Charge delocalization of 1,4-benzenedicyclometalated ruthenium: A comparison between tris-bidentate and bis-tridentate complexes

Article abstract of DOI:10.1021/ic202295b

A dimetallic biscyclometalated ruthenium complex, [(bpy) ₂Ru(dpb)Ru(bpy)₂]²⁺ (bpy = 2,2′- bipyridine; dpb = 1,4-di-2-pyridylbenzene), with a tris-bidentate coordination mode has been prepared. The electronic properties of

Full text of DOI:10.1021/ic202295b

Article
pubs.acs.org/IC
Charge Delocalization of 1,4-Benzenedicyclometalated Ruthenium: A
Comparison between Tris-bidentate and Bis-tridentate Complexes
Long-Zhen Sui,†,‡,§ Wen-Wen Yang, Chang-Jiang Yao, Hai-Yan Xie, 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
‡
School of Life Science and Technology, Beijing Institute of Technology, Beijing 100081, People’s Republic of China
*
S Supporting Information
ABSTRACT: A dimetallic biscyclometalated ruthenium com-
2
+
plex, [(bpy) Ru(dpb)Ru(bpy) ] (bpy = 2,2′-bipyridine; dpb =
2
2
1
,4-di-2-pyridylbenzene), with a tris-bidentate coordination
mode has been prepared. The electronic properties of this
complex were studied by electrochemical and spectroscopic
analysis and DFT/TDDFT calculations on both rac and meso
2
+
isomers. Complex [(bpy) Ru(dpb)Ru(bpy) ] has a similar
2
2
1
,4-benzenedicyclometalated ruthenium (Ru−phenyl−Ru)
structural component with a previously reported bis-tridentate
complex, [(tpy)Ru(tpb)Ru(tpy)] (tpy = 2,2′;6′,2″-terpyridine;
2
+
tpb = 1,2,4,5-tetra-2-pyridylbenzene). The charge delocaliza-
tions of these complexes across the Ru−phenyl−Ru array were
investigated and compared by studying the corresponding one-electron-oxidized species, generated by chemical oxidation or
electrochemical electrolysis, with DFT/TDDFT calculations and spectroscopic and EPR analysis. These studies indicate that
3
+
3+
both [(bpy) Ru(dpb)Ru(bpy) ] and [(tpy)Ru(tpb)Ru(tpy)] are fully delocalized systems. However, the coordination mode
2
2
of the metal component plays an important role in influencing their electronic properties.
INTRODUCTION
systems with increasing electron delocalization. However, when
■
the bridging ligand becomes redox-noninnocent, corresponding
open-shell systems are no longer mixed-valent and classical
The studies of organometallic complexes with a covalent bond
between the ligand and metal center have been the focus of many
18
1
Marcus−Hush theory is not applicable. Elucidation of the charge
delocalization degree in these systems needs detailed experimental
studies in combination with computational calculations. We re-
research activities. Organic ligands in these complexes are
electron-rich, and some representative examples are phenyl
1
2
3
anion, phenylacetylide, phenylvinylene anion, and anionic5
3
+
4
cently disclosed a dimetallic open-shell complex, 1 (Figure 1),
nitrogen donors. As for the metal components, ruthenium,
6
7
8
bridged by a redox-noninnocent 1,2,4,5-tetra-2-pyridylbenzene
iron, platinum, and iridium are most employed partly because
these metals form stable complexes with anionic ligands. These
complexes display appealing electronic properties and are
identified as promising materials for applications in organic
19
(
tpb) ligand. This complex was determined to be a fully
electron-delocalized system across the central Ru−phenyl−Ru
array. It displays multiple charge-transfer transitions in the near-IR
(NIR) region. Herein, we report on a structurally related system
9
10
11
catalysis, molecular electronics, nonlinear optics, and dye-
3
+
12
2 , where two ruthenium metals are connected with a 1,4-phenyl
sensitized solar cells. One particularly interesting property of
these complexes is that the extensive orbital overlap between the
metal center and ligand complicates the redox process. In other
3
+
dianion bridge similar to that in 1 . However, the coordination
environment of the metal was changed from bis-tridentate to tris-
bidentate. The charge delocalizations of these two complexes
across the central Ru−phenyl−Ru array were investigated and
compared with multiple experimental and theoretical techniques.
13
words, these ligands are redox-noninnocent. A specific redox
process could be associated with the metal component or ligand
unit or an admixture of them.
+
+
In addition, the studies of monometallic complexes 3 and 4 have
also been included for comparison. It should be noted that
different coordination modes of octahedral ruthenium complexes,
namely, bis-tridentate or tris-bidentate, play an important role in
determining their electronic structures and photophysical
Dimetallic systems with the above-mentioned organometallic
components with an anionic ligand are of special interest because
of the redox noninnocent nature of the ligand. 1,4-Diethynyl-
14
15
16
phenylene, 1,4-divinylphenylene, 4,4′-biphenyl dianion, 2,7-
17
pyrene dianion and compounds with similar structural features
are representative bridging ligands for the construction of these
complexes. Assuming metal-confined redox behavior, the intro-
duction of an anionic ligand strengthens the electron coupling
between individual redox termini and affords mixed-valence
2+
behaviors. For instance, the tris-bidentate complex [Ru(bpy)3]
Received: August 27, 2011
Published: January 11, 2012
©
2012 American Chemical Society
1590
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Ru(bpy) Cl in the presence of AgOTf in a 1:1 mixed solvent
2
2
t
of N,N-dimethylformamide (DMF) and BuOH, followed by
anion exchange with KPF and chromatography using neutral
6
+
alumina, afforded monometallic complex 4 and dimetallic
complex 2 in a yield of 40% and 18%, respectively. It must be
2
+
stressed that the use of alumina is crucial for the successful
2
+
2+
isolation of the dimetallic complex 2 . We failed to purify 2
in attempts using silica gel for flash column chromatography.
+
However, this is not an issue for the monometallic complex 4
2
+
and the previously reported bis-tridentate dimetallic complex 1 .
2
+
We found that a solution of 2 in acetonitrile was contaminated
by a small amount of the monometallic complex 4 after standing
+
for several hours at room temperature, as monitored by thin-layer
chromatography. However, this stability issue should not affect
the following electrochemical and spectroscopic measurements.
+
1
Complex 4 displays well-defined H NMR peaks. However, the
1
2+
H NMR spectrum of the dimetallic complex 2 is rather
complex and could not be assigned. It is very likely that it is
composed of two diastereomers (rac and meso, Figure S1 in the
Supporting Information) in a 1:1 ratio. We used this sample
directly for the following electrochemical and spectroscopic
measurements. The relative configurations of ruthenium
components of two diastereomers are supposed to play a
27
minor role in affecting their optoelectronic properties. Although
we have not been able to experimentally separate the rac and
2
+
meso isomers of 2 , the following computational calculations
have been performed on both isomers to estimate the differences
in their electronic structures.
3
+
3+
+
+
Figure 1. Cyclometalated complexes 1 , 2 , 3 , and 4 .
(
bpy = 2,2′-bipyridine) is brightly emissive with a lifetime on the
order of microseconds at room temperature. On the other
hand, the bis-tridentate complex [Ru(tpy)2] (tpy = 2,2′:6′,2″-
terpyridine) is virtually nonemissive at room temperature. How-
ever, it can be readily functionalized at the 4′ position of the tpy
ligand and incorporated into supramolecular architectures with
Electrochemical Studies and Density Functional
Theory (DFT) Calculations. The electronic properties of
the above-prepared samples were first studied by electrochemical
20
2
+
+
2+
analysis. Cyclic voltammetry (CV) profiles of 4 and 2 are
+
shown in Figure 2. Monometallic complex 4 displays an anodic
redox couple at +0.55 V and two cathodic waves at −1.52 and
−1.78 V vs Ag/AgCl, respectively. All of these peaks exhibit good
chemical reversibility. In comparison, two consecutive anodic
waves at +0.18 and +0.62 V are evident in the CV profile of the
21
well-defined structures. The effect of the coordination mode in
cyclometalated ruthenium complexes with a covalent Ru−C bond
22
has also been addressed recently. More importantly, the coor-
dination mode and even the stereochemistry in chiral systems
can influence the nature of mixed-valent systems derived from
2
+
dimetallic complex 2 . The potential difference (ΔE) between
two waves is 440 mV. This indicates a high thermodynamic
stability of the in situ electrochemically generated open-shell
23
polypyridine ligands.
3
+
complex 2 , with a comproportionation constant K of 2.87 ×
c
7
2+
RESULTS AND DISCUSSION
1
0 . We note that complex 1 with two bis-tridentate ruthenium
centers displays two similar anodic waves albeit at slightly negative
19
■
2
+
Synthesis. The syntheses of tridentate complexes 1 and
+
19
3
have been reported previously. The syntheses of bidentate
potentials (+0.12 and +0.55 V vs Ag/AgCl, ΔE = 430 mV).
2+
+
complexes 2 and 4 started from the preparation of ligand 5.
As outlined in Scheme 1, the bridging ligand 1,4-di-2-
However, it should be kept in mind that the ΔE value should not
be taken as a parameter for measuring the degree of electronic
coupling between redox termini. A number of other factors, such
as the electrostatic repulsion between like-charged metal centers,
ion pairing with the electrolyte, and antiferromagnetic exchange,
Scheme 1. Synthesis of 5, 4 , and 22+
+
28
contribute to the degree of the ΔE value, not to mention the
different coordination modes in the cases of 1 and 2. The
2
+
cathodic scan of 2 displays four closely spaced one-electron
+
2+
waves. Overall, anodic waves of 4 and 2 are ascribed to the
II/III
Ru
process, with appreciable contribution from oxidation of
1
,12,14−17
the cyclometalating ligand.
Cathodic waves are associated
with the reduction of bpy ligands.
To assist in the determination of the electronic structures,
+
2+
pyridylbenzene (5, dpb) was obtained in 70% yield from the
palladium-catalyzed Stille coupling between 1,4-dibromoben-
zene and 2-pyridyltributylstannane in the presence of
DFT calculations were performed on complexes 4 , rac-2 , and
2
+
meso-2 at the B3LYP/LANL2DZ/6-31G* level (see the
Experimental Section for details). Their lowest unoccupied
molecular orbital (LUMO) and highest occupied molecular
2
4
anhydrous LiCl. We note that 5 has previously been used
25
2+
to prepare a dinuclear cyclometalated iridium complex and a
orbital (HOMO) diagrams, together with those of 1 , are
26
diboron compound. The reaction of 5 with 2 equiv of
shown in Figure 3. More orbital graphics could be found in
1
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1
−4 is provided in Table 1. The extensive orbital overlap
between metal centers and the cyclometalating phenyl ring has
been found for the previously reported complex 1 .
2+
19
Table 1. Calculated Mulliken Populations of HOMOs of
Complexes Studied
Ru1
phenyl
Ru2
12+
0.25
0.26
0.29
0.48
0.53
0.29
0.33
0.31
0.34
0.34
0.25
0.23
0.28
rac-22+
meso-22+
3+
4+
2
+
The frontier orbital energy level alignment of complexes 1 ,
2
+
2+
rac-2 , and meso-2 is shown in Figure 4. The calculation
+
2+
Figure 2. Cyclic voltammograms (black lines) of (a) 4 and (b) 2 in
n
acetonitrile containing 0.1 M Bu NClO at a scan rate of 100 mV/s.
4
4
The red lines are differential pulse voltammograms with a step
potential of 5 mV and an amplitude of 50 mV. The working electrode
is a glassy carbon, the counter electrode is a platinum wire, and the
reference electrode is Ag/AgCl in saturated aqueous NaCl.
2+
Figure 4. Frontier orbital energy level alignment of complexes 1 , rac-
2+
2+
2
, and meso-2 .
methods for these complexes are identical, and they have the
2
+
same charge (2+). Both HOMO and LUMO of 1 (−8.69 and
Figures S2 and S3 in the Supporting Information. All LUMOs
2
+
+
2+
−6.46 eV, respectively) are more stabilized than those of 2
of 4 and 2 are dominated by bpy ligands, which is in
(
−8.40 eV and −5.97 eV, respectively). However, the
agreement with the assignment of their cathodic CV waves to
2+ 2+
+
difference of the LUMO levels between 1 and 2 (0.49
the reduction of these ligands. The HOMO of 4 has major
eV) is larger than that of the HOMO levels (0.29 eV). As a
result, the calculated energy gap of 1 (2.23 eV) is narrower
than that of 2 (2.43 eV for both isomers). The different
energy levels of the calculated LUMOs of 1 and 2 can be
contribution from both the metal center and the cyclo-
metalating phenyl ring, with Mulliken population values of 0.53
and 0.34, respectively. The electron densities of HOMOs of
2
+
2
+
2
+
2+
rac-22 and meso-2 distribute across the central Ru−phenyl−
Ru array. The Mulliken populations of the two ruthenium
centers and the cyclometalating phenyl fragment are 0.26, 0.23,
+
2+
2
+
easily understood. The LUMO of 1 is mainly associated with
2
+
the bridging tpb ligand. However, the LUMO of 2 is mainly
associated with the auxiliary bpy ligands. Consistent results
were reflected from the above electrochemical analysis, which
showed that the first reduction wave of 1 occurred at a less
negative potential than that of 2 (−1.36 and −1.52 V vs
2
+
2+
and 0.33 for rac-2 and 0.29, 0.28, and 0.31 for meso-2 ,
2
+
respectively. We know from this calculation that rac-2 and
meso-22 have similar albeit slightly different HOMO
compositions. A comparison of the HOMO compositions for
+
2+
2
+
+
2+
2+
2+
Figure 3. Isodensity plots of selected frontier orbitals for complexes 4 , rac-2 , meso-2 , and 1 . All orbitals have been computed at an isovalue of
.02.
0
1
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Ag/AgCl). The slightly different HOMO levels of 12+ and 22+
could not be rationalized at this stage. As a matter of fact, th2e+
electrochemical results showed that the RuII/III process of 1
560 nm is ascribed to the MLCT transitions from HOMO−1 and
2
+
2+
HOMO−2 (S −S for rac-2 and S and S for meso-2 ). The
5
8
5
7
latter peak at 490 nm is mainly from HOMO−3 and HOMO−4
2
+
2+
2+
occurred at less positive potential than that of 2 (+0.12 and
0.18 V vs Ag/AgCl), which is contradictory with the
(S −S for rac-2 and S and S for meso-2 ). All of these
10
13
11
12
+
occupied orbitals are dominated by the metal component
(Figures S2 and S3 in the Supporting Information).
calculated results.
Electronic Absorption Spectra and Time-Dependent
DFT (TDDFT) Calculations. The electronic absorption spectra
NIR Transition Analysis of Open-Shell Complexes. The
2
+
7
large K value of 2 (2.87 × 10 ) allows us to precisely titrate it
c
+
2+
of 4 and 2 were recorded in acetonitrile and are shown in
with 1 or 2 equiv of oxidant, cerium ammonium nitrate (CAN).
Corresponding visible/NIR absorption spectral changes are
2
+
Figure 5, together with that of 1 . We previously found that
2
+
provided in Figure 6a,b. When a solution of 2 in acetonitrile
was gradually treated with CAN up to 1 equiv (Figure 6a),
MLCT transitions decrease continually with the concomitant
emergence of broad bands between 900 and 2700 nm. When
the amount of CAN was gradually increased to 2 equiv, NIR
transitions decrease gradually until they vanish (Figure 6b).
Clearly, the new generated NIR bands after treatment with
1
equiv of CAN are associated with the one-electron-oxidized
3
+
open-shell system 2 . It seems that several overlapping peaks
are present in those NIR bands. Thus, they were deconvoluted
into three peaks at 1059, 1283, and 1827 nm, respectively
(
Figure 6e), by assuming Gaussian shapes. From a comparison
3
+
3+
Figure 5. Electronic absorption spectra of complex 2 (black line), 4+
2
+
of the deconvoluted NIR transitions of 2 and 1 (Figure
6e,f), we note that those of the tris-bidentate complex 2 are
broader, weaker, and in a lower energy region than those of the
bis-tridentate complex 1 . Oxidative titration experiments were
3
+
2
+
(red line), and 1 (blue line) in acetonitrile.
3
+
the bis-tridentate complex 12+ exhibits a separate low-energy band
at 805 nm in addition to the conventionally observed metal-to-
ligand charge-transfer (MLCT) transitions in the visible region.
+
+
also carried out on the monometallic complex 3 and 4
(Figure 6c,d), which manifested the decrease of the MLCT
bands in the visible region and the emergence of ligand-to-
metal charge-transfer (LMCT) transitions around 800 nm
upon one-electron oxidation. It should be noted that the
energies of those new LMCT bands are much higher than those
of the NIR transitions of 1 and 2 , which excludes the
possibility of mixing some LMCT bands in Figure 6e,f.
NIR transitions could also be observed during oxidative
2
+
However, in the case of the tris-bidentate complex 2 , no similar
19
band is present. According to TDDFT calculations, the peak at
8
05 nm of 12 is of HOMO → LUMO origin. The HOMO
+
2
+
2+
compositions of 1 and 2 are quite similar, as described above.
2
+
3+
3+
However, as has been described above, the LUMO of 1 is
mainly associated with the bridging tpb ligand and is more
stabilized (−6.46 eV) than the auxiliary ligand bpy-dominated
LUMO of 22 (−5.97 eV for both rac-2 and meso-2 ). This
+
2+
2+
2+
2+
spectroelectrochemical analysis of 1 and 2 (Figures S5−S10
in the Supporting Information). For instance, when a solution
of 1 in acetonitrile was electrolyzed by applying various
difference is likely responsible for the absence of a low-energy
2
+
2+
absorption band for 2 . The absorption bands in the visible
+
2+
region for 4 and 2 are ascribed to the MLCT transitions
mixing with some contribution from the ligand-to-ligand charge-
transfer (LLCT) transitions due to the significant involvement of
the cyclometalating phenyl fragment in the HOMOs. In
potentials increasing stepwisely from +0.05 to +0.5 V vs Ag/
AgCl, multiple transitions in the NIR region appeared (Figure
S5 in the Supporting Information). When the potential was
further increased from +0.5 to +0.8 V, these new NIR
transitions went down. The shape and energy of these NIR
bands in the one-electron-oxidized state (1 ) is very similar to
those observed during chemical oxidation with CAN. Similar
spectroelectrochemical experiments were carried out with the
comparison, 22 has higher molar absorptivity than 4 and its
low-energy side extends into the NIR region. Bands in the
ultraviolet region are attributed to the intraligand transitions from
both bpy and dpb ligands. The observed red shift of the MLCT
+
+
3
+
2
+
2+
transitions and narrower optical energy gap of 1 , compared to
tris-bidentate complex 2 and in different solvents. Figure 7
those of 22 and 4 , agree well with the above electrochemical
+
+
3+
3+
shows the absorption spectra of 1 and 2 recorded during
spectroelectrochemical experiments in CH CN, CH Cl , and
DMF, respectively. It is very clear that the NIR transitions of
and calculation results.
3
2
2
In order to examine the possible difference of absorption
2
+
3+
3+
between the rac and meso isomers of 2 , TDDFT calculations
were performed on the above DFT-optimized structure in the gas
phase. Their predicted absorption spectra are shown in Figure S4
in the Supporting Information, and their corresponding excitation
energy, oscillator strength (f), and dominant contributing
transitions are collected in Table S1 in the Supporting
1
and 2 exhibit only slight differences in the different
solvents used, which means that they are virtually solvent-
independent. These facts suggest than complexes 1 and 2
belong to a fully delocalized open-shell system.
3
+
3+
From the above electrochemical, DFT, and spectroscopic
analysis, we know that the bridging biscyclometalating ligand of
2 is redox-noninnocent like that in 1. The observed NIR bands
of 1 and 2 cannot be interpreted as intervalence charge-
transfer (IVCT) transitions, and the classical Marcus−Hush
theory is not applicable for these systems. Computational
calculations on the one-electron-oxidized complex 2 were
then carried out to elucidate the nature of its electronic
structure and the observed NIR transitions. DFT and TDDFT
2
+
2+
Information. Overall, rac-2 and meso-2 have very similar
absorption patterns. However, the meso isomer was predicted to
have relatively higher oscillator strength. The predicted HOMO
3
+
3+
→
LUMO excitation at 680 nm is responsible for the observed
3
+
low-energy edge around 700 nm. Two MLCT transition peaks,
although overlapping with each other, could be distinguished
2
+
around 560 and 490 nm for complex 2 . The former peak at
1
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Figure 6. Absorption spectral changes of 22+ (a and b), 3 (c) and 4 (d) in acetonitrile upon one-electron (a, c, d) and two-electron (b) oxidation
+
+
by adding different equivalents of CAN while keeping the concentration of complexes constant. (e) and (f) are deconvolution plots of the NIR
spectra of 23 (a) and 1 (b) generated by adding 1 equiv CAN in acetonitrile. The black lines are experimentally observed spectra. The blue lines
+
3+
are individual deconvoluted peaks. The red lines are the sum of blue lines.
these complexes is delineated in Table 2. rac-23+ and meso-23+
have very similar spin distributions. Ruthenium atoms have
almost identical spin densities (0.385 each), which is in
3
+
accordance with the electron delocalization nature of 2 . A
significant portion of spin resides in the cyclometalating phenyl
3
+
3+
ring (a total of 0.251 for rac-2 and 0.249 for meso-2 ), with the
carbon atoms directly connecting with the metal center having
the highest density.
TDDFT calculations were performed on the above DFT-
optimized structure of both rac-23+ and meso- 2 at the same
UB3LYP/LanL2DZ/6-31G* level in vacuo. Calculated low-
energy excitations with oscillator strengths (f) larger than 0.001
are provided in Table S2 in the Supporting Information.
Corresponding spin orbitals involved in these transitions are
given in Figure S11 in the Supporting Information. The
3+
3
+
3+
predicted low-energy transitions of rac-2 and meso-2 are
very similar. The major NIR absorption peak of 23+ at 1283 nm
is well predicted by TDDFT results (S of rac-2 and meso-
3
+
5
3
+
2
), in terms of both energy and strength. This transition is
mainly associated with the excitation of a β electron from
β-HOSO (highest occupied spin orbital) and β-HOSO−4 to
β-LUSO (lowest unoccupied spin orbital). The experimentally
observed lowest-energy peak at 1827 nm is likely associated
Figure 7. Absorption spectra (a) 13+ and (b) 23+ in indicated solvents
CH CN, CH Cl , or DMF) recorded during oxidative spectroelec-
trochemistry measurements.
with the S and S excitations, which involve excitation of a β
(
1
3
3
2
2
3
+
electron to β-LUSO from β-HOSO and β-HOSO−1 for rac-2
3+
and from β-HOSO and β-HOSO−2 for meso-2 . All of these
excitations could be interpreted as the charge-transfer transition
from the metal centers to the biscyclometalating phenyl ring.
However, the metal centers in different occupied orbitals have
different orbital configurations. For instance, in the case of rac-
calculations have previously been employed to interpret
29
organometallic systems with redox-noninnocent ligands. We
carried out DFT calculations on both rac and meso isomers of
3
+
30
2
at the UB3LYP level with the effective core potential
31
3+
LanL2DZ basis set for ruthenium and 6-31G* for other atoms
2 , β-HOSO, β-HOSO−1, and β-HOSO−4 consist of
ruthenium atoms mainly with d , d , and d configurations,
respectively. In the case of meso-2 , ruthenium atoms of
32
3+
in vacuo. The Mulliken spin-density plots of rac-2 and meso-
xy
xz
yz
3+
3+
3+
2
, together with that of 1 , are shown in Figure 8. It is clear
that the spins of these complexes are evenly distributed along the
central Ru−phenyl−Ru array, which points to a strong electron
delocalization in these systems. The spin-density population of
β-HOSO, β-HOSO−2, and β-HOSO−4 are dominated by d ,
xz
2
2
d , and d
, respectively. TDDFT calculations do not predict
xy
3z −r
the observed shoulder band at 1059 nm. The same situation
1
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3
+
3+
3+
Figure 8. Spin-density plots of 1 , rac-2 , and meso-2 .
Thus, in 13+ and 2 , the major peaks at 1147 and 1283 nm,
respectively, are assigned to MMCT bands. The nearby peaks on
the higher side (1012 and 1059 nm, respectively) are attributed to
MBCT bands. The peak in the lowest-energy region could be due
to vibration signatures, which tend to play a more important role
in fully delocalized systems. As for the small band at 860 nm in
3+
Table 2. Calculated Spin-Density Distributions of
3+
3+
3+
Complexes 1 , rac-2 , and meso-2 on the Level of
a
UB3LYP/LanL2DZ/6-31G*
3
+
tridentate complex 1 , it is likely resulting from the red shift of
spin density
2+
the band at 805 nm before oxidation (1 ; see the oxidative
titration spectral changes in Figure 5a of ref 19). We note that
such a band is not present in both 2 and 2 with bidendate
coordination. More complicated analysis using three-state models
and simulation of the absorption spectra will be carried out in the
near future.
Electron Paramagnetic Resonance (EPR) Studies. EPR
spectroscopy is a useful tool for analyzing the spin distributions
for ruthenium complexes with redox-noninnocent ligands. A free
organic spin has a g factor g = 2.0023. A low-spin Ru species is
atom
13+
rac-23+
meso-23+
2
+
3+
Ru1
Ru2
C1
0.326
0.326
0.050
0.043
0.043
0.050
0.043
0.043
0.272
0.384
0.385
0.063
0.027
0.036
0.062
0.027
0.036
0.251
0.385
0.385
0.062
0.035
0.027
0.062
0.035
0.027
0.249
C2
C3
C4
C5
III
C6
e
phenyl
usually EPR-inactive at room temperature because of rapid spin−
lattice relaxation. However, it could exhibit a rhombic or axial
EPR signal at low temperature as solid or frozen solutions. The
anisotropy Δg (=g − g ) and the deviation of the isotropic g
a
The spin density is determined by the difference of the Mulliken
charges of α and β electrons (α − β).
3
+
19
1
3
was previously found for comeplx 1 . We acknowledge that
factor ⟨g⟩ (=[(g12 + g + g )/3] ) from g reflects the spin
2
2
2
3
1/2
e
TDDFT results did not fully agree with the observed NIR
distribution of the complex and symmetry at the metal center as a
3
+
3+
transitions for complexes 1 and 2 . However, their major
absorption peak has been well predicted, in terms of both
energy and strength. In this sense, TDDFT results provide
useful and instructive information regarding the charge
delocalizations of these open-shell systems.
result of spin−orbital coupling and low-symmetry ligand-field
34
effects. The larger of the Δg value and the deviation, the higher
the amount of spin is on the metal center. For example, a true
metal-centered spin of a cetecholatoruthenium(III) complex has a
34a
3+
⟨
g⟩ value of 2.476 with Δg = 0.833. However, complex 1
Classical Marcus−Hush two-state theory neglects the
importance of the bridging ligand in mediating electron transfer
between individual redox sites, and it predicts the presence of a
single IVCT band, although the band shape is dependent on the
degree of electronic coupling. Considering the redox-noninnocent
exhibits a rhombic EPR signal at 77 K with ⟨g⟩ and Δg values of
19
2
.144 and 0.34, respectively. The substantially low ⟨g⟩ and Δg
3+
values of complex 1 are a result of significant participation of
ligand oxidation. However, the pronounced rhombicity of the
EPR signal indicates that the amount of spin on the metals is
3
+
3+
3+
nature of the bridging ligand in systems 1 and 2 and the
more than that on the organic ligand. In comparison, complex 2
failure of TDDFT calculations to fully predict their NIR
displays an axial EPR signal at low temperature with g = g =
1
2
33
transitions, we turn to a three-state model developed by
Brunschwig, Creutz, and Sutin for the explanation of NIR
2.106 and g = 1.857 (Figure 9). The ⟨g⟩ and Δg values are
3
transitions of 13 and 2 . This model adopts a third bridge state
beyond the donor and acceptor states. When the bridge state lies
higher in energy than the other two states, the three-state model
predicts the presence of two open-shell system-associated NIR
transitions. The low-energy band is metal-to-metal charge transfer
+
3+
(
MMCT) in character, and the high-energy transition is metal−
bridging ligand charge transfer (MBCT). We know that oxidation
of the dianionic phenyl bridge is supposed to be more difficult
II/III
than the Ru
process in cyclometalated complexes. In the
electrochemical analysis of some cyclometalated ruthenium
1
2,16,17
complexes,
beyond the Ru
a second oxidation wave (mostly irreversible)
process at a more positive potential is often
II/III
Figure 9. EPR signal of 23 at 77 K in acetonitrile. The spectrometer
+
9
frequency ν is 9.519 × 10 Hz.
observed and that wave is mainly associated with oxidation of the
anionic ligand. In this sense, the bridge-dominated state in
calculated to be 2.026 and 0.249, which are comparable to those
systems 13 and 2 should be higher in energy than the metal-
+
3+
3+
of 1 . Both rhombic and axial EPR signals have been
III
5
dominated state and the three-state model is applicable to them.
documented for a variety of Ru complexes with t2g electronic
1
595
dx.doi.org/10.1021/ic202295b | Inorg. Chem. 2012, 51, 1590−1598

Inorganic Chemistry
Article
3
4
configuration. Larger distortions from the octahedral field pro-
reference electrode. The working electrode was glassy carbon, and a
platinum coil was used as the counter electrode.
3
+
bably exist in the tridentate complex 1 than the bidentate com-
3+
Oxidative Spectroelectrochemistry. Oxidative spectroelectro-
chemistry was performed in a thin-layer cell (optical length = 0.2 cm)
in which an indium−tin oxide glass electrode was set in an indicated
solvent containing [1](PF ) or [2](PF ) (the concentration is
plex 2 , as suggested by their different g splittings. Nevertheless,
low ⟨g⟩ and Δg values of these two complexes signify an appre-
ciable amount of ligand participation at their singly occupied
molecular orbitals, which is consistent with the presence of a
redox-noninnocent bridging ligand.
6
2
6 2
−4
around 1 × 10 M) and 0.1 M TBAP as the supporting electrolyte. A
platinum wire and Ag/AgCl in a saturated aqueous solution were used
as the counter and reference electrodes, respectively. The cell was put
into a PE Lambda 750 UV/vis/NIR spectrophotometer to monitor
spectral changes during electrolysis.
CONCLUSION
■
To conclude, we present in this paper studies of the electronic
properties of the 1,4-benzenedicyclometalated ruthenium
complex with either bis-tridentate or tris-bidentate coordination
mode. The electrochemical studies of the bidentate complex
Computational Methods. DFT calculations were carried out
3
0
using the B3LYP exchange correlation functional and implemented
3
5
in the Gaussian 03 program package. The electronic structures of the
complexes were determined using a general basis set with the Los
3
1
2
+
Alamos effective core potential LanL2DZ basis set for ruthenium
and 6-31G* for other atoms in vacuo.
[
(bpy) Ru(dpb)Ru(bpy) ] establish that the in situ generated
2 2
3
2
one-electron-oxidized complex has a relatively large compro-
portionation constant (2.87 × 10 ) in acetonitrile, which is
comparable to that of the tridentate complex [(tpy)Ru(tpb)-
7
Synthesis. NMR spectra were recorded in the designated solvent
on a Bruker Avance 400 MHz spectrometer. Spectra are reported in ppm
1
values from residual protons of the deuterated solvent for H NMR (7.26
3+
Ru(tpy)] . However, the absorption spectra of [(bpy) Ru-
13
2
ppm for CDCl and 1.92 ppm for CD CN) and C NMR (77.00 ppm
3
3
2+
2+
(
dpb)Ru(bpy)2]
and [(tpy)Ru(tpb)Ru(tpy)] are signifi-
cantly different. The separate and distinct low-energy band at
05 nm of the bis-tridentate complex is not present in the tris-
for CDCl ). MS data were obtained with a Bruker Daltonics Inc. Apex II
3
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.
8
bidentate complex. We attribute this difference to the different
LUMO compositions and energy levels of these complexes.
DFT calculations indicate that the bridging-ligand-associated
LUMO of [(tpy)Ru(tpb)Ru(tpy)] is much more stabilized
than the bpy-dominated LUMO level of [(bpy) Ru(dpb)Ru-
bpy) ] . As a matter of fact, this difference significantly affects
the electronic properties of these two complexes.
2
+
Synthesis of 5. To 50 mL of degassed toluene were added 1,4-
dibromobenzene (566 mg, 2.4 mmol), 2-pyridyltributylstannane (4.4 g,
2
1
2 mmol), Pd(PPh )Cl2 (224 mg, 0.32 mmol), and LiCl (1.0 g,
2+
3
(
2
24 mmol). After bubbling with nitrogen, the system was refluxed in a
sealed pressure tube for 4 days. The mixture was concentrated and
subjected to flash column chromatography on silica gel to afford 390.8
2
+
[
(bpy) Ru(dpb)Ru(bpy) ] could be precisely titrated with
2 2
1
CAN to give one-electron-oxidized complex [(bpy) Ru(dpb)-
mg of 5 in a yield of 70% (eluent: 6:1 petroleum ether/ethyl acetate). H
2
Ru(bpy)2]3 and two-electron-oxidized complex [(bpy) Ru-
+
NMR (400 MHz, CDCl ): δ 7.25 (m, 2H), 7.77 (m, 4H), 8.14 (s, 4H),
2
3
13
4+
7
.72 (d, J = 4.2 Hz, 2H). C NMR (100 MHz, CDCl ): δ 120.6, 122.3,
3
(
dpb)Ru(bpy) ] . Alternatively, oxidative electrolysis of both
2
+
127.2, 127.3, 136.7, 139.7, 149.7, 156.8. EI-MS: 232 for [M] .
complexes also generated open-shell species of these complexes
and the corresponding NIR transition energy was virtually
independent of the solvents used (acetonitrile, CH Cl , and
Synthesis of [4](PF ) and [2](PF ) . To 10 mL of dry acetone
6
6 2
were added Ru(bpy) Cl ·2H O (104 mg, 0.2 mmol) and AgOTf
2
2
2
2
2
(
154 mg, 0.6 mmol), and the system was refluxed for 3 h before cooling
DMF). TDDFT calculations suggest that the major NIR peak of
to room temperature. The mixture was filtered to remove unwanted
3
+
[
(bpy) Ru(dpb)Ru(bpy) ] is associated with the charge-transfer
2 2
precipitates, and the filtrate was concentrated to dryness. To the residue
t
transitions from the metal components to the biscyclometalating
benzene ring. EPR studies imply that an appreciable amount of
free spin is distributed on the briding ligand. These studies
establish that a 1,4-benzene dianion, when covalently coupled to
two ruthenium atoms with either a bis-tridentate or tris-bidentate
coordination mode, behaves as a redox-noninnocent bridging
ligand and the corresponding open-shell complex is a fully
delocalized system across the Ru−phenyl−Ru motif. However,
the bis-tridentate complex is more appealing to us, because of
higher stability, linear configuration, the presence of low-energy
absorption, and the separation of MMCT and MBCT bands in
the NIR region. Future work will involve spectral simulation of
NIR spectra using a three-state model and studies of a series of
tridentate complexes with different substituents on the auxiliary
tpy ligands.
were added ligand 5 (23 mg, 0.1 mmol), DMF (10 mL), and BuOH
(10 mL), and the mixture was refluxed in a sealed pressure tube for 48 h.
After cooling to room temperature, the solvent was removed under
reduced pressure, and the residue was dissolved in the proper amount of
methanol. After the addition of an excess of KPF , the resulting
6
precipitate was collected by filtering and washing with water and Et O.
2
The obtained solid was subjected to flash column chromatography on
neutral Al O (eluent: 50:1 → 10:1 → 2:1 CH Cl /CH CN) to give 31
2
3
2
2
3
mg of complex [4](PF ) (40%) and 24 mg of complex [2](PF )
6
6 2
1
(
18%). Characterization data for [4](PF6). H NMR (400 MHz,
CD CN): δ 6.95 (t, J = 6.3 Hz, 1H), 7.04 (s, 1H), 7.16−7.26 (m, 4H),
3
7.42 (m, 2H), 7.56 (d, J = 8.2 Hz, 1H), 7.59 (d, J = 5.6 Hz, 1H), 7.66 (t,
J = 7.8 Hz, 1H), 7.71 (t, J = 7.8 Hz, 1H), 7.76−7.87 (m, 6H), 7.93 (d,
J = 8.2 Hz, 1H), 7.99 (t, J = 7.8 Hz, 1H), 8.08 (d, J = 8.2 Hz, 1H), 8.13
(
1
d, J = 5.5 Hz, 1H), 8.30 (d, J = 8.2 Hz, 1H), 8.34 (d, J = 8.2 Hz,
H), 8.39 (d, J = 8.2 Hz, 1H), 8.46 (d, J = 8.2 Hz, 1H), 8.49 (d, J =
+
4
.6 Hz, 1H). MALDI-MS: 645.3 for [M − PF ] . Anal. Calcd for
6
C H N RuPF ·H O: C, 53.53; H, 3.62; N, 10.41. Found: C, 53.57; H,
3.48; N, 10.27. Characterization data for [2](PF ) . MALDI-MS: 1202.4
36
27
6
6
2
EXPERIMENTAL SECTION
Spectroscopic Measurements. All optical ultraviolet/visible
UV/vis) absorption spectra were obtained using a TU-1810DSPC
■
(
6 2
+
for [M − PF ] . Anal. Calcd for C H N Ru P F ·3H O: C, 48.00; H,
3
6
56 42 10
2
2
12
2
.45; N, 10.00. Found: C, 47.82; H, 3.16; N, 9.93.
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.
ASSOCIATED CONTENT
■
*
S
Supporting Information
DFT-optimized structures of rac and meso diastereomers
Electrochemical Measurements. All CV were taken using a
CHI620D potentiostat. All measurements were carried out in 0.1 M of
Bu NClO /acetonitrile at a scan rate of 100 mV/s with a Ag/AgCl
2
+
2+
of 2 , selected frontier molecular orbital graphics of 2 , simulation
2
+
of absorption spectra of 2 , oxidative spectroelectrochemistry
4
4
1
596
dx.doi.org/10.1021/ic202295b | Inorg. Chem. 2012, 51, 1590−1598

Inorganic Chemistry
Article
of 12+ and 2 , calculated low-energy excitations of rac-2 and
meso-23 and involved frontier spin orbitals, and NMR and MS
spectra of new compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
2+
3+
(9) (a) Li, J.; Siegler, M.; Lutz, M.; Spek, A. L.; Gebbink, R. J. M. K.;
van Koten, G. Adv. Synth. Catal. 2010, 352, 2474. (b) van der Boom,
M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759.
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(10) (a) Keller, J. M.; Glusac, K. D.; Danilov, E. O.; McIlroy, S.;
Sreearuothai, P.; Cook, A. R.; Jiang, H.; Miller, J. R.; Schanze, K. S.
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AUTHOR INFORMATION
■
*
Corresponding Author
E-mail: zhongyuwu@iccas.ac.cn.
3
(
0, 355.
12) (a) Bomben, P. G.; Koivisto, B. D.; Berlinguette, C. P. Inorg.
Author Contributions
These authors contributed equally.
§
Chem. 2010, 49, 4960. (b) Koivisto, B. D.; Robson, K. C. D.;
Berlinguette, C. P. Inorg. Chem. 2009, 48, 9644. (c) Robson, K. C. D.;
Koivisto, B. D.; Yella, A.; Sporinova, B.; Nazeeruddin, M. K.;
Baumgartner, T.; Gratzel, M.; Berlinguette, C. P. Inorg. Chem. 2011,
ACKNOWLEDGMENTS
■
Y.-W.Z. thanks 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.
5
(
2
0, 5494.
13) (a) Ward, M. D.; McCleverty, J. A. J. Chem. Soc., Dalton Trans.
002, 275. (b) Boyer, J. L.; Rochford, J.; Tsai, M.-K.; Muckerman,
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