Synthesis，crystal structure，and fluorescence of a Zn（II）supramolecular complex with 5-（pyridin-2-yl）-3，3′-bi（1H-1，2，4-triazole）and benzene-1，2，4，5-tetracarboxylic acid

GE Zhiwei，WU Na，CHEN Jing

（College of Chemistry，Tianjin Normal University，Tianjin 300387，China）

In recent years，the rational design and assembly of coordination supramolecular complexes have attracted great interest due to their fascinating structural diversity as well as their potential applications in magnetism，photoluminescence，guest inclusion，gas adsorption，catalysis，and so on[1-4].Basically，the construction of such metallosupramolecular assemblies mainly depends on the skilful selection of metal ions and organic ligands.However，it is still a challenge to accurately control their final structures because of the high unpredictability of the subtle processes of such coordination assemblies，which may be influenced by a variety of reaction parameters and weaker intermolecular interactions.Generally，the selection of organic ligands plays the key role in constructing and regulating the coordination structures.Accordingly，ligands with certain functional groups，such as pyridyl，carboxylate，and triazolyl，etc.are especially crucial and have been widely explored so far.One of the most effective strategies for achieving diverse metallosupramolecular architectures is to introduce the mixed-ligands system into the assembled process.In this regard，triazolyl derivatives and polycarboxyl ligands have been proved to be good candidates for the construction of interesting coordination supramolecular networks[5-8].Herein，an unsymmetric triazolyl-based derivative 5-（pyridin-2-yl）-3，3′-bi（1H-1，2，4-triazole）（H2pbt）and a polycarboxyl ligand benzene-1，2，4，5-tetracarboxylic acid （H4BTCA）that may show different binding features，are selected to assemble with zinc（II） acetate to afford a new supramolecular complex.

1 Experimental

1.1 Materials and methods

All reagents and solvents were commercially available and used as received.Elemental analyses of C，H，and N were carried out with a CE-440（USA，Leemanlabs）analyzer.FT-IR spectrum（KBr pellet）was taken on an AVATAR-370（Nicolet）spectrometer.Powder X-ray diffraction（PXRD）data were recorded on a Bruker D8 Advance diffractometer at 40 kV and 100 mA for a Cu-target tube（λ =0.154 06 nm）.The calculated PXRD pattern was obtained from the single-crystal X-ray diffraction data by using the PLATON package.Solid state fluorescence spectrum of the microcrystalline sample was measured on a CaryEclipsespectrofluorimeter（Varian）at room temperature.

1.2 Preparation of the complex

A CH3OH solution（5 mL） of H2pbt（10.7 mg，0.05 mmol） and H4BTCA （12.7 mg，0.05 mmol） was carefully layered onto a water solution （5 mL）of Zn（OAc）2·2H2O （21.9 mg，0.1 mmol）in a straight glass tube.After evaporating the solvents slowly for about 1 week，colourless block single crystals suitable for X-ray analysis were produced in 49%yield（15.6 mg，basedon H2pbt）.Analysis（%），calculated for C19H23ZnN7O14：C，35.72，H，3.63，N，15.35；found：C，35.58，H，3.57，N，15.41.IR（KBr，cm-1）：3 429 b，2 369 w，1 717 s，1 635 s，1 569 s，1 521 w，1 458 s，1 419 w，1 373 w，1 337 w，1 292 w，1 272 w，1 125 s，1 104 w，1 040 s，982 vs，802 s，757 s，725 s，668 w，640 w.

1.3 X-ray crystallography

X-ray single-crystal diffraction data for the title complex were collected on a Bruker Apex II CCD diffractometer at ambient temperature with Mo Kα radiation（λ =0.071 073 nm）.There was no evidence of crystal decay during data collection.Semiempirical absorption correction was applied （SADABS）[9]，and the program SAINT was used for integration of the diffraction profiles10].The structure was solved by direct methods using the SHELXS program of SHELXTL package and refined with SHELXL[11-12].The final refinement was performed by full-matrix least-squares methods with anisotropic thermal parameters for all non-H atoms on F2.All H atoms were initially determined in difference Fourier maps，which were then constrained to an ideal geometry，and refined as riding atoms：C—H=0.093 nm，O—H=0.085 nm and N—H=0.086 nm with Uiso（H）=1.2Ueq（C），1.5Ueq（O）or 1.2Ueq（N）.Further crystallographic details are summarized in Table 1，and selected bond lengths and angles are shown in Table 2.

Tab.1 Crystallographic data for the title complex

Tab.2 Selected bond length and angle for the title complex

2 Results and discussion

2.1 Description of the crystal structure

As illustrated in Figure 1，the title complex has a mononuclear coordination structure.The central ZnIIatom is six-coordinated and shows a distorted octahedral geometry.Two oxygen （O1 and O9）and two nitrogen atoms（N1 and N2）from one unidentate H2BTCA，one water molecule，and one chelating H2pbt ligand form the equatorial plane，and other two water ligands（O10 and O11）occupy the axial positions with the O10—Zn1—O11 angle of 175.9（1）°.The Zn—O bond lengths are in the range of 0.206 6 （2）-0.213 2（2）nm and the Zn—N bond lengths are 0.214 7（3） and 0.223 6（2） nm.The H2pbt ligand adopts the chelating coordination fashion by using the nitrogen atoms from the pyridyl and the adjacent triazolyl ring.With regard to H2BTCA2-，only one carboxylate group is coordinated to the central ZnIIion in the monodentate fashion.

Fig.1 A view of asymmetric unit of title complex showing displacement ellipsoids drawn at 30%probability level

As depicted in Figure 2，the adjacent mononuclear molecules are further interlinked to construct a two-dimensional supramolecular structure via N6—H6···O8v[symmetry operation （v）=2-x，1-y，2-z]between the triazolyl nitrogen of H2pbt and the carboxyl oxygen of H2BTCA2-as well as O9—H9B···O7i[symmetry operation（i）=2-x，2-y，2-z]，O10—H10B···O5iv[symmetry operation （iv）=-1+x，-1+y，z]，and O11—H11B···O5iii[symmetry operation （iii）=x，-1+y，z]between the water ligands and different carboxyl groups.Additionally，the uncoordinated water molecules are also linked to the above two-dimensional pattern via hydrogen bonding of O4—H4…O14vi，N4—H4B···O12iii，O10—H10A···O13，O12—H12A···O2ii，O12—H12B…O1 and O13—H13B…O8i（see Table 3）.Besides，aromatic stacking interactions are found between the triazolyl groups（N2—N4 and C6—C7/N5—N7 and C8—C9）and benzene （C10—C15）planes with the centroid-to-centroid distance of 0.378 0（2）/0.382 8（2）nm，centroid-toplane distances of 0.329 2（2）-0.342 1（2）nm，and the dihedral angle of 3.0（2）/4.6（2）°，as well as between the parallelpyridyl（N1andC1—C5）ringswiththecentroidto-centroid distance and centroid-to-plane distance of 0.363 4（2）nm and 0.330 4（2）nm separately.Further investigation on the crystal packing indicates that these two-dimensional layers are arranged in a parallel fashion in the lattice，as shown in Figure 3.

Fig.2 A side view of two-dimensional supramolecular structure of title complex constructed via hydrogen bonds

Tab.3 H—bonding geometries for the title complex

Fig.3 Packing diagram of title complex

2.2 PXRD and Fluorescence

The phase purity of the bulk sample for the title complex was confirmed by PXRD，which shows a good agreement of the experimental and calculated patterns（see Figure 4）.It has been demonstrated that coordination complexes with d10electronic configuration metals are promising candidates for photoactive hybrid materials.Thus，the fluorescent property of the title complex was studied in solid state at room temperature.As expected，the result shows that this complex may serve as a good candidate for potential photoactive material.As shown in Figure 5，the maximal emission is observed at 380 nm upon excitation at 290 nm，which can be ascribed to the ligand-centered transitions.

Fig.4 Experimental and simulated PXRD patterns for the title complex.

Fig.5 Emission spectrum of the title complex at room temperature upon excitation at 290 nm

3 Conclusions

In summary，a new supramolecular complex with mixed-ligands H2pbt and H4BTCA has been prepared，the structure of which is regulated by multiple interactions including coordination，H-bonding and aromatic stacking.This result proves that H2pbt and H4BTCA could serve as excellent building blocks to construct supramolecular complexesand such a mixed-ligands synthetic strategy may further flourish the current field of crystal engineering.It will also encourage us to make more efforts on the supramolecular systems with mixed organic ligands.The fluorescent property of the title complex reveals that it may be applied as a potential luminescence material.

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