Charge delocalization in a cyclometalated bisruthenium complex bridged by a noninnocent 1,2,4,5-Tetra(2-pyridyl)benzene ligand

Article abstract of DOI:10.1021/ja205879y

Two ruthenium atoms are covalently connected to the para positions of a phenyl ring in 1,2,4,5-tetra(2-pyridyl)benzene (tpb) to form a linear Ru-tpb-Ru arrangement. This unique structure leads to appealing electronic properties for the biscyclometalated c

Full text of DOI:10.1021/ja205879y

ARTICLE
pubs.acs.org/JACS
Charge Delocalization in a Cyclometalated Bisruthenium Complex
Bridged by a Noninnocent 1,2,4,5-Tetra(2-pyridyl)benzene Ligand
Chang-Jiang Yao, Yu-Wu Zhong,* and Jiannian Yao*
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
S Supporting Information
b
ABSTRACT: Two ruthenium atoms are covalently connected
to the para positions of a phenyl ring in 1,2,4,5-tetra(2-pyridyl)-
benzene (tpb) to form a linear RuꢀtpbꢀRu arrangement. This
unique structure leads to appealing electronic properties for the
biscyclometalated complex [(tpy)Ru(tpb)Ru(tpy)]2+, where
tpy is 2,20;60,200-terpyridine. It could be stepwise oxidized at
substantially low potential (+0.12 and +0.55 V vs Ag/AgCl) and
with a noticeably large comproportionation constant (1.94 ꢁ
107). In addition to the routinely observed metal-to-ligand
charge-transfer transitions, [(tpy)Ru(tpb)Ru(tpy)]2+ displays a separate and distinct absorption band at 805 nm with appreciable
absorptivity (ε = 9000 Mꢀ1 cmꢀ1). This band is assigned to the charge transition from the RuꢀtpbꢀRu motif to the pyridine rings
of tpb with the aide of density functional theory (DFT) and time-dependent DFT calculations. Complex [(tpy)Ru(tpb)Ru(tpy)]2+
was precisely titrated with 1 equiv of cerium ammonium nitrate to produce [(tpy)Ru(tpb)Ru(tpy)]3+, which shows intense multiple
NIR transitions. The electronic coupling parameters Hab of individual NIR components are determined to be 5812, 4942, 4358, and
3560 cmꢀ1. DFT and TDDFT calculation were performed on [(tpy)Ru(tpb)Ru(tpy)]3+ to elucidate its electronic structure and
spin density population and the nature of the observed NIR transitions. Electron paramagnetic resonance studies of
[(tpy)Ru(tpb)Ru(tpy)]3+ exhibit a discernible rhombic signal with the isotropic g factor of Ægæ = 2.144. These results point to
the strong orbital interaction of tpb with metal centers and that tpb behaves as a redox noninnocent bridging ligand in
[(tpy)Ru(tpb)Ru(tpy)]2+. Complex [(tpy)Ru(tpb)Ru(tpy)]3+ is determined to be a RobinꢀDay class III system with full charge
delocalization across the RuꢀtpbꢀRu motif.
’ INTRODUCTION
According to Robin and Day,4 three categories of MV systems
are distinguished on the basis of the extent of electronic coupling
between individual redox centers. Class I systems are composed
of noninteracting centers. Species in Class II systems exhibit
weak or moderate coupling between individual redox compo-
nents. Class III systems consist of strongly coupled centers in
which electrons fully delocalize across the whole molecule and
electron transfer between redox sites take places without an
activation barrier. Among them, linear class III systems with full
charge delocalization would be very attractive for applications in
molecular electronics.5 However, most MV complexes reported
to date are classified as the class II system.
Polyazine transition-metal complexes are very useful and
interesting for building MV systems.6 These complexes often
exhibit rich photophysical properties and multiple well-defined
redox processes. Construction and investigations of linear poly-
azine multimetallic complexes have been the focus for several
decades and recently gained renewed interest due to their poten-
tial use in molecular electronics.7 We are particularly interested in
construction of new MV systems with cyclometalated polyazine
The understanding of charge delocalization in conjugated
compounds is of particular importance in designing and devel-
oping new semiconductor materials for optoelectronic applica-
tions.1 In this regard, mixed-valence (MV) systems, with purely
organic or organometallic components, have attracted growing
interest since they provide simple models for the investigation of
basic charge delocalization and electron transfer processes.2 One
of the most appealing motives in studying MV systems is that
their charge delocalization degree could be qualitatively and
quantitatively assessed by a combination of electrochemical and
spectroscopic techniques. As demonstrated by enormous experi-
mental data and sophisticated theoretical calculations,2 the charge
delocalization of MV systems greatly depends on a number of
factors, such as the distance between redox centers, coordination
environments of the metal components, and ability of the bridg-
ing ligand to delocalize the electronic charge. These studies are of
significant importance in understanding many naturally occur-
ring photoinduced electron/energy transfer processes and relevance
to molecular electronics and switches. Despite these efforts, design
and realization of new MV systems with full charge delocalization is
still challenging.3
Received: June 24, 2011
Published: August 24, 2011
r
2011 American Chemical Society
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Chart 1. Pyrazine- vs 1,4-Benzene-Bridged Mixed-Valent
Bisruthenium Complexesa
aThe counteranions and ancillary ligands are omitted.
Scheme 1. Synthesis of Compounds 1ꢀ3
Figure 1. Cyclic voltammograms (CV, black lines) of (a) 2+ and
(b) 32+ in acetonitrile containing 0.1 M nBu4NClO4 at a scan rate of
100 mV/s. The red lines are differential pulse voltammograms with a
step potential of 5 mV and an amplitude of 50 mV. The working
electrode is glassy carbon, the counter electrode is a Pt wire, and
the reference electrode is Ag/AgCl in saturated aq. NaCl. For CV
profiles with wider potential windows, see Figures S1 and S2 in the
Supporting Information.
ruthenium complexes.8 The anionic nature of the cyclometalat-
ing ligand significantly changes the properties of these complexes.9
They have been reported to greatly enhance metalꢀmetal coupl-
ing in MV systems,10 as compared to noncyclometalated ones. In
addition, cyclometalated ruthenium complexes have been used as
efficient sensitizers for solar cell applications in numerous recent
research activities.11
Complexes with redox noninnocent ligands, either ancillary or
bridging ligands, show appealing and unique electronic properties
and have been the focus of a number of research groups.12
According to the definition by Jørgensen, a ligand is called non-
innocent if it does not allow the oxidation state of the metal to be
defined.13 Two types of noninnocent ligands are well known.
One class is 1,2-dioxolene, 1,2-dithiolene, o-phenylene diamine,
and other electronically similar ligands.14 Another is the anionic
carbon ligand, such as phenylacetylene, phenylvinylene, or the phenyl
group when covalently connected to a metal center.15 Depend-
ing on the nature of the ligands and metals, redox processes of
complexes with noninnocent ligands could occur from the ligand
motif or the metal center or both components. For interpretation
of the MV state of multimetallic complexes with noninnocent
ligands, which show extended conjugation across the molecule,
a combination of electrochemical, spectroscopic, magnetic, and
theoretical analysis must be invoked.
Figure 2. Isodensity plots of selected HOMO and LUMO orbitals for
complex 2+. All orbitals have been computed at an isovalue of 0.02.
We report in this article on new cyclometalated bisruthenium
complexes [(tpy)Ru(tpb)Ru(tpy)]2+ and the one-electron oxi-
dized species [(tpy)Ru(tpb)Ru(tpy)]3+, where tpy is 2,20;60,200-
terpyridine and tpb is the bridging 1,2,4,5-tetra(2-pyridyl)-
benzene (tpb) ligand. In contrast to the familiar CruetzꢀTaube
ion bridged by a pyrazine ligand with dative bonds,16 namely,
{[Ru(NH3)5](pyrazine)[Ru(NH3)5]5+}, ruthenium atoms in
[(tpy)Ru(tpb)Ru(tpy)]2+ and [(tpy)Ru(tpb)Ru(tpy)]3+ are con-
nected to the bridging ligand through covalent RuꢀC bonds
(Chart 1). Interestingly, complex [(tpy)Ru(tpb)Ru(tpy)]2+
shows peculiar and unique electronic properties that have rarely
been reported in other polyazine transition metal complexes, as
revealed by detailed electrochemical, spectroscopic, and theoretical
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Figure 3. Isodensity plots of selected HOMO and LUMO orbitals for complex 32+. All orbitals have been computed at an isovalue of 0.02.
between 1,2,4,5-tetrabromobenzene and 2-(tributylstannyl)-
pyridine in 34% yield. The reaction of 2 equiv of (tpy)RuCl3
(tpy = 2,20;60,200-terpyridine) with 1 in the presence of AgOTf,
followed by subsequent counteranion exchange, afforded cyclo-
metalated bisruthenium complex 32+ in 17% yield. In addition,
monoruthenium complex 2+ was prepared from 1 equiv of
(tpy)RuCl3 with 1 for a comparison study. The identities of new
1
compounds were confirmed by H NMR, mass spectrometry,
and microanalysis (see the Experimental Section for details).
Electrochemical Studies and DFT Calculations. Electroche-
mical techniques, such as cyclic voltammetric (CV) and differential
pulse voltammetric (DPV) analysis, are frequently employed to
study the electronic communication between metal centers of
symmetric dimetallic systems. If no electronic coupling is present
between two metals, the voltammetric profile exhibits a single
metal-based redox wave. On the other hand, two separated
sequential redox waves from the metals suggest the presence of
an efficient charge delocalization between them. However, this
principle should be taken with great care. Numerous studies18
have proven that the electrochemical data are largely dependent
on the measurement conditions, particularly the solvent and
supporting electrolyte used. Nevertheless, the separation differ-
ence between two redox waves (ΔEo) may serve as a parameter
for qualitatively estimating the extent of electronic coupling between
two metal centers if under the same conditions of measurement.
The anodic CV profile of monoruthenium complex 2+ is
shown in Figure 1a, which displays one reversible redox couple
at +0.60 V vs Ag/AgCl. This is a typical value for a cyclometalated
RuII/III redox process.8ꢀ11 On the other hand, two sequential
redox couples at +0.12 and +0.55 V are evident on the CV of
bisruthenium complex 32+ (Figure 1b), with a potential difference
(ΔEo) of 430 mV between two half-wave potentials. Differential
Figure 4. (a) Electronic absorption spectra of tpb 1 in dichloromethane
pulse voltammetry (DPV) of 32+ (red line in Figure 1b) also con-
firms this result. Noteworthy is that both redox processes of 32+
occur at less positive potentials than that of 2+. This phenomenon
is rarely observed in MV systems, because the redox process of the
monometallic complex usually takes place at a potential between
those of the two splitting waves of the homologous dimetallic
complex. In addition, the first oxidation potential at +0.12 V vs
Ag/AgCl of complex 32+ is too low to be simply ascribed to a
cyclometalated RuII/III redox process.9ꢀ11 We reason that attach-
ment of two cyclometalated ruthenium atoms makes the anionic
bridging ligand sufficiently electron rich to become noninnocent.
In other words, the redox processes of 32+ shown in Figure 1b are
attributable to an admixture of oxidations from both the metal
center and the bridging ligand. We notice that the electrochemical
behaviors of 32+ are in stark contrast with another biscyclometa-
lated bisruthenium complex [(tpy)Ru(dpdpz)Ru(tpy)](PF6)2
(black line) and complexes 2+ (red line) and 32+ (blue line) in
acetonitrile. (b and c) First 100 absorptions with oscillator strength
values higher than 0.005 as predicted by TDDFT for complexes 2+ and
32+, respectively.
calculation studies. In addition, strong electronic coupling was
found to be present between individual redox centers as inves-
tigated on complex [(tpy)Ru(tpb)Ru(tpy)]3+ with electroche-
mical and electron paramagnetic resonance (EPR) studies and by
analyzing near-infrared transitions with the aide of theoretical
calculations.
’ RESULTS AND DISCUSSION
Synthesis. As depicted in Scheme 1, tpb (1) was synthesized
through the palladium-catalyzed Stille cross-coupling reaction17
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Table 1. Excitation Energy (E), Oscillator Strength (f), Dominant Contributing Transitions, and Associated Percent Contribution
and Assignment of Complex 2+a
Sn
E/eV
E/nm
f
dominant transitions (percent contributionb)
assignmentb
MLtpyCT
3
5
6
1.97
2.24
2.56
630
552
484
0.0073
0.051
0.15
HOMOꢀ1 f LUMO (74%)
HOMOꢀ2 f LUMO+1 (87%)
HOMOꢀ2 f LUMO (39%)
HOMOꢀ1 f LUMO+1 (30%)
HOMO f LUMO+2 (84%)
HOMO f LUMO+3 (56%)
HOMOꢀ1 f LUMO+2 (34%)
HOMOꢀ2 f LUMO+3 (63%)
HOMOꢀ1 f LUMO+3 (84%)
HOMOꢀ1 f LUMO+4 (61%)
HOMOꢀ1 f LUMO+5 (50%)
HOMOꢀ3 f LUMO+2 (56%)
HOMOꢀ5 f LUMO+2 (27%)
HOMOꢀ5 f LUMO+2 (61%)
HOMOꢀ2 f LUMO+14 (52%)
HOMOꢀ2 f LUMO+8 (41%)
HOMO f LUMO+10 (61%)
HOMOꢀ6 f LUMO+2 (37%)
HOMO f LUMO+10 (30%)
HOMOꢀ9 f LUMO (72%)
MLtpyCT
MLtpyCT
MLtpyCT
MLtpbCT/ILtpbCT
MLtpbCT/ILtpbCT
MLtpbCT
MLtpbCT
MLtpbCT
MLtpyCT
MLtpyCT
ILtpb
7
9
2.63
2.87
471
432
0.095
0.0088
13
15
19
21
33
3.02
3.09
3.29
3.36
3.77
411
401
377
369
329
0.015
0.072
0.092
0.045
0.16
ILtpb
36
47
48
49
51
3.82
4.06
4.13
4.13
4.16
324
305
300
300
298
0.069
0.045
0.055
0.087
0.24
ILtpb
Mc
MLtpyCT
ILtpb
ILtpb
ILtpb
52
4.17
297
0.28
ILtpy
a Computed at the TDDFT/LANL2DZ level of theory. b The actual percent contribution = (configuration coefficient)2 ꢁ 2 ꢁ 100%.
reported by us recently,8a where dpdpz is 2,3-di(2-pyridyl)-5,6-
diphenylpyrazine and the metal center binds to the bridging
ligand in a tridentate C∧N∧N fashion. The first two oxidation
processes of this complex take place at +0.65 and +0.84 V vs
Ag/AgCl. This suggests the position of the RuꢀC bond plays a
significant role in determining the electronic properties of biscy-
clometalated ruthenium complexes. The tridentate N∧C∧N
coordination mode with the bridging tpb ligand in complex 32+
is essential for its peculiar properties.
(LUMO+9). The electron density of the HOMO of 32+ dis-
tributes across the RuꢀtpbꢀRu motif, with tpb even playing a
more important role. The Mulliken population of the cyclome-
talating phenyl fragment is 0.29, which is larger than that of each
metal center (0.25). This supports the above electrochemical
results that a considerable portion of oxidation processes of 32+
may arise from the bridging tpb component.
Electronic Absorption Spectra and TDDFT Calculations.
To further probe the electronic properties of complexes 2+ and
32+, their electronic absorption spectra were subsequently recorded
(Figure 4a). To aid in the assignment of the optical absorptions
more precisely, the nature of the low-energy transitions was
studied by time-dependent DFT (TDDFT) calculations. The
first 100 predicted transitions of complexes 2+ and 32+ are shown
in Figure 4b and 4c, respectively. Tables 1 and 2 give major pre-
dicted transitions with the excitation energy (E), oscillator
strength (f), dominant configuration contribution, and assign-
ment. Monoruthenium complex 2+ displays multiple sharp and
intense excitations below 350 nm and a broad transition between
450 and 650 nm. According to TDDFT predications (Figure 4b
and Table 1) and in agreement with previous assignment for
similar cyclometalated ruthenium complexes,8ꢀ11 absorptions in
the UV region are ascribed to intraligand (IL) πꢀπ* transitions
of both tpy and tpb ligands mixed with a small portion of a metal-
centered transition. The relatively shallow absorptions between
350 and 450 nm are attributable to an admixture of metal-to-
ligand charge-transfer (MLCT) transitions from ruthenium to
both tpy and tpb ligands. The broad transition centered at 500 nm
has major contribution from MLtpyCT transition (excitations from
HOMO-1 and HOMO-2 to LUMO and LUMO+1) mixed with
some MLtpbCT transition and ILtpbCT (intramolecular charge-
transfer) transitions. The latter two transitions are due to excita-
tion from HOMO to LUMO-2 and LUMO-3, respectively.
Density functional theory (DFT) calculations were performed
on the B3LYP/LANL2DZ level to assist determination of their
electronic structures. Selected frontier orbital structures of 2+
and 32+ with electron density distributions are shown in Figures 2
and 3, respectively. More orbital graphics can be found in Figures
S3ꢀS6 in the Supporting Information. The lowest unoccupied
molecular orbital (LUMO) and LUMO+1, LUMO+4, LUMO
+5, and LUMO+8 of 2+ have major contributions from the
ancillary tpy ligand, while LUMO+2, LUMO+3, and LUMO+10
are dominated by the tpb ligand. In addition, the ruthenium com-
ponent contributes dominantly to LUMO+14. The highest occu-
pied molecular orbital (HOMO) of 2+ has contributions from
both the metal component and the center phenyl ring of tpb
ligand. Mulliken population analysis of the HOMO composition
reveals a value of 0.48 and 0.34 for the metal center and the
cyclometalating phenyl fragment. HOMO-1 and HOMO-2 are
dominated by the metal component. As for the lower occupied
orbitals, HOMO-3, HOMO-5, and HOMO-6 have a major
contribution from the tpb ligand while HOMO-9 is dominated
by the ancillary tpy ligand. Interestingly, the situation is signifi-
cantly different in the case of bisruthenium complex 32+. The
bridging tpb ligand, instead of tpy, contributes dominantly in the
LUMO of 32+. Higher unoccupied orbitals have a major con-
tribution from either tpy (LUMO+1, LUMO+2, LUMO+3, and
LUMO+4), tpb (LUMO+5 and LUMO+14), or the metal center
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Table 2. Excitation Energy (E), Oscillator Strength (f), Dominant Contributing Transitions, and Associated Percent Contribution
and Assignment of Complex 32+a
Sn
E/eV
E/nm
f
dominant transitions (percent contributionb)
assignmentb
3
1.72
1.88
2.05
2.21
2.35
718
661
606
560
527
0.075
0.0056
0.017
0.29
HOMO f LUMO (85%)
CT(RuꢀtpbꢀRu f Ltpb
)
5
HOMO f LUMO+4 (92%)
HOMOꢀ1 f LUMO+1 (56%)
HOMOꢀ1 f LUMO (85%)
HOMOꢀ2 f LUMO+4 (48%)
HOMOꢀ3 f LUMO+3 (39%)
HOMO f LUMO+5 (65%)
HOMOꢀ5 f LUMO+1 (27%)
HOMOꢀ1 f LUMO+4 (23%)
HOMOꢀ4 f LUMO+3 (67%)
HOMOꢀ1 f LUMO+5 (77%)
HOMOꢀ1 f LUMO (65%)
HOMOꢀ2 f LUMO+9 (50%)
HOMOꢀ1 f LUMO+9 (65%)
HOMO f LUMO+14 (65%)
HOMOꢀ6 f LUMO+4 (90%)
CT(RuꢀtpbꢀRu f Ltpy
MLtpyCT
)
6
9
MLtpbCT
14
0.085
MLtpyCT
MLtpyCT
17
21
2.53
2.67
489
464
0.090
0.033
CT(RuꢀtpbꢀRu f Ltpb
MLtpyCT
MLtpyCT
MLtpyCT
MLtpbCT
MLtpbCT
Mc
)
33
38
43
49
53
55
57
2.81
2.98
3.19
3.30
3.33
3.37
3.40
442
416
388
375
372
368
364
0.043
0.047
0.28
0.018
0.11
Mc
0.019
0.064
ILtpb
LtpbLtpyCT
a Computed at the TDDFT/LANL2DZ level of theory. b The actual percent contribution = (configuration coefficient)2 ꢁ 2 ꢁ 100%.
Figure 6. Deconvolution of the NIR spectra of 33+ generated by adding
1 equiv of cerium ammonium nitrate in acetonitrile. See text for details.
Irregular noises at 6000 cmꢀ1 were deleted intentionally before
deconvolution.
the best of our knowledge, in the case of polyazine ruthenium
complexes at room temperature. This is because the spinꢀorbit
coupling effect of ruthenium is not as heavy as that of osmium.
Taking into account the unusual observation in the electroche-
mical analysis of 32+ and the TDDFT prediction of the presence
of a low-energy transition at 718 nm from the HOMO f LUMO
excitation (f = 0.075), we assign the observed absorption at 805 nm
mainly to the CT transition from the RuꢀtpbꢀRu component to
the pyridine rings of tpb ligand.
Figure 5. Absorption spectral changes of bisruthenium complex 32+ in
acetonitrile upon one-electron (a) and two-electron (b) oxidation by
adding different equivalents of cerium ammonium nitrate (up to 2
equiv) while keeping the concentration of 32+ constant. The irregular
peaks at 1700 nm are due to artifacts.
NIR Transition Analysis of Mixed-Valence Species. As is
clear from the above discussion, the bridging ligand in complex
32+ is redox noninnocent. Thus, one-electron-oxidized inter-
mediate 33+ could not be described as a true MV system with
defined metal oxidation state. Nevertheless, the comproportio-
nation constant Kc could be determined to be 1.94 ꢁ 107 by the
equation Kc = 10ΔE (mV)/59 for a room-temperature case,2 where
ΔE is the potential splitting observed in Figure 1b (430 mV). The
substantially large Kc value indicates the high thermodynamic
In comparison, the IL and MLCT transitions of 32+ shift
bathochromically and their absorptivity increases considerably.
However, the most striking absorption feature of 32+ is the
emergence of a separate new band at 805 nm. This kind of spec-
trum is reminiscent of absorption of polyazine osmium complexes,
which often display 3MLCT transitions at a lower energy region
in addition to 1MLCT transitions in the visible region. However,
3MLCT electronic transitions have never been documented, to
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Table 3. Parameters for the Deconvoluted NIR Transitions of 33+
λmax/nm
νmax/cmꢀ1
εmax (Mꢀ1 cmꢀ1
)
Δν1/2(exp) (cmꢀ1
)
Δν1/2(theor)a (cmꢀ1
)
Γb
Habc (cmꢀ1
)
860
11624
9884
8717
7120
5100
6800
16500
3950
1560
1160
1160
1850
5181
4778
4487
4055
0.70
0.76
0.74
0.54
5812
4942
4358
3560
1012
1147
1405
a The theoretical Δν1/2 value equals (2310νmax
)
1/2. b Γ = 1 ꢀ Δν1/2exp/Δν1/2theo. c Hab = 1/2 νmax for a Robin and Day class III system.
Figure 7. Isodensity plots of selected HOSO and LUSO orbitals for complex 33+. All orbitals have been computed at an isovalue of 0.02.
these bands are remarkably narrow, with the observed full width
at half-height (Δν1/2) of 1560, 1160, 1160, and 1850 cmꢀ1
Table 4. Calculated Spin Density Distribution of Complex
3+ on the Level of B3LYP/LanL2DZ/6-31G*a
,
3
respectively. The theoretical Δν1/2 values of these bands were
determined to be 5181, 4778, 4487, and 4055 cmꢀ1, respectively,
according to Hush’s expression (Δν1/2theor = (2310νmax)
1/2).19
It is clear that the experimental values of Δν1/2 are much narrower
than the theoretical ones. The Γ parameters, introduced by Creutz,
Sutin, and co-workers,20 of these bands are all bigger than 0.5
(0.70, 0.76, 0.74, and 0.54, respectively), as determined by Γ =
1 ꢀ Δν1/2exp/Δν1/2theor. On the basis of these facts and taking
into account the peculiar results observed in the electrochemical,
spectroscopic, and DFT/TDDFT analysis, we conclude that 33+
is a Robin and Day class III system. The electronic coupling
parameter Hab is thus calculated to be 5812, 4942, 4358, and
3560 cmꢀ1, respectively, for the above four observed NIR bands,
according to the equation Hab = 1/2νmax for a Robin and Day
class III system. It should be noted here that the observed NIR
bands for a class III system are more correctly described as charge-
resonance bands rather than intervalence charge-transfer (IVCT)
transitions.
atom
spin density
atom
spin density
Ru1
Ru2
C1
0.326
0.326
0.050
0.043
C3
C4
C5
C6
0.043
0.050
0.043
0.043
C2
a Spin density is determined by the difference of the Mulliken charges of
α and β electrons (α ꢀ β).
stability of the in-situ-generated one-electron-oxidized MV inter-
mediate. This allows us to precisely titrate 32+ with 1 or 2 equiv of
oxidant, cerium ammonium nitrate (CAN), and monitor corre-
sponding vis/NIR absorption spectral changes (Figure 5). When
a solution of 32+ in acetonitrile was gradually treated with 0.25ꢀ1
equiv of CAN, both charge-transfer transitions at 588 and
805 nm decrease continually and substantially. At the same time,
the emergence of multiple overlapping bands between 800 and
2000 nm is evident. When the amount of CAN was gradually
increased to 2 equiv, two new peaks at 470 and 600 nm appear,
meanwhile all transitions in the near-infrared (NIR) region
decrease gradually until they disappear completely. The former
two peaks could be assigned to the charge-transfer transitions
from the tpy and the tpb ligand to the oxidized RuꢀtpbꢀRu
motif, and the multiple NIR peaks are associated with the MV
system 33+. By assuming Gaussian shapes, the NIR transitions
could be at least deconvoluted into four distinct bands at 860,
1012, 1147, and 1405 nm (Figure 6). Corresponding parameters
and analysis of these NIR bands are collected in Table 3. All of
Observation of multiple NIR bands for MV system 33+ deserves
further comment. The presence of multiple IVCT bands in the
spectrum of a MV system is not an uncommon occurrence, though
they are not routinely observed. For instance, a mixed-valent
diaza[2,2]ferrocenophane compound exhibits two IVCT bands
associated with two different stable conformers present in the
system.21 A number of bisosmium MV complexes have been re-
ported to display up to five IVCT bands due to the strong spinꢀ
orbit coupling effect of the osmium atom.22 However, as for
ruthenium complexes, only a few MV systems disclose the ob-
servation of multiple NIR transitions. For example, complexes
{[Cl3RuII(tppz)RuIIICl3]ꢀ}23 and {[Ru(NH3)5](bqd)[Ru-
(NH3)5]5+},24 where tppz is 2,3,5,6-tetrakis(2-pyridyl)pyrazine
and bqd is p-benzoquinone diimine, have been reported to show
multiple IVCT transitions. Complex 33+ represents another new
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Table 5. Calculated Low-Energy Excitations of Complex 33+
method
Sn
E/eV
E/nm
f
dominant transitions (configuration coefficient)
B3LYP/LanL2DZ/6-31G*/vacuum
2
5
6
7
33
2
4
5
6
2
4
5
6
0.87
1.07
1.65
1.91
2.69
0.89
0.99
1.05
1.66
0.88
0.98
1.05
1.71
1418
1157
749
0.0075
0.2314
0.0023
0.054
β-HOSO-2 f β-LUSO (0.95)
β-HOSO f β-LUSO (0.95)
β-HOSO-5 f β-LUSO (0.97)
α-HOSO f α-LUSO (0.98)
α-HOSO-1 f α-LUSO (0.62)
β-HOSO-2 f β-LUSO (0.93)
β-HOSO-3 f β-LUSO (0.99)
β-HOSO f β-LUSO (0.95)
β-HOSO-5 f β-LUSO (0.97)
β-HOSO-2 f β-LUSO (0.91)
β-HOSO-3 f β-LUSO (0.99)
β-HOSO f β-LUSO (0.94)
β-HOSO-5 f β-LUSO (0.97)
647
460
0.185
B3LYP/LanL2DZ/6-31G*/ CPCM(CH3CN)
B3LYP/SDD/6-31G*/ CPCM(CH3CN)
1393
1245
1179
748
0.0152
0.0012
0.2905
0.0032
0.0168
0.0013
0.2931
0.0031
1411
1269
1178
724
MV system with multiple discernible NIR bands with high
intensity. We reason that in the case of complex 33+ it is a result of
the strong orbital interaction of the biscyclometalating bridging
tpb ligand with the metal center and considerable participation of
tpb into the oxidation process.
Given the redox noninnocent nature of the bridging ligand, it
is somewhat ambiguous to analyze 33+ on the basis of the classical
Hush theory and the RobinꢀDay classification, which are best
applicable to systems with well-defined redox centers. Thus, we
performed DFT and TDDFT calculations of complex 33+ to
further elucidate the nature of its electronic structure and the
observed NIR transitions. DFT and TDDFT calculations have
proven useful in the interpretation of organometallic systems
with redox noninnocent ligands.15e,25 DFT calculations for com-
plex 33+ were performed at the UB3LYP level with the LanL2DZ
basis set for ruthenium and 6-31G* for other atoms in vacuum.
TDDFT calculations were carried out on the DFT-optimized struc-
ture at the same UB3LYP/LanL2DZ/6-31G* level. Two RuꢀC
bonds in the DFT-optimized structure of 33+ have identical
length (1.954 Å). Compared with complex 32+, the RuꢀC bond
of the one-electron-oxidized 33+ is shortened by 0.041 Å. How-
ever, all RuꢀN bonds are lengthened by 0.04ꢀ0.05 Å. Some
selected α- and β-spin frontier orbitals of 33+ are shown in
Figure 7. Most of them have very similar composition to the
frontier orbitals described for complex 32+ (Figure 3), albeit with
a small change in relative ordering. For example, the electronic
configurations of α-LUSO (lowest unoccupied spin orbital),
α-HOSO (highest occupied spin orbital), α-HOSO-1, β-LUSO,
β-HOSO, and β-HOSO-2 of MV complex 33+ are very similar to
LUMO, HOMO, HOMO-1, HOMO, HOMO-5, and HOMO-2
of complex 32+, respectively. The Mulliken spin density popula-
tion analysis of complex 33+ is summarized in Table 4. Two
ruthenium atoms have identical spin density (0.326 each), which
is in agreement with the RobinꢀDay class III assignment for 33+.
A significant portion of spin resides in the cyclometalating phenyl
ring (a total of 0.272), pointing to a strong electron delocalization
across the RuꢀdpbꢀRu array. This is also corroborated by EPR
analysis presented below.
Figure 8. EPR signal of 33+ at 77 K in acetonitrile. Spectrometer
frequency ν is 9.519 ꢁ 109 Hz.
associated with excitation of a β electron to β-LUSO from
β-HOSO-2 and β-HOSO levels, respectively. These transitions
could be interpreted as the charge-transfer transition from the
metal centers to the bridging phenyl ring. It is important to note
that β-HOSO-2 and β-HOSO levels have different metal orbital
configurations (dxz and dxy, respectively), which could be parti-
ally responsible for the observation of multiple NIR bands.
TDDFT calculations do not well predict the bands at 1012 and
860 nm. However, the band at 860 nm may have some LMCT
character, since a β-HOSO-5 f β-LUSO transition at 749 nm is
predicted by TDDFT results. In addition, the observed band at
524 nm could be assigned to a MLCT transition associated with
α-HOSO-1 f α-LUSO excitation. Since the above TDDFT
results did not predict the origin of the shoulder peak at 1012 nm,
we performed new DFT and TDDFT calculations with inclusion
of solvation effects in acetonitrile using the conductor-like
polarizable continuum model (CPCM).26 However, very similar
results were obtained as those calculated in vacuum (table 5). When
the SDD basis set27 with effective core potential was employed
for the ruthenium atom in combination CPCM (table 5), TDDFT
calculations still cannot predict the peak at 1012 nm. It seems
that TDDFT calculations could not fully agree with the experi-
mentally observed NIR transitions of complex 33+ and new
calculation methods are desirable in the future. Nevertheless,
TDDFT results predict well the major band at 1147 nm and
provide instructive information regarding the charge resonance
of complex 33+.
Table 5 presents the calculated low-energy excitations of 33+
with oscillator strength (f) larger than 0.001. The two lowest
energy transitions (S2 at 1418 nm and S5 at 1157 nm) are in good
agreement with the experimentally observed bands at 1405 and
1147 nm in terms of both energy and strength. They are mainly
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EPR Studies. The strong metalꢀmetal coupling in 33+ is also
supported by EPR studies. Complex 33+, obtained after one-
electron oxidation of 32+ by adding 1 equiv CAN, is EPR silent at
room temperature. However, it exhibits a rhombic EPR signal at
77 K typical for a low-spin RuIII species (Figure 8). The electron g
bands of [(tpy)Ru(tpb)Ru(tpy)]3+, which are completely absent
in [(tpy)Ru(tpb)Ru(tpy)]2+ and the two-electron oxidized species
[(tpy)Ru(tpb)Ru(tpy)]4+, would make it a promising candidate
as a NIR electrochromic material.33 These appealing electronic
properties of complexes [(tpy)Ru(tpb)Ru(tpy)]2+ and [(tpy)Ru-
(tpb)Ru(tpy)]3+ provide important guides for the design and
synthesis of new MV systems with potential applications in mole-
cular electronics, and construction of other MV complexes bridged
by biscyclometalating ligands is underway in this laboratory.
factors g1, g2, and g3 are 2.302, 2.156, and 1.962, respectively. The
2
isotropic g factor of Ægæ = 2.144, derived according to Ægæ = [(g1
+
g22 + g3 )/3]1/2. The total g anisotropy Δg = g1 ꢀ g3 = 0.34. The
isotropic g factor of 33+ is much lower than those for the
CruetzꢀTaube ion (2.298)28 and a bisruthenium RobinꢀDay
class III system bridged by 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine
(2.477).29 We also notice that a true metal-centered spin of a
cetecholatoruthenium(III) complex has a Ægæ value of 2.476 with
Δg = 0.833.30 The pronounced rhombicity of the EPR signal and
substantial low Ægæ and Δg values of complex 33+ points to a
significant participation of ligand oxidation. This is in agreement
with the spin density population analysis and experimental findings
observed in the electrochemical and spectroscopic studies.
2
’ EXPERIMENTAL SECTION
Spectroscopic Measurements. All optical ultravioletꢀvisible
(UVꢀvis) absorption spectra were obtained using a TU-1810DSPC
spectrometer of Beijing Purkinje General Instrument Co. Ltd. at room
temperature in denoted solvents, with a conventional 1.0 cm quartz cell.
UVꢀvisꢀNIR spectra were recorded using a JASCO V-570 UV/vis/
NIR spectrophotometer.
Electrochemical Measurements. All cyclic voltammetry (CV)
measurements were taken using a CHI620D potentiostat. All measure-
ments were carried out in 0.1 M Bu4NClO4/acetonitrile at a scan rate of
100 mV/s with a Ag/AgCl reference electrode. The working electrode is
a glassy carbon electrode, and a platinum coil is used as the counter
electrode.
Computational Methods. DFT calculations are carried out using
the B3LYP exchange correlation functional34 and implemented in the
Gaussian 03 program package.35 The electronic structures of the com-
plexes were determined using a general basis set with the Los Alamos
effective core potential LanL2DZ or the SDD basis set for ruthenium
and 6-31G* for other atoms in vacuum.36 In case the solvation effects are
included, the conductor-like polarizable continuum model (CPCM)
with solvent = acetonitrile and united-atom KohnꢀSham (UAKS) radii
were employed.26
’ CONCLUSION
To summarize, we present in this contribution a new biscy-
clometalated bisruthenium complex [(tpy)Ru(tpb)Ru(tpy)]2+
and its one-electron-oxidized species [(tpy)Ru(tpb)Ru(tpy)]3+
bridged by 1,2,4,5-tetra(2-pyridyl)benzene. A combination of
electrochemical, spectroscopic, DFT/TDDFT calculation, and
EPR studies implies that tpb behaves as a redox noninnocent
bridging ligand when coupled to two ruthenium atoms via
covalent RuꢀC bonds and the MV complex [(tpy)Ru(tpb)-
Ru(tpy)]3+ is determined to be a RobinꢀDay class III system
with full charge delocalization across the RuꢀtpbꢀRu motif.
Complex [(tpy)Ru(tpb)Ru(tpy)]2+ exhibits very peculiar and
unique electronic properties that have not been commonly
observed in most transition metal polyazine complexes. The
electrochemical studies show that the first two oxidation pro-
cesses take place at a substantially lower potential (+0.12 and
+0.55 V vs Ag/AgCl) than common cyclometalated ruthenium
complexes and with a remarkably large comproportionation
constant (1.94 ꢁ 107). These features would allow us to readily
access different oxidation states of [(tpy)Ru(tpb)Ru(tpy)]2+
and facilitate realization of redox-controlled single-molecule
conductance.31 In addition, low oxidation potential and rever-
sible redox process would make such compounds promising hole
transporting materials for optoelectronic applications.32 In terms
of electronic absorption transitions, complex [(tpy)Ru(tpb)-
Ru(tpy)]2+ distinguishes itself from other ruthenium polyzaine
complexes as well. In addition to the routinely observed MLCT
transitions in the visible region for those compounds, it displays a
separate and distinct band on the lower energy side (805 nm)
with appreciable absorptivity (ε = 9000 Mꢀ1 cmꢀ1). This band is
assigned to the charge transition from the RuꢀtpbꢀRu motif to
the pyridine rings of tpb. Finally, observation of multiple NIR
bands in the MV system [(tpy)Ru(tpb)Ru(tpy)]3+ is worthy of
particular attention. This represents one of the few examples of
ruthenium MV complexes that display multiple NIR transitions.
The strong orbital interaction of the biscyclometalating bridging
ligand with the metal center and the significant participation of
tpb into the oxidation processes is thought to be responsible
for the appearance of multiple NIR transitions. According to
TDDFT calculations, these bands are predominantly associated
with the charge-transfer transition from the metal centers to the
biscyclometalating phenyl ring. The high intensity of the NIR
Synthesis. NMR spectra were recorded in the designated solvent on
a Bruker Avance 400 MHz spectrometer. Spectra are reported in
1
ppm values from residual protons of deuterated solvent for H NMR
(δ 7.26 ppm for CDCl3 and 1.92 ppm for CD3CN) and 13C NMR
(δ 77.00 ppm for CDCl3). MS data were obtained with a Bruker Daltonics
Inc. ApexII FT-ICR or Autoflex III MALDI-TOF mass spectrometer.
The matrix for MALDI-TOF measurement is α-cyano-4-hydroxycin-
namic acid (CCA). Microanalysis was carried out using a Flash EA 1112
or Carlo Erba 1106 analyzer at the Institute of Chemistry, CAS.
Synthesis of 1. To a solution of 1,2,4,5-tetrabromobenzene
(0.3 mmol, 118 mg) and 2-(tributylstannyl)pyridine (3.0 mmol, 1.1 g)
in dry toluene (20 mL) were added PdCl2(PPh3)2 (0.08 mmol, 56.1 mg)
and anhydrous LiCl (6 mmol, 254 mg) under a N2 atmosphere. The
mixture was bubbled with nitrogen for 10 min before the vial was capped
and heated at 150 ꢀC for 48 h. The solvent was removed under reduced
pressure, and the residue was subject to flash column chromatography
on silica gel (eluent, CH2Cl2/ethyl acetate/NH4OH 50/100/0.05) to
afford 41 mg of 1 as a white solid. The yield is 34%. 1H NMR (400 MHz,
CDCl3): δ 7.09 (dd, J = 11.4, 7.8 Hz, 4H), 7.13ꢀ7.15 (m, 4H),
7.45ꢀ7.49 (m, 4H), 8.06 (s, 2H), 8.60 (t, J = 4.2 Hz, 4H). 13C NMR
(100 MHz, CDCl3): δ 122.3, 125.8, 133.4, 136.2, 140.4, 150.1, 159.2.
ESI-MS (m/z): 387.3 for [M + H]+. EI-HRMS (m/z): calcd 387.1610
for C26H19N4 ([M + H]+), found 387.1607.
Synthesis of [2](PF6). To 50 mL of dry acetone were added
Ru(tpy)Cl3 (0.1 mmol, 44 mg) and AgOTf (0.3 mmol, 78 mg). The
mixture was then refluxed under a N2 atmosphere for 3 h. The mixture was
filtered to afford a purple black solution, and the filtrate was concentrated to
dryness. To the residue were added 1,2,4,5-tetra(pyridin-2-yl)benzene, 1
(0.1 mmol, 38.6 mg), 20 mL of DMF, and 20 mL of t-BuOH. The system
was heated under microwave irradiation (375 W) for 30 min before cooling
to room temperature. The solvent was removed under reduced pressure,
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ARTICLE
and the residue was dissolved in 3 mL of methanol. After adding an excess of
KPF6, the resulting precipitate was collected by filtering and washing with
water and Et2O. The crude solid was purified through flash column
chromatography on silica gel followed by anion exchange with KPF6 to
give 24 mg of monoruthenium complex [2](PF6) as a black solid
(c) D’Alessandro, D. M.; Keene, F. R. Chem. Soc. Rev. 2006, 35, 424. (d)
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101, 2655. (f) Kaim, W.; Klein, A.; Gl€ockle, M. Acc. Chem. Res. 2000, 33,
755.
1
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Bizzarri, C.; Galloni, P.; Floris, B.; Nowik, I.; Herber, R. H.; Marrani,
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Fox, M. A.; Robert, R. L.; Baines, T. E.; Guennic, B. L.; Halet, J.-F.; Hartl,
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Soc. 2008, 130, 3566. (c) Bonvoisin, J.; Fabre, M. J. Am. Chem. Soc. 2007,
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F.; Ruiz, J.; Astruc, D. Angew. Chem., Int. Ed. 2009, 48, 3141. (k) Gao, L.-B.;
Kan, J.; Fan, Y.; Zhang, L.-Y.; Liu, S.-H.; Chen, Z.-N. Inorg. Chem. 2007,
46, 5651.
(eluent, CH3CN/H2O/aq.KNO3 100/5/0.03). The yield is 28%. H
NMR (400 MHz, CDCl3): δ 6.57 (t, J = 6.4 Hz, 2H), 6.84 (d, J = 8.3 Hz,
2H), 7.06 (t, J = 5.6 Hz, 4H), 7.20 (t, J = 11.2 Hz, 3H), 7.30 (d, J = 5.0 Hz,
2H), 7.51 (t, J = 6.4 Hz, 2H), 7.72 (t, J = 7.6 Hz, 2H), 7.77 (d, J = 7.7 Hz,
2H), 7.97 (t, J = 7.6 Hz, 2H), 8.27 (t, J = 7.9 Hz, 1H), 8.44 (d, J = 8.0 Hz,
2H), 8.77 (t, J = 8.5 Hz, 2H). ESI-MS (m/z): 720.14 for [M ꢀ PF6]+.
Anal. Calcd for C41H28F6N7PRu: C, 56.95; H, 3.26; N, 11.34. Found:
C, 56.55; H, 3.73; N, 11.75.
Synthesis of [3](PF6)2. To 50 mL of dry acetone were added
Ru(tpy)Cl3 (0.1 mmol, 44 mg) and AgOTf (0.3 mmol, 78 mg). The
mixture was then refluxed for 3 h. The mixture was filtered to afford a
purple-black solution, and the filtrate was concentrated to dryness. To
the residue were added 1,2,4,5-tetra(pyridin-2-yl)benzene, 1 (0.05 mmol,
19.3 mg), 20 mL of DMF, and 20 mL of t-BuOH. The mixture was
bubbled with nitrogen for 10 min before the vial was capped and heated
at 130 ꢀC for 48 h. After cooling to room temperature, the solvent was
removed under reduced pressure. The residue was then dissolved in
3 mL of methanol. After adding an excess of KPF6, the resulting pre-
cipitate was collected by filtering and washing with water and Et2O. The
crude solid was purified through flash column chromatography on silica
gel to give 11 mg of bisruthenium complex [3](PF6)2 as a black solid.
(4) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967, 8, 357.
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Sarkar, B.; Braunstein, P. Chem. Commun. 2009, 4387.
1
(eluent, CH3CN/H2O/aq.KNO3 100/10/0.1). The yield is 17%. H
(6) (a) Newell, M.; Thomas, J. A. Dalton Trans. 2006, 705. (b)
Halpin, Y.; Cleary, L.; Cassidy, L.; Horne, S.; Dini, D.; Browne, W. R.;
Vos, J. G. Dalton Trans. 2009, 4146. (c) Slater, J. W.; D’Alessandro,
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Sarkar, B.; Kaim, W.; Klein, A.; Schwederski, B.; Fiedler, J.; Duboc-Toia, C.;
Lahir, G. K. Inorg. Chem. 2003, 42, 6172. (f) D’Alessandro, D. M.; Dinolfo,
P. H.; Davis, M. S.; Hupp, J. T.; Keene, F. R. Inorg. Chem. 2006, 45, 3261.
(7) (a) Park, J.; Pasupathy, A. N.; Goldsmith, J. I.; Chang, C.; Yaish,
Y.; Petta, J. R.; Rinkoski, M.; Sethna, J. P.; Abru~na, H. D.; McEuen, P. L.;
Ralph, D. C. Nature 2002, 417, 722. (b) Tang, J.; Wang, Y.; Klare, J. E.;
Tulevski, G. S.; Wind, S. J.; Nuckolls, C. Angew. Chem., Int. Ed. 2007,
46, 3892. (c) Seo, K.; Konchenko, A. V.; Lee, J.; Bang, G. S.; Lee, H.
J. Am. Chem. Soc. 2008, 130, 2553. (d) Flores-Torres, S.; Hutchison,
G. R.; Stoltzberg, L. J.; Abru~na, H. D. J. Am. Chem. Soc. 2006, 128, 1513.
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J. C.; Abrun~a, H. D. Inorg. Chem. 2009, 48, 991.
NMR (400 MHz, CDCl3): δ 6.64 (s, 4H), 7.14 (s, 4H), 7.19 (d, J =
5.3 Hz, 4H), 7.51 (t, J = 7.7 Hz, 4H), 7.56 (d, J = 4.5 Hz, 4H), 7.79 (t, J =
7.6 Hz, 4H), 8.32 (t, J = 7.8 Hz, 2H), 8.41 (s, 4H), 8.52 (d, J = 8.0 Hz,
4H), 8.84 (d, J = 7.8 Hz, 4H). MALDI-MS (m/z): 1053.1 for [M ꢀ
2PF6ꢀH]+. Anal. Calcd for C56H38F12N10P2Ru2 2H2O: C, 48.77;
3
H, 3.07; N, 10.16. Found: C, 48.64; H, 2.90; N, 10.34.
’ ASSOCIATED CONTENT
Supporting Information. CV profiles of 2+ and 32+ with
S
b
a wider potential window, selected frontier molecular orbital
graphics of 2+ and 32+, full list of authors of ref 35, and NMR and
MS spectra of new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
(8) (a) Zhong, Y.-W.; Wu, S.-H.; Burkhardt, S. E.; Yao, C.-J.; Abru~na,
H. D. Inorg. Chem. 2011, 50, 517. (b) Yao, C.-J.; Sui, L.-Z.; Xie, H.-Y.;
Xiao, W.-J.; Zhong, Y.-W.; Yao, J. Inorg. Chem. 2010, 49, 8347. (c) Wu,
S.-H.; Burkhardt, S. E.; Yao, J.; Zhong, Y.-W.; Abru~na, H. D. Inorg. Chem.
2011, 50, 3959. (d) Wang, L.; Yang, W.-W.; Zheng, R.-H.; Shi, Q.;
Zhong, Y.-W.; Yao, J. Inorg. Chem. 2011, 50, 7074.
Corresponding Author
*Email: zhongyuwu@iccas.ac.cn (Y.-W.Z.); jnyao@iccas.ac.cn
(J.Y.)
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Sortais, J.-B.; Barloy, L.; Pfeffer, M. Eur. J. Inorg. Chem. 2009, 817. (c)
J€ager, M.; Smeigh, A.; Lombeck, F.; G€orls, H.; Collin, J.-P.; Sauvage,
J.-P.; Hammarstr€om, L.; Johannsson, O. Inorg. Chem. 2010, 49, 374. (d)
Wadman, S. H.; Lutz, M.; Tooke, D. M.; Spek, A. L.; Hartl, F.; Havenith,
R. W. A.; van Klink, G. P. M.; van Koten, G. Inorg. Chem. 2009, 48, 1887.
(e) Duati, M.; Tasca, S.; Lynch, F. C.; Bohlen, H.; Vos, J. G.; Stagni, S.;
Ward, M. D. Inorg. Chem. 2003, 42, 8377. (f) Yang, W.-W.; Wang, L.;
Zhong, Y.-W.; Yao, J. Organometallics 2011, 30, 2236.
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S.; Collin, J.-P.; James, S.; Sauvage, J.-P. J. Am. Chem. Soc. 1998, 120, 3717.
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Steenwinkel, P.; Grove, D. M.; Veldman, N.; Spek, A. L.; van Koten, G.
Organometallics 1998, 17, 5647.
’ ACKNOWLEDGMENT
We thank the National Natural Science Foundation of China
(No. 21002104), National Basic Research 973 program of China
(Nos. 2011CB932301 and 2011CB808402), and Institute of
Chemistry, Chinese Academy of Sciences (“100 Talent” Program)
for funding support.
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