H2PO4–- and Solvent-Induced Polymorphism of an Amide

Sep 28, 2016 - A simple [Pt(N^C^N)Cl] complex functionalized with an amide group was prepared, and its absorption and emission properties were examine...
2 downloads 0 Views 6MB Size
Download PDF
Article pubs.acs.org/IC

H2PO4−- and Solvent-Induced Polymorphism of an AmideFunctionalized [Pt(N^C^N)Cl] Complex Zhong-Liang Gong and Yu-Wu Zhong* Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Science, 2 Bei Yi Jie, Zhong Guan Cun, Haidian District, Beijing 100190, China S Supporting Information *

ABSTRACT: A simple [Pt(N^C^N)Cl] complex functionalized with an amide group was prepared, and its absorption and emission properties were examined in different solvents in response to various anions. On the one hand, in the presence of H2PO4−, the solution of the complex shows distinct color changes in CH3CN, together with a ratiometric emission change from a green emission band at 537 nm to a deep red emission band at 680 nm. On the other hand, two-step spectral changes were observed in response to H2PO4− in CH2Cl2, with the green emission being attenuated first followed by the appearance of enhanced and yellow-green emissions at a lower-energy region. These recognition processes are highly selective for H2PO4− against other common anions including F−, Cl−, Br−, I−, OAc−, NO3−, and HSO4−. In addition, the platinum complex displays multistage emission polymorphism in mixed CH3CN/H2O solvent of various ratios. The hydrogen-bonding interaction between H2PO4− and the amide unit was confirmed by NMR analysis. In the solid state, this platinum complex emits red light. However, the composite material of the platinum complex with H2PO4− shows purely monomeric yellow emissions. The solidstate materials were further analyzed by single-crystal X-ray and Fourier-transform IR analysis. These studies suggest that this simple platinum complex is useful for the selective recognition of H2PO4− and as solid-state emitting materials with tunable emission colors.

■

INTRODUCTION Polypyridyl platinum(II) complexes with a square-planar geometry have received much interest due to their appealing emission and photophysical properties.1−3 To date, they have been investigated in a wide range of applications, including optoelectronic devices,4−6 chemosensing,7−9 self-assembly,10,11 and bioimaging.12,13 One interesting feature of the squareplanar platinum(II) complexes is that they tend to form intermolecular stacking in both solution and solid states, leading to various degrees of Pt−Pt and π−π interactions.10,14,15 As a result, the square-planar platinum(II) complexes often show interesting polymorphism characterized by excimeric emissions in the deep red to near-infrared (NIR) region and monomeric emissions in a higher-energy region. The interconversion among different state of emissions can be triggered by concentration,16,17 solvent composition,18,19 organic vapors,20,21 temperature,22,23 counteranions (for cationic complexes),24,25 pH value,26,27 and polyelectrolytes,28,29 among others. Dihydrogen phosphate (H2PO4−) is an important anion in biological entities and environments. The molecular recognition and sensing of H2PO4− has thus received considerable interests. 30−40 Among them, fluorescence detection of phosphate anion has been very popular due to its simplicity and sensitivity.41 A great number of emissive organic fluorophores or metal complexes have been reported to recognize H2PO4− with emission “turn-ON”, “turn-OFF”, or © XXXX American Chemical Society

emission-energy shifted (ratiometric) responses. Most of these probes contain specific hydrogen-bonding functional groups, such as amide,31 urea,32 sulfonamide,36 imidazole,38 or imidazolium units,32,35 which are complexed with H2PO4− to give rise to corresponding spectral responses. The detection of H2PO4− by a displacement protocol is also possible, in which the interaction of the anion with a coordinated metal complex liberates the emissive ligand.39,41 In spite of these advances, new fluorescent probes are still desirable to detect H2PO4− with high selectivity against basic anions such as F− and OAc−. In addition, ratiometric fluorescent detections of H 2 PO 4 − characterized by low-energy red emissions would be appealing considering their potential applications in bioimaging.12,13 Anion recognition and sensing by transition-metal complexes are well-established.42−45 In particular, ruthenium and iridium complexes functionalized with polarized N−H groups were reported to show spectroscopic response toward H2PO4−.33,38,46−48 However, the use of emissive platinum complexes for anion detections has remained largely unexplored, in contrast to their applications for metal ion sensing.7,8,49,50 Recently, Yam and co-workers reported some calixarene-based diplatinum terpyridine complexes with various receptor sites (amide, urea, and sulfonamide) for the luminescence sensing of anions (H2PO4−, F−, and OAc−).51 Received: April 28, 2016

A

DOI: 10.1021/acs.inorgchem.6b01059 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

the wavelength region between 350 and 480 nm, which are due to the metal-to-ligand charge transfer (MLCT) transitions.16,53,54 The weak absorptions between 480 and 540 nm are attributed to the direct population of the triplet ligandcentered (3LC) state.16,53,54 In the presence of 1 equiv of H2PO4−, big changes occurred to the absorption spectrum of 3 in CH3CN and CH2Cl2 (Figure 1b). The MLCT transitions decreased considerably, and new absorptions in the lower-energy side (480−600 nm) appeared. The changes in CH3CN are significant, leading to a distinct color change of the solution from pale yellow to orange (Figure 1c). The absorption spectra in other solvents did not change in response to H2PO4−. Interestingly, the response of 3 to H2PO4− in CH3CN and CH2Cl2 is highly selective. No absorption spectral changes were observed in the presence of other anions (F−, Cl−, Br−, I−, OAc−, NO3−, and HSO4−, Figure 1c,d). Figure 2a shows the stepwise absorption spectral changes of 3 in CH3CN in response to different equivalent of H2PO4−. The decrease of the MLCT band at 415 nm and the increase of the new absorptions between 460 and 600 nm were clearly recorded with an isosbestic point at 450 nm. The spectra essentially did not change any more when more than 1.0 equiv of H2PO4− was present. The absorption of 3 in CH3CN in the presence of 1.0 equiv of H2PO4− obeys the Beer−Lambert Law (Figure 2b). Figure 2c shows the emission spectral changes of 3 in CH3CN in response to different equivalent of H2PO4− (excited at 450 nm). When H2PO4− was added, the emission at 537 nm (primarily of the 3LC character; Φ = 13%; τ1 = 490 ns (90%) and τ2 = 160 ns (10%) under deaerated condition)16,53,54 gradually decreased, and a new emission band at 680 nm increased. Accordingly, the emission color turned red from green (Figure 2e). This is a typical ratiometric emission

The molecular structure of the probe is relatively complex, and the anion selectivity remains to be improved. We present in this contribution a simple [Pt(N^C^N)Cl] complex functionalized with an amide group, which shows multicolor emissions in response to H2PO4− and solvent compositions (CH3CN, CH2Cl2, and water). The response to H2PO4− is characterized by a high selectivity over other basic anions such as F− and OAc−, and the complex can be treated as a probe for colorimetric and luminescence sensing of H2PO4−. In addition, the solid-state emission properties of the complex itself and its composite materials with H2PO4− are presented.

■

RESULTS AND DISCUSSION The neutral platinum complex 3 was designed and synthesized (Scheme 1). The acetylation of the known compound 1, 3,5Scheme 1. Synthesis of Complex 3

di(pyrid-2-yl)aniline,52 afforded ligand 2 in 76% yield. The reaction of 2 with K2PtCl4 in refluxing acetic acid16,53,54 gave the desired product 3 in good yield. The presence of the amide hydrogen-bonding functional group is expected to give rise to stimuli-responsive absorption and emission spectral changes. The absorption spectrum of 3 was measured in a number of different organic solvents (CH3OH, CH3CN, C6H5Cl, CH2Cl2, tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO); Figure 1a). Complex 3 shows a set of absorption bands in

Figure 1. (a, b) Absorption spectra of 3 in the absence (a) or presence (b) of 1.0 equiv of H2PO4− in different solvent. (c, d) Absorption spectra of 3 in CH3CN (c) or CH2Cl2 (d) in the absence or presence of 1.0 equiv of H2PO4− and other anions (F−, Cl−, Br−, I−, OAc−, NO3−, and HSO4−). (inset) The pictures of the solutions of 3 in the absence or presence of 1.0 equiv of H2PO4− in CH3CN. The concentration is 1.5 × 10−4 M for all measurements. Tetrabutylammonium salts of all anions are used. B

DOI: 10.1021/acs.inorgchem.6b01059 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 2. (a) Absorption spectral changes of 3 (1.5 × 10−4 M) in CH3CN in response to H2PO4− (up to 2.5 equiv). (inset) The absorbance changes at 415 and 490 nm. (b) Absorption spectra of 3 in CH3CN at different concentrations (up to 0.25 mM) in the presence of 1.0 equiv of H2PO4−. (inset) The changes of the absorbance at 490 nm. (c) Emission spectral changes of 3 (1.5 × 10−4 M) in CH3CN in response to H2PO4− (up to 2.5 equiv). Excitation wavelength is 450 nm. (d) Emission intensity changes at 537 and 680 nm. (e, left to right) Pictures of 3 in the presence of 0, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0 equiv of H2PO4− under the illumination of 365 nm. (f) Excitation spectra of the emission at 537 and 680 nm of 3 (1.5 × 10−4 M) + 1.0 equiv of H2PO4− in CH3CN. (g) Concentration-dependent emission spectra of 3 + 1.0 equiv of H2PO4− in CH3CN. Excitation wavelength is 450 nm.

Figure 3. Absorption (a, b) and emission (d, e) spectral changes of 3 (1.5 × 10−4 M) in CH2Cl2 in response to H2PO4−. Excitation wavelength is 440 and 470 nm for (d) and (e), respectively. (c) Absorption spectra of 3 in CH2Cl2 at different concentration (up to 0.50 mM) in the presence of 1.0 equiv of H2PO4−. (inset) The changes of the absorbance at 490 nm. (f) The pictures of 3 in the absence (left) or presence (right) of 10 equiv of H2PO4− under the illumination of 365 nm.

response for ion sensing. Again, 1 equiv of H2PO4− is enough to saturate the signal changes. In accordance with the absorption spectral studies, no changes occurred to the emission spectra of 3 in the presence of other anions. On the basis of the absorption and emission spectral changes, the

mixture of H2PO4− and 3 may form a dimeric adduct in the ground state connected by two H2PO4− anions via hydrogen bonding (1:1 ratio between two components). The effective Pt−Pt and/or π−π interactions16−29 in the dimer result in the deep red emission at 680 nm. C

DOI: 10.1021/acs.inorgchem.6b01059 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry The emission at 537 and 680 nm of the 1:1 [3 + H2PO4−] adduct in CH3CN has significantly different excitation spectrum (Figure 2f). Specifically, the low-energy absorption band between 450 and 600 nm makes a big contribution to the emission at 680 nm, while the emission at 537 nm is mainly associated with the MLCT absorptions. This also suggests that the emission at 680 nm originates from the dimeric adduct formed in the ground state. The emission spectra of the [3 + H2PO4−] adduct is independent of the concentration when the concentration is higher than 1 × 10−5 M (Figure 2g). However, when the solution is highly diluted, for example, with a concentration of 2.5 × 10−6 M, the emission at 680 nm is negligible. This is possibly caused by the difficulty in forming the dimeric adduct at the very low concentration. Interestingly, the spectral changes of 3 in response to H2PO4− in CH2Cl2 are significantly different with respect to those in CH3CN (Figure 3). When less than 1.0 equiv of H2PO4− was present in the solution of 3 in CH2Cl2, the MLCT absorption at 420 nm decreased, and new absorption band at the lower-energy side increased (Figure 3a). The emission at 530 nm (Φ = 25%, τ = 1020 ns under deaerated condition) decreased distinctly at this stage (Figure 3d). When more than 1.0 equiv of H2PO4− was present, both MLCT and low-energy absorptions decreased (Figure 3b). However, the emission was enhanced again, and the emission wavelength was shifted to 552 nm (Figure 3e). As a result, the emission color turned yellow-green from green (Figure 3f). It should be pointed out that the absorption of 3 in CH2Cl2 in the presence of 1.0 equiv of H2PO4− does not obey the Beer−Lambert Law (Figure 3c), indicating the formation of molecular ensembles or macromolecular aggregates at the ground state. To examine the interaction of 3 with H2PO4− in solution, the 1 H NMR spectral changes of 3 in the presence of various equivalents of H2PO4− in CD2Cl2 or CD3CN were monitored. When the equivalents of H2PO4− were increased from 0 to 2.5 equiv in CD2Cl2, the amide N−H proton signal gradually shifts from 7.7 to 10.8 ppm (Figure 4a). This distinct downfield shift suggests the formation of hydrogen bonding of the amide N−H proton with H2PO4−. Most NMR peaks remain sharp during the titration experiments, but some signals become broad when the equivalents of H2PO4− were increased. This suggests that the formation of molecular ensembles, instead of macromolecular aggregates, is more possible in CH2Cl2. The sample precipitated from the solution when more H2PO4− was added. When the NMR titration experiments were performed in CD3CN, precipitate appeared when ∼0.6 equiv of H2PO4− was added, and the remaining NMR signals became very weak. However, the downfield shift of the amide N−H proton of 3 could be observed before the appearance of the precipitate (Figure 4b), again suggesting the formation of hydrogen bonding between 3 and H2PO4−. Note that the samples used for the NMR titration experiments are more than 10 times more concentrated with respect to those used for the previous spectroscopic studies. This is the reason why precipitates were observed in the early stage of the NMR titration experiments, while clear and complete spectral changes were observed during the spectroscopic studies. The interesting emission polymorphism of 3 in different solvents in the presence of H2PO4− prompted us to examine its emission in the solid state. Three samples (I, II, and III, Figure 5) were analyzed. Sample I was the originally isolated compound. Samples II and III were obtained by precipitation from the solution of 3 in CH2Cl2 or CH3CN, respectively, after

Figure 4. 1H NMR spectral changes of 3 (2 × 10−3 M) in CD2Cl2 (a) or CD3CN (b) in the presence of various equiv of H2PO4− (up to 2.5 equiv in CD2Cl2 and 0.6 equiv in CD3CN).

adding 1 equiv of Bu4NH2PO4. Sample I is yellow in color and emits red light (from an admixture of the monomeric and excimeric emission) under illumination at 365 nm. The colors of samples II and III are deeper than sample I, being orange and dark brown, respectively. Interestingly, samples II and III both show bright yellow emissions. The incorporation of 1 equiv of Bu4NH2PO4 seems to greatly suppress the excimeric emission of 3 in the solid state. Samples II and III show very similar Fourier-transform infrared (FTIR) spectra (Figure 6). However, some differences are present between the spectrum of sample I and those of samples II and III. All samples show broad signals at ∼3430 cm−1 from hydrogen-bonding networks. Sample I shows an amide N−H stretching signal at 3258 cm−1, which is significantly attenuated for samples II and III. The frequency of the amide carbonyl stretching vibration of samples II and III (1672 and 1674 cm−1, respectively) is slightly lower with respect to that of sample I (1677 cm−1). These results suggest the presence of hydrogen bonding between H2PO4− and the amide group in samples II and III.55 The enhanced C−H stretching signals between 3000 and 2800 cm−1 of samples II and III are attributed to the n-butyl groups from Bu4NH2PO4, which was mixed with 3 to prepare these two samples. D

DOI: 10.1021/acs.inorgchem.6b01059 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

presence of some water molecules is observed in the crystal structure (Figure 7), which should come from the small

Figure 7. Single-crystal packing of 3. The thermal ellipsoids are set at a 50% probability level. Carbon: gray; nitrogen: blue; oxygen: red; platinum: navy; chlorine: green.

amount of water in the solvent. Three molecules of 3 are held together by one water molecule with three hydrogen bonding between water and the amide oxygen of one complex, the amide hydrogen of another complex, and the chloro atom of a third complex. Extensive π−π stacking is present in the crystal structure to form a layered packing as shown in Figure 7b. The shortest intermolecular Pt−Pt distance is 6.885 Å. This suggests that no direct Pt−Pt interaction is present in the crystal structure, which normally requires a short Pt−Pt distance less than 3.5 Å.18 Because of the lack of single-crystal structures of potential adducts of 3 with H2PO4−, the exact binding mode between two components (or the recognition mechanism) could not be clearly established at this stage. The H2PO4− ion is believed to bind with complex 3 via hydrogen bonding to form a dimeric adduct in CH3CN in the ground state, which gives rise to the deep red emission at 680 nm upon excitation. The association of 3 with H2PO4− in CH2Cl2 may result in the formation of low-molecular aggregates in the ground state, which lead to the emission quenching or enhancement depending on the equivalent of H2PO4−. The hydrogen bonding between 3 and H2PO4− is supported by the above 1H NMR analysis in solution and the mass and FTIR data of samples I−III. As was shown above in Figure 7, the amide group of 3 functions as both hydrogen-bond donor and acceptor with water molecules present in the crystal. Similar hydrogen-bonding interactions could be present for potential 3·H2PO4− adducts, since each H2PO4− anion has two hydrogen-bond donor sites and two hydrogen-bond acceptor sites. This may partially explain why simple hydrogen acceptor ions such as F− and OAc− could not be recognized by 3.

Figure 5. Pictures of the sample I (a, b), sample II (c, d), and sample III (e, f) of 3 under daylight (a, c, e) or illumination at 365 nm (b, d, f). Sample I was obtained as isolated. Samples II and III were obtained from the solution of 3 in CH2Cl2 or CH3CN, respectively, in the presence of 1.0 equiv of H2PO4−. (g) Solid-state emission spectra excited at 450 nm.

Figure 6. FTIR spectra of samples I, II, and III. (inset) The enlarged plot between 1750 and 1500 cm−1.

The attempt failed to get a single crystal of 3 in CH2Cl2 or CH3CN in the presence of H2PO4−. However, a single crystal suitable for X-ray diffraction analysis was obtained by slow evaporation of a CH3CN solution of 3. Interestingly, the E

DOI: 10.1021/acs.inorgchem.6b01059 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 8. (a−c) Absorption and (d−f) emission spectral changes of 3 (1.0 × 10−4 M) in mixed solvent of CH3CN and water. Excitation wavelength is 450 nm. (g, left to right) Pictures of 3 under the illumination of 365 nm in CH3CN/water mixture with a ratio of 10/0, 9/1, 7/3, 6/4, 5/5, 4/6, 3/ 7, 2/8, and 1/9, respectively.

Interestingly, except H2PO4−, no distinct emission spectral changes were observed for 3 in response to other phosphate sources such as HPO42−, H3PO4, and PO43− (Figure S1), suggesting a special hydrogen binding mode of H2PO4− with 3. A possible binding mode is shown in Figure S2, where two molecules of 3 were held together by two H2PO4− anions in a face-to-face antiparallel fashion through multiple hydrogen bonds. However, this proposal is very tentative. New evidence to establish the exact binding mode are necessary. Note that when MeOH was added to a solution of 3 in the presence of H2PO4− in either CH3CN or CH2Cl2, the original emission spectrum of 3 in the absence of H2PO4− can be recovered (Figure S3). For the solution in CH2Cl2, only ∼5 volume percent of MeOH is needed to recover the original emission. For the solution in CH3CN, a larger amount of MeOH is needed (∼25%). These results suggest that the binding hydrogen bonding between 3 and H2PO4− was disrupted by MeOH and that the recognition process is reversible. This experiment also rules out the possibility of other recognition mechanism such as the displacement of the chloride ligand by solvent or H2PO4−. The situation of the emission properties of the solid samples is different with respect to that in solution. Elemental analysis data suggest that samples II and III are composed of 1:1 ratio of 3 with Bu4NH2PO4. The signal of the 3·H2PO4 adduct was detected by mass spectrometry. Samples I−III show different powder XRD spectra (Figure S4). The signals of sample I in the region of 5° < 2θ < 20° are not observed or significantly weakened for samples II and III, suggesting a low degree of crystallity of samples II and III. The absorption spectra of samples II and III in solution are very similar to that of 3 in the presence of H2PO4− in CH2Cl2 or CH3CN, respectively

(Figure S5a). In the solid state, the absorption spectra of samples II and III are slightly expanded and shifted to the lowenergy region (Figure S5b), which is in accordance with the color difference of the three samples as displayed in Figure 5. The excitation spectra of these samples show that the lowenergy absorption bands of samples II and III are not responsible for their emissions (Figure S5c). These suggest that the effect of Bu4NH2PO4 in samples II and III is simply to disrupt the Pt−Pt and π−π interactions in the solid state, which leads to the monomer-dominated emissions in these two samples. Stimulated by the crystal structure of 3, we were interested to know the effect of water in influencing its absorption and emission properties. Figure 8a−f shows the absorption and emission spectral changes in mixed CH3CN/H2O solvent of different ratios. Three-step changes were observed when the ratio of water was increased. In the first step, the MLCT absorption maxima shifted from 412 to 385 nm, and the emission at 530 nm was significantly quenched (Figure 8a and 8d). In the second step, the MLCT absorption decreased a little, and some shallow absorptions in the lower-energy side appeared (Figure 8b). Meanwhile, the emission was enhanced again when the CH3CN/H2O ratio was changed from 7/3 to 3/7. The emission band at a ratio of 3/7 is rather broad, consisting of both monomeric and excimeric emissions (Figure 8e). When the water content was further increased, distinct absorptions in the lower-energy side were observed (Figure 8c), suggesting the formation of aggregates in the ground state. In the meantime, the emission became dominated by the excimeric emission at ∼700 nm (Figure 8f). The emission color changes from green to red in this three-step process (Figure 8g). This experiment also confirms that the previous F

DOI: 10.1021/acs.inorgchem.6b01059 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry spectral response of 3 toward H2PO4− in organic solvents (Figures 1−3) is not a result of the presence of small amount of water molecules in the solvent. To induce distinct emission spectral changes, a large amount of water is needed with respect to the equivalent of 3.

1.55−1.63 (m, 8H), 2.18 (s, 3H), 3.18 (t, J = 8.0 Hz, 8H), 7.24 (t, J = 6.4 Hz, 2H), 7.67 (d, J = 8.0 Hz, 2H), 7.73 (s, 2H), 7.92 (t, J = 7.2 Hz, 2H), 8.71 (s, 1H), 9.13 (d, J = 5.2 Hz, 2H). ESI-MS: 633.20 for [3 + H2PO4 + H2O]+, 597.55 for [3 + H2PO4 + H2O − Cl]+. Anal. Calcd for C18H14N3OPtCl·Bu4NH2PO4·3H2O: C, 44.76; H, 6.41; N, 6.14. Found: C, 45.09; H, 6.20; N, 5.77%. To the solution of 3 (0.030 mmol, 16 mg) in 3 mL of CH3CN was added a solution of (Bu)4NH2PO4 (0.030 mmol, 10 mg) in 2 mL of CH3CN. The mixture was stirred at room temperature for 5 min. The resulting precipitate was collected by filtration and was washed with CH3CN to afford 12 mg of sample III as a brown solid in 43% yield. 1H NMR (400 MHz, CD2Cl2): δ 0.90 (t, J = 7.2 Hz, 12H), 1.30−1.36 (m, 8H), 1.50−1.55 (m, 8H), 2.18 (s, 3H), 3.14 (t, J = 8.4 Hz, 8H), 7.14 (t, J = 6.4 Hz, 2H), 7.72 (d, J = 8.0 Hz, 2H), 7.81 (s, 2H), 7.92 (t, J = 7.2 Hz, 2H), 9.00 (d, J = 5.2 Hz, 2H), 9.77 (s, 1H). ESI-MS: 597.55 for [3 + H2PO4 + H2O − Cl]. Anal. Calcd for C18H14N3OPtCl·Bu4NH2PO4·H2O: C, 46.60; H, 6.21; N, 6.39. Found: C, 46.48; H, 6.03; N, 6.12%. Spectroscopic Measurements. Absorption spectra were recorded at room temperature on a TU-1810DSPC spectrometer from Beijing Purkinje General Instrument Co, Ltd. Luminescence spectra were recorded on an F-380 spectrofluorimeter from Tianjin Gangdong Sci. & Tech. Development Co, Ltd, with a red-sensitive photomultiplier tube R928F. The luminescence lifetime was obtained on FLS920 spectrometer from Edinburgh instruments Co, Ltd. The absolute emission quantum yields in solution were determined on FLS980 spectrometer from Edinburgh instruments Co, Ltd. All measurements were performed in a quartz cuvette with path length of 1 cm. The solid-state emission spectra were recorded on Hitachi F4500 spectrometer from Techcomp (China) Ltd. X-ray Crystallography. The X-ray diffraction data were collected using a Rigaku Saturn 724 diffractometer on a rotating anode (Mo Kα radiation, 0.710 73 Å) at 173 K. The structure was solved by the direct method using SHELXS-9758 and refined with Olex2.59 Crystallographic data for 3: C18H14ClN3OPt·H2O, M = 536.88, triclinic, space group P1̅, a = 7.3316(8), b = 10.9505(11), c = 11.703(2) Å, α = 63.101(3)°, β = 84.681(5)°, γ = 84.531(5)°, U = 832.8(2) Å3, T = 173.15 K, Z = 2, 10 174 reflections measured, radiation type Mo Kα, radiation wavelength 0.710 73 Å, final R indices R1 = 0.0301, wR2 = 0.0742, R indices (all data) R1 = 0.0310, wR2 = 0.0752.

■

CONCLUSION A simple [Pt(N^C^N)Cl] complex functionalized with an amide group was prepared, which shows interesting polymorphism in response to H2PO4− and solvent compositions. Multicolor emissions are possible, including green, yellowgreen, and red, by controlling the solvent compositions and the amount of H2PO4−. It behaves as a colorimetric and ratiometric fluorescent sensor for H2PO4− with high selectivity against other basic anions. In addition, this platinum complex emits red light in the solid state. However, the composite materials of the platinum complex with H2PO4− show purely monomeric yellow emissions. This solid-state polymorphism makes it potentially useful in light-emitting devices.56,57

■

EXPERIMENTAL SECTION

Synthesis. NMR spectra were recorded in the designated solvent on Bruker Avance 400 MHz spectrometer. Spectra are reported in parts per million values from residual protons of deuterated solvent. Mass data were obtained with a Bruker Daltonics Inc. Microanalysis was performed using Flash EA 1112 or Carlo Erba 1106 analyzer at the Institute of Chemistry, Chinese Academy of Sciences. Compound 1, 3,5-di(pyrid-2-yl)aniline, was prepared according to the known procedure.52 Synthesis of N-(3,5-Di(pyrid-2-yl)phenyl)acetamide (2). To the solution of 3,5-di(pyridin-2-yl)aniline (124 mg, 0.50 mmol) in 8 mL of dry CH2Cl2 was added dropwise 1 mL of Ac2O at room temperature within 5 min. The solution was stirred for 2 h. The solvent was removed by rotary evaporation. To the residue was added 20 mL of saturated aqueous NaHCO3, followed by extraction with copious CH2Cl2 (15 mL × 3). The organic layer was combined and dried with MgSO4. The crude product was purified by column chromatography on silica gel to give 110 mg of 2 as a white solid in 76% yield. 1H NMR (400 MHz, CDCl3): δ 2.20 (s, 3H), 7.24 (t, J = 7.6 Hz, 2H), 7.48 (s, 1H), 7.77 (t, J = 7.6 Hz, 2H), 7.87 (d, J = 8.0 Hz, 2H), 8.27 (s, 2H), 8.40 (s, 1H), 8.70 (d, J = 4.4 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ 24.7, 118.93, 121.0, 121.3, 122.6, 136.9, 139.3, 140.7, 149.6, 156.8, 168.7. HR-MS m/z calcd for C18H15N3O: 289.1215; found: 289.1212. Synthesis of Complex 3. To the mixture of compound 2 (58 mg, 0.20 mmol) in 5 mL of AcOH was added K2PtCl4 (91 mg, 0.22 mol) under N2 atmosphere. The suspension was heated to reflux for 3 d. After the mixture cooled to room temperature, the precipitate was collected by filtration and washed successively with water, ethanol, and ethyl ether. To the obtained crude product was added 100 mL of CH2Cl2, and the suspension was stirred. The insoluble impurity was removed by filtration. The filtrate was concentrated to afford 80 mg of 3 in 77% yield as a yellow solid. 1H NMR (400 MHz, CD2Cl2): δ 2.25 (s, 3H), 7.29 (t, J = 6.4 Hz, 2H), 7.49−7.53 (m, 4H), 7.59 (s, 1H), 7.89 (t, J = 7.2 Hz, 2H), 9.10−9.30 (m, 2H). ESI-MS: 483.2 for [M − Cl]+. Anal. Calcd for C18H14N3OPtCl·0.5CH2Cl2: C, 39.58; H, 2.69; N, 7.49. Found: C, 39.78; H, 2.47; N, 7.27%. Preparations of Samples I, II, and III. The above-obtained complex 3 was directly used as sample I. To the solution of 3 (0.030 mmol, 16 mg) in 3 mL of CH2Cl2 was added a solution of Bu4NH2PO4 (0.030 mmol, 10 mg) in 3 mL of CH2Cl2. The mixture was stirred at room temperature for 5 min. The volume of the solution was concentrated to ∼2 mL, followed by the addition of 10 mL of petroleum ether. The resulting precipitate was collected by filtration and was washed with small amount of CH2Cl2 and petroleum ether to afford 16 mg of sample II as an orange solid in 57% yield. 1H NMR (400 MHz, CD2Cl2): δ 0.95 (t, J = 7.2 Hz, 12H), 1.35−1.42 (m, 8H),

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01059. Emission spectra of 3 in the presence of different phosphate species, a possible binding mode of 3 with H2PO4− in CH3CN, the emission spectra of 3 in the presence of H2PO4− in 1/1 mixed solvent of CH2Cl2 or CH3CN with MeOH, powder XRD spectra of samples I−III, NMR and mass spectra of new compounds (PDF) The crystallographic data of 3 (CIF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grant Nos. 21472196, 21601194, 21271176, and 21521062) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB 12010400) for funding support. G

DOI: 10.1021/acs.inorgchem.6b01059 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

■

Complexes in Room-Temperature Fluid Solution. Inorg. Chem. 1999, 38, 3264−3265. (18) Yam, V. W.-W.; Wong, K. M.-C; Zhu, N. Solvent-Induced Aggregation through Metal···Metal/π···π Interactions: Large Solvatochromism of Luminescent Organoplatinum(II) Terpyridyl Complexes. J. Am. Chem. Soc. 2002, 124, 6506−6507. (19) Lu, W.; Chen, Y.; Roy, V. A. L.; Chui, S.-Y.; Che, C.-M. Supramolecular Polymers and Chromonic Mesophases Self-Organized from Phosphorescent Cationic Organoplatinum(II) Complexes in Water. Angew. Chem., Int. Ed. 2009, 48, 7621−7625. (20) (a) Zhang, X.; Wang, J.-Y.; Ni, J.; Zhang, L.-Y.; Chen, Z.-N. Vapochromic and Mechanochromic Phosphorescence Materials Based on a Platinum(II) Complex with 4-Trifluoromethylphenylacetylide. Inorg. Chem. 2012, 51, 5569−5579. (b) Zhang, X.; Li, B.; Chen, Z.-H.; Chen, Z.-N. Luminescence Vapochromism in Solid Materials Based on Metal Complexes for Detection of Volatile Organic Compounds (VOCs). J. Mater. Chem. 2012, 22, 11427−11441. (21) Jiang, B.; Zhang, J.; Ma, J.-Q.; Zheng, W.; Chen, L.-J.; Sun, B.; Li, C.; Hu, B.-W.; Tan, H.; Li, X.; Yang, H.-B. Vapochromic Behavior of a Chair-Shaped Supramolecular Metallacycle with Ultra-Stability. J. Am. Chem. Soc. 2016, 138, 738−741. (22) Kozhevnikov, V. N.; Donnio, B.; Bruce, D. W. Phosphorescent, Terdentate, Liquid-Crystalline Complexes of Platinum(II): StimulusDependent Emission. Angew. Chem., Int. Ed. 2008, 47, 6286−6289. (23) Chan, K. H.-Y.; Chow, H.-S; Wong, K. M.-C.; Yeung, M. C.-L.; Yam, V. W.-W. Towards Thermochromic and Thermoresponsive Near-infrared (NIR) Luminescent Molecular Materials Through the Modulation of Inter- and/or Intramolecular Pt−Pt and π−π interactions. Chem. Sci. 2010, 1, 477−482. (24) Taylor, S. D.; Howard, W.; Kaval, N.; Hart, R.; Krause, J. A.; Connick, W. B. Solid-State Materials for Anion Sensing in Aqueous Solution: Highly Selective Colorimetric and Luminescence-based Detection of Perchlorate Using a Platinum(II) Salt. Chem. Commun. 2010, 46, 1070−1072. (25) Chen, Y.; Che, C.-M; Lu, W. Phosphorescent Organoplatinum(II) Complexes With a Lipophilic Anion: Supramolecular Soft Nanomaterials Through Ionic Self-assembly and Metallophilicity. Chem. Commun. 2015, 51, 5371−5374. (26) Chung, C. Y.-S.; Yam, V. W.-W. Dual pH- and TemperatureResponsive Metallosupramolecular Block Copolymers with Tunable Critical Micelle Temperature by Modulation of the Self-Assembly of NIR-Emissive Alkynylplatinum(II) Complexes Induced by Changes in Hydrophilicity and Electrostatic Effects. Chem. - Eur. J. 2013, 19, 13182−13192. (27) Koo, C.-K.; Lam, B.; Leung, S.-K.; Lam, M. H.-W.; Wong, W.-Y. “Molecular Pivot-Hinge” Based on the pH-Regulated Intramolecular Switching of Pt−Pt and π−π Interactions. J. Am. Chem. Soc. 2006, 128, 16434−16435. (28) Yu, C.; Wong, K. M.-C; Chan, K. H.-Y.; Yam, V. W.-W. Polymer-Induced Self-Assembly of Alkynylplatinum(II) Terpyridyl Complexes by Metal···Metal/π···π Interactions. Angew. Chem., Int. Ed. 2005, 44, 791−794. (29) Chan, K.; Chung, C. Y.-S.; Yam, V. W.-W. Conjugated Polyelectrolyte-Induced Self-Assembly of Alkynylplatinum(II) 2,6Bis(benzimidazol-2′-yl)pyridine Complexes. Chem. - Eur. J. 2015, 21, 16434−16447. (30) Wang, Y.-W.; Liu, S.-B; Yang, Y.-L; Wang, P.-Z; Zhang, A.-J; Peng, Y. A Terbium(III)-Complex-Based On−Off Fluorescent Chemosensor for Phosphate Anions in Aqueous Solution and Its Application in Molecular Logic Gates. ACS Appl. Mater. Interfaces 2015, 7, 4415−4422. (31) Kondo, S.-i.; Takai, R. Selective Detection of Dihydrogen Phosphate Anion by Fluorescence Change with Tetraamide-Based Receptors Bearing Isoquinolyl and Quinolyl Moieties. Org. Lett. 2013, 15, 538−541. (32) Zhang, D.; Jiang, X.; Yang, H.; Su, Z.; Gao, E.; Martinez, A.; Gao, G. Novel Benzimidazolium−Urea-based Macrocyclic Fluorescent Sensors: Synthesis, Ratiometric Sensing of H2PO4− and Improvement

REFERENCES

(1) Li, K.; Ming Tong, G. S.; Wan, Q.; Cheng, G.; Tong, W.-Y.; Ang, W.-H.; Kwong, W.-L.; Che, C.-M. Highly Phosphorescent Platinum(II) Emitters: Photophysics, Materials and Biological Applications. Chem. Sci. 2016, 7, 1653−1673. (2) Yam, V. W.-W.; Au, V. K.-M.; Leung, S. Y.-L. Light-Emitting SelfAssembled Materials Based on d8 and d10 Transition Metal Complexes. Chem. Rev. 2015, 115, 7589−7728. (3) Cummings, S. D. Platinum Complexes of Terpyridine: Synthesis, Structure and Reactivity. Coord. Chem. Rev. 2009, 253, 449−478. (4) Kalinowski, J.; Fattori, V.; Cocchi, M.; Williams, J. A. G. LightEmitting Devices Based on Organometallic Platinum Complexes as Emitters. Coord. Chem. Rev. 2011, 255, 2401−2425. (5) Cebrián, C.; Mauro, M.; Kourkoulos, D.; Mercandelli, P.; Hertel, D.; Meerholz, K.; Strassert, C. A.; De Cola, L. Luminescent Neutral Platinum Complexes Bearing an Asymmetric N^N^N Ligand for High-Performance Solution-Processed OLEDs. Adv. Mater. 2013, 25, 437−442. (6) Cocchi, M.; Kalinowski, J.; Fattori, V.; Williams, J. A. G.; Murphy, L. Color-Variable Highly Efficient Organic Electrophosphorescent Diodes Manipulating Molecular Exciton and Excimer Emissions. Appl. Phys. Lett. 2009, 94, 073309. (7) (a) Wong, K. M.-C.; Yam, V. W.-W. Luminescence Platinum(II) Terpyridyl Complexes: From Fundamental Studies to Sensory Functions. Coord. Chem. Rev. 2007, 251, 2477−2488. (b) Wenger, O. S. Vapochromism in Organometallic and Coordination Complexes: Chemical Sensors for Volatile Organic Compounds. Chem. Rev. 2013, 113, 3686−3733. (c) Kobayashi, A.; Kato, M. Vapochromic Platinum(II) Complexes: Crystal Engineering toward Intelligent Sensing Devices. Eur. J. Inorg. Chem. 2014, 2014, 4469−4483. (8) Guerchais, V.; Fillaut, J.-L. Sensory Luminescent Iridium(III) and Platinum(II) Complexes for Cation Recognition. Coord. Chem. Rev. 2011, 255, 2448−2457. (9) (a) Yang, W.; Karatay, A.; Zhao, J.; Song, J.; Zhao, L.; Xing, X.; Zhang, C.; He, C.; Yaglioglu, H. G.; Hayvali, M.; Elmali, A.; Kucukoz, B. Near-IR Broadband-Absorbing trans-Bisphosphine Pt(II) Bisacetylide Complexes: Preparation and Study of the Photophysics. Inorg. Chem. 2015, 54, 7492−7505. (b) Liu, Y.; Guo, H.; Zhao, J. Ratiometric Luminescent Molecular Oxygen Sensors Based on uniLuminophores of C^N Pt(II)(acac) Complexes that Show Intense Visible-light Absorption and Balanced Fluorescence/Phosphorescence Dual Emission. Chem. Commun. 2011, 47, 11471−11473. (10) Wong, K. M.-C.; Yam, V. W.-W. Self-Assembly of Luminescent Alkynylplatinum(II) Terpyridyl Complexes: Modulation of Photophysical Properties through Aggregation Behavior. Acc. Chem. Res. 2011, 44, 424−434. (11) Liu, N.; He, Q.; Bu, W. Self-Assembly of Star Micelle into Vesicle in Solvents of Variable Quality: The Star Micelle Retains Its Core−Shell Nanostructure in the Vesicle. Langmuir 2015, 31, 2262− 2268. (12) Baggaley, E.; Weinstein, J. A.; Williams, J. A. G. Lighting the Way to See Inside the Live Cell with Luminescent Transition Metal Complexes. Coord. Chem. Rev. 2012, 256, 1762−1785. (13) Mauro, M.; Aliprandi, A.; Septiadi, D.; Kehr, N. S.; De Cola, L. When Self-assembly Meets Biology: Luminescent Platinum Complexes for Imaging Applications. Chem. Soc. Rev. 2014, 43, 4144−4166. (14) Aliprandi, A.; Genovese, D.; Mauro, M.; De Cola, L. Recent Advances in Phosphorescent Pt(II) Complexes Featuring Metallophilic Interactions: Properties and Applications. Chem. Lett. 2015, 44, 1152−1169. (15) Anderson, B. M.; Hurst, S. K. Platinum Stacking Interactions in Homoleptic Platinum Polymers. Eur. J. Inorg. Chem. 2009, 2009, 3041−3054. (16) Williams, J. A. G.; Beeby, A.; Davies, E. S.; Weinstein, J. A.; Wilson, C. An Alternative Route to Highly Luminescent Platinum(II) Complexes: Cyclometalation with N∧C∧N-Coordinating Dipyridylbenzene Ligands. Inorg. Chem. 2003, 42, 8609−8611. (17) Connick, W. B.; Geiger, D.; Eisenberg, R. Excited-State SelfQuenching Reactions of Square Planar Platinum(II) Diimine H

DOI: 10.1021/acs.inorgchem.6b01059 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

and Luminescence Sensing of Anions. Chem. - Eur. J. 2016, 22, 3738− 3749. (52) Gong, Z.-L.; Zhong, Y.-W. Stepwise Coordination Followed by Oxidation Mechanism for the Multichannel Detection of Cu2+ in an Aqueous Environment. Organometallics 2013, 32, 7495−7502. (53) Wang, Z.; Turner, E.; Mahoney, V.; Madakuni, S.; Groy, T.; Li, J. Facile Synthesis and Characterization of Phosphorescent Pt(N∧C∧N)X Complexes. Inorg. Chem. 2010, 49, 11276−11286. (54) Farley, S. J.; Rochester, D. L.; Thompson, A. L.; Howard, J. A. K.; Williams, J. A. G. Controlling Emission Energy, Self-Quenching, and Excimer Formation in Highly Luminescent N∧C∧N-Coordinated Platinum(II) Complexes. Inorg. Chem. 2005, 44, 9690−9703. (55) Banerji, B.; Chatterjee, M.; Pal, U.; Maiti, N. C. Molecular Details of Acetate Binding to a New Diamine Receptor by NMR and FT-IR Analyses. J. Phys. Chem. A 2016, 120, 2330−2341. (56) (a) Mei, J.; Leung, N. L. C.; Kwok, R. T. K.; Lam, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission: Together We Shine, United We Soar. Chem. Rev. 2015, 115, 11718−11940. (b) Li, Q.; Li, Z. AIE Probes Towards Biomolecules: The Improved Selectivity With the Aid of Graphene Oxide. Sci. China: Chem. 2015, 58, 1800−1809. (57) Feng, X. J.; Peng, J.; Xu, Z.; Fang, R.; Zhang, H.-r.; Xu, X.; Li, L.; Gao, J.; Wong, M. S. AIE-Active Fluorene Derivatives for SolutionProcessable Nondoped Blue Organic Light-Emitting Devices (OLEDs). ACS Appl. Mater. Interfaces 2015, 7, 28156−28165. (58) Sheldrick, G. M. A Short History of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112−122. (59) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: A Complete Structure Solution,Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42, 339−341.

of the Anion Binding Performance Via a Synergistic Binding Strategy. Chem. Commun. 2013, 49, 6149−6151. (33) Beer, P. D. Transition-Metal Receptor Systems for the Selective Recognition and Sensing of Anionic Guest Species. Acc. Chem. Res. 1998, 31, 71−80. (34) Chowdhury, B.; Khatua, S.; Dutta, R.; Chakraborty, S.; Ghosh, P. Bis-Heteroleptic Ruthenium(II) Complex of a Triazole Ligand as a Selective Probe for Phosphates. Inorg. Chem. 2014, 53, 8061−8070. (35) Yoon, J.; Kim, S. K.; Singh, N. J.; Lee, J. W.; Yang, Y. J.; Chellappan, K.; Kim, K. S. Highly Effective Fluorescent Sensor for H2PO4−. J. Org. Chem. 2004, 69, 581−583. (36) Chen, Q.-Y.; Chen, C.-F. A Highly Selective Fluorescent Chemosensor for H2PO4− Based on a Calix[4]arene Tetraamide Derivative. Eur. J. Org. Chem. 2005, 2005, 2468−2472. (37) Hargrove, A. E.; Nieto, S.; Zhang, T.; Sessler, J. L.; Anslyn, E. V. Artificial Receptors for the Recognition of Phosphorylated Molecules. Chem. Rev. 2011, 111, 6603−6782. (38) Zheng, Z.-B.; Duan, Z.-M.; Ma, Y.-Y.; Wang, K.-Z. Highly Sensitive and Selective Difunctional Ruthenium(II) Complex-Based Chemosensor for Dihydrogen Phosphate Anion and Ferrous Cation. Inorg. Chem. 2013, 52, 2306−2316. (39) Liu, J.; Lin, Q.; Zhang, Y. M.; Wei, T. B. A Reversible Fluorescent Chemosensor for Fe3+ and H2PO4− With “on-off-on” Switching in Aqueous Media. Sci. China: Chem. 2014, 57, 1257−1263. (40) Gale, P. A.; Caltagirone, C. Anion Sensing by Small Molecules and Molecular Ensembles. Chem. Soc. Rev. 2015, 44, 4212−4227. (41) Zhang, D.; Cochrane, J. R.; Martinez, A.; Gao, G. Recent Advances in H2PO4− Fluorescent Sensors. RSC Adv. 2014, 4, 29735− 29749. (42) Chang, K.-C.; Sun, S.-S; Odago, M. O.; Lees, A. J. Anion Recognition and Sensing by Transition-Metal Complexes with Polarized N-H Recognition Motifs. Coord. Chem. Rev. 2015, 284, 111−123. (43) Mercer, D. J.; Loeb, S. J. Metal-Based Anion Receptors: An Application of Second-Sphere Coordination. Chem. Soc. Rev. 2010, 39, 3612−3620. (44) Amendola, V.; Fabbrizzi, L. Anion Receptors that Contain Metals as Structural Units. Chem. Commun. 2009, 513−531. (45) Beer, P. D.; Hayes, E. J. Transition Metal and Organometallic Anion Complexation Agents. Coord. Chem. Rev. 2003, 240, 167−189. (46) Beer, P. D.; Chen, Z.; Goulden, A. J.; Grieve, A.; Hesek, D.; Szemes, F.; Wear, T. Anion Recognition by Novel Ruthenium(II) Bipyridyl Calix[4]arene Receptor Molecules. J. Chem. Soc., Chem. Commun. 1994, 1269−1271. (47) Plitt, P.; Gross, D. E.; Lynch, V. M.; Sessler, J. L. DipyrrolylFunctionalized Bipyridine-Based Anion Receptors for Emission-Based Selective Detection of Dihydrogen Phosphate. Chem. - Eur. J. 2007, 13, 1374−1381. (48) (a) Saha, D.; Das, S.; Karmakar, S.; Dutta, S.; Baitalik, S. Synthesis, Structural Characterization and Anion-, Cation- and Solvent-Induced Tuning of Photophysical Properties of a Bimetallic Ru(II) Complex: Combined Experimental and DFT/TDDFT Investigation. RSC Adv. 2013, 3, 17314−17334. (b) Karmakar, S.; Mardanya, S.; Pal, P.; Baitalik, S. Design of Multichannel OsmiumBased Metalloreceptor for Anions and Cations by Taking Profit from Metal-Ligand Interaction and Construction of Molecular Keypad Lock and Memory Device. Inorg. Chem. 2015, 54, 11813−11825. (49) Qiu, D.; Li, M.; Zhao, Q.; Wang, H.; Yang, C. Cyclometalated Platinum(II) Terpyridylacetylide with a Bis(arylamine) Donor as a Proton-Triggered Luminescence Chemosensor for Zn2+. Inorg. Chem. 2015, 54, 7774−7782. (50) Chung, S.-K.; Tseng, Y.-R; Chen, C.-Y; Sun, S.-S. A Selective Colorimetric Hg2+ Probe Featuring a Styryl Dithiaazacrown Containing Platinum(II) Terpyridine Complex through Modulation of the Relative Strength of ICT and MLCT Transitions. Inorg. Chem. 2011, 50, 2711−2713. (51) Hau, F. K.-W.; Lo, H.-S.; Yam, V. W.-W. Synthesis and Photophysical Studies of Calixarene-Based Alkynylplatinum(II) Terpyridine Complexes with Various Receptor Sites for Colorimetric I

DOI: 10.1021/acs.inorgchem.6b01059 Inorg. Chem. XXXX, XXX, XXX−XXX