Syntheses, Structure and Properties of a Strontium(II) Complex with 5-Sulfoisophthalic Acid Monosodium①

FENG Jin-Hu WANGXio-Feng ZHANG Zhi-Qing WU Gng②

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Syntheses, Structure and Properties of a Strontium(II) Complex with 5-Sulfoisophthalic Acid Monosodium①

FENG Jian-HuaaWANGXiao-FengbZHANG Zhi-QiangaWU Ganga②

a(239012)b(211171)

A new Sr(II) complex, [Sr(Hsip)(H2O)] (1, NaH2sip = 5-sulfoisophthalic acid monosodium), has been synthesized by solvothermal reaction of SrCO3and NaH2sip at 120 ℃, and characterized by single-crystal X-ray diffraction studies, elemental analysis and FT-IR. Single- crystal X-ray diffraction reveals that compound 1 has a 3D architecture, and there is only one crystallographically independent Sr(II) ion in 1. The coordination geometry of Sr(II) is a distorted tetragonal anti-prism. The whole Hsip2-ligand performs a6-coordination model. In the solid state, complex 1 shows luminescence with the maximum emission intensity at 417 nm upon excitation at 320 nm. Thermal stability of complex 1 was also investigated.

Sr(II) complex, sulfonate group, luminescence property, thermal stability;

1 INTRODUCTION

Over the past decades, scientists have paid an amount of attention in design and synthesis of metal- organic frameworks (MOFs) in supramolecular chemistry and crystal engineering owing to their unique architectures[1, 2]and potential applications in heterogeneous catalysis[3-5], luminescence, magne- tism, catalysis, gas storage and separation[6, 7]. In the process of self-assembly of MOFs, the coordination preference of the ligands and the coordination geometry of metal centers are generally the primary considerations. Rational selection of ligands and metal ions is an effective way to construct metal- organic frameworks with novel structures and pro- perties[8, 9]. Those ligands containing carboxylate group have been widely used to construct metal- organic frameworks because they can adopt a variety of coordination modes, such as bidentate bridging, bidenate chelating, tridenate bridging and so on, resulting in diverse multidimensional architectures, and giving rise to various metal-organic frameworks (MOFs) with specific topologies and useful pro- perties[10-14].

Generally, alkaline earth metals and transition- metal ions differ in their coordination numbers and coordination geometries, which exhibit much higher coordination numbers and more flexible coordi- nation geometries, resulting in their easy generation of structural flexibility and devise structures. Furthermore, some alkaline earth complexes were found to exhibit interesting properties. Therefore, it is a good strategy to use alkaline earth metals and carboxylic acids as building blocks to construct metal-organic frameworks[15, 16]. On the other hand, in the designed synthesis of alkaline earth complexes, 5-sulfoisophthalicacid sodium (NaH2sip) is an excellent aromatic ligand, which can not only afford two carboxylate groups, but also one sulfonate group, supplying with seven O coordination sites. Therefore, it has strong potential to act as a bridging ligand. Herein, we report the synthesis and structural cha- racterization of one new three-dimensional complex [Sr(Hsip)(H2O)] (1). The luminescence property in the solid state at room temperature and thermo- gravity of 1 were investigated.

2 EXPERIMENTAL

2. 1 General procedures

All commercially available chemicals were of reagent grade and used as received without further purification. Infrared (IR) spectra were recorded on a Nicolet 6700 FT-IR spectrophotometer by using KBr disks in the range of 4000～400 cm–1. C, S and H analyses were made on a Perkin-Elmer 240C elemental analyzer. Thermogravimetric analyses (TGA) were carried out with a SDT Q600 instrument under 100.0 mL/min flowing nitrogen at a heating rate of 20.00 ℃/min from room temperature to 800 ℃. The luminescent spectra for the solid samples were recorded at room temperature on a CaryEclipse 300 spectrophotometer with a xenon arc lamp as the light source. In the measurements of the emission and excitation spectra, the pass width was 5.0 nm. Powder X-ray diffraction patterns were performed with a Bruker D8 ADVANCE X-ray diffractometer with Cu-radiation at 40 kV and 40 mA.

2. 2 Preparation of [Sr(Hsip)(H2O)] (1)

A mixture of 5-sulfoisophthalicacid sodium (NaH2sip: C8H5NaO7S, 26.8 mg,0.1 mmol) and SrCO3(8.9 mg, 0.06 mmol) in distilled water (5 mL) was stirred at room temperature for about 10 min. Afterwards, CH3OH(6 mL) was added to the clear solution and sealed in a 25 mL Teflon-linedstainless-steel container, which was heated at 120 ℃ for 3 days.After the sample was cooled to room temperature, colorless rod crystalswere obtained (yield 6.0 mg). Anal. Calcd. (%) for compound 1 (C8H6O8SSr): C, 27.47; H, 1.73; S: 9.17. Found (%): C, 27.51; H, 1.67; S: 9.19. FT-IR (KBr pellet, cm-1): 3631(m), 3545(m), 1708(s), 1611(s), 1561(s), 1435(m), 1395(s), 1282(s), 1228(s), 1115(m), 1063(m), 888(w), 796(w), 781(m), 750(m), 693(m), 673(m), 628(s), 529(w).

2. 3 X-ray crystallography

The crystal collection for complex 1 was carried out on a Bruker CCD area detector at room tempera- ture, using graphite-monochromated Mo-radia- tion (= 0.71073 ?). The structure of complex 1 was solved by direct methods, and then refined anisotropically with SHELXTL-97 using a full- matrix least-squares procedure based on2values[17, 18]. The non-hydrogen atoms were located from the trial structure. A total of 15486 reflections were collected in the range of 2.34≤≤27.56o by using an-scan mode, of which 2405 were unique withint= 0.0918 and 1798 were observed with> 2(). This complex crystallizes in monoclinic, space group21/with= 7.2189(4),= 17.3962(10),= 9.0953(5) ?,= 109.708(4)o,= 4, C8H6SrN4O8S,= 349.81,= 2.161 g/cm3,(000) = 688 and= 5.241 mm-1, the final= 0.0403 and= 0.0939 for 1798 observed reflections with> 2() and= 0.0596 and= 0.0990 (= 1/[2(F2) + (0.0528)2+ 0.0300], where= (F2+ 2F2)/3) for all data, and= 0.981.The selected bond lengths and bond angles are listed in Table 1.

3 RESULTS AND DISCUSSION

3. 1 IR spectrum of [Sr(Hsip)(H2O)] (1)

In the IR spectrum of 1, the(O–H) stretching frequency of a broad band around 3400 cm-1suggests the existence of coordinated H2O mole- cule(s). A carboxyl group of H2sip-1is protonated since a strong band around 1708 cm-1for -COOH is observed. The characteristic vibration bands at 1228 and 1115 cm-1are attributed to theas(SO3) vibra- tion[19]. IR spectrum of 1 shows characteristic bands of carboxyl groups at 1611 and 1561 cm-1for the asymmetric stretching vibration and 1435 and 1,395 cm-1for symmetric stretching vibration[20]. The difference of 166 cm-1for the asymmetric stretching and symmetric stretching (Δasym– Δsym) indicates that the carboxylate group in Hsip2-anions is coordinated to the Sr(II) centers in a bridging mode[21, 22]. The value of Δ(as(-COO-) –s(-COO-)) amounts 216 cm-1for 1, also suggesting that there is a carboxylate group coordinated to the metal ion in an unidentate mode[21, 23, 24], which is in consistent with the crystal structure of 1.

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

Symmetry transformation: #1: 2 –, –, 1 –

3. 2 Crystal structure of 1

The structure of complex 1 is a 3network. X-ray analysis reveals that compound 1 crystallizes in monoclinic space group21/. The asymmetric unit consists of an independent Sr(II) cation, a Hsip2-anion, and a coordinated water molecule (The O(8) atom is disordered). As shown in Fig. 1, an inde- pendent Sr(II) is coordinated by eight oxygen atoms from six different Hsip2-anions and one water molecule. The coordination geometry of the central Sr(II) ion is a distorted tetragonal anti-prism. The Sr–O bond lengths are in the range from 2.496(3) to 2.796(16) ? and the O–Sr–O bond angles fall in the 48.24(8)～163.38(10)orange(Table 1), which are within the ranges reported for Sr(II) complexes[25, 26].

Fig. 1 . ORTEP view of coordination environment of Sr(II) atom in 1 with 50% probability displacement. The hydrogen atoms are omitted for clarity (O1B: #1: 2 – x, – y, 1 – z)

In complex 1, there is only one unique kind of coordination mode of the ligand in this structure. However, two carboxylate groups of Hsip2-adopt two different coordination modes (Scheme 1). One takes a2-2:1bridging coordination model, while the other exhibits a1-1:0monodentate coordina- tion model. The dihedral angles between the central benzene ring and two carboxylate groups are 13.0o (2-2:1) and 11.0o (1-1:0), respectively, indica- ting there may be relevance about the conformation and coordination models. The sulfonate group acts as a3-bridge coordinating with three different Sr(II) ions, in which each oxygen of the sulfonate group takes a monodentate coordination mode (Scheme 1). Therefore, the whole Hsip2-ligand performs a6-coordination mode in 1 (Scheme 1). The Sr(II) ions are bridged by3-sulfonate groups, forming a one-dimensional ladder chain along the-axis, as illustrated in Fig. 2, with consecutive Sr···Sr dis- tances of 5.5555(7) and 5.8211(8) ?.

Scheme 1. Coordination model of Hsip2-

Further, the 1metallic chains are linked to gene- rate a 2network structure (Fig. 3) by carboxylate groups, which take a2-CO2bridging mode. In the 2network, the repeating units can be described as formed by [Sr4(CO2)2(SO3)2] and [Sr2(SO3)2]. Finally, the three-dimensional network with one- dimensional channel is generated by2-CO2and1-CO2(Fig. 4).

3. 3 Luminescent property of complex 1

The luminescent behavior of complex 1 was investigated at room temperature in the solid state. The emission spectrum is shown in Fig. 5. Complex 1 exhibits a luminescence emission at 417 nm upon excitation at 320 nm, which is similar to that of the ligand under the same excitation wavelength. The observed emission of complex 1 is probably due to* intraligand fluorescence since a similar emis- sion was also observed for the ligand itself.

Fig. 3 . Two-dimensional layer structure in 1

Fig. 4 . Three-dimensional network structure of 1

Fig. 5 . Emission spectrum of complex 1 in the solid state

3. 4 Thermogravimetric analyses

The thermal stability of complex 1 (Fig. 6) has been determined from room temperature to 800 ℃ in a N2atmosphere by thermogravimetric (TG). Complex 1 is stable up to 393 ℃, at which the H2O molecule begins to be completely removed. The experimental mass loss of 5.20% is consistent with the calculated value of 5.15% for the elimination of one H2O molecule. The solid residue formed between 393 ℃ and around 620 ℃ is suggested to be SrCO3and SrSO4because of the decomposition of ligand Hsip2-, releasing one benzene molecule. At higher temperature, from 626 to 646 ℃, the decomposition of SrCO3occurs with an endothermic effect. The final residue may be the mixture of SrOand SrSO4.

Furthermore, PXRD of 1 was performed, as indicated in Fig. 7. All the peaks of 1 in measured pattern match those in simulated pattern from single-crystal diffraction data, suggesting the as- synthesized crystals are pure. Dehydrationexperi- ment wasalso performed for 1 and powder X-ray diffraction (PXRD) was used to check the phases (Fig.7). 1 was heated at about 390℃ for 1 hto release water completely.Fig.7 shows the powdered dehydrated phase has a PXRD pattern almost the same as that of complex 1 (Fig.7), indicating that the framework of 1is stable after the removal of coordinated water.

Fig. 6 . TG curve of 1

Fig. 7 . XRD patterns of complex 1 simulated from single-crystal X-ray data, PXRD of 1 and the dehydrated

4 CONCLUSION

We have synthesized and structurally characteri- zed an alkaline earth complex [Sr(Hsip)(H2O)] (1). There is only one crystallographically independent strontium ion with eight-coordinateddistorted tetragonal antiprism in 1. In ligand Hsip2-, one carboxylate group takes a2-2:1bridging coor- dination model, while the other takes a1-1:0monodentate coordination model, and the sulfonate group acts as a3-bridge coordinating with three different Sr(II) ions. The whole Hsip2-acts as a6-coordination bridge to link Sr(II) ions to give rise to a 3network. We also investigated the thermal stability and luminescent properties of complex 1.

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8 October 2014; accepted27 November 2014 (CCDC 991485)

①This project was supported by the National Natural Science Foundation of China (41472047), Science Research Starting Project of Chuzhou University (2014qd035) and Student Creative Project of Chuzhou University

. Wu Gang, born in 1963, E-mail: wugangczu@163.com

10.14102/j.cnki.0254-5861.2011-0527