Stepwise Coordination Followed by Oxidation Mechanism for the

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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-toligand 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-tometal 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.

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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 environment 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 © 2013 American Chemical Society

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 properties, 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 Received: October 11, 2013 Published: November 18, 2013 7495

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Stille coupling between 3,5-dibromoaniline and (pyrid-2yl)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 2chloroacetyl chloride afforded compound 1 in good yield. The reaction of [(ttpy)RuCl3]22 (ttpy = 4′-tolyl-2,2′:6′,2″-terpyridine) 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,Ndiisopropylethylamine (DIPEA). New complexes were characterized 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 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). To develop a multichannel sensor of Cu2+, we then examined the absorption spectra of 3(PF6) in the absence or presence of

We present in this contribution the polypyridyl ruthenium complex 3(PF6) (Figure 1) modified with a dipicolylamine

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.

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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. 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. 7496

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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. It is important to mention that the near-IR absorption detection has the advantage of low interference with the environment 24 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 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

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.

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(PF 6 ) was characterized with an intense and broad absorption band 7497

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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). 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

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).

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 7498

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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. 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+. 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

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).

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 electrochemical 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 assignment. The interference of ions, such as Cd2+, Co2+, and Zn2+, is caused by the competition of these ions to bind to the DPA unit. 7499

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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 MALDITOF measurements was 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. 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. 1 H 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. Synthesis of Compound 1. To a mixture of 3,5-bis(pyrid-2yl)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 as a light yellow solid in 75% yield. 1H 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. 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. 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 afford complex 3(PF6) as a black solid in 39% yield. 1H NMR (400 MHz, CD3CN): δ 2.52 (s, 3H), 3.58 (s, 2H), 4.04 (s, 4H), 6.67 (t, J =

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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 monolayerformation34 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.

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EXPERIMENTAL SECTION

Spectroscopic Measurements. Absorption spectra were recorded using a PerkinElmer Lambda 750 UV/vis/near-IR spectrophotometer at room temperature in denoted solvents, with a conventional 1.0 cm quartz cell. Spectroelectrochemistry was performed in a thinlayer cell (optical length 0.2 cm) in which an ITO glass electrode (