YAN Jun-Zhi LU Li-Ping

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Syntheses, Structures, and Luminescence Properties of Two New Open-framework Zinc Phosphites Based on 1,2,4-Triazole Derivatives①

YAN Juan-Zhia, bLU Li-Pinga②

a(030006)b(030032)

With 1,2,4-triazolederivatives as structure directing agents, two new open- framework zinc phosphites, [Zn(atrz) (HPO3)]n(1) and [Zn(dmatrz) (HPO3)]n(2)(atrz= 4-amino-1,2,4-triazole, dmatrz= 4-amino-3,5-dimethyl-1,2,4-triazole) have been synthesized and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, powder andsingle-crystal X-ray diffractions. Both compounds are isostructure and crystallize in the21/space group of monoclinic system. Compound 1:9.629(1),7.384(1),10.274(1)?,110.729(3),683.26(2)?3,4, M = 229.44, D= 2.230 g/cm3,(000)456,1.10,3.79mm–1,0.0181and0.0466 for 1121 observed reflections (＞ 2())Compound 2:10.786(2),8.921(1),9.749(1)?,107.3,895.6(3)?3,4, M = 257.49, D= 1.910 g/cm3,(000)520,1.00,2.90mm–1,0.018and0.051 for 1581 observed reflections (＞ 2())Both compounds are built up into 4.8-net 2open-frameworks of vertex-linked ZnO4and HPO3units (3.57 × 4.53 ?2for 1 and 4.43 × 5.90 ?2for 2). The structures consist of left-, right-handed helical chains that are connected through oxygen atoms to form an undulated 2sheet stack, which can be topologically regarded as 4.82nets.Solid-state luminescence properties and thermo gravimetric analyses of these two compounds were investigated, respectively.

zinc phosphites, 1,2,4-triazole, structure, luminescence property;

1 INTRODUCTION

Open-framework structures with extra-large chan- nels (pore size larger than 12 polyhedra)are of great interest because of their versatile architectures[1-3]and potential applications in many fields, such as sensors, separation, molecular magnetism, photolu- minescence, catalysis, and others[4-6]. Zinc pho- sphites are now a well-established family of open frameworks, where 3open-frameworks, 2layers, 1chains and 0clusters have been reported in detail. Different rings channels have been obtained such as 4-membered rings (4R)[7, 8], 8R, 12R, 16R, 24R , 26R, 28R, 40R, 48R, 56R, 64R, and 72R[9-11]. Compared with 4-connected phosphate group, three- connected -HPO3groups can reduce the M–O–P connectivity and generate more “open” interrupted architectures with larger pore sizes and lower framework densities[12-14]. However, the number of zinc phosphite frameworks with nanometer-scale channels is limited[5]. To accurately synthesize a tunable framework with different templates is equally difficult. It is of great interest and also challenging to develop an efficient systematic synthesis for fine-tuning porosity. It is well known that 1,2,4-triazole and its derivatives are very interesting bridging ligands because of their poten- tial binding modes (-1,2,-2,4, and-1,2,4)[15-18]. More diversified constructions may be shown. In most cases, as far as we know, however, only one triazole zinc phosphite, [Zn(C2H3N3)(HPO3)]n, has been constructed by 1,2,4-triazole (trz)[19].

Herein, we report the syntheses, characterizations, thermal behaviors and luminescence properties of Zn(atrz)(HPO3)]n(1) and [Zn(dmatrz) (HPO3)]n(2). Both compounds display the same 8-ring open-framework structures with 3.57 × 4.53 ?2(1) and 4.43 × 5.90 ?2(2).

2 EXPERIMENTAL

2. 1 Instruments and materials

Both triazole ligands were synthesized according to the literature method[20].All other reagents were purchased commercially and used without further purification. The FT-IR infrared spectra were recor- ded from KBr pellets in the range of 4000～400 cm?1on a Bio-Rad FTS 135 spectrometer. Elemental analysis was carried out on a Perkin-Elmer Optima 3300 DV inductive coupled plasma spectrometer (ICP) and an Elementar Vario EL III analyzer. X-ray powder diffraction (PXRD) data were collected on a SCXminiFlex II X’pert PRO diffractometer using Cu-radiation (= 1.540598 ?) at 30 kV and 15 mA. Thermogravimetric analyses (TGA) were performed on a Mettler Toledo TGA/SDTA 851E analyzer in air at a heating rate of 10 °C·min?1. Luminescence spectrum was recorded on a CARY Eclipse (Varian, USA) fluorescence spectrophoto- meter at room temperature.

2. 2 Syntheses of the compounds

[Zn(atrz)(HPO3)]n(1) ZnCl2(0.27 g, 2.0 mmol) was added to a stirred solution of atrz (0.168 g, 2.0 mmol) in 20.0 mL of water; and then H3PO3(0.164 g, 2.0 mmol) in water (2.0 mL) was slowly added. The resulting solution was stirred for 6.0 hours, and then filtered. The filtrate was slowly evaporated at room temperature. After 4 weeks, colorless block crystals suitable for X-ray analysis were obtained (yield: 47.8%). Analysis calculated for C2H5ZnN4O3P (%): C, 10.52; H, 1.76; N, 24.53; P, 13.5; Zn, 28.5. Found (%): C, 10.73; H, 1.78; N, 23.62; P, 12.9; Zn, 28.8. IR (cm?1): 3320(s), 3114(s), 2392(m), 1631(s), 1466(m), 1387(w), 1124(s), 1101(s), 1014(s), 888(m), 633(s), 561(s), 554(m).

[Zn(dmatrz)(HPO3)]n(2) Compound2 was prepared in a similar method to that of 1, but atrz was replaced by dmatrz (0.224 g, 2.0 mmol). Colorless block crystals were obtained (yield: 55.3%). Analysis calculated for C4H9ZnN4O3P(%): C, 18.66; H, 3.52; N, 21.76; P, 12.03; Zn, 25.39. Found: C, 18.35; H, 3.56; N, 21.23; P, 11.8; Zn, 25.6. IR (cm?1): 3280(s), 3177(m), 2354(s), 2335(m), 1650(s), 1537(m), 1269(w), 1169(s), 1095(s), 1014(s), 1033(m), 775(m), 611(s), 598(w).

2. 3 X-ray crystallography

Single-crystal X-ray diffraction data for 1were collected on a Bruker Smart Apex II with CCD area detector diffractometer Mo-(= 0.71073 ?) at room temperature and data processing was accom- plished with the SAINT processing program[21]. The data of 2 were collected at 100(2) K on Beijing Synchrotron Radiation Facility (BSRF) beamline 1W2B which was mounted with a MARCCD-165 detector with storage ring working at 2.2 GeV (= 0.72 ?), and the data were collected by the program MARCCD and processed using HKL2000[22]. The structures were solved by direct methods and refined by full-matrix least-squares technique using the SHELXS-2014[23].After all non-H atoms were refined anisotropically, hydrogen atoms attached to C and N atoms were added theoretically and treated as riding on the concerned atoms. The final cycle of full-matrix least-squares refinement was based on observed reflections and variable parameters. For compound 1, a total of 7011 reflections were obtained in the range of 2.3<<31.4° with 1086 unique ones (int= 0.054), of which 1211 were observed (＞ 2()). The final= 0.0181,= 0.0466 (= 1/[2(F2) + (0.0219)2], where= (F2+ 2F2)/3), (Δ)max= 0.30, (Δ)min= –0.25 e/?3, (D/)max= 0.001 and= 1.10. For compound 2, a total of 2845 reflections were obtained in the range of 2.97<<25.05° with 1463 unique ones (int= 0.0739), of which 1581 were observed (＞ 2()). The final= 0.018,= 0.051 (= 1/[2(F2) + (0.0323)2+ 0.108], where= (F2+ 2F2)/3), (Δ)max= 0.40, (Δ)min= –0.33 e/?3, (D/)max= 0.001 and= 1.00. The selected bond lengths, bond angles and hydrogen bonds for 1 and 2 are listed in Tables 1 and 2, respectively.

Table 1. Selected Bond Lengths (?) and Angles (o) for Compounds 1 and 2

Symmetry codes for 1: i ?1,+1/2, ?+3/2; ii ?+1, ?+1, ?+1; iii ?+1,?1/2, ?+3/2. 2: i –+1,?1/2, ?+1/2; ii, ?+5/2,?1/2

Table 2. Hydrogen Bond Geometry (?, °) of Compounds1 and 2

3 RESULTS AND DISCUSSION

3. 1 IR spectroscopy

The IR spectra of compounds 1 and 2 were similar and showed typical peaks, with the exception of the characteristic bands observed at 2391 and 2374 cm–1, which are attributed to the stretching vibration of P–H bonds in phosphite anions (Fig. 1)[4, 24].The intense bands at 1124, 1101, 1014 cm–1for 1 and 1169, 1095, 1014 cm–1for 2 are associated with the stretching vibrations of P–O bonds, whereas the bands at 424～630 cm–1for 1 and 427～623 cm–1for 2 are associated with the bending vibrations of P–O bonds. The stretching vibration bands of -NH2groups appeared at about 3314～3109 cm–1, and the bending bands of -NH2and -CH2are present at about 1649～1378 cm–1.

Fig. 1. IR spectra of 1 and 2

3. 2 Crystal structures of 1 and 2

As compounds1 and 2 havesimilar structures, only the structure of 1 is selected to describe. X-ray single-crystal structural analysis reveals that the asymmetric unit of 1 contains 11 non-hydrogen atoms, one zinc atom, one phosphorus atom, three oxygen atoms and one 4-amino-1,2,4-triazole (atrz) molecule (Fig. 2), which contains ZnO3N tetrahe- dron and HPO3pseudo-pyramids as the polyhedral building units. The Zn–N bond corresponds to a direct link between zinc and the triazole template ((Zn–N) = 2.007(2) ? (1), 2.030(2) ? (2)) in a monodentate fashion. The Zn species makes three Zn–O–P links to the P atoms nearby (av(Zn–O) = 1.9381(1) ? (1), 1.9263(1) ? (2)).P(1) makes three bonds to the Zn neighbors (av(P–O) = 1.5235(1) ? (1), 1.5153 ? (2)), with the terminal P–H bond ((P–H) = 1.30(2) ? (1), 1.35(2) ? (2)) as its fourth vertex. P–O and P–H bond distances are in agreement with those observed in the known zinc phosphites[13, 19, 25]. The existence of P–H bonds is also confirmed by the characteristic band of phosphite anions ((H–P), 2391 cm?1(1), 2374 cm?1(2)) in the IR spectrum. The average Zn–O–P bond angles of the three bridging O atoms are 128.89° (1) and 133.27° (2).The geometrical parameters of the organic components of 1 and 2 are typical. Fourier difference maps clearly located two H atoms attached to the C atom, thus the triazole “template” is neutral, in accordance with the charge balancing requirement.

Fig. 2. Coordination units of 1 (left) and 2 (right) with 40% thermal ellipsoids.

Symmetry codes: for 1: i: ?1,+1/2, ?+3/2; ii: ?+1, ?+1, ?+1. 2: i: –+1,?1/2, ?+1/2; ii:, ?+5/2,?1/2

As illustrated in Fig. 3 (left), the ZnO3N and HPO3groups are connected to form a 2layer. The adjacent layers are stacked in an -ABAB- sequence along thedirection. ZnO3N and HPO3are con- nected to form 4- and 8-rings.Each 4-ring has two up and two down Zn–N and P–H groups. Each 4-ring is surrounded by four 8-rings, and each 8-ring is surrounded by four 4-rings, which form a 24.8-net sheet parallel to theplane. The atrz molecules reside alternately above and below the layer. The opening of the 8-ring is about 3.57? × 4.53? (1) and 4.43? × 5.90? (2) (the shortest O···O diagonal distance, taking the van der Waals radii into account). If each four-ring is taken as a four-connected node, this 2sheet has a 4.82topology (Fig. 3 (right)).

Fig. 3. Representation of the 4.8-net sheet parallel to theplane: connectivity of HPO3(purpe) and ZnO3N (blue) tetrahedron to 4-ring and 8-ring loops (left); schematic drawing of the 2D 4.82networks (right)

It is noteworthy that, within each 2sheet of compound2, the 4-rings are not parallel to each other (Fig. 4 (left)) and a helical chain can be extracted from this layer structure as reported previously for 4.82rings. The left- and right-handed helical chains are interconnected through O(1) atoms to form the layer. The central axis of each helical chain is a twofold screw axis along theaxis. It can be clearly seen that the layer exhibits an undulated pattern (Fig. 4right). The cyclic hydrophobic rings of the atrz/dmatrz molecules exclusively protrude into the interlayer region.

Fig. 4. Framework of 2 viewed along the [010] direction, showing two types of helical channels and chiral channels which are alternately arranged along theaxis (left). Left- and right-handed helical chains are isolated by omitting bridging O(1) in compound 2 (right). Color code: Zn: light blue; P: pink; O: red; C: grey; N: blue

Fig. 5. Packing diagram of compound 1 with the hydrogen bonds indicated by dashed lines

As show in Fig. 5, the layers of compound 1 are held together by N–H···O hydrogen bonds (3.139(3)～3.330(3) ?). Compared with 1, besides the hydrogen bond N–H···O (3.0303(2) ?), N–H···N (3.062(2) ?) and C–H···O (3.362(2)～3.385(3) ?) hydrogen bonds are also important to define the molecular packing and stabilize the structure of 2.

Interestingly, both crystal structures have very similar structures to the [Zn(C2H3N3) (HPO3)]n[18](C2H3N3=1,2,4-triazole (trz)). These three phases contain equivalent 4.8-ring polyhedral networks, encapsulating the same organic species, which templates in essentially the same way in each case. It is delicately different that 1,2,4-triazole families take binding modes: [Zn(atrz)(HPO3)]nand [Zn(dmatrz)(HPO3)]nare coordinated with N1rather than N4 because -NH2occupies the 4-position of 1,2,4-triazole. As a part of systematic research, 4-amino-3,5-diethanyl-1,2,4-triazole (deatrz) and 4-amino-3,5-propyl-1,2,4-triazole (dpatrz) have been surveyed. However, the corresponding compounds can not be synthesized, which attribute to the bonding requirement of the ligand.

3. 3 XRD and TGA

Simulations based on compounds1 and 2 single crystal structures were in excellent agreement with X-ray powder data, indicating phase purity and high crystallinity (Fig. 6).

Fig. 6. TGA curves and simulated and experimental powered X-ray diffraction patterns of 1 and 2

The initial thermogravimetric analysis was per- formed under flowing N2at a heating rate of 10 °C·min-1in 23～800 °C. As shown in Fig. 6, relatively high thermal stability was found for two compounds in air up to 325 °C (1) and 380 °C (2) and similar thermal behaviors are consistent with their similar structure skeleton. The TGA curve of 1 revealed that a sharp weight loss of 18.6% occurs between 325 and 340 °C, which is in good agreement with half of atrz (calcd. weight loss 18.4%), followed by a gradual weight loss of 17.5% between 340 and 800 °C, with a total weight loss of 36.1% (calcd. 36.7%). The remaining residue of the sample is amorphous after the calcination and their phases are unidentified. For 2, a sharp weight loss of 10.3% occurs between 380 and 400 °C, followed by a gradual weight loss of 21.6% between 400 and 800 °C. The total weight loss of 31.6%, far to the calculated percentage of the organic component (43.4%) which attributes to partial dmatrz, was combusted[26].

3. 4 Luminescence properties

The solid state luminescence properties of ligands atrz, dmatrz, compounds 12were investigated at room temperature. As shown in Fig. 7,the free atrz/dmatrz ligand, 12display similar shoulder peaks at ca. 423 nm with 255 nm excitation, which can be attributed to anintraligand emission state[27]. Furthermore, considerable enhancement of the intensity for these peaks in the metal complex may be attributed to the increased rigidity of the ligand when it is bound to a metal center, compared with that of the free one, which effectively reduces the loss of energy[28].

4 CONCLUSION

Two new open-framework zinc phosphates of 1,2,4-triazole family, [Zn(atrz)(HPO3)]n(1) and [Zn(dmatrz)(HPO3)]n(2), have been synthesized by solution evaporation methods. The ZnO3N and HPO3groups are connected to form a 2layer, which can be topologically regarded as 4.82nets. High thermal stability was found for two compounds in air up to 325 °C (1) and 380 °C (2). Zinc ions of compounds can increase the relevant ligands photoluminescence properties.

Fig. 7. Solid-state photoluminescence spectra of ligands and compounds 1～2

ACKNOWLEDGEMENT

The authors thank Dr Gao Zeng-Qiang at line 3W1Aof BSRF for his help with the single-crystal X-ray diffraction data collection and reduction. The authors thank Dr. Wei Cao at the Scientific Instrument Center of Shanxi University of China for her help with the single-crystal X-ray diffraction data collection.

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25 December 2017;

22 March 2018 (CCDC 1811502 for 1 and 1811503 for 2)

the National Natural Science Foundation of China (No. 21571118)

. E-mail: luliping@sxu.edu.cn

10.14102/j.cnki.0254-5861.2011-1929