Electronic communication between two amine redox centers bridged by a bis(terpyridine)ruthenium(II) complex

Article abstract of DOI:10.1021/ic200701j

Two bis(terpyridine)ruthenium(II) complexes 2 and 3 appended with one or two di-p-anisylamino groups, respectively, were synthesized and fully characterized. Their electronic properties were studied by electrochemical and spectroscopic analyses. Electronic communication between individual amine sites of 3 was estimated by intervalence charge-transfer band analyses.

Full text of DOI:10.1021/ic200701j

COMMUNICATION
pubs.acs.org/IC
Electronic Communication between Two Amine Redox Centers
Bridged by a Bis(terpyridine)ruthenium(II) Complex
Chang-Jiang Yao, 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
b
40-bromoterpyridine and 4,40-dimethoxydiphenylamine (see the
ABSTRACT: Two bis(terpyridine)ruthenium(II) com-
plexes 2 and 3 appended with one or two di-p-anisylamino
groups, respectively, were synthesized and fully character-
ized. Their electronic properties were studied by electro-
chemical and spectroscopic analyses. Electronic communi-
cation between individual amine sites of 3 was estimated by
intervalence charge-transfer band analyses.
Supporting Information for details). The reaction of RuCl3 with 2
equiv of 1, followed by anion exchange with KPF6, afforded
complex 3 in 81% yield. Complex 2, with one di-p-anisylamino
substituent, was also prepared for a comparison study. The
identities of new compounds were confirmed by 1H NMR, mass
spectrometry, and micꢀroanalysis. Besides, a single crystal of
complex 3 with a NO3 counteranion was obtained by slowly
diffusing n-hexane into a solution of 3 in CHCl3, and its X-ray
crystallographic structure is shown in Figure 1.12 The NꢀN
ꢀ
distance between two amino N atoms was found to be 12.26 Å.
The electronic absorption spectra of compounds 1ꢀ3 and
benchmark complex [Ru(tpy)2](PF6)2 are shown in Figure 2a.
Ligand 1 displays an intraligand (IL) πꢀπ* transition at 281 nm
and a shoulder between 310 and 400 nm, which is assigned to the
IL charge-transfer (ILCT) transition from the di-p-anisylamino
group to the tpy moiety.10 In comparison, complexes 2 and 3
both exhibit a broad and intense band in the visible region, in
addition to the slightly red-shifted and more intense IL and ILCT
transitions. The broad band in the visible region is attributable to
an admixture of metal-to-ligand charge-transfer (MLCT) and
ligand-to-ligand charge-transfer (LLCT) transitions.10
The electronic properties of these compounds were further
studied by electrochemical analysis (Table S1 and Figures
S1ꢀS4 in the Supporting Information and Figure 3). The cyclic
voltammogram (CV) of ligand 1 reveals a redox couple at +0.97
V and an irreversible anodic peak at +1.41 V vs Ag/AgCl
(Figure 3a). The first peak is chemically reversible, as shown
by the red line, where the potential was scanned back at +1.2 V.
However, a relatively large separation (110 mV) between the
anodic peak and the return cathodic peak suggests a somewhat
slow electron-transfer process. This peak is ascribed to the
N/N•+ process of the di-p-anisylamino site. The second peak
at +1.41 V is assigned to the further oxidation of the in situ
generated radical cation to a dication species (N2+), with the
production of new chemical species, which gives rise to a new
peak near 0 V. Similar processes have been reported to occur
from various triarylamines.13,14 We did not determine the
identity of the decomposed product at this stage. However, some
literatures prove that a carbazole derivative is produced during
similar processes.14 We note that this decomposed product is
also evident in the CV profile of complex 3 (Figure 3d). The
anodic scan of complex 2 displays two redox couples at +0.98 and
+1.44 V, respectively (Figure 3c). The former peak is attributable
ixed-valence (MV) compounds have been the subject of
M
enormous research activities in the past 4 decades.1 In
comparison with the extensively studied inorganic and organo-
metallic binuclear complexes,2 organic MV compounds have
remained relatively unexplored. Among numerous organic redox
groups,3 triarylamines have gained increasing attention in the
studies of MV systems,4 in addition to their widespread applica-
tions in optoelectronic devices.5 On the other hand, transition-
metal polyazine complexes have attracted extensive interest
because of their superior photophysical and electrochemical
properties.6 Linear coordination oligomers consisting of multiple
octahedral Ru atoms have recently been studied as single-
molecular wires.7 We report herein the electronic communica-
tion of two di-p-anisylamino redox centers coupled through a
polyazine transition-metal motif [Ru(tpy)2], where tpy is
2,20:60,200-terpyridine. The effect of [Ru(tpy)2] on electronic
delocalization is assessed by analyzing the intervalence charge-
transfer (IVCT) transition associated with optically induced hole
transfer from one triarylamine center to the other. To the best of
our knowledge, this is the first example to use transition-metal
polypyridyl complexes as the bridge unit to build triarylamine
MV systems. However, organic MV systems bridged by a metal
porphyrin8 or a platinum alkynyl9 unit have been documented.
The following considerations were taken into account when
complex 3 in Scheme 1 was designed. First, 4-methoxy substit-
uents were chosen to ensure reversible oxidation of triarylamines.
Second, the Ru atom was chosen because the anodic redox
potential of [Ru(tpy)2]2+ is well-separated from those of triar-
ylamines and the ligand lability is not an issue in this complex.
Third, amine atoms are directly connected to tpy ligands to
ensure strong coupling between them. We note that there are
several reports on the Ru(tpy)ꢀtriarylamine conjugated
systems.10 However, none of these intended to study the
electronic coupling between amine redox centers. Ligand 1
containing a di-p-anisylamino motif was obtained in 78% yield
through a Pd-catalyzed CꢀN bond formation11 between
Received: April 6, 2011
Published: July 07, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ic200701j Inorg. Chem. 2011, 50, 6847–6849
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Inorganic Chemistry
COMMUNICATION
Scheme 1. Synthesis of Compounds 1ꢀ3a
a The counteranions are PF6
.
ꢀ
Figure 3. CVs of 1 (a), [Ru(tpy)2](PF6)2 (b), 2 (c), and 3 (d) in a
mixture of acetonitrile/CH2Cl2 (1:1) containing 0.1 M Bu4NClO4 as
the supporting electrolyte at a scan rate of 100 mV sꢀ1
.
larger cathodic current than the anodic current of the peak at
+0.98 V in the black line of Figure 2c is probably a result of the
contamination caused by the irreversible N•+/N2+ process.
However, this is not an issue when the potential was scanned
back before the N•+/N2+ process (red line). It is noteworthy that
the RuII/III process of 2 is 120 mV more positive than that of
[Ru(tpy)2](PF6)2 (Figure 3b). This suggests that the di-p-
anisylamino unit is positively charged and withdraws the electron
density from the metal center.
CV profiles of complex 3 are shown in Figure 3d. The first two
redox couples at +0.96 and +1.23 V vs Ag/AgCl could be ascribed
to the stepwise oxidation of two di-p-anisylamino units into
corresponding radical-cation species. The potential separation
(ΔE) between two peaks is 270 mV. The comproportionation
constant15 Kc for the equilibrium [NꢀRuIIꢀN] + [N•+ꢀRuIIꢀ
N•+] T [NꢀRuIIꢀN•+] is 2.55 ꢁ 104, which indicates the high
thermodynamic stability of the electrochemically generated MV
compound [NꢀRuIIꢀN•+]. The irreversible peak at +1.56 V is
attributable to the further oxidation of the in situ produced
ammonium radical cations, as is found in free ligand 1. The
reversible redox couple at +1.64 V is ascribed to the RuII/III
process of 3. This process occurs at a more positive potential
than that of 2 and [Ru(tpy)2](PF6)2 because of the presence of
two appended ammonium cations. The cathodic scans of 2 and 3
both display two redox couples from the reduction of tpy
ligands. It is clear that the electrochemical energy gap of 2 or
3, determined by the potential difference between the first anodic
and first cathodic peaks, is narrower than that of [Ru(tpy)2]-
(PF6)2.
Figure 1. Thermal ellipsoid plot with 50% probability of [3](NO3)2.
H atoms and solvents and counteranions are omitted.
To further probe the electronic delocalization of 2 and 3, their
UV/vis/near-IR (NIR) absorption spectral changes upon the
gradual addition of a SbCl5 solution in CH2Cl2 are monitored
(Figure 2b,c). Upon the gradual addition of the oxidant, the di-p-
anisylamino unit in complex 2 was stepwisely oxidized to N•+, as
evidenced by the emergence of a new band at 810 nm arising
from a typical triarylamine radical-cation species.4 At the same
time, the MLCT/LLCT transitions display a slight red shift and
an intense band at 1360 nm appears. The latter peak is assigned
to an electron transfer from the metal component to the
triarylamine center (M f N•+). The observation of a clear
Figure 2. (a): UV/vis absorption spectra of 1ꢀ3 in CH2Cl2 and
[Ru(tpy)2](PF6)2 in acetonitrile; (b) and (c): UV/vis/NIR electronic
absorption spectral changes of 2 (b) and 3 (c) CH2Cl2 after gradual
addition of SbCl5; (d): Deconvoluted IVCT transition band (red line) of
MV complex of 3.
to the N/N•+ process of the di-p-anisylamino moiety, and the
latter peak is assigned to the RuII/III process, which overlaps with
the irreversible N•+/N2+ oxidation at +1.35 V. The relatively
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Inorganic Chemistry
COMMUNICATION
isosbestic point at 520 nm suggests a clean conversion of the
N/N•+ process. A further addition of SbCl5 causes no further
change of the spectrum.
EPR plots of 2 and 3 after one-electron oxidation. This material is
available free of charge via the Internet at http://pubs.acs.org.
The absorption spectral changes of complex 3 in response to the
addition of SbCl5 are shown in Figures 2c and S5 and S6 in the
Supporting Information. Upon the gradual addition of the oxidant,
one of the di-p-anisylamino sites was first oxidized to the N•+
species. The absorption spectrum shows changes similar to those in
complex 2 except that a clear shoulder band around 1160 nm grows
gradually (highlighted with a red circle). Upon a further addition of
the oxidant, the second triarylamine unit was oxidized as well. The
previously observed shoulder disappears gradually, and the N•+
band around 800 nm shifts bathochromically slightly. On the basis
of these facts, the shoulder band at 1160 nm is assigned to the IVCT
transition of the chemically generated MV complex of 3. Part of the
NIR transitions of the one-electron-oxidized intermediate of com-
plex 3 is shown in Figure 2d. Because the IVCT band is overlapped
with the intense M f N•+ transition (around 1400 nm), we assume
that the IVCT band is symmetrical and Gaussian-shaped. Thus, the
black line from the experimental data is deconvoluted into the IVCT
band (red line) and the M f N•+ transition (blue line), as shown in
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: zhongyuwu@iccas.ac.cn.
’ ACKNOWLEDGMENT
We thank the National Natural Science Foundation of China
(Grant 21002104), the National Basic Research 973 program of
China (Grant 2011CB932301), and the Institute of Chemistry,
Chinese Academy of Sciences (“100 Talent” Program), for
funding support.
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Figure 2d. The IVCT band centers at 1240 nm (νmax = 8050 cmꢀ1
)
)
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1/2/rab,16 where rab is the diabatic electron-transfer
ν
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assembling organic redox centers with transition-metal com-
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fication of the electronic coupling between two amine redox sites
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information in terms of their applications as conducting molec-
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Cl39N20O20Ru2, M = 3844.05, triclinic, a = 10.858(2) Å, b = 18.726(4)
ꢀ
Å, c = 20.548(4) Å, R = 97.57(3)°, β = 102.43(3)°, γ = 90.03(3)°, U =
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4042.3(14) Å , T = 173 K, space group P1, Z = 1, 11 514 reflections
ꢀ
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’ ASSOCIATED CONTENT
S
Supporting Information. Syntheses and characteriza-
(17) Heckmann, A.; Amothor, S.; Lambert, C. Chem. Commun.
2006, 2959.
b
tion, NMR spectra of new compounds, CV profiles of 1ꢀ3,
CIF file of 3, plots of spectral changes of 3 upon oxidation, and
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