一个二重互穿的镉配合物：合成、结构和双功能荧光传感性质

刘国成 于蕙瑄 高 越 唐 爽 冯 聪

（渤海大学化学系，锦州 121000）

0 Introduction

In recent years,coordination polymers have attracted attention due to their wide applications in gas adsorption[1-2],dye degradation[3-5],separation and enrichment[6-8],electrochemical[9-11]and fluorescence sensing[12-14].There are many factors that affect the properties and structures of coordination polymers,of which organic ligands have a strong influence on the complexes[15].At present,someamide ligands have been reported,and bispyridyl bisamide ligands are very important ones,but V-type bispyridyl bisamide ligands containing supramolecular hydrogen bond sites （-OH）were rarely reported[16].In addition,the aromatic carboxylic acid as a co-ligand also has a certain influence on the structure and properties of the complexes[17-18].Therefore,it is meaningful to study the construction of complex based on V-type bispyridyl bisamide and polycarboxylate mixed ligands.

On the other hand,some coordination polymers based on d10metal ions show fluorescent sensing property[19].However,many of them show single sensing object[20].In 2016,Li′s group used 1,3-di（4-pyridyl）-propane and 1,4-bis（1,2,4-triazol-l-yl）butane ligands to successfully synthesize two compounds,both of which have good fluorescence sensing properties for Fe3+ions;In 2017,our group used N,N′-bis（4-methylenepyridin-4-yl）-1,4-benzenedicarboxamide to synthesize a series of Cd-complexes which have fluorescence sensing propertiesfor methanol[21-22].Therefore,the investigation on the synthesis of sensing materials with multifunctional detection is very meaningful.

In this work,we use a V-type bispyridyl bisamide,N,N′-bis（4-pyridin-3-yl）-5-hydroxyisophthalamide （L）,as the neutral ligand to combine with 1,4-benzenedicarboxylic acid （1,4-H2BDC）under hydrothermal condition,a 2-fold parallel-interpenetrated coordination polymer of{[Cd（BDC）（L）]·1.5H2O}n（1）was obtained.The title complex shows fluorescence sensing properties for Fe3+and dichloromethane,which is expected to become a new fluorescent probe material.

1 Experimental

1.1 General procedures

All chemicals purchased were of reagent grade and used without further purification.Ligand L was synthesized by the method of the literature[23].FT-IR spectra were taken on a Magna FT-IR 560 spectrometer （500～4 000 cm-1）with KBr pellet.Fluorescence spectra were performed on an F-4500 fluorescence/phosphorescence spectrophotometer at room temperature.

1.2 Synthesis of{[Cd(BDC）(L）]·1.5H 2O}n

A mixture of CdCl2（0.1 mmol）,L （0.1 mmol）,H2BDC （0.1 mmol）,NaOH （0.2 mmol）,H2O （10 mL）was sealed to a Teflon-lined stainless steel autoclave（25 mL）and kept at 120 ℃ for 4 days.After the mixture was slowly cooled to room temperature,colorless block crystals of 1 suitable for X-ray diffraction were obtained in 34%yield （based on Cd）.Anal.Calcd.for C26H21CdN4O8.5（%）:C,48.96;H,3.32;N,8.78.Found（%）:C,49.43;H,3.14;N,8.80.IR （KBr,cm-1）:3 291w,3 178w,3 097w,1 676m,1 557s,1 488s,1 375s,1 287 m,1 219m,843s,805w,743m,693m.

1.3 X-ray crystallography

The data were collected on a Bruker Smart ApexⅡ CCD diffractometer with Mo Kα （λ=0.071 073 nm）at 296 K（-21≤h≤28,-21≤k≤17,-22≤l≤22）in the range of 1.800°～28.204°by using an ω-2θ scan mode.A total of 15 753 reflections were collected,of which 6 171 were independent （Rint=0.018 8）and 5 241 reflections were used in the succeeding refinement.The structure was solved by the direct method with SHELXS-2014 and refined by the Full-matrix least-squares on F2using the SHELXL-2014[24-25].The hydrogen atoms of the organic ligands were placed in calculated positions （C-H 0.093 nm）and treated as riding atoms,and all the non-hydrogen atoms were refined anisotropically.One of H atom in O1W is disordered in two sites,H1B and H1C with the occupancy factor of 0.5.O2W is half-occupied.The crystal data and structure refinement details for 1 are listed in Table 1.Selected bond lengths and angles are given in Table 2.

CCDC:1847700.

Table 1 Crystal data and structure refinement for the title complex

Continued Table 1

Table 2 Selected bond lengths(nm）and angles(°）for the title complex

2 Results and discussion

2.1 Description of the structure

Complex 1 belongs to the C2/c space group and the asymmetric unit contains one Cdギcation,one L,one 1,4-BDC anion,one and a half of lattice water molecules.Each Cdギcenter is six-coordinated and possesses a distorted octahedral coordination geometry surrounded by two pyridyl N atoms belonging to two L ligands with Cd-N bond lengths of 0.225 36（17）and 0.227 58（18）nm,four O atoms from two BDC anions,respectively （Fig.1）.The bond lengths of Cd-O are in the range of 0.222 55（14）～0.262 66（16）nm.

Fig.1 Coordination environment of the Cdギion in 1 with ellipsoid probability of 50%

Table 3 Hydrogen-bonding geometry for the title complex

In 1,each 1,4-BDC anion connects the adjacent Cdギto form a wave-like [Cd-BDC]nchain with the Cd…Cd distance of 1.123 9 nm （Fig.2a）.A pair ofμ2-L coordinates two Cdギions to generate [Cd-L]2loop with the Cd…Cd distance of 1.41 nm,which links the above[Cd-BDC]nchains forming a two-dimensional 63connected layer （Fig.2b and 2c）.Finally,the parallel 2D layers are extended into a 3D polycatenation array（Fig.3）.The outstanding point is the catenation fashion in such a way that each six-membered ring interlocks two six-membered rings from two adjacent parallel wave-like layer nets up and down and vice versa（Fig.3）.In addition,the polycatenation array is stabilized by the hydrogen bonding interactions between hydroxyl,carboxylic,amino groups and lattice water molecules.The corresponding hydrogen bonding parameters of the complex are listed in Table 3.

Fig.2 View of 1D[Cd-BDC]n chain （a）,2D layer （b）and simplified 2D structure （c）of 1

Fig.3 Details of interpenetrated structure （a）,stacking structure （b）and 2-fold-interpenetrated structure （c）of 1

2.2 PXRD and IR spectrum

In order to examine the phase purity of the bulk material of 1,powder X-ray diffraction （PXRD）experiment was performed at room temperature.As shown in Fig.4,the as-synthesized sample and simulated patterns are in good agreement with each other,proving the consistency of synthesized bulk material and the measured single crystal[26].The slight difference in peak intensity may be due to the different orientation of the crystals in the powder sample.The IR spectrum of the title complex is shown in Fig.5.The absorption peak at 3 178 cm-1should be attributed to the characteristic absorption of-OH groups of water molecules[27].The characteristic peaks at 1 557 and 1 488 cm-1indicate the ν（C-N）stretching vibrations on the pyridine rings of bipyridine ligand.The characteristic peaks of 1 219 to 843 cm-1are the characteristic absorption peaks of benzene rings of organic ligands[28].

Fig.4 PXRD patterns of complex 1

Fig.5 FT-IR spectrum of the complex 1

2.3 Thermogravimetric and photoluminescent properties of 1

The TG curve of complex 1 is shown in Fig.6.The weight loss of 1 happened in two steps.The first weight loss was observed in the range of 50～172 ℃with a weight loss of approximately 4.03%,demonstrating the removal of lattice water molecules（Calcd.4.23%）.The second weight loss （53.28%）occurred in the range of 142～531 ℃,indicating the collapse of the organic skeleton （Calcd.52.43%）.The solid state fluorescence of the complex was studied at room temperature （Slit:2.5 nm,Voltage:700 V）.As shown in Fig.7,at the excitation wavelength of 310 nm,the title complex has a maximum emission peak at 375 nm.Compared with the luminescence of L,the maximum emission peak of the complex is red-shifted of 5 nm.It was considered that there is no obvious contribution for the polycarboxylate to the luminescence emission of the complex in the presence of the N-containing ligand.Therefore,the luminescence of the title complex can be attributed to ligand-metal charge transfer[29].

Fig.6 TGA curve of complex 1

Fig.7 Solid-state luminescence of complex 1

2.4 Fluorescent sensing property

The fluorescent responses of 1 for organic solvents and metal ions were performed at same experimental conditions （Slit:2.5 nm,Voltage:700）according to the reported similar methods[30-34].To explore the fluorescent response of 1 to various organic solvents,5 mg of 1 was ball milled and dispersed in various organic solvents （3 mL）including methanol,ethanol,ethylene glycol,n-propanol,isopropanol（IPA）,butanol,dichloromethane （DCM）,cyclohexane,tetrahydrofuran （THF）,acetonitrile,N,N′-dimethylformamide （DMF）,1,4-dioxane.Compared to the other organic solvents,the fluorescence intensity of the suspension of 1 has been strongly enhanced by DCM in the fluorescence response （Fig.8）.This effect may be attributed to the energy transfer from the ligand to the methylene chloride molecule after excitation and the interaction between the complex skeleton and the organic small molecule[30].To investigate the relationship between fluorescence intensity and DCM concentration,we gradually added DCM in 3 mL of ethanol,adding 50μL each time.As shown in Fig.9,as the concentration of DCM increases,the fluorescence intensity of the suspension of 1 also increases.In addition,we also explored the fluorescence response of 1 to different metal ions.As seen in Fig.10,theas-synthesized sampleof 1 （2 mg）wasimmersed in different aqueous solutions （3 mL）of containing 0.01 mol·L-1of metal ions （Pb2+,Cd2+,Ag+,Zn2+,Co2+,Cu2+,Fe2+and Fe3+）to form stable suspension.The luminescence intensity of 1 suspension showed an excellent quenching effect with addition of Fe3+.The fluorescence quenching effect may be attributed to the electron transfer of the ligand to the metal,i.e.,the interaction between the Lewis basic site of the ligand and the Fe3+[35].In addition,with the gradual addition of Fe3+from 0 to 1 700 μL （100 μL each time,1 mmol·L-1）,the fluorescence intensity of the suspension of 1 gradually decreases （Fig.11）.

Fig.8 Emission spectra of 1 in different organic solvents under excitation at 310 nm

Fig.9 Emission spectra of DCM@1 in ethanol with gradual addition of DCM under excitation at 310 nm

Fig.10 Emission spectra of Mn@1 in aqueous solutions containingmetal ionsunder excitation at 310 nm

Fig.11 Emission spectra of Fe3+@1 in H2O with gradual addition of Fe3+under excitation at 310 nm

3 Conclusions

In summary,a new 2-fold parallel-interpenetrated Cdギcoordination polymer was hydrothermally synthesized.The obvious change of 1 in the luminescence intensities caused by Fe3+and dichloromethane relative to other metal ions and organic solvents implies the possible application of 1 for recognizing and sensing Fe3+ions and dichloromethane,demonstrating its potential applications in the functional material fields.