Anthracene-bridged binuclear ruthenium complexes: Electrochemical and spectroscopic evidence of electronic communication through the Π system

Article abstract of DOI:10.1021/ic900100w

Six dinuclear cyclometalated ruthenium complexes, 1-6, based on diphenylanthracene (DPA) and anthracene (AN) as bridging ligands have been synthesized and fully characterized electrochemically and spectroscopically. The anodic electrochemistry of the homobinuclear ruthenium complexes, 1 -6, has been examined in three different nonaqueous solvents (ACN, DMF, and CH ₂CI₂). The ability of the anthracene derivatives to transmit electronic effects between the two redox units has been demonstrated by the observed splitting of the voltammetric signals ascribed to the metal centers. The electronic communication has also been evidenced by the presence of intervalence charge transfer transition bands in the near-infrared region of the spectrum due to an intramolecular electron transfer process mediated by the bridge when the mixed valence species (Ru^II/Ru^III) are electrochemically generated. Cyclic voltammetric measurements have been carried out under different conditions of solvent and supporting electrolyte. Differences in ?,E°′ the potential separation of the formal potentials of the metal-based anodic processes, have been observed and found to depend on the medium employed. These differences have been ascribed to different degrees of ion pairing. Such effects can be, in turn, modulated as a function of not only the polarity and donor strength of the solvent but also of the coordinating capacity of the anion employed as a supporting electrolyte.

Full text of DOI:10.1021/ic900100w

796 Inorg. Chem. 2010, 49, 796–804
DOI: 10.1021/ic900100w
Anthracene-Bridged Binuclear Ruthenium Complexes: Electrochemical and
Spectroscopic Evidence of Electronic Communication Through the π System
ꢀ
ꢁ
~
Neus Vila, Yu-Wu Zhong, Jay C. Henderson, and Hector D. Abruna*
Baker Laboratory, Department of Chemistry and Chemical Biology, Cornell University, Ithaca,
New York 14853-1301
Received December 23, 2008
Six dinuclear cyclometalated ruthenium complexes, 1-6, based on diphenylanthracene (DPA) and anthracene (AN)
as bridging ligands have been synthesized and fully characterized electrochemically and spectroscopically. The anodic
electrochemistry of the homobinuclear ruthenium complexes, 1-6, has been examined in three different nonaqueous
solvents (ACN, DMF, and CH2Cl2). The ability of the anthracene derivatives to transmit electronic effects between the
two redox units has been demonstrated by the observed splitting of the voltammetric signals ascribed to the metal
centers. The electronic communication has also been evidenced by the presence of intervalence charge transfer
transition bands in the near-infrared region of the spectrum due to an intramolecular electron transfer process
mediated by the bridge when the mixed valence species (RuII/RuIII) are electrochemically generated. Cyclic
voltammetric measurements have been carried out under different conditions of solvent and supporting electrolyte.
Differences in ΔE°0, the potential separation of the formal potentials of the metal-based anodic processes, have been
observed and found to depend on the medium employed. These differences have been ascribed to different degrees of
ion pairing. Such effects can be, in turn, modulated as a function of not only the polarity and donor strength of the
solvent but also of the coordinating capacity of the anion employed as a supporting electrolyte.
Introduction
complexes toward the medium (solvent/electrolyte),2-24 an
experimental factor which has been carefully discussed in
previous reports by several authors, has been demonstrated
in a number of systems. Furthermore, although the compro-
portionation constant, Kc, determined from electrochemi-
cal data, is indicative of the strength of the metal-metal
Long-range communication, including electron and en-
ergy transfer between remote metal centers in multimetallic
assemblies, is one of the most fundamental aspects of infor-
mation conveying in molecular systems and of great potential
use in molecular electronics. Mixed-valence compounds,
particularly species containing metallic centers in different
oxidation states, have been widely employed to study intra-
molecular electron transfer (ET) processes between donor
and acceptor sites. Robin and Day1 classified mixed-valence
systems, including molecules, metal complexes, and solid-
state materials, as a function of the strength of the electronic
interaction (coupling) between donor and acceptor sites.
Such classification takes into account three situations for
valence exchange. In this sense, species included in class I
essentially display zero metal-metal interaction. Class II
includes species for which a nonzero barrier for valence
exchange might exist. The sensitivity of mixed-valence metal
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pubs.acs.org/IC
Published on Web 12/22/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 797
interaction in a way similar to the electronic interaction
energy and intensity of the intervalence charge transfer
(IVCT), some reports have demonstrated that these two
manifestations of metal-metal interactions can differ in mag-
nitude depending on the nature of the bridging ligands.3-5,18-39
Mixed-valence species categorized as class III are based on
electron delocalized systems with possibly fractional oxida-
tion states. Electron transfer between redox sites occurs in the
absence of an activation barrier. A new class of species, class
II-III or a borderline situation, was introduced in 1998 by
Meyer and Chen3 to address the behavior of mixed-valence
ruthenium and osmium complexes.
It has been previously well-documented that mixed-va-
lence metal complexes, bridged by conjugated organic li-
gands and capped with redox-active organometallic termini,
allow facile intramolecular electron transfer to occur along
the molecular backbone. The study of ET through mixed-
valence compounds is a very active field of research. A large
number of compounds have been synthesized, allowing for
the study of various factors such as the distance between the
redox centers, solvent effects, and the nature of the bridging
ligand.1,40-46 Linear organometallic compounds with ex-
tended π-conjugated backbones are attractive potential can-
didates as molecular wires that might operate as connectors
allowing electron flow to occur between different elements of
nanoscale electronic devices by virtue of the possible charge
delocalization along the conjugated molecular backbone.
Electrochemical measurements and spectroscopic studies in
the near-infrared region are often employed to evaluate the
extent of electronic communication between remote electro-
active organometallic groups and to gain insight into metal-to-
metal charge transfer processes in mixed-valence species with
extended π delocalization. Important contributions to this
issue have been provided by Keene, reporting differences in
the physical properties of diastereoisomers of several dinuclear
complexes,6,7 which could explain some particular solvent
effects on the IVCT bands observed.18,45 Keene has also
focused on different factors such as structural influences or
the effect of anions on the IVCT bands, providing new insights
into crucial factors that can affect intermetal communication.
Cyclometalated transition metal complexes have found
widespread interest as species with promising properties in
various fields owing to the strong σ-donating ability of the
cyclometalating ligand. Some examples of binuclear ruthe-
nium and osmium complexes linked by cyclometalating
ligands47,48 have been previously reported and compared
to the analogous species built with bis(terpyridine)-type
ligands.28,29 In most of the species investigated, metal-metal
couplings were found to be much stronger for the cyclome-
talated derivatives, and this has been ascribed to better
overlap of the molecular orbitals for these complexes. Typi-
cally, the coordinating ligand forms a bond to the metal
center, and then, intramolecular C-H activation takes place
to yield a five-membered chelate ring.47-49
It is well-known that E°0 shifts are sensitive to both solvent
and supporting electrolyte effects,50-54 and there have been
recent systematic studies illustrating that the effects of changes
in the supporting electrolyte anion on the ΔE°0 values of
oxidations involving cationic products can be large.3,12,55,56
In this manuscript, we report on the spectral and electro-
chemical characterization of six binuclear ruthenium com-
plexes based on anthracene derivatives as bridging ligands
(see Chart 1). The influence of the medium on the compro-
portionation constants determined from electrochemical
data is demonstrated, suggesting that the metal-to-metal
distance and the possibility of direct d-d overlap in the case
of small bridging ligands are not the only factors contributing
to the magnitude of Kc in binuclear complexes, as recently
pointed out by other authors. Our particular interest in these
compounds arises from their potential application as materi-
als for molecular electronics and data storage.
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798 Inorganic Chemistry, Vol. 49, No. 3, 2010
Chart 1. Binuclear Complexes 1-6
NMR (300 MHz) spectra were recorded on a Varian Mercury
300 MHz FT-NMR spectrometer and referenced relative to
residual proton resonances.
1,5-dibromo-9,10-diphenylanthracene, 2,6-dibromo-9,10-an-
thracene, 2,7-dibromo-9,10-anthracene, and 1,5-dibromo-
9,10-anthracene were prepared following literature proce-
dures.55-59
The preparation of the binuclear ruthenium complexes, 1-6,
consisted of the separate synthesis of the anthracene and
diphenylanthracene precursors and the mononuclear complex,
[Ru(bpy)2(Br-phpy)](PF6), as described below.
Instrumentation. Cyclic voltammograms were performed
using either a BAS Epsilon potentiostat or a PAR 273A
potentiostat. Three-compartment electrochemical cells were
employed, and all measurements were carried out at room
temperature under a nitrogen atmosphere. A glassy carbon
electrode (3 mm diameter) was employed as a working electrode
and a platinum wire as a counter electrode. All potentials are
referenced to a Ag/AgCl reference electrode.
Synthesis of 2,7-Bis(ethynyl)-9,10-diphenylanthracene. 2,7-
Dibromo-9,10-diphenylanthracene was converted into 2,7-bis-
(trimethylsilyl)-9,10-diphenylanthracene by following a litera-
ture procedure.55-59 2,7-Bis(ethynyl)-9,10-diphenylanthracene
was synthesized from 2,7-dibromo-9,10-anthraquinone, which
was obtained via Sandmeyer reaction from the corresponding
2,7-diamino-9,10-anthraquinone afforded by following a pro-
cedure described in the literature.60 To prepare 2,7-bis(ethynyl)-
9,10-diphenylanthracene derivatives, dibromo-9,10-anthra-
quinones were first reacted with phenyl magnesium bromide
in anhydrous tetrahydrofuran at room temperature and subse-
quently treated with Zn powder in aqueous acetic acid solu-
tion at 120 °C. On the other hand, to afford the simplest
anthracene cores, 9,10-anthraquinones were reduced with so-
dium borohydride, yielding 9,10-dihydroxy-9,10-dihydroan-
thracenes in methanol that were subsequently converted to the
UV/vis/NIR spectra were performed on a Shimazdu UV-
3101PC spectrophotometer.
Materials. All solvents and chemicals for synthesis were
commercially available, of reagent-grade quality, and used
without any further purification, except as noted below.
Experiments involving air- or water-sensitive reagents were
carried out using standard Schlenck line techniques under a
nitrogen atmosphere.
2,6-Diamino-anthraquinone (97%þ), anthrone (97%þ),
copper(II) bromide (99%þ), tetra-n-butyl ammonium hexa-
fluorophosphate (n-Bu4NPF6; 98%), and tetra-n-butyl-ammo-
nium chloride (n-Bu4NCl) were purchased from Aldrich
Chemical Co. and used as received. Tetra-n-butyl-ammonium
tetrakis(pentafluorophenyl)borate (n-Bu4NB(C6F5)4) was pre-
pared as described by Geiger and co-workers.53,54 1,5-Diamino-
anthraquinone (85%þ) was from Aldrich Chemical Co.
2,7-Dinitro-9,10-anthraquinone, 2,7-diamino-9,10-anthraqui-
none, 2,7-dibromo-9,10-anthraquinone, 2,6-dibromo-9,10-an-
thraquinone, 1,5-dibromo-9,10-anthraquinone, 2,6-dibromo-9,
10-diphenylanthracene, 2,7-dibromo-9,10-diphenylanthracene,
(57) Criswell, T. R; Klanderman, B. H. J. Org. Chem. 1974, 39, 770.
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Reibenspies, J. H.; Rillema, P. Inorg. Chem. 2004, 6383.

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 799
corresponding anthrones by dehydratation in aqueous hydro-
chloric acid. Reduction of the anthrones was performed by
reaction with sodium borohydride, yielding the dibromo sub-
stituted anthracenes. A Sonogashira coupling reaction between
2,7-dibromo-9,10-diphenylanthracene and ethynyltrimethylsi-
lane followed by deprotection of the trimethylsilyl groups by
refluxing the mixture for 12 h in the presence of 2 equiv of
potassium fluoride in methanol afforded the 2,7-bis(ethynyl)-
9,10-diphenylanthracene derivative. In turn, 2,6- and 1,5-bis-
(ethynyl)-9,10-diphenylanthracene derivatives were synthesized
by following the same methodology from 2,6- and 1,5-diamino-
9,10-anthraquinones, which were from commercial sources.
2,7-Bis(ethynyl)-9,10-diphenylanthracene. 1HNMR(300MHz,
CDCl3): δ 7.65 (d, 2 H), 7.54 (d, 2 H), 7.45 (dd, 2 H), 7.38 (ddd,
4 H), 7.15 (m, 6 H), 2.95 (s, 2 H).
0.01 mmol), CuI (10 mg, 0.05 mmol), triethylamine (1 mL), and
0.09 mmol of the bis(ethynyl)-9,10-anthracene derivative in
5 mL of DMF were heated under argon for 48 h. Afterward,
the solvent was removed in vacuo. To the red residue was added
NH4PF6 (100 mg) in acetonitrile (5 mL). The solution was
stirred and evaporated to dryness. The complex was dissolved
-
in CH2Cl2, while the excess PF6 salt remained undissolved.
After filtration, the solvent was removed, and the resulting dark
red solid was purified by column chromatography on silica gel
with 4:6 toluene/acetonitrile as the mobile phase. The complex
was completely desorbed in a concentrated NH4PF6 solution in
acetonitrile. After removal of the solvent, the residue was
redissolved in CH2Cl2, and then the solution was concentrated
and diethyl ether was added to precipitate the complex.
Complex 1. A 25% yield from 2,7-bis(ethynyl)-9,10-dipheny-
2,6-Bis(ethynyl)-9,10-diphenylanthracene. 1HNMR(300MHz,
CDCl3): δ 7.63 (d, 2 H), 7.56 (d, 2 H), 7.53 (dd, 2 H), 7.38 (dd, 4 H),
7.17 (m, 6 H), 2.78 (s, 2 H).
1
lanthracene was acheived. H NMR (300 MHz, d6-DMSO): δ
8.58-8.76 (m, 8 H), 8.05-8.15 (m, 6 H), 7.85-7.97 (m, 10 H),
7.42-7.77 (m, 16 H), 7.23-7.36 (m, 8 H), 6.94-7.13 (m, 6 H),
6.74-6.90 (m, 6 H), 6.30 (d, J = 8.02 Hz, 2 H). 13C NMR (75
MHz, d6-DMSO) δ: 175.4, 158.1, 152.3, 149.5, 148.7, 146.9,
140.2, 138.0, 136.5, 134.3, 133.1, 132.4, 129.8, 129.0, 127.8,
127.3, 124.9, 122.3, 121.6, 115.3, 101.2, 98.79. ES-MS m/z:
1654.9 (M - PF6)þ, 1242.4 (M - PF6 - Ru(bpy)2)þ, 755.1 (M
- 2PF6)2þ. Anal. Calcd for C92H62F12N10P2Ru2: C, 61.40; H,
3.45; N, 7.79. Found: C, 61.62; H, 3.63; N, 7.91.
1,5-Bis(ethynyl)-9,10-diphenylanthracene. 1H NMR (300
MHz, CDCl3): δ 7.75 (d, 2 H), 7.45 (m, 6 H), 7.30 (m, 8 H),
2.88 (s, 2 H).
Synthesis of 2,7-Bis(ethynyl)anthracene. To obtain 2,7-Bis-
(ethynyl)anthracene, 2,7-dibromo-9,10-anthraquinone was first
reduced with sodium borohydride, yielding 2,7-dibromo-9,10-
dihydroxy-9,10-dihydroanthracene in methanol that was con-
verted to the corresponding anthrones by dehydratation in
aqueous hydrochloric acid. Reduction of the anthrone was
performed by reaction with sodium borohydride, yielding the
dibromo substituted anthracene. A similar procedure was em-
ployed to obtain the 2,6- and 1,5-Bis(ethynyl) derivatives from
the appropriate precursors.
Complex 2. A 21% yield from 2,7-bis(ethynyl)anthracene was
achieved. 1H NMR (300 MHz, d6-DMSO): δ 6.32 (d, J = 8.45
Hz, 2 H), 6.75-6.85 (m, 6 H), 7.01-7.05 (m, 6 H), 7.36-7.78 (m,
8 H), 7.84-7.92 (m, 14 H), 7.94-8.15 (m, 10 H), 8.58-8.76 (m, 8
H). 13C NMR (75 MHz, d6-DMSO): δ 176.3, 157.5, 153.3, 149.7,
148.9, 147.3, 139.2, 137.3, 136.5, 134.3, 133.9, 132.1, 130.1,
129.5, 129.0, 124.2, 121.6, 120.8, 118.9, 100.9, 98.99. ES-MS
m/z: 1502.2 [M - (PF6)]þ, 1090.3 [M - PF6 - Ru(bpy)2]þ, 678.9
[M - 2PF6]2þ. Anal. Calcd for C80H54F12N10P2Ru2: C, 58.33;
H, 3.28; N, 8.51. Found: C, 58.22; H, 3.48; N, 8.41.
2,7-Bis(ethynyl)anthracene: 80% yield from 2,7-dibromoan-
thracene. 1H NMR (300 MHz, CDCl3): δ 8.52 (s, 1 H), 8.37 (s, 1
H), 8.23 (d, J = 8.43 Hz, 2 H), 8.05 (d, J = 1.97 Hz, 2 H), 7.47
(dd, J = 8.43 Hz, J = 1.97 Hz, 2 H), 2.98 (s, 2 H).
2,6-Bis(ethynyl)anthracene: 82% yield from 2,6-dibromoan-
thracene. 1H NMR (300 MHz, CDCl3): δ 8.45 (s, 2 H), 8.15 (d,
J = 8.23 Hz, 2 H), 7.99 (d, J = 1.85 Hz, 2 H), 7.77 (dd, J = 8.23
Hz, J = 1.85 Hz, 2 H), 3.08 (s, 2 H).
1,5-Bis(ethynyl)anthracene: 77% yield from 1,5-dibromoan-
thracene. 1H NMR (300 MHz, CDCl3): δ 8.89 (s, 2 H), 8.10 (m, 2
H), 7.67 (m, 4 H), 2.88 (s, 2 H).
Complex 3. A 22% yield from 2,6-bis(ethynyl)-9,10-dipheny-
lanthracene was achieved. H NMR (300 MHz, d6-DMSO): δ
1
8.45-8.76 (m, 8 H), 8.15-8.37 (m, 6 H), 7.85-8.04 (m, 10 H),
7.40-7.73 (m, 16 H), 7.25-7.36 (m, 8 H), 6.74-7.03 (m, 6 H),
6.44-6.70 (m, 6 H), 6.30 (d, J = 8.02 Hz, 2 H). 13C NMR (75
MHz, d6-DMSO): δ 173.6, 157.6, 151.1, 149.7, 148.1, 146.9,
141.2, 137.5, 137.1, 135.9, 134.1, 133.4, 130.8, 129.0, 128.8,
127.9, 125.9, 123.1, 122.6, 114.7, 100.2, 97.9. ES-MS m/z:
1654.3 (M - PF6)þ, 1241.9 (M - PF6 - Ru(bpy)2)þ, 755.5 (M
- 2PF6)2þ. Anal. Calcd for C92H62F12N10P2Ru2: C, 61.40; H,
3.45; N, 7.79. Found: C, 61.572; H, 3.45; N, 8.05.
Preparation of the Mononuclear Brominated Ruthenium Com-
plex, [Ru(bipy)2(p-Brphpy)]PF6. The preparation of the mono-
metallic ruthenium complex was performed from commercial
ruthenium trichloride following a synthetic procedure described
in the literature.60-63 cis-[Ru(bipy)2Cl2] was first prepared by
refluxing a mixture of ruthenium trichloride and bipyridine in
the presence of LiCl in dimethylformamide overnight.60,61 Sub-
sequently, reaction with an excess of 2-phenylpyridine in the
presence of silver(I) tetrafluoroborate afforded [Ru(bipy)2-
(phpy)](BF4).62 An exchange reaction was carried out in a
methanolic solution containing NH4PF6 that afforded [Ru-
(bipy)2(phpy)](PF6) as a red dark crystalline solid. The singly
brominated compound, [Ru(bipy)2(p-Brphpy)]PF6, was ob-
tained upon treatment of the complex with 1.1 equiv of
N-bromosuccinimide in acetonitrile at room temperature, as
previously reported.63
Complex 4. A 27% yield from 2,6-bis(ethynyl)anthracene was
achieved. 1H NMR (300 MHz, d6-DMSO): δ 6.28 (d, J = 8.15
Hz, 2 H), 6.75-6.95 (m, 6 H), 7.11-7.25 (m, 6 H), 7.28-7.65 (m,
8 H), 7.72-7.85 (m, 14 H), 7.99-8.35 (m, 10 H), 8.58-8.76 (m, 8
H). 13C NMR (75 MHz, d6-DMSO): δ 177.2, 156.4, 152.8, 147.7,
146.9, 142.9, 138.9, 136.2, 134.5, 134.3, 132.9, 132.0, 131.1,
130.5, 129.6, 126.2, 122.6, 121.8, 113.7, 100.1, 97.6. ES-MS m/
z: 1502.7 [M - (PF6)]þ, 1089.9 [M - PF6 - Ru(bpy)2]þ, 678.6
[M - 2PF6]2þ. Anal. Calcd for C80H54F12N10P2Ru2: C, 58.33;
H, 3.28; N, 8.51. Found: C, 58.29; H, 3.37; N, 8.75.
Complex 5. A 17% yield from 1,5-bis(ethynyl)-9,10-dipheny-
1
lanthracene was achieved. H NMR (300 MHz, d6-DMSO): δ
Palladium-Catalyzed Cross Coupling Reaction for the Synth-
esis of the Homobimetallic Compounds (1-6). The homobinuc-
lear complexes, 1-6, were afforded following a procedure
slightly modified from the literature for similar binuclear com-
plexes.64 A solution of the monobrominated mononuclear
ruthenium complex (150 mg, 0.19 mmol), Pd(PPh3)4 (10 mg,
8.45-8.76 (m, 8 H), 8.15-8.37 (m, 6 H), 7.85-8.04 (m, 10 H),
7.40-7.73 (m, 16 H), 7.25-7.36 (m, 8 H), 6.74-7.03 (m, 6 H),
6.44-6.70 (m, 6 H), 6.30 (d, J = 8.15 Hz, 2 H). 13C NMR (75
MHz, d6-DMSO): δ 178.4, 160.4, 154.5, 150.5, 149.7, 147.3,
144.5, 136.8, 136.4, 135.1, 133.8, 131.7, 130.8, 129.6, 127.3,
126.7, 124.2, 121.3, 121.4, 117.8, 100.9, 99.4. ES-MS m/z:
1654.2 (M - PF6)þ, 1242.9 (M - PF6 - Ru(bpy)2)þ, 755.9 (M
- 2PF6)2þ. Anal. Calcd for C92H62F12N10P2Ru2: C, 61.40; H,
3.45; N, 7.79. Found: C, 61.27; H, 3.51; N, 7.71.
(61) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 3334.
(62) Constable, E. C.; Holmes, J. M. J. Organomet. Chem. 1986, 301, 203.
(63) Coudret, C.; Fraysse, S.; Launay, J.-P. Chem. Commun. 1998, 663.
(64) Fraysse, S.; Coudret, C.; Launay, J.-P. J. Am. Chem. Soc. 2003, 125
(19), 5880.
Complex 6. A 25% yield from 1,5-bis(ethynyl)anthracene was
achieved. 1H NMR (300 MHz, d6-DMSO): δ 6.32 (d, J = 8.25

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Vila et al.
800 Inorganic Chemistry, Vol. 49, No. 3, 2010
Scheme 1. Synthesis of the Binuclear Ruthenium Complexes through a Sonogashira Cross-Coupling Reaction
Hz, 2 H), 6.65-6.82 (m, 6 H), 7.04-7.15 (m, 6 H), 7.26-7.70 (m,
8 H), 7.84-7.92 (m, 14 H), 7.92-8.15 (m, 10 H), 8.45-8.76 (m, 8
H). 13C NMR (75 MHz, d6-DMSO): δ 178.0, 155.5, 151.3, 149.4,
148.1, 145.2, 138.8, 137.5, 136.2, 134.8, 133.2, 132.4, 130.9,
129.8, 129.0, 125.2, 122.7, 120.1, 114.1, 101.2, 99.1. ES-MS m/
z: 1501.7 [M - (PF6)]þ, 1089.5 [M - PF6 - Ru(bpy)2]þ, 679.2
[M - 2PF6]2þ. Anal. Calcd for C80H54F12N10P2Ru2: C, 58.33;
H, 3.28; N, 8.51. Found: C, 58.06; H, 3.54; N, 8.74.
and diphenylanthracene precursors and the mononuclear
complex, [Ru(bpy)2(Br-phpy)](PF6), by following syn-
thetic procedures described in the literature for similar
compounds.69,70 The brominated cyclometalated mono-
nuclear ruthenium complex, [Ru(bpy)2(Br-phpy)](PF6),
was prepared as previously reported.63 Substituted diami-
noanthraquinones were converted into the dibromo deri-
vatives via Sandmeyer reaction (Scheme 2, step a).57,58
Diphenylanthracene cores were obtained by reaction
of the appropriate dibromoanthraquinones55,58,59 with
phenyl magnesium bromide (Scheme 2, step b) in anhy-
drous tetrahydrofuran at room temperature and subse-
quently treated with Zn powder in an aqueous acetic
acid solution at 120 °C.56 Sonogashira coupling between
the dibromo-9,10-diphenylanthracene derivatives and
(trimethylsilyl)acetylene afforded the corresponding di-
(trimethylsily)-9,10-diphenylanthracenes60,61 (Scheme 2,
step c), which were readily deprotected (Scheme 2, step d)
by stirring the mixture for 2 h in the presence of 2 equiv of
tetrabutylammonium fluoride in tetrahydrofuran to
eventually yield the desired bis(ethynyl)-9,10 diphenylan-
thracene cores.60,61 Anthracene bridging ligands were
prepared by reduction of the anthraquinone cores with
sodium borohydride (Scheme 2, step e) as previously
reported.57,58 Preparation of the bis(ethynyl)anthracene
derivatives was performed by a synthetic methodology
analogous to that employed to afford the bis(ethynyl)-
9,10-diphenylanthracene derivatives described above.
Electrochemistry. Electrochemical studies of the homo-
binuclear compounds, 1-6, were initially performed in
methylene chloride solution containing 0.1 M n-Bu4NPF6
as a supporting electrolyte. All of the homobinuclear
ruthenium complexes investigated displayed similar elec-
trochemical behavior in this medium. Whereas the mono-
nuclear analog [Ru(bipy)2(phpy)](PF6) displays a single
reversible oxidation wave on the anodic scan at E0 =
0.591 V versus Ag/AgCl (inset Figure 1), two-reversible
metal-based one-electron oxidation waves are observed
Results and Discussion
As mentioned earlier, we report on the synthesis and
electrochemical and spectroscopic characterization of six
homobinuclear cyclometalated ruthenium complexes, 1-6
(Chart 1). Significant electronic communication between the
metallic units, through the different anthracene cores, was
evidenced not only by cyclic voltammetry but also by spectro-
scopic measurements in the NIR region when the mixed-
valence species were electrochemically generated.
The ability of anthracene to mediate electronic effects has
been previously reported and frequently justified, in a quali-
tative way, by the possibility of a mesomeric structure.65,66
However, all previous reports have focused on anthracene
derivatives substituted on C9 and C10 since 9,10-diethynylan-
thracene is readily available, either from the 9,10-bis-TMS-
anthracene67 or the 9,10-bis-dimethylcarbinolanthracene.68 In
this paper, we have systematically studied the influence of the
substitution at different positions of the anthracene core, such
as C2-C6, C2-C7, and C1-C5, on the electronic commu-
nication between the redox units (see Chart 1).
Syntheses and Characterization. As illustrated in
Scheme 1, the homobinuclear ruthenium complexes
1-6 were synthesized from the appropriate substituted
bis(ethynyl)anthracene derivative and the brominated
mononuclear ruthenium complex by a palladium-cata-
lyzed cross-coupling reaction.
The preparation of the binuclear ruthenium complexes,
1-6, consisted of the separate synthesis of the anthracene
(65) Taylor, P. N.; Wylie, A. P.; Huuskonen, J.; Anderson, H. L. Angew.
Chem., Int. Ed. 1998, 37, 986.
(66) Piet, J. J.; Taylor, P. N.; Anderson, H. L.; Osuka, A.; Warman, J. M.
J. Am. Chem. Soc. 2000, 122, 1749.
(67) Kobayashi, E.; Jiang, J.; Furukawa, J. Polym. J. 1990, 22, 266.
(68) Thorand, S.; Krause, N. J. Org. Chem. 1998, 63, 8551.
(69) Svorstol, I.; Hoiland, H.; Songstad, J. Acta Chem. Scand. B 1984, 38,
e
885.
(70) Fraysse, S.; Coudret, C.; Launay, J.-P. J. Am. Chem. Soc. 2003, 125
(19), 5880.

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 801
Scheme 2. Overall Synthetic Pathway to Obtain Anthracene and Diphenylanthracene Derivatives
for all of the binuclear ruthenium complexes investigated
(Figures 1 and 2).
The effectiveness of the ET process is related to the
stability of the mixed-valence state and can be discussed
on the basis of the comproportionation constant Kc
(eq 1), which can be determined from electrochemical
measurement of the difference in formal potentials (ΔE )
of the two one-electron oxidation waves and can be
expressed as
In addition, a quasi-reversible anodic process ascribed
to the formation of the diphenylanthracene radical
cation, DPA•þ, occurs at þ 1.65 V versus Ag/AgCl
(Figure 1). The formal potentials of the metal-based
oxidation processes for compounds 1-6 are summarized
in Table 1. The first oxidation occurs at lower potentials
for all of the binuclear complexes, 1-6, compared to the
mononuclear complex, [Ru(bpy)2(phpy)](PF6). On the
other hand, the second anodic process becomes more
difficult in all cases.
The splitting displayed in the metal-based anodic oxi-
dations, separated into two one-electron waves, is a clear
indication of the stability of the electrochemically gener-
ated mixed-valence intermediates.
Particularly for 1, the second one-electron oxidation
occurs at a potential that is 264 mV more positive than the
first monoelectronic couple, reflecting a strong degree of
electronic communication between the redox units.
0
0
Mn þ Mðn -2Þ a 2Mðn -1Þ
2
Kc ¼ 10ΔE=59mV ¼ ½Mðn -1Þꢀ =½Mnꢀ½Mðn -2Þꢀ
ð1Þ
The Kc values determined from electrochemical data
evidence the stability of the electrochemically generated
mixed-valence species. A shorter metal-metal distance
would be anticipated for the dinuclear complexes substi-
tuted on C2 and C7 of the anthracene cores with respect to
the analogous complexes substituted on C2 and C6. The
relatively weaker electronic coupling for the anthracene
linkages substituted on C1 and C5 is attributed, at least in
part, to steric hindrance, preventing coplanarity of the
metallic moieties and the central anthracene core. All of
these aspects are borne out by the electrochemical data
(Table 1), where it is evident that the 2,7 substitution gives
0
0
rise to the strongest interaction (largest ΔE values), while
the 1,5 substitution gives rise to the smallest differences.
Dichloromethane was initially employed as a solvent to
perform cyclic voltammetric measurements. Its weakly
coordinating properties, particularly its very weak ability
to act as a donor, favors its use for studies of inorganic/
coordination compounds. However, its low dielectric
constant, 8.9 at 25 °C,68 can cause some restrictions on
its general use as a solvent when ionic species are involved.
Salts of small cations and anions are generally insoluble in
this solvent, whereas salts with cations containing large
aliphatic substituents are readily soluble in dichloro-
methane but are known to be only weakly dissociated.69
Owing to the low degree of dissociation of salts in this
solvent, a considerable fraction of the dissolved salts will
exist as various ion-paired species.
Figure 1. Cyclic voltammetry of the homobinuclear complex 1 in
CH2Cl2/0.1 M n-Bu4NPF6 at a glassy carbon electrode (d = 3 mm). Scan
rate: 100 mV s-1. Ref.: Ag/AgCl. Counter electrode: Platinum wire. Inset:
Cyclic voltammetry of the mononuclear complex, [Ru(bipy)2(phpy)]-
(PF6) for comparison.

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Vila et al.
802 Inorganic Chemistry, Vol. 49, No. 3, 2010
Figure 2. Cyclic voltammetry of the homobinuclear complexes, 1-6, in CH2Cl2/0.1 M n-Bu4NPF6 at a glassy carbon electrode (d = 3 mm). Scan rate:
100 mV s-1. Ref.: Ag/AgCl. Counter electrode: Platinum wire.
Table 1. Electrochemical Data for the Homobinuclear Compoundsa Including
the Mononuclear [Ru(bpy)2(phpy)](PF6) Complex for Comparison
anion would be expected to result in changes in the
separation between the formally RuIIRuII/RuIIRuIII
and RuIIRuIII/RuIIIRuIII couples and could be correlated
0
E1 (V)
0
0
0
0
compound
E2 (V)
ΔE0 (mV)
KC
with differences in ion-pairing effects.6,7,12,19,50,67,75-77 In
this sense, it should be pointed out that the largest ΔE0
0
2,7Ru-9,10-DPA, 1
2,7Ru2-9,10-AN, 2
2,6Ru2-9,10-DPA, 3
2,6Ru2-9,10-AN, 4
1,5Ru2-9,10-DPA, 5
1,5Ru2-9,10-AN, 6
[Ru(bpy)2(ppy)]
0.404
0.389
0.410
0.480
0.419
0.474
0.591
0.668
0.647
0.654
0.670
0.540
0.602
264
258
244
190
121
128
2.9 ꢁ 104
2.3 ꢁ 104
1.4 ꢁ 104
1.7 ꢁ 103
1.1 ꢁ 102
1.5 ꢁ 102
values have been found when low donor solvents have
been employed in the presence of weakly coordinating
anions used as supporting electrolytes, such as n-Bu4N-
[B(C6F5)4]. In contrast, the use of small anions such as
0
0
chloride led to a significant decrease in the ΔE . On the
other hand, changing from a low polarity solvent to a
higher polarity solvent, such as acetonitrile, shows the
effects of solvation over ion pairing and leads to smaller
a Potential data in volts versus Ag/AgCl are from single scan cyclic
voltammograms recorded in CH2Cl2/0.1 M n-Bu4NPF6.
Although ΔE°0 values of compounds containing
equivalent redox sites linked by a bridging ligand have
been commonly employed as an indication of the strength
of the electronic interaction between electron transfer
sites, it is well-known that the effects, not only of the
solvent but also of the counterion on ΔE°0 values (and
corresponding comproportionation constants, Kc) may
be large, and systematic investigations of the combined
medium effects in low-polarity solvents have been re-
ported.69-77 In order to address several medium effects
that could be affecting our measurements, we extended
our initial studies in dichloromethane for 2 to other
solvent/electrolyte media (Figure 3). Specifically, we also
employed acetonitrile and DMF as solvents and chloride,
PF6-, and [B(C6F5)4] as anions with tetra-n-butyl ammo-
nium as the cation in all cases. A set of electrochemical
measurements was performed, and the results obtained
are summarized in Table 2.
ΔE0 values. Furthermore, the overall change in ΔE0 is
mostly due to the shift of the second oxidation processes.
This could be rationalized by taking into account the
existence of electrostatic interactions in which the greater
positive charge density of the dicationic species would
result in higher ion-pairing effects.53,55
0
0
UV/Vis/NIR Spectroelectrochemistry
The UV-vis-NIR spectra of the binuclear compounds
were recorded. The absorption spectra of the binuclear
ruthenium complexes exhibited very intense bands near
300 nm (black line, Figure 4) assigned to ligand-centered
(LC) π-π* transitions, and moderately intense bandslocated
in the visible region from 350 to 600 nm ascribed to metal-to-
ligand charge-transfer (MLCT) transitions. The mixed-
valence species, derived from all of the compounds, were
electrochemically generated and their formation followed by
absorption spectroscopy. Thus, a stepwise coulommetric
titration was performed on a 1 mM solution of compounds
1-6 in CDCl3/0.1 M n-Bu4NPF6, and absorption spectra
were regularly recorded during the oxidation process.
Figures 4 and 5 show the evolution of the spectra of com-
pounds 3 and 5, respectively, during the electrochemical
oxidation process. The characteristic MLCT broad bands
progressively decreased in amplitude, and a new band, cen-
tered at around 750 nm, emerged. This band was ascribed to
ligand-to-metal charge-transfer transitions (LMCT). This
band reached its maximum intensity when complete oxida-
tion to the RuIIIRuIII form was achieved. In addition, and
particularly important in the present context, broad IVCT
transitions were found in the NIR region for all of the
Some previous electrochemical studies with mono- and
binuclear ruthenium(II) complexes have reported on the
dependence of the redox potentials of the complexes
studied on the anion of the supporting electrolyte.75 Ion
pairing between the cationic complex and electrolyte
(71) Algra, G. P.; Balt, S. Inorg. Chem. 1981, 20, 1102.
(72) Ito, K.; Iwamoto, E.; Yamamoto, Y. Bull. Chem. Soc. Jpn. 1982, 55,
2143.
(73) Abbott, A. Chem. Soc. Rev. 1993, 435.
ꢂ
(74) Pospısil, L.; King, B. T.; Michl, J. Electrochim. Acta 1998, 44, 103.
´
(75) Yeomans, B. D.; Kelso, L. S.; Tregloan, P. A.; Keene, F. R. Eur. J.
Inorg. Chem. 2001, 239.
(76) Hupp, J. T. Inorg. Chem. 1990, 29, 5010.
(77) Ruiz, J.; Astruc, D. C. R. Acad. Sci. Paris 1998, 2, 21.

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 803
Figure 3. Cyclicvoltammetryof the homobinuclear complex, 2, in CH2Cl2 (a) 0.1 M n-Bu4NPF6, (b) 0.1M n-Bu4N[B(C6F5)4], and(c) 0.1Mn-Bu4NCl ata
glassy carbon electrode (d = 3 mm). Scan rate: 100 mV s-1. Ref: Ag/AgCl. Counter electrode: platinum wire.
Table 2. Electrochemical Data for the Homobinuclear Compound, 2a in Three
Different Solvents
E1 (V) E2 (V) ΔE0 (mV)
KC
0
0
0
0
0
2,7Ru2-9,10-AN, 2
CH2Cl2/n-Bu4NPF6
CH2Cl2/n-Bu4NCl
CH2Cl2/n-Bu4N[B(C6F5)4] 0.400
0.389
0.414
0.647
0.530
0.752
0.575
0.497
0.640
0.563
0.520
0.658
258
116
352
166
92
238
148
102
262
2.3 ꢁ 104
9.2 ꢁ 101
2.0 ꢁ 106
4.4 ꢁ 102
3.6 ꢁ 101
1.1 ꢁ 104
3.2 ꢁ 102
5.3 ꢁ 101
2.8 ꢁ 104
CH3CN/n-Bu4NPF6
CH3CN/n-Bu4NCl
CH3CN/n-Bu4N[B(C6F5)4] 0.402
0.419
0.405
DMF/n-Bu4NPF6
DMF/n-Bu4NCl
DMF/n-Bu4N[B(C6F5)4]
0.415
0.418
0.396
a Potential data in volts versus Ag/AgCl are from single scan cyclic
voltammograms.
Figure 5. UV-vis-NIR spectra in CDCl3 of 1,5(Ru)2-9,10-DPA be-
fore oxidation (black line), after half-oxidation (1 electron, green line),
and after full oxidation (2 electrons, bright blue line).
Table 3. Intervalence Charge Transfer (IVCT) Transitions and Experimental and
Theoretical Bandwidth Values for the Homobinuclear Ruthenium Compexes,
1-6
Γc
a
b
compd
λ (nm)
Δν1/2(exp)
Δν1/2(theo)
2,7Ru2-9,10-DPA, 1
2,7Ru2-9,10-AN, 2
2,6Ru2-9,10-DPA, 3
2,6Ru2-9,10-AN, 4
1,5Ru2-9,10-DPA, 5
1,5Ru2-9,10-AN, 6
1580
1520
1725
1638
1322
1310
2740
3330
2520
2615
3900
4050
3825
3900
3660
3755
4180
4200
0.28
0.15
0.31
0.30
0.07
0.04
a Full width at half-height in cm-1 (experimental). b Full width at
half-height in cm-1 computed from Hush’s equation. c Γ is estimated
from eq 2.
Figure 4. UV-vis-NIR spectra in CDCl3 of 2,6(Ru)2-9,10-DPA be-
fore oxidation (black line), after half-oxidation (1 electron, red line), and
after full oxidation (2 electrons, blue line).
which bandwidths are compared with the theoretical values
1/2
) .
expected from Hush’s expression79 Δν1/2 = (2310νmax
bimetallic compounds investigated. The IVCT bands were
centered at 1580 (compound 1), 1520 (compound 2), 1725
(compound 3), 1638 (compound 4), 1322 (compound 5), and
1310 (compound 6), as summarized in Table 3. The intensity
of these bands increased continuously during the oxidation
process until complete formation of the mixed valence
compounds was achieved upon consumption of one electron.
These transitions are assigned to bridge-mediated intramole-
cular electron transfer when the mixed-valence species were
formed. These IVCT transitions are absent in the RuIIRuII
complexes and entirely disappear in the RuIIIRuIII species
(i.e., the fully oxidized forms), as shown in Figures 4 and 5.
Analyses of the IVCT bands are presented in Table 3, in
Experimental values were of the same order of magnitude
for all of the complexes studied, 1-6, and slightly narrower
than predicted by Hush’s theory, suggesting the categoriza-
tion of these complexes as class II mixed-valence species
according to the Robin and Day classification.
The Γ values, a parameter introduced by Sutin40,78 and
co-workers to provide an estimation of the electron coup-
ling in mixed-valence species, are also included in Table 3.
Such a parameter has been calculated from eq 2 and is
commonly used as a measure of the delocalization degree
(78) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1.

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Vila et al.
804 Inorganic Chemistry, Vol. 49, No. 3, 2010
in mixed-valence complexes. Its value is readily calculated
from the deviation of the bandwidth from its theoretical
value as specified in eq 2. Values within the range 0 < Γ < 0.5
are expected for class II mixed-valence species, whereas larger
Γ values are expected for the most delocalized systems.
strength of the metal-metal electronic communication
mediated by a bridging ligand since a number of experimental
factors (such as the solvent and supporting electrolyte) can
affect, as we have shown, the measured redox potentials for
these oxidation processes.4,11 As a result, several discrepancies
have been encountered when comparing electrochemical and
spectroscopic data in order to establish the extent of localiza-
tion/delocalization through the anthracene-bridged π-conju-
gated systems employed in the binuclear ruthenium
complexes (1-6) described herein. In this sense, the analysis
of IVCT bands provides a more reliable estimate of the extent
of interaction through the Γ parameter (within the range 0 <
Γ < 0.5 in all the cases), which leads us to conclude that the
bimetallic complexes investigated belong to class II in the
Robin and Day classification.
Γ ¼ 1 -Δν1=2exp=Δν1=2theo
ð2Þ
Thus, for weakly coupled (localized) class II systems, Γ varies
from 0 to 1.0. Its value progressively increases, in the same
direction as the electronic coupling, exhibiting values in the
range 0.1 < Γ < 0.5 for moderately coupled class II systems.
Conclusions
We have synthesized and characterized, via electrochemical
and spectroscopic techniques, a family of anthracene-bridged
binuclear ruthenium complexes whose degree of metal-metal
interactions could be modulated by changes in the nature of
the bridge and substitution pattern. We have also studied the
Acknowledgment. We thank the support of the NSF via
the NIRT program (Grant CHE-04030806) and the
Cornell Center for Chemical Interfacing, a Phase I Center
for Chemical Innovation (CHE-0847926). This work was
also supported by the Cornell Center for Materials Re-
search. N.V. gratefully acknowledges financial support
dependence of the ΔE0 values for the oxidation processes of
0
homobinuclear mixed-valence complexes on the medium. As
has been previously pointed out, the comproportionation
constants estimated from electrochemical data can be used
as an indication of the thermodynamic stability of the mixed-
valence species. However, as previously reported by Keene
and co-workers, particular caution should be taken when
trying to establish relationships between the Kc values and the
ꢁ
from the Ministerio de Ciencia e Innovacion from Spain
for a Postdoctoral Fellow. The authors also thank Pro-
fessor Iluminada Gallardo for several measurements
carried out in her lab and for helpful discussions.
Supporting Information Available: Synthetic procedure and
characterization of compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
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