Biscyclometalated ruthenium complexes bridged by 3,3′,5,5′- tetrakis(N-methylbenzimidazol-2-yl)biphenyl: Synthesis and spectroscopic and electronic coupling studies

Article abstract of DOI:10.1021/ic300054j

A series of biscyclometalated ruthenium complexes bridged by the title ligand were prepared by either an oxidative dimerization of corresponding monometallic complexes or treatment of the bridging ligand with Ru(L)Cl ₃ (L = capping ligand). The electronic properties of these complexes were examined by electrochemical and spectroscopic analysis and DFT/TDDFT calculations. The degree of metal-metal electronic coupling of these complexes was estimated on the basis of intervalence charge-transfer transition analysis of corresponding mixed-valent complexes. These studies indicated that the electronic coupling was strongly dependent on the electronic nature of the terminal ligands. A hole-transfer superexchange mechanism was used to understand the underlying electron-transfer processes.

Full text of DOI:10.1021/ic300054j

Article
pubs.acs.org/IC
Biscyclometalated Ruthenium Complexes Bridged by
3,3′,5,5′-Tetrakis(N-methylbenzimidazol-2-yl)biphenyl: Synthesis
and Spectroscopic and Electronic Coupling Studies
Jiang-Yang Shao, Wen-Wen Yang, Jiannian Yao, and Yu-Wu Zhong*
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
ABSTRACT: A series of biscyclometalated ruthenium com-
plexes bridged by the title ligand were prepared by either an
oxidative dimerization of corresponding monometallic com-
plexes or treatment of the bridging ligand with Ru(L)Cl3 (L =
capping ligand). The electronic properties of these complexes
were examined by electrochemical and spectroscopic analysis
and DFT/TDDFT calculations. The degree of metal−metal
electronic coupling of these complexes was estimated on the
basis of intervalence charge-transfer transition analysis of corre-
sponding mixed-valent complexes. These studies indicated that
the electronic coupling was strongly dependent on the electronic nature of the terminal ligands. A hole-transfer superexchange
mechanism was used to understand the underlying electron-transfer processes.
triazole,14 dimethylamino,15 or diphenylphosphine16 groups as
auxiliary ligands resulted in different degrees of electron delocaliz-
INTRODUCTION
■
Since disclosure of the Creutz−Taube ion,1 {[Ru(NH3)5]-
(pyrazine)[Ru(NH3)5]5+}, tremendous efforts have been devoted
ation in these complexes. We report in this article the synthesis and
to the studies of dimetallic mixed-valent (MV) complexes.2 The
electronic property studies of a series biscyclometalated ruthenium
complexes bridged by 3,3′,5,5′-tetrakis(N-methylbenzimidazol-2-
yl)biphenyl, where N-methylbenzimidazole moieties are used as
the auxiliary ligands to support metal−metal electronic communi-
cation through the biphenyl backbone. It should be noted that MV
systems with benzimidazole-containing noncyclometalated ruthe-
nium centers have been reported previously.17 However, biscyclo-
metalated ruthenium complexes supported by benzimidazole ligands
have not been documented, to the best of our knowledge.
degree of electronic coupling between individual metal centers de-
pends on a number of key factors, including the distance between
metal centers, the coordination environments of metal components,
and the ability of the bridging ligand to delocalize the electronic
charge.3 According to the degree of electronic coupling (from weak
to moderate and very strong), MV systems can be distinguished as
Robin and Day class I−III categories.4 MV complexes can be taken
as simple model systems for studying electron-transfer processes
between organic bridge-connected electronic donors and acceptors.
These studies provide useful information relevant to molecular
electronics and many naturally occurring photoinduced electron/
energy-transfer processes.3
One of the interesting redox centers for MV systems is a
ruthenium metal supported by a covalent bond with carbon,5
nitrogen,6 or oxygen anions7 (so-called cyclometalated ruthe-
niums).8 As a result of the electron-rich nature of these anionic
ligands, corresponding RuII/III processes of these complexes take
place at a much lower potential than those surrounded by all dative
bonds. However, the exact redox potential of the RuII/III process is
dependent on the auxiliary ligands. For instance, cyclometalated
ruthenium complexes with pyridine,9 triazole,10 or benzimidazole11
ligands exhibit distinctly different metal-based redox events. It has
been reported that MV systems with cyclometalated rutheniums
displayed enhanced electronic coupling versus their noncyclometa-
lated counterparts.12 However, the degree of metal−metal electronic
coupling in MV systems with cyclometalated rutheniums is also
strongly dependent on the auxiliary ligands. The use of pyridine,13
RESULTS AND DISCUSSION
■
Synthesis. We have studied three diruthenium complexes
(22+, 42+, and 72+; Scheme 1) bridged by the title ligand in this
paper. Experimental details and characterization data for synthesis of
these complexes are provided in the Experimental Section. Complex
22+ was obtained via the oxidative coupling of the monometallic
complex 1+ in the presence of AgBF4.12 However, attempts to pre-
pare complex 42+ capped by the electron-donating ligands bis-
(N-methylbenzimidazolyl)pyridine (Mebip) from 3+ failed under
the same reaction conditions. The only isolated product is the one-
electron-oxidized complex 32+, which has been reported in a
previous paper by us.11 The significantly lower oxidation potential of
3+ compared to 1+ may account for this difference (+0.26 vs +0.48
V vs Ag/AgCl).11 We then turned our attention to the bridging
Received: January 10, 2012
Published: March 9, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of the Compounds Studied
ligand 5, which was obtained from the reaction of biphenyl-3,3′,5,5′-
tetracarboxylic acid with N-methyl-1,2-phenylenediamine in good
yield (70%). The reaction of 2 equiv of (Mebip)RuCl3 with 5 in the
presence of AgOTf, followed by a subsequent counteranion
exchange, afforded the desired cyclometalated diruthenium complex
42+ in 27% yield. It should be noted that the diruthenium complex
22+ could also be prepared from the reaction of 5 with (tpy)RuCl3
(tpy = 2,2′:6′,2″-terpyridine) in moderate yield, which actually is a
better alternative to the synthesis of 22+ than the oxidative coupling
method in our hand. Finally, a diruthenium complex 72+ capped
with the electron-withdrawing ligands trimethyl-4,4′,4″-tricarbox-
ylate-2,2′:6′,2″-terpyridine (Me3tcbtpy) was prepared using the same
method.
coupling between metal centers of a series of structurally related
dimetallic systems. A single metal-based two-electron redox wave
suggests that the electronic coupling between two metals is weak.
On the other hand, two separate sequential one-electron redox
couples from the metals point to an efficient charge delocalization
between them. However, electrochemical data are largely dependent
on the measurement conditions, particularly the solvent and
supporting electrolyte used.18 This principle should be taken with
great care.
The anodic CV and DPV profiles of 22+, 42+, and 72+ in N,N-
dimethylformamide (DMF) are shown in Figure 1. Although these
complexes have a common bridging ligand, their electrochemical
properties are distinctly different. The capping ligands of different
electronic properties make a big difference. Complex 22+ exhibits
two sequential redox couples at +0.58 and +0.66 V vs Ag/AgCl with
a potential difference (ΔE) of 76 mV between two half-wave
Electrochemical Studies. Electrochemical techniques, such
as cyclic voltammetric (CV) and differential pulse voltammetric
(DPV) analysis, are often used to qualitatively study the electronic
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Density Functional Theory (DFT) Calculations. DFT
calculations were performed on the above complexes at the
level of B3LYP/LANL2DZ/6-31G*/vacuum to study their
electronic structures (see the Experimental Section for details).
Selected frontier orbital structures with electron density
distributions are shown in Figures S4−S7 in the Supporting
Information and 2. The lowest unoccupied molecular orbitals
Figure 2. Isodensity plots of LUMO and HOMO for complex 72+.
Calculation method: B3LYP/LANL2DZ/6-31G*/vacuum isovalue = 0.02.
(LUMOs) of all complexes are dominated by corresponding
capping ligands. The highest occupied molecular orbitals
(HOMOs) of all complexes have contributions from both the
metal component and the center biphenyl backbone. A com-
parison of the Mulliken population of the HOMO composi-
tions of three complexes is delineated in Table 1. For instance,
Figure 1. CV and DPV profiles of (a) 22+, (b) 42+, and (c) 72+ in DMF
containing 0.1 M Bu4NClO4 at a scan rate of 100 mV/s. DPV was
measured with a step potential of 5 mV and an amplitude of 50 mV.
The working electrode was glassy carbon, the counter electrode was a
platinum wire, and the reference electrode was Ag/AgCl in saturated
aqueous NaCl. For CV profiles with wider potential windows, see
Figures S1−S3 in the Supporting Information.
n
Table 1. Calculated Mulliken Population of HOMOs of the
Complexes Studied
potentials. Complex 72+ capped with Me3tctpy shows similar two
waves at more positive potentials (+0.74 and +0.83 V) and with a
slightly larger potential separation (ΔE = 88 mV). The compro-
portionation constants Kc for 22+ and 72+ were determined to be 20
and 31, respectively, according to the equation Kc = 10ΔE(mV)/59 for
a room temperature case. However, complex 42+ with the electron-
donating Mebip ligands only displays one reversible redox couple at
+0.40 V, which should be associated with two inseparable one-
electron waves. All of these peaks are attributed to the cyclometala-
ted RuII/III redox process mixing with some portion of ligand
oxidation.10−16 Complexes 22+ and 42+ exhibit a cathodic wave at
−1.40 and −1.46 V vs Ag/AgCl, respectively (Figures S1 and S2 in
the Supporting Information). They are ascribed to the reduction of
corresponding capping ligands. However, reductions of the capping
ligands of 72+ occur at much less negative potentials (−0.97 and
−1.09 V vs Ag/AgCl; Figure S3 in the Supporting Information)
because of their electron-withdrawing nature. The redox couples
at the more negative potential in Figure S3 in the Supporting
Information could be attributed to the reduction of the bridging
ligand. These assignments are supported by theoretical calculations
presented below.
Ru1
phenyl1
phenyl2
Ru2
22+
42+
72+
0.22
0.22
0.21
0.19
0.19
0.19
0.19
0.17
0.19
0.22
0.22
0.21
complex 22+ has populations of 0.22 and 0.19 for each Ru atom
and bridging phenyl ring, respectively. Complexes 42+ and 72+ have
similar HOMO compositions. This suggests that the bridging
biphenyl unit accounts for a considerable HOMO composition for
these complexes, and the electrochemical waves shown in Figure 1
are a result of oxidation of both ruthenium and the biphenyl back-
bone. This feature has been previously found for many biscy-
clometalated ruthenium complexes.5,12,13
The calculated energy-level alignment of the above complexes is
shown in Figure 3. Compared to that of 22+, the HOMO of 42+ is
destabilized and the HOMO of 72+ is stabilized. This correlates well
with the electrochemical behavior of these complexes (Figure 1). In
comparison, complex 72+ has the smallest energy gap (2.50 eV).
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Figure 3. Energy-level alignment diagram of the complexes calculated.
Because of the electron-withdrawing nature of the capping ligand of
72+, the degree of LUMO stabilization is larger than that of the
HOMO, which results in a decrease of the energy gap of 72+
compared to that of 22+ or 42+. This is also reflected in their absorp-
tion spectra presented below.
Electronic Absorption Spectra and Time-Dependent
DFT (TDDFT) Calculations. The electronic absorption spectra
of the dimetallic complexes 22+, 42+, and 72+ are shown in Figure 4,
together with those of the monometallic complexes 1+ and 3+ for
comparison. Absorptions in the UV and visible regions are ascribed
to the intraligand (IL) π−π* transition of both cyclometalating and
auxiliary ligands and metal-to-ligand charge-transfer (MLCT) transi-
tions, respectively. The shapes and energies of the absorptions of
22+ and 42+ resemble those of 1+ and 3+, respectively, albeit with an
increase in the molar absorptivities and a slight red shift of the
MLCT transitions. For instance, the main MLCT peaks of 22+ and
42+ center at 520 and 528 nm, respectively, which are slightly
bathochromically shifted compared to those of 1+ and 3+ (500 and
512 nm, respectively). The MLCT transitions of 1+ and 3+ have
been previously analyzed in great detail with the aide of TDDFT
calculations.11 We believe that the same assignments should also be
applicable to 22+ and 42+. For complex 22+, the band at 520 nm is
attributed to both the bridging-ligand- and tpy-targeted MLCT
transitions, and the former plays a more important role. The peak at
395 nm is mainly associated with the tpy-targeted MLCT transi-
tions. For complex 42+, the visible band between 400 and 600 nm is
associated with both the bridging-ligand- and Mebip-targeted
MLCT transitions. The absorption features of 72+ are somewhat
different from others. We performed TDDFT calculations on the
DFT-optimized structure of 72+ at the level of B3LYP/LANL2DZ/
6-31G*/CPCM/CH3CN theory (see the Experimental Section) to
help assign its absorption (Table S1 in the Supporting Information).
The involved molecular orbital diagrams are given in Figures S6 and
S7 in the Supporting Information. As can be found from Figure 4b,
the strengths of the predicted excitations agree well with the
observed absorption spectrum of 72+. However, the energies of the
predicted excitations are blue-shifted by about 30−40 nm. The
broad and shallow absorption at 720 nm is mainly associated with
the HOMO−2 → LUMO and HOMO−1 → LUMO+1 excita-
tions (S2 in Table S1, Supporting Information). They can be inter-
preted as the capping-ligand-targeted MLCT transitions. Absorption
bands between 510 and 660 nm have similar MLCT character from
HOMO−5 → LUMO+2, HOMO−4 → LUMO+3, HOMO−
1 → LUMO+3, and HOMO−2 → LUMO+2 excitations (S12 and
S15 in Table S1, Supporting Information). As far as the intense
absorption band between 380 and 510 nm is concerned, in addition
Figure 4. (a) Electronic absorption spectra of 1+ (black line), 22+ (red
line), and 72+ (blue line) in acetonitrile. (b) TDDFT-predicted vertical
excitations of 72+. (c) Electronic absorption spectra of 3+ (black line)
and 42+ (red line) in acetonitrile.
to the capping ligand, the bridging-ligand-targeted MLCT tran-
sitions make a big contribution as well (LUMO+8 and LUMO+9).
The red shift of the observed MLCT transitions of 72+ vs 22+ or 42+
is consistent with its reduced energy gap, as inferred from the above
DFT calculations.
Near-Infrared (NIR) Transition Analysis of MV Species.
MV systems often display a characteristic intervalence charge-
transfer (IVCT) band in the NIR region, which is not observable in
corresponding homovalent complexes. The degree of electronic
coupling of MV systems can be estimated on the basis of the IVCT
band analysis. The visible (vis)/NIR absorption spectral changes of
complexes 22+, 42+, and 72+ upon oxidative titration with cerium
ammonium nitrate (CAN) are shown in Figure 5. Table 2 sum-
marizes the parameters obtained upon IVCT analysis. When a solu-
tion of 22+ in acetonitrile was gradually treated with up to 1 equiv of
CAN, MLCT transitions in the visible decreased a little. At the same
time, the emergence of a broad absorption band in the NIR region
was evident (Figure 5a). When the amount of CAN was gradually
increased to 2 equiv, MLCT transitions continued to decrease and
the new NIR band decreased gradually as well until it disappeared
completely (Figure 5b). Thus, this new NIR band specifically
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Figure 5. Absorption spectral changes of (a and b) ruthenium complexes 22+, (c and d) 42+, and (e and f) 72+ in acetonitrile upon first-electron (a, c,
and e) and second-electron (b, d, and f) oxidation by adding different equivalents of CAN while keeping the concentrations of 22+, 42+, or 72+
constant. Shown in insets of a, c, and e are NIR bands of 23+, 43+, or 73+ with Gaussian-fitted lines (blue lines) as a function of the wavenumbers,
respectively. Asterisks denote artifacts due to nonperfect background compensation.
Table 2. Parameters for the IVCT Transitions of the Complexes Studied
a
b
c
λmax/nm
νmax/cm−1
εmax (M−1 cm−1
)
Δν1/2(exp) (cm−1
)
Δν1/2(theo) (cm−1
)
Γ
rab (Å)
Hab (cm−1
)
23+
43+
73+
1730
1715
1828
5780
5830
5470
2500
1040
8875
3170
2140
3330
3654
3670
3555
0.13
0.72
0.063
11.16
11.15
11.17
395
210
740
a
b
c
1/2
The theoretical Δν1/2 value equals (2310νmax
system.
) . Γ = 1 − Δν1/2exp/Δν1/2theo. Hab = 0.0206(εmaxνmaxΔν1/2)
1/2/rab for a Robin and Day class II
associated with the one-electron-oxidized species 23+ is attributed to
the IVCT transition. It should also be mentioned that a new
shoulder band around 900 nm is observable in Figure 5b, which is
ascribed to the ligand-to-metal charge-transfer transition.
A Gaussian-fitted line of the IVCT band is shown in the inset of
Figure 5a. The observed full width at half-height (Δν1/2) is 3170
cm−1. The theoretical Δν1/2 value of this band is determined to be
3654 cm−1, according to Hush′s expression [Δν1/2theo
=
(2310νmax)1/2],19 which is slightly wider than the experimental
Δν1/2 value. The Γ parameter, introduced by Sutin and co-workers,20
of this band is 0.13, as determined by Γ = 1 − Δν1/2exp/Δν1/2theo
.
The vis/NIR absorption spectral changes of 42+ and 72+ upon
oxidative titration with CAN are shown in Figure 5c−f, which
evidence the appearance and disappearance of the IVCT band in a
similar region. However, it is clear that MV complex 43+ displays a
much weaker IVCT band, while 73+ exhibits a much more intense
one (εmax = 8875 M−1 cm−1) than that of 23+. The stepwise
oxidation process of 7 (72+ → 73+ → 74+) could also be realized by
the electrochemical method in different solvents (Figures S8−S10 in
the Supporting Information). For instance, when a solution of 72+
Figure 6. NIR electronic absorption spectra of 73+ in different solvents
obtained in spectroelectrochemical measurements. *: Artifacts caused
by a nonperfect background compensation.
studied (CH3CN, DMF, and CH2Cl2), which proved that the
IVCT bands in CH3CN and CH2Cl2 had similar energies and
shapes. However, the energy of the IVCT band in DMF is distinctly
red-shifted. The spectroelectrochemical measurements of 22+
and 42+ did not proceed well because of their limited solubility in
the presence of the electrolyte. On the basis of these analyses,
we conclude that 23+ and 43+ are Robin and Day class II systems,
while 73+ is either a class II system or on the II/III borderline. The
electronic coupling parameters (Hab) of 23+, 43+, and 73+ were thus
calculated to be 395, 210, and 740 cm−1, respectively, according to
n
in acetonitrile containing 0.1 M Bu4ClO4 was stepwise applied,
with a potential from +0.4 to +0.75 V vs Ag/AgCl with an indium−
tin oxide glass electrode, the first-electron-oxidation process took
place smoothly, as evidenced by the appearance of the IVCT band
in the NIR region (Figure S8a in the Supporting Information).
When the potential was further increased to induce the second-
electron oxidation, the disappearance of the IVCT band was
observed (Figure S8b in the Supporting Information). Figure 6
shows a comparison of the IVCT bands of 73+ in different solvents
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the Hush formula21 Hab = 0.0206(εmaxνmaxΔν1/2)1/2/rab, where rab
was taken to be the DFT-calculated Ru−Ru distance.
Table 3. Calculated Spin-Density Distributions of 23+, 43+,
and 73+ on the Level of UB3LYP/LanL2DZ/6-31G*/
a
Vacuum
As can be seen from the above analysis, the intermetallic
electronic coupling of the complexes studied is in the order of
73+ > 23+ > 43+. The effect of the terminal ligands on the
electronic coupling process at the MV state has been previously
demonstrated.22 In a similar MV system with cyclometalated
rutheniums as the charge-bearing sites, Launay and co-workers
found that the presence of electron-rich tert-butyl groups on the
terminal ligands reduced the intermetallic coupling,13a which
coincides with our results. To further aid in the understanding
of the electron-transfer processes in these systems, DFT cal-
culations on the open-shell complexes 23+, 43+, and 73+ were pre-
formed on the level of UB3LYP/LanL2DZ/6-31G*/vacuum theory
with the input files generated from the previously DFT-optimized
structures of 22+, 42+, and 72+. The dihedral angles between two
central phenyl rings of 22+, 42+, and 72+ are 43.48°, 45.93°, and
43.92°, respectively. In the open-shell complexes 23+ and 43+, these
angles decrease to 34.53° and 34.57°, respectively. However, this
angle increases to 47.53° in the case of 73+, which is appreciably
larger than those of 23+ and 43+. These results contradict a common
sense that a more planar bridge enhances an electron-transfer
process because complex 73+ with the biggest dihedral angle exhibits
the strongest electron coupling among the three complexes studied.
The spin-density plots of 23+, 43+, and 73+ are shown in Figure 7.
Table 3 summarizes the Mulliken spin-density populations of three
spin density
atom
23+
43+
73+
Ru1
Ru2
C1
0.338
0.338
0.319
0.320
0.406
0.406
−0.020
0.054
−0.024
0.061
−0.011
−0.006
0.019
C2
C3
−0.014
0.066
−0.013
0.073
C4
−0.009
0.019
C5
−0.014
0.054
−0.013
0.061
C6
−0.006
0.006
C1+2+3+4+5+6
0.126
0.145
C7
−0.020
0.054
−0.024
0.061
−0.011
−0.006
0.019
C8
C9
−0.014
0.066
−0.013
0.073
C10
−0.009
0.019
C11
−0.014
0.054
−0.013
0.061
C12
−0.006
0.006
C7+8+9+10+11+12
N1, N2, N3, or N4
C13, C14, C15, or C16
0.126
0.145
0.004
0.006
0.016
−0.015
−0.016
0.011
a
The spin density is determined by the difference of the Mulliken
charges of α and β electrons (α − β).
observed in many biscyclometalated complexes.5,13−16 In stark
contrast, complex 73+ has dominant spin contributions from two
metal centers (0.406 each). The spin distribution on the biphenyl
fragment is negligible. The smaller dihedral angles between two
central phenyl rings of 23+ and 43+ than that of 73+ is caused by the
spin delocalization in 23+ and 43+. Electron paramagnetic resonance
(EPR) analysis can provide useful information on the spin distri-
butions of metal complexes. However, to our disappointment, we
failed to obtain distinct EPR signals for complexes 23+, 43+, and 73+
even at 77 K. The calculated spin distributions of 23+, 43+, and 73+
shown in Figure 7 seem in contradiction with the experimental
findings at the first sight. Complexes 23+ and 43+ exhibit a large
degree of spin delocalization, but their electronic couplings are
much weaker than 73+ according to the above IVCT band analysis.
However, we should keep in mind that the spin delocalization
across the bridge does not necessarily stand for a strong electronic
coupling between two termini, but it does mean that some amount
of the bridge participates in the involved oxidation process. For
delocalized systems with conventional noncyclometalated ruthe-
niums as the redox centers, the bridge does not contribute to the
spin distribution.
The electron-transfer processes in these complexes could be
rationalized by a hole-transfer-tunneling mechanism. In the
electron-tunneling regime, two transfer processes could be
envisaged: electron-transfer or hole-transfer superexchange24
(Figure S11 in the Supporting Information). In the electron-
transfer mechanism, the electron moves from the donor state to
the LUMO of the bridge, followed by subsequent transfer to
the acceptor state. On the other hand, if the process is initiated
from the electron transfer from the bridge’s HOMO to the
acceptor state, it is termed the hole-transfer superexchange
Figure 7. Spin-density plots of 23+, 43+, and 73+. H atoms are omitted
for clarity.
complexes. The free spins in 23+ and 43+ are delocalized across the
Ru−biphenyl−Ru array. Each Ru atom and phenyl ring in 23+ has
spin populations of 0.338 and 0.126, respectively. In the complex
4
3+ with electron-rich Mebip terminal ligands, the phenyl rings have
even larger contributions (0.145 each). Although DFT methods
have an artificial preference for delocalization,23 it does suggest that
appreciable amounts of spin in 23+ and 43+ are associated with the
biphenyl backbone. This phenomenon has been previously
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Computational Methods. DFT calculations were carried out
using the B3LYP exchange correlation functional27 and implemented
in the Gaussian 03 program package.28 The electronic structures of the
complexes were determined using a general basis set with the Los
Alamos effective core potential LanL2DZ basis set for ruthenium29
and 6-31G* for other atoms in vacuum.30 In the case where the
solvation effects are included, the conductor-like polarizable con-
tinuum model (CPCM) with acetonitrile as the solvent and united-
atom Kohn−Sham radii were employed.31
mechanism. Qualitatively speaking, the smaller the energy gap
between the donor state and LUMO of the bridge (ΔEET), the
more important role the electron-transfer superexchange will
play. On the other hand, the smaller the energy difference
between the acceptor state and HOMO of the bridge (ΔEHT),
the bigger contribution the hole-transfer superexchange will
make. In the biphenyl-bridged biscyclometalated ruthenium
series, there is a strong orbital overlap between metals and
bridging ligands and the energy gap between them is small
because of the anionic nature of the bridging ligand.5,10−16
Thus, a hole-transfer superexchange is believed to play a more
important role in the series 23+, 43+, and 73+ with a similar
bridging state. Figure S12 in the Supporting Information shows
the highest single occupied molecular orbital isodensity plots of
23+, 43+, and 73+, which are very similar to the HOMOs of 22+,
42+, and 72+. In the case of the complex 43+ with electron-
donating benzimidazole capping ligands, the donor or acceptor
state is destabilized more than that of 23+ (as has been dis-
cussed in the electrochemical analysis and DFT results). As a
result, the ΔEHT value increases and the intermetallic coupling
becomes weaker. For the same reason, complex 73+ with
electron-withdrawing capping ligands leads to the decrease of
the ΔEHT value and the increase of the electronic coupling. We
know from these results that the ΔEHT value plays a more
important role than the planarity of the bridge on the degree of
the electronic coupling in these complexes.
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 the deuterated solvent for H
NMR (7.26 ppm for CDCl3 and 1.92 ppm for CD3CN). Mass
spectrometry (MS) data were obtained with a Bruker Daltonics Inc.
Apex II FT-ICR or Autoflex III MALDI-TOF mass spectrometer. The
matrix for matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF) measurement is α-cyano-4-hydroxycinnamic acid.
Microanalysis was carried out using a Flash EA 1112 or Carlo Erba
1106 analyzer at the Institute of Chemistry, Chinese Academy of
Sciences.
Synthesis of [2](PF6)2. To 15 mL dry of nBuOH were added
[Ru(tpy)(Mebib)](PF6)11a (1+; 62.5 mg, 0. 77 mmol) and AgBF4 (151.3
mg, 0.77 mmol). The mixture was refluxed for 5 h. After cooling to room
temperature, the solvent was removed under reduced pressure, and the
residue was dissolved in a proper amount of methanol. After the addition of
an excess of KPF6, the resulting precipitate was collected by filtration and
washing with water and Et2O successively. The obtained solid was subjected
to flash column chromatography on silica gel (eluent: CH3CN/H2O/
aqueous KNO3, 100/10/0.2) followed by anion exchange with KPF6 to
1
give 15.6 mg of [2](PF6)2 in a yield of 25%. H NMR (400 MHz,
CONCLUSION
CD3CN): δ 4.55 (s, 12H), 5.83 (d, J = 8.4 Hz, 4H), 6.83 (t, J = 7.7 Hz,
4H), 7.00 (t, J = 6.5 Hz, 4H), 7.14 (t, J = 7.6 Hz, 4H), 7.35 (d, J = 5.5 Hz,
4H), 7.48 (d, J = 7.9 Hz, 4H), 7.62 (t, J = 7.6 Hz, 4H), 8.38 (d, J = 8.0 Hz,
4H), 8.43 (d, J = 7.8 Hz, 2H), 8.84 (d, J = 8.3 Hz, 4H), 8.98 (s, 4H).
MALDI-TOF for [M − PF6 − H]+: m/z 1485.0 (calcd m/z 1485.23).
Anal. Calcd for C74H54F12N14P2Ru2·2H2O: C, 53.30; H, 3.51; N, 11.76.
Found: C, 53.20; H, 3.42; N, 11.62. Complex [2](PF6)2 was also prepared
in a yield of 54% starting from Ru(tpy)Cl3 and 5 in a procedure similar to
that for the synthesis of [4](PF6)2 as shown below.
■
To summarize, we present in this contribution the studies of a
series of biscyclometalated ruthenium complexes bridged by the
title ligand. A combination of electrochemical, spectroscopic,
DFT, and IVCT transition analysis proved that the electronic
nature of capping ligands plays a very important role in determining
the electronic coupling degree of these MV complexes. Complex
43+ with electron-rich benzimidazole capping ligands exhibits a
weaker coupling, while complex 73+ with electron-deficient capping
ligands shows an enhanced electronic coupling. This trend can be
rationalized on the basis of a hole-transfer-tunneling mechanism.
Although biscyclometalated ruthenium systems bridged with various
conjugated ligands have been reported in many publications,5,12−16
the effect of the capping ligand on the degree of intermetallic elec-
tronic coupling has not been examined in depth. Our results
presented in this paper are believed to greatly stimulate such studies.
In addition, cyclometalated ruthenium complexes have recently
been investigated as promising NIR electrochromic materials25 and
efficient sensitizers for solar cell applications.26 Compounds des-
cribed in this paper, especially complex 7 with the capping ligands
Me3tcbtpy that can be converted into carboxylic acid anchor groups
after a simple transformation,26c exhibited interesting vis/NIR
absorption features under different oxidation states and will be
useful for such purposes.
Synthesis of 5. A mixture of biphenyl-3,3′,5,5′-tetracarboxylic acid
(100 mg, 0.3 mmol) and N-methyl-1,2-phenylenediamine (250 mg,
1.28 mmol) in 10 mL of polyphosphoric acid was stirred at ca. 210 °C
for 8 h. After cooling to room temperature, the reaction mixture was
poured into 30 mL of water and neutralized by a 5 M aqueous NaOH
solution. The resulting precipitate was collected by filtration and
washing with water. The obtained crude product was subjected to flash
column chromatography on silica gel (eluent: CH2Cl2/acetone, 1/1)
to give 142 mg of compound 5 as a slightly yellow solid in a yield of
1
70%. H NMR (400 MHz, CDCl3): δ 3.97 (s, 12H), 7.36 (m, J = 6.6
Hz, 8H), 7.44 (d, J = 7.4 Hz, 4H), 7.85 (d, J = 7.4 Hz, 4H), 8.17
(s, 2H), 8.29 (s, 4H). EI-MS: m/z 673 for [M − H]+. EI-HRMS. Calcd
for C44H33N8: m/z 673.2828. Found: m/z 673.2838.
Synthesis of [4](PF6)2. To 10 mL of dry acetone were added
Ru(Mebip)Cl311b (55 mg, 0.1 mmol) and AgOTf (80 mg, 0.31 mmol).
The mixture was refluxed for 2 h before cooling to room temperature.
The precipitate was removed by filtration, and the filtrate was
concentrated to dryness. To the residue were added 3,3′,5,5′-tetrakis
(1-methylbenzimidazol-2-yl)biphenyl (33 mg, 0.05 mmol), DMF
t
EXPERIMENTAL SECTION
(10 mL), and BuOH (10 mL). The mixture was then refluxed for
■
another 24 h. After cooling to room temperature, the solvent was
removed under reduced pressure. The residue was dissolved in a
proper amount of methanol. After the addition of an excess of KPF6,
the resulting precipitate was collected by filtration and washing with
water and Et2O. The obtained solid was subjected to flash column
chromatography on silica gel (eluent: CH3CN/H2O/aqueous KNO3,
100/10/0.1) followed by anion exchange with KPF6 to give 25 mg of
[4](PF6)2 in a yield of 27%. MALDI-TOF for [M − 2PF6 − CH3 −
H]2+: m/z 1537.9 (calcd m/z 1538.35). Anal. Calcd for
C86H66F12N18P2Ru2·2H2O·Et2O: C, 55.33; H, 4.13; N, 12.90. Found:
C, 55.33; H, 4.18; N, 12.59.
Spectroscopic Measurements. All optical ultraviolet/visible
(UV/vis) 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. UV/vis/NIR spectra were recorded using a PE Lambda 750 UV/
vis/NIR spectrophotometer.
Electrochemical Measurements. All cyclic voltammograms were
taken using a CHI620D potentiostat. All measurements were carried out
in 0.1 M of Bu4NClO4/DMF at a scan rate of 100 mV/s with a Ag/AgCl
reference electrode. The working electrode was glassy carbon, and a
platinum coil was used as the counter electrode.
4349
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Inorganic Chemistry
Article
Synthesis of [7](PF6)2. To 40 mL of ethanol were added 405 mg
of RuCl3·3H2O and 265 mg of Me3tctpy. The mixture was refluxed for
8 h before cooling to room temperature. The resulting precipitate was
collected by filtration and washing with water to give 412.2 mg of
Ru(Me3tctpy)Cl3 (67%), which was used for the next transformation
without further purification. To 10 mL of dry acetone were added
Ru(Me3tctpy)Cl3 (64.6 mg, 0.105 mmol) and AgOTf (100 mg, 0.38
mmol). The mixture was refluxed for 2 h before cooling to room
temperature. The AgCl precipitate was removed by filtration, and the
filtrate was concentrated to dryness. To the residue were added 5 (35
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was then refluxed for 24 h under a nitrogen atmosphere. After cooling
to room temperature, the solvent was removed under reduced pres-
sure. The residue was dissolved in a proper amount of methanol, followed
by the addition of an excess of KPF6. The resulting precipitate was collected
by filtration and washing with water and Et2O. The obtained solid was
subjected to flash column chromatography on silica gel (eluent: CH3CN/
H2O/aqueous KNO3, 100/10/0.2) to give 59 mg of [7](PF6)2 (57%).
MALDI-TOF for [M − PF6 − 2H]+: m/z 1834.8 (calcd m/z 1835.26).
Anal. Calcd for C86H66F12N14O12P2Ru2·4H2O: C, 50.35; H, 3.64; N, 9.56.
Found: C, 50.07; H, 3.70; N, 9.67.
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ASSOCIATED CONTENT
* Supporting Information
■
S
CV profiles of 22+, 42+, and 72+ with a wider potential window,
DFT/TDDFT calculation results, oxidative spectroelectrochemistry
of 72+, an electron-transfer versus hole-transfer superexchange
mechanism, and SOMO surfaces. This material is available free of
charge via the Internet at http://pubs.acs.org.
(11) (a) Yang, W.-W.; Zhong, Y.-W.; Yoshikawa, S.; Shao, J.-Y.;
Masaoka, S.; Sakai, K.; Yao, J.; Haga, M.-a. Inorg. Chem. 2012, 51, 890.
(b) Concepcion, J. J.; Tsai, M.-K.; Muckerman, J. T.; Meyer, T. J.
J. Am. Chem. Soc. 2010, 132, 1545.
AUTHOR INFORMATION
Corresponding Author
*E-mail: zhongyuwu@iccas.ac.cn.
■
(12) (a) Beley, M.; Collin, J.-P.; Louis, R.; Metz, B.; Sauvage, J.-P.
J. Am. Chem. Soc. 1991, 113, 8522. (b) Patoux, C.; Launay, J.-P.; Beley,
M.; Chodorowski-Kimmers, S.; Collin, J.-P.; James, S.; Sauvage, J.-P.
J. Am. Chem. Soc. 1998, 120, 3717.
(13) (a) Fraysse, S.; Coudret, C.; Launay, J.-P. J. Am. Chem. Soc.
2003, 125, 5880. (b) Gao, L.-B.; Kan, J.; Fan, Y.; Zhang, L.-Y.; Liu,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Grant 21002104), the National Basic Research 973 program of
China (Grant 2011CB932301), the Scientific Research
Foundation for the Returned Overseas Chinese Scholars,
State Education Ministry of China, and the Institute of
Chemistry, Chinese Academy of Sciences (“100 Talent”
Program), for financial support.
S.-H.; Chen, Z.-N. Inorg. Chem. 2007, 46, 5651. (c) Vila,
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N.; Zhong,
Y.-W.; Henderson, J. C.; Abruna, H. D. Inorg. Chem. 2010, 49, 796.
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(d) Yao, C.-J.; Sui, L.-Z.; Xie, H.-Y.; Xiao, W.-J.; Zhong, Y.-W.; Yao, J.
Inorg. Chem. 2010, 49, 8347. (e) Yao, C.-J.; Zhong, Y.-W.; Yao, J.
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