4-Tert-butyl-N?-((2-Hydroxynaphthalen-1-yl)methylene) Benzohydrazide: Synthesis, Crystal Structure and Recognition Properties①

LIU Tian-Bao PENG Yan-Fen GUI Mei-Fang CHANG Jie WANG Xin

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4-Tert-butyl-N?-((2-Hydroxynaphthalen-1-yl)methylene) Benzohydrazide: Synthesis, Crystal Structure and Recognition Properties①

LIU Tian-Bao②PENG Yan-Fen②GUI Mei-Fang CHANG Jie WANG Xin

(247000)

A new naphthol-based compound 1, C22H22N2O2, has been designed and synthesi- zed.The structure of the title compound 1 was confirmed by IR,1H NMR,13C NMR, H RMS, and X-ray single-crystal diffraction.The crystal belongs to the monoclinic system, space group21/, with= 12.888(9),= 15.543(10),= 9.119(6) ?,= 94.05(3)°,= 1822(2) ?3,= 4,D= 1.263 g/cm3,M= 346.41,= 0.081 mm-1,(000) = 736.0, the final= 0.0452 and= 0.1142 for 3404 observed reflections with (> 2()).The crystal structure of 1 is stabilized by O–H···N, N–H···O, C–H···O hydrogen bonds and-interactions.The spectroscopic studies of 1 toward various metal ions were also investigated in 25% (V/V) ethanol aqueous solution, and the result showed that it can selectively recognize Zn2+with fluorescence enhancement.

naphthol, crystal structure, Zn2+, recognition property;

1 INTRODUCTION

Zinc is the second most abundant essential trace element in the human body, and is indispensable to living organism in a minute quantity[1-3].Impor- tantly, excess or deficiency Zn2+will lead to the physiological disorders and health issues such as skin disease, diabetes, epilepsy, ischemic stroke and Alzheimer’s disease[4, 5].Thus, the detection and quantification of Zn2+have become a hot issue in chemical analysis.Given that zinc does not produce spectroscopic or magnetic signals because of its 31040electronic configuration[6], the fluorescence method remains the primary method of the choice for Zn2+determination[7].In recent years, a number of fluorescent probes for Zn2+recognition have been developed and reported in literature[8-13].However, the development of sophisticated and applicable fluorescent probes for Zn2+is still a challenge.Naphthol and its derivatives are often used as a fluorophore for designing fluorescence probes to detect metal ions due to their good photo stabilities[14-16].In this study, 4-tert-butyl-N?-((2- hydroxynaphthalen-1-yl)methylene)benzohydrazide 1 was prepared by the reaction of 4-tert-butyl- benzhydrazide with 2-hydroxy-1-naphthaldehyde as the starting materials (Scheme 1), and its crystal structure and spectral properties were also investigated.

Scheme 1. Synthetic route for the fluorescent sensor 1

2 EXPERIMENTAL

2.1 Materials and Instruments

The melting points were determined on XT-4 microscopic melting point apparatus with a digital thermometer and uncorrected.IR spectra were recorded on a Nicolet FTIR iS10 infrared spectro- meter as KBr pellets with absorption reported in cm-1.1H NMR and13C NMR spectra were acquired on a Bruker 400 MHz FT-NMR spectrophotometer in DMSO-6using TMS as an internal reference.HRMS were obtained with an AgilentTechnologies 6540 UHD Accurate-Mass Q-TOF LC/M.Fluore- scence spectra measurements were performed on a Shimadzu RF-5301PC spectrofluorophotometer.4-tert-Butylbenzhydrazide was prepared according to literature procedures[17].All other reagents were purchased from commercial suppliers and used without further purification.

2.2 Preparation of 4-tert-butyl-N?-((2-hydroxynaphthalen-1-yl)me-thylene)benzohydrazide 1

2-Hydroxy-1-naphthaldehyde (0.18 g, 1.05 mmol) and 4-tert-butylbenzhydrazide (0.20 g, 1.05 mmol) were dissolved in absolute ethanol (15 mL) at room temperature.Then the mixture was refluxed for 5 h.After the solvent was removed under reduced pressure, the crude solid was recrystallized from ethanol (95%) to obtain a yellow solid at 81% yield (0.29 g), m.p.: 268～270 °C.IR(cm-1)max: 3450(OH, m), 3210(NH, m), 2927(CH3, m), 1633(C=O, s), 1544(s), 1469(s), 1475(s), 1319(s), 1200(s), 954(m), 819(s), 744(s), 662(w).1H NMR(400M, DMSO-6)(ppm): 12.85(s, 1H, NH), 12.16(s, 1H, OH), 9.50(s, 1H, =C–H), 8.21(d, 1H,= 8.00 Hz, Ar.C–H), 7.88～7.94(m, 4H, Ar.C–H), 7.57～7.63(m, 3H, Ar.C–H), 7.40(t, 1H,= 8.00 Hz, Ar.C–H), 7.24(d, 1H,= 8.00 Hz, Ar.C–H), 1.31(s, 9H, 3CH3).13C NMR(100M, DMSO–6)(ppm): 162.9, 158.5, 155.5, 147.0, 133.1, 132.1, 130.3, 129.5, 128.3, 128.2, 127.9, 125.9, 124.0, 121.0, 119.4, 109.0, 35.2, 31.4.HRMS(ESI): calcd.for C22H23N2O2(M+H+) 347.1759.Found: 347.1752.

2.3 X-ray crystallography of CDT

Suitable single crystals of 1 for X-ray structural analysis were obtained by slow evaporation of a solution of compound 1 in CH3CH2OH (95%) at room temperature.A crystal with dimensions of 0.50mm × 0.50mm × 0.30mm was obtained.The unit cell determination and data collection were performed with Moradiation (= 0.71073 ?) on a Bruker D8 VENTURE diffractometer equipped with ascan mode at 120(2) K in the range of 2.594≤≤27.542°.A total of 3404 reflections were collected with 4174 independent ones (int= 0.0633), of which 27476 observed reflections with> 2() were used in the succeeding refinements with –15≤≤16, –20≤≤20 and –11≤≤11.

The crystal structures were solved by direct methods with SHELXS-2013 program and refine- ments on2were performed with SHELXL-2013 program by full-matrix least-squares techniques[18].Anisotropic thermal parameters were applied to all non-hydrogen atoms.All H atoms were initially located in a difference Fourier map.All H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms.A full- matrix least-squares gave the final= 0.0452 and= 0.1142 (= 1/[2(F2) + (0.0487)2+ 0.7190], where= (F2+ 2F2)/3), (Δ/)max= 0.000 and=1.076.The final Fourier map had no hole deeper than –0.25 e·?-3and peak greater than 0.30 e·?-3.

3 RESULTS AND DISCUSSION

3.1 Crystal structure of compound 1

The molecular structure of probe 1 with atomic numbering is shown in Fig.1.The compound crys- tallizes in monoclinic space group21/.The selected bond lengths and bond angles are listed in Table 1.The bond length of O(2)–C(12) is shorter than that of O(1)–C(1), because O(2) and C(12) of the carbonyl group are both2hybridized.The N(1)–C(11) bond (1.2863(18) ?) is also found to be shorter than the N(2)–C(12) bond (1.3515(18) ?) for the similar reason.In the crystal structure, the naphthalene ring (plane Ⅳ) was constituted by two benzene rings: plane I made up of C(1)/C(2)/ C(3)/ C(4)/C(9)/C(10) atoms and plane II consisting of C(4)/C(5)/C(6)/C(7)/C(8)/C(9) atoms.Plane III con- tained six atoms C(13)/C(14)/C(15)/C(16)/ C(17)/ C(18).The dihedral angle between planes III and Ⅳ is 5.995(12)°.Therefore, the two planes are almost flat.The torsion angles of O(1)–C(1)–C(10)–C(11), C(1)–C(10)–C(11)–N(1), N(2)–N(1)–C(11)–C(10), C(11)–N(1)–N(2)–C(12) and N(1)–N(2)–C(12)–O(2) are –3.87(19), 10.10(18), –177.36(10), –169.53(12) and 0.43(19)°, respectively.

Fig.1. Molecular structure of probe 1

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

The independent infinite chains were connected by hydrogen bonds andinteractions, which stabili- zed the crystal structure (Fig.2).The centeriod-to- centeriod, perpendicular distances and dihedral angle of the naphthalene ring C(1)–C(10) and the adjacent naphthalene rings are 4.3412 ?, 3.4523 ? and 0.000°.The intramolecular hydrogen bond O(1)–H(1)···N(1) is expected to stabilize the conformation of the molecular conformation (Table 2).The crystal packing in probe 1 is stabilized by weak nonclassical N(2)–H(2)···O(2) and C(11)–H(11)···O(2) contacts (N(2)···O(2) = 2.851(2) ?, C(11)···O(2) = 3.181(2) ?), which resulted in the formation of inversion- relatedcrystal (Fig.2a).Then, 1D chains are expan- ded into a 2D network structure bystacking interactions (Fig.2b).

Fig.2. a) 1D chain of 1.b) 2D network of 1

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°) of the Title Compound

Symmetry code: (a), 1/2–, 1/2+

3.2 Recognition properties of probe 1

The fluorescence spectra of probe 1 were investigated in EtOH/H2O (v/v = 4:1).A 2.0 × 10-5mol/L stock solution was prepared by dissolving appropriate amount of probe 1 in EtOH/H2O (v/v = 4:1).Stock solutions (1.0 × 10-5mol/L) of Ag+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Hg2+, K+, Mg2+, Na+, Pb2+, Mn2+, Fe3+, Ni2+, Cu2+and Zn2+were prepared by dissolving their nitrate or chloride salts in deionized water.The excitation wavelength is 429 nm, the optical grating is 3 nm, and the pH has no changes.To get insight into fluorescence intensity changes with an increase of Zn2+concentration, the fluore- scence spectra changes of probe 1 towards Zn2+were evaluated.As shown in Fig.3, upon the addition of Zn2+to the solution of probe 1, the emission intensity at 486 nm clearly increased.Moreover, there was a good linear relationship between the fluorescence intensity of probe and the concentration of Zn2+in the range of 0 to 10 μmol·L-1(inset in Fig.3).According to the reported definition (S/N = 3)[19], the detection limit of probe 1 for Zn2+was found to be 2.7 × 10-7mol/L.These results indicated that probe 1 was highly sensitive to the Zn2+ions.The binding constantsof 1-Zn2+were calculated by the Benesi-Hildebrand equation[20].Depending on the slope, the result obtained was1= 2.3 × 10-5L/mol, signifying that probe 1 had a great binding affinity to Zn2+.In order to further study the binding of probe with Zn2+, Job’s method[21]was employed by using the emission changes at 486 nm as a function of molar fraction of Zn2+.A maximum emission was observed when the molar fraction of Zn2+reached 0.5 (Fig.4), indicating that Zn2+ions form a 1:1 complex with the sensing compound.

To further understand the fluorescent properties of probe 1 with Zn2+, the fluorescence spectrum of probe 1 in the presence of 16 light and heavy important biological metal ions (Ag+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Hg2+, K+, Mg2+, Na+, Pb2+, Mn2+, Fe3+, Ni2+, Cu2+and Zn2+) is presented in Fig.5.As expected, there were no obvious changes in the fluorescence intensity after adding other metal ions to probe 1 solution, including Ag+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Hg2+, K+, Na+, Pb2+, Mn2+, Fe3+, Ni2+, Mg2+and Cu2+.Interestingly, probe 1 exhibited selectivity only for Zn2+and no fluorescent re- sponse to other metal ions.Competition experiments of Zn2+ions mixed with other metal ions were carried out from fluorescence spectra and the results are shown in Fig.6.The presence of metal ions, such as Ag+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Hg2+, K+, Na+, Pb2+, Mn2+and Mg2+, did not interfere with the enhanced fluorescence.However, the presence of Fe3+would seriously weaken the enhanced fluorescence and Cu2+and Ni2+totally quenched the fluorescence, possibly because the complex formed between these metals and the probes was too stable to be replaced by Zn2+.These results suggested thatprobe 1 had good selectivity toward Zn2+.

Fig.3. Fluorescence changes of probe 1 (20 μmol·L-1) with different concentrations of Zn2+(0～45 μmol·L-1) in EtOH/H2O (v/v = 4:1) with an excitation at 486 nm. Inset: the linear relationship of probe 1 towards Zn2+(below 10 μmol·L-1)

Fig.4. Job’s plot of probe 1 and Zn2+([1] + [Zn2+] = 20 μmol·L-1)

Fig.5. Fluorescence response of probe 1 (20 μmol/L) to different metal ions (20 μmol/L)

Fig.6. Fluorescence intensity changes of 1 (20 μmol/L) upon the addition of various metal ions in and without the presence of Zn2+(20 μmol/L) (1, Zn2+; 2, Ag+; 3, Al3+; 4, Ba2+; 5, Ca2+; 6, Cd2+; 7, Co2+; 8, Hg2+; 9, K+; 10, Mg2+; 11, Na+; 12, Pb2+; 13, Mn2+; 14, Fe3+; 15, Cu2+; 16, Ni2+)

According to the above results and some reports[8, 22], a possible mechanism of compound 1 with Zn2+is shown in Scheme 2.The weak fluorescence of probe 1 is due to the free rotation around the N–N bond.In contrast, complexation with Zn2+inhibits the rotation of N–N bond and produces a chelation-enhanced fluorescence (CHEF) effect.

Scheme 2. Proposed sensing mechanism of probe 1 for Zn2+

4 CONCLUSION

4-Tert-butyl-N?-((2-hydroxynaphthalen-1-yl)meth-ylene)benzohydrazide 1 has been designed and syn- thesized.The single crystals were grown by slow evaporation at room temperature.The molecular structure and crystal packing are stabilized by hydrogen bonds andstacking to generate a 2D network.The preliminary fluorescence showed the molecule may be a good probe for Zn2+.At the mean time, compound 1 is possible to act as a probe for Zn2+in practice applications as it exhibits certain water solubility.

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2 January 2018;

23 April 2018 (CCDC 1814099)

① This work was supported by the National Natural Science Foundation of China (No.21271035), the Natural Science Foundation of Anhui Province (No.KJ2016A512), Key projects of Anhui Province University Outstanding Youth Talent Support Program (No.gxyqZD2016372) and the Natural Science Foundation of Chizhou University (No.2017ZRZ002)

Liu Tian-Bao, E-mail: tianbaoliu1979@126.com; Peng Yan-Fen, E-mail: pengyanfen1978@126.com

10.14102/j.cnki.0254-5861.2011-1941