Mononuclear and dinuclear ruthenium complexes of 2,3-Di-2-pyridyl-5,6- diphenylpyrazine: Synthesis and spectroscopic and electrochemical studies

Article abstract of DOI:10.1021/ic101629w

Reported here are a new bridging ligand, 2,3-di-2-pyridyl-5,6- diphenylpyrazine (dpdpz), and its complexation with one or two ruthenium atoms. This ligand was designed so that it could bind to metal species in either a N^∧N bidentate fashion or a C^∧N^∧N tridentate mode to form a metallacycle. The reaction between dpdpz and (tpy)RuCl₃ (tpy = 2,2′:6′,2″-terpyridine) afforded C^∧N^∧N-type mono- and dinuclear cyclometalated complexes in moderate yields. On the other hand, N^∧N-type mono- and dinuclear noncyclometalated complexes could be isolated from the reaction of dpdpz with (bpy)₂RuCl₂ (bpy = 2,2′-bipyridine). An asymmetric diruthenium complex, bridged by dpdpz, was prepared with one ruthenium atom cyclometalated and another one noncyclometalated. The electronic properties of these complexes were probed by electrochemical and spectroscopic techniques. They exhibited multiple reversible redox processes. However, the formal potentials and electrochemical energy gap are greatly dependent on the binding nature and number of ruthenium atoms. As indicated by electrochemical and spectroelectrochemical studies, diruthenium complexes bridged by dpdpz exhibited electronic coupling between the two metal centers. A comparison of the electronic absorption and emission properties of these complexes is also presented.

Full text of DOI:10.1021/ic101629w

Inorg. Chem. 2011, 50, 517–524 517
DOI: 10.1021/ic101629w
Mononuclear and Dinuclear Ruthenium Complexes of 2,3-Di-2-pyridyl-
5,6-diphenylpyrazine: Synthesis and Spectroscopic and Electrochemical Studies
,†
†
‡
†
,‡
ꢀ
~
Yu-Wu Zhong,* Si-Hai Wu, Stephen E. Burkhardt, Chang-Jiang Yao, and Hector D. Abruna*
†Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China, and
‡Baker Laboratory, Department of Chemistry and Chemical Biology, Cornell University, Ithaca,
New York 14853-1301, United States
Received August 10, 2010
Reported here are a new bridging ligand, 2,3-di-2-pyridyl-5,6-diphenylpyrazine (dpdpz), and its complexation with
one or two ruthenium atoms. This ligand was designed so that it could bind to metal species in either a N∧N
bidentate fashion or a C∧N∧N tridentate mode to form a metallacycle. The reaction between dpdpz and (tpy)RuCl3
(tpy = 2,20:60,200-terpyridine) afforded C∧N∧N-type mono- and dinuclear cyclometalated complexes in moderate
yields. On the other hand, N∧N-type mono- and dinuclear noncyclometalated complexes could be isolated from the
reaction of dpdpz with (bpy)2RuCl2 (bpy=2,20-bipyridine). An asymmetric diruthenium complex, bridged by dpdpz,
was prepared with one ruthenium atom cyclometalated and another one noncyclometalated. The electronic
properties of these complexes were probed by electrochemical and spectroscopic techniques. They exhib-
ited multiple reversible redox processes. However, the formal potentials and electrochemical energy gap are
greatly dependent on the binding nature and number of ruthenium atoms. As indicated by electrochemical and
spectroelectrochemical studies, diruthenium complexes bridged by dpdpz exhibited electronic coupling between the
two metal centers. A comparison of the electronic absorption and emission properties of these complexes is also
presented.
Introduction
attractive building blocks for the construction of molecular
architectures to perform light and redox-related functions.
A simple, deliberate, and general way to construct poly-
nuclear coordination oligomers or polymers is to assemble
metal centers using appropriate bridging ligands. In this sense,
one of the most extensively studied polypyridine bridging
ligands, 2,3,5,6-tetra-2-pyridylpyrazine (tppz; Figure 1), has
been shown to yield a great number of linear and rigid
molecular assemblies when bound to octahedral transition-
metalions suchas FeII, RuII, OsII, and CoII ina bis-terdentate
fashion.2 These compounds exhibit a high degree of electron-
ic coupling and are most attractive for the generation of
highly conductive one-dimensional molecular wires because
of the extended π conjugation and their ability to mediate
electron transfer through multiple redox states of the transition
metals. Unfortunately, these complexes are mostly nonemissive
at room temperature. In comparison, polynuclear RuII, OsII,
ReI, and CuI complexes based on the bidentate bridging
Because of their appealing photophysical and electrochemical
properties, transition-metal polypyridine complexes have been
extensively employed for building macromolecular assemblies
and molecular wire prototypes.1 Complexation of transition
metals with polypyridine ligands provides molecular struc-
tures with rigid and well-defined geometries. Optimal orbital
overlap between metals and organic ligands gives rise to
electronic delocalization along the entire molecule. This
narrows the highest occupied molecular orbital (HOMO)-
lowest unoccupied molecular orbital (LUMO) gap and gives
rise to intense absorption inthe visibleregion of thespectrum.
These properties make transition-metal polypyridine complexes
*To whom correspondence should be addressed. E-mail: zhongyuwu@
iccas.ac.cn (Y.-W.Z.), hda1@cornell.edu (H.D.A.).
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Juris, A.; Venturi, M. Chem. Rev. 1996, 96, 759. (d) Barigelletti, F.; Flamigni, L.
Chem. Soc. Rev. 2000, 29, 1. (e) Ziessel, R. Synthesis 1999, 1839. (f) Balzani, V.;
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r
2010 American Chemical Society
Published on Web 12/09/2010
pubs.acs.org/IC

518 Inorganic Chemistry, Vol. 50, No. 2, 2011
Zhong et al.
detailed comparisons of the electrochemical and spectro-
scopic properties between the cyclometalated and non-
cyclometalated ruthenium complexes with the same bridging
ligand are still rare. In this manuscript, we report on 2,3-di-2-
pyridyl-5,6-diphenylpyrazine (dpdpz, 1; Figure 1), as a new
bridging ligand, and its complexation with ruthenium atoms.
This ligand was designed so that it could behave as either a
cyclometalating or a noncyclometalating ligand for ruthenium
atoms. The difference between tppz and dpdpz is that two of
the lateral pyridines in tppz are replaced by two phenyl rings
in dpdpz. Compared to tppz, which has two N∧N∧N-type tri-
dentate coordination sites, dpdpz is expected to bind transition
metals in a C∧N∧N-type tridentate fashion to form cyclomet-
alated complexes,8 if the phenyl rings participate in the coordi-
nation with metals. However, if the phenyl rings remain intact,
dpdpz could behave as a N∧N-type bidentate binding ligand.
It should be mentioned that a similar bridging ligand, dibenzo-
[a:c](dipyrido[2,3-h:20,30-j])phenazine (dbdpzH2), was reported
a few years ago.9 However, attempts to prepare a sym-
metric dicyclometalated diruthenium complex were not
successful.
Figure 1. Bridging ligands.
ligands 2,3-di-2-pyridylpyrazine (dpp), 2,3-di-2-pyridylquin-
oxaline (dpq), and 2,3-di-2-pyridylbenzo[g]quinoxaline (dpb)
and their derivatives are emissive.3 Most of these complexes
absorb strongly and emit in the vis/near-IR (NIR) region.
They have been investigated in many photoinduced energy/
electron-transfer processes and as light-harvesting materials.
All of these studies have demonstrated that the development
of new polypyridine bridging ligands with comparable and
complementary electronic properties is still of great interest
to the materials science and chemistry communities.4
Recently, much attention has been paid to cyclometalated
ruthenium complexes, which have at least one Ru-C σ bond
present in the complex. The introduction of a negative charge
in the ligand strongly affects the electronic and photophysical
properties of the complexes. In contrast to noncyclometalated
counterparts, cyclometalated ruthenium complexes generally
exhibit much lower metal-based oxidation potentials5 and
stronger metal-metal electronic coupling between individual
metal centers.6 It should be noted that bis(triazole)- or bis-
(tetrazole)pyridine, as reported by Vos and co-workers, can
also act as σ-donor ligands, and the corresponding RuII(tpy)-
type complexes show attractive emissive properties.7 However,
Results and Discussion
Synthesis. 1 has been reported as a spectrophotometric
reagent for copper and iron ions by Khuhawar and co-
workers.10 However, to the best of our knowledge, only
limited studies on the synthesis and characterization of its
transition-metal complexes have been described. 1 was
readily prepared from the condensation of meso-1,2-
diphenylethylenediamine with 2,20-pyridil and subsequent
dehydrogenation (Scheme 1). The precipitate, which was
separated from the reaction mixture, was analytically
pure and used for subsequent transformations without
further purification. The reaction of 1 with 1 equiv of
(tpy)RuCl3 (tpy = 2,20:60,200-terpyridine) in the presence
of AgOTf gave the desired cyclometalated monoruthe-
nium complex 2 in moderate yield (52%). When 1 was
treated with 2 equiv of (tpy)RuCl3, the biscyclometalated
diruthenium complex 3 was isolated in 36% yield after
flash column chromatography.
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The reaction of 2 equiv of cis-(bpy)2RuCl2 (bpy=2,20-
bipyridine) with dpdpz under microwave irradiation gave
a mixture of monoruthenium complex 4 and diruthenium
complex 5. In this case, the dpdpz ligand coordinates to
RuII atoms in a N∧N bidentate fashion. After purification
through flash column chromatography, 4 and 5 were
obtained in 28% and 36% yield, respectively. Alterna-
tively, monoruthenium complex 4 was prepared from
1 equiv of cis-(bpy)2RuCl2 and dpdpz in good yield (90%).
(4) (a) Han, F. S.; Higuchi, M.; Kurth, D. G. J. Am. Chem. Soc. 2008, 130,
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C. J.; Da Re, R. E.; Veauthier, J. M.; Schelter, E. J.; Milligan, A. E.; Scott, B. L.; Bauer,
E. D.; Thompson, J. D.; Morris, D. E.; John, K. D. J. Am. Chem. Soc. 2006, 128,
7230. (f) Sauers, A. L.; Ho, D. M.; Bernhard, S. J. Org. Chem. 2004, 69, 8910.
(5) Coudret, C.; Fraysse, S.; Launay, J.-P. Chem. Commun. 1998, 663.
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1993, 1444. (b) Beley, M.; Collin, J.-P.; Louis, R.; Metz, B.; Sauvage, J.-P. J. Am.
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J. Am. Chem. Soc. 1998, 120, 3717. (e) Yao, C.-J.; Sui, L.-Z.; Xie, H.-Y.; Xiao, W.-J.;
The identities of 2-5 were confirmed by their H NMR
and mass spectrometry (MS) spectra and elemental analyses.
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van Klink, G. P. M.; van Koten, G. Inorg. Chem. 2009, 48, 1887. (d) Wadman, S. H.;
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Inorg. Chem. Commun. 2000, 3, 68.

Article
Inorganic Chemistry, Vol. 50, No. 2, 2011 519
Scheme 1. Synthesis of the dpdpz Ligand 1 and Ruthenium Complexes 2-6
Judging from the 1H NMR spectrum, diruthenium com-
plex 5 was obtained as a mixture of two diastereomers
(meso and rac) with a diastereoselectivity of 2:1. No
attempts have been made to separate or distinguish these
two diastereomers because the relative configuration of
the ruthenium centers in similar diastereomers has a trivial
effect on the luminescent and electrochemical properties.11
The asymmetric diruthenium complex 6, with one ruthenium
being cyclometalated and another noncyclometalated,
could be prepared from the reaction of either cyclometa-
lated monoruthenium complex 2 with cis-(bpy)2RuCl2
or noncyclometalated monoruthenium complex 4 with
(tpy)RuCl3.
Electrochemical Studies. The electrochemical proper-
ties of complexes 2-6 were studied by cyclic voltammetry
(CV). The CV profiles are shown in Figure 2, and their
electrochemical data are summarized in Table 1, together
with some related complexes for comparison. The anodic
scan of 2 displays two redox couples at þ0.67 and þ1.31 V
vs Ag/AgCl (entry 1). The former peak is assigned to
the ruthenium-based II/III redox process, and the latter
one could be attributed either to a RuIII/IV couple or to a
ligand-based oxidative decomposition.8 It is well-accepted
that oxidation of the cyclometalated ruthenium atom takes
place at much less positive potentials than noncyclometataed
ruthenium centers, and this phenomenon could even be
used to judge the formation of a Ru-C bond. For example,
the RuII/III redox process of [(tpy)Ru(dbdpz)](PF6) was
reported to occur at þ0.82 V vs SCE (entry 6).9 The higher
σ-donating character of the anionic carbon ligands, com-
pared to a neutral nitrogen ligand, destabilizes the metal-
centered HOMO level and, hence, gives a much lower
oxidation potential. The cathodic scan of 2 shows two revers-
ible redox couples at -1.33 and -1.71 V vs Ag/AgCl. The
wave at -1.33 V is due to reduction of the dpdpz ligand,
and reduction of tpy occurs at -1.71 V. As deduced from
the CV data, the HOMO-LUMO energy gap of 2 is esti-
mated to be 2.00 eV. In comparison, the tppz-bridged,
noncyclometalated ruthenium analogue, [(tpy)Ru(tppz)]-
(PF6)2 (entry 7), exhibits the first oxidation and reduction
peaks at þ1.50 and -0.95 V vs SCE, respectively, result-
ing in a HOMO-LUMO energy gap of 2.45 eV. We note
that the cyclometalated complex has a much narrower
energy gap, which could be advantageous for applications
in molecular wires.
The CV profile of 3 displays two well-resolved redox
couples at þ0.65 and þ0.84 V vs Ag/AgCl in the anodic
scan (190 mV difference, entry 2), which suggests the pres-
ence of two Ru-C bonds in the complex and effective
metal-metal electronic communication through the dpdpz
bridging ligand. In addition, two irreversible processes
were also observed at higher potentials. The compropor-
(11) (a) Slater, J. W.; D’Alessandro, D. M.; Keene, F. R.; Steel, P. J.
Dalton Trans. 2006, 1954. (b) MacDonnell, F. M.; Kim, M.-J.; Wouters, K. L.;
Konduri, R. Coord. Chem. Rev. 2003, 242, 47.
tionation constant Kc for the equilibrium RuII-RuII
RuIII-RuIII T 2 RuII-RuIII is 1659, as determined by
þ

520 Inorganic Chemistry, Vol. 50, No. 2, 2011
Zhong et al.
at þ1.71, þ1.40, -0.39, -0.86, -1.43, and -1.86 V (vs SCE;
310 mV difference in the two anodic waves and an energy
gap of 1.79 eV), we can find that the cyclometalated com-
plex, again, displays a narrower energy gap. However, the
degree of metal-metal communication is weaker based
on the potential separation of the formal potentials of the
two anodic waves.
The noncyclometalated monoruthenium complex 4
displays one ruthenium-related reversible oxidation wave
at þ1.38 V vs Ag/AgCl and three ligand reduction waves
at -0.97, -1.40, and -1.73 V (entry 3). The wave at -0.97 V
is due to reduction of the dpdpz ligand, and waves at
-1.40 and -1.73 V could be assigned to reduction of the
two bpy ligands. The anodic scan of diruthenium complex
5 shows two well-resolved redox couples at þ1.45 and
þ1.62 V vs Ag/AgCl (entry 4). The potential difference
between these two waves is 170 mV, which again suggests
strong metal-metal electronic communication through
the dpdpz bridging ligand. The comproportionation con-
stant Kc for the mixed-valence equilibrium is 761. The
cathodic scan of 5 exhibited up to six redox couples at
-0.60, -1.07, -1.42, -1.52, -1.79, and -1.89 V vs Ag/AgCl.
The first two waves are due to reduction of the dpdpz
ligand, and the latter four waves could be assigned to
reduction of the four bpy ligands. The electrochemical
energy gap, as determined from the potential gap between
the first oxidation and first reduction wave in CV data, is
2.35 eV for 4 and 2.05 eV for 5, respectively. The electro-
chemical properties of 4 and 5 are virtually the same as
those of the dpp-bridged ruthenium monomer and dimer
(entries 9 and 10), respectively.
It is interesting to take a closer look at the CV profile of
the asymmetric diruthenium complex 6, as shown in the
bottom of Figure 2. The redox couple at þ0.81 V is due to
oxidation of the cyclometalated RuII/III process, followed
by an irreversible oxidation process at þ1.32 V. The latter
wave was assigned to either the cyclometalated RuIII/IV
couple or the ligand-based oxidative decomposition based
on its quasi-reversible nature8 and as observed in the
similar redox processes of cyclometalated complexes 2
and 3. The redox couple at a more positive potential
(þ1.57 V) is likely from the noncyclometalated ruthenium
center. It should be noted that the RuII/III process of the
cyclometalated ruthenium center of the asymmetric com-
plex 6 occurs at a more positive potential than that of the
first RuII/III process of the symmetric biscyclometalated
complex 3 (þ0.81 vs þ0.65 V). Moreover, the RuII/III
process of the noncyclometalated ruthenium center of 6
also takes place at a more positive potential than that of the
first RuII/III process of the symmetric bis-noncyclometalated
complex 5 (þ1.57 vs þ1.45 V). This indicates the presence
of electronic coupling between the two metal centers in
the asymmetric diruthenium complex 6. In the cathodic
scan of 6, five ligand-based redox couples were observed
at -0.82, -1.25, -1.55, -1.73, and -1.90 V vs Ag/AgCl
(entry 5). The first two waves are likely from reduction of
the dpdpz ligand, and the latter three waves are due to
reduction of the bpy and tpy ligands. By comparison of
the first reduction potential of diruthenium complexes 3,
5, and 6 (-1.03, -0.60, and -0.82 V, respectively), it is
evident that the bis-noncyclometalated complex 5 could
be reduced at the least negative potential and the biscyclo-
metalated complex 3 at the most negative potential.
Figure 2. Cyclic voltammograms of complexes 2-6 in acetonitrile con-
taining 0.1 M Bu4NClO4 at a scan rate of 100 mV/s. The working elec-
trode is a glassy carbon, the counter electrode is a platinum wire, and the
reference electrode is Ag/AgCl in saturated aqueous NaCl.
Kc=10ΔE (mV)/59. The cathodic scan showed four couples
at -1.03, -1.43, -1.73, and -1.86 V vs Ag/AgCl. The first
two waves could be ascribed to reduction of the dpdpz
ligand, and the latter two partially overlapped processes
are from tpy. Coordination of the second ruthenium center
makes the first reduction process of dpdpz 300 mV less
negative because of a higher degree of electron delocali-
zation of the dimetallic complex. The electrochemical
energy gap is 1.68 eV for diruthenium complex 3, which
is 320 mV narrower relative to the ruthenium monomer 2.
This again reflects the significant effect of transition
metals on the π-electron delocalization across the entire
molecule. By comparing 3 with the tppz-bridged noncyclo-
metalated diruthenium analogue, [(tpy)Ru(tppz)Ru(tpy)]-
(PF6)4 (entry 8), which similarly shows six redox couples

Article
Inorganic Chemistry, Vol. 50, No. 2, 2011 521
E1/2(ligand-based)
Table 1. Electrochemical Data of Complexes 2-6 and Some Related Compoundsa
entry
complex
E
1/2(anodic)
bridging ligand
bpy or tpy
ΔEb (eV)
1
2
3
4
5
6
7
8
9
10
2, [(tpy)Ru(dpdpz)](PF6)
3, [(tpy)Ru(dpdpz)Ru(tpy)](PF6)2
4, [(bpy)2Ru(dpdpz)](PF6)2
0.67, 1.31
0.65, 0.84, 1.33, 1.60
1.38
1.45, 1.62
0.81, 1.32, 1.57
0.82
-1.33
-1.03, -1.43
-0.97
-0.60, -1.07
-0.82, -1.25
-1.07
-1.71
2.00
1.68
2.35
2.05
1.63
1.89
2.45
1.79
2.39
2.04
-1.73, -1.86
-1.40, -1.73
5, [(bpy)2Ru(dpdpz)Ru(bpy)2](PF6)4
6, [(bpy)2Ru(dpdpz)Ru(tpy)](PF6)3
[(tpy)Ru(dbdpz)](PF6)c
-1.42, -1.52, -1.79, -1.89
-1.55, -1.73, -1.90
-1.56, -1.62
d
[(tpy)Ru(tppz)](PF6)2
[(tpy)Ru(tppz)Ru(tpy)](PF6)4
[(bpy)2Ru(dpp)](PF6)2
[(bpy)2Ru(dpp)Ru(bpy)2](PF6)4
1.50
-0.95, -1.40
-0.39, -0.86
-1.01
-1.60
d
1.40, 1.71
1.38
1.43, 1.61
-1.43, -1.86
e
-1.49, -1.72
e
-0.61, -1.09
a All measurements were carried out in acetonitrile containing 0.1 M Bu4NClO4 as the supporting electrolyte. Unless otherwise noted, the potential
was reported as the E1/2 value vs Ag/AgCl. b The electrochemical energy gap is determined by the potential difference between the first oxidation and first
reduction wave. c E1/2 vs SCE. See ref 9. d E1/2 vs SSCE. See ref 2a. e E1/2 vs SSCE. See ref 3m.
attributed to the ancillary bpy-based intraligand π-π*
transition. The peak at 360 nm is likely from the intrali-
gand transition of the dpdpz ligand. Complex 5, with two
noncyclometalated ruthenium atoms, exhibits multiple
MLCT bands between 400 and 600 nm (blue line). On
the other hand, the asymmetric diruthenium complex 6
shows an intense ruthenium-to-cyclometalating dpdpz
ligand charge-transfer band at 550 nm (magenta line).
This band is of similar intensity, albeit with a 24 nm blue
shift, when compared to that of the symmetric biscyclo-
metalated complex 3. By comparison of all five spectra
together, it is clear that the metal-to-cyclometalating
ligand charge-transfer transition is much broader and
Figure 3. UV/vis absorption spectra of 2-6 in acetonitrile.
more intense and red-shifted, as observed in either sym-
metric biscyclometalated complex 3 or asymmetric com-
plex 6. This property would make them good potential
light-harvesting materials.
This is reasonable because complex 3 has two negatively
charged ligands, which makes it difficult to reduce. The
electrochemical energy gap of the asymmetric diruthenium
complex 6 (1.63 eV) is similar to that of the biscyclo-
metalated complex 3 (1.68 eV).
Emission Spectroscopic Studies. Among the five ruthe-
nium complexes synthesized above, noncyclometalated
complexes 4 and 5 are emissive at room temperature. No
emission was detected from the cyclometalated com-
plexes 2, 3, and 6 in a fluidic acetonitrile solution at room
temperature, contrasting with the bis(triazole)- or bis-
(tetrazole)pyridine-based ruthenium complexes cited in
ref 7. However, it might be possible that the photomul-
tiplier tube employed (R928F) is not sufficiently red-
sensitive to detect any low-energy weak emission from
these complexes, if present. The emission spectra of 4 and
5 are presented in Figure 4, and the positions of the
luminescence maxima are summarized in Table 2. Mono-
nuclear ruthenium complex 4 exhibited a broad lumines-
cence band with a maximum at 709 nm. The luminescence
quantum yield of 4 was 0.057%. The emission spectral
changes of 4 upon an increase in the acidity of the aqueous
solution, by adding H2SO4, are shown in Figure 5. Clearly,
protonation of the free nitrogen atom in 4 quenches its
emission. The emission could be recovered by the addition of
the appropriate amount of aqueous NaOH (Figure S4 in
the Supporting Information). Similar results for ruthe-
nium complexes containing dpp bridging ligands have
been reported.12 The weak band at 675 nm appears too
narrow to be real, and it could be due to an instrumental
Electronic Absorption Spectroscopic Studies. The elec-
tronic absorption spectra of complexes 2-6 are shown in
Figure 3, and the corresponding absorption maxima and
molar absorption coefficients are presented in Table 2.
Generally speaking, transitions in the UV region are
largely due to intraligand π-π* excitations, while the
intense and broad bands in the visible region are ascribed
to metal-to-ligand charge-transfer (MLCT) transitions.8
Monocyclometalated ruthenium complex 2 displayed a
broad MLCT band centered at 515 nm (black line). This
band could arise from the transition from ruthenium to
the cyclometalating dpdpz ligand. In comparison, the
corresponding MLCT band in biscyclometalated com-
plex 3 appears much broader and more intense (red line),
in addition to a large red shift compared to 2. This reflects
a greater degree of electron delocalization across the
molecule in the case of the bimetallic complex, which
correlates well with the electrochemical results described
above. As far as the noncyclometalated monoruthenium
complex 4 is concerned, two MLCT bands, centered at
446 and 490 nm, are evident in the visible region (olive line).
They are assigned to the ruthenium to bpy and dpdpz
ligand charge transfer, respectively. Because the dpdpz
ligand is easier to reduce than bpy, as evidenced in the
electrochemical studies, the MLCT band of dpdpz would
be expected to be at lower energy. The peak at 299 nm is
(12) (a) Hosek, W.; Tysoe, S. A.; Gafney, H. D.; Baker, A. D.; Strekas,
T. C. Inorg. Chem. 1989, 28, 1228. (b) Nazeeruddin, M. K.; Kalyanasundaram,
K. Inorg. Chem. 1989, 28, 4251.

522 Inorganic Chemistry, Vol. 50, No. 2, 2011
Zhong et al.
Table 2. Absorption and Emission Data of Complexes 2-6 and Some Related Compoundsa
entry
complex
absorption λmax/nm (ε/105 M-1cm-1
)
emissionb λmax/nm (quantum yield)
1
2
3
4
2, [(tpy)Ru(dpdpz)](PF6)
276 (0.37), 319 (0.37), 360 (0.20), 427 (0.053), 515 (0.13)
276 (0.71), 316 (0.70), 384 (0.37), 574 (0.48)
299 (0.63), 360 (0.23), 446 (0.10), 490 (0.088)
285 (0.87), 360 (0.41), 423 (0.19), 496 (0.22)
282 (0.91), 374 (0.39), 438 (0.14), 550 (0.44)
285 (0.65), 354 (0.051), 420 (0.087), 455 (0.11)
430 (0.12), 470
3, [(tpy)Ru(dpdpz)Ru(tpy)](PF6)2
4, [(bpy)2Ru(dpdpz)](PF6)2
5, [(bpy)2Ru(dpdpz)Ru(bpy)2](PF6)4
6, [(bpy)2Ru(dpdpz)Ru(tpy)](PF6)3
[Ru(bpy)3](PF6)2
709 (0.057%)
791 (0.023%)
5
6
612 (5.9%)
675 (5.1%)
755 (0.31%)
7c
8c
[(bpy)2Ru(dpp)](PF6)2
[(bpy)2Ru(dpp)Ru(bpy)2](PF6)4
425 (0.17), 525 (0.21)
a All spectra were recorded in a conventional 1.0 cm quartz cell in acetonitrile. b The excitation wavelength is 400 nm. The quantum yield is calculated
by comparing it with a standard sample [Ru(bpy)3](PF6)2, which has a quantum yield of 5.9% in N2-saturated acetonitrile. c See ref 3n.
Figure 4. Normalized emission spectra of complexes 4 (black line) and
Figure 6. Absorption spectral changes of diruthenium complex 5 upon
5 (red line) in acetonitrile. The excitation wavelength was 400 nm.
electrolysis at þ1.55 V vs Ag/AgCl in CD3CN containing 0.1 M n-Bu4NClO4.
Asterisks indicate artifacts due to errors in compensation of solvent
absorption.
the low-lying and nonradiative cyclometalated ruthenium
moiety state.13
Spectroelectrochemical Studies. Studies of mixed-valence
dimetallic complexes have attracted much attention.14 These
studies are pertinent to the understanding of naturally occur-
ring electron-transfer processes and molecular electronics.
As mentioned in the previous sections, symmetric diruthe-
nium complexes 3 and 5 displayed large differences in the
formal potentials (ΔE°0), suggesting the presence of strong
electronic communication between the two metal centers in
these complexes. To further probe the electronic coupling
between ruthenium atoms, spectroelectrochemical mea-
surements were carried out. Upon electrolysis of the bis-
noncyclometalated complex 5 at þ1.55 V vs Ag/AgCl at a
Figure 5. Emission spectral changes of monoruthenium complex 4 at
platinum mesh electrode (Figure 6), partial diminution of the
different pH values in water. The excitation wavelength was 400 nm. The
visible MLCT band intensity was evident along with the
concomitant emergence of a broad band centered at 1920 nm
(5200 cm-1). Upon further electrolysis at þ1.80 V, this band
disappeared (Figure S5 in the Supporting Information). This
NIR band could be assigned to an intervalence charge-
transfer (IVCT) transition of the in situ generated mixed-
valence complex. The observed full width at half-height (Δν1/2)
was 2900 cm-1, which is narrower than, although of the same
order of magnitude as, the theoretical value of 3465 cm-1, as
weak bands at 675 and 820 nm are due to an instrumental artifact.
artifact. From the emission titration in Figure 5, the pKa
value for the excited state of the protonated 4 was calcu-
lated to be 2.5 (see the Supporting Information for details).
When the second ruthenium atom is coordinated to 4,
the emission of the resulting dinuclear complex 5 moves
into the NIR region, displaying a broad luminescence
band with a maximum between 700 and 880 nm, albeit
with a lower quantum yield of 0.023%. It should be noted
that both of the above emission spectra are independent
of the excitation wavelength, and the quantum yield is
around 15% less in an air-purged solution. As far as the
asymmetric dinuclear complex 6 is concerned, attach-
ment of the cyclometalated ruthenium component leads
to the complete disappearance of emission, which we
believe is the result of fast energy transfer from the excited
state of the noncyclometalated ruthenium component to
calculated from the Hush formula Δν1/2 =(2310νmax)
1/2 for
€
€
(13) Ott, S.; Borgstrom, M.; Hammarstrom, L.; Johansson, O. Dalton
Trans. 2006, 1434.
(14) (a) D’Alessandro, D. M.; Keene, F. R. Chem. Rev. 2006, 106, 2270.
(b) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J. Chem. Rev. 2001, 101, 2655.
(c) Kaim, W.; Lahiri, G. K. Angew. Chem., Int. Ed. 2007, 46, 1778. (d) Chisholm,
M. H.; Patmore, N. J. Acc. Chem. Res. 2007, 40, 19. (e) Kaim, W.; Klein, A.;
€
Glockle, M. Acc. Chem. Res. 2000, 33, 755. (f) D'Alessandro, D. M.; Keene, F. R.
Chem. Soc. Rev. 2006, 35, 424. (g) Launay, J.-P. Chem. Soc. Rev. 2001, 30, 386.

Article
Inorganic Chemistry, Vol. 50, No. 2, 2011 523
weakly or moderately coupled class II mixed-valence sys-
tems.15 The electronic coupling parameter Hab is calculated to
be 650 cm-1, according to the Hush formula:16 Hab=2.06ꢀ
the working electrode. The electrode was polished prior to use
with 1 μm diamond paste (Buehler) and rinsed thoroughly with
water and acetone. A large-area platinum wire coil was used as
the counter electrode. All potentials were referenced to a satu-
rated Ag/AgCl electrode without regard for the liquid junction
potential. All measurements were carried out with a 0.3 mM
concentration of the corresponding complex in acetonitrile at a
scan rate of 100 mV/s, with 0.1 M Bu4NClO4 as the supporting
electrolyte.
10-2(εmax max
ν
Δν1/2)1/2/rab, where εmax equals 0.031 ꢀ 105 M-1
cm-1, νmax equals 5200 cm-1, Δν1/2 equals 2900 cm-1, and
ꢀ
˚
rab is estimated to be 6.85 A according to a reported crystal
structure of a similar diruthenium complex bridged by dpb.17
One-electron oxidation of biscyclometalated complex 3
at þ0.76 V vs Ag/AgCl did not give rise to a distinct IVCT
band in the NIR region (Figure S6 in the Supporting Infor-
mation). Another possibility is that it could be too broad and
weak, if indeed present, to be clearly discerned.
Synthesis. All reactions were carried out under an atmosphere
of dry nitrogen using standard Schlenk techniques. Dry tetra-
hydrofuran was distilled from sodium/benzophenone, and other
solvents (analytical grade) were used without further purifica-
tion. NMR spectra were recorded in the designated solvent on a
Varian 300 or Varian 400 spectrometer. MALDI-TOF positive-
ion data were obtained with a Waters MALDI micro-MX mass
spectrometer run in a reflection mode. Cyclometalated com-
plexes were synthesized followed by a published procedure with
slight modifications (see ref 9).
Conclusions
In summary, the dpdpz ligand, described in this contribution,
was found to be a versatile bridging ligand for the synthesis of
new transition-metal complexes with implications in molec-
ular wires and the construction of more sophisticated supra-
molecular systems and architectures. This ligand could be
readily prepared in a one-step reaction from commercially
available starting materials. It is of great interest that dpdpz
can bind to ruthenium atoms with different coordination
modes, namely, in a N∧N-type bidentate fashion or in a
C∧N∧N tridentate mode, to form a metallacycle. We have
prepared five dpdpz-containing mononuclear and dinuclear
ruthenium complexes, and each metal could be cyclometa-
lated or noncyclometalated. Combined electrochemical and
spectroscopic studies showed that these complexes exhibit
intriguing structural and electronic features. In comparison,
cyclometalated complexes have rigid linear conformation,
narrower energy gaps, and lower formal potentials of the
metal centers. These properties make them potential candi-
dates as molecular wires and as light-harvesting materials.
Noncyclometalated complexes were found to emit at room
temperature. However, attachment of a cyclometalated ruthe-
nium atom completely quenched the emission. This behavior
would make the asymmetric diruthenium complex a good candi-
date for the dynamic study of fast energy-transfer processes.
Future work will focus on the preparation and application of
new complexes based on dpdpz and related ligands.
2,3-Di-2-pyridyl-5,6-diphenylpyrazine (dpdpz, 1). A solution
of meso-1,2-diphenylethylenediamine (42 mg, 0.2 mmol) and
2,20-pyridil (42 mg, 0.2 mmol) in 5 mL of ethanol was refluxed
for 24 h under a N2 atmosphere. After cooling, the mixture was
open to air and stirred for another 2 days at room temperature.
The yellow precipitate was collected after filtration to give 50 mg
of product with a yield of 62%. 1H NMR (300 MHz, CDCl3):
δ 7.20-7.26 (m, 2H), 7.30-7.35 (m, 6H), 7.58-7.62 (m, 4H), 7.80
(t, J=7.5 Hz, 2H), 8.08 (d, J=7.8 Hz, 2H), 8.36 (d, J=3.9 Hz,
2H). ESI-HRMS. Calcd for [M þ H]þ: m/z 387.1610. Found:
m/z 387.1613. Anal. Calcd for C26H18N4: C, 80.81; H, 4.69; N,
14.50. Found: C, 80.79; H, 4.59; N, 14.47.
[(tpy)Ru(dpdpz)](PF6) (2). To a solution of (tpy)RuCl3 (44 mg,
0.10 mmol) in 30 mL of acetone was added 91 mg (0.35 mmol) of
AgOTf, and the mixture was refluxed for 2 h. After cooling to room
temperature, the mixture was filtered through a pad of Celite,
and then concentrated under reduced pressure. To the residue were
then added the dpdpz ligand 1 (39 mg, 0.10 mmol), 10 mL of N,
N-dimethylformamide(DMF), andtBuOH, respectively. The result-
ing mixture was refluxed for another 20 h before cooling and
removal of some insoluble impurities through filtering. The solvent
was removed at reduced pressure. The residue was dissolved in
10 mL of MeOH, and then excess NH4PF6 was added. The precipi-
tate generated was collected and purified through flash column
chromatography on silica gel to give 45 mg of 2 as a brownish solid
in a yield of 52%. 1H NMR (300 MHz, CD3CN): δ 5.84 (dd, J=
7.5 and 1.5 Hz, 1H), 6.40 (td, J=7.8 and 1.5 Hz, 1H), 6.48 (td, J=
7.2 and 1.5 Hz, 1H), 7.05 (m, 2H), 7.14 (m, 2H), 7.34 (d, J=9.0 Hz,
1H), 7.51 (m, 2H), 7.62 (m, 6H), 7.83 (td, J=7.80 and 1.5 Hz, 2H),
7.90 (m, 2H), 8.18 (m, 3H), 8.46 (d, J= 8.4 Hz, 2H), 8.67 (d, J=
7.8 Hz, 2H), 8.70 (m, 1H, overlapped). MALDI-MS: 720 ([M -
2PF6]2þ). Anal. Calcd for C41H28F6N7PRu: C, 56.95; H, 3.26; N,
11.34. Found: C, 56.71; H, 3.16; N, 11.09.
Experimental Section
Spectroscopic Measurements. All optical UV/vis absorption
spectra were obtained using a TU-1810DSPC spectrometer of
Beijing Purkinje General Instrument Co. Ltd. at room tempera-
ture in denoted solvents, with a conventional 1.0 cm quartz cell.
Emission spectra were recorded using a F-380 spectrofluori-
meter of Tianjin Gangdong Sic. & Tech Development Co. Ltd.,
with a red-sensitive photomultiplier tube R928F. Samples for
emission measurement were obtained in quartz cuvettes of 1 cm
path length. Luminescence quantum yields were determined
using [Ru(bpy)3](PF6)2 in a degassed acetonitrile solution as
the standard (φ = 5.9%);18 the estimated uncertainty of φ is
(10% or better.
[(tpy)Ru(dpdpz)Ru(tpy)](PF6)2 (3). To a solution of (tpy)RuCl3
(90 mg, 0.21 mmol) in 30 mL of acetone was added 195 mg
(0.70 mmol) of AgOTf, and the mixture was refluxed for 2 h. After
cooling to room temperature, the mixture was filtered through a
pad of Celite and then concentrated under reduced pressure. To the
residue were then added the dpdpz ligand 1(39 mg, 0.10 mmol), 15 mL
t
of DMF, and BuOH, respectively. The resulting mixture was
Electrochemical Measurements. All CV was taken using an
Epsilon BAS CV-27 or a CHI620D potentiostat. One-compartment
electrochemical cells with a provision for gas addition were employed.
A glassy carbon electrode with a diameter of 3 mm was used as
refluxed for another 20 h before cooling and removal of some
insoluble impurities through filtering. The solvent was removed at
reduced pressure. The residue was dissolved in 10 mL of MeOH, and
then excess NH4PF6 was added. The precipitate generated was collec-
ted after filtering and purified through flash column chromatography
on silica gel to give 49 mg of 3 as a purple solid in a yield of 36%. 1H
NMR (300 MHz, CD3CN): δ 6.02 (dd, J = 7.5 and 1.5 Hz, 2H),
6.67 (m, 4H), 7.23 (m, 6H), 7.70-7.90 (m, 12H), 8.23 (t, J=8.1 Hz,
2H), 8.36 (dd, J= 7.8 and 1.5 Hz, 2H), 8.52 (d, J= 8.4 Hz, 4H),
8.72 (d, J=7.8 Hz, 6H). MALDI-MS: 1054 ([M - 2PF6]2þ), 720
(15) (a) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391. (b) Hush, N. S.
Electrochim. Acta 1968, 1005.
(16) Hush, N. S. Coord. Chem. Rev. 1985, 64, 135.
(17) D’Alessandro, D. M.; Junk, P. C.; Keene, F. R. Supramol. Chem.
2005, 17, 529.
(18) Nakamura, K. Bull. Chem. Soc. Jpn. 1982, 55, 1639.

524 Inorganic Chemistry, Vol. 50, No. 2, 2011
Zhong et al.
([M - 2PF6 - Ru - tpy]þ). Anal. Calcd for C56H38F12N10P2Ru2
[(bpy)2Ru(dpdpz)Ru(tpy)](PF6)3 (6).To a solution of (tpy)RuCl320
(35 mg, 0.08 mmol) in 30 mL of acetone was added 73 mg of AgOTf
(0.28 mmol). The mixture was refluxed for 2 h under N2. After cooling
to room temperature, the generated AgCl precipitate was removed by
filtering through Celite. The filtrate was then concentrated
under reduced pressure. To the residue were added mono-
ruthenium complex 2 (87 mg, 0.08 mmol), 10 mL of DMF, and
tBuOH, respectively. The resulting mixture was refluxed under
a N2 atmosphere for another 24 h before cooling and removal
of the solvent under reduced pressure. The residue was dis-
solved in 5 mL of MeOH, followed by the addition of 15 mL of
water and an excess of KPF6. The precipitate generated was
collected after filtering and purified through flash column
chromatography on silica gel to give 41 mg of 6 as a deep-
purple solid in a yield of 33%. MALDI-MS: 1423.9 ([M - H -
PF6]þ), 1278.0 ([M - 2PF6]2þ), 976.2 ([M - 2H - bpy -3PF6]3þ).
3
Et2O: C, 50.85; H, 3.41; N, 9.88. Found: C, 50.82; H, 3.32; N, 10.08.
[(bpy)2Ru(dpdpz)](PF6)2 (4). To a solution of dpdpz (193 mg,
0.50 mmol) in 25 mL of ethylene glycol was added 260 mg of
Ru(bpy)2Cl2 2H2O19 (0.50 mmol), and the mixture was heated
3
under microwave irradiation for 30 min. After cooling to room
temperature, an excess of an aqueous solution of KPF6 was added.
The precipitate generated was collected and purified through flash
column chromatography on silica gel to give 488 mg of 4 as a deep-
yellow solid in 90% yield. 1H NMR (400 MHz, CD3CN): δ 6.01 (d,
J=7.5 Hz, 1H), 6.45 (t, J=7.4 Hz, 1H), 6.72 (t, J=6.5 Hz, 1H),
6.83 (d, J=5.5 Hz, 1H), 6.88-6.94 (m, 2H), 7.16 (t, J=7.5 Hz,
2H), 7.22 (d, J=7.3 Hz, 5H), 7.33 (d, J=5.4 Hz, 1H), 7.41 (t, J=
8.7 Hz, 2H), 7.54-7.67 (m, 6H), 7.93-7.98 (m, 2H), 8.13-8.19
(m, 3H), 8.25 (d, J=7.7 Hz, 1H), 8.30 (d, J=5.4 Hz, 1H), 8.38 (t,
J = 7.6 Hz, 2H), 8.50 (d, J = 8.1 Hz, 1H), 8.56 (d, J = 5.5 Hz,
1H), 8.61 (d, J=4.6 Hz, 1H). ESI-MS. Calcd for [M-PF6]þ: m/z
Anal. Calcd for C61H44F18N11P3Ru2 2H2O: C, 45.67; H, 3.02; N,
3
9.60. Found: C, 45.46; H, 2.83; N, 9.83.
945.3. Anal. Calcd for C46H34N8P2F12Ru H2O: C, 49.87; H,
3
3.28; N, 10.11. Found: C, 49.87; H, 3.17; N, 9.67.
Acknowledgment. We thank the Institute of Chemistry,
Chinese Academy of Sciences (“100 Talent” Program), and
the National Natural Science Foundation of China (Grant
21002104) for funding support. This work was also supported
by the Center for Molecular Interfacing, an NSF Phase I Center
for Chemical Innovation award (0847926), and the Cornell
Center for Materials Research.
[(bpy)2Ru(dpdpz)Ru(bpy)2](PF6)4 (5). To a solution of dpdpz
(38.6 mg, 0.10 mmol) in 10 mL of ethylene glycol was added
103 mg of Ru(bpy)2Cl2 2H2O (0.20 mmol). The mixture was
3
heated under microwave irradiation for 20 min. After cooling to
room temperature, an excess of an aqueous solution of KPF6
was added. The precipitate generated was collected and purified
through flash column chromatography on silica gel to afford
65.5 mg of 5 as a brownish solid in a yield of 36%. Because
of the presence of a mixture of diastereomers, it is difficult to
Supporting Information Available: pH titration and pKa*
determination of 4, absorption and emission spectral changes
of complex 4 at different pH values in water, spectroelectro-
chemical analysis of 5 and 3, and NMR and MS spectra of new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
assign the H NMR signals. However, it is evident from the
spectrum that two diastereomers exist in a rough ratio of
2:1. MALDI-MS: 1503.0 ([M - 2PF6 -H]2þ). Anal. Calcd for
C66H50F24N12P4Ru2 H2O: C, 43.77; H, 2.89; N, 9.28. Found: C,
43.70; H, 2.95; N, 9.22.
3
(19) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17,
(20) Sullivan, B. P.; Calvert, J. M.; Meyer, T. J. Inorg. Chem. 1980, 19,
3334.
1404.