochemical experiments have been carried out using a quartz

OTTLE cell with a 0.03 cm path length. Temperature control was

achieved by a special cell holder with quartz windows, in which

two nitrogen fluxes (one at room temperature and the other at low

temperature) were regulated by two needle valves. All the spectra

have been recorded by a Cary 5. All the experimental details for

the spectroelectrochemical setup have been reported elsewhere.²⁶

All fluorescence spectra were recorded on a Fluorolog-3 spectro-

fluorometer (ISA-Jobin Yvon Hariba, Edison, NJ) using a slit width

of 1 nm and a resolution of 1.5 nm. All UV-visible spectra were

recorded on a Milton Roy Spectronic 3000 array spectrophotometer.

The absorbance and fluorescence spectra of the complexes were

obtained with a 40 µM acetonitrile (ACN) solution, and the relative

fluorescence efficiency was calibrated with Ru(bpy)₃²⁺as a standard

(λ_exc) 500 nm). ECL measurements were obtained as previously

reported²⁷with 1 mM [Ru(BTB)Ru]²⁺or [Ru(4-TBN)Ru]³⁺ACN

solutions, 0.1 M in supporting electrolyte TBAH (from Fluka). To

generate the annihilation reaction, the working electrode was pulsed

between the first oxidation and reduction peak potentials of [Ru-

(BTB)Ru]²⁺and [Ru(4-TBN)Ru]³⁺with a pulse width of 0.1 s.

The resulting emission spectra were obtained with a charged-

coupled device (CCD) camera (Photometrics CH 260, Photometrics-

Roper Scientific, Tucson, AZ) that was cooled to -100 °C.

Integration times were 4 min. The CCD camera and grating system

were calibrated with a mercury lamp prior to each measurement.

Instrumentation and Procedures. All the obtained complexes

were characterized by elemental analysis and spectroscopic methods.

Elemental analyses were performed on a ThermoQuest Flash 1112

Series EA instrument. ESI-mass spectra were performed on a

Waters ZQ-4000 instrument; acetonitrile was used as the solvent.

The routine NMR spectra (¹H, ¹³C) were always recorded using a

Varian Mercury Plus 400 instrument (¹H, 400.1; ¹³C, 100 MHz).

The spectra were referenced internally to residual solvent resonance

and were recorded at 298 K for characterization purposes. Bidimen-

sional ¹H,¹³C correlation spectra were measured via gs-HSQC and

1

gs-HMBC experiments,²¹whereas H,¹H correlations were deter-

mined by gs-COSY experiments.²²

Electrochemical and Spectroelectrochemical Measurements.

Tetraethylammonium tetrafluoborate (TEATFB; from Fluka) and

tetrabutylammonium hexafluorophosphate (TBAH; from Fluka)

were used as received as supporting electrolyte. Dry vacuum

distilled N,N-dimethylformamide (DMF) was purified using sodium

anthracenide to remove any traces of water and oxygen, according

to the method of Aoyagui and co-workers.²³Acetonitrile (ACN),

after being refluxed over CaH₂, was distilled under vacuum at room

temperature with a high refluxing ratio, utilizing a 1 m length

distillation column filled with glass rings, and it was stored in a

specially designed Schlenk flask over 3 Å activated molecular

sieves, protected from light. The solvent was distilled via a closed

system into an electrochemical cell containing the supporting

electrolyte and the species under examination, soon before perform-

ing the experiment. All the other chemicals were of reagent grade.

Electrochemical experiments were carried out in an airtight single-

compartment cell described elsewhere,²⁴by using platinum as

working and counter electrodes and a silver spiral as a quasi-

reference electrode. The drift of the quasi-reference electrode was

negligible for the time required by an experiment. All the E_1/2

potentials have been directly obtained from cvc’s as average of

the cathodic and anodic peak potentials for one-electron peaks and

by digital simulation for those processes closely spaced in multi-

electron voltammetric peaks. The E_1/2values, referred to an aqueous

saturated calomel electrode (SCE), have been determined by adding

at the end of each experiment ferrocene as an internal standard

and measuring them with respect to the ferrocenium/ferrocene

couple standard potential. The cell containing the supporting

electrolyte and the electroactive compound was dried under vacuum

at 100-110 °C for at least 60 h before each experiment. The

pressure measured in the electrochemical cell prior to performing

the trap-to-trap distillation of the solvent was typically (1-2) ×

10^-5mbar. Voltammograms were recorded with an AMEL model

552 potentiostat controlled by an AMEL model 568 programmable

function generator. The potentiostat was interfaced to a Nicolet

3--

ECL yields were determined against the standard, i.e., Ru(bpy)₃

Ru(bpy)₃^-ECL system in ACN solution containing 0.1 M TBAH

(with φ_ecl) 0.05),²⁸taking also into account the differences in the

charges passed through the studied solution. In a given solution,

two or three records were made to check the temporal stability of

the system studied. Since the compounds can undergo several

reduction and oxidation processes, scans with different limits were

performed over the available potential window to include 2, 3, or

4 redox processes.

X-ray Crystallography. Crystal data and collection details are

reported in Table 1. The diffraction experiments were carried out

on a Bruker APEX II diffractometer (for [Ru(4-TBN)][PF₆]‚2CH₂-

Cl₂and [Ru(4-(H)TBN)][CF₃SO₃]₂‚CH₃CN‚H₂O) and on a Bruker

SMART 2000 diffractometer (for [Ru(4-(Me)TBN)][PF₆]_1.56[CF₃-

SO₃]_0.44), equipped with a CCD detector and using Mo KR radiation.

Data were corrected for Lorentz polarization and absorption effects

(empirical absorption correction SADABS).²⁹Structures were

solved by direct methods and refined by full-matrix least-squares

on the basis of all data using F².³⁰H atoms were placed in calculated

positions, except H(9n) in [Ru(4-(H)TBN)][CF₃SO₃]₂‚CH₃CN‚H₂O,

which was located in the Fourier map and refined with the N(9)-

H(9n) distance restrained to 0.86 Å; conversely, it has not been

possible to locate the H atoms for the water molecule in [Ru(4-

(H)TBN)][CF₃SO₃]₂‚CH₃CN‚H₂O. H atoms were treated isotropi-

(25) Antigona was developed by Dr. Loic Mottier.

(26) Lee, S.-M.; Kowallick, R.; Marcaccio, M.; McCleverty, J. A.; Ward,

M. D. J. Chem. Soc., Dalton Trans. 1998, 3443.

(21) Wilker, W.; Leibfritz, D.; Kerssebaum, R.; Beimel, W. Magn. Reson.

Chem. 1993, 31, 287.

(22) Hurd, R. E. J. Magn. Reson. 1990, 87, 422.

(23) Saji, T.; Yamada, T.; Aoyagui, S. J. Electroanal. Chem. 1975, 61,

147.

(24) Marcaccio, M.; Paolucci, F.; Paradisi, C.; Roffia, S.; Fontanesi, C.;

Yellowlees, L. J.; Serroni, S.; Campagna, S.; Denti, G.; Balzani, V.

J. Am. Chem. Soc. 1999, 121, 10081.

(27) McCord, P.; Bard, A. J. J. Electroanal. Chem. 1991, 318, 91

(28) Wallace, W. L.; Bard, A. J. J. Phys. Chem. 1979, 83, 1350.

(29) Sheldrick, G. M. SADABS, Program for Empirical Absorption

Correction; University of Go¨ttingen: Go¨ttingen, Germany, 1996.

(30) Sheldrick, G. M. SHELX97-Program for the refinement of Crystal

Structure; University of Go¨ttingen: Go¨ttingen, Germany, 1997.

698 Inorganic Chemistry, Vol. 45, No. 2, 2006

Ruthenium(II) Polypyridine Complexes

Table 1. Crystal Data and Experimental Details for [Ru(4-TBN)][PF₆]‚2CH₂Cl₂, [Ru(4-(Me)TBN)][PF₆]_1.56[CF₃SO₃]_0.44, and

[Ru(4-(H)TBN)][CF₃SO₃]₂‚CH₃CN‚H₂O

param

[Ru(4-TBN)[PF₆]‚2CH₂Cl₂

[Ru(4-(Me)TBN)][PF6]_1.56[CF₃SO₃]_0.44

[Ru(4-(H)TBN)][CF₃SO₃]2‚CH₃CN‚H₂O

Formula

fw

C₃₅H₂₇Cl₄F₆N₁₀PRu

975.51

C_34.44H₂₆F_10.68N₁₀O_1.33P_1.56RuS_0.44

967.48

C₃₇H₂₉F₆N₁₁O₇RuS₂

1018.90

T, K

100(2)

293(2)

296(2)

λ, Å

0.710 73

monoclinic

P2₁/c

8.5520(4)

21.0605(10)

21.1224(9)

90

92.827(2)

90

3799.7(3)

4

0.710 73

monoclinic

P2₁/c

14.294(3)

17.306(4)

16.038(3)

90

101.22(3)

90

3891.5(14)

4

0.710 73

triclinic

P1h

9.392(4)

10.915(4)

20.692(8)

84.953(6)

88.306(6)

83.028(6)

2096.9(14)

2

cryst system

space group

a, Å

b, Å

c, Å

R, deg

â, deg

γ, deg

cell vol, Å³

Z

D_c, g cm^-3

µ, mm^-1

F(000)

1.705

0.809

1952

1.651

0.587

1935

1.614

0.563

1028

cryst size, mm

θ limits, deg

reflcns collcd

0.22 × 0.14 × 0.13

1.93-25.03

23 704

0.18 × 0.14 × 0.11

1.45-25.03

34 357

0.19 × 0.14 × 0.11

1.89-25.03

18 484

indepndt reflcns

6698 (R_int) 0.0402)

6698/3/512

1.041

0.0412

0.1076

6859 (R_int) 0.1020)

6859/58/606

1.050

0.0534

0.1388

7359 (R_int) 0.0959)

7359/207/581

0.972

0.0734

0.2144

data/restraints/params

goodness on fit on F²

R₁(I > 2σ(I))

wR₂(all data)

largest diff peak and hole, e‚Å^-3

1.143/-1.036

1.163/-0.734

0.870/-1.486

Scheme 1. Synthetic Pathway Followed for the Preparation of Tetrazolate Complexes

cally using the 1.2-fold U_isovalue of the parent atom except methyl

protons, which were assigned the 1.5-fold U_isovalue of the parent

C atom. All non-hydrogen atoms were refined with anisotropic

displacement parameters, unless otherwise stated. One of the two

CH₂Cl₂molecules in [Ru(4-TBN)][PF₆]‚2CH₂Cl₂is disordered over

two positions. Moreover, substitutional disorder is present in [Ru-

(4-(Me)TBN)][PF₆]_1.56[CF₃SO₃]_0.44, where the site of one of the two

positions were split and refined using similar distance and similar

U restraints and one occupancy parameter/disordered group.

Results and Discussion

Synthesis, NMR, Structural Characterization, and

Reactivity of Complexes. The synthetic procedure for the

preparation of the target mono- and dinuclear complexes [Ru-

(4-TBN)]⁺, [Ru(4-TBN)Ru]³⁺, and [Ru(BTB)Ru]²⁺(Scheme

1) involved the preliminary reaction of the ruthenium

precursor [Ru(tpy)(bpy)Cl][PF₆] with a slight molar excess

-

anions is partially occupied by PF₆and partially by CF₃SO₃

.

Attempts to refine the structure using only one type of anion resulted

in a lower fitting of the model with the experimental data with

high residual electron density nearby the anion. Disordered atomic

Inorganic Chemistry, Vol. 45, No. 2, 2006 699

Stagni et al.

Chart 2. N-1 and N-2 Coordination Isomers of Tetrazolate Complexes

tions from the average plane in the range +0.003 to -0.004

Å) and almost coplanar with the bonded aromatic ring

[dihedral angle N(6)-C(26)-C(27)-C(32) ) 7.3(6)°]. A

similar conformation of the 5-substituted tetrazolate ligand

had been previously observed in the iron complex [CpFe-

(CO)(P(OCH₃)₃)(N₄CC₆H₄CN)]^17cand clearly indicates inter-

ring conjugation within the ligand. The formation of C-H‚‚‚

π and π‚‚‚π interactions in the crystals of [Ru(4-TBN)]-

[PF₆]‚2CH₂Cl₂is mainly hampered by the separation action

of the PF₆^-anion and the two CH₂Cl₂molecules. Only few

interactions are, therefore, present, and they can be classified

as weak on the basis of the rather long centroid-centroid

distances (>4.4 Å) and the large slip angles (>40°).³¹

The NMR characterization of all the mononuclear species

has been accomplished with the use of ¹H gs-COSY, ¹H,¹³C

gs-HSQC, and ¹H,¹³C gs-HMQC two-dimensional tech-

niques, leading to the clear distinction of the resonances due

to the 5-substituted tetrazolate ligand (see Table 3) from those

due to the tpy and bpy moieties. Complete NMR (¹H, ¹³C)

data and spectra are reported in the Supporting Information.

The complexation reaction of tetrazolate salts can lead to

the formation of different coordination isomers (Chart 2).

In the particular case of 5-aryl- or 5-pyridyl-substituted

tetrazolate ligands, the ¹³C NMR resonance of the tetrazolate

carbon (Ct) represent a reliable parameter to determine

whether the metal fragment is N-1 [δ(Ct) ) 151-155 ppm]

or N-2 coordinated [δ(Ct) ) 161-165 ppm] to the tetrazolate

moiety (see Chart 2).^17,32Relative to [Ru(4-TBN)]⁺, the

exclusive formation of the less hindered N-2 coordinated

isomer is indicated by the presence of a single tetrazolate

(Ct; see Table 3) carbon resonance at 163.4 ppm, which falls

in the chemical shifts range (161-165 ppm) typical of N-2

coordinated tetrazolate complexes^17,32and of N-2 methyl

derivatives of organic 5-aryltetrazoles.³³In addition, the

presence of interannular conjugation in the 5-aryltetrazolate

ligand of complex [Ru(4-TBN)]⁺is indicated by the chemical

shifts separation [∆(δ(C_meta)- δ(C_ortho)) ) 6.4 ppm] between

the carbons meta and ortho (relative to the tetrazole group;

see Scheme 1) of the benzonitrile ring. The magnitude of

this parameter, which has been introduced by Butler and co-

workers³³for determining the extent of interannular conjuga-

tion in a series of organic 5-aryltetrazoles, is even greater

than what we recently reported [∆(δ(C_meta)-δ(C_ortho)) ) 5.1

ppm] in the case of the analogue organometallic complex

[Cp(CO)(P(OMe)₃)FeN₄CC₆H₄CN],^17cthe structural deter-

Figure 1. Molecular structure of the cation [Ru(4-TBN)]⁺, with key atoms

labeled. Displacement ellipsoids are at 30% probability level.

Table 2. Selected Bond Lengths (Å) and Angles (deg) for

[Ru(4-TBN)]⁺, [Ru(4-(Me)TBN)]²⁺, and [Ru(4-(H)TBN)]²⁺

param

[Ru(4-TBN)]⁺[Ru(4-(Me)TBN)]²⁺[Ru(4-(H)TBN)]²⁺

Ru(1)-N(1)

Ru(1)-N(2)

Ru(1)-N(3)

Ru(1)-N(4)

Ru(1)-N(5)

Ru(1)-N(7)

N(6)-N(7)

N(7)-N(8)

N(8)-N(9)

N(9)-C(26)

N(6)-C(26)

C(26)-C(27)

2.076(3)

2.063(3)

2.069(3)

1.962(3)

2.074(3)

2.086(3)

1.332(4)

1.309(4)

1.339(4)

1.344(5)

1.337(5)

1.472(5)

2.079(4)

2.047(4)

2.060(4)

1.975(4)

2.067(4)

1.352(5)

1.299(5)

1.341(5)

1.330(6)

1.325(6)

1.462(7)

2.070(6)

2.050(6)

2.071(6)

1.965(5)

2.065(6)

2.084(6)

1.347(8)

1.307(8)

1.324(9)

1.337(9)

1.316(9)

1.480(10)

N(6)-N(7)-N(8)

N(7)-N(8)-N(9)

N(8)-N(9)-C(26)

N(9)-C(26)-N(6)

C(26)-N(6)-N(7)

C(30)-C(33)-N(10)

113.1(3)

107.0(3)

105.4(3)

112.1(3)

102.4(3)

178.3(5)

111.5(4)

105.5(4)

109.3(4)

108.4(4)

105.3(4)

176.9(9)

111.8(6)

105.1(6)

109.7(6)

108.4(7)

105.1(6)

174.0(12)

of a silver salt, such as AgPF₆, in refluxing acetone. The

removal of the precipitated AgCl afforded a deep red filtrate,

probably containing the solvato complex [Ru(tpy)(bpy)-

(acetone)][PF₆]₂, which was thoroughly combined with an

acetone solution of the desired 5-substituted tetrazolate ligand

[4-TBN]^-or [BTB]^2-.

The reaction mixtures was stirred at reflux for 10-15 h,

and the target complexes were all purified by alumina filled

column chromatography. The confirmation of the composi-

tion of each of the target mono- and dinuclear complexes

was provided by the positive ion ESI mass spectra. Moreover,

the molecular structure of [Ru(4-TBN)]⁺has been confirmed

by X-ray diffraction studies (Figure 1, Table 2). The molecule

is composed of a Ru(tpy)(bpy) fragment to which the

5-substituted tetrazolate ligand is N-2 coordinated. The

bonding parameters for the former parallel very well the ones

reported for other complexes containing the Ru(tpy)(bpy)

unit.^5a,9c-dIn particular, the bond distance of ruthenium to

the central tpy nitrogen [Ru(1)-N(4) ) 1.962(3) Å] is ca.

0.1 Å shorter than the other Ru-N distances (2.06-2.09

Å), and this is typical of the tpy coordination to ruthenium

and osmium complexes.^8aThe tetrazolate ring is flat (devia-

(31) Janiak, C. Dalton Trans. 2000, 3885.

(32) (a) Jackson, W. G.; Cortez, S. Inorg. Chem. 1994, 33, 1921 and

references therein. (b) Takach, N. E.; Holt, E. M.; Alcock, N. W.;

Henry, R. A.; Nelson, J. H. J. Am. Chem. Soc. 1980, 102, 2968.

(33) Butler, R. N. In ComprehensiVe Heterocyclic Chemistry; Potts, K. T.,

Ed.; Tetrazoles; Pergamon Press: Oxford, U.K., 1984; Vol. 5, pp 791-

838 and references therein.

700 Inorganic Chemistry, Vol. 45, No. 2, 2006

Ruthenium(II) Polypyridine Complexes

1

Table 3. Selected H and ¹³C NMR Data^afor All the Complexes Reported in This Paper with Atom Labeling Presented in Scheme 1

complex

δ(Ct)

δ(C_meta

)

δ(C_ortho

)

∆[δ(C_meta)-δ(C_ortho)]

δ(H_ortho

)

δ(H_meta)

[Ru(4-TBN)]⁺

163.4

156.6

156.4

163.1

164.3

133.5

134.3

134.0

134.7

126.8

127.2

128.9

130.5

128.3

126.8

6.3

5.4

3.5

6.4

7.86

7.85

7.62

b

7.63

7.85

b

[Ru(4-(H)TBN)]²⁺

[Ru(4-(Me)TBN)]²⁺

[Ru[4-TBN]Ru]³⁺

[Ru[BTB]Ru]²⁺

7.64

^aCD₃CN as solvent, room temperature, chemical shifts expressed in ppm. ^bOverlapping with bpy and tpy resonances.

Scheme 2. (a) Methylation Reactions and (b) Selected ¹³C NMR Signals of the Methylated Compound [Ru(4-Me)TBN]²⁺(Right) Compared to Those

of the Starting Complex [Ru(4-TBN)]⁺(Left)

mination of which showed the tetrazolate moiety and the

phenyl ring to be coplanar.

encumbrance from the metal fragmentscausing the out of

plane rotation of the aromatic 5-substituent ring. The

consequent loss or, at least, strong reduction of interannular

conjugation is indicated by the separation between the

chemical shifts of the benzonitrile carbons meta and ortho

in the complex [Ru(4-(Me)TBN)]²⁺[∆(δ(C_meta)-δ(C_ortho))

) 3.5 ppm; see Table 3 and Scheme 2b, right], which is

significantly lower than that displayed by the precursor [Ru-

(4-TBN)]⁺[∆(δ(C_meta)- δ(C_ortho)) ) 6.4 ppm; see Scheme

2b, left]. Further support to our hypotheses is given by the

crystal structure of the methylated dication [Ru(4-(Me)-

TBN)]²⁺(Figure 2 and Table 2), in which the tetrazole and

the benzonitrile rings are considerably out of plane [dihedral

angle N(6)-C(26)-C(27)-C(32) ) 118.9(6)°]. The large

variation of this torsion angle is the only major change

introduced in the structure of [Ru(4-(Me)TBN)]⁺after

methylation, compared to the precursor [Ru(4-TBN)]⁺, all

the other bonding parameters being almost identical.

Reactivity toward Electrophiles. The presence of three

imine-type nitrogen atoms in the N-2 coordinated five-

membered ring of the 5-substituted tetrazolate ligands offers

different sites which could undergo alkylation and/or pro-

tonation reactions. In particular, electrophilic additions onto

organometallic complexes such as [Cp(CO)(L)FeN₄CC₆H₄-

CN] (L ) CO, PPh₃, P(OMe)₃)^17cwere reported to cause

significant modifications to the structural and electronic

properties of the coordinated 5-aryltetrazolate moiety. Thence,

complex [Ru(4-TBN)]⁺was treated with 1 equiv of methyl

triflate, leading to the formation of the dication [Ru(4-(Me)-

TBN)]²⁺(Scheme 2), in which the tetrazolate carbon (Ct)

¹³CNMR resonance is found at 156.4 ppm and falls in the

expected chemical shift range (152-157 ppm) for 5-aryltet-

razoles having a substituent at the nitrogen adjacent to

tetrazole carbon.^32,33

In perfect agreement with the analogous organometallic

compounds,¹⁷the analysis of the NMR data suggests that

the methylation reaction occurs regioselectively at the N-4

nitrogen (see Scheme 2a)sthe one which suffers less

Similarly, the protonation of [Ru(4-TBN)]⁺(Scheme 3)

affords the dication complex [Ru(4-(H)TBN)]²⁺in which the

hydrogen substituent is found on the N-4 tetrazolate nitrogen,

as clearly indicated by the ¹³C NMR data [δ(Ct) ) 156.4

Inorganic Chemistry, Vol. 45, No. 2, 2006 701

Stagni et al.

an intermediate value [N(6)-C(26)-C(27)-C(32) ) 24.6-

(11)°] between the ones found in [Ru(4-TBN)]⁺[7.3(6)°]

and [Ru(4-(Me)TBN)]²⁺[118.9(6)°], and this is probably due

to the smaller steric hindrance of the hydrogen compared to

the methyl group. A similar trend is also observed in the

¹³C NMR data, where the separation of meta and ortho

carbons for [Ru(4-(H)TBN)]²⁺[∆(δ(C_meta)-δ(C_ortho)) ) 5.4

ppm] is intermediate between the precursor [Ru(4-TBN)]⁺

[∆(δC_meta)-δ(C_ortho)) ) 6.4 ppm] and the methylated com-

pound [Ru(4-(Me)TBN)]²⁺[∆(δ(C_meta)-δ(C_ortho)) ) 3.5

ppm].

As we might expect,¹⁷the protonated mononuclear species

[Ru(4-(H)TBN)]²⁺can be reconverted into the cationic

precursor [Ru(4-TBN)]⁺by treatment with a stoichiometric

amount of a base such as triethylamine. It is important to

point out that the reversibility of the protonation reaction

(Scheme 3) offers the opportunity to modulate the structural

and electronic properties of the aromatic 5-substituted

tetrazolate ligands by a simple protonation-deprotonation

mechanism.

Figure 2. Molecular structure of [Ru(4-(Me)TBN)]²⁺, with key atoms

labeled. Displacement ellipsoids are at 30% probability level.

1

Dinuclear Complexes. The H NMR spectrum of the

dinuclear complex [Ru(BTB)Ru]²⁺(Supporting Information,

Figure S4a) is found to be closely similar to that of

mononuclear compound [Ru(4-TBN)]⁺. The presence of the

“fully tetrazolate” bridging ligand BTB is demonstrated by

the singlet at 7.64 ppm, which is assigned to the four

chemically equivalent BTB protons. The ¹³C NMR spectrum

of [Ru(BTB)Ru]²⁺(Table 3 and Figure S4b) displays a single

tetrazole carbon (Ct) resonance at 164.3 ppm, clearly

indicating the selective N-2 tetrazole coordination of each

of the peripheral Ru(tpy)(bpy) fragment. In the case of the

dinuclear species [Ru(4-TBN)Ru]³⁺, the presence of the

nonequivalently bridging ligand [4-TBN^-] led to a quite

complicated ¹H NMR spectrum in which it was not possible

to clearly distinguish the resonance frequencies of protons

ortho and meta (see Figure S5a and Scheme 1 for labeling)

from those of the bpy and tpy rings. Concerning ¹³C NMR

spectroscopy, the comparison of the ¹³C NMR data of the

dinuclear complex [Ru(4-TBN)Ru]³⁺(Figure S5b) with the

ones of the mononuclear building block [Ru(4-TBN)]⁺and

of the dinuclear organometallic analogues [Cp(CO)(L)FeN₄-

CC₆H₄NFe(L)(CO)Cp]^{+ 17a}allowed the assignment of ¹³C

NMR carbon resonances of the tetrazolate ligand [4-TBN^-],

which were consistent both with the selective coordination

of the Ru(tpy)(bpy) unit to the N-2 tetrazole nitrogen

[δ(Ct) ) 163.1 ppm] and with the presence of interannular

conjugation [∆(δ(C_meta)-δ(C_ortho)) ) 6.4 ppm] across the

bridging linker [4-TBN^-].

Figure 3. Molecular structure of [Ru(4-(H)TBN)]²⁺, with key atoms

labeled. Displacement ellipsoids are at 30% probability level.

ppm] and the crystal structure of the salt [Ru(4-(H)TBN)]-

[CF₃SO₃]₂‚CH₃CN‚H₂O (Figure 3 and Table 2). Hydrogen

-

bonding is present within the crystal, involving one CF₃SO₃

anion and the N-H tetrazole group [N(9)‚‚‚O(6) ) 2.774-

(12) Å, N(9)-H(9n)‚‚‚O(6) ) 159(8)°] and the second

-

CF₃SO₃anion and H₂O [O(3)‚‚O(7) ) 2.554(12) Å; the

water hydrogens have not been located]. As for the meth-

ylated analogous [Ru(4-(Me)TBN)]⁺, no significant differ-

ences in the bond distances and bond angles of [Ru(4-

(H)TBN)]²⁺compared to [Ru(4-TBN)]⁺are present. It is

only important to notice that the dihedral angle between the

two aromatic rings of the 5-substituted tetrazole ligand shows

UV-Vis Absorption and Emission Properties. The

electronic spectra of all the complexes, which were measured

Scheme 3. Protonation of [Ru(4-TBN)]⁺

702 Inorganic Chemistry, Vol. 45, No. 2, 2006

Ruthenium(II) Polypyridine Complexes

Table 4. Absorption^aand Emission^a,bBands (CH₃CN as Solvent, Room Temperature) of All Complexes

λ/nm (10^-4ꢀ/M^-1cm^-1

)

complex

LC-MC

MLCT

503 (2.21)

425 sh (0.97), 482 (1.73)

459 (1.91)

464 (1.37)

416 sh (2.22), 454 (2.58)

418 sh (1.03), 480 (2.14)

λ_em/nm

[Ru(tpy)(bpy)Cl]⁺

[Ru(4-TBN)]⁺

240 (7.10), 282 (6.62), 294 (7.45), 317 (6.80)

248 (6.95), 272 (5.49), 289 (5.90), 313 (6.88)

248 (5.61), 272 (5.49), 289 (5.90), 310 (5.26)

242 (5.80), 275 (5.24), 291 (5.78), 310 (5.30)

239 (8.04), 271 (10.4), 287 (11.6), 310 (10.4)

232 (7.05), 271 (8.27), 291 (10.5), 312 (10.3)

nd

672

664

616

676

680

[Ru(4-(H)TBN)]²⁺

[Ru(4-(Me)TBN)]²⁺

[Ru[4-TBN]Ru]³⁺

[Ru[BTB]Ru]²⁺

^aConcentrations: 4 × 10^-5M. ^bExcitation wavelength: 480 nm.

This aspect, which is of importance for any future study

(as for example, in the bioinorganic chemistry field) of the

mononuclear derivative [Ru(4-TBN)]⁺, is currently under

investigation together with the electrochemical behavior of

this species upon addition of electrophiles (vide infra).

However, all compounds exhibit a fluorescence efficiency

(Φ) in the range between 0.008 (i.e. [Ru(4-(Me)TBN)]²⁺and

[Ru(4-TBN)Ru]³⁺) and 0.003 (i.e. [Ru(4-TBN)]⁺and [Ru-

(BTB)Ru]²⁺). Such values are about 1 order of magnitude

lower than that found for Ru(bpy)₃²⁺, which shows a

fluorescence efficiency of 0.06 under the same experimental

conditions.^1aConcerning the dinuclear complexes [Ru(BTB)-

Ru]²⁺and [Ru(4-TBN)Ru]³⁺, the relatively good lumines-

cence efficiency and the weak electronic coupling between

the metal centers (see spectroelectrochemical results) prompted

us to consider these molecules as good candidates for ECL

studies.^6a

Figure 4. Emission spectra (excitation wavelength, 480 nm; sample

concentration, 4 × 10^-5M in acetonitrile) recorded at 25 °C of mononuclear

complexes [Ru(4-TBN)]⁺(blue trace), [Ru(4-(Me)TBN)]²⁺(red trace), and

[Ru(4-(H)TBN)]²⁺(black trace).

Electrochemical Behavior of Ligands and Complexes.

Uncoordinated Ligands. It is well established that the redox

processes in Ru(II)-polypyridine complexes are mainly

localized either on the metal center (oxidations) or on the

ligands (reductions).^1,8a,34It is, therefore, of fundamental

importance to know the electrochemical behavior of the

uncoordinated ligands to understand the pattern of the ligand-

based redox series for the complexes.

The electrochemistry of the uncoordinated ligand bpy³⁵

has been previously studied under the same experimental

conditions used for this work. We investigated the redox

behavior of the ligands 4-TBN and BTB, to our knowledge

for the first time, either as their tetraphenylarsonium salts

or N-protonated species, while tpy was reinvestigated in

DMF under strictly aprotic conditions, both at room and low

temperature.

4-TBN. The free ligand exists as the N-protonated species

4-TBNH and was investigated in ACN and DMF solutions

both in its deprotonated form (i.e. 4-TBN) and as 4-TBNH.

In acetonitrile the 4-TBNH shows two one-electron reduc-

tions but no oxidation process is observed up to + 2.5 V

(vs SCE). Figure 5a shows the CV of 4-TBNH, in DMF at

24 °C and a scan rate, ν, of 1 V/s; the first reduction wave

exhibits some degree of electrochemical irreversibility, most

likely due to internal rearrangements related to proton

in acetonitrile solution, are summarized in Table 4. In all

cases, the UV region is dominated by intense ligand-centered

(LC) π-π* transitions, the energies of which are analogous

to those of similar Ru(tpy)(bpy)-based complexes.^5aIn

addition, as typical for Ru(II)-polypyridyl chromophores,

each spectrum displays the metal-to-ligand charge-transfer

(MLCT) transitions as the lowest energy (400-600 nm)

absorptions. In general, this broadened band is centered

between 450 and 480 nm; in some cases (see Table 4) the

energy differences between d(Ru)-π*(tpy) and d(Ru)-

π*(bpy) transitions are indicated by the presence of a

shoulder in the range 416-425 nm. The increase of the ionic

charge on going from complex [Ru(4-TBN)]⁺to the dication

complexes [Ru(4-(H)TBN)]²⁺and [Ru(4-(Me)TBN)]²⁺re-

sults in a shift of the maximum of the MLCT bands to higher

energy (i.e. λ_max^MLCT) 464 nm for [Ru(4-(Me)TBN)]²⁺and

459 nm for [Ru(4-(H)TBN)]²⁺), while marginal changes

occurred to the ligand-centered transitions (see Supporting

Information, Figure S6).

The emission spectra of all compounds were recorded in

acetonitrile at room temperature. Some preliminary studies

on the luminescence of all complexes indicated that, by

excitation at 480 nm, they show the characteristic emission

from ³MLCT excited states. In particular, the luminescence

properties of mononuclear complex [Ru(4-TBN)]⁺(λ_em

)

672 nm) can be tuned by addition of electrophiles (Figure

4). Indeed, the protonated compound [Ru(4-(H)TBN)]²⁺

showed a weaker emission peak at slightly higher energy

(λ_em) 664 nm), while a significantly stronger and blue-

shifted luminescence is exhibited by the methylated deriva-

tive [Ru(4-(Me)TBN)]²⁺(λ_em) 616 nm).

(34) Marcaccio, M.; Paolucci, F.; Paradisi, C.; Roffia, S.; Fontanesi, C.;

Yellowlees, L. J.; Serroni, S.; Campagna, S.; Denti, G.; Balzani, V.

J. Am. Chem. Soc. 1999, 121, 10081.

(35) (a) Roffia, S.; Casadei, R.; Paolucci, F.; Paradisi, C.; Bignozzi, C. A.;

Scandola, F. J. Electroanal. Chem. 1991, 302, 157. (b) Roffia, S.;

Marcaccio, M.; Paradisi, C.; Paolucci, F.; Balzani, V.; Denti, G.;

Serroni, S.; Campagna, S. Inorg. Chem. 1993, 32, 3003.

Inorganic Chemistry, Vol. 45, No. 2, 2006 703

Stagni et al.

tions: the first is completely irreversible even at 200 V/s

while the further two processes are reversible. Although the

third reduction occurs at the edge of the electrolyte/DMF

discharge, it can still be observed as a reversible process.

Figure 5b shows the cyclic voltammetric curve of BTB at

24 °C and at 10 V/s. Just before the second voltammetric

peak, there is a shoulder whose nature is still under

investigation but is probably related to a species generated

in a follow-up reaction.

tpy. The voltammetry of uncoordinated terpyridine, previ-

ously investigated,³⁶shows a one-electron reduction process.

It was reinvestigated in DMF, to compare it to the other

ligands and complexes under the same strictly aprotic

conditions, and it exhibits two reduction peaks both at room

and low temperature. The first reduction is a one-electron

Nernstian process while the second one-electron reduction

shows chemical irreversibility at room temperature but

becomes reversible at low temperature and at scan rates

higher than 100 V/s.

Figure 5c shows the CV of tpy at -56 °C and 100 V/s.

The E_1/2values of the two processes are given in Table 5.

The separation between them is 520 mV, and this value falls

within the typical range (500-600 mV) for the electronic

coupling energy³⁷into the same orbital of polypyridyl

ligands. The chemical follow-up reaction is probably a proton

uptake process of the doubly reduced tpy with the solvent.

This reaction is a rather fast process even at low temperature,

and the voltammetric pattern is similar to other polypyridyl

ligands in the uncoordinated state^35,38that are known to

undergo fast protonation reactions in the doubly reduced

state.

Ruthenium Mono- and Dinuclear Complexes. [Ru(4-

TBN)]⁺. Within the framework of the general strategy for

the investigation of multinuclear species, showing a large

number of redox processes, this species was studied since it

represents a precursor or a model for the behavior of the

dinuclear ones. The electrochemistry of the complex [Ru-

(4-TBN)]⁺was investigated in ACN both at room temper-

ature and at -45 °C. In both cases it shows four one-electron

reduction processes and a one-electron oxidation. The first

two reductions are completely reversible whereas the suc-

cessive two are affected by chemical follow-up reactions.

With increase of the sweep rate, the extent of the chemical

irreversibility decreases, even though it still remains at 100

V/s. As a consequence of the chemical irreversibility of the

third process, an anodic peak appears on the reverse scan,

at about -1.9 V (Figure 6). The chemical process associated

with the third reduction probably involves protonation, either

by the solvent or the electrolyte, of one of the tetrazolate

nitrogens, whose basicity is increased when the other two

ligands are reduced.³⁹

Figure 5. Cyclic voltammetric curves of uncoordinated ligands: (a)

4-TBNH, 1 mM in a 0.05 M TEATFB/DMF solution, working electrode

Pt disk, diameter 125 µm, T ) 24 °C, scan rate 1 V/s; (b) BTB 1 mM in

a 0.05 M TEATFB/DMF solution, working electrode Pt disk, diameter 125

µm, T ) 24 °C, scan rate 10 V/s; (c) tpy 2 mM in a 0.07 M TEATFB/

DMF solution, working electrode Pt disk, diameter 125 µm, T ) -56 °C,

scan rate 100 V/s.

Table 5. Half-Wave Potentials E_1/2(vs SCE) of the Uncoordinated

Ligands and Mono- and Dinuclear Ruthenium Complexes

oxidn E_1/2/V

redn E_1/2/V

species

1

2

I

II

III

IV

V

tpy^a

-1.98 -2.50

4-TBNH^a,b

BTB^a

-0.87 -2.24

-2.03^c-2.85 -3.05

-1.42 -1.71 -2.19 -2.34

[Ru(4-TBN)]^{+ b}

1.02

[Ru(4-TBN)Ru]^{3+ b}1.02 1.32 -1.28 -1.63 -1.96 -2.19 -2.36

-1.40 -1.70

1.01 1.03 -1.41 -1.71 -2.19 -2.45

-1.45 -1.74 -2.24 -2.52

-2.47

[Ru(BTB)Ru]^{2+ b}

^aTEATFB/DMF solution. ^bTBAH/ACN solution. ^cCathodic peak po-

tential.

migration onto the nitrogens of the tetrazole ring, while the

second reduction is Nernstian. The standard potentials of the

processes are collected in Table 5, together with those of

the other species.

BTB. The BTBH₂ligand has been investigated as the

deprotonated species (i.e. the BTB species; see Chart 1) in

DMF at room and low temperature. It shows three reduc-

(36) Saji, T.; Aoyagui, S. J. Electroanal. Chem. 1975, 58, 401.

(37) Marcaccio, M.; Paolucci, F.; Paradisi, C.; Carano, M.; Roffia, S.;

Fontanesi, C.; Yellowlees, L. J.; Serroni, S.; Campagna, S.; Balzani,

V. J. Electroanal. Chem. 2002, 532, 99 and references therein.

(38) Krejcˇik, M.; Vlcˇek, A. A. J. Electroanal. Chem. 1991, 313, 243.

(39) Further investigations concerning the nature and the mechanism of

such reaction are currently being undertaken.

704 Inorganic Chemistry, Vol. 45, No. 2, 2006

Ruthenium(II) Polypyridine Complexes

Figure 7. Cyclic voltammetric curve of 1 mM of dinuclear complex [Ru-

(4-TBN)Ru]³⁺in 0.05 M TBAH/ACN solution: working electrode Pt disk;

diameter 125 mm; T ) 25 °C; scan rate ) 1 V/s.

Figure 6. Cyclic voltammetric curve for 1 mM mononuclear complex

[Ru(4-TBN)]⁺in a 0.06 M TBAH/ACN solution: working electrode Pt

disk; diameter 125 µm; T ) 25 °C; scan rate ) 1 V/s.

The oxidation process appears chemically and electro-

chemically reversible at 1 V/s. As the scan rate is increased,

the process reveals some degree of electrochemical irrevers-

ibility. The simulation of the cyclic voltammetric curve,

comprising the oxidation and the first reduction wave,

confirms that the oxidation is not a fully Nernstian process.

A satisfactory agreement of the simulated curve with the

experimental one is obtained when the heterogeneous

constant k_hfor the oxidation process is 4 × 10^-2cm/s and

the electron-transfer coefficient R of the Butler-Volmer

equation is 0.34. By contrast, the reduction process is

satisfactorily simulated as a fast (diffusion-controlled) elec-

tron transfer. This can be understood on the basis of quantum

molecular orbital calculations, carried out at DFT level (see

Supporting Information), where it is shown that the HOMO

spans over the metal centers and part of the bridging ligand,

in particular on the tetrazolate ring (see the corresponding

dinuclear species, Figure 8a). The sluggishness of the electron

transfer is therefore most probably associated with molecular

rearrangement of the tetrazolate ligand, following the oxida-

tion process. However, the reductions are centered onto the

polypyridine ligands (vide infra), as also evidenced by the

calculations, and these are intrinsically fast processes.

[Ru(4-TBN)Ru]³⁺. On the basis of the behavior of the

mononuclear species, which is one of the two constituent

moieties of the complex [Ru(4-TBN)Ru]³⁺, it is possible to

rationalize the electrochemistry of the dinuclear species,

whose cyclic voltammetric curve is shown in Figure 7.

The bridging ligand can be considered as electronically

asymmetric because the two coordinating nitrogens are

chemically different: one belongs to the tetrazolyl ring and

bears a partial negative charge while the other can be

considered to be a nitrile-N for its coordinating properties.

The voltammetric curve shows five reduction peaks and

two oxidation ones. Concerning the oxidations, the two peaks

are both reversible one-electron processes, centered on the

two Ru(II) centers. The sizable separation between the

oxidations suggests a quite strong metal-metal bridge-

mediated interaction, but it could also be ascribed to the

electronically asymmetric coordinating ends of the bridging

ligands, giving rise to two nonequivalent ruthenium centers

Figure 8. Molecular orbital surfaces of [Ru(4-TBN)Ru]³⁺showing the

localization of relevant orbitals involved in the redox processes: (a) HOMO

(first oxidation) mainly centered on the ruthenium coordinated to tetrazolate;

(b) LUMO (first reduction) centered on the tpy; (c) third unoccupied MO

(third reduction process) centered on bpy ligand.

with different oxidation potentials. Probably this potential

separation arises from an interplay of these two effects, with

the second more important. A comparison of the E_1/2

potentials of the first oxidation of this dinuclear complex

with that of the oxidation of the mononuclear precursor

species [Ru(4-TBN)]⁺shows that the processes happen at

the same value. Therefore the first oxidation of the dinuclear

is centered on the ruthenium bound to tetrazolate moiety of

the bridging ligand. A DFT calculation also supports this

assignment of the oxidation processes to the HOMO mainly

centered onto the ruthenium coordinated to tetrazole (Figure

8a) and the second highest occupied MO onto the second

metal center, coordinated to the nitrile end of the bridge.

[Ru(BTB)Ru]²⁺. The voltammetric behavior of the di-

nuclear complex carrying the symmetric BTB bridging ligand

is shown in Figure 9, and it consists of five voltammetric

peaks: one in the oxidation region and the remaining in

reduction. All the peaks are two-electron processes, as

evidenced by chronoamperometric measurements. The elec-

Inorganic Chemistry, Vol. 45, No. 2, 2006 705

Stagni et al.

set of peaks (i.e. between the second and third one), which

reflects the energy of electron pairing into polypyridyl

ligands. Thus the reduction centered onto 4-TBN is not

observed since it is outside the negative limit of the useful

potential window. On the basis of the assignments of

reductions for [Ru(4-TBN)]⁺, we can obtain those for [Ru-

(4-TBN)Ru]³⁺. The processes within the first two peaks are

localized onto the two tpy’s and the two bpy’s. As expected,

molecular orbital calculations indicated that the first reduction

is localized on the tpy bound to ruthenium coordinated to

the nitrile end of the 4-TBN bridging ligand (Figure 8b).

Again, the first bpy to undergo reduction (i.e. the third one)

is that coordinated to the ruthenium-nitrile moiety, as shown

in Figure 8c by the prevalent localization of the third

unoccupied MO. The fifth reduction (i.e. the third peak) has

a potential which does not match any process in the

corresponding mononuclear species, and then it can be

confidently attributed to the bridging ligand 4-TBN. The

coordination to two metal centers shifts the unobservable

process in the mononuclear to one occurring at less negative

potentials, falling within the useful potential window. The

process in the fourth peaks and the two in the fifth peak can

be attributed as the second reduction of three out of four

polypyridyl ligands (tpy and bpy), on the basis of the

comparison of the E_1/2values in Table 5.

Figure 9. Cyclic voltammetric curve of 1 mM of dinuclear complex [Ru-

(BTB)Ru]²⁺in 0.05 M TBAH/ACN solution: working electrode Pt; T )

25 °C; scan rate ) 1 V/s.

trochemical behavior is very similar to that of the mono-

nuclear [Ru(4TBN)]⁺species which is, in fact, the building

block of the dinuclear complex [Ru(BTB)Ru]²⁺. The cyclic

voltammetric curve shown in Figure 9 suggests that the first

two reduction peaks are strongly affected by adsorption or

precipitation phenomena. Moreover, the processes in the last

two peaks are partially affected by chemical follow-up

reactions.

Finally, the localization of the reductions for the [Ru-

(BTB)Ru]²⁺is quite straightforward comparing the CV

curves and the potentials of processes. The first pair of two-

electron peaks represents the first reduction of the two tpy’s

and two bpy ligands. The last two peaks (four electrons) are

the electron couplings onto the same redox orbital centered

on each polypyridine.

UV-Vis-NIR Spectroelectrochemistry of Dinuclear

Complexes. The UV-vis-NIR spectroelectrochemical in-

vestigation of the two dinuclear complexes was carried out

at room and low temperature (-32 °C) in ACN and DMF

for the oxidation and the reduction processes respectively,

under strictly aprotic conditions as for the voltammetric

study. The investigation only of the oxidation processes will

be discussed, since it is related to the evaluation of the

intramolecular metal-metal communication. The reductions

are mainly centered on the ligands, and the results merely

confirmed the assignments of the various processes previ-

ously made on the basis of electronic and electrochemical

arguments.

[Ru(4-TBN)Ru]³⁺. The UV-vis-NIR spectrum of the

complex [Ru(4-TBN)Ru]³⁺shows two main regions: the

MLCT transitions in the visible, centered at about 23 000

cm^-1, and the rather intense π-π* intraligand absorptions

in the UV. As discussed above, the species undergoes two

well-separated one-electron oxidations, and upon performing

the first oxidation process, we observe spectral changes

consisting of a decrease in the intensity of MLCT bands and

at the same time the appearance of new transitions in the

NIR and UV regions together with a modification of those

attributed to π-π* intraligand transitions. As shown in

Figure 10a, the band in the NIR around 12 000 cm^-1is rather

broad and weak. This is clearer in the inset of Figure 10a,

The electron-transfer processes in each of the voltammetric

peaks are very close each other, and the evaluation of the

E_1/2values was carried out by digital simulation of the

voltammetric curves. Accordingly, the two processes com-

prised in each peak have a separation in the range 30-60

mV (see Table 5), which is slightly larger than the separation

due to the statistical factor for systems with very weakly

interacting redox centers.⁴⁰Thus, there is little delocalization

across the BTB bridge, suggesting the same holds true for

the 4-TBN-bridged species, as also indicated by DFT

calculations and spectroelectrochemical results.

To understand the main localization of the reduction

processes for the mono- and dinuclear complexes, the

behavior of the uncoordinated ligands must be taken into

account, together with a comparison of the E_1/2values of

the corresponding processes for related complexes and results

of the quantum molecular calculations. The reduction

processes of the mononuclear species [Ru(4-TBN)]⁺can be

assigned as follows: The first is attributed to tpy, since it is

slightly easier to reduce than bpy, and hence, the subsequent

reduction is centered onto bpy. The third process might be

reasonably assigned to 4-TBN, and the last represents the

second reduction (electron pairing into the same redox

orbital) of the tpy. However, if the voltammetric curve (see

also the E_1/2values in Table 5) of the mononuclear is

compared with those of the dinuclear species [Ru(4-TBN)-

Ru]³⁺and [Ru(BTB)Ru]²⁺, the last two processes of the

mononuclear should be assigned to the second reduction onto

tpy and bpy, respectively. Such an attribution is also

supported by the separation between the first and the second

(40) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J. Am. Chem.

Soc. 1978, 100, 4248.

706 Inorganic Chemistry, Vol. 45, No. 2, 2006

Ruthenium(II) Polypyridine Complexes

Figure 11. UV-vis-NIR spectroelectrochemistry of a 0.6 mM [Ru(BTB)-

Ru]²⁺in 0.08 M TBAH/ACN solution: (a) spectra of the pristine compound

(blue trace), one-electron-oxidized species (green trace), and doubly oxidized

species (red trace) with inset showing the difference spectra in the NIR

region of the one- and two-electron-oxidized species (with same colors);

(b) Gaussian deconvolution of the difference spectrum of one-electron-

oxidized product as in the inset of (a).

Figure 10. UV-vis-NIR spectroelectrochemistry of a 0.6 mM [Ru(4-

TBN)Ru]³⁺in 0.08 M TBAH/ACN solution: (a) spectra of the pristine

compound (blue trace), one-electron-oxidized species (green trace), and

doubly oxidized complex (red trace) with inset showing the difference

spectra (spectrum of the pristine complex subtracted) in the NIR region of

the one- and two-electron-oxidized species; (b) Gaussian deconvolution of

the difference spectrum of one-electron-oxidized product as in the inset of

(a).

Again, the Gaussian deconvolution of the difference spectrum

for the first oxidation product (Figure 11b) gives three

components, only one of which (9200 cm^-1, i.e., λ ) 1090

nm) is rather broad and disappears upon the subsequent

oxidation, while the other two increase. This is a clear

indication that this weak band is an IVCT band. The

intensities of the deconvoluted IVCT bands of the dinuclear

complexes [Ru(4-TBN)Ru]³⁺and [Ru(BTB)Ru]²⁺are 1090

M^-1and 935 M^-1cm^-1, respectively. According to the Hush

model^7c,41the magnitude of the metal-metal coupling

energies is 500 and 313 cm^-1, respectively. The separation

between the metal centers, for the estimation of electronic

coupling, was obtained from the optimized molecular

structures of the species at the DFT level. The energy values

so obtained can be considered as a lower limit, since the

redox centers involved are not strongly localized on the metal

atoms but partially involve the tetrazole rings of the bridging

ligands, as shown by molecular orbital results above;

therefore, the appropriate dipole distance in calculating the

IVCT oscillator strength is shorter, which results in larger

electronic metal-metal coupling.⁴¹

where the difference spectra of the singly and doubly

oxidized species are shown. Upon the second oxidation, the

MLCT band disappears completely, the spectral evolution

in the UV region continues, and, most importantly, the weak

and broad NIR band collapses. This is a clear indication that

this band is an intervalence charge transfer band (IVCT),

only observed in the mixed-valence species, which grows

with the oxidation of one of the two metal centers and

disappears completely upon the removal of the second

electron from the dinuclear species. In Figure 10b the

difference spectrum of the first oxidation product (green

trace) has been deconvoluted into three Gaussian components

of which the broadest one centered at λ ) 800 nm (12 550

cm^-1), which disappears upon the second oxidation, is the

IVCT band.

[Ru(BTB)Ru]²⁺. The spectroelectrochemistry of the di-

nuclear complex [Ru(BTB)Ru]²⁺is shown in Figure 11, and

the behavior is very close to that described above for the

other dinuclear species. During the two one-electron oxida-

tion processes the MLCT absorptions gradually decrease and

eventually disappear. The π-π* intraligand bands move to

higher energy, and some transitions develop in the NIR

region (Figure 11a). Although the two oxidation processes

for this complex are very close to each other, careful control

of the oxidation potential during the spectroelectrochemical

investigation allowed us to obtain the mixed-valence species.

Electrogenerated Chemiluminescence. The ECL spectra

of [Ru(BTB)Ru]²⁺and [Ru(4-TBN)Ru]³⁺obtained by al-

ternating generation of the one electron oxidized and reduced

(41) Crutchley, R. J. AdV. Inorg. Chem. 1994, 41, 273 and references

therein.

Inorganic Chemistry, Vol. 45, No. 2, 2006 707

Stagni et al.

energy (about 30 nm for [Ru(BTB)Ru]²⁺). This difference

is probably due to the lower resolution in the ECL instrument

where the spectra were collected. In addition, the observed

ECL emission band shows significant emission in the longer

wavelength region as compared to the fluorescence spectrum.

Such a lower energy emission, as previously observed for

other systems,⁴²might be due to the formation of side

products generated during the annihilation. At the same time

the fact that no change in the absorption and fluorescence

spectra was observed before and after extensive potential

cycling demonstrates that this is not the case. The longer

wavelength component in the ECL could also be due to the

formation of excited-state aggregates at the concentration

used to obtain the spectrum, although this also appears

unlikely. ECL yields were determined against a standard (the

ECL system containing 1 mM of [Ru(bpy)₃]²⁺in 0.1 M

TBAH/ACN with φ_ECL= 0.05)⁴³by comparison of the

measured integrated photon intensities, taking into account

the differences in the electric charges passed through the

solution studied.

Figure 12. ECL spectra of the reference compound [Ru(bpy)₃]²⁺(black

trace), [Ru(BTB)Ru]²⁺(blue trace), and [Ru(4-TBN)Ru]²⁺(red traces). The

dashed red line for [Ru(4-TBN)Ru]²⁺is the spectrum obtained by

annihilation of one-electron-oxidized and -reduced forms; the full red line

is obtained for the doubly oxidized and reduced forms of complex. All

ECL spectra were collected for 1 mM ACN solutions and TBAH as

supporting electrolyte, 25 °C, and accumulation time 4 min.

forms in ACN containing 0.1 M of TBAH as supporting

electrolyte are shown in Figure 12. Emission was not seen

by eye indicating that either the ECL yield is not very high

or it falls in the red region. However, the results were

completely reproducible over a wide number of experiments

performed with the same solution. This would indicate that

the systems studied are quite stable and therefore promising

for future use in devices. On the basis of the absorption and

emission wavelengths of the compound, the energy needed

to generate the first singlet-excited state is about 1.77 eV

for [Ru(BTB)Ru]²⁺and 1.86 eV for [Ru(4-TBN)Ru]³⁺.

These energy values are therefore those that must be

generated in the electrochemical annihilation reaction of the

oxidized and reduced species (radical ions) to generate the

excited state and ECL emission. In a typical ECL experiment,

the radical anion reacts with the radical cation; however, in

our case, it was also possible to study the effect of the

generation of doubly reduced and doubly oxidized species

for [Ru(BTB)Ru]²⁺and [Ru(4-TBN)Ru]³⁺. The best result

for compound [Ru(BTB)Ru]²⁺, in terms of ECL intensity,

was obtained when the potential was stepped between 1.4

and -1.6 V (vs silver wire quasi reference electrode). In

this potential range the first two-electron reduction and the

first two-electron oxidation were included. No difference in

the ECL signal was observed when extending the potential

window further while a lower intensity was obtained for

1.1/-1.8 and 1.1/-1.6 potential ranges, with the last interval

leading to the smallest signal of all. Compound [Ru(4-TBN)-

Ru]³⁺showed very interesting results both considering the

one-electron oxidation/reduction and the doubly oxidized/

reduced forms. The ECL spectra of [Ru(BTB)Ru]²⁺and [Ru-

(4-TBN)Ru]³⁺are characterized by broad emission bands

between 640 and 850 nm and between 600 and 860 nm,

respectively. The maxima for both compounds were at 710

nm for [Ru(BTB)Ru]²⁺and 680 nm for [Ru(4-TBN)Ru]³⁺.

When compared to the fluorescence, the ECL spectra show

some differences, being slightly broader in shape with the

λ_maxat about the same value or eventually shifted to lower

The ECL efficiencies of [Ru(BTB)Ru]²⁺and [Ru(4-TBN)-

2+

Ru]³⁺are about 40% and 150% of that of Ru(bpy)₃

,

respectively. The 4-TBN dinuclear complex shows an

efficiency much higher than that of related ruthenium

complexes containing tpy as ligand, which are known to have

a low luminescence efficiency at room temperature.^1c,8c,d

The ECL efficiency (in photons emitted/electron trans-

ferred between reduced and oxidized forms of the parent

molecule) is related directly to the yield of the excited state

on annihilation (φ_es) and to the emission quantum yield (φ_o)

of a given emitter (φ_ecl) φ_esφ_o). The result implies that the

yield for the formation of the excited states is close to 100%.

This is promising for future development of these systems.

The enthalpy for the annihilation reaction is sufficient to

generate the emitting state. For the process ∆H°_ann

)

∆G°_ann+ T∆S°, this value is based on the half-wave

potentials of the first oxidation and reduction wave in the

(ox/red)

cyclic voltammogram (∆E_1/2

≈ 2.5 V for both com-

pounds), and if we consider an entropy effect = 0.1 eV, we

obtain a value of 2.4 eV which is in both cases (compound

[Ru(BTB)Ru]²⁺and [Ru(4-TBN)Ru]³⁺) is higher than the

minimum energy required for the population of the excited

state from which the emission takes place.

Conclusions

Tetrazole-based compounds are a kind of a “missing link

in the family tree” represented by five-membered N-

heterocyclic compounds commonly incorporated in polypyr-

idine complexes of Ru(II). The introduction of 5-aryltetra-

zolate ligands leads to a class of complexes that are

interesting from different viewpoints. First, the planar

arrangement which is adopted by the tetrazolate ligand

4-TBN in the corresponding mononuclear complex [Ru(4-

TBN)]⁺might indeed represent a promising feature for the

(42) Lai, Y. R.; Kong, X.; Jenekhe. S. A.; Bard, A. J. J. Am. Chem. Soc.

2003, 125, 12631.

(43) Rubinstein, L.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 6641.

708 Inorganic Chemistry, Vol. 45, No. 2, 2006

Ruthenium(II) Polypyridine Complexes

use of this compound as a metallointercalator for DNA or

related nucleotides. Note that the geometry of the ligand, as

well as its electronic properties, can be modified either

permanently or in a reversible way. Furthermore, NMR

evidence indicates that the main structural and electronic

features of the 5-aryl-substituted ligands are also maintained

in the corresponding dinuclear complex [Ru(4-TBN)Ru]³⁺.

The spectral, photophysical, and redox properties of the

complexes have been investigated. In the electrochemistry

of those species, the reduction processes are mainly centered

on the ligands and the sites of the various processes have

been identified on the basis of the behavior of the uncoor-

dinated ligands and a comparison between the species herein

studied. The oxidations are mainly metal-centered, and the

electrochemical and UV-vis-NIR spectroelectrochemical

data indicate that these tetrazolate bridging ligands mediate

the electronic interactions between the two ruthenium centers.

Finally, the photophysical properties and especially the very

efficient ECL of the complexes investigated are very

encouraging for a future development of these species in

electrochemiluminescent devices.

Acknowledgment. We thank Dr. Simone Zanarini for

helping us with some ECL measurements. We also thank

Professor Alberto Arcioni for helpful discussions. The

financial support from MIUR (PRIN 2004035330 and

PRINs“New strategies for the control of reactions: interac-

tions of molecular fragments with metallic sites in uncon-

ventional species”), University of Bologna, the National

Science Foundation (Grants CHE 0304925 and 0202136),

and BioVeris Corp. is gratefully acknowledged.

Supporting Information Available: An experimental section,

reporting NMR data and spectra, UV-vis absorption spectra of

mononuclear complexes, and computational details, and X-ray

crystallographic files in CIF format for the structure determinations

of complexes [Ru(4-TBN)]⁺, [Ru(4-(Me)TBN)]²⁺, and [Ru(4-(H)-

TBN)]²⁺. This material is available free of charge via the Internet

at http://pubs.acs.org.

IC0514905

Inorganic Chemistry, Vol. 45, No. 2, 2006 709

Article Doi

A new family of ruthenium(II) polypyridine complexes bearing 5-aryltetrazolate ligands as systems for electrochemiluminescent devices

DOI: 10.1021/ic0514905

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Article abstract of DOI:10.1021/ic0514905

Full text of DOI:10.1021/ic0514905

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