Solvothermal Synthesis, Crystal Structure and Luminescence Property of a 1D Silver(I) Coordination Polymer①

ZHANG Nn GUO Yu-Hu YU You-Zhu WANG Zhen NIU Yong-Sheng② WU Xin-Li

a (Henan Joint International Research Laboratory of Nanocomposite Sensing Materials, School of Chemical and Environmental Engineering, Anyang Institute of Technology, Anyang 455000, China)

b (College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China)

ABSTRACT An unprecedented one-dimensional coordination polymer [Ag2(PhPO3H)2(DPPP)]n (1) based on phenylphosphonic acid (PhPO3H2) and 1,3-bis(diphenyphosphino)propane (DPPP) has been synthesized and characterized by IR, elemental analyses, thermogravimetric analyses and X-ray diffraction technique. Single-crystal X-ray diffraction analyses revealed that the complex crystallizes in triclinic system, space group P 1-. The coordination geometry of each Ag(I) ion is planar trigonal. Two kinds of rings were found in 1, such as the four-membered ring with Ag2O2 and the eight-membered ring with Ag2O4P2 constructed by PhPO3H– ligands, which were then connected by DPPP ligands forming a zigzag type chain. Moreover, the light absorption and luminescent property of 1 were also investigated.

Keywords: silver complex, crystal structure, coordination polymer, photoluminescent property;

1 INTRODUCTION

Crystal engineering of coordination polymers (CPs) has been a field of rapid growth in coordination and material chemistry not only because of their intriguing topologies but also for their interesting physical and chemical properties[1-6], such as catalysis[7,8], gas storage/adsorption[9-11], separa- tions[12-15], drug delivery[16,17]and so on. Coordination polymers are polymeric, supramolecular assemblies of metal ions (or metal clusters) and organic ligands linked through coordination bonds. Though crystal engineering affords us powerful approaches for preparing CPs, it is still a long-term challenge to rationally design and synthesize CPs with desired properties[18-20]. Hence, more and more scientists were attracted into this area and a large number of structures with potential applications were reported. However, among the reported CPs, silver(I) phosphonate based CPs are much less explored because of their sensitivity to light and tendency to yield highly insoluble polymers/oligomers[21-25].

As is well known, the flexibility of ligand plays a very important role in the construction of CPs because the shape, symmetry and length of a ligand can be affected by its flexibility. Rigid ligands have the benefit to construct CPs with expected structures. However, it is not easy to predict the structures of CPs constructed from flexible ligands mainly because flexible ligands can adopt a variety of conformations in coordination process especially by the affection of central metal ions. A large number of interesting CPs incorporating flexible ligands with metal ions has already been obtained. Taking account of the above, we select a flexible ligand, 1,3-bis(diphenyphosphino)propane (DPPP), which contains four rigid benzene ring pieces and one flexible propyl chain. On the other hand, DPPP has strong coordination affinities to Ag(I) because phosphine belongs to soft base and Ag(I) belongs to soft acid according to the soft and hard acid-base theory in coordination chemistry[26]. To the best of our knowledge, the Ag(I)-based CPs assembled with PhPO3H2and DPPP ligands have not been found in the literature. In this work, the solvothermal reactions of CF3COOAg with PhPO3H2and DPPP in mixed organic solvent at 65 °C gave rise to a new Ag(I)-based CP, namely [Ag2(PhPO3H)2- (DPPP)]n(1). Additionally, the thermal stability and photolu- minescent property of 1 were also investigated.

2 EXPERIMENTAL

2. 1 Materials and general methods

All starting reagents were of AR grade and used as received without further purification. The crystal data were collected on a Bruker Apex II CCD diffractometer. IR spectrum was recorded with KBr pellets on a Tensor 27 OPUS (Bruker) FT-IR spectrometer in the 4000～400 cm–1range. Analyses of C, H and P were determined on a Perkin-Elmer 240 Elemental analyzer. The powder X-ray diffraction patterns (PXRD) were conducted at 40 kV and 100 mA on a Rigaku D/Max-2500 diffractometer, using a graphite-monochromator and a Cu- target tube under ambient conditions. Thermogravimetric analysis (TGA) experiments were recorded at a heating rate of 10 °C/min from 40 to 800 °C on a NETZSCH STA 449F3 thermal analyzer under N2. The solid UV-Vis spectra were measured on a UV25500 UV-VIS-NIR Spectrophotometer (Shimadzu Corp.). The luminescence excitation/emission spectra were measured at room temperature on a Hitachi F-4600 fluorescence spectrophotometer.

2. 2 Synthesis of [Ag2(PhPO3H)2(DPPP)]n (1)

A mixture of CF3COOAg (90 mg, 0.41 mmol), PhPO3H2(16 mg, 0.1 mmol), DPPP (41 mg, 0.1 mmol), MeCN (2 mL), CH3OH (2 mL), DMF (2 mL) and H2O2(30%, 100 uL) was sealed in a Teflon-lined stainless-vessel (15 mL) and heated at 65 °C for 24 hours, then cooled to room temperature at a rate of 5 °C·h−1. The colorless blocks of 1 were collected and washed thoroughly with MeCN and dried in air. Yield: 43 mg (46%, based on PhPO3H2). Analysis calculated for C39H38Ag2O6P4(%): C, 49.71; H, 4.06; P, 13.15. Found (%): C, 49.67; H, 3.97; P, 13.22. IR (KBr, cm–1): 3436(m), 3049(w), 2902(w), 2700(w), 1630(m), 1477(w), 1432(s), 1392(w), 1232(m), 1130(s), 1051(s), 908(s).

2. 3 Crystallographic measurements and structure determination

Table 1. Selected Bond Lengths (Å) and Bond Angles (º) for 1

3 RESULTS AND DISCUSSION

3. 1 Description of the crystal structure

Fig.1. Asymmetric unit in complex 1. All H atoms are omitted for clarity (except H atoms on the O atoms)

Fig.2. Four-membered ring with Ag2O2 (i), eight-membered ring with Ag2O4P2 (ii) and the zigzag type chain (iii) in complex 1. All H atoms are omitted for clarity (except H atoms on the O atoms)

3. 2 IR spectrum

The absorption at 3436 cm–1can be attributed to the stretching vibration of O–H bond of groups -PO3H[29]. The peak at 3049 cm–1should be assigned to the stretching vibrations of the C–H bonds of benzenes. The strong absorption at 1200～900 cm–1is the typical stretching vibration of -PO3. The peaks of 1130 and 1051 correspond to the asymmetric and symmetric stretching vibration of PO2respectively, whereas the peak at 908 cm–1can be ascribed to the stretching vibration of P–OH bond[30], indicating the existence of -PO3H groups in complex 1.

3. 3 Powder X-ray diffraction (PXRD) and thermal stability

The powder X-ray diffraction of complex 1 was also performed at room temperature. The pattern calculated from single-crystal X-ray data of 1 was in good agreement with the observed ones in almost identical peak positions (Fig.3), which confirmed the pure phase of 1. The different reflection intensity between the simulated and experimental patterns could be ascribed to the powder size and variation in preferred orientation for the powder samples during the collection of experimental PXRD data.

Fig.3. Simulated and experimental PXRD patterns for complex 1

To study the thermal stability of complex 1, thermogravi- metric analysis (TGA) was performed (Fig.4). The TGA curve exhibits compound 1 shows good thermal stability under 280 °C. One obvious weight loss step was observed from 280 to 500 °C with the weight loss of about 70.96%, which is equivalent to the release of organic ligands (calcd.: 70.39%). Finally, the residue remains about 28.91% according to the produced compound of silver phosphate (calcd.: 29.53%).

Fig.4. TGA curve of complex 1

3. 4 Diffuse-reflectance UV-Vis spectra and photoluminescence properties

The UV-Vis absorption spectra of DPPP, PhPO3H2and complex 1 were recorded in the solid state at room tem- perature (Fig.5). As shown in the absorption spectra of DPPP and PhPO3H2, both of them have two absorption peaks (260 and 295 nm for DPPP; 218 and 269 nm for PhPO3H2), which can mainly be ascribed to the B-band showing the charac- teristic absorption band of aromatic compounds, and the R-band showing the characteristic absorption band of the conjugated bond with heteroatom[31], corresponding to the π → π* and n → π* transitions[32,33]. Accordingly, the absorp- tion peaks for 1 (254 and 333 nm) are very similar to the free DPPP ligand, which should be mainly assigned as π → π* or n → π* transitions of DPPP ligand.

Fig.5. Solid UV-Vis absorption spectra for DPPP, PhPO3H2 and complex 1 at room temperature

As shown in Fig.6, the solid-state photoluminesent properties of DPPP and complex 1 have been investigated in the solid state at room temperature (The photoluminesent spectra of PhPO3H2are not shown in Fig.6 because of its very weak photoluminescent property). Complex 1 shows a main emission peak at 475 nm (λex= 291 nm), whereas the main emission peak of DPPP is 459 nm (λex= 260 nm). The emission characteristic of complex 1 is similar to that of the free DPPP ligand, which indicates that intraligand excitation is responsible for the emission of 1. Comparison of emission spectra of DPPP and complex 1 reveals that the emission peak of complex 1 exhibits a red-shift with 16 nm, which mainly originates from the ligand-to-metal charge transfer[32].

Fig.6. Emission spectra of DPPP ligand and complex 1 at room temperature

4 CONCLUSION

In summary, we successfully synthesized a new Ag(I)- based CP 1 from the solvothermal reaction of CF3COOAg with DPPP and PhPO3H2ligands in mixed organic solvent at 65 °C. A four-membered ring with Ag2O2and an eight- membered ring with Ag2O4P2were observed in the structure of complex 1. Moreover, complex 1 shows good stability and photoluminescent property. This work may further enrich the family of Ag(I)-based coordination polymers .