4-吡啶-NH-1,2,3-三唑构筑的两个Zn（Ⅱ）配合物的合成、结构和荧光性质

雷 雪 陈云舟 李元祥 贾丽慧 陈云峰

（武汉工程大学化学与环境工程学院，武汉 430073）

0 Introduction

Coordination polymers （CPs）,known as a type of organic-inorganic hybrid materials,are very promising for developing multifunctional luminescent materials[1-3].Both the inorganic and the organic moieties can provide the platforms to generate luminescence,while metal-ligand charge transfer also contributed to luminescence[4].The size and nature of metal ions,the orientation and arrangement of ligands,even the supramolecular interactions can affect the fluorescent properties of CPs[5-8].Therefore,controlling these interactionsbyexploring variousmetalsand organic ligands is crucialfor tuning the luminescence properties for a particular application.

In the recent years,a great deal of attention has been devoted to the synthesis of N-heterocyclic-based coordination polymers due to their surprising structural variability[9-12].With the development of the Cuガ catalyzed azide-alkyne cycloaddition （CuAAC）reaction[13-14],1,2,3-triazole which could exhibit abundant coordination modes has been appreciated as a metal coordination ligand[15-19].Since the extended π-systems of small florescence molecules facilitate the ligandcentered π→π*transition[20-23],we have intended to add aromatic groups to 1,2,3-triazole and incorporate this moiety into coordination polymers.Based on our previous work about the syntheses of 1,2,3-triazoles and related complexes[24-28],we chose the small molecular 4-pyridyl-NH-1,2,3-triazole as the ligand.On the other hand,zinc complexes havereceived much attention because they not only exhibit interesting structures but also show better luminescent properties[29-32].Herein,we report the syntheses,structures,and luminescent properties of two coordination complexes[Zn2（L）2Cl4]（1）and[Zn（L）2Cl2]·2H2O （2）.

1 Experimental

1.1 Material and measurements

All reagents were purchased and used without further purification.Thermogravimetric analysis （TGA）was performed on a NETZSCH TG 209 F3 thermogravimetric analyzer in flowing N2（20 mL·min-1）with a heating rate of 10℃·min-1.The FT-IR spectra were recorded from KBr pellets in the range of 4000～450 cm-1on an Agilent FT-IR spectrometer.The luminescence spectra for powdered samples and aqueous samples were recorded on Perkin Elmer Florescence Spectrometer LS S5 with a xenon arc lamp as the light source with the pass width of emission and excitation spectra of 15 and 2.5 nm.The UV-Vis spectra for samples （0.1 mmol·L-1in methanol）were recorded on Perkin Elmer UV WinLab 6.0.4.0738/Lambda35 1.27.Elemental analysis of carbon,nitrogen,hydrogen was performed using an Elementar Vario MICRO CUBE（Germany）analyzer.PXRD data for complexes 1 and 2 were collected in the range of 5°～50°for 2θ on crystalline samples using a XPERT-PRO diffractometer with Cu Kα radiation （λ=0.154 18 nm,40 kV,40 mA）in flat-plate geometry atroom temperature.The experimental powder X-ray diffraction pattern was compared to the calculated one from the single-crystal structure to identify the phase of the sample （Fig.S6～S7）.

1.2 Preparations of complexes 1 and 2

A mixture of ZnCl2·2H2O （0.15 mmol）,L （0.15 mmol）and acetonitrile （5 mL）was placed in a 20 mL glass bottle.The mixture was sealed and heated at 80℃ for 48 hours and cooled to room temperature naturally.Grey block crystals of 1 was obtained by filtration.IR （KBr,cm-1）：3 112 （s）,2 963 （m）,1 609（m）,1 139（m）,711（w）.Elemental analysis Calcd.for C14H12Cl4N8Zn2（%）：C 29.77,H 2.14,N 19.84;Found（%）：C 29.61,H 2.04,N 19.65.To a solution of 25.8 mg ZnCl2·2H2O （0.15 mmol）in 1 mL distilled water was added 22.1 mg L （0.15 mmol）in 4 mL acetonitrile.The mixture was left to evaporate in air until colorless block crystals of 2 were obtained after 3 days.IR（KBr,cm-1）：3 342（s）,3 042（m）,2 732（m）,2 362（m）,1 615（m）,1 445（m）,1 337（w）,1 243（m）,787（s）,714（w）cm-1.Elemental analysis Calcd.for C14H16O2Cl2N8Zn（%）：C 36.19,H 3.47,N 24.12;Found（%）：C 36.02,H 3.22,N 24.00.

1.3 Single-crystal X-ray crystallography

Single crystals of complexes 1 and 2 suitable for single-crystal X-ray diffraction were grown up from acetonitrile solution under hydrothermal reaction and slow evaporation at room temperature.The crystallographic data for the single crystals of 1 and 2 were collected on a CrysAlis PRO 1.171.39.7a（Rigaku OD,2015）employing graphite-monochromated Mo Kα radiation （λ=0.071 073 nm）.Empirical absorption correction using sphericalharmonics,implemented in SCALE3 ABSPACK scaling algorithm.All structures were solved by direct methods using the Olex2 program with the SHELXTL package and refined with SHELXL[33-34].Hydrogen atoms were added geometrically and refined with riding model position parameters and fixed isotropic thermal parameters.Crystallographic data for 1 and 2 are listed in Table 1,selected bond lengths and angles,and hydrogen bond lengths and angles are listed in Table S1 and S2.

Table 1 Crystallographic data of 1 and 2

CCDC：1554502,1;1554503,2.

1.4 Density functional theory calculations

The initial structures of complex 1 and complex 2 were obtained from the single-crystal data and the initial structure of free ligand L was obtained from that of complex 2 by deleting other atoms.The geometry optimization were performed at B3LYP/6-31G（d,p）[35-37]theory level in methanol solvent with PCM model by using Gaussian 16 software[38].At the optimized structures,harmonic vibrational frequencies（all real）were calculated to confirm that all optimized structures correspond to energy minima.Molecular orbitals cube files were generated and visualized with GaussView 6.0.16.

2 Results and discussion

2.1 Crystal structures

X-ray structural analyses reveal that the 1 and 2 crystallize in different space groups,P1 and P21/n,respectively.In the structure of 1,Zn2+exhibits five coordination （the degree of trigonality,τ=0.43）[39]with one terminal Cl,two μ2-Cl and two N atoms from a chelating L （Fig.1a）.There are three kinds of H-bonds within the molecular units.One is the strong intermolecular H-bond,N4-H4…Cl1 （H4…Cl1 0.229 1（55）nm,N4-H4…Cl1 162.55（558）°）,connecting the neighbor units to form dimer in ac plane.The second H-bond originates from C2-H2…Cl2,with dC2-H2=0.278 2（60）nm and ∠C2-H2…Cl2=169.00（572）°.The third is an intramolecular H-bonds,C7-H7…Cl1（H7…Cl1 0.273 5（48）nm,C7-H7…Cl1 160.98（370）°）（Fig.S1）.Besides the H-bonds,there is one offset π-π stacking interaction between two pyridine （C5N）rings with a centroid-to-centroid distance of 0.362 0（1）nm and an interplane separation of 0.333 4（1）nm （Fig.1b）.These intermolecular interactions are important for the crystallization of 1.The Zn1…Zn1idistance is 0.357 2（1）nm and the Zn1-Cl2-Zn1iangle is 93.880（39）°[40-41].

Fig.1 Structure of 1：（a）Coordination environment of Zn（Ⅱ）;（b）π-π interactions

In the structure of 2,Zn2+exhibits six coordinated with two terminal Cl,four N atoms from chelating L（Fig.2a）.Four intermolecular H-bonds were found.The first is C4-H4…Cl1,with dH4…Cl1=0.281 6（33）nm and∠C4-H4…Cl1=159.07（260）°.As a result,the adjacent mono-zinc（Ⅱ）blocks are assembled into H-bonded one dimensional chains along a direction with the shortest intrachain separation of 0.804 5（1）nm （Fig.S2a）.As for the bc plane,three kinds of intermolecular H-bonds connect the chains.Each H2O acts as a double H-bond donor leading to intermole-cular O1W-H1WA…Cl1 （H…Cl 0.232 2（39）nm,O-H…Cl 170.33（363）°）and O1W-H1WB…Cl1 （H…Cl 0.254 0（32）nm,O-H…Cl 164.82（33）°）.Moreover,the water molecular serve as an H-bond acceptor,which generate strong hydrogen bonding N4-H4A…O1W,with dH4A…O1W=0.189 7（31）nm and ∠N4-H4A…O1W=174.90（311）°（Fig.2b）.The intramolecular H-bond is C1-H1…N3 （dH1…N3=0.263 7（24）nm, ∠C1-H1…N3=149.28（195）°）（Fig.S2b）.

Fig.2 Structure of 2：（a）Coordination environment of Zn（Ⅱ）;（b）Various hydrogen bonds generated by H2O

2.2 Fluorescent properties

The phase purities of 1 and 2 have been confirmed by PXRD （Fig.S6 and S7）.Firstly,the photoluminescence spectra of the free ligand L and the two Zn（Ⅱ）coordination complexes were measured in methanol（10 μmol·L-1, λex=301 nm）.The free ligand L and the corresponding Zn coordination complexes exhibit similar emission spectra centered at λem=361 nm （Fig.3）.Compared with free L and Zn2+ion,the enhancementoffluorescence intensity came up in the complexes 1 and 2,and the mononuclear 2 is stronger than the dimer 1.It is probably due to the coordination of L to the Zn（Ⅱ）center,increasing the conformational rigidity of the ligand,thereby reducing the nonradiative decay of the intraligand （π-π*）excited state[42].

Fig.3 Emission spectra of the complexes and the ligand recorded at room temperature in methanol

Fig.4 Room-temperature solid-state emission spectra for L,1 and 2

Further,the solid-state photoluminescent properties of Zn（Ⅱ）coordination complexes 1 and 2 have been investigated together with free ligand L at room temperature （Fig.4）.Ligand L displays strong green photoluminescence with a maximum emission peak at 517 nm upon excitation at 260 nm,which could probably be attributed to the π*-π and π*-n transitions[43-45].For the complexes 1 and 2,excitation of the microcrystalline samples leads to the generation of strong blue fluorescent emissions with the same maximal peak occurring at 358 nm （λex=260 nm）.Obviously,this characteristic emission can probably be attributed to the intraligand charge transitions.As for the correlation ofstructure and fluorescent properties,the π-π packing in the L ligand and its coordination complexes have influence on the fluorescent properties[46].From the photoluminescence spectra of the free ligand L （Fig.S8）,the big change was observed in the methanol solution and solid state at room temperature,which is due to the enhancement of π-π packing in ligand under the condition of solid state.The observed blue-shift of the emission bands between the L ligand and the corresponding complexes 1～2 might tentatively originate from the weakened structure packing of ligand L because of the coordination with the metal ions.

2.3 Molecular orbitals

It can be concluded reasonably that the HOMO of ligand is π bonding orbital,and the HOMO of complex 2 is mainly based on coordinated chloride ions,while the HOMO of complex 1 is based on both coordinated chloride ions and the π-bonding orbitals of ligand.In contrast,the LUMOs of ligand and complexes 1 and 2 are mainly contributed by the πantibonding orbits of ligand （Fig.5）.As a consequence,the UV-Vis and luminescence spectra of complex 1 and 2 should be mainly assigned to charge transfer between chloride ions and ligand in nature[47].

Fig.5 Frontier molecular orbits of complexes 1 and 2 from HOMO to LUMO

3 Conclusions

In summary,two new pyridyl triazole complexes of Zn（Ⅱ）were synthesized and their photophysical properties were explored.Structurally,the free ligand L coordinated with Zn（Ⅱ）in different ways,resulting in the mono-and di-nuclear Zn2+complexes.With Cloccupying the axial positions,the complexes exhibit weaker π-π stacking compared with small-molecular ligand,which led to blue shift of the ligand in solid state.

Acknowledgements:The authors are grateful to Prof.ZHANG Yue-Xingand Prof.LIU Jun-Liangon density functional theory calculations.

Supporting information is available at http：//www.wjhxxb.cn

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