Coordination Behavior of Bis-Imidazole and Various Carboxylate Ligands towards Zn ( II ) and Cd ( II ) Ions : Synthesis , Structure , and Photoluminescence Study

Four coordination polymers (CPs) based on bis-imidazole ligands (1,2-bimb and 1,2-bmimb), namely, {[Zn(1,2-bimb)(2,5-dtpa)] H2O}n (1), {[Cd2(1,2-bimb)2(5-hipa)2] 2H2O} (2), {Zn2(1,2-bimb)(L)(CH3COO) DMF·2H2O}n (3) and {Cd(1,2-bmimb)(3-npa)}n (4), have been synthesized by solvothermal reactions (1,2-bimb = 1,2-bis((1H-imidazol-1-yl)methyl)benzene, 1,2-bmimb = 1,2-bis((2-methyl-1H-imidazol-1-yl)methyl)benzene, 2,5-H2dtpa = 2,5-diaminoterephthalic acid, 5-H2hipa = 5-hydroxyisophthalic acid, H3L= 3,3′,3′′-(2,4,6-trioxo-1,3,5-triazinane-1,3,5-triyl)tripropanoic acid, 3-H2npa = 3-nitrophthalic acid) and structurally verified by single-crystal X-ray diffraction analyses and further characterized by powder X-ray diffraction (PXRD), elemental analyses and infrared spectroscopy (IR). Complex 1 and 2 show a dinuclear 2D layered structure. Complex 4 exhibits a two-dimensional network consisting of [Cd(3-npa)]n and [Cd(1,2-bmimb)]n chains. Both 1,2 and 4 display a 4-connected sql topology sheet, which can be further expanded into a 3D supramolecular network through π···π interaction between layers. Complex 3 features a 3D (3,6)-connected {42·6}·{44·610·8}-3,6T24 topology structure consisting of 2D bilayers. Structural comparison reveals that it is not only the substituents at different positions of ancillary ligands and the primary bis(imidazole) linkers that play crucial roles in the control of the final structures. Besides, the photoluminescence properties of 1–4 have been investigated in the solid state at room temperature.


Introduction
The design and synthesis of new coordination polymers (CPs) have attracted enormous attention in recent years, not only for their intriguing structures and diverse topologies, but also for their potential applications in gas adsorption and separation, catalysis, magnetism, luminescent sensor and so on [1][2][3][4][5][6].Among them, the luminescent properties of coordination polymers are widely studied.As we all know, the organic ligands often contain aromatic rings or conjugated groups that are subject to excitation, giving rise to photoluminescence (PL) upon irradiation.Moreover, the metal center also contributes to optical emission, such as lanthanides and inorganic cluster.Utilizing their emission properties, various types of sensors are being developed.Luminescent sensors for detecting cations, anions, small molecules, explosives and temperature have been applied to various fields [7,8].Many researchers attempted to control the self-assembly process of CPs and expected to synthesize precise structures with specific properties.However, it is difficult to design and predict a new CP structure because of various factors, such as solvents, pH values, temperature, template, the structural characteristics of organic ligands, the nature of the metal ions, the counterions and other various experimental conditions [9][10][11][12][13].Among these various factors, the selection of suitable organic ligands is more effective and is a prerequisite for controlling the synthesis of novel coordination materials [14].
An effective way to carry out the reaction is to use mixed ligands such as carboxylic acid ligands and N-containing ligands in the same system [15].More and more CPs from 1D to 3D have already been synthesized by using polyimidazole ligands as spacers, such as 1,3,5-tris(1-imidazolyl)-benzene, 1,4-bis(imidazolyl)benzene in combination with various polycarboxylate ligands as linkers [16].Furthermore, we have been striving to synthesize systematically mixed-ligand CPs by adjusting the carboxylic acid chain with different substituents and the flexible/rigid properties of N-containing ligands, and to explain the topological relationships of the CPs [17,18].Previous studies showed that carboxylate auxiliary ligands can provide cooperative coordination together with the N-containing ligands to meet the requirements of the coordination geometries of the metal ions in the assembly process.Although many bis-imidazol ligands and different poly-acids have been widely used to synthesize new coordination polymers [19][20][21][22][23][24][25], there are relatively rare in exploring the effect of the introduction of substituents and different carboxylic acid positions on the structure.
Herein, four mixed-ligand CPs have been successfully synthesized (Scheme 1), namely {[Zn ( emission properties, various types of sensors are being developed.Luminescent sensors for detecting cations, anions, small molecules, explosives and temperature have been applied to various fields [7,8].Many researchers attempted to control the self-assembly process of CPs and expected to synthesize precise structures with specific properties.However, it is difficult to design and predict a new CP structure because of various factors, such as solvents, pH values, temperature, template, the structural characteristics of organic ligands, the nature of the metal ions, the counterions and other various experimental conditions [9][10][11][12][13].Among these various factors, the selection of suitable organic ligands is more effective and is a prerequisite for controlling the synthesis of novel coordination materials [14]. An effective way to carry out the reaction is to use mixed ligands such as carboxylic acid ligands and N-containing ligands in the same system [15].More and more CPs from 1D to 3D have already been synthesized by using polyimidazole ligands as spacers, such as 1,3,5-tris(1-imidazolyl)-benzene, 1,4-bis(imidazolyl)benzene in combination with various polycarboxylate ligands as linkers [16].Furthermore, we have been striving to synthesize systematically mixed-ligand CPs by adjusting the carboxylic acid chain with different substituents and the flexible/rigid properties of N-containing ligands, and to explain the topological relationships of the CPs [17,18].Previous studies showed that carboxylate auxiliary ligands can provide cooperative coordination together with the N-containing ligands to meet the requirements of the coordination geometries of the metal ions in the assembly process.Although many bis-imidazol ligands and different poly-acids have been widely used to synthesize new coordination polymers [19][20][21][22][23][24][25], there are relatively rare in exploring the effect of the introduction of substituents and different carboxylic acid positions on the structure.

Materials and Physical Measurements
The ligands 1,2-bimb and 1,2-bmimb were prepared according to reported methods [26,27].The auxiliary ligands and other materials were purchased from commercial sources and without further purification.The FT-IR spectra were measured on a NEXUS 670 FTIR spectrometer (Thermo Nicolet Corporation, Madison, WI, USA) in the range of 600-4000 cm −1 using KBr pellets.Elemental analyses (EA) were carried out on a CE instruments EA 1110 elemental analyzer (Carlo-Erba Corporation, Sandwich, Italy).TGA were carried out on an SDT Q600 instrument (TA Instruments, New Castle, DE, USA) at a heating rate of 5 • C under a N 2 atmosphere.The powder X-ray diffraction (PXRD) spectra of complexes 1-4 were measured on a Rigaku D/Max-2500 diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu-Kα radiation.Fluorescence spectra were obtained by a Hitachi F-4500 fluorescence spectrophotometer (Hitachi Limited, Tokyo, Japan) at room temperature.

X-ray Crystallography
Diffraction intensity date was carried out on a Siemens SMART diffractometer equipped with a CCD area detector using Mo-Kα monochromatic radiation (λ = 0.71073 Å Siemens Limited, Berlin, Germany).The absorption correction was based on multiple and symmetry-equivalent reflections in the date set using the program SADABS (Version 2.03) [28].The structures were solved by direct methods with SHELXS-97 (Version 6.10) and refined with the full-matrix least-squares technique using the SHELXS-97 program [29].All non-hydrogen atoms were refined anisotropically [30].The hydrogen atoms attached to carbon and oxygen atoms were included in the structure factor calculations.Crystallographic date for complexes 1-4 are summarized in Table 1 and selected bond lengths and angles are listed in Tables S1-S4.CCDC (1839112 for 1, 1839113 for 2, 1839114 for 3, 1840894 for 4) contains the supplementary crystallographic for this paper.There data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336033; E-mail: deposit@ccdc.cam.ac.uk).

Results and Discussion
Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the triclinic with space group P1.The asymmetric unit consists of one crystallographically independent Zn II ion, one 1,2-bimb ligand, one 2,5-dtpa anion and one lattice water molecule.As shown in Figure 1a, each Zn II ion located in a distorted [ZnO 5 N 2 ] decahedron geometry, surrounded by five oxygen atoms of three 2,5-dtpa anions, two nitrogen atoms from two 1,2-bimb ligands.The bond lengths around the Zn II ions are in the range of 2.186(5)-2.566(4)Å.There are two coordination modes of 2,5-dtpa anions in complex 1.Both the two carboxylate group in 2.5-dtpa (mode I) anion ligated to four Zn II ions in µ 2 -η 2 :η 1 coordination mode, while the carboxylate groups in another 2,5-dtpa (mode II) adopt µ 1 -η 1 :η 1 coordination mode (Figure 1b).Two adjacent zinc atoms are connected by two oxygen atoms (O1, O2) from 2,5-dtpa anion forming one binuclear Zn cluster.Further, the structure is built around  1c).As shown in Figure 1d, the π . . .π interactions between the benzene rings of 2,5-dtpa and 1,2-bimd ligands with distance of 3.753 Å play important role in the formation of three-dimensional network structure.
Topologically, if the binuclear Cd cluster is viewed as four-connected nodes and each 1,2-bimb or 5-hipa serves as a two-connected spacer, complex 2 can be simplified as a 2D sql topology with a point symbol of {4 4 •6 2 } (Figure 2e).
Topologically, if the binuclear Cd cluster is viewed as four-connected nodes and each 1,2-bimb or 5-hipa serves as a two-connected spacer, complex 2 can be simplified as a 2D sql topology with a point symbol of {4 4 •6 2 } (Figure 2e).

Crystal Structures of {Zn2(1,2-bimb)(L)(CH3COO)•DMF•2H2O}n (3)
Structural analysis reveals that complex 3 crystallizes in the triclinic system, P1 space group.There are two Zn II ions, one H3L anion, one 1,2-bimb ligand, one coordinated acetate anion, one free  Structural analysis reveals that complex 3 crystallizes in the triclinic system, P1 space group.There are two Zn II ions, one H 3 L anion, one 1,2-bimb ligand, one coordinated acetate anion, one free DMF molecular, and two uncoordinated water molecules in the asymmetric unit.As shown in Figure 3a, the two Zn II ions are both located in a tetrahedron coordination geometry.Zn1 is coordinated by two oxygen atoms from two H 3 L anions, another oxygen atom from acetate anion, one nitrogen atom from one 1,2-bimb ligand.The coordination patterns of Zn1 and Zn2 are very similar except that one of the oxygen atoms is from the H 3 L ligand.The Zn-O/N bond lengths fall in the range of 1.928(2)-2.011(2)Å.Two Zn II ions are bridged by one carboxylates from one H 3 L anion with Zn1•••Zn2 separation of 4.6985(7) Å.The large atomic radius of Zn II ion makes the deprotonated H 3 L anion ligand adopt a µ 3 -η 1 :η 1 :η 1 :η 1 η 0 :η 1 coordination mode (Figure 3b).The 3-connected H 3 L anion liagand link the Zn II ions forming a 2D [Zn(H 3 L)] n sheet (Figure 3c).The 1,2-bimb act as bridging linkers, and link neighboring 2D [Zn(H 3 L)] n sheets together into a 3D framework (Figure 3d).

Structure Discussion
It is well known that carboxyl groups of different substitution positions or different substituents have a significant influence in terms of structure and function [31][32][33].Regarding 1,2 and 4, the three kinds of dicarboxylate (para-substituted in 1, meta-substituted in 2, ortho-substituted in 4) with different coordination fashions and steric hindrances result in distinct architectures, respectively (Scheme 2).However, the structural analysis of complex 1,2 and 4 shows that the introduction of different substituents (-NH2, -OH, -NO2) has no effect on the formation of the final structure.Ignoring the effect of substituents, Complex 1,2 and 4 form a sql topology from dicarboxylate ligands with different substitution positions.As ancillary ligands, carboxylic acid ligands play an important linking role in the formation of structures.
Moreover, the primary ligand is indispensable in forming the structure.In the structures of 1 and 2, the primary ligand connects adjacent metals to form a bimetallic node, which is of great significance for the formation of a three-dimensional supramolecular structure.The π•••π interaction between the primary and ancillary ligands reinforces the stability of the structure.In complex 4, the primary ligands not only connect the metal nodes to form a 1D "Z" chain, but also form a π•••π stacking with adjacent primary ligands to build up a three-dimensional supramolecular structure.Scheme 2. The diverse coordination modes of auxiliary ligands of complexes 1, 2 and 4.

X-ray Power Diffraction Analysis and Thermal Analysis
The PXRD patterns were obtained at room temperature to confirm that the crystal structures are truly representative of the bulk samples in the solid state.For complexes 1-4, the peak positions of

Structure Discussion
It is well known that carboxyl groups of different substitution positions or different substituents have a significant influence in terms of structure and function [31][32][33].Regarding 1,2 and 4, the three kinds of dicarboxylate (para-substituted in 1, meta-substituted in 2, ortho-substituted in 4) with different coordination fashions and steric hindrances result in distinct architectures, respectively (Scheme 2).However, the structural analysis of complex 1,2 and 4 shows that the introduction of different substituents (-NH 2 ,-OH, -NO 2 ) has no effect on the formation of the final structure.Ignoring the effect of substituents, Complex 1,2 and 4 form a sql topology from dicarboxylate ligands with different substitution positions.As ancillary ligands, carboxylic acid ligands play an important linking role in the formation of structures.
Moreover, the primary ligand is indispensable in forming the structure.In the structures of 1 and 2, the primary ligand connects adjacent metals to form a bimetallic node, which is of great significance for the formation of a three-dimensional supramolecular structure.The π•••π interaction between the primary and ancillary ligands reinforces the stability of the structure.In complex 4, the primary ligands not only connect the metal nodes to form a 1D "Z" chain, but also form a π•••π stacking with adjacent primary ligands to build up a three-dimensional supramolecular structure.

Structure Discussion
It is well known that carboxyl groups of different substitution positions or different substituents have a significant influence in terms of structure and function [31][32][33].Regarding 1,2 and 4, the three kinds of dicarboxylate (para-substituted in 1, meta-substituted in 2, ortho-substituted in 4) with different coordination fashions and steric hindrances result in distinct architectures, respectively (Scheme 2).However, the structural analysis of complex 1,2 and 4 shows that the introduction of different substituents (-NH2, -OH, -NO2) has no effect on the formation of the final structure.Ignoring the effect of substituents, Complex 1,2 and 4 form a sql topology from dicarboxylate ligands with different substitution positions.As ancillary ligands, carboxylic acid ligands play an important linking role in the formation of structures.
Moreover, the primary ligand is indispensable in forming the structure.In the structures of 1 and 2, the primary ligand connects adjacent metals to form a bimetallic node, which is of great significance for the formation of a three-dimensional supramolecular structure.The π•••π interaction between the primary and ancillary ligands reinforces the stability of the structure.In complex 4, the primary ligands not only connect the metal nodes to form a 1D "Z" chain, but also form a π•••π stacking with adjacent primary ligands to build up a three-dimensional supramolecular structure.

X-ray Power Diffraction Analysis and Thermal Analysis
The PXRD patterns were obtained at room temperature to confirm that the crystal structures are truly representative of the bulk samples in the solid state.For complexes 1-4, the peak positions of

X-ray Power Diffraction Analysis and Thermal Analysis
The PXRD patterns were obtained at room temperature to confirm that the crystal structures are truly representative of the bulk samples in the solid state.For complexes 1-4, the peak positions of the experimental PXRD patterns closely match the simulated one based on the single-crystal X-ray date, demonstrating the high phase purity of all complexes (Figures S1-S4).
To study the stabilities of CPs, thermogravimetric (TGA) analysis of complexes 1-4 were carried out under N 2 atmosphere from 30 to 600 • C (Figure 5).In the care of complex 1, the weight loss of 3.71% from 100 to 150 • C is attributed to the loss of lattice water molecules (calc.3.5%).The decomposition of the organic ligands starts at 200 • C, leaving a remaining residue of ZnO (obsd: 16.1%, calcd.: 15.7%).For complex 2, the lattice water molecules decompose around 110 • C.And then the organic ligands of the polymers begin to pyrolyze.Ultimately leads to thermal decomposition and produces CdO residues.For 3, the weight loss 12.6% occurs at 220 • C, which conforms to the loss of free DMF and H 2 O molecules (calcd.: 12.4%) followed by decomposition of the polymers.The final residue is ZnO.The TGA curve of 4 indicates it is stable up to about 210 • C, and then it begins to decompose upon further heating, leaving a remaining residue of CdO (obsed: 22.7%, calcd.: 21.8%).the experimental PXRD patterns closely match the simulated one based on the single-crystal X-ray date, demonstrating the high phase purity of all complexes (Figures S1-S4).
To study the stabilities of CPs, thermogravimetric (TGA) analysis of complexes 1-4 were carried out under N2 atmosphere from 30 to 600 °C (Figure 5).In the care of complex 1, the weight loss of 3.71% from 100 to 150 °C is attributed to the loss of lattice water molecules (calc.3.5%).The decomposition of the organic ligands starts at 200 °C, leaving a remaining residue of ZnO (obsd: 16.1%, calcd.: 15.7%).For complex 2, the lattice water molecules decompose around 110 °C.And then the organic ligands of the polymers begin to pyrolyze.Ultimately leads to thermal decomposition and produces CdO residues.For 3, the weight loss 12.6% occurs at 220 °C, which conforms to the loss of free DMF and H2O molecules (calcd.: 12.4%) followed by decomposition of the polymers.The final residue is ZnO.The TGA curve of 4 indicates it is stable up to about 210 °C, and then it begins to decompose upon further heating, leaving a remaining residue of CdO (obsed: 22.7%, calcd.: 21.8%).

Photoluminescence Properties
In general, the coordination polymers of d 10 transition metal centers have been displayed photoluminescence application in luminescent materials and photochemical sensors [34][35][36][37].Meanwhile, the luminescence properties are affected by the coordination pattern of organic ligands and different metal ions.The fluorescence properties of complexes 1-4, free 1,2-bimb and 1,2-bmimb ligand have been examined in the solid state at room temperature, as shown in Figure 6.The 1,2-bimb and 1,2-bmimb ligands display photoluminescent emissions at 483 and 447 nm upon excitation at 300nm, which may be attributed to π* → π transition.Under similar excitation condition, complexes 1-4 exhibit intense emission bands at 483 nm for 1, 464 nm for 2, 438 nm for 3, and 442 nm for 4, respectively.The measured luminescence quantum yields are 10.2% for 1, 15.9% for 2, 12.4% for 3 and 5.7% for 4.These broad emissions of complexes 1-4 could be regarded as arising from IL (intraligand) transitions or with the admixture of IL and MLCT (metal-to-ligand charge transfer) characters [38][39][40][41].

Photoluminescence Properties
In general, the coordination polymers of d 10 transition metal centers have been displayed photoluminescence application in luminescent materials and photochemical sensors [34][35][36][37].Meanwhile, the luminescence properties are affected by the coordination pattern of organic ligands and different metal ions.The fluorescence properties of complexes 1-4, free 1,2-bimb and 1,2-bmimb ligand have been examined in the solid state at room temperature, as shown in Figure 6.The 1,2-bimb and 1,2-bmimb ligands display photoluminescent emissions at 483 and 447 nm upon excitation at 300nm, which may be attributed to π* → π transition.Under similar excitation condition, complexes 1-4 exhibit intense emission bands at 483 nm for 1, 464 nm for 2, 438 nm for 3, and 442 nm for 4, respectively.The measured luminescence quantum yields are 10.2% for 1, 15.9% for 2, 12.4% for 3 and 5.7% for 4.These broad emissions of complexes 1-4 could be regarded as arising from IL (intraligand) transitions or with the admixture of IL and MLCT (metal-to-ligand charge transfer) characters [38][39][40][41].

Conclusions
In summary, four coordination polymers based on the mixed ligand system have been synthesized and characterized.These complexes show fascinating 2D and 3D structures.The structural diversities of the complexes indicate that the primary and auxiliary ligands show remarkable effects on the formation of the final structures.S1: Selected bond distances (Å) and angles (°) for 1; Table S2: Selected bond distances (Å) and angles (°) for 2; Table S3: Selected bond distances (Å) and angles (°) for 3; Table S4: Selected bond distances (Å) and angles (°) for 4.
Author Contributions: K.L. and L.D. conceived and designed the experiments; K.L. performed the experiments; Y.Z. and S.J. analyzed the data; L.W. supervised the work.All the authors have contributed to manuscript revision.

Conclusions
In summary, four coordination polymers based on the mixed ligand system have been synthesized and characterized.These complexes show fascinating 2D and 3D structures.The structural diversities of the complexes indicate that the primary and auxiliary ligands show remarkable effects on the formation of the final structures.Complexes 1, 2 and 4 exhibit the same 2D sql topology network, which further constructs a 3D supramolecular framework through π•••π stacking interactions.Complex 3 shows a 3D {4 2 •6}•{4 4 •6 10 •8}-3,6T24 topology net.Furthermore, photoluminescence measurements reveal that complexes 1-4 are potential photochemical materials, due to their intense luminescence emission.Meanwhile, this work prompted us to obtain more CPs by using a reliable synthetic route using the mixed ligand.

Crystals 2018, 8 ,
x FOR PEER REVIEW 8 of 15 DMF molecular, and two uncoordinated water molecules in the asymmetric unit.As shown in Figure3a, the two Zn II ions are both located in a tetrahedron coordination geometry.Zn1 is coordinated by two oxygen atoms from two H3L anions, another oxygen atom from acetate anion, one nitrogen atom from one 1,2-bimb ligand.The coordination patterns of Zn1 and Zn2 are very similar except that one of the oxygen atoms is from the H3L ligand.The Zn-O/N bond lengths fall in the range of 1.928(2)-2.011(2)Å.Two Zn II ions are bridged by one carboxylates from one H3L anion with Zn1•••Zn2 separation of 4.6985(

Crystals 2018, 8 ,
x FOR PEER REVIEW 10 of 15 act as glue to assemble the adjacent sheets in an -ABA-fashion resulting in a 3D supramolecular framework (Figure4g).

Scheme 2 .
Scheme 2. The diverse coordination modes of auxiliary ligands of complexes 1, 2 and 4.

Scheme 2 .
Scheme 2. The diverse coordination modes of auxiliary ligands of complexes 1, 2 and 4.

Table 1 .
Crystallographic date and structure refinement results for complexes 1-4.Empirical formula C 22 H 22 N 6 O 5 Zn C 44 H 40 N 8 O 12 Cd 2 C 31 H 40 N 8 O 14 Zn 2 C 24 H 21 N 5 O 6 Cd