The synthesis, characterization and application of the binol-cages of R-/S-enantiomers
College of Chemistry, Zhengzhou University, Zhengzhou 450001, Henan, China.
*Correspondence to: Prof. Linlin Shi, College of Chemistry, Zhengzhou University, No.100 Science Avenue, Zhengzhou 450001, Henan, China. E-mail:
In the past few years, significant efforts have been made to create and self-assemble covalent organic cages with increased complexity and functionality. However, although supramolecule cages have been widely recognized as probes to identify metal ions, the detection of mercury ions has not been fully developed. Here, we have designed and synthesized a pair of chiral cages with custom cavities based on the unique rigid structure of 1, 10-binaphthol (binol). Meanwhile, the supramolecular cage has excellent performance in high sensitivity and selectivity for detecting mercury ions. The UV titration results indicate that the binding ratio of the host to guest is 1:5. The titration curve conforms to the nonlinear fitting of the Hill function, which can obtain the binding constant K =
The global attention on environmental pollution caused by heavy metal ions persists due to their potential to inflict severe harm on both animals and humans. Many highly sensitive and selective probes have been developed and successfully used in practical life in order to detect metal ions in the environment. For example, probes synthesized by Mohandoss et al. were widely used for detecting Fe3+, Al3+, Cu2+, and Ag+ ions[2-4]. As is well known, mercury is extremely harmful to organisms and the environment, especially to mammals. It can not only enter the human body directly through the skin, digestive system, or respiratory system but also accumulate in the environment, ultimately entering the human body through the food chain and causing serious damage to the central nervous and endocrine systems of the human body. In addition, all stable forms of Hg (i.e., metallic, inorganic, and organic) cause neurological poisoning in fish, wildlife, and humans. It causes several disorders in the liver, kidney, heart, stomach, brain, intestine, and immune system[5-7]. According to the United States Environmental Protection Agency (US EPA), the maximum possible limit of Hg2+ ions in drinking water is 2 ppb, but the amount of Hg2+ ions in drinking water excess to the limit may pose a risk to human health and the environment (Environmental Protection Agency, 2001). Despite these drawbacks, mercury and mercury salts still widely exist in nature in the form of elemental mercury, inorganic, and organic compounds. Human activities, industrial production, and natural phenomena can all lead to the production of various forms of mercury compounds[8-10]. Therefore, it is necessary to seek efficient, simple, and fast new ways to detect mercury ions in the atmosphere and wastewater in order to protect the Earth's environment[11-25].
In recent years, mercury ions have been detected by a large number of analytical methods, including cold vapor atomic fluorescence spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), gas chromatography, neutron activation analysis (NAA), and high-performance liquid chromatography. However, these technologies often require cumbersome operations and expensive instruments. As a result, the development of more efficient and facile approaches for mercury ion detection is highly appealing yet challenging.
Supramolecular chemistry, as an interesting field, has successively attracted much attention due to its unique structures and extensive applications in sensing[31,32], catalysis[33,34], separation[35,36], gas adsorption[37,38], biological imitation, and so on[40,41]. The intriguing three-dimensional molecular cages have proven to be superior alternative architectures, garnering considerable interest owing to their outstanding structures, properties, applications, and the diverse array of charming skeletons successfully generated via coordinative self-assembly, such as octahedrons[42,43], tetrahedrons[44-47], spirals, capsules, etc. Additionally, subcomponent self-assembly, as the most powerful bottom-up strategy, has attracted great attention by preparing highly organized structures with different functions in an "order-out-of-chaos" and well-controlled manner[50-53]. Comparatively, the discrete supramolecular structures formed by covalent bonds remain underdeveloped, possibly due to the dynamic and unstable interactions[54,55]. In this context, molecular cages formed by imine condensation have developed rapidly[56-58] since the groundbreaking work of Quan et al. in 1991. Despite the impressive progress made by chemists in this field, the involvement of multiple functional components in a single supramolecular system causes further difficulties for synthesis and its evocative biological heuristic process. Meanwhile, there are relatively few examples of detecting mercury ions as fluorescent probes, although covalent supramolecular cages have been widely reported as examples of fluorescent probes[60-64]. Therefore, it is crucial to create novel covalent supramolecular cages and use them for mercury ion detection.
Herein, we purposefully designed and constructed covalent organic cages with (R)- or (S)-binaphthyl skeletons as ligands and tri(2-aminoethyl)amine as vertices by amine aldehyde condensation-driven collaborative self-assembly. Notably, the ether chains were introduced on three symmetric binaphthyl ligands to form 20-crown-6. In addition, mercury ions were successfully recognized through fluorescence testing. To our delight, these covalent organic nanocages, which possess chemiluminescent properties, can serve as a fluorescence sensor to detect these ions. The successful synthesis of the cage had been demonstrated by NMR, mass spectrometry, Fourier transform infrared (FT-IR), and molecular simulations.
RESULTS AND DISCUSSION
As shown in Scheme 1, our investigation was commenced by using the compound 1 [3, 6, 9, 12-tetraoxatetradecane-1, 14-diol] and compound 3 (R)- or (S)-BINOL (binaphthol) as the starting substrates. Initially, (S)-4 was treated with potassium t-butoxide at 75 °C, and the resultant system was reacted with pentaethylene glycol di-p-toluenesulfonate 2 to afford the product (S)-5 in 40% yield. A Suzuki coupling reaction of bromide (S)-5 with 4-formylphenylboronic acid gave an important precursor (S)-7 in 30% yield, which has two aldehyde functionalities attached to the binaphthyl backbone. On the basis of the successful synthesis of (S)-7, covalent organic nanocage cage (S)-8 was prepared by co-self-assembly with tris(2-aminoethyl)amine in a yield of 55% as a yellow solid. The same method was used to synthesize (R)-8 for a 53% yield. The chemical structures of these chiral cages were adequately characterized by 1H NMR, mass spectrometry, and correlation spectroscopy (COSY) spectral information.
The desired covalent organic cages were successfully synthesized according to 1H NMR spectroscopy
Figure 1. Part of the 1H NMR (600 MHz, CDCl3, 22 oC) spectra of (S)-7 (A); (S)-8 (B); (R)-8 (C); (R)-7 (D) and experimental ESI-MS of the (S)-8 of [8+2H]2+ (blue) and simulated mass spectra (red) (E).
FT-IR characterization has also proven to be an important tool for detection of changes in functional groups [Figure 2]. The infrared spectrum of compound (S)-7 displayed a strong aldehyde stretching vibrational peak at 1,695 nm alongside a stretching vibration peak at 1,642 nm due to the imine formation.
The UV-Vis and fluorescence spectra also show the structural characteristics of the cage, as shown in Figure 3; the UV-Vis absorption spectra for the cage display two main peaks at 322 nm and 280 nm, which correspond to the n-π*transitions (C=N) and π-π* transitions (C=C). The UV-Vis absorption spectra for the ligand exhibited a unique peak at 337 nm, which was attributed to the existence of n-π* transitions (C=O). Furthermore, the emission wavelengths of (S)-8 and (S)-7 are 414 nm and 458 nm in the fluorescence spectra, respectively.
A simulated molecular model of cages was constructed due to many unsuccessful efforts to obtain the crystal structure of cages [Figure 4 and Supplementary Figure 24]. Analysis of the simulated cages showed the expected [2+3] structure, with two amine moieties connected by three side arms based on binol in the top and bottom views. The structure of cages had a C3 symmetric topology and three C2 axes perpendicular to the C3 axis.
Figure 4. The simulated crystal structures of (S)-8 (A) and (R)-8 (B). (N, blue, O, red, C, gray, and hydrogens are omitted for clarity).
The inherent chirality of imine cages was characterized by circular dichroism (CD) spectroscopy. As shown in Figure 5, (R)-8 and (S)-8 exhibited mirror CD spectra with highly pronounced Cotton effects in the wavelength range of 240 to 400 nm. The CD spectra of cages displayed one strong band at 286 nm, which indicated the absorption of naphthalene groups on the binaphthyl backbone. Therefore, we have successfully synthesized the covalent supramolecular cages through NMR, ESI-MS, molecular simulations, CD, and FT-IR characterization methods.
Figure 5. (A) The UV-Vis spectra of (R)-8 and (S)-8 in CDCl3 (c = 0.01 mM); (B) CD spectra of (R)-8 and (S)-8 in CDCl3 (c = 0.1 mM).
The heavy metal ion Hg2+ poses a significant threat to the environment due to its high affinity for thiol groups in proteins and enzymes. Additionally, it is important to note that mercury ions are not biodegradable. As a result, even at extremely low concentrations, they can have detrimental effects on the kidneys and brain, potentially leading to various diseases. The US EPA stipulates that the maximum concentration of mercury ions in drinking water is 10 μM. Consequently, in this work, the cage can be used as a fluorescence probe because adding mercury ions to the solution does not decompose the cage
Figure 6. Fluorescence titration of (R)-8 (10 µM) upon addition of Hg2+ ion up to 10 µM in CHCl3 (A) and the plot of fluorescence intensity versus Hg2+ concentration (B) (λex = 322 nm, Silt = 2, 2); (C) Mercury ion detection mechanism.
Figure 7. (A) Emission spectra and (B) bar graph of (R)-8 (10 µM) with various metal ions (10 µM) in CHCl3 (λex = 322 nm, slit width = 2, 2 nm).
In order to further explore the sensing ability of the cage for the Hg2+ ion, selective recognition experiments of the cage (0.2 mM) were also conducted by using different concentrations of Hg2+ (0.7, 1.4, 2.1 mM) in CHCl3 [Figure 8]. The results, as depicted in Figure 8, exhibit distinct changes in color that can be obviously observed by the naked eye. The color of the CHCl3 with spiked Hg2+ transformed from colorless to light yellow. The ability of cages to detect mercury ions in different pH solutions was also tested separately, and similar results were observed
In conclusion, we have demonstrated the formation of two chiral organic cages using chiral binaphthyl as the main building block. The structures of the chiral cages were characterized using 1H NMR, 2D NMR, mass spectroscopy, molecular simulation, CD, and FT-IR characterization analyses. Furthermore, the absorbance of cages exhibited a good linear correlation with Hg2+ concentrations with detection limits of
We acknowledge the Molecular Scale Lab for mass spectrometry characterization.Authors’ contributions
Designing the experiments, writing the manuscript, and being responsible for the whole work: Shi L, Guo Y, Song MP
Revising the first draft of the manuscript: Hao XQ
Performing the experiments and synthesizing the substrates: Li T, Ding L, Kang YAvailability of data and materials
Please refer to the Supplementary Materials for the experimental procedures and characterization of the new compound.Financial support and sponsorship
This research was funded by the National Natural Science Foundation of China (No. 22101267); the Natural Science Foundation of Henan Province (202300410477); the China Postdoctoral Science Foundation (No. 2021M692905).Conflicts of interest
All authors declared that there are no conflicts of interest.Ethical approval and consent to participate
Not applicable.Consent for publication
© The Author(s) 2023.Supplementary Materials
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Cite This Article
Li T, Ding L, Kang Y, Hao XQ, Guo Y, Shi L, Song MP. The synthesis, characterization and application of the binol-cages of R-/S-enantiomers. Chem Synth 2023;3:45. http://dx.doi.org/10.20517/cs.2023.39
Li T, Ding L, Kang Y, Hao XQ, Guo Y, Shi L, Song MP. The synthesis, characterization and application of the binol-cages of R-/S-enantiomers. Chemical Synthesis. 2023; 3(4): 45. http://dx.doi.org/10.20517/cs.2023.39
Li, Tianyu, Luyao Ding, Yihong Kang, Xin-Qi Hao, Yujing Guo, Linlin Shi, Mao-Ping Song. 2023. "The synthesis, characterization and application of the binol-cages of R-/S-enantiomers" Chemical Synthesis. 3, no.4: 45. http://dx.doi.org/10.20517/cs.2023.39
Li, T.; Ding L.; Kang Y.; Hao X.Q.; Guo Y.; Shi L.; Song M.P. The synthesis, characterization and application of the binol-cages of R-/S-enantiomers. Chem. Synth. 2023, 3, 45. http://dx.doi.org/10.20517/cs.2023.39
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