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Fluorescent Recognition of Zn2+ by Two Diastereomeric Salicylaldimines: Dramatically Different Responses and Spectroscopic Investigation Ting Song,† Yuan Cao,† Gang Zhao,*,† and Lin Pu*,‡ †

College of Chemical Engineering, Sichuan University, Chengdu, 610065, People’s Republic of China Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States

‡

S Supporting Information *

ABSTRACT: Fluorescence responses of two BINOL-based diastereomeric salicylaldimines toward a variety of metal cations have been studied in methanol solution. It is revealed that both compounds show great fluorescence enhancements in the presence of Zn2+ but not with any other metal ions. Moreover, these two diastereomers exhibit dramatically different responses toward Zn2+ under the same conditions. That is, one can produce much stronger fluorescence enhancement also at a longer wavelength than the other. This fluorescence recognition of Zn2+ also shows distinctive color changes under a UV lamp. Mass and NMR spectroscopic analyses have been used to study the mechanism, which indicates the formation of 2+nZn2+ complexes (n = 2, 3). This work has shed new light on the mechanism of an enantioselective fluorescent recognition of chiral amines promoted by Zn2+.

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light on the mechanism of an enantioselective fluorescent recognition of chiral amines promoted by Zn2+.9

INTRODUCTION Zn is an important metal ion present in the environment and biological systems.1−5 It is found to be the second most abundant transition metal element in the human body, playing a significant role in various fundamental biological processes, such as gene expression, apoptosis, enzyme regulation, and neurotransmission. In order to detect Zn2+, the use of fluorescence-based molecular probes has been extensively investigated, and a number of sensitive and selective fluorescent sensors have been developed.6−8 For example, it has been demonstrated that coordination of the salicylaldimine-based molecules with Zn2+ can inhibit the excited-state isomerization of CN bonds, leading to fluorescent enhancement.8 Thus, a few of the salicylaldimine-based molecules have been used for the fluorescent detection of Zn2+. 2+

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General Data. 1H and 13C NMR spectra were measured using a Bruker AM400 NMR spectrometer. Proton chemical shifts of NMR spectra were given in ppm relative to the internal reference tetramethylsilane (Si(CH3)4) (1H, 0.00 ppm). High-resolution mass spectrometry (HRMS) data were recorded on a Bruker micrOTOF QII mass spectrometer. Fluorescence emission spectra were obtained by using a Shimadzu RF-5301PC spectrofluorophotometer at 298 K unless noted otherwise. Absorption spectra were recorded on a PerkinElmer Lambda 35 UV−vis spectrometer using quartz cells with an absorption path length (l) of 10.0 mm. Melting points were determined on an X-4 digital display micro melting point apparatus (Fukai,China). Optical rotations were acquired on a PerkinElmer M341 polarimeter. Compound (S)-1 was synthesized by following the literature.10 The enantiomers of D/L-alaninol were purchased from Adamas. Unless otherwise noted, materials were obtained from commercial suppliers and were used without further purification. All of the solvents used in the optical spectroscopic studies were either HPLC- or spectroscopic-grade. Zn(CH3COO)2·2H2O was used as the Zn2+ source unless otherwise noted. Preparation and Characterization of (Sa,R)-3. (S)-1 (342 mg, 1 mmol) and D-alaninol (166 mg, 2.2 mmol) were dissolved in dry dichloromethane (CH2Cl2) under nitrogen, and the mixture was stirred at room temperature overnight. After evaporation of the solvent, the crude product was purified by recrystallization in methanol, which gave brown bulk crystals upon slow cooling. The

We have used the chiral 1,1′-binaphthol (BINOL) molecules to synthesize two diastereomeric salicylaldimine compounds that are found to show highly selective fluorescence enhancement in the presence of Zn2+ but not with other metal cations. In addition, the two diastereomers exhibit remarkably different fluorescent responses toward Zn2+. Spectroscopic studies have been conducted to probe this new type of stereoselective fluorescent responses toward Zn2+. This work has shed new © 2017 American Chemical Society

EXPERIMENTAL SECTION

Received: December 14, 2016 Published: March 27, 2017 4395

DOI: 10.1021/acs.inorgchem.6b03062 Inorg. Chem. 2017, 56, 4395−4399

Article

Inorganic Chemistry crystals were filtered and washed with a minimum amount of methanol. After drying under vacuum, (Sa,R)-3 was obtained in 89% yield (406 mg). 1H NMR (CDCl3, 400 MHz): δ 13.37 (s, 2H), 8.66 (s, 2H), 7.94 (s, 2H), 7.87−7.84 (m, 2H), 7.30−7.28 (m, 4H), 7.18− 7.16 (m, 2H), 3.60−3.51 (m, 6H), 1.18 (d, 6H, J = 4 Hz). 13C NMR (CDCl3, 100 MHz): δ 65.21, 154.60, 135.22, 133.42, 129.00, 128.40, 127.64, 124.82, 123.46, 120.83, 116.62, 66.98, 66.83, 18.21. HRMS (ESI+): calcd for C28H29N2O4 [M + H]+ 457.2122, found 457.2104. Mp: 158−160 °C. [α]20D = −75.3 (c 1, CH3OH). Preparation and Characterization of (Sa,S)-3. The procedure was similar to the preparation of (Sa,R)-3, but L-alaninol (instead of Dalaninol) was used and the reaction time was much longer (1.5 d). After evaporation of the solvent, the crude product was dissolved in a minimum amount of CH2Cl2, to which was slowly added an excess amount of n-hexane to precipitate out (Sa,S)-3. The light yellow solid was collected by filtration and washed with n-hexane. After drying under vacuum, (Sa,S)-3 was obtained in 91% yield (415 mg). 1H NMR (CDCl3, 400 MHz): δ 13.19 (s, 2H), 8.65 (s, 2H), 7.96 (s, 2H), 7.89−7.87 (m, 2H), 7.30−7.28 (m, 4H), 7.21−7.19 (m, 2H), 3.64− 3.51 (m, 6H), 1.20 (d, 6H, J = 4 Hz). 13C NMR (CDCl3, 100 MHz): δ 165.28, 154.57, 135.32, 133.59, 128.98, 128.49, 127.63, 124.75, 123.50, 120.87, 116.63, 67.14, 66.89, 18.23. HRMS (ESI+): calcd for C28H29N2O4 [M + H]+ 457.2122, found 457.2121. Mp: 151−153 °C. [α]20D = 58.8 (c 1, CH3OH). Preparation of Samples for Fluorescence Measurement. A 2.0 mM stock solution of a sensor in CH2Cl2 and 2.0 mM Zn(CH3COO)2·2H2O in CH3OH were freshly prepared for each measurement. For the fluorescence enhancement study, a sensor solution was mixed with varying equivalents of Zn2+ stock solution at room temperature in a 5 mL volumetric flask. The resulting solution was allowed to stand at room temperature for 0.5 h before being diluted to the desired concentration (0.02 mM). All the fluorescence spectra were taken within 2 h. Preparation of Samples for Job Plots. The Job plot for the interaction of (Sa,R)-3 with Zn2+ was obtained by measuring the fluorescence response of (Sa,R)-3 with varying ratios of Zn2+ versus (Sa,R)-3 while the total concentration of (Sa,R)-3+Zn2+ was maintained. A 2.0 mM stock solution of the sensor in CH3OH and 2.0 mM Zn(CH3COO)2·2H2O in CH3OH were freshly prepared for each measurement. For the fluorescence enhancement study, 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 μL of (Sa,R)-3 stock solution were mixed with 100, 90, 80, 70, 60, 50, 40, 30, 20, 10, and 0 μL of Zn2+ stock solution in a 5 mL volumetric flask. The resulting solution was allowed to stand at room temperature for 0.5 h before being diluted to the desired concentration (0.04 mM). All the fluorescence spectra were taken within 2 h. Preparation of Samples for Absorption Measurement. A stock solution of a sensor (2.0 mM in CH2Cl2) was treated with various equivalents of Zn(CH3COO)2·2H2O (2.0 mM in CH3OH) at room temperature in a 5 mL volumetric flask. The resulting solution was allowed to stand at room temperature for 0.5 h before being diluted to the desired concentration (0.02 mM). All the absorbance spectra were taken within 2 h. Preparation of Samples for NMR Titration. To an NMR tube containing a sensor (0.2 mL, 25 mM) in CDCl3 was gradually added Zn(CH3COO)2·2H2O (0.1 M in CD3OD) to obtain 1:0, 1:0.5, 1:1, 1:1.5, and 1:2 molar ratios of the sensor:Zn2+, respectively. The total volume was made up to 0.5 mL, and the final concentration of the sensor was 10 mM in CDCl3/CD3OD (4:1). The resulting solution was allowed to stand at room temperature for 3 h before measurement.

Scheme 1. Synthesis of the BINOL-Based Diastereomeric Salicylaldimines (Sa,R)-3 and (Sa,S)-3

13.2−13.3, indicating strong intramolecular hydrogen bonding with the imine nitrogens, and the imine proton signals were observed at δ 8.6−8.7. Both (Sa,R)-3 and (Sa,S)-3 show little fluorescence in methanol solution, which can be attributed to two fluorescent quenching mechanisms in these molecules including the excited-state intramolecular proton transfer between the BINOL hydroxyl groups and the imine nitrogens12 and the excited-state isomerization of the CN bonds.8 We studied the fluorescence response of (Sa,R)-3 in the presence of various metal ions in methanol solution. As shown in Figure 1, when

Figure 1. Fluorescence responses of (Sa,R)-3 (2.0 × 10−5 M) toward 1.0 equiv of various metal ions (a) and bar graphs of the fluorescence intensity at 523 nm (b). (Solvent: CH3OH with 1% CH2Cl2; λexc = 420 nm, slit = 5/3 nm.)

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this compound was treated with a variety of metal cations, only Zn2+ generated large fluorescence enhancement at λ = 523 nm. Similarly, when (Sa,S)-3 was treated with these metal cations, only Zn2+ generated fluorescence enhancement at λ = 505 nm (Figure S3a and b, SI). However, as shown in Figure 2, these two diastereomers responded to Zn2+ very differently. (Sa,R)-3 produced much stronger fluorescent enhancement also at a longer wavelength than its diastereomer (Sa,S)-3. That is, up to 1700-fold fluorescent enhancement at λ = 523 nm was observed when (Sa,R)-3 was treated with Zn2+ (2.0 × 10−5

RESULTS AND DISCUSSION As shown in Scheme 1, we have synthesized the two diastereomeric salicylaldimine compounds (Sa,R)-3 and (Sa,S)-3 from the condensation of the BINOL-based dialdehyde (S)-110 with the chiral amino alcohol (R)- and (S)-2, respectively, in methylene chloride at room temperature.11 These two compounds give very similar 1H NMR spectra, with the BINOL hydroxyl proton signals observed at δ 4396

DOI: 10.1021/acs.inorgchem.6b03062 Inorg. Chem. 2017, 56, 4395−4399

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Inorganic Chemistry

fluorescence enhancement of both compounds in the presence of Zn2+ reached a maximum at about 1.5 equiv of Zn2+. This is consistent with a Job plot in Figure 5 for the fluorescence

Figure 2. Fluorescence spectra of (Sa,R)-3 and (Sa,S)-3 in the absence and in the presence of 1 equiv of Zn2+ (2.0 × 10−5 M in CH3OH with 1% CH2Cl2; λexc = 420 nm, slit = 5/3 nm).

Figure 5. Job plot of (Sa,R)-3 with Zn2+ on the basis of the fluorescence signal at 523 nm. (The total concentration of (Sa,R)3+Zn2+ was 4.0 × 10−5 M. Solvent: CH3OH, λexc = 420 nm, slit: 3/3 nm.)

M) (Figure 1b), while Zn2+ enhanced the fluorescence of (Sa,S)-3 by 430-fold at λ = 505 nm (Figure S3b, SI). Figure 3 compares the photos of (Sa,R)-3 + Zn2+(1 equiv) with that of (Sa,S)-3 + Zn2+ (1 equiv). It shows that (Sa,R)-3 +

response of (Sa,R)-3 toward Zn2+ obtained by measuring the fluorescence responses of (Sa,R)-3 with varying ratios of Zn2+ versus the mole fraction of (Sa,R)-3 while the total concentration of (Sa,R)-3+Zn2+ was maintained. As shown in Figure 5, the maximum fluorescence enhancement of (Sa,R)-3 promoted by Zn2+ occurs at a ratio of 2:3. The fluorescence titration plots of (Sa,S)-3 with Zn2+ are given in Figure S6 (SI). The job plot of (Sa,S)-3 with Zn2+ also shows that the fluorescence intensity of (Sa,S)-3 with Zn2+ reached the maximum at a ratio of 2:3 (Figure S7, SI). On the basis of these fluorescence analyses, it is proposed that formation of 2+3 complexes between the BINOL-based salicylaldimine compounds and Zn2+ has produced the maximum fluorescence enhancement. We have obtained the high-resolution mass spectra for the reaction of (Sa,R)-3 and (Sa,S)-3 with 0−2 equiv of Zn2+ in methanol solution (Figure S9, SI). In the mass spectra, signals corresponding to a proposed 2 + 2 complex 4 were observed with 4+H at 1037.25 (calcd 1037.24) and 4+H−H2O at 1019.31 (calcd 1019.23). A simulated isotope distribution for the 2+2 complex is found to match the observed (Figure S10, SI). A signal corresponding to a proposed 2+3 complex was also observed for 4+Zn+2CH3OH−H2O at 1146.15 (calcd 1146.21).

Figure 3. From left to right: (Sa,R)-3, (Sa,R)-3+Zn2+(1 equiv), (Sa,S)3, and (Sa,S)-3+Zn2+(1 equiv) (2.0 × 10−5 M in CH3OH/1% CH2Cl2) under a UV lamp irradiation at 365 nm.

Zn2+(1 equiv) exhibits intense yellow emission under a UV lamp (365 nm), but (Sa,S)-3 + Zn2+(1 equiv) gives much weaker green emission. That is, the fluorescent responses of the two diastereomers toward Zn2+ can be visually discriminated. In compounds (Sa,R)-3 and (Sa,S)-3, the chiral carbon centers of the amino alcohol units are far away from the chiral BINOL core. How could the difference in the remote chiral centers of the amino alcohol units lead to the observed dramatic differences in the fluorescence response of these two diastereomers toward Zn2+? In order to probe this new type of stereoselective fluorescent response, we have carried out a series of spectroscopic studies. Fluorescence titration of (Sa,R)-3 and (Sa,S)-3 with various amounts of Zn2+ was conducted. As shown in Figure 4, the

In order to gain additional information for the interaction of the sensor with Zn2+, we studied the reaction by using 1H NMR spectroscopy. Figure 6 gives the 1H NMR spectra when (Sa,R)3 (10 mM) was treated with 0−2 equiv of Zn(OAc)2 in CDCl3/CD3OD (4:1, v/v). It shows the formation of a major symmetric product with a singlet at δ 8.08 observed for the Zn2+-coordinated imine groups (Figures S11 and S12, SI). An unusual upfield shift for the methyl group of (Sa,R)-3 from δ 1.26 to δ 0.57 upon coordination with Zn2+ was observed. This indicates that coordination of Zn2+ with the ligand has placed the methyl groups of the amino alcohol units in the shielding

Figure 4. Fluorescence intensity of (Sa,R)-3 and (Sa,S)-3 at 523 nm versus the equivalence of Zn(AcO)2·2H2O (solvent: CH3OH/1% CH2Cl2; λexc = 420 nm, slit: 5/3 nm). 4397

DOI: 10.1021/acs.inorgchem.6b03062 Inorg. Chem. 2017, 56, 4395−4399

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Inorganic Chemistry

with the available oxygen atoms to form 2+nZn2+ complexes, among which the fluorescence enhancement reaches a maximum for the 4+Zn2+ complex (a 2 + 3 complex). Formation of the bisBINOL complexes such as 4 and 4+nZn2+ requires much higher organization of the structural units of compounds (Sa,R)-3 and (Sa,S)-3, and a small difference in the structural units could be amplified in these higher ordered structures. In these bisBINOL complexes, the chiral carbon centers of the amino alcohol units are brought close to each other, which should enhance the influence of these chiral centers on the structure and property and contribute to the observed large difference in fluorescence. Earlier, we reported that (S)- or (R)-1 in combination with Zn2+ can be used to conduct the enantioselective fluorescent recognition of chiral amines including chiral amino alcohols.9 The observed very different fluorescent responses of the two diastereomeric salicylaldimine compounds (Sa,R)-3 and (Sa,S)3 toward Zn2+ in this work have provided an important clue to understand the previously observed chiral recognition of the amino alcohols. That is, in the presence of Zn2+, (S)-1 should react with the enantiomers of the chiral amino alcohols to form the corresponding diastereomeric salicylaldimine+Zn(II) complexes such as 4 and 4+nZn2+ with different extents of reactions as well as different fluorescence properties.

Figure 6. 1H NMR spectra of (Sa,R)-3 (10 mM) titrated with Zn(AcO)2·2H2O (0−2 equiv) in CDCl3/CD3OD (4:1, v/v). (The 1H NMR spectra were taken after the solution was allowed to stand at room temperature for 3 h.)

region of the aromatic rings. Similar NMR signals are observed for the reaction of (Sa,S)-3 with Zn(OAc)2 except smaller differences in chemical shifts and lower conversion of the reaction. That is, the equilibrium for the reaction of the diastereomer (Sa,R)-3 with Zn2+ favors the formation of the coordinated product much more than the other diastereomer. The 1H NMR spectra demonstrate that in the presence of 2 equiv of Zn2+ at room temperature for 3 h, the ratio of the product versus the starting material was 2.3:1 for (Sa,R)-3 and 1.2:1 for (Sa,S)-3. This large difference in the extent of product formation should have contributed to their very different fluorescence responses. The results obtained by fluorescence, mass, and NMR spectroscopic analyses for the reaction of 3 with Zn2+ are consistent with the proposed formation of the 2 + 2 complex 4, which can further coordinate with Zn2+ to generate the 2+3Zn2+ and 2+nZn2+ complexes. An approximate structural model of the 2+2 complex 4 is shown in Figure 7 obtained by using the Spartan program (molecular mechanics MMFF).13 It shows that the methyl groups on the amino alcohol units are located near the shielding region of the naphthalene rings, accounting for the observed significantly upfield shifted 1H NMR signal. Additional Zn2+ ions could also be associated with 4 by coordination

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CONCLUSIONS In summary, we have demonstrated that two diastereomeric salicylaldimine compounds can be used as highly selective fluorescent sensors toward Zn2+. They also exhibit very different fluorescent responses toward Zn2+, with one giving much greater fluorescence enhancement at a different wavelength than the other. On the basis of the NMR and mass spectroscopic analyses, it is proposed that formation of the 2+2 complex 4 and its further coordination with Zn2+ should account for the observed fluorescence enhancement as well as the stereoselective response. The different fluorescent responses of the two diastereomeric salicylaldimines upon Zn2+ coordination provide a better understanding of the mechanism of the previously reported enantioselective fluorescent recognition of chiral amino alcohols by a BINOL dialdehyde.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b03062. Additional NMR, UV−vis, fluorescence and mass spectroscopic data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail (G. Zhao): [email protected]. *E-mail (L. Pu): [email protected]. ORCID

Lin Pu: 0000-0001-8698-3228 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.Z. thanks the support by the Sichuan University High Level Talent Project and Sichuan Province 1,000 Talents Plan

Figure 7. Molecular model of 4. 4398

DOI: 10.1021/acs.inorgchem.6b03062 Inorg. Chem. 2017, 56, 4395−4399

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and the Use of Their Titanium Complexes as Pre-Catalysts for the Asymmetric Trimethylsilylcyanation of Benzaldehyde. Russ. Chem. Bull. 2008, 57, 1981−1988. (12) A review: Formosinho, S. J.; Arnaut, L. G. Excited-State Proton Transfer Reactions II. Intramolecular Reactions. J. Photochem. Photobiol. A: Chem. 1993, 75, 21−48. (13) Spartan’10; Wavefunction, Inc.: Irvine, CA, 2010.

Project. L.P. thanks the partial support of the U.S. National Science Foundation (CHE-1565627). We also thank Public Center of Experimental Technology of Chengdu Institute of Biology, Chinese Academy of Sciences, for spectroscopic analyses of the compounds.

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DOI: 10.1021/acs.inorgchem.6b03062 Inorg. Chem. 2017, 56, 4395−4399