(College of Chemistry and Chemical Engineering,Jinzhong University,Jinzhong,Shanxi 030619,China)

Abstract:Two new metal-organic coordination polymers(MOCPs),namely,[Cd(HDCPN)(1,4-bib)0.5(H2O)2]n(1)and{[Zn(HDCPN)(1,2-bimb)]·H2O}n(2)have been synthesized based on H3DCPN and Cd（Ⅱ）/Zn（Ⅱ） under solvothermal condition(H3DCPN=6-(3,5-dicarboxylphenyl)nicotinic acid,1,4-bib=1,4-bis(1-imidazoly)benzene,1,2-bimb=1,2-bis(imidazol-1-ylmethyl)benzene)and characterized by X-ray single-crystal diffraction analysis,IR spectrum analysis,thermogravimetric analysis and powder X-ray diffraction analysis.The results show that complexes 1 and 2 are 1D chain structures,which are further extended into a 3D structure by π…π interactions between HDCPN ligand and imidazole ligands.In addition,solid fluorescence of 1 and 2 were detected.The fluorescence experiments demonstrate that 1 has highly selective recognition for Fe3+,CrO42-and Cr2O72-in aqueous solution.Meanwhile,the sensing mechanisms of 1 for Fe3+,CrO42-and Cr2O72-ions were also studied.CCDC:1532247,1;1999116,2.

Keywords:6-(3,5-dicarboxylphenyl)nicotinic acid;metal-organic coordination polymer;luminescent property

In recent years,the rapid detection of dangerous chemicals has attracted much attention among researchers,such as toxic organic small molecules,nitro-aromatic explosive and heavy metal ions,which pose serious threat to human health and the environment.As a new type of crystalline materials,metal-organic coor-dination polymers(MOCPs)have received extensive attention because of their potential applications in luminescence sensing[2],gas storage and separation[3],magnetism[4],and so on.Among them,fluorescent sensing presents great application prospect owing to its high selectivity,short response time,high sensitivity and operability[5].For example,some MOCPs show excellent luminescence sensing for heavy metal ions(Cu2+,Pb2+,Cr6+,etc.),anions and organic molecules[6-8].

It is generally known that Fe3+,CrO2-and CrO2-427 ions are very important ions.Fe3+plays important role in oxygen metabolism,oxygen absorption and electron transfer,and CrO42-and Cr2O72-ions have been widely used as oxidants.But an excess intake of Fe3+and Cr6+ions can cause cytotoxicity and carcinogenicity to the human body[9-10].Therefore,the design of fluorescent materials with selective and sensitive recognition of Fe3+,CrO42-and Cr2O72-ions is very imperative.Meanwhile,many fluorescence MOCPs are unstable in aqueous medium,so it remains a huge challenge to synthesize water-stable fluorescent MOCPs materials.

Scheme 1 Structures of the organic ligands

1 Experimental

1.1 Materials and physical measurements

All the chemicals used in the experiments were commercially available.The IR spectra were recorded on a FTIR-8400s spectrometer.A Vario MACRO cube elemental analyzer was used to measure the contents of C,H and N.The powder X ray diffraction patternswere confirmed on a Rigaku D/Max-2500 PC diffractometer(Mo Kα,0.071 073 nm)at 50 kV,30 mA with a 2θ range of 5°～50°.Thermogravimetric analysis was tested by using METTLER TGA analyzer.The luminescent spectra were collected on a F-4600 fluorescence spectrometer.

1.2 Syntheses of the complexes

A mixture of H3DCPN(0.007 5 mmol,2.2 mg),1,4-bib(0.007 5 mmol，1.6 mg),Cd(NO3)2·4H2O(0.015 mmol,4.6 mg)and H2O/Ethanol/DMA(2∶1∶1,V/V,1.0 mL)was added into a glass tube and vacuumized,sealed and heated to 80℃for 96 h,and then cooled slowly to room temperature.Colorless block crystals were collected(Yield:41%,based on Cd).Anal.Calcd.for C20H16CdN3O8(%):C,44.55;H,2.97;N,7.80.Found(%):C,44.51;H,2.32;N,7.75.IR(KBr pellet,cm-1):3 463(m),1 707(m),1 628(m),1 593(s),1 521(m),1 454(m),1 382(m),1 160(m),839(w),789(m),734(m),684(w),470(w)(Supporting information,Fig.S1).

The synthetic process of complex 2 is similar to that of 1 except for using 1,2-bimb(0.02 mmol,4.8 mg)instead of 1,4 bib.And the colorless crystals were obtained.Yield:48%.Anal.Calcd.for C28H23ZnN5O7(%):C,55.45;H,3.79;N,11.53.Found(%):C,55.31;H,3.83;N,11.51.IR(KBr,cm-1):3 439(m),3 119(m),2 482(w),1 777(s),1 593(s),1 540(s),1 364(s),1 349(s),1 096(s),935(m),828(m),767(vs),621(s)(Fig.S1).

1.3 Single crystal X-ray diffraction

Single-crystal diffraction data of 1 and 2 were collected on a Bruker ApexⅡCCD diffractometer(Mo Kα,0.071 073 nm)at 296 and 293 K,respectively.The structures were solved by direct methods and refined using the full matrix least squares method based on F2by the SHELXL-2015[12].Non-hydrogen atoms were refined anisotropically.Hydrogen atoms from carbon atoms were generated by geometrical considerations.The solvent molecules in 1 and 2 are highly disordered and were removed by the SQUEEZE program of PLATON[13].The primary crystallographic data,selected bond lengths,bond angles and fractional atomic coordinates are given in Table 1,S1 and S2.

CCDC:1532247,1;1999116,2.

Table 1 Crystallographic data of complexes 1 and 2

2 Results and discussion

2.1 Descriptions of crystal structures

2.1.1Crystal structure of 1

Complex 1 crystallizes in the triclinic system,with P1 space group.There are one Cd（Ⅱ）ion,one partly deprotonated H3DCPN ligand,half of a 1,4-bib molecule and two coordinated water molecules in the asymmetric unit of 1(Fig.1).The Cd1（Ⅱ）ion is seven coordinated by six O-atoms(Cd1-O1 0.270 00(25)nm,Cd1-O2 0.226 19(23)nm,Cd1-O5A 0.253 17(18)nm,Cd1 O6A 0.231 14(21)nm,Cd1-O1W 0.231 19(20)nm,Cd1 O2W 0.246 48(18)nm)from two different H3DCPN ligands and two coordinated water molecules,one nitrogen atom from a 1,4 bib molecule(Cd1 N2 0.222 36(30)nm),presenting a distorted pentagonal bipyramid geometry.The angles of O-Cd-O and O-Cd-N are from 50.978(77)°to 176.415(70)°.

The carboxylate groups of H3DCPN in 1 are partially deprotonated and adopt chelating(η2)coordination modes(Scheme 2a).The two carboxylate groups of HDCPN2-ligand and 1,4-bib linker connect the adjacent Cd（Ⅱ）ions to form 1D chains(Cd…Cd 1.405 61(17)nm)(Fig.2a and 2b),which were further extended into a 3D structure by π…πinteractions(Cg…Cg:0.367 13(35)and 0.365 05(28)nm,Fig.2c and 2d).

As he was inspecting it with some surprise, the fish opened its mouth and said: Listen to me, fisher; if you will just throw me back into the water I ll turn your poor little cottage into a splendid castle

Fig.1 Coordination environment of Cd（Ⅱ）in 1

Scheme 2 Coordination configurations of the H3DCPN ligands

Fig.2 (a,b)One-dimensional chain structure of complex 1;(c)3D framework of 1 viewed along c axis;(d)π…π interaction between molecules of complex 1

2.1.2Crystal structure of 2

Complex 2 crystallizes in the monoclinic system space group P21/n.The asymmetric unit consists of one independent Zn（Ⅱ）ion,one H3DCPN ligand,a 1,2-bimb linker,and a lattice water(Fig.3).The Zn1（Ⅱ）ion presents a distorted{ZnN2O4}octahedral geometry,and is bridged by four O atoms(Zn1-O1 0.221 79(23)nm,Zn1-O2A 0.197 72(21)nm,Zn1-O3B 0.207 32(24)nm,Zn1-O4B 0.284 32(22)nm)from three distinct HDCPN2-ligands,two N atoms(Zn1-N2 0.197 41(27)nm,Zn1 N5A 0.198 24(27)nm)from two different 1,2-bimb linkers.The angles of O-Zn-N and N-Zn-N are from 81.603(91)°to 176.984(118)°.

The carboxylate groups of H3DCPN in 2 are partially deprotonated,and adopt chelating(η2)and bridging(μ2-η1∶η1)coordination modes(Scheme 2b).Four carboxylate groups from four HDCPN2-ligands connect with two Zn（Ⅱ）ions to construct a binuclear[Zn2(COO)4]cluster(Fig.4a)with the distance of Zn…Zn being 0.386 20(9)nm,which is extended by HDCPN2-ligandand 1,2-bimb linker to generate a 1D chain(Fig.4b),and is further expanded to a 3D framework through hydrogen bonds(Table S3)and π…π interactions(Cg…Cg:0.349 43(180)nm)between HDCPN2-ligand and 1,2-bimb linker(Fig.4c).

Fig.3 Coordination environment of Zn（Ⅱ）in 2

Fig.4 (a)[Zn2(COO)4]cluster of 2;(b)1D chain structure of complex 2;(c)3D structure of 2;(d)π…π interaction between molecules of 2

2.2 IR analysis,thermalanalysisand X ray powder diffraction analysis

The IR measurements of 1 and 2 were carried out in a wavenumber range of 4 000～400 cm-1.The absorption peaks at 1 628 and 1 382 cm-1for 1 and 1 593 and 1 364 cm-1for 2 are attributed to asymmetrical and symmetricalstretchingvibrationsof COOHfrom HDCPN2-ligand,respectively.The presence of strong peak at 1 650～1 710 cm-1shows that the H3DCPN ligand is partially deprotonated[14](Fig.S1).The thermal stability curves of 1 and 2 are displayed in Fig.S2.For 1,the weight loss of 6.3%(Calcd.6.7%)below 100℃corresponds to the release of two coordinated water molecules,and the structure began to collapse above264℃.2 presented the first weight loss of 2.5%(Calcd.2.9%)under 248℃,which is attributed to the release of a lattice water.The structure began to decompose quickly at about 270℃.Furthermore,X-ray powder diffraction analyses of 1 and 2 indicate that the experimental diffraction patterns are basically consistent with simulated ones,indicating the good phase purity(Fig.S3).

2.3 Luminescent properties

The solid state fluorescence emission spectra of H3DCPN,1 and 2 were investigated at room temperature(Fig.5).The maximum emission peaks of H3DCPN,1 and 2 are at 420 nm(λex=280 nm),392 nm(λex=280 nm),387 nm(λex=280 nm),respectively.Compared to H3DCPN ligand,the obvious blue-shifts of the emission peaks of 1 and 2 may be assigned to the increase of the conformational rigidity of H3DCPN ligand,and the decrease of non-radioactive decay due to the coordination between Cd（Ⅱ）/Zn（Ⅱ）ions and H3DCPN[15].

Because complex 1 remains stable in aqueous solution and shows luminescent properties in the solid state at ambient temperature,the fluorescence sensing properties of 1 to different metal cation and anion were measured in aqueous solution.Ground samples of 1(3 mg)were immersed in 3 mL M(NO3)naqueous solutions(0.01 mol·L-1,M=Cd2+,Zn2+,Ba2+,Co2+,K+,Al3+,Cu2+,Pb2+,Ni2+,Na+,Ag+and Fe3+).As displayed in Fig.6,the luminescence intensity of Mn+@1 suspensions mainly depends on different cations.It is worth mentioning that Fe3+shows distinct quenching effects with the quenching rates being more than 98%.Meanwhile,the anti-interference experiment was also detected in the presence of other ions(Fig.S4).The results revealed that the fluorescence intensity of 1 was changed slightlyin the absence of Fe3+,and the fluorescence showed obviously quenched behavior after adding Fe3+into the above solutions.

Fig.5 Solid-state fluorescence emissions for H3DCPN ligand,1 and 2 at room temperature

Fig.6 Photoluminescence intensities of 1 dispersed in M(NO3)naqueous solutions

Theluminescentsensingsensitizationswere further examined by changing Fe3+concentration,titration experiments revealed that the luminescent intensities of 1 were gradually decreased as the concentration of Fe3+increased.When the concentration of Fe3+reached 0.80 mmol·L-1,the luminescence intensity of 1 was obviously quenched.The nonlinear curve fitting between the concentrations of Fe3+and luminescence intensity(I0/I)can be followed the exponential equations:I0/I=0.177 1exp(cM/0.161 7)+1.023 4,where I0is the fluorescence intensity of 1@H2O and I is the luminescent intensity of 1@Fe3+suspensions,cMis the concentration of Fe3+(mmol·L-1)(Fig.7).For Fe3+,in the low concentration range,the plot of I0/I vs cMis nearly linear and can be fitted by the Stern-Volmer(SV)equation I0/I=1+KSVcM.The fluorescence quenching constant(KSV)was calculated to be 4.74×104L·mol-1.The detection limit(3σ/KSV)of 1 for Fe3+was calculated to be 1.32×10-4mol·L-1(σ is the standard deviations for five repetitive fluorescence experiment of blank solutions),which is similar to the reported values of other luminescent materials in the literature[16-17].And the low detection limit demonstrates that 1 shows highly sensitive detection toward Fe3+.

Fig.7 Luminescent emission spectra of 1 dispersed in aqueous solution upon incremental addition of Fe3+

To further investigate the luminescence sensing of 1 for different anions,3 mg samples were dispersed in 3 mL KmX aqueous solution(0.01 mol·L-1,X=Cl-,HCO-,I-,SO2-,Br-,SCN-,HPO-,CO2-,HPO2-,342434 CrO42-,and Cr2O72-).As shown in Fig.8,the luminescence of Cr2O72-and CrO42-anions for 1@H2O suspension was almost completely quenched,and the quenching rate was as high as 98.2%and 97.1%,respectively.

Fig.8 Photoluminescence intensities of complex 1 dispersed in the solutions containing different anions

As shown in Fig.9 and 10,when the concentrations of Cr2O72-and CrO42-were 0.6 and 0.7 mmol·L-1,respectively,the fluorescence intensity of 1 was almost completely quenched.The curves of the luminescence intensity and the concentrations of CrO2-and CrO2-274were fitted by the exponential equations:I0/I=0.178 6exp(cCr2O27-/0.148 1)-0.654 1 and I0/I=2.919 1exp(cCrO24-/0.356 0)-1.768 3,respectively(Fig.9 and 10,Inset).

Fig.9 Luminescent emission spectra of 1 dispersed in aqueous solution upon incremental addition of CrO2-27

Fig.10 Luminescent emission spectra of 1 dispersed in aqueous solution upon incremental addition of CrO2-4

At low concentrations,the SV plots for Cr2O72-and CrO42-were approximately linear and the KSVvalues of Cr2O72-and CrO42-were calculated to be 2.194×103and 1.096×103L·mol-1.The detection limit for Cr2O72-and CrO42-could reach 1.98×10-3and 4.38×10-3mol·L-1.And the structure of 1 immersed in aqueous solutions containing Fe3+,Cr2O72-and CrO42-ions is stable,which can be confirmed by the PXRD patterns of the samples before and after fluorescence experiments(Fig.S5).Moreover,three cycles of the Fe3+/Cr2O72-/CrO42-sensing experiments indicated that 1 showed good cyclability(Fig.S6～S8).These results indicate that 1 is expected to be used as a potential fluorescent material for detecting Cr2O72-and CrO42-in aqueous system[18-19].

The quenching mechanism of Fe3+/CrO2-/CrO2-274 for 1 can be ascribed to the competitive adsorption between the framework of 1 and metal ions,which can be confirmed by the UV-Vis absorption spectra of 1,Fe3+,Cr2O72-and CrO42-,since there were partial overlaps between the excitation spectra of 1 and the absorption spectra of Fe3+/Cr2O72-/CrO42-(Fig.S9).Besides,the electron-transfer and weak interaction between the analyte and Fe3+/Cr2O72-/CrO42-ions also play a part role[20-22].

3 Conclusions

In summary,two fluorescent MOCPs have been prepared based on 6 (3,5 dicarboxylphenyl)nicotinic acid ligand and Cd（Ⅱ）/Zn（Ⅱ）in the presence of imidazole ligands,and their structures are one dimensional chain.Complexes 1 and 2 present good stability.In particular,1 shows sensitive and selective sensing for Fe3+,Cr2O72-and CrO42-in aqueous solutions,which means that it can be employed as a chemical sensor indetecting Fe3+,Cr2O72-and CrO42-.

Acknowledgments:The authors gratefully acknowledge the financial support of this work by the fund for Natural Science Foundation for Young Scientists of Shanxi Province(Grant No.201901D211443),the fund for Shanxi“1331 Project”Key Innovative Research Team(Grant No.PY201817),the Jinzhong University“1331 Project”Key Innovation Team(Grant No.jzxycxtd2019005)and Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi(STIP)(No.2020L0590).

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