Ruthenium-bis-terpyridine Complex with two redox-asymmetric amine substituents: Potential-controlled reversal of the direction of charge-transfer

Article abstract of DOI:10.1021/om500904k

A ruthenium-bis-terpyridine complex [Ru(NPhtpy)(Ntpy)]²⁺ (2²⁺) with two redox-asymmetric amine units has been prepared, where NPhtpy is 4-(di-p-anisylaminophen-4-yl)-2,2:6,2′-terpyridine and Ntpy is 4-(di-p-anisylamino)-2,2:6,2′-terpyridine. This complex displays two consecutive redox couples at +0.82 and +1.02 V vs Ag/AgCl, which are assigned to the N^?+/0 processes of the amine components of the NPhtpy and Ntpy ligands, respectively. The mono-oxidized complex 2³⁺ obtained by oxidative electrolysis shows the presence of the charge transfer from ruthenium(II) to the oxidized aminium radical cation of the NPhtpy ligand (MN_NPhtpyCT) around 1000 nm. In the dioxidized form (2⁴⁺), the MN_NPhtpyCT transition decreased distinctly and an opposite charge transfer from ruthenium(II) to the oxidized aminium radical cation of the Ntpy ligand (MN_NtpyCT) appeared at 1380 nm. Complexes [Ru(NPhtpy)(tpy)]²⁺ (tpy is 2,2:6,2′-terpyridine), [Ru(Ntpy)(tpy)]²⁺, and [Ru(NPhtpy)₂]²⁺ have been prepared and studied for the purpose of comparison. TDDFT calculations show that the involvement of the intraligand charge transfer from both NPhtpy and Ntpy ligands is responsible for the enhancement of the visible absorptions of these complexes with respect to [Ru(tpy)₂]²⁺. DFT and TDDFT calculations have been performed on 2³⁺ and 2⁴⁺ to provide information on the spin distributions and the nature of the near-infrared absorptions. Complex 2³⁺ shows an isotropic EPR signal at room temperature, consistent with an unpaired electron localized on the nitrogen atom.

Full text of DOI:10.1021/om500904k

Article
pubs.acs.org/Organometallics
Ruthenium-bis-terpyridine Complex with Two Redox-Asymmetric
Amine Substituents: Potential-Controlled Reversal of the Direction of
Charge-Transfer
Hai-Jing Nie, Chang-Jiang Yao, Meng-Jia Sun, 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, China
*
S Supporting Information
ABSTRACT: A ruthenium-bis-terpyridine complex [Ru(NPhtpy)-
Ntpy)]2 (2 ) with two redox-asymmetric amine units has been
+
2+
(
prepared, where NPhtpy is 4′-(di-p-anisylaminophen-4-yl)-2,2′:6′,2″-
terpyridine and Ntpy is 4′-(di-p-anisylamino)-2,2′:6′,2″-terpyridine.
This complex displays two consecutive redox couples at +0.82 and
•
+/0
+
1.02 V vs Ag/AgCl, which are assigned to the N
processes of the
amine components of the NPhtpy and Ntpy ligands, respectively. The
3
+
mono-oxidized complex 2 obtained by oxidative electrolysis shows
the presence of the charge transfer from ruthenium(II) to the
oxidized aminium radical cation of the NPhtpy ligand (MNNPhtpyCT) around 1000 nm. In the dioxidized form (2 ), the
4
+
MNNPhtpyCT transition decreased distinctly and an opposite charge transfer from ruthenium(II) to the oxidized aminium radical
2
+
cation of the Ntpy ligand (MN CT) appeared at 1380 nm. Complexes [Ru(NPhtpy)(tpy)] (tpy is 2,2′:6′,2″-terpyridine),
Ntpy
2
+
2+
[
Ru(Ntpy)(tpy)] , and [Ru(NPhtpy) ] have been prepared and studied for the purpose of comparison. TDDFT calculations
2
show that the involvement of the intraligand charge transfer from both NPhtpy and Ntpy ligands is responsible for the
2+
enhancement of the visible absorptions of these complexes with respect to [Ru(tpy) ] . DFT and TDDFT calculations have
2
3
+
4+
been performed on 2 and 2 to provide information on the spin distributions and the nature of the near-infrared absorptions.
Complex 23 shows an isotropic EPR signal at room temperature, consistent with an unpaired electron localized on the nitrogen
+
atom.
4
,9
INTRODUCTION
bearing sites in mixed-valence systems have been well-known.
■
We expect that the future hybrid materials of these two
components will possess some intriguing properties, in addition
Electron transfer and/or charge transfer are fundamental
processes in photosynthesis systems and optoelectronic devices.
Since the pioneering experimental and theoretical studies by
Creutz, Taube, Hush, and others, mixed-valence chemistry has
become a classical method to examine the charge transfer
1
0
1
2
to the known properties, such as enhanced light adsorption,
11 12
electrochromism, multistage redox processes, 14and inter-
6
,13
valence charge-transfer (IVCT),
among others.
3
We present herein a combined experimental and theoretical
study of complex 2(PF ) (Scheme 1), where the Ru(tpy)
component is used to bridge two redox-active amine units. This
complex is designed so that the two amine substituents have
different electronic properties and thus different oxidation
processes between redox sites. To date, a great number of
n
6
2
2
mixed-valent compounds with a general formula of [M -BL-
a
n+1
4
Mb ] have been reported, where BL is an organic bridging
ligand and Ma and Mb are redox-active inorganic or
organometallic components. These studies provide very useful
information on the effect of the BL on the degree of charge
delocalization of the system.
•+/0
potentials of individual N
processes. When these two amine
units are stepwise oxidized by potential-controlled electrolysis,
the charge transfer of the complex is examined by analyzing the
resulting near-infrared (NIR) absorptions. Interestingly, a
potential-controlled reversal of the direction of charge transfer
has been observed. Complexes 3(PF ) through 6(PF ) with
related structures have been synthesized and studied for the
purpose of comparison (Scheme 1). In addition, density
functional theory (DFT) and time-dependent DFT (TDDFT)
Recently, a conceptually reversed system using an inorganic
component to bridge two redox-active organic species has
attracted some attention. For instance, we have used the well-
5
6
2
6 2
known polypyridine ruthenium complexes Ru(tpy) (tpy =
2
2
,2′:6′,2″-terpyridine) or Ru(bpy) (bpy = 2,2′-bipyridine) as
3
the bridging component to mediate the electronic coupling
6
between two redox-active triarylamine units. Polypyridine
2+
3+
4+
calculations have been performed on 2 , 2 , and 2 with
different total charges, which provide complementary and
transition-metal complexes are useful optoelectronic materials
because of their appealing photophysical and electrochemical
7
properties. Triarylamines are versatile organic materials with
8
good electron-donating and hole-transporting abilities. The
Received: September 2, 2014
uses of polypyridine complexes or triarylamines as the charge-
©
XXXX American Chemical Society
A
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Scheme 1. Synthesis of 2(PF ) − 5(PF ) and the chemical structure of the known complex 6(PF )
6
2
6 2
6 2
insightful information on the compositions of frontier
molecular orbitals, spin density populations, and the nature of
the electronic absorptions of these complexes.
RESULTS AND DISCUSSION
■
Synthesis. Complexes 2(PF6)2 through 5(PF6)2 were
synthesized as outlined in Scheme 1. Ligand 4′-(di-p-
anisylaminophen-4-yl)-2,2′:6′,2″-terpyridine (1, NPhtpy) was
obtained in 70% yield through the Pd-catalyzed C−N coupling
15
between 4′-(p-bromophenyl)-2,2′:6′,2″-terpyridine and di(p-
anisyl)amine. The reaction of [Ru(Ntpy)Cl ] (Ntpy =4′-(di-p-
3
16
anisylamino)-2,2′:6′,2″-terpyridine) with 1 in ethylene glycol
under microwave heating, followed by anion exchange using
KPF , gave complex 2(PF ) in 54% yield. Complexes 3(PF )
6 2
Figure 1. CVs of ligands Ntpy and 1 and complexes 2(PF ) through
6
2
6
6 2
6
(PF ) and DPV of 5(PF ) (green curve) at a glassy carbon in 0.1 M
6 2 6 2
and 6(PF ) were prepared according to the reported
6
2
Bu NClO /CH CN.
1
6
4
4
3
procedures. Complex 4(PF ) was obtained from the reaction
6
2
of [Ru(tpy)Cl ] with ligand 1 in acceptable yield. The
3
symmetric complex 5(PF ) was obtained from the reaction
results, the two consecutive redox couples of the asymmetric
6
2
of ligand 1 with hydrated RuCl . The details of synthesis and
characterization data of these compounds are provided in the
Experimental Section.
diamine complex 2(PF6)2 at +0.82 and +1.02 V can be
3
•+/0
unambiguously assigned to the N
processes of the amine
components of the NPhtpy and Ntpy ligand, respectively. The
Electrochemical Studies. Figure 1 shows the anodic cyclic
voltammograms (CVs) of the above synthesized ligands and
comproportionation constant K for the equation [N −N ] +
c
1
2
•
+
•+
•+
[N -N ] → 2[N1 -N ] was determined to be 2450 by K =
1
2
2
c
ΔE (mV)/59
ruthenium complexes in CH CN. The electrochemical data are
10
, where ΔE (200 mV) is the potential splitting
3
•
+/0
delineated in Table 1. Ligands NPhtpy and Ntpy display a
between two N
processes and N and N stand for the
1 2
•
+/0
N
redox couple at +0.81 and +0.97 V vs Ag/AgCl,
amine nitrogen atom of the NPhtpy and Ntpy ligand of
respectively. The insertion of a phenyl group between the
dianisylamino group and the tpy ligand in NPhtpy decreases
2(PF ) , respectively.
The difference between the anodic and cathodic peak
potentials of each N
80−110 mV, which is somewhat larger than the theoretical
value (59 mV) for an electrochemically reversible one-electron
redox couple. This may be caused by the relatively slow
6
2
•
+/0
•+/0
the N
potential by 160 mV with respect to that of Ntpy.
process (ΔE ) falls in the range of
p
When these two ligands are chelated with Ru(tpy) to give
•+/0
complexes 4(PF ) and 3(PF ) , the N
potentials only vary
6
2
6 2
a little (+0.82 and +0.98 V, respectively). On the basis of these
B
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a
Table 1. Electrochemical Data
a
E1/2/V vs Fc+/0
E1/2/V vs Ag/AgCl (ΔE /mV)
p
compound
anodic
cathodic
anodic
+0.35
cathodic
1
, NPhtpy
+0.81 (80)
b
Ntpy
+0.97 (105), +1.41
+0.51
b
b
2
3
4
5
6
(PF6)2
+0.82 (90), +1.02 (85), +1.31, +1.36, +1.48
−1.30, −1.54
−1.28, −1.52
−1.24, −1.49
−1.26, −1.46
−1.34, −1.58
−1.22, −1.46
+0.36, +0.56
+0.55
−1.76, −2.00
−1.71, −1.95
−1.69, −2.04
−1.70, −1.90
−1.79, −2.03
−1.77, −1.91
b
(PF6)2
(PF6)2
(PF6)2
(PF6)2
+0.98 (80), +1.35, +1.44
b
+0.82 (88), +1.30, +1.36
+0.37
b
+0.81 (110), +1.30, +1.37
+0.37
b
+0.96 (87), +1.23 (95), +1.56, +1.64
+0.51, +0.78
[
Ru(tpy) ](PF )
+1.32
+0.87
2
6 2
a
Data was recorded in CH CN. The potential is reported as the E value vs Ag/AgCl or ferrocene+/0 (Fc+/0). ΔE refers to the difference between
3
1/2
p
b
the anodic and cathodic peak potentials. Anodic peak potential of irreversible processes.
electron transfer kinetics of the N•+/0 process under the
measurement conditions. However, each N• wave has the
same current height between the anodic and cathodic peaks,
suggestive of chemically reversible processes. The Ntpy-based
+/0
•
+/0
N
potential of 2(PF ) is 40 mV positively shifted with
6 2
•+/0
respect to that of 3(PF ) . The NPhtpy-based N
potential
6
2
of 2(PF ) is essentially the same with respect to that of
6
2
4
(PF ) . This however does not necessarily mean that an
6 2
electronic communication is present between two amine units
of 2(PF ) . The potential splitting of consecutive redox
6
2
processes depends on a variety of different factors apart from
the resonance contribution, including the inductive contribu-
tion and electrostatic contribution, among others.
17
Figure 2. Electronic absorption spectra of [Ru(tpy) ](PF ) and
2
6 2
2
(PF ) through (PF ) in CH CN.
6 2 6 2 3
The symmetric complex 5(PF ) with two NPhtpy ligands
6
2
shows a redox couple at +0.81 V vs Ag/AgCl. This wave should
be a result of the oxidations of both amine units. The potential
separation between two N• processes is insignificant, which
is also supported by its differential pulse voltammogram
Table 2. Electronic Absorption Data in CH CN
3
+/0
5
−1
compound
(PF6)2
λabs,max [nm] (ε × 10 M cm−1
)
2
3
4
5
6
277 (0.64), 301 (0.70), 515 (0.35)
274 (0.65), 303 (0.74), 495 (0.25)
272 (0.33), 308 (0.50), 496 (0.20)
(
DPV). This suggests that little electronic coupling is present
(PF6)2
(PF6)2
(PF6)2
(PF6)2
between two amine units, if there is any. No IVCT transition
has been observed in the following spectroelectrochemical
measurements either. In stark contrast, the diamine complex
275 (0.53), 310 (0.71), 404 (0.18), 516 (0.42)
302 (0.94), 350 (0.21), 512 (0.25)
6
N
(PF ) with shorter N−N distance displays two consecutive
6
2
[Ru(tpy) ](PF )
6 2
270 (0.48), 307 (0.78), 475 (0.17)
2
•
+/0
16a
processes at +0.96 and +1.23 V.
In the more positive potential region, some redox processes
are observed for complexes 2(PF ) , 4(PF ) , and 5(PF ) ,
6
2
6 2
6 2
(MLCT) transitions. We note that these complexes are
essentially nonemissive in fluidic solution at room temperature.
The above amine-containing complexes were then subjected
to stepwise oxidative electrolysis at a transparent indium−tin-
oxide (ITO) glass electrode, and the NIR absorption spectral
changes were monitored by a UV/vis/NIR spectrometer. When
the potential was gradually increased from +0.60 to +0.95 V vs
Ag/AgCl to oxidize the amine unit of the NPhtpy ligand of
2(PF ) , the absorption bands in the visible region significantly
which consist of the irreversible oxidation of the aminium
1
8
III/II
radical cations and the Ru
Supporting Information, SI). The Ru
complexes are slightly positively shifted with respect to that of
processes (Figure S1 in the
III/II
potentials of these
19
[
Ru(tpy) ](PF ) (Table 1). This is reasonable considering
2 6 2
III/II
that the di-p-anisylamino unit was oxidized prior to the Ru
processes, and the resulting aminium radical cations behave as
electron-withdrawing substituents. In the cathodic scan, these
complexes all show two ligand-based reduction waves (Figure
S1).
6
2
decreased (Figure 3a), with the concomitant appearance of the
•+
6,9,16,18
N -localized transitions at 750 nm
and some broad NIR
Spectroscopic Studies. The UV/vis absorption spectra of
absorptions between 900 and 1700 nm. The NIR absorptions at
least contain two components around 1000 and 1300 nm. The
former is assigned to the charge transfer from the ruthenium
ion to the oxidized aminium radical cation (of the NPhtpy
2
(PF ) through 6(PF ) and [Ru(tpy) ](PF ) are displayed
6 2 6 2 2 6 2
in Figure 2. When substituted with amine groups, the visible
absorptions of 2(PF ) through (PF ) are distinctly expanded
6
2
6 2
II
•+
with respect to that of [Ru(tpy) ](PF ) . The absorption
ligand), which is abbreviated as the Ru → N MNNPhtpyCT
transition. The latter band has not been observed for complexes
4(PF6)2 and 5(PF6)2 upon one-electron oxidations. The
assignment of this band will be discussed below.
2
6 2
maxima of 2(PF ) through 6(PF ) locate at 515, 495, 496,
6
2
6 2
516, and 512 nm, respectively, which are around 20 or 40 nm
red-shifted with respect to that of [Ru(tpy) ](PF ) (475 nm,
2
6 2
Table 2). The following TDDFT results suggest that the visible
absorptions of the amine-containing complexes have significant
contributions from the intraligand charge transfer (ILCT)
transitions, in addition to the metal-to-ligand charge transfer
When the potential was further increased from +0.95 to
+1.15 V to oxidize the amine unit of the Ntpy ligand of
2(PF ) , the MNNPhtpyCT transition at 1000 nm decreased, and
6
2
an intense absorption band at 1380 nm appeared (Figure 3b).
C
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Figure 3. Absorption spectral changes of (a,b) 2(PF ) , (c) 4(PF ) , and (d) 5(PF ) upon stepwise oxidative electrolysis at an ITO glass electrode
6
2
6
2
6 2
in CH Cl . The applied potentials were referenced versus Ag/AgCl. The inset in (b) shows the enlarged plots in the region between 800 and 1200
2
2
nm. The small and weak bands of the black curve in the NIR region of (c) are due to a nonperfect compensation of the background absorption.
The latter band is assigned to the charge transfer from the
ruthenium ion to the oxidized aminium radical cation of the
Ntpy ligand (MNNtpyCT). This assignment is also supported by
4
+
the following TDDFT results of 2 . In addition, the
MNNtpyCT has been observed at a similar position for complex
16
3
(PF ) after one-electron oxidation. The decrease of the
6 2
MNNPhtpyCT transition and the simultaneous appearance of the
MNNtpyCT transition indicate that the direction of charge
4+
3+
transfer in 2 is reversed with respect to that in 2 . When the
potential was decreased from +1.15 V to +0.60 V, a reverse
absorption spectral change can be observed, suggesting the
reversibility of the redox reactions.
3+
Figure 4. Gaussian-fitting of the NIR absorption of 2 . The black
When complexes 4(PF ) and 5(PF ) were subjected to
6
2
6 2
curve is the experimental data. The red curve is the sum of the
similar oxidative electrolysis, the visible absorptions decreased
deconvoluted subbands.
•+
significantly (Figure 3c and d). At the same time, the N -
localized transitions at 750 nm and the MNNPhtpyCT transition
at 1000 nm appeared. The molar absorptivities of these two
band is from the charge transfer from the amine site of Ntpy to
the ruthenium ion (NNtpyMCT). In this way, the ruthenium ion
is involved in the indirect charge transfer from one side of the
molecule to the other, as has been proposed in a three-state
4+
3+
transitions of 5 roughly double with respect to those of 4 . In
addition, no IVCT transition can be distinguished for 5(PF6)2
during the spectroelectrochemical measurements. This suggests
that no efficient electronic coupling is present between the two
amine units of 5(PF ) , consistent with the above electro-
20
model. Because of the uncertainty in the assignment of this
band, no further quantitative analysis has been performed.
The above analysis suggests that the presence of the
6
2
chemical studies. Previous studies showed that an IVCT band
3+
3+
additional phenyl groups within the ligands of 2 and 5
could be observed for the shorter diamine complex 6(PF )
3+
6
2
with respect to 6 quenches the electron transfer. Direct
electronic interaction is present for complex 6 . For complex
16a
after one-electron oxidation.
The NIR transitions of 2 were deconvoluted by Gaussian
3+
3
+
3+
2
with one additional phenyl group, the degree of direct
−1
functions (Figure 4). Two subbands at 9800 and 7500 cm 1can
be distinguished. The more intense band at 9800 cm− is
assigned to the MNNPhtpyCT transition. The small band at 7500
interaction is decreased or maybe the electron transfer is a two-
step mechanism involving the ruthenium ion. The presence of
two of those phenyl groups completely prevents the electron
−1
3+
cm could be a weak IVCT band between two distal amine
transfer for complex 5 .
−
1
2+
sites. However, this band is too narrow (1500 cm ) for a class
II compound and two weak (εmax = 700 M cm ) for a class
III compound. In addition, in the transformation of 2 to 2
Figure 3b), the decrease of this weak band has not been
DFT and TDDFT Studies. The DFT calculations of 2 and
52+ have been performed on the level of theory of B3LYP/
LANL2DZ/6-31G*/CPCM (see details in the Experimental
Section). Figure 5 shows the calculated energy diagram and
isodensity plots of frontier molecular orbitals of 2 . The
highest occupied molecular orbital (HOMO), HOMO−1, and
−1
−1
3+
4+
(
2
+
observed. Thus, the assignment of this small band as the IVCT
transitions is not unambiguous. Another possibility is that this
D
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Figure 5. DFT-calculated energy diagram of 22+ and the isodensity plots of frontier molecular orbitals.
HOMO−2 are dominated by the NPhtpy ligand, the Ntpy
ligand, and the ruthenium ion, respectively. These three orbitals
are well-separated from each other, with the eigenvalues of
other hand, the predicted S − S excitations are mainly
6
8
associated with the ILCT transitions from the HOMO−1
orbital, namely the amine unit of the Ntpy ligand. The S and
9
−
5.32, −5.82, and −6.36 eV, respectively. This result is
consistent with the above electrochemical assignment of
(PF ) . The lowest unoccupied molecular orbital (LUMO)
S10 excitations have major contributions from the HOMO−3
→
LUMO and HOMO−3 → LUMO+2 transitions, which are
2
6
2
mainly of the MLCT character. These results suggest that the
visible absorptions of 22+ have major contributions from the
ILCT transitions of both NPhtpy and Ntpy ligands and the
MLCT transitions. Similarly, the TDDFT results of complex
and LUMO+1 and LUMO+2 are all dominated by the tpy unit
of both ligands.
DFT calculations have also been performed on the open-
shell states 23 and 2 with the same level of theory. Figure 6
+
4+
2
+
5
show that the visible absorptions have combined
contributions from the ILCT transitions of the NPhtpy ligand
and the MLCT transitions (see details in Table S1 and Figure
2
+
2+
S2). Similar situation should apply to complexes 3 and 4 .
Table 3. TDDFT-Predicted Excitations of 23+ and 24+
a
dominant transitions (percent
complex No.
λ/nm
f
contribution)
23+
D1
D2
D3
D4
D5
D6
T1
T2
T3
T4
T5
T6
1521.8
922.2
889.0
837.0
749.1
691.8
1092.5
1044.1
1036.1
827.4
758.4
752.1
0.0003
0.1742
0
β-HOSO → β-LUSO (99%)
β-HOSO-1 → β-LUSO (96%)
β-HOSO-2 → β-LUSO (99%)
β-HOSO-3 → β-LUSO (97%)
β-HOSO-5 → β-LUSO (97%)
β-HOSO-4 → β-LUSO (100%)
β-HOSO-1 → β-LUSO (72%)
β-HOSO-2 → β-LUSO (96%)
β-HOSO → β-LUSO (69%)
β-HOSO-4 → β-LUSO (99%)
β-HOSO-3 → β-LUSO+1 (96%)
β-HOSO-3 → β-LUSO (97%)
0.0003
0.2989
0
Figure 6. DFT-calculated spin density plots of 23+ and 24+.
24+
0.0005
0.0015
0.2723
0.2813
0.2967
0.0068
shows the Mulliken spin density distributions (α − β) of these
3
+
two complexes. The spin of 2 is dominated by the NPhtpy
4
+
ligand. The spins of 2 are dominated by both NPhtpy and
Ntpy ligands. The ruthenium ion makes little contributions to
3
+
4+
the spin distributions of 2 and 2 . These results are also
consistent with the above electrochemical and spectroscopic
studies. However, the overestimation of charge delocalization of
the DFT methods should be mentioned here, and these results
should be considered with care.
a
Computed at the TDDFT/UB3LYP/LANL2DZ/6-31G*/CPCM
level of theory. D = doublet; T = triplet; f = oscillator strength;
HOSO = highest occupied spin orbital; LUSO = lowest unoccupied
spin orbital.
To rationalize the origins of the absorption spectra of the
above complexes, TDDFT calculations have been carried out
Table 3 and Figure 7 show the TDDFT results of the open-
shell complexes 2 and 2 . An IVCT transition with a very
2+
2+ 3+
4+
3+
4+
on 2 , 5 , 2 , and 2 . The predicted S (S = singlet) and S
1
6
−
S10 excitations are mainly responsible for the visible
weak oscillator strength (f = 0.0003) is predicted by the D
excitation (λ = 1521.8 nm) of 2 , with the dominant β spin
1
2+
3+
absorptions of 2 (Tables S1). The S excitation is dominated
1
by the HOMO → LUMO transition, which is mainly of the
ILCT character associated with the NPhtpy ligand. On the
transition from the highest occupied spin orbital (HOSO) to
the lowest unoccupied spin orbital (LUSO). The predicted D2
E
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Figure 7. TDDFT-predicted excitations (red curves) of (a) 23+ and (b) 2 . The observed absorption spectra (black curves) are included for
4+
comparison. See details in Table 3
excitation of 23+ of the β-HOSO-1 → β-LUSO character (λ =
22.2 nm, f = 0.1742) is responsible for the MNNPhtpyCT
transitions. In addition, the D excitation (λ = 749.1 nm, f =
(III) complexes are well-known to be EPR inactive at room
9
temperature due to fast spin relaxation by spin−orbital coupling
2
1
or display rhombic or axial EPR signals at low temperature.
5
•
+
4+
0.2989) is consistent with the N -localized transitions. Other
Complex 2 +shows a similar but much weaker EPR signal with
3
respect to 2 .
transitions are with negligible oscillator strengths.
4
+
For complex 2 , the predicted T (λ = 1036.1 nm, f =
3
0
.2723) excitation associated with the β-HOSO → β-LUSO
CONCLUSION
■
transition has a major contribution from the MNNtpyCT
transitions. This excitation is responsible for the observed
absorption band at 1380 nm, although the predicted excitation
is 0.3 eV higher in energy with respect to the observed data.
The T and T excitations associated with the β-HOSO-4 → β-
In summary, a bis-terpyridine ruthenium complex with two
redox-asymmetric amine units has been designed and
synthesized. Because of the different electronic natures of
these two amine units, they can be stepwisely oxidized with a
potential separation of 200 mV. This design is reminiscent of
the redox-asymmetric mixed-valent systems with either organic
4
5
LUSO and β-HOSO-3 → β-LUSO+1 transitions are consistent
•+
22
with the N -localized transitions of both NPhtpy and Ntpy
ligands. These TDDFT results are basically in agreement with
the previous absorption assignment of these two complexes.
Electron Paramagnetic Resonance (EPR) Analysis.
Complex 2 , obtained by chemical oxidation using cerium
ammonium nitrate, shows a strong isotropic EPR signal at g =
or inorganic redox sites. In the dioxidized state, the direction
of the metal-to-aminium charge transfer is reversed with respect
to that in the mono-oxidized state. A weak NIR absorption
band at 7500 cm− was observed for the mono-oxidized state,
which is likely from the intervalence or amine-to-metal charge
transfer transition. This supports the presence of a direct or
indirect electronic communication between two amine sites of
this complex. In addition, mono- and dioxidized states can be
distinguished by the significantly different NIR absorption
signals, suggesting their potential uses as electrochromic
1
3+
2
.006 at room temperature in CH CN (Figure 8). This
3
indicates that the unpaired electron is localized on the nitrogen
atom, instead of the ruthenium ion. Polypyridine ruthenium-
11
materials and molecular logic gates.
EXPERIMENTAL SECTION
■
Synthesis. NMR spectra were recorded in the designated solvent
on a Bruker Avance 400 MHz spectrometer. Spectra are reported in
ppm values from residual protons of deuterated solvent. Mass data
were obtained with a Bruker Daltonics Inc. Apex II FT-ICR or
Autoflex III MALDI-TOF mass spectrometer. The matrix for 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. 4′-(p-
1
5
Bromophenyl)-2,2′:6′,2″-terpyridine, [Ru(Ntpy)Cl ], and 3(PF )
3
6 2
16
were prepared according to the previously reported procedures.
Synthesis of 4′-(Di(p-anisyl)amino-phen-4-yl)-2,2′:6′,2″-ter-
Figure 8. EPR spectrum of 23+ in CH CN at rt.
pyridine (1). A suspension of 4′-(p-bromophenyl)-2,2′:6′,2″-terpyr-
3
F
dx.doi.org/10.1021/om500904k | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
idine (97 mg, 0.25 mmol), di(p-anisyl)amine (69 mg, 0.30 mmol),
reference electrode, respectively. The cell was put into the
spectrometer to monitor the spectral change during electrolysis.
Electrochemical Measurement. All electrochemical measure-
ments were taken using a CHI 660D potentiostat with one-
compartment electrochemical cell under an atmosphere of nitrogen.
tris(dibenzylideneacetone)dipalladium (Pd (dba) , 11 mg, 0.013
2
3
mmol), bis(diphenylphosphino)ferrocene (dppf, 7.0 mg, 0.013
t
mmol), and NaO Bu (29 mg, 0.30 mmol) in 10 mL dry toluene was
heated at 140 °C for 48 h under a N atmosphere. The system was
2
cooled to room temperature. The solvent was removed under vacuum
and the crude product was purified by chromatography on silica gel
All measurements were carried out in CH
3
CN containing 0.1 M
Bu NClO as the supporting electrolyte at a scan rate of 100 mV/s.
4 4
n
(
eluting with petroleum ether/ethyl acetate/NH OH 50/15/1) to
The working electrode was a glassy carbon with a diameter of 3 mm.
The electrode was polished prior to use with 0.05 μm alumina and
rinsed thoroughly with water and acetone. A large area platinum wire
coil was used as the counter electrode. All potentials are referenced to
4
1
yield 94 mg of 1 as a gray solid (70%). H NMR (400 MHz, CDCl ):
δ 3.82 (s, 6H), 6.87 (d, J = 8.8 Hz, 4H), 7.01 (d, J = 8.8 Hz, 2H), 7.11
3
(
7
d, J = 9.2 Hz, 4H), 7.35 (t, J = 6.0 Hz, 2H), 7.76 (d, J = 8.8 Hz, 2H),
.89 (t, J = 7.8 Hz, 2H), 8.67 (d, J = 8.0 Hz, 2H), 8.71 (s, 2H), 8.73 (d,
+
/0
Ag/AgCl electrode in saturated aqueous NaCl or Fc
.
13
J = 4.0 Hz, 2H). C NMR (100 MHz, CDCl ): 156.4, 156.1, 155.7,
Computational Methods. DFT and TDDFT calculations are
3
2
3
carried out using the B3LYP, exchange correlation functional and
1
1
5
49.7, 149.6, 149.0, 140.4, 136.8, 129.5, 127.8, 126.9, 123.6, 121.3,
19.9, 117.9, 114.7, 55.42. EI-HRMS: calcd for C H N O
2
2
4
implemented in the Gaussian 09 program package. The electronic
structures of complexes were determined using a general basis set with
the Los Alamos effective core potential LanL2DZ basis set for
3
5
28
4
36.22068. Found: 536.22026.
Synthesis of Complex 2(PF ) . To 10 mL ethylene glycol were
6
2
2
5
ruthenium, and 6-31G* for other atoms. No symmetry constraints
were used in the optimization (nosymm keyword was used). Solvent
effects (CH Cl ) are included in all calculations with the conductor-
added ligand 1 (27 mg, 0.050 mmol) and [Ru(Ntpy)Cl ] (33.0 mg,
0
=
was treated with an excess of aq. KPF . The resulting precipitate was
collected by filtering and washing with water and Et O. The obtained
solid was subjected to flash column chromatography on silica gel
3
.050 mmol). The mixture was refluxed by microwave heating (power
375 W) for 30 min. After cooling to room temperature, the system
2
2
2
6
like polarizable continuum model (CPCM). Frequency calculations
have been performed with the same level of theory to ensure the
optimized geometries to be local minima. All orbitals have been
6
2
3
computed at an isovalue of 0.02 e/bohr . The TDDFT-predicted
(
eluent: saturated aq. KNO /H O/CH CN, 1/10/600) to give 37 mg
3 2 3
1
spectra were generated using GaussView 5.0.
EPR Measurements. EPR measurements were performed on a
of 2(PF ) as an orange solid in 54% yield. H NMR (400 MHz,
CD CN): δ 3.83 (s, 6H), 3.89 (s, 6H), 7.03 (d, J = 8.8 Hz, 4H), 7.04
6
2
3
Bruker ELEXSYS E500−10/12 spectrometer at room temperature in
(
6
(
7
m, 4H), 7.15 (d, J = 9.2 Hz, 4H), 7.22 (d, J = 8.8 Hz, 4H), 7.23 (t, J =
.0 Hz, 2H), 7.31 (d, J = 5.2 Hz, 2H), 7.50 (d, J = 8.8 Hz, 4H), 7.55
d, J = 5.2 Hz, 2H), 7.74 (t, J = 7.6 Hz, 2H), 7.88 (s, 2H), 7.92 (t, J =
.5 Hz, 2H), 8.09 (d, J = 8.8 Hz, 2H), 8.09 (d, J = 8.0 Hz, 2H), 8.59
9
3+
CH CN. The spectrometer frequency is 9.7 × 10 Hz. Complexes 2
3
4+
and 2 were obtained by chemical oxidation of 2(PF ) with 0.5 or 1.5
6
2
equiv cerium ammonium nitrate.
+
(
1
d, J = 8.0 Hz, 2H), 8.87 (s, 2H). MALDI-MS: 1242.9 for [M − PF ] ,
6
+
ASSOCIATED CONTENT
Supporting Information
098.0 for [M − 2PF ] . Anal. Calcd for C H F N O P Ru: C,
■
6
64 52 12
8
4 2
5
5.37; H, 3.78; N, 8.07. Found: C, 55.42; H, 3.96; N, 7.70.
Synthesis of Complex 4(PF ) . Using the similar procedure for
*
S
6
2
CVs of 2(PF ) , 4(PF ) , and 5(PF ) , DFT and TDDFT
6
2
6 2
6 2
the synthesis of 2(PF ) , complex 4(PF ) was obtained from the
reaction of [Ru(tpy)Cl ] (22.0 mg, 0.050 mmol) and ligand 1 (27 mg,
0
6
2
6
2
results, and NMR and mass spectra of new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
3
1
.050 mmol) as an orange solid in 48% yield. H NMR (400 MHz,
CD CN): δ 3.74 (s, 12H), 6.92 (d, J = 9.2 Hz, 8H), 6.97 (d, J = 8.8
3
Hz, 4H), 7.12 (d, J = 8.8 Hz, 8H), 7.21 (t, J = 6.0 Hz, 4H), 7.67 (d, J =
AUTHOR INFORMATION
Corresponding Authors
*E-mail: zhongyuwu@iccas.ac.cn (Y.-W.Z.).
E-mail: jnyao@iccas.ac.cn (J.Y.).
5
.2 Hz, 4H), 7.95 (t, J = 7.8 Hz, 4H), 8.05 (d, J = 8.8 Hz, 4H), 8.88 (d,
■
13
J = 8.0 Hz, 4H), 9.19 (s, 4H). C NMR (100 MHz, CD COCD ):
3
3
5
1
1
1
8
4.6, 114.7, 118.2, 119.8, 123.5, 124.1, 124.2, 126.5, 127.2, 127.3,
27.4, 128.2, 135.6, 137.8, 137.8, 139.3, 148.0, 150.8, 152.1, 152.2,
*
+
55.0, 155.3, 156.9, 158.0, 158.3. MALDI-MS: 1318.9 for [M − PF ] ,
6
+
Notes
173.9 for [M − 2PF ] . MALDI-HRMS calcd for C H N O Ru:
6
50 38
7
2
The authors declare no competing financial interest.
64.2157. Found: 864.2165.
Synthesis of Complex 5(PF ) . Using the similar procedure for
6
2
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China
grants 21271176, 91227104, 21472196, and 21221002), the
National Basic Research 973 program of China (grant
011CB932301), and the Strategic Priority Research Program
the synthesis of 2(PF ) , complex 5(PF ) was obtained from the
6
2
6
2
■
reaction of ligand 1 (27 mg, 0.050 mmol) and RuCl ·3H O (7.0 mg,
3
2
1
0
.025 mmol) as an orange solid in 65% yield. H NMR (400 MHz,
(
CD CN): δ 3.74 (s, 6H), 6.92 (d, J = 9.2 Hz, 4H), 6.98 (d, J = 8.8 Hz,
3
2
H), 7.13 (d, J = 8.8 Hz, 4H), 7.20 (t, J = 5.6 Hz, 2H), 7.23 (t, J = 5.6
Hz, 2H), 7.58 (d, J = 5.2 Hz, 2H), 7.69 (d, J = 5.2 Hz, 2H), 7.96 (t, J =
.8 Hz, 4H), 8.04 (d, J = 8.8 Hz, 2H), 8.46 (t, J = 8.2 Hz, 1H), 8.71 (d,
J = 8.0 Hz, 2H), 8.89 (d, J = 8.0 Hz, 2H), 8.97 (d, J = 8.0 Hz, 2H),
2
of the Chinese Academy of Sciences (grant XDB 12010400) for
funding support.
7
9
1
1
.21 (s, 2H). 13C NMR (100 MHz, CD COCD ): 54.7, 114.8, 118.4,
3 3
REFERENCES
19.9, 124.3, 126.6, 127.3, 127.4, 128.3, 137.8, 139.4, 147.8, 150.9,
■
+
52.2, 155.3, 157.0, 158.5. MALDI-MS: 1015.0 for [M − PF ] , 871.0
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dx.doi.org/10.1021/om500904k | Organometallics XXXX, XXX, XXX−XXX

Organometallics
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