Two macrocycle-based sensors for anions sensing

Two macrocyclic bis-benzimidazolium salts 2 and 4 (23-membered for 2 and 25-membered for 4) were prepared, and their structures were confirmed by X-ray crystallography, 1H NMR and 13C NMR spectroscopy. The research of anion recognitions using 2 or 4 as hosts were carried out with the methods of fluorescence and ultraviolet spectroscopy, 1H NMR titrations, MS and IR spectra. The experiment results show that 2 can detect acetate anion and 4 can detect nitrate anion with favorable selectivity and sensitivity.

Recognition of acetate anion using compound 2 as a sensor. The      In ultraviolet titration experiments (Fig. 6), the ultraviolet absorption of 2 enhanced gradually along with the increasing concentration of OAc − . The stability constant (K S ) was calculated according to the following relation of non-linear least square analysis of the titration curves for 1:1 complexation 56 .
where ΔA is the discrepancy between the absorbance with or without OAc − (ΔA = A − A 0 ), B is a floating parameter, C OAc − and C 2 are the concentrations of guest and host. As displayed in Fig. S4, the plot of ΔA versus C OAc − showed good non-linear relationship for OAc − , which indicates the formation of 1:1 complexation between 2 and OAc − . The stability constant K S was 2.3 × 10 4 M −1 (R = 0.991). Noteworthy, Job's plot at 254 nm also further demonstrated to form a 1:1 complexation for 2·OAc − (Inset of Fig. 6) [57][58][59] . In the recognition of OAc − using 2 as a host, the K SV value (1.2 × 10 4 M −1 ) from the fluorescence method and the K S value (2.3 × 10 4 M −1 ) from the ultraviolet method are consistent with each other 60 . Compared to the literatures, the binding constants of 2 to OAc − are in the middle of the values of literatures reported, and the detection limit of 2 to OAc − is close to the minimum in literatures reported (the binding constants and detection limits of these reports being in the range of 6.9 × 10 2 M −1 to 5.9 × 10 5 M −1 and 1.2 × 10 −7 mol/L to 1.0 × 10 −6 mol/L) 10,11,[19][20][21]23,24,26 .  Interactions of acetate anion with 2. Through analyzing the cavity size and structure characteristics of 2 (H(1)···H(2) separation being 3.72 Å in Fig. 2), the size of OAc − (the distance of two oxygen atoms in OAc − being 2.25 Å) 61 is suitable with that of 2. As shown in Fig. 7, Ha of host 2 is the most possible binding site for OAc − and the acting force between 2 and OAc − may be C-H···O hydrogen bonds. To gain related information about the binding mode of 2 and OAc − , 1 H NMR titration experiments was performed in DMSO-d 6 ( Fig. 8). As displayed in Fig. 8(iv), the chemical shifts of Ha shifted downfield by 0.10 ppm, and the chemical shifts of Hb-He shifted upfield by 0.02-0.03 ppm in the presence of 1 equiv. of OAc − . Additionally, the chemical shifts of Hf decreased by 0.03 ppm. These results indicated there existed C-H···O hydrogen bonds between 2 and OAc − . By comparison of Fig. 8(v,vi), chemical shifts of protons (Ha-Hf) did not have further change in the presence of more OAc − , which proved the formation of 1:1 complexation between 2 and OAc − . In HRMS of 2·OAc − (Fig. S14), m/z (747.29) of [2 − 2(PF 6 − ) + OAc − ] + was observed, which further confirmed that a 1:1 complexation between 2 and OAc − was formed. All these results accorded with the survey of the Job's plot experiment (Fig. 5). To further know the complexation property of 2 with OAc − , IR spectra of free 2, OAc − and 2·OAc − were measured (Fig. S16). The C-H flexural vibration bands moved from 841 cm −1 in free 2 to 837 cm −1 in 2·OAc − , and the C-H stretching vibration moved from 2959 cm −1 in free OAc − to 2962 cm −1 in 2·OAc − . The C=O bands of OAc − moved from 1677 cm −1 in OAc − to 1667 cm −1 in 2·OAc − . The C-N bands moved from 1587 cm −1 in free 2 to 1584 cm −1 in 2·OAc − .
Through the comprehensive analysis of 1 H NMR titrations, HRMS spectra, IR spectra and structure of 2, the binding force between 2 and OAc − is mainly attributed to C-H···O hydrogen bonds because of strong affinity of hydrogen atom toward oxygen atom. Upon the combination of 2 and OAc − , the fluorescence intensity of 2  The spectral differences in 1 H NMR titration experiments of 4 are depicted in Fig. 9. Ha' has a large shift (0.12 ppm) with the addition of 1 equiv. of NO 3 − as shown in Fig. 9(iv). At the same time, the signals of Hb' and Hc' shifted upfield by 0.02 ppm upon addition of 1 equiv. of NO 3 − . These changes should be attributed to the formation of C-H···O hydrogen bonds between NO 3 − and C(2)-H of benzimidazolium (Fig. 10). In addition, the chemical shifts of the other protons in 4 do not obviously change upon the addition of more equivalents of NO 3 − (Fig. 9(v)). This indicates that a 1:1 complex between 4 and NO 3 − was formed. In high resolution mass spectrometry (HRMS) analysis of 4·NO 3 − (Fig. S15), m/z (778.30) of [4 − 2(PF 6 − ) + NO 3 − ] + is observed, which provides additional evidence for the formation of a 1:1 complex between 4 and NO 3 − . These results are consistent with the result of the Job's plot experiment (Inset of Fig. S11).
To further understand the complexation behavior of 4 with NO 3 − , the infrared spectra (IR) of 4, NO 3 − and 4·NO 3 − were measured. In the infrared spectra (Fig. S17), we observed that N=O absorption bands of NO 3

Conclusion
In summary, two new 23-membered and 25-membered macrocyclic compounds 2 and 4 with bis-benzimidazolium groups have been synthesized and characterized. The anion recognition abilities of 2 or 4 have been investigated. The fluorescence and ultraviolet titrations show that 2 can detect effectively acetate anion. The K SV value (1.2 × 10 4 M −1 ) and K S value (2.3 × 10 4 M −1 ) for 2·OAc − are similar to each other. Even if the detection limit is down to 2.1 × 10 −7 mol/L, the detection of 2 to OAc − is sensitive. Analogously, 4 has special selectivity for nitrate anion. The K SV value (1.5 × 10 4 M −1 ) and K S value (2.5 × 10 4 M −1 ) for 4·NO 3 − are too similar to each other. Even if the detection limit is down to 2.6 × 10 −7 mol/L, the detection of 4 to NO 3 − is sensitive. The differences between 2 and 4 in the course of anion recognitions are mainly related to the structural characteristics of the macrocycles. The distance between two C(2)-H of benzimidazolium in 2 (3.72 Å) is larger than that in 4 (3.32 Å), therefore, 2 can effectively match with OAc − , and 4 can match effectively with NO 3 − (the distance between two oxygen atoms in OAc − (2.25 Å) being larger than in NO 3 − (2.10 Å)). The experiment results reveal that compounds 2 and 4 have good application prospects in anion recognitions. Our ongoing research is aiming at recognizing anions more efficiently, and the development of some environment-friendly, highly selective and highly sensitive chemosensors are underway.

Experimental
General procedures. Total commercially available chemicals for synthesis and test were of reagent grade. A Boetius Block apparatus was used for the report of melting points. A PerkinElmer Spectrum 100 FT-IR spectrophotometer was used for the report of Infrared (IR) spectra. A Varian spectrometers was used for the report of 1 H NMR and 13 C NMR spectra. The measurement of the elemental analyses was carried out on a Perkin-Elmer 2400C Elemental Analyzer. Ultraviolet spectra were recorded on a PerkinElmer Lamber35 UV spectrophotometer. The fluorescence spectra were carried out in a Shimadzu RF-5301PC fluorescence spectrophotometer. A VG ZAB-HS mass spectrometer was used to record EI mass spectra. 10 mL of HBr/acetic acid solution (31 wt%) was added to the glacial acetic acid (30 mL) solution of durene (6.710 g, 50.0 mmol) and paraformaldehyde (3.075 g, 102.5 mmol). The mixture was heated at 120 °C for 8 hours, and then this mixture was poured into 50 mL of H 2 O to precipitate a   2 mmol) were added to 100 mL of CH 3 CN, and this suspension was stirred under refluxing for 1 hour. To above mixture was dropwise added a CH 3 CN (50 mL) solution of 1,4-di(bromomethyl)-durene (2.880 g, 9.0 mmol), and the reaction was carried out at 80 °C for 72 hours. After CH 3 CN was removed via rotary evaporation, CH 2 Cl 2 (100 mL) was added to the residue, which was washed with water (3 × 100 mL). The CH 2 Cl 2 solution was dried over anhydrous MgSO 4 . After CH 2 Cl 2 was removed via rotary evaporation, a white powder of 1,4-di(benzimidazol-1-y l-methyl)-durene was gotten. Yield: 3.014 g (86%). Anal. Calcd for C 26   X-ray structure determinations. A Bruker Apex II CCD diffractometer were used for the collection of diffraction data of 2 and 4 62 . The structure was solved with the SHELXS program 63 . Figures 1 and 2 were formed via employing Crystal-Maker 64 . Other details for structural analysis and crystallographic data was listed in Table S1.