Stepwise coordination followed by oxidation mechanism for the multichannel detection of Cu2+ in an aqueous environment

Article abstract of DOI:10.1021/om400999h

The cyclometalated ruthenium-dipicolylamine (DPA) derivative 3(PF ₆) has been synthesized. In the presence of 1 equiv of Cu ²⁺ in an aqueous environment, a new redox peak at -0.03 V vs Ag/AgCl appeared. This peak is assigned to the Cu^II/I process as a result of the complexation of Cu²⁺ with the DPA unit. In the presence of 2 equiv of Cu²⁺, the metal-to-ligand charge-transfer absorption of 3(PF₆) at 516 nm significantly decreased and a new absorption peak at 750 nm appeared. Accordingly, the solution turned from purple to yellow. The new absorption at 750 nm is assigned to the ligand-to-metal charge-transfer absorption, as a result of the oxidation of the ruthenium component by Cu ²⁺. These optical and electrochemical changes have not been observed in the presence of the other 13 metal ions examined. A single-crystal X-ray structure of 3·Cu·CH₃CN·3ClO₄ has been obtained and used for the elucidation of the stepwise recognition mechanism (coordination followed by oxidation), together with the electrochemical and spectroscopic studies of the two model compounds 2(PF₆) and 4 with only the ruthenium component or the DPA unit.

Full text of DOI:10.1021/om400999h

Article
pubs.acs.org/Organometallics
Stepwise Coordination Followed by Oxidation Mechanism for the
Multichannel Detection of Cu2+ in an Aqueous Environment
Zhong-Liang Gong†,‡ and Yu-Wu Zhong*,†
†Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese
Academy of Sciences, 2 Bei Yi Jie, Zhong Guan Cun, Beijing 100190, People’s Republic of China
‡University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
S
* Supporting Information
ABSTRACT: The cyclometalated ruthenium−dipicolylamine
(DPA) derivative 3(PF6) has been synthesized. In the presence
of 1 equiv of Cu2+ in an aqueous environment, a new redox
peak at −0.03 V vs Ag/AgCl appeared. This peak is assigned to
the CuII/I process as a result of the complexation of Cu2+ with
the DPA unit. In the presence of 2 equiv of Cu2+, the metal-to-
ligand charge-transfer absorption of 3(PF6) at 516 nm
significantly decreased and a new absorption peak at 750 nm
appeared. Accordingly, the solution turned from purple to yellow. The new absorption at 750 nm is assigned to the ligand-to-
metal charge-transfer absorption, as a result of the oxidation of the ruthenium component by Cu2+. These optical and
electrochemical changes have not been observed in the presence of the other 13 metal ions examined. A single-crystal X-ray
structure of 3·Cu·CH3CN·3ClO4 has been obtained and used for the elucidation of the stepwise recognition mechanism
(coordination followed by oxidation), together with the electrochemical and spectroscopic studies of the two model compounds
2(PF6) and 4 with only the ruthenium component or the DPA unit.
the binding of a given ion to the receptor results in a change of
the redox potential of the receptor ligand. Ferrocene (Fc)10 and
tetrathiafulvalene (TTF)11 moieties are frequently used as the
redox-active sites, because they display chemically reversible
redox processes at low potentials (Fc0/+, about +0.45 V vs Ag/
AgCl; TTF0/+ and TTF+/2+, about +0.34 and +0.78 V vs Ag/
AgCl, respectively).11 The redox potential changes upon ion
binding can be easily monitored. The sensitivity is usually high,
and the signal readout is immediate. In addition to Fc and TTF,
other redox-active units, such as quinone and cobaltacene,12
have also been used for electrochemical sensing. In addition to
using a redox-active ligand, other electrochemical detection
methods for metal ions, such as stripping analysis, are also
available.13
On the other hand, polypyridyl transition-metal complexes
have received much attention in the field of ion sensors.14
Taking advantage of their distinguished photophysical proper-
ties, a great number of polypyridyl complexes have been used as
optical sensors for various anions and metal cations.15 This
includes the recent examples reported by van der Boom, who
successfully used OsII monolayers for the optical detection of
Fe3+ and Cr6+.16 Electrochemical sensors based on polypyridyl
metal complexes for anions have also long been known, as
pioneered by Beer, Loeb, Fabbrizzi, and others.17 However, the
use of polypyridyl complexes for the electrochemical detection
of metal ions is limited.18
INTRODUCTION
■
The design and synthesis of highly selective and sensitive
chemosensors for metal ions have been the focus of intensive
research activities, due to their essential roles in various
biological and environmental processes.1 Cu2+ is the third most
abundant transition-metal ion in the human body (after Fe2+
and Zn2+) and plays essential roles in many biological
processes. A number of diseases and disorders, such as Wilson’s
diseases, hypoglycemia, gastrointestinal distress, dyslexia, and
liver or kidney damage, may be caused upon exposure to high
concentrations of Cu2+, which is capable of displacing other
metal ions of enzyme cofactors.2 In addition, Cu2+ also is a
significant environmental pollutant.3 The U.S. Environmental
Protection Agency (EPA) has set the safe limit of copper in
drinking water at 1.3 mg/L (ca. 20 μM).4 For these reasons, the
development of highly selective and sensitive detection
methods for Cu2+ ion has great significance for environment
protection and human health.
To date, a large number of chemosensors for the selective
detection of Cu2+ have been designed on the basis of
fluorescent,5 chromogenic,6 or electrochemical7 signal changes.
Among them, the fluorescent sensing of Cu2+, in conjugation
with cell imaging, has recently been very successful.5,8 For
future practical applications, a new detection method with high
selectivity and multichannel responses in an aqueous environ-
ment is still desirable.
Electrochemical sensing is a promising method for the
detection of anions and metal ions.9 A common strategy is to
design a redox-active ligand containing a recognition site, where
Received: October 11, 2013
Published: November 18, 2013
© 2013 American Chemical Society
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We present in this contribution the polypyridyl ruthenium
complex 3(PF6) (Figure 1) modified with a dipicolylamine
Stille coupling between 3,5-dibromoaniline and (pyrid-2-
yl)tributylstanne gave 3,5-bis(pyrid-2-yl)aniline in 69% yield
(Figure 1). The treatment of 3,5-bis(pyrid-2-yl)aniline with 2-
chloroacetyl chloride afforded compound 1 in good yield. The
reaction of [(ttpy)RuCl3]22 (ttpy = 4′-tolyl-2,2′:6′,2″-terpyr-
idine) with 1 in the presence of AgOTf, followed by anion
exchange using KPF6, gave the complex 2(PF6) in 62% yield.
The desired complex 3(PF6) was isolated from the reaction of
2(PF6) with DPA in the presence of NaI and N,N-
diisopropylethylamine (DIPEA). New complexes were charac-
terized by NMR, microanalysis, and mass data. In addition, the
single-crystal X-ray structure of 3·Cu·(CH3CN)·3ClO4 has
been obtained to further support the structure of these newly
prepared complexes, which will be presented in a later section
of this paper.
Detection of Cu2+ in Aqueous Environment. The
electrochemical characteristics and detectability of receptor
3(PF6) to Cu2+ vs various other metal ions were investigated by
cyclic voltammetric (CV) and differential pulse voltammetric
(DPV) measurements in CH3CN/H2O (4/1 v/v, 50 mM
HEPES, pH 7.2) solution containing 0.1 M Bu4NClO4
electrolyte (Figure 2). The complex 3(PF6) shows a chemically
reversible redox couple at +0.43 V vs Ag/AgCl, assigned to the
RuIII/II redox process.20 This peak is partially overlapped with
an irreversible wave at +0.53 V, as shown in the DPV plot in
Figure 2b, which is possibly caused by the oxidation of the
amine atom. When 1.0 equiv of Cu2+ was added to the receptor
solution, the electrochemical behavior changed dramatically.
Three consecutive redox couples at −0.03, +0.39, and +0.66 V
were observed. When various other metal ions (Cd2+, Co2+,
Fe2+, Fe3+, Hg2+, Ni2+, Zn2+, Mn2+, Mg2+, Na+, and K+,
respectively) were added, there was no redox couple evident
between −0.30 and +0.20 V. One exception is Ag+, which itself
has an irreversible oxidation peak at +0.13 V (possibly an Ag0/+
process; Figure S1 in the Supporting Information). In the
potential region between +0.30 and +1.0 V, the DPV signals are
different in the presence of various metal ions.
The competition experiments were performed in the
presence of 1.0 equiv of Cu2+ and various other ions, and the
currents were measured at −0.03 V (Figure 2b,c). The presence
of Cd2+, Co2+, Mn2+, Ni2+, or Zn2+ did more or less interfere
with the signal readout. In the presence of 10 equiv of Cd2+,
Co2+, or Zn2+, the signal readout was strongly interfered with
(Figure S2, Supporting Information). However, the new peak at
−0.03 V can be clearly discerned as long as the Cu2+ is present.
Moreover, the electrochemical sensing will be complemented
by the optical sensing discussed below. On the basis of these
facts, complex 3(PF6) can specifically signal the presence of
Cu2+ in an aqueous environment by the appearance of a new
redox wave at −0.03 V vs Ag/AgCl.
Figure 1. Synthesis of 3(PF6).
(DPA) motif for Cu2+ detection in an aqueous environment.
The recognition process is accompanied by good selectivity and
multichannel responses (electrochemical, chromogenic, and
colorimetric). In comparison to known compounds for the
detection of Cu2+ ion, this system has a unique recognition
mechanism featuring a stepwise coordination followed by
oxidation process.
RESULTS AND DISCUSSION
■
Design and Synthesis. A prerequisite for an applicable
electrochemical sensor is that the receptor itself possesses
chemically reversible redox processes at low potentials. The
recognition process will be complicated by the electrochemical
behavior from the solvent and analyte at high detection
potentials. We thus selected cyclometalated polypyridyl
ruthenium complexes as the platforms for designing an
electrochemical sensor.19 In comparison to conventional
RuN6-type ruthenium complexes, cyclometalated ruthenium
complexes have a Ru−C bond present in the molecule and a
RuN5C-type coordination, which significantly decreases the
RuIII/II potential.20 In addition, cyclometalated polypyridyl
ruthenium complexes often display rich metal-to-ligand or
ligand-to-metal charge-transfer (MLCT or LMCT) transitions
in the visible region, which can be used as the optical readout
for ion recognition. However, this feature is not present in
conventional electrochemical sensors based on Fc or TTF
compounds.
Figure 3 shows the DPV signal changes of 0.25 mM 3(PF6)
upon titration with various amounts of Cu2+. The current at
−0.03 V increased gradually and continuously when up to 1.0
equiv of Cu2+ was added. However, the current remained
constant when additional Cu2+ was added. The appearance of
the peak at −0.03 V is believed to be caused by the 1:1 binding
of Cu2+ with 3(PF6) (the mechanism will be discussed later).
The detection limit is evaluated to be 5.3 μM using 3σ/m,23
where σ is the standard deviation of the blank signals and m is
the slope of the linear calibration plot (Figure S3, Supporting
Information).
The complex 3(PF6) was designed for cation sensing, where
a DPA unit was connected to the cyclometalating phenyl ring
via an acetyl amide linker. The DPA unit is a versatile receptor
for many cations, including Cu2+.21 We hope the cation−DPA
binding will result in significant changes of the RuIII/II potential
and the absorption spectrum and thus form the basis of an
electrochemical and optical sensor.
To develop a multichannel sensor of Cu2+, we then examined
the absorption spectra of 3(PF6) in the absence or presence of
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Figure 4. (a) Absorption spectra of 1.0 × 10−5 M 3(PF6) (red curve)
in the presence of 4.0 equiv of Cu2+ (blue curve), Ag+, Cd2+, Co2+,
Fe2+, Fe3+, Hg2+, Zn2+, Ni2+, Mn2+, Ba2+, Mg2+, Ca2+, K+, and Na+ in
CH3CN/H2O (4/1 v/v, 50 mM HEPES, pH 7.2). The metal ions
were added as the perchlorate salts. The inset shows the colors of the
solutions. (b) The absorbance intensity ratio (A − A0)/A0 monitored
at 750 nm during the competition experiment measurements. A0
stands for the absorbance in the absence of any ions, and A stands for
the absorbance in the presence 4.0 equiv of Cu2+ and various other
ions.
between 450 and 600 nm with an extinction coefficient (ε) of
1.73 × 104 M−1 cm−1 at λmax 516 nm, which was ascribed to the
MLCT transitions of cyclometalated ruthenium complexes.20
The absorption spectra or the colors of the receptor solutions
were not affected at all upon the addition of 4.0 equiv of Ag+,
Cd2+, Co2+, Fe2+, Fe3+, Hg2+, Zn2+, Ni2+, Mn2+, Ba2+, Mg2+,
Ca2+, K+, and Na+ ions (Figure S4, Supporting Information).
However, when 4.0 equiv of Cu2+ was added, a dramatic change
was noted in the absorption spectrum. The MLCT band at 516
nm significantly decreased and a new absorption peak appeared
at 750 nm (ε = 0.54 × 104 M−1 cm−1). Furthermore, the
solution of 3(PF6) changed from purple to yellow (Figure 4,
easily discerned by the naked eye). The spectral changes were
typically measured 2 min after the addition of Cu2+. The
competition experiments proved that the optical sensing of
Cu2+ was not interfered with by all 12 other ions tested (Figure
4b), except in the coexistence of Fe2+, where the absorbance at
750 nm decreased to some extent. The complex 3(PF6) can
thus be used for chromogenic and colorimetric detection,
complementary to the electrochemical detection discussed
above, of Cu2+ among 15 ions examined.
Figure 2. (a) CVs of 0.5 mM 3(PF6) in the absence or presence of 1.0
equiv of Cu2+. (b) DPVs of 0.5 mM 3(PF6) in the absence or presence
of 1.0 equiv of various ions in CH3CN/H2O (4/1 v/v, 50 mM HEPES,
pH 7.2). The metal ions were added as the perchlorate salts. (c)
Current intensity ratio (I − I0)/I0 monitored at −0.03 V during the
competition experiment measurements. I0 stands for the current in the
absence of any ions, and I stands for the current in the presence of 1.0
equiv of Cu2+ and various other ions.
It is important to mention that the near-IR absorption
detection has the advantage of low interference with the
environment24 and the colorimetric detection is very
convenient and straightforward. In addition, complex 3(PF6)
is essentially nonemissive in either the absence or presence of
Cu2+. We also did not expect to develop a fluorescent sensor
based on complex 3(PF6).
Figure 3. (a) DPV signal changes of 0.25 mM of 3(PF6) in CH3CN/
H2O (4/1 v/v, 50 mM HEPES, pH 7.2) upon titration with Cu2+
(from 0 to 5.0 equiv). (b) Variation of current at −0.03 V vs the
amount of Cu2+ and the Boltzmann-fitted curve with a R2 value of
0.992.
different metals in the CH3CN/H2O (4/1 v/v, 50 mM HEPES,
pH 7.2) solution (Figure 4a). Complex 3(PF6) was
characterized with an intense and broad absorption band
Figure 5a shows the absorption titration of 3(PF6) with Cu2+
(varied from 0 to 4 equiv) in CH3CN/H2O (4/1 v/v, 50 mM
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Figure 5. (a) Absorption spectral changes of 1.0 × 10−5 M 3(PF6) in
CH3CN/H2O (4/1 v/v, 50 mM HEPES, pH 7.2) upon titration with
Cu2+ (from 0 to 4.0 equiv). (b) Variation of the absorption intensity at
750 nm vs the amount of Cu2+ and the Boltzmann-fitted curve with an
R2 value of 0.953. (c) Job plot of 3(PF6)−Cu2+ in CH3CN/H2O (4/1
v/v, 50 mM HEPES, pH 7.2) with a total concentration of 5.0 × 10−5
M. The absorbance was measured at 750 nm.
Figure 6. Thermal ellipsoid plot of the single-crystal X-ray structure of
3·Cu·CH3CN·3ClO4 with 30% probability ellipsoids. Two ClO4− ions
have been omitted for clarity. Selected bond lengths (Å) and angles
(deg): Cu−O1, 2.171; Cu−N2, 1.922; Cu−N3, 2.047; Cu−N4, 1.937;
Cu−N5, 1.960; C1−O1, 1.217; C1−N1, 1.313; H1−O2, 2.025;
∠N2−Cu−N5, 174.4; ∠N2−Cu−O1, 78.7; ∠N2−Cu−N3, 84.6;
∠N2−Cu−N4, 83.4.
HEPES, pH 7.2) solution. The absorption spectrum of 3(PF6)
and the color of the solution did not change obviously when
the amount of added Cu2+ ion was below 1.0 equiv. At first
sight, this is in contrast with the above electrochemical
measurements (Figure 3). As will be clarified in Recognition
Mechanism, the first 1 equiv of Cu2+ is simply used to bind to
the DPA unit. This leads to significant changes in the
electrochemical signals, but the absorption spectrum essentially
remains unchanged. However, when more than 1.0 equiv of
Cu2+ was added, the absorption spectrum changed dramatically
(Figure 5b). A new peak at 750 nm increased gradually and the
initial MLCT band at 516 nm decreased slowly. The
appearance of isosbestic points at 476 and 614 nm points to
a clean transformation. The solution color changed from purple
to yellow gradually. When more than 2 equiv of Cu2+ was
added, the absorption at 750 nm essentially remained
unchanged. This indicates that 2.0 equiv of Cu2+ was needed
for the saturation of the new absorption band at 750 nm, which
was further confirmed by the Job plot shown in Figure 5c.
When the spectral titration was performed in the presence of 10
equiv of Cd2+, the Job plot indicates that less than 2 equiv of
Cu2+ is needed to saturate the absorption at 750 nm (Figure S5,
Supporting Information).
the oxygen atom (O2) of one perchlorate anion, with an O2−
H1 distance of 2.025 Å.
To further probe the recognition mechanism of 3(PF6)
toward Cu2+, we examined the electrochemical and spectral
characteristics of the two model compounds 2(PF6) and 4 in
the absence or presence of 1 equiv of Cu2+ (Figures 7 and 8).
Recognition Mechanism. The above studies established
that the complex 3(PF6) is able to selectively detect Cu2+ in an
aqueous environment by the appearance of a new DPV signal at
−0.03 V vs Ag/AgCl and a new absorption band at 750 nm
(more than 1 equiv of Cu2+ is needed), accompanied by a color
change from purple to yellow. To gain insight into the analyte−
receptor binding mode, a single crystal obtained from a 1/1
mixture of 3(PF6) and Cu(ClO4)2 in acetonitrile/ethyl ether
mixed solvent was analyzed by X-ray diffraction. The single
crystal has the formula 3·Cu·CH3CN·3ClO4, and the X-ray
structure is shown in Figure 6. The DPA acetyl unit of the
ruthenium complex acts as a quadridentate ligand to bind to the
Cu2+ ion via the three N atoms of the DPA unit and the
carbonyl O atom. The Cu2+ ion has a distorted five-coordinate
trigonal-bipyramidal configuration, with one site occupied by a
CH3CN molecule. The Cu−N bond lengths range from 1.922
to 2.047 Å, which are shorter with respect to the Cu−O1 bond
(2.171 Å). A similar five-coordinate configuration of Cu2+ has
been documented previously.25 The C1−O1 and C1−N1 bond
lengths of the amide group are 1.217 and 1.313 Å, respectively.
A H bond is present between the amide hydrogen (H1) and
Figure 7. (a) Absorption spectral changes of 2(PF6) in CH3CN after
the addition of 1 equiv of Cu2+ (red curve) or electrolysis at an ITO
glass electrode at +0.7 V vs Ag/AgCl (blue curve). (b) DPV signal
changes of 2(PF6) in CH3CN upon the addition of 1 equiv of Cu2+
(red curve).
Compound 2(PF6) is a cyclometalated ruthenium complex
without the DPA unit, which is an intermediate obtained during
the synthesis of 3(PF6) (Figure 1). Compound 4 was prepared
according to the known procedure26 and has only the DPA
acetyl amide structure. When 1 equiv of Cu2+ was added to
2(PF6), the MLCT band around 500−600 nm decreased
significantly, and a new absorption band at 758 nm appeared
(Figure 7a). Similar spectral changes were observed when
2(PF6) was electrolyzed at +0.7 V vs Ag/AgCl using an
indium−tin oxide (ITO) glass electrode. These spectral
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changes have been previously observed in Figure 5 for complex
3(PF6) upon the addition of up to 2 equiv of Cu2+. The
complex 2(PF6) displays a RuIII/II DPV wave at +0.52 V vs Ag/
AgCl. This wave shifted to +0.49 V when 1 equiv of Cu2+ was
added, accompanied by some signal changes in the more
positive potential region. However, no peak is evident in the
region between −0.3 and +0.3 V, where the detection peak is
located for the sensing of Cu2+ by complex 3(PF6) (Figures 2
and 3). A significant change between +0.8 and +1.4 V was
observed, as shown in Figure 7b. We did not know the reason
at this stage. These waves are possibly associated with a RuIV/III
process or the oxidation of the amine unit.
Compound 4 shows some irreversible oxidation peaks at
potentials more positive than +0.9 V. When 1 equiv of Cu2+
was added, a new redox couple at +0.01 V appeared (Figure
8a), reminiscent of the detection peak at −0.03 V for the
Figure 9. Stepwise detection mechanism of Cu2+ by 3(PF6).
It should be pointed out that the appearance of the CuII/I
wave as the readout signal is a serendipitous discovery for us.
Our original expectation was to detect the metal ion by a
change in the RuIII/II potential. It turned out that the new CuII/I
wave was a more effective readout signal.
Figure 8. (a) CV signal changes of 4 in CH3CN/H2O (4/1 v/v, 50
mM HEPES, pH 7.2) after the addition of 1 equiv of Cu2+ (red curve).
(b) Absorption spectral changes of 4 in CH3CN/H2O upon the
addition of 1 equiv of Cu2+ (red curve).
In the second step of the process, the [RuII−CuII]3+ complex
was oxidized by another 1 equiv of Cu2+ to afford a [RuIII−
CuII]4+ complex, where the RuII was transformed into RuIII.
During this step, the detection of Cu2+ was read out by a color
change of the solution from purple to yellow and a significant
decrease of the MLCT band at 516 nm and the appearance of a
new absorption band at 750 nm. The latter band is assigned to
the LMCT absorption associated with the oxidized RuIII
component. Previous studies have shown that the LMCT
absorptions of cyclometalated polypyridyl RuIII complexes are
in the region 600−900 nm.28 The oxidative mechanism is also
supported by the results shown in Figure 7. The slight
interference of Fe2+ is possibly caused by the consumption of
Cu2+ via partial oxidation of Fe2+ to Fe3+.
sensing of Cu2+ by complex 3(PF6). This signal, however, is not
evident when other ions were added to the solution of 4
(Figure S6, Supporting Information). There is no absorption at
all in the visible to near-IR region for compound 4, in either the
absence or presence of 1 equiv of Cu2+ (Figure 8b).
On the basis of the above results, the detection mechanism of
Cu2+ by 3(PF6) is rationalized in Figure 9, which involves two
consecutive processes. In the first step, the selective
coordination of 1 equiv of Cu2+ with the DPA acetyl amide
unit results in a heterodimetallic [RuII−CuII]3+ complex. The
detection is read out by the appearance of a new electro-
chemical peak at −0.03 V vs Ag/AgCl. This new peak is
assigned to the CuII/I process. This is rather reasonable, because
a similar electrochemical response has been observed for
compound 4 with the addition of 1 equiv of Cu2+ (Figure 8).
The CuII/I process of a five-coordinate CuN5-type complex was
reported to possess the CuII/I process at −0.07 V vs SCE,27
which also supports the above electrochemical signal assign-
ment. The interference of ions, such as Cd2+, Co2+, and Zn2+, is
caused by the competition of these ions to bind to the DPA
unit.
The oxidation of cyclometalated RuII complexes into RuIII
complexes by Cu2+ sources has long been recognized by van
Koten and co-workers.29 The CuII/I potential of Cu(ClO4)2 is
around +1.0 V vs Ag/AgCl in CH3CN (Figure S4, Supporting
Information),30 which is clearly more positive than the RuIII/II
potential of 2(PF6) and 3(PF6) (around +0.5 V). This means
that Cu2+ can easily oxidize RuII of a cyclometalated ruthenium
complex. It is worth noting that the detection of Cu2+ by
oxidative transformation has recently attracted considerable
interest among a number of research groups.31 Probes
containing triarylamine or ferrocene units can be oxidized by
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Cu2+ to lead to various spectroscopic and electrochemical signal
changes.
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 measurements was α-cyano-4-hydroxycinnamic acid. Micro-
analysis was carried out using a Flash EA 1112 or Carlo Erba 1106
analyzer at the Institute of Chemistry, Chinese Academy of Sciences.
3,5-Dibromoaniline and (pyrid-2-yl)tributylstanne were purchased
from commercial sources and used as received.
3,5-Bis(pyrid-2-yl)aniline. To a mixture of 3,5-dibromoaniline
(502 mg, 2.0 mmol), Pd(PPh3)Cl2 (84.0 mg, 0.12 mmol), and LiCl
(672 mg, 16.0 mmol) were added (pyrid-2-yl)tributylstanne (1.62 mg,
4.4 mmol) and 20 mL of distilled toluene under an N2 atmosphere.
After bubbling with N2 for 15 min, the system was heated to 120 °C in
a sealed pressure tube for 48 h. The reaction mixture was cooled to
room temperature. The insoluble precipitate was removed by filtration.
The filtrate was concentrated and subjected to flash column
chromatography on silica gel (eluent: CH2Cl2/ethyl acetate 1/1) to
afford 343 mg of 3,5-bis(pyrid-2-yl)aniline as a pale solid in 69% yield.
1H NMR (400 MHz, CDCl3): δ 7.23 (d, J = 6.4 Hz, 2H), 7.44 (s, 2H),
7.75 (t, J = 8 Hz, 2H), 7.81 (d, J = 8 Hz, 2H), 7.93 (s, 1H), 8.69 (d, J =
4.4 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ 114.36, 116.09, 120.87,
122.34, 136.80, 141.05, 147.62, 149.61, 157.49. EI-MS (m/z): 247 for
[M]+. HR-EIMS (m/z): calcd 247.1109 for C16H13N3, found
247.1113.
CONCLUSION
■
In conclusion, we have developed a new chemosensor based on
cyclometalated ruthenium and DPA motifs, which can
selectively detect Cu2+ against various metal ions through
chromogenic, colorimetric, and electrochemical responses in an
aqueous environment. The detection limit is around 5 μM.
When the optical and electrochemical detection methods are
taken together, the interference of other metal ions is
insignificant. The stepwise detection mechanism involving the
complexation of Cu2+ with the DPA unit (associated with the
electrochemical output), followed by the oxidation of the
ruthenium component (associated with the optical output), is
unique in comparison to all known Cu2+ sensors. This provides
a new strategy for the design of ion sensors with multichannel
responses.
Because of the low solubility of ruthenium complexes in
water, the application of the chemosensor may be limited.
However, one appealing advantage of electrochemical sensing is
that the solution-based technology can be transformed into
mono- or multilayer films immobilized on electrode surfaces,32
by the well-known electropolymerization33 or monolayer-
formation34 method. This will expand the application of the
current technology. Using this method, a sensor device can be
imagined if the substrate−probe binding is reversible and the
substrate can be removed by ligands with stronger binding
strength. Such work is under way in this laboratory and will be
reported in due course.
Synthesis of Compound 1. To a mixture of 3,5-bis(pyrid-2-
yl)aniline (61.8 mg, 0.25 mmol) and Et3N (30.4 mg, 0.30 mmol) in 20
mL of dichloromethane was added dropwise 2-chloroacetyl chloride
(33.9 mg, 0.30 mmol). The solution was stirred for 1 h at 0−5 °C. The
solvent was removed under reduced pressure, and the residue was
subjected to flash column chromatography on silica gel (eluent
dichloromethane/ethyl acetate, 1/1) to afford 60.0 mg of compound 1
1
as a light yellow solid in 75% yield. H NMR (400 MHz, CDCl3): δ
4.24 (s, 2H), 7.27−7.29 (m, 2H), 7.76−7.80 (m, 2H), 7.88 (d, J = 8.0
Hz, 2H), 8.33 (s, 2H), 8.45 (s, 1H), 8.51 (s, 1H), 8.71 (s, J = 4.0 Hz,
2H). 13C NMR (100 MHz, CDCl3): δ 43.12, 119.09, 121.03, 122.18,
122.78, 137.04, 138.07, 140.90, 149.75, 156.49, 164.33. EI-MS (m/z):
323 for [M]+. HR-EIMS (m/z): calcd 323.0825 for C18H14N3OCl,
found 323.0829.
EXPERIMENTAL SECTION
■
Spectroscopic Measurements. Absorption spectra were re-
corded using a PerkinElmer Lambda 750 UV/vis/near-IR spectropho-
tometer at room temperature in denoted solvents, with a conventional
1.0 cm quartz cell. Spectroelectrochemistry was performed in a thin-
layer cell (optical length 0.2 cm) in which an ITO glass electrode (<10
Ω/square) was set in the indicated solvent containing 0.1 M
Bu4NClO4 and the compound to be measured (the concentration is
around 1 × 10−4 M). A platinum wire and Ag/AgCl in saturated
aqueous solution was used as a counter electrode and a reference
electrode, respectively. The cell was put into the spectrophotometer to
monitor spectral changes during electrolysis.
Synthesis of Complex 2(PF6). To 20 mL of dry acetone were
added [(ttpy)RuCl3] (37.0 mg, 0.070 mmol) and AgOTf (63.0 mg,
0.25 mmol). The mixture was refluxed for 2 h before cooling to room
temperature. The resulting AgCl precipitate was filtered. The filtrate
was concentrated, and the residue was dissolved in 6 mL of DMF. The
solution was then transferred by syringe to a pressure vessel charged
with compound 1 (23.0 mg, 0.070 mmol) in 6 mL of dry t-BuOH. The
mixture was bubbled with N2 for 15 min before the vessel was capped
and heated at 120 °C for 12 h. After it was cooled, the reaction mixture
was filtered and the solvent was removed under vacuum. To the
residue was added 3 mL of methanol, followed by addition of an excess
of KPF6. The resulting precipitate was collected by filtering and
washing with water and ether. This crude product was purified by
column chromatography on silica gel (eluent CH2Cl2/C2H5OH 200/
1) to give the complex 2(PF6) as a brown solid in 62% yield. 1H NMR
(400 MHz, CD3CN): δ 2.52 (s, 3H), 4.34 (s, 2H), 6.68 (t, J = 6.4 Hz,
2H), 6.96 (t, J = 6.4 Hz, 2H), 7.11 (d, J = 5.4 Hz, 4H), 7.55 (d, J = 7.7
Hz, 2H), 7.62 (t, J = 7.7 Hz, 2H), 7.71 (t, J = 8.0 Hz, 2H), 8.11 (t, J =
4.0 Hz, 4H), 8.45 (s, 2H), 8.56 (d, J = 8.0 Hz, 4H), 8.92 (s, 1H), 8.99
(s, 2H). MALDI-TOF (m/z): 746.9 [M − PF6]+. Anal. Calcd for
C40H30ClF6N6OPRu·H2O: C, 52.78; H, 3.54; N, 9.23. Found: C,
52.86; H, 3.62; N, 9.06.
Electrochemical Measurements. CV and DPV measurements
were taken using a CHI 620D potentiostat with a one-compartment
electrochemical cell under an atmosphere of nitrogen. All measure-
ments were carried out in 0.1 M nBu4NClO4 in the indicated solvents.
The working electrode was glassy carbon with a diameter of 3.0 mm,
which 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 a Ag/
AgCl electrode in saturated aqueous NaCl without regard for the
liquid junction potential.
X-ray Crystallography. The X-ray diffraction data were collected
using a Rigaku Saturn 724 diffractometer with a rotating anode (Mo
Kα radiation, 0.71073 Å) at 173 K. The structure was solved by direct
methods using SHELXS-9735 and refined with Olex2.36 Crystallo-
graphic data for 3·Cu·CH3CN·3ClO4: C54H45Cl3CuN10O13Ru, Mr =
Synthesis of Complex 3(PF6). To a mixture of 2(PF6) (22.3 mg,
0.025 mmol), N,N-diisopropylethylamine (DIPEA, 0.10 mL), and NaI
(2.4 mg, 0.015 mmol) in 10 mL of CH3CN was added dipicolylamine
(6.0 mg, 0.030 mmol). The solution was stirred and refluxed for 17 h.
The solvent was removed under reduced pressure, and the residue was
subjected to flash column chromatography on silica gel (eluent
CH3CN/H2O/aqueous KNO3 200/5/1), followed by anion exchange
using KPF6 and reprecipitation in a mixture of CH2Cl2/MeOH, to
1312.96, triclinic, space group P1, a = 12.094(2) Å, b = 13.575(3) Å, c
̅
= 21.239(4) Å, α = 92.92(3)°, β = 98.09(3)°, γ = 133.90(3)°, U =
3133.6(11) Å3, T = 173 K, Z = 2, 9747 reflections measured, radiation
́
type Mo Kα, radiation wavelength 0.71073 Å, final R indices R1 =
0.1645 and wR2 = 0.3825, R indices (all data) R1 = 0.2177 and wR2 =
0.4162.
Synthesis. NMR spectra were recorded in the designated solvent
on a Bruker Avance 400 MHz spectrometer. Spectra are reported in
1
afford complex 3(PF6) as a black solid in 39% yield. H NMR (400
MHz, CD3CN): δ 2.52 (s, 3H), 3.58 (s, 2H), 4.04 (s, 4H), 6.67 (t, J =
7500
dx.doi.org/10.1021/om400999h | Organometallics 2013, 32, 7495−7502

Organometallics
Article
́
́
(6) (a) Ballesteros, E.; Moreno, D.; Gomez, T.; Rodrıguez, T.; Rojo,
6.8 Hz, 2H), 6.96 (t, J = 6.4 Hz, 2H), 7.10 (d, J = 5.4 Hz, 2H), 7.14 (d,
J = 5.4 Hz, 2H), 7.32 (t, J = 5.2 Hz, 2H), 7.49 (d, J = 7.8 Hz, 2H), 7.55
(d, J = 7.8 Hz, 2H), 7.63 (t, J = 7.7 Hz, 2H), 7.71 (t, J = 7.8 Hz, 2H),
7.78 (t, J = 7.6 Hz, 2H), 8.12 (t, J = 8.4 Hz, 4H), 8.56 (d, J = 8.0 Hz,
2H), 8.66 (s, 2H), 8.75 (d, J = 4.4 Hz, 2H), 8.99 (s, 2H), 11.11 (s,
1H). MALDI-TOF (m/z): 910.2 [M − PF6]+. Anal. Calcd for
C52H42F6N9OPRu·2H2O: C, 57.25; H, 4.25; N, 11.55. Found: C,
57.29; H, 4.20; N, 11.33.
́
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ASSOCIATED CONTENT
* Supporting Information
■
S
A CIF file giving X-ray crystallographic data for 3·Cu·CH3CN·
3ClO4 and figures giving electrochemical data of AgClO4 and
Cu(ClO4)2, competition experiments of 3(PF6)-Cu2+ in the
presence of 10 equiv of other ions, a plot for the determination
of the detection limit, color changes of 3(PF6) in response to
the presence of various ions, a Job plot of 3(PF6)-Cu2+ in the
presence of 10 equiv of Cd2+, DPV signal changes of 4 in the
presence of other ions, and NMR and MALDI-MS spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for Y.-W.Z.: zhongyuwu@iccas.ac.cn.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(grants 91227104, 21271176, 21002104, and 212210017), the
National Basic Research 973 program of China (grant
2011CB932301), and the Institute of Chemistry, Chinese
Academy of Sciences (“100 Talent” Program and grant CMS-
PY-201230), for funding support.
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dx.doi.org/10.1021/om400999h | Organometallics 2013, 32, 7495−7502