The first colorimetric receptor for the B4O72− anion based on nitro substituted phenanthroimidazole ferrocene derivatives

Four phenanthroimidazole ferrocene derivatives (2a–2d) were designed, synthesized and characterized by 1H NMR, 13C NMR and high-resolution mass spectroscopy (HRMS). Recognition of 12 anions by 2a–2d was investigated by UV-Vis absorption analysis, showing that 2b and 2d sensed B4O72− selectively among the tested anions with an obvious color change observed. 1H NMR titrations and theoretical calculations demonstrated that 2b binds B4O72− through O⋯H hydrogen bonding and O⋯B interactions between the nitro moiety and the B4O72− anion.


Introduction
Sodium borate (Na 2 B 4 O 7 $10H 2 O) is an important boroncontaining mineral and is used widely in the elds of glassmaking, metallurgy, detergents, cosmetics, pesticides and medicines. The borate anion, or tetraborate anion, is a bicyclic structure that is composed of two BO 4 and two BO 3 moieties. A receptor for sensing the borate anion is urgently needed because about ve hundred thousand tons of sodium borate is produced in China every year. The massive industrial production leads to the production of a large amount of waste water containing borate anions, and no receptor to detect this anion exists.
Imidazole is a ve-membered heterocyclic aryl system that coordinates easily with metal ions to form imidazole complexes. Different structural ferrocenyl imidazole derivatives can coordinate with cations, such as Ag(I), Cu(I), 1 Pd(II), Pt(II), 2 Mo(II), 3 Mn(II), Co(II), 4 Pb(II) 5 and Hg(II), 6 to form complexes, and can be used in the recognition of cations. Imidazole can also interact with anions by hydrogen bonding, electrostatic forces and other non-covalent interactions to function as a complexing unit of an anion sensor. Ferrocene is a good sensing unit because of its excellent electrochemical properties. In 2002, Tomas and coworkers reported a simple ferrocenyl imidazolium, in which ferrocene bonded to the nitrogen of imidazole by a methylene group could interact with Cl À , Br À , NO 3 À and HSO 4 À anions, 7 but with broad selectivity. In recent years, a series of acyclic and trinuclear ferrocene-based imidazolium receptors noticeably improved selectivity toward the F À anion. [8][9][10] Structural characterization of other ferrocenyl imidazole derivatives showed that specic recognition properties involve the ferrocene being linked directly with the C2 of imidazole, with the C4 and C5 fused to a conjugative system. One of the receptors discovered was 2-ferrocenyl-1H-anthra [1,2-d]imidazole-6,11(5aH,11aH)-dione, which was capable of detecting [CN À ] aq as low as $0.1 ppm. 11 The ferrocene-imidazopyrene dyad is a simple but effective dual redox and uorescent ion pair receptor that can detect Hg 2+ and H 2 PO 4 À ions simultaneously. 12 Another ion pair sensor is the bisferrocene-benzobisimidazole triad, which acts as a multichannel receptor for selective sensing of Hg 2+ and HSO 4 À ions. 13 We are interested in studying the synthesis of ferrocenyl imidazole sensors 14 and the application of these sensors as cation and anion receptors. 15,16 Ferrocene moiety in these receptors raised the selectivity. It was due to the electronic properties and rigid structure of ferrocene. 2-Ferrocenyl-1Hphenanthro-[9,10-d]imidazole (2a) is an electrochemical switching device. 17 When studying the UV-Vis spectral response of 2a to different ions, we found that the absorbance at 400 nm in the presence of borate and hydroxyl anions showed signicant increases, but no color change to the solution was observed. In this report, a strategy was designed to introduce a nitro group to ferrocenyl imidazophenanthrene and link an acetyl group to the cyclopentadienyl (Cp) to yield novel receptors (2b-2d) that display better sensing responses to the borate anion.

Synthesis of receptors
Methods for the syntheses of the receptors (2a-2d) are shown in Scheme 1. 2-Nitrophenanthraquinone (d) was prepared by the nitration of phenanthraquinone (c). 18 In the presence of NH 4 OAc, formylferrocene and 1 0 -acetyl-1-formylferrocene 2.2 X-ray diffraction single crystal structure of 2a The X-ray diffraction single crystal structure of 2a is shown in Fig. 1. The Fe-C bond length ranges between 2.043 and 2.059 A. The dihedral angle of the two Cps is 0.217 , and the dihedral angle between Cp and phenanthroimidazole is 5.055 . The structure shows that Cp and phenanthroimidazole are almost parallel and form a larger conjugative system.
The rigid structure of ferrocene and phenanthroimidazole provides a specic space conguration to facilitate selectivity toward anions.

UV-Vis spectrum
The anion recognition properties of the receptors 2a-2d toward F À , Cl À , Br À , were evaluated by UV-Vis spectroscopy. The solvent used in ion recognition manipulation was a mixture of DMSO and water (V DMSO : V H 2 O ¼ 4 : 1). The solutions of 2a-2d were prepared in DMSO (c ¼ 1.0 Â 10 À3 mol L À1 ). All the tested anions were dissolved in water (c ¼ 1.0 Â 10 À2 mol L À1 ). The receptor 2a was studied by UV-Vis spectroscopy. The absorbance at 400 nm increased dramatically when B 4 O 7 2À and OH À were added to 2a ( Fig. 2(a)), but both anions showed the same yellow color. The receptor 2c possesses one acetyl group more than 2a.
When the interaction of 2c with 12 anions was investigated, we observed that the maximum absorbance in the UV-Vis spectrum of 2c in the presence of B 4 O 7 2À was at 400 nm, whereas the absorbance maximum of the peak for 2c in the presence of OH À had blue shied ( Fig. 2(c)). No color change was observed. Addition of B 4 O 7 2À to 2b, which has one more nitro group than 2a, caused an immediate change in the color of the solution from light yellow to violet (Fig. 3). The change in the spectrum was caused by a bathochromic shi in the absorption maxima from 436 to 516 nm ( Fig. 2(b)) and a change in the molar extinction coefficient from 4265 to 3890 L (mol cm À1 ) À1 , when 10 equivalents in concentration of B 4 (Fig. 4). Correspondingly, the absorption maxima shied from 436 to 496 nm ( Fig. 2(d)) and the molar extinction coefficient from 5628 to 6694 L (mol cm À1 ) À1 , when 10 equiv. of B 4 O 7 2À was added.
For the tested anions, receptors 2b and 2d possessed sensing properties only toward B 4 O 7 2À . Thus, introduction of a nitro group to phenanthroimidazole resulted in a prominent color change, whereas the acetyl group showed no effect. Vis spectrum did not change either (Fig. 7(b)). This observation    , and K a is the binding constant.

Mechanism study
The mechanism of 2b binding to B 4 O 7 2À was determined by    According to the 1 H NMR titration experiments (Fig. 11) , as an electron-decient element bind readily with two oxygen atoms of the nitro. This event leads to all peaks in the spectra of 2b shiing upeld initially. The peaks of H 1 (9.4 ppm) and H 4  , which support the results of the theoretical calculations.

Detection of B 4 O 7 2À in lake water
To evaluate whether our colorimetric probe can detect B 4 O 7 2À anion in actual samples, we found some samples in the lakes of Yuanming Yuan (the Old Summer Palace) in Beijing. The insoluble substances were removed by ltration. Then 2b was    added in. We observed that sample 1 and 2 showed the same yellow color (Fig. 12(b) and (d)). UV-Vis spectroscopy was used to study the two samples and it showed that no obvious change was found (Fig. 13). It manifested that concentration of B 4 O 7 2À anion in sample 1 and 2 was below the limit of detection. When B 4 O 7 2À anion was added into sample 1 and 2, the color changed from yellow to violet immediately and the change in the spectrum was caused by a bathochromic shi in the absorption maxima from 436 to 516 nm ( Fig. 12(c, e) and 13(a, b) the blue curves). It means that the receptor could be used to determine the B 4 O 7 2À anion in actual samples.

Conclusions
The syntheses of new ferrocenyl phenanthroimidazole derivatives 2a-2d have been achieved using simple and short synthetic routines from ferrocene. The introduction of a nitro group to receptor 2a to afford 2d gave rise to a prominent color change, and 2b and 2d formed 1 : 1 complexes with the borate anion. Both 2b and 2d showed good selectivity toward the borate anion when testing the 12 anions, and selectivity was visibly detected by a color change of the reaction solution.

Synthesis of target compounds 2b-2d
A general method was used to synthesize 2b-2d, and we provide the method for synthesis of 2b. In a 100 mL three neck ask, 1formylferrocene (0.32 g, 1.5 mmol), 2-nitrophenanthraquinone (0.38 g, 1.5 mmol), ammonia acetate (0.58 g, 7.5 mmol) and 20 mL anhydrous methanol were mixed. The temperature was held at $65 C and the solution le overnight. The mixture was cooled to room temperature and poured into a 30 mL mixture of ice and water. A brown-red solid was precipitated when the pH value was adjusted to 8. Aer ltration and washing with dichloromethane, the brown-red product 2b was obtained.   This journal is © The Royal Society of Chemistry 2018