基于二羧基二苄胺和二咪唑基联苯的钴（Ⅱ）配位聚合物的合成、结构及对水中Fe3+的检测

徐田阳 赵亚丽 李家明 史忠丰＊,

(1北部湾大学石油与化工学院，钦州 535011)

(2广西师范大学化学与药学学院，桂林 541004)

(3北部湾大学国际教育与外国语学院，钦州 535011)

0 Introduction

As is known to all,Fe3+is a class of ample trivalent metal ion for all organisms and plays an significant role in environmental and biological systems due to its importance and function in various crucial processes,such as electron transfer in DNA and RNA formation and biological metabolisms[1-2].Both iron shortage or excess will give rise to various serious function condition disorders,such as agrypnia,skin diseases,decreased immunity,and iron deficiency anemia(IDA).Huntington′s,Alzheimer′s and Parkinson′s diseases also have been related to the abnormal distribution of the iron[3-4].Though Fe3+is very important for organisms,excess Fe3+will cause environmental pollution[5].So,how to effectively sense Fe3+ion is an extremely important issue for life system that needs to be given more attention[6-7].Nowadays,Fe3+ions are commonly analyzed using large-scale instruments such as atomic emission/absorption spectrometry,inductively coupled plasma-mass spectrometry(ICP-MS),X-ray dispersion,voltammetry,some of which are limited in their characterization.However,all of these ways have limitations,such as sophisticated instrumentation,complicated pretreatment procedures,being time-consuming,and easily interfered by other metal ions,which make them less efficient for fast,facile and exclusive determination of Fe3+ion in the daily life.Thus,it is very necessary to develop novel techniques that can be easily applied to exclusively detect Fe3+ion[8-9].

The coordination polymers(CPs)or metal-organic frameworks(MOFs)as a new type of crystalline materials has been aroused widespread concern in view of their extensive applications in catalysis[10-11],magnetism[12-13],gas storage[14-15],fluorescent sensor[16-24],and so on.Among them,luminescent MOF-based chemosensors have afforded great interest due to their particular aspects such as monitoring in real-time,quick response,high selectivity,and high sensitivity,so numerous CPs or MOFs have been synthesized as sensors for the detection of ions,explosives,and small molecules in water system.

The mixed-ligand MOFs derived from 4,4′-oxydibenzoic acid or 4,4′-iminodibenzoic acid and N-donor linkers have attracted intensive interest due to their ability to incorporate the virtues of different functional groups and to easily obtain controlled architecture by changing one of the ligands.Meanwhile,the organic aromatic dicarboxylic acid ligands,4,4′-oxydibenzoic acid or 4,4′-iminodibenzoic acid,have been extensively applied in the construction of MOFs due to their versatile coordination modes and high structural stability.Herein we present a new synthetic approach for a metal-organic coordination framework by employing the Co2+and a newly ligand H3bcba(4,4′-biscarboxyl-N,N-dibenzylamine)which has two carboxylate groups at the terminal position,and one aliphatic amine group between the benzoxy groups.These different coordination groups would react with equatorial and axial positions of paddle-wheel type dimer to extend an open framework.In addition,the N-containing auxiliary ligand bis(imidazole)has been proven to be good candidates for constructing multifunctional MOFs due to their length and flexibility.We have selected 1,1′-biphenyl-4,4′-diylbis(4-methyl-1H-imidazole)(bdmi)as the organic linker,which features three special characteristics:(i)as a relatively rigid ligand,bdmi has two coordination fashions,and the free rotation of the imidazolyl ring and benzene ring can improve the flexibility of the polymeric frameworks；(ii)the long size makes it a potential candidate to generate CPs of entangled topology；(iii)the good fluorescent characteristic of bdmi may endow the resulting products some interesting luminescent properties.

Based on the above consideration,to investigate the effect of the coordination modes of H3bcba and bdmi ancillary ligand on the structural assembly and diversity,we designed and obtained a new CP,{[Co(H3bcba)(bdmi)]·H2O}n(1)(Scheme 1),which has been characterized by single-crystal X-ray diffraction,IR spectroscopy,thermogravimetry,and elemental analysis.Its luminescence properties have been investigated.

1 Experimental

1.1 Materials and methods

Scheme 1 Synthesis of 1

All chemicals were commercially available and used as received without further purification.H3bcba was synthesized according to the reference[25].Powder X-ray diffraction(PXRD)data were collected on a Bruker D8 ADVANCE X-ray diffractometer with CuKαradiation(λ=0.154 18 nm)at generator voltage of 40 kV,generator current of 40 mA with a scanning range of 5°～50°.The simulation of PXRD pattern was carried out by the single-crystal data and diffraction-crystal module of the Mercury 2.0 program available free of charge via http://www.iucr.org.The purity and homogeneity of the bulk products were determined by comparing the simulated and experimental PXRD patterns.The elemental analyses(C,H,N)were performed on a Perkin-Elmer 240C apparatus.FT-IR spectra were recorded in a range of 4 000～450 cm-1on a PerkinElmer Frontier spectrometer.Thermogravimetric analyses(TG)were performed under nitrogen with a heating rate of 10℃·min-1using a PerkinElmer Thermogravimetric Analyzer TGA4000.Photoluminescence spectra were measured on a Varian Cary Eclipse fluorescence spectrophotometer with a xenon arc lamp as the light source.In the measurement of the emission spectrum and the excitation spectrum,the widths of the excitation slit and the emission slit were 5 and 10 nm,respectively.The crystal structure was determined by a Rigaku XtaLAB Mini(ROW)diffractometer.

1.2 Synthesis of{[Co(Hbcba)(bdmi)]·H2O}n(1)

A mixture of Co(NO3)2·6H2O(29.1 mg,0.1 mmol),H3bcba(28.5 mg,0.2 mmol),bdmi(31.5 mg,0.1 mmol),NaOH(8.0 mg,0.2 mmol),H2O(10 mL)and C2H5OH(3 mL)was placed in a Teflon-lined stainless steel vessel(25 mL),and then the vessel was sealed and heated at 145℃for 3 days.After gradually cooling to room temperature at a rate of 10℃·h-1,red blockshaped crystals of 1 were collected from filtration,washed with distilled water and dried in the air(Yield:45% based on Co).Elemental analysis Calcd.for C36H33N5O5Co(%):C,64.04；H,4.89；N,10.38.Found(%):C,64.01；H,4.92；N,10.35.IR(cm-1):3 409s,3 120s,1 608m,1 520m,1 353s,1 290m,1 135m,995m,767m,659m.

1.3 Photoluminescent sensing experiments

The luminescence properties of 1 were investigated in the solid state and various analytes at room temperature.For sensing of cations and anions,2.0 mg of a grounded powder samples of 1 was immersed in 2.0 mL aqueous solution of M(NO3)x(Mx+=Ag+,Al3+,Cd2+,Co2+,Cr3+,Cu2+,K+,Li+,Mn2+,Mg2+,Ni2+,Pb2+,Zn2+,Fe2+and Fe3+,1.0 mmol·L-1)or,1.0 mmol·L-1).Then the solid-liquid mixture was ultrasonicated for 30 min to form steady turbid suspension of 1@Mx+or 1@Xy-for the fluorescence measurements.The fluorescent intensities of these 1@Mx+or 1@Xysuspensions were immediately recorded at room temperature and compared.

1.4 Fluorescence titration experiments for Fe3+

The 2.0 mg grounded powder sample of 1 was dispersed in 2.0 mL H2O solution of target analyte Fe(NO3)3with different concentrations(0～0.26 mmol·L-1).Then the solid-liquid mixture was ultrasonicated for 30 min to form steady turbid suspension of 1@Fe3+for the fluorescence measurements.The fluorescent intensities of these 1@Fe3+suspensions were immediately recorded at room temperature and compared.

1.5 Fluorescence experiments at different pH values for 1

The aqueous solutions with pH value of 0 and 12 were configured respectively.A powder sample of 1(2.0 mg)was dispersed in these aqueous solution(2.0 mL),then the solid-liquid mixture was ultrasonicated for 30 min to form steady turbid suspension of 1,and the fluorescence emission spectrum of 1 was recorded.

1.6 X-ray crystallography

Single-crystal data collections were performed on a Rigaku XtaLAB Mini(ROW)diffractometer with graphite-monochromatized MoKαradiation(λ=0.071 073 nm)at 296(2)K.Using Olex2[26],the structure was solved with the SHELXTL[27]structure solution program using Intrinsic Phasing and refined with the SHELXL[28]refinement package using Least Squares minimization.All non-hydrogen atoms were refined with anisotropic displacement parameters.C,N and O-bound H atoms were placed in calculated positions(dC-H=0.093 nm for benzene and imidazole-CH,0.097 nm for methylene-CH2-,0.096 nm for methyl-CH3；dN-H=0.086 nm for-NH；dO-H=0.085 nm for H2O)and were included in the refinement in the riding model approximation,withUiso(H)set to 1.2Ueq(C or N)for benzene and imidazole-CH,methylene-CH2-and-NH,withUiso(H)set to 1.5Ueq(C or O)for methyl-CH3and H2O.The disordered atoms(N1,C8 and C20)were split in two parts and refined with an occupancy ratio of 0.63:0.37.DFIX and EADP instructions from ShelXL have been applied to constrain the disordered atoms associated NC,C-C distances and their anisotropic displacement parameters.Further details of the structure determinations are summarized in Table S1(Supporting information).Selected bond lengths and bond angles for 1 are listed in Table S2.Hydrogen bonds for 1 are listed in Table S3.

CCDC:1973191.

2 Results and discussion

2.1 Crystal structure of 1

Fig.1 (a)Coordination environment for Co2+in 1；(b)Wave-shaped 1D chain of H3bcba-Co-bdmi；(c)View of simplified 4-connected(4,4)topology of 1

The single-crystalX-ray diffraction structural analysis has revealed that 1 is a two-dimensional metalorganic framework,whose asymmetric unit is comprised of one Co2+,one partly deprotonated divalent anion Hbcba2-ligand,one bimb ligand,and one free H2O.As shown in Fig.1a,each crystallographic independent Co（Ⅱ）metal centers with a distorted octahedral(CoN2O4)geometry lies in the crystal structure(Fig.1a and Table S2).Co1 lies in a distorted octahedral coordination sphere,the equatorial plane of which consists of three carboxylate oxygen atoms(O1,O3iiand O4ii)(Symmetry code:ii-1/2-x,1/2+y,1/2-z)from two symmetry-related Hbcba2-ligands and one bdmi nitrogen atom(N2),another carboxylate oxygen atom(O2)and symmetryrelated bdmi nitrogen atom(N5i)(Symmetry code:i5/2-x,-1/2+y,1/2-z)in the axial coordination site.The distances of the Co-O bonds range from 0.203 5(4)to 0.247 5(4)nm,the lengths of the Co-N bond are 0.205 6(5)and 0.208 6(4)nm,both of which are in the normal range.It is worth mentioning that the Co1-O2 distance is 0.247 5(4)nm,suggesting a non-negligible interaction with the uncoordinated carboxylate oxygen atom which can be described as a semi-chelating coordination mode,implying the Co1 ion in a distorted octahedron environment.

The partly deprotonated Hbcba2-ligand connects with two Co（Ⅱ）ions,with two carboxylate groups acting as bidentate chelate fashion.The Hbcba2-ligands bend to coordinate Co（Ⅱ）ions to form a 1D framework with a Co…Co separation of 1.591(4)nm(Fig.1b).In addition,it should be noted that these independent sets of bdmi spacers,which establish a physical bridge between Co（Ⅱ） ions with a Co…Co separation of 1.731(4)nm.Both Hbcba2-and bdmi reside at an alternate horizontal and vertical of arrangement,resulting in the formation of an irregular parallelogram 2D structure with dimensions of 1.59 nm×1.73 nm alongc-axis,which are occupied by the distorted solvent water molecules(Fig.1c).In 1,theμ2-bridging bdmi ligands also adopt thecis-conformation,different from each other by the dihedral angle between the imidazolyl and phenyl rings.

From a topological point of view[29],each Co（Ⅱ）links two Hbcba2-ligands and two bdmi ligands as a four-connected node.The Hbcba2-or bdmi ligands bridging two metal ions serve as linkers.Thus,the 2D structure of 1 can be classified as a four-connected(4,4)grid layer topology for(Co)4(Hbcba2-)2(bdmi)2.

Intermolecular O-H…N,N-H…O,C-H…N and C-H…O hydrogen bonds are originated from the noncoordinated imino group-NH(N1,acceptor and donor),the solvate H2O(O5,donor and acceptor)and the carboxylate group(O1/O2/O3 and O4,acceptors).These hydrogen bonds alternate by translation along thecaxis and link the grid layers in reversely alternating parallel arrangements,supporting the supramolecular architecture.

Three C-H…πinteractions,i.e.C4-H4…Cg8ix(6-membered ring(Cg8)C21-C22-C23-C24-C25-C26,Symmetry code:ix1-x,1-y,1-z),C20-H20E…Cg5i(5-membered ring(Cg5)N4-C33-C34-N5-C35,Symmetry code:i5/2-x,-1/2+y,1/2-z),and C36-H36C…Cg6x(6-membered ring(Cg6)C2-C3-C4-C5-C6-C7,Symmetry code:x3/2-x,1/2+y,1/2-z)have distances of 0.294,0.243 and 0.281 nm,respectively,which interlink the adjacent chains.These interactions also exist between flanking benzene rings of Hbcba2-,bdmi and imidazole.Thus,the molecules are extended into an interwoven 3D supramolecular architecture through O-H…N,NH…O,C-H…N,C-H…O and C-H…πinteractions.

2.2 Ions sensing

As seen in the experimental section,we conducted a series of spectral measurements to explore the fluorescence response of 1 which interacted with 15 different cations and 18 different anions.Interestingly,the fluorescence emission of1 was almostentirely quenched in the Fe(NO3)3solution,while a moderate strength reduction were produced in 1@Al3+,1@Cr3+,1@S2O82-,1@Cr2O72-,1@CrO42-and 1@BF-suspensions,but a slight intensity changes were observed in other ions(Fig.2a,b).Due to the significant quenching phenomenon of 1 towards Fe3+,the quenching effect of Fe3+was subsequently examined as a function of Fe(NO3)3in a concentration range of 0～0.26 mmol·L-1.As shown in Fig.2c,when the Fe3+concentration was increased from 0 to 0.26 mmol·L-1,the fluorescence gradually decreased,and the quenching efficiency was nearly 93.8% when the concentration of Fe3+ions reached 0.26 mmol·L-1.

Fig.2 (a)Photoluminescence intensities of 1 in aqueous solution with various inorganic cations；(b)Photoluminescence intensities of 1 in aqueous solution with various inorganic anions；(c)Emission spectra and the Stern-Volmer plot(Inset)for 1 in aqueous solution of different Fe3+concentrations；(d)Fluorescence intensity of 1 in aqueous solution with the introduction of diverse other metal ions(red)and introduction of Fe（Ⅲ）(blue)

As is well known,the fluorescent quenching efficiency can be quantitatively accounted for the Stern-Volmer formula:I0/I=1+KsvcM(I0:initial fluorescent intensity of 1；I:fluorescence intensity after adding iron ion；cM:concentration of Fe3+ions；Ksv:Stern-Volmer constant(L·mol-1)).According to the Stern-Volmer plots,there was a good linear relationship in the range of Fe3+concentration from 0 to 0.26 mmol·L-1(Ksv=13 670 L·mol-1,R2=0.990 9).According to the equation 3σ/k(σ:standard error；k:slope),the detection limit of 1 detection Fe3+was calculated to be 2.17 μmol·L-1.It implies that 1 can selectively sense Fe3+ions[30-31].

Considering multiple cations in industrial wastewater,anti-interference experiments were subsequently performed to inspect the influence of mixed solutions containing Fe3+ions and various other cations on the luminescence.As shown in Fig.2d,initially,the fluorescence intensity of 1 showed a negligible change with the addition of other metal ions.However,the luminescence intensities rapidly decreased with the addition of Fe3+ions to the mixed solution of 1 and other metal ions.The decrease of the fluorescence intensities indicate that 1 can detect Fe3+ions even in the presence of other metal ions.In order to adapt to detection in more complicated environments,we further explored the fluorescence intensity of 1 with different pH values(Fig.3).As expected,1 exhibited satisfactory fluorescence intensity when the pH value ranged from 3 to 12,indicating that we could carry out fluorescence detection by 1 in a wide range of acid-base medium.Therefore,these results show that 1 has better anti-interference ability and can be applied to detect Fe3+ions in real en-vironment.

Fig.3 Emission spectra of 1 with different pH values

2.3 PXRD measurements

To verify the purity and stability of 1,we explored the PXRD experiments of 1 at room temperature.We added 1 to the Fe(NO3)3(1mmol·L-1)solution,soaked it for 12 hours,multiple rinsed and collected 1,and finally dried it at room temperature for 48 hours before performing a PXRD test.At the same time,after being immersed in the aqueous solution for 48 hours,the samples of 1 were collected and dried at room temperature forPXRD test.ThePXRD measurementresults showed that under the above two conditions,compared with the initial and simulated structures indexes,the structure of the sample after immersion remained basically unchanged(Fig.4).These results show that 1 has very high water-stability and no structural change has occurred after the detection of Fe3+,so it can be used to detect Fe3+in aqueous solutions.

Fig.4 PXRD patterns of as-synthesized 1 and simulated result as reference as well as 1 soaking in H2O and 1@Fe3+

2.4 Hirshfeld surface analysis of 1

Fig.5 (a)Hirshfeld surface mapped of dnormof 1；(b)Detail of two-dimensional fingerprint plot of 1

By using CrystalExplorer software[32]to calculate the Hirshfeld surface,dnormand 2D fingerprints were further drawn to clarify the molecular interactions and surface environment.There is an obvious correspondence between the two maps[33-34].As shown in Fig.5a,the red parts represent intense interactions that similar to hydrogen bond and coordination bond effects.In order to quantitatively study the effect of intermolecular interaction on the molecular surface,a two-dimensional fingerprint was analyzed[35].For compound 1,48.0% H…H interactions and 26.8% C…H/H…C interactions accounted for approximately half of the total.Many types of hydrogen bonds might be the main cause of O…H/H…O interactions,which value achieve 14.0%.The other hydrogen bonds of C-H…N possibly contained 3.5% N…H/H…N interactions.Besides,compound 1 had an C…C(π…π)interaction being only 1.5%,which was very significant in the formation of structure(Fig.5b).So,it is found that the Hirshfeld surface analysis results are well consistent with the crystal structure of 1.

3 Conclusions

In summary,a 4-connected(4,4)topology 2D Co（Ⅱ）-based luminescent CP(1)has been successfully obtained by solvothermal way.1 exhibits exceptional fluorescence and it can be used to detect Fe3+ions in aqueous solution with high selectivity and anti-interference.The quenching constant(KSV)value of 1 towards Fe3+was as high as 13 670 L·mol-1,and the limit of detection of Fe3+could reach 2.17 μmol·L-1.These results indicate that 1 can be applied as a potential luminescent sensing material for quantitative detection of Fe3+ion in biological and environmental areas.

Acknowledgements:The support of the Natural Science Foundation of Guangxi(Grant No.2018GXNSFAA281174)are gratefully acknowledged.The authors also acknowledge the financial supports from the Natural Science Foundation of Qinzhou University(Grant No.2016PY-GJ01)and the Opening Project of Guangxi Colleges and Universities Key Laboratory of Beibu Gulf Oil and Natural Gas Resource Effective Utilization(Grants No.2017KLOG11,2017KLOG14).

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