Synthesis, Crystal Structure and Optical Properties of 2-(3-(Hexyloxy)-5-Methylphenoxy)-N-(4-Nitrophenyl)acetamideforAnionDetection

: In this study, 2-(3-(hexyloxy)-5-methylphenoxy)-N-(4-nitrophenyl)acetamide (sensor L1) was synthesized and characterized by FT–IR, ESI–MS, 1 H and 13 C NMR spectroscopy, elemental analysis, and single crystal X-ray techniques. The crystal structure and space group of sensor L1 was monoclinic and P 2 1 , respectively. The crystal packing of sensor L1 was dominantly linked by two strong hydrogen bonds forming a six membered ring pattern. The binding properties of sensor L1 and various anions (F − , Cl − , Br − , CH 3 COO − , C 6 H 5 COO − , and H 2 PO 4 − ) were investigated by UV–Vis and 1 H NMR spectroscopy in DMSO. The proton resonance signals of sensor L1 and F − greatly changed positions when compared to those of anions. The solution color of sensor L1 changed from pale yellow to orange in the presence of F − . The UV–Vis results indicate that sensor L1 and F − ions underwent an internal charge transfer process. The stoichiometric complex was conﬁrmed by Job’s method, revealing a 1:1 formation for sensor L1 and ﬂuoride. Our results show that sensor L1 is highly selective for ﬂuoride ions over other anions.


Introduction
Currently, anion sensors have been the subject of considerable interest for their numerous roles in various processes in the fields of biology, catalysis, industry, and environmental protection. Anion sensors have also been used in devices such as television screens, as well as in monitoring and microelectronic sensors [1][2][3][4][5]. Researchers are interested in anion sensors with different dimensional structures, such as 2-and 3-dimensional patterns on carbon nanotubes and MOF structures [6,7]. For example, many binding units in the molecules amide, amine, thiourea, pyrrole, urea, hydroxyl, and boronic moieties are bound to anions through hydrogen bonding, anion π interaction, electrostatic force, cleavage bonds, and Lewis acid-base interactions [8][9][10][11][12][13][14][15][16]. Fluoride ions are important anions because they assist in the growth of bones and teeth. However, when a considerable number of fluoride ions remain in the human body, it can cause a variety of diseases such as bone, kidney, and gastrointestinal disorders [17,18]. The sensor-anion recognition can be observed in many processes such as internal charge transfers, twisted intramolecular charge transfers, and excited state intramolecular proton transfers [19][20][21]. 3,5-dihydroxytoluene is known as an orcinol compound. Orcinol derivatives have important roles in many applications, such as for pharmaceutical and medical functions [22][23][24][25][26][27]. Among orcinol crystal structures, there are many polymorphs, pseudopolymorphs, and co-crystals of orcinols with different co-formers such as urea, acridine, and 4-cyanopyridine. They exhibit three possible conformations (i.e., "anti-anti", "syn-anti", and "syn-syn") through the packing modes of hydrogen bonds [28]. The co-crystal of an orcinol with 1,2-di(4-pyridyl)ethane can be observed in the pattern of 1D zig-zag chains that are interconnected to form a 2D layer structure by C-H-π and π-π interactions. The other co-crystal of an orcinol with 1,2-di(4pyridyl)ethylene is formed by a 0D four component complex through hydrogen bonds [23]. The advantage of crystals of orcinol compounds and their derivatives is that they allow for the design of synthetic sensors for anion detection. Orcinolic sensors consist of substituent groups, such as -NO 2 withdrawing groups, that help binding sites provide more acidic protons of the NH group in the molecule and result in a strong binding interaction with anions. Among the related literatures, Singh and coworkers prepared two new receptors based on 4-nitrobenzylidenes with a highly selective binding ability to F − , AcO − , and H 2 PO 4 − [29]. Erdemir et al. synthesized two new colorimetric receptors bearing one or two nitrophenyl urea moieties that specifically bound to the F − ion, rather than other anions [30].

Materials
All of the synthesises were used without further purification. In the titration experiment, all anions were added in the form of tetrabutylammonium (TBA) salts, which were purchased from Sigma-Aldrich Co., St. Louis, USA. Column chromatography was performed using silica gel (70-230 mesh).

Apparatus
Elemental analysis (CHNS/O) was carried out by Thermo Scientific TM FLASH 2000, Cambridge, United Kingdom. MS was performed by Agilent 1100 Series LC/MSD Trap Karlsruhe, Germany. Elemental analysis informed the percent of elements in the product. Infrared spectra (4000-400 cm −1 ) were obtained by a Perkin Elmer system 2000 Fourier transform infrared spectrometer, Singapore. Infrared spectra express the functional groups of the molecules. 1 H and 13 C NMR spectra were obtained by a Bruker, AVANCE III HD 400 MHz NMR spectrometer, Fällanden, Switzerland. 1 H and 13 C NMR spectra exhibit the resonance signals of the types of protons and carbons. UV-Vis spectra were recorded on a Shimadzu spectrophotometer, Kyoto, Japan. UV-Vis spectra display the absorption bands relate to the type of transitions between HOMO and LUMO.

X-ray Crystallography
Crystal of sensor L1 was selected under microscope and mounted on a Bruker D8 QUEST CMOS PHOTON II, Bremen, Germany, with graphite monochromated Mo-Kα (λ = 0.71073 Å) radiation at 296 K. Data reduction was performed with SAINT and SAD-ABS [31]. The integrity of the symmetry was determined using PLATON [32]. The structure was solved using the ShelXT structure solution program with combined Patterson and dualspace recycling methods [33] and refined using least squares with ShelXL [34]. All non-H atoms were refined anisotropically. Mercury software was used to generate the graphic molecules [35].
Then, the reaction mixture was stirred and refluxed at 110 • C for 16 h. The reaction mixture was extracted with H 2 O and 20 mL CH 2 Cl 2 three times. The combined organic phase was dried with anhydrous Na 2 SO 4 . After the solvent was removed, the residue was purified by column chromatography over silica gel using dichloromethane:ethyl acetate (9:1) as an eluent to afford a yellow solid with a yield of 85%. mp. = 142-143 • C. 1

Crystal Structure and Structural Description
The light-yellow block-shaped crystals of sensor L1 were grown in the mixture of dichloromethane and hexane (1:1) solvents at room temperature. The crystal data and experimental refinement parameters of sensor L1 have been summarized in Table 1. Figure 1 exhibits the molecular structure of sensor L1 with atom labeling. The aromatic ring of 3,5-dihydroxyl toluene is combined with 4-nitrobenzene moiety by the bridging group of methylene amide with torsion angles O1-C8-C9-N1 of 17.77 • and O1-C8-C9-O3 of −163.61 • . In this conformation, the -NH group and oxygen atom from the previous hydroxyl group (O1) are on the same side, while the -CO group is on the opposite side. The dihedral angle between 3,5-dihydroxyl toluene and 4-nitrobenzene mean planes is 5.60 • , indicating that the aromatics are almost coplanar. Additionally, another hydroxyl group of 3,5-dihydroxyl toluene at the position of C3 reacted with hexyl bromide, producing alkyl ether with torsion angles C2-C3-O2-C16 of 2.09 • .  The alkyl chain (C16-C21) is in a zig-zag configuration with torsion angles in the range of 177.47-179.69 • , and the dihedral angle between the plane of the alkyl chain and the aromatic plane of 3,5-dihydroxyl toluene is 7.83 • . This indicates that the two functional groups on the toluene ring are almost coplanar.
The crystal packing is strongly supported by two hydrogen bonds between the -NH amide group, and the -CH in nitrobenzene with oxygen of the carbonyl group in amide of adjacent molecules (N-H···O, and C-H···O) with a six membered ring pattern as shown in Figure 2 and Table 2.  In addition, the crystal structure is also supported by a weak hydrogen bond between C-H of the methylene group and carbon atoms (C-H···C), which are close to the high electronegativity atoms, N and O ( Figure 2 and Table 2). The crystal packing through strong and weak hydrogen bonding interaction among the adjacent molecules in the direction of x, y + 1, z shows a one-dimensional array along the b-axis in Figure 3a. If a weaker hydrogen bond (C-H···C) with neighbors is added in the direction of −1 − x, −1/2 + y, −1 − z, the crystal structure will appear different to that in Figure 3b-d. The selected bond length and bond angle of sensor L1 have been provided in Table 3.

Interaction between Sensor L1 and Various Anions Using UV-Visible Spectroscopy
The anion binding abilities of sensor L1 were studied using UV-Vis titration with various anions such as F − , Cl − , Br − , CH 3 COO − , C 6 H 5 COO − , and H 2 PO 4 − (as tetrabutylammonium salts). The experiment was performed by preparing 5 × 10 −5 M of sensor L1 in a dimethyl sulfoxide solution. There were two absorption bands of sensor L1 that exhibited a shoulder band at approximately 284 nm and a strong band at 327 nm. Figure 5 shows the absorption spectra changes of sensor L1 in the presence of different fluoride ions (0-20 equiv.). With the addition of an increasing concentration of F − ions to the sensor L1 solution, the absorption bands at 284 and 327 nm gradually decreased. In addition, a new red shift absorption band at 446 nm was observed, corresponding to a new species of the deprotonated form in the solution. An isosbestic point at 378 nm indicated that the two formations of sensor L1 were normal and deprotonated species in the equilibrium [37,38]. These results indicate that the mechanism of sensor L1 and F − ions occurred under an internal charge transfer process [39]. The addition of other anions in the solution of sensor L1 caused a decrease only in the absorption bands at 284 and 327 nm because of weak interaction, as shown in Figure 6.

Interaction between Sensor L1 and Various Anions Using 1 H NMR Spectroscopy
The interaction behavior of sensor L1 (5 × 10 −3 M) with various anions such as F − , Cl − , Br − , CH 3 COO − , C 6 H 5 COO − and H 2 PO 4 − (tetrabutylammonium salt) was investigated using 1 H NMR experiments in DMSO-d 6 , as shown in Figure 9. The selected chemical shift assignments of sensor L1 have been listed in Table 4. Sensor L1 exhibited a sharp signal of the NH amide proton at 10.64 ppm. Upon addition of 4 equiv. F − to sensor L1, the NH amide proton disappeared when under the deprotonation process, as shown in Figure 9A. In addition, the signals of aromatic protons and CH 2 protons shifted upfield due to an enhancement of electron density around their region. The 1 H NMR spectrum of sensor L1 with an F − ion considerably changed when compared with that of other anions. In the case of CH 3 COO − , C 6 H 5 COO − , and H 2 PO 4 − , the NH proton of sensor L1 also disappeared. However, the aromatic protons and CH 2 protons hardly changed. In the presence of Cl − and Br − , the NH proton slightly shifted downfield due to weak hydrogen bonding interaction, and the aromatic protons and CH 2 protons remained in unchangeable positions ( Figure 9C,D). In addition to sensor L1 with a concentration of F − ions (0-4 equiv.), as shown in Figure 10, it was found that the NH signal disappeared upon the addition of F − ions at a 0.5 equivalent. It was assumed that the NH proton of the amide group disappeared from the molecule because of the strength basicity of F − ions. In addition, a new triplet peak was observed at 16.2 ppm, corresponding to the species of HF 2 − ions [38] upon the addition of 3-4 equivalents of F − ion, as shown in Figure 10E,F. The aromatic protons and methyl protons gradually moved upfield with the addition of F − ions from 0.5 equiv. to 4 equiv.

Determination of Association Constants (K a ) and Detection Limit of Sensor L1 with F − Ions
Under a 1:1 binding complexation between sensor L1 and F − ions, the association constant (K a ) was evaluated by the Benesi-Hildebrand equation as follows [40]: The association constant of sensor L1 and F − ions was determined to be 1.92 × 10 3 M −1 . In the other anions, this cannot be calculated since their size and shape might not be suitable for the binding site, and interactions between sensor L1 and other anions are also much weaker. From the association constants, sensor L1 showed the highest affinity binding to fluoride ions.
The detection limit between sensor L1 and F − was calculated using a method present in the literature [41]. From the equation, the maximum and minimum absorbances were described as an absorbance at 430 nm of sensor L1 with 20 equivalents F − and an absorbance at 430 nm of sensor L1, respectively. ( Figure 11) The detection limits of F − with sensor L1 was determined to be 0.77 mM.

Conclusions
In summary, sensor L1 was successfully designed and synthesized, including the product of crystals. The crystal structure revealed that the planes of the two functional groups (the alkyl chain and nitro benzene) were almost coplanar with the plane of toluene. The crystal packing was mainly supported by strong and weak hydrogen bonds, creating a one-dimensional array along the b-axis. From the 1 H NMR results, the new HF 2 − species of L1 was found at 16.2 ppm following the addition of fluoride ions. This result occurred via a deprotonation process. From UV-Vis titration in DMSO, it could be observed that sensor L1 displayed an isosbestic point at 378 nm and a new absorption band at 446 nm, leading to a deprotonated form. Sensor L1 has a stronger interaction with fluoride ions than with other anions. Therefore, sensor L1 can be used as a fluoride sensor.
Author Contributions: Conceptualization, methodology, funding acquisition and writing-original draft, B.W.; methodology, and writing, P.J.; writing, and methodology, T.D.; methodology, S.S. and K.C. All authors have read and agreed to the published version of the manuscript.