1,4-Benzene-bridged covalent hybrid of triarylamine and cyclometalated ruthenium: A new type of organic-inorganic mixed-valent system

Article abstract of DOI:10.1039/c2cc32471g

A 1,4-benzene-bridged covalent hybrid of triarylamine and cyclometalated ruthenium 1²⁺ was isolated as a bench-stable open-shell substance. The free spin in this complex is mainly associated with the triarylamine unit, as indicated by EPR and D

Full text of DOI:10.1039/c2cc32471g

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1,4-Benzene-bridged covalent hybrid of triarylamine and cyclometalated
ruthenium: a new type of organic–inorganic mixed-valent systemw
Chang-Jiang Yao,a Ren-Hui Zheng,a Qiang Shi,a Yu-Wu Zhong*ab and Jiannian Yaoa
Received 10th March 2012, Accepted 20th April 2012
DOI: 10.1039/c2cc32471g
A 1,4-benzene-bridged covalent hybrid of triarylamine and
cyclometalated ruthenium 12+ was isolated as a bench-stable
open-shell substance. The free spin in this complex is mainly
associated with the triarylamine unit, as indicated by EPR and
DFT calculations and electrochemical analysis. It exhibits an
intense intervalence-charge-transfer transition around 1050 nm
that is not present in 1+ and 13+
.
Open-shell compounds with a general formula of [M1-bridge-M2]nÆ
have attracted considerable attention since the pioneering
work of Creutz and Taube on mixed-valent (MV) chemistry.1
The interest arises from their implications in a wide range of
fields such as molecular electronics, optoelectronics, and artificial
photosynthesis.2 In most reported examples, M1 = M2 and they
stand for either a purely organic3 or organometallic/inorganic
redox center.4 Only a few studies have been devoted to those
with redox/energetically asymmetric centers.5 MV systems
with strong electronic coupling provide a simple means to
absorb light in the near infrared (NIR, 750–3000 nm),6 which
is of great importance in many military and civilian uses.7 In
particular, those exhibiting distinct redox-switchable NIR
absorptions are of paramount interest as intelligent materials.8
However, NIR materials with high stability, low redox-switching
potentials, and strong molar absorptivities are still in great
demand. We present in this communication a covalent hybrid
1n+ (n = 1–3, Fig. 1) of triarylamine and cyclometalated
ruthenium.9 It should be noted that both redox species exhibit
Fig. 1 Hybrids of triarylamine and ruthenium. Counter-anions
À
are PF6
.
intense intervalence-charge-transfer transition that is not present
in 1+ and 13+
.
Details for the preparation of 1 are provided in the ESI.w
On the basis of known procedures for the synthesis of cyclo-
metalated ruthenium complexes,9 the reaction of Ru(tpy)Cl3
and 1-(di-p-anisylamino)-3,5-di(2-pyridyl)benzene in the presence
of AgOTf followed by anion exchange with KPF6 was carried
out. To our surprise, the only isolated product was the open-
shell complex 12+ instead of the expected 1+ with one counter-
anion. Although we failed to obtain a single crystal of this
complex for X-ray analysis at this stage, the identity of 12+ has
been fully proved. The polarity (as judged by thin layer
chromatography) and microanalysis of 12+ agree well with
the presence of two counter-anions. No distinct 1H NMR
spectrum of 12+ could be recorded, which indicates that it is
a paramagnetic substance. More importantly, it displays a
singlet electron paramagnetic resonance signal at g = 2.0348
at room temperature (Fig. 2). The density functional theory
(DFT)-calculated spin density plot of 12+ is shown in Fig. 2
(see details in the ESIw), which suggests that the triarylamine
+
chemically reversible redox processes (N/Nꢀ and RuII/III) at
appreciably low potentials. Upon one-electron oxidation,
1,4-benzene-bridged bistriarylamine10 and biscyclometalated
ruthenium11 have previously been reported to generate fully
delocalized systems with strong NIR absorptions. As will be
presented in this communication, open-shell compound 12+
was isolated as a bench-stable material and it displays an
a Beijing National Laboratory for Molecular Sciences,
CAS Key Laboratory of Photochemistry, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, P. R. China
b State Key Laboratory of Coordination Chemistry,
Nanjing University, Nanjing 210093, P. R. China.
E-mail: zhongyuwu@iccas.ac.cn; Fax: +86-10-62559373;
Tel: +86-10-62652950
w Electronic supplementary information (ESI) available: Synthesis
and characterization and computational results of 1 and 2. See DOI:
10.1039/c2cc32471g
unit provides a major contribution to the free spin in 12+
.
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5680 Chem. Commun., 2012, 48, 5680–5682
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Fig. 2 EPR signal (left) and calculated spin density (right) of 12+
.
It should be kept in mind that a cyclometalated ruthenium-
dominated spin often exhibits a rhombic or axial EPR signal.11
For comparison, cyclometalated complex 2+ with an opposite
Ru–C bond was prepared and studied. As will be presented
below, complex 2+ and a previously reported non-cyclometalated
ruthenium complex12 32+ containing a triarylamine motif exhibit
significantly different electronic properties than 1. Besides,
other transition metal complexes bearing a triarylamine ligand
have been documented as well.13
Fig. 4 Vis/NIR absorption spectra of 12+ in different solvents.
on the triarylamine motif. This suggests that the couples at
+0.41 and +1.15 V of 2+ could be due to the RuII/III and
+
N/Nꢀ processes, respectively. This assignment is reasonable
but the order of these two processes is opposite to that for 1.
The RuII/III redox potential for most cyclometalated mono-
ruthenium complexes is in the range of +0.5 to +0.7 V vs.
Ag/AgCl.9 The presence of the electron-donating amine group
makes the oxidation of RuII in 2+ easier. The N/Nꢀ+ process
takes place at a relatively high potential due to the electron-
withdrawing metal component. Oxidation of the amine unit in
the non-cyclometalated complex 32+ was previously found to
occur at the same range of potential (+0.98 V vs. Ag/AgCl).12
In addition to these events, both 1 and 2 exhibit a terpyridine-
based reduction wave around À1.5 V and an irreversible wave
at a higher potential, which is possibly due to the further
The electronic properties of the above complexes were first
studied by cyclic voltammetric (CV) analysis (Fig. 3). Complex
12+ exhibits two consecutive reversible couples at +0.27 and
+
+0.68 V vs. Ag/AgCl. They are assigned to the N/Nꢀ and
RuII/III processes, respectively. The DFT-optimized structure of
1+ shows a triarylamine-dominated highest occupied molecular
orbital (HOMO) and a much lower lying cyclometalated
ruthenium-associated HOMO-1 (Fig. S1 and S2w), which are
consistent with the above assignment. The remarkably low
+
potential for the N/Nꢀ process in this complex is possibly
+
oxidation of Nꢀ species.
The most interesting property of 12+ is its absorption
spectrum (Fig. 4 and 5). In addition to the metal-to-ligand-
charge-transfer transitions (MLCT) in the visible region, it
caused by the electron delocalization and/or the inductive
effect of the metal component. In contrast, the CV profile of
complex 2+ evidences two redox couples at +0.41 and
+1.15 V vs. Ag/AgCl. The HOMO and HOMO-2 of 2+ have
major contributions from the cyclometalated ruthenium unit
(Fig. S5 and S6w). The HOMO-1 has comparable contri-
butions from the triarylamine and ruthenium motifs. A
lower-lying HOMO-3 level is found to be mainly localized
displays an intense absorption band in the NIR region (emax
=
2.0 Â 104 MÀ1 cmÀ1 in acetonitrile), which could be attributed
to the intramolecular charge transfer transition from the metal
+
to the amine radical cation center (Ru - Nꢀ electronic
transition). This is corroborated by the time-dependent DFT
(TDDFT) computations of 12+, which suggest that its
NIR absorptions are associated with the 187B - 189B and
Fig. 3 CV profiles of (a) 12+ and (b) 2+ in acetonitrile containing
0.1 M Bu4NClO4 as the supporting electrolyte at a scan rate of
100 mV sÀ1. The working electrode was a glassy carbon, the counter
electrode was a platinum wire, and the reference electrode was
Ag/AgCl in aqueous NaCl.
Fig. 5 Vis/NIR absorption spectral changes upon (a) one-electron
reduction of 12+ with stepwise addition of NH2NH2ÁH2O in aceto-
nitrile and (b, c) one-electron oxidation of 12+ or 2+ with stepwise
addition of SbCl5 in dichloromethane and (d) the second-electron
oxidation of 2+ with stepwise addition of SbCl5.
c
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188B - 189B excitations (Table S1 and Fig. S3w). This
assignment is also supported by data shown below. The
Ru - Nꢀ transition is only slightly solvent dependent both
These features will make them excellent candidates for NIR
electrochromic devices.16 Work is under way to construct
other [M-bridge-N]nÆ hybrid systems where a metal compo-
nent and an organic amine unit are covalently connected by a
conjugated bridge.
+
in terms of shape and energy (Fig. 4, 1084, 1050, 1066, and
1084 nm for CH2Cl2, CH3CN, THF, and DMF, respectively).
This suggests that the Ru–N coupling is very strong. The
electronic coupling parameter Hab in acetonitrile is roughly
estimated to be 2760 cmÀ1, according to the Hush formula14
Notes and references
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Hab = 2.06 Â 10À2 (emax
nmaxDn1/2)
1/2/(rab), where rab is taken
to be the DFT-calculated Ru–N distance (6.16 A). However,
this value is underestimated because the effective electron transfer
distance is much smaller than the geometrical separation15 due to
a charge delocalization and it can be taken only as a low limit of
the actual value.
Fig. 5a and b show the vis/NIR spectral changes of 12+
upon one-electron reduction or oxidation in acetonitrile or
dichloromethane. When 12+ was reduced to 1+ by slowly
+
adding 1 equiv. hydrazine hydrate, the Ru - Nꢀ NIR
transition decreased continuously until it disappeared. At the
same time, a peak at 540 nm increased, which could be
interpreted as the recovery of the ligand-to-ligand-charge-
transfer (triarylamine - pyridines, associated with the S3
+
and S6 excitations of 1+, Table S1w) transitions after Nꢀ
was reduced back to a neutral amine. When 12+ was oxidized
to 13+ by slowly adding one equivalent of SbCl5, the
+
Ru - Nꢀ NIR transition decreased as well. Interestingly,
a very intense peak appeared at 714 nm, which arises from the
+
electronic transition of the triarylamine radical cation (Nꢀ
)
itself. TDDFT results of 13+ predict that this peak is mainly
associated with its 187B - 189B excitation with an admixture
of the 182B - 188B excitation (Table S1 and Fig. S4w). A
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oxidized species of 32+ 12
It is clear from these results that the
.
NIR band at 1050 nm is only observed in the form of 12+
,
but not 1+ and 13+, which agrees with the nature of the
+
Ru - Nꢀ transition.
As for complex 2+, it does not show any NIR absorption
itself. However, upon transformation into 22+ by adding
1 equiv. SbCl5, MLCT transitions of 2+ at 520 nm decreased
and a broad and shallow band between 600 and 1300 nm
appeared (Fig. 5c, emax = 2600 MÀ1cmÀ1). This band is
ascribed to the ligand-to-metal-charge-transfer (LMCT) tran-
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excitation, as suggested by the TDDFT computations of 22+
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+
23+ and an intense peak at 904 nm arising from the Nꢀ
species showed up (Fig. 5d). TDDFT results indicate that this
band is mainly associated with its 187B - 188B excitation
(Table S1 and Fig. S8w). These interpretations are consistent
with the assignment of the electrochemical behaviors of 2+
(Fig. 3b), where the RuII/III process proceeded before oxida-
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In conclusion, we present in this contribution a new type of
mixed-valent system consisting of mixed organic and inorganic
asymmetric redox species. The covalent hybrid 1n+ of triaryl-
amine and cyclometalated ruthenium displays two consecutive
and widely separated redox couples at considerably low
potentials and strong redox-switchable NIR absorptions.
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c
5682 Chem. Commun., 2012, 48, 5680–5682
This journal is The Royal Society of Chemistry 2012