Synthesis and Crystal Structure of a New Pr(III)–K(I) Heterometallic Oxalate Coordination Polymer①

HONG Dong-Feng YIN Dng-Yue WANG Jun-Ru② DAI Jing-Kun

a (College of Science, Northwest A&F University, Yangling 712100, China)

b (College of Chemistry and Chemical Engineering,Luoyang Normal University, Luoyang 471022, China)

1 INTRODUCTION

Over the past few years, the design and synthesis of heterometallic coordination polymers with novel structures have attracted much attention because of their potential applications in many fields such as photoluminescence, electricity, magnetics, catalysis,gas storage, molecular recognition, etc[1-8]. To date,the construction of 3d-4f heterometallic coordination polymers based on lanthanide and transition metal ions continue to receive much interest in the context of crystal engineering[9-14]. By contrast, the introduction of alkali metals into lanthanide complex systems is still rare and seems significant for their various topologies and promising applications in non-liner optics, ion-exchange, molecular recognition and so on[15-17].

Among varieties of polycarboxylate ligands, as the simple and good bridging ligand, the oxalate anion can adopt variable coordination modes such as monodentate, bidentate, and combinations of both mono- and bidentate to fulfil the structural diversities of the final frameworks[18-20]. Moreover, it is able to bridge the homo-metal and hetero-metal complexes. To the best of our knowledge, the reported lanthanide-alkali-heterometallic frameworks involving Pr(III) and K(I) cations with oxalate anions have never been reported up to now.

In this paper, we report the synthesis and structure of a new heterometallic coordination polymer,[K2Pr2(C2O4)4·H2O]n, in which oxalates exhibit three different coordination behaviours. Simultaneously,thermal property of the complex is also described in detail.

2 EXPERIMENTAL

All reagents were obtained commercially and used without further purification. The hydrothermal reaction was performed in a 25 mL Teflon-lined autoclave under autogenous pressure. Elemental analyses for C, H, and N were carried out on a Flash 2000 elemental analyzer. The IR spectra were recorded as KBr pellets on a Nicolet Avatar-360 spectrometer in the range of 4000～400 cm-1.Thermogravimetric analyses (TGA) were carried out on a SDTQ600 thermogravimetric analyzer. A platinum pan was used for heating the sample with a heating rate of 10 ℃/min under a N2atmosphere.

2.1 Synthesis of the complex

[K2Pr2(C2O4)4·H2O]n(1)A mixture of 3,4-thiophenedicarboxylic acid (17.2 mg, 0.1 mmol), K2C2O4·H2O (18.4 mg, 0.1 mmol)and Pr(NO3)2·6H2O (43.5 mg, 0.1 mmol) in distilled water (8 mL) was placed in a Teflon-lined stainless steel vessel, heated to 180 ℃ for 7 days, and then cooled to room temperature. Light green block crystals of 1 were collected (yield: 13% based on Pr).Elemental analysis calcd. (%) for C8H2K2O17Pr2: C,13.15; H, 0.27. Found (%): C, 13.10; H, 0.31. IR data (KBr, cm-1): 3356(br), 3221(w), 2915(w),1594(s), 1543(w), 1488(m), 1310(s), 1263(m),1102(w), 1030(w), 971(w), 872(w), 810(m), 741(w),678(w).

2.2 X-ray structure determination

The structure of 1 was determined by single-crystal X-ray diffraction technique. Diffraction data were collected on a Bruker SMART Apex CCD diffractometer with Mo-Ka radiation (λ = 0.71073 ?) at 293 K using an ω scan mode. Data reduction and absorption correction were made with SADABS software[21]. The structure was solved by direct methods and refined by full-matrix least-squares techniques using SHELXL-97[22]. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were placed at the calculation positions. Selected bond distances and bond angles of 1 are summarized in Table 1. Crystal data for 1: monoclinic, space group P21/n, a =12.060(4), b = 8.493(3), c = 15.776(5) ?, β =93.235(3)°, V = 1613.4(9) ?3, Z = 4, Mr= 730.12, Dc= 3.006 Mg/m3, μ = 6.581 mm-1, F(000) = 1368, the final R = 0.0283 and wR = 0.0866 for 2939 observed reflections with I > 2σ(I) and R = 0.0312 and wR =0.0891 for all data. (Δρ)max= 0.673 and (Δρ)min=–2.250 e/?3. The goodness-of-fit indicator (S) is 1.065.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

Symmetry transformation: #1: –x+1, –y+1, –z; #2: –x+1, –y+2, –z; #3: x, y+1, z #4: x–1/2, –y+3/2, z+1/2; #5: –x+1,–y+3, –z+1; #6: –x+3/2, y+1/2, –z+1/2; #7: –x+3/2, y–1/2, –z+1/2; #8: x+1/2, –y+3/2, z–1/2; #9: x, y–1, z

3 RESULTS AND DISCUSSION

Single-crystal X-ray structural analysis reveals that 1 crystallizes in the monoclinic space group P21/n. There are two independent Pr(III) [Pr(1) and Pr(2)] ions, two independent K(I) ions [K(1) and K(2)], three and two half oxalate anions and one coordinated water molecule in the asymmetric unit.An ORTEP view of 1 is shown in Fig. 1, the Pr(1)and Pr(2) cations are nine-coordinated with distorted tricapped trigonal prism geometries: nine O atoms from four chelate oxalate anions and one monodentate oxalate anion. The Pr–O bond lengths range from 2.473(5) to 2.654(5) ? with an average value of about 2.5273 ?, which are within the normal ranges found in other Pr(III) complexes[23]. Owing to the larger atomic radius and lesser d electrons, the K(I) ion can also bond with the oxygen atoms easily and show a higher coordination number. In complex 1, there are two kinds of coordination environments for K(I) ions. K(1) ion is eight-coordinated and is defined by eight oxalate oxygen atoms from four oxalate anions. On the other hand, the K(2) ion is seven-coordinated by six oxygen atoms from five oxalate anions and one coordinated aqua ligand. The K–O bond lengths range from 2.356(6) to 3.212(6)?, with an average value of about 2.6937 ?.Interestingly, oxalates in 1 exhibit a surprising efficiency toward the Pr(III) and K(I) ions because it coordinate through all of its carboxylate-oxygen atoms, and finally provide three coordination modes(Fig. 2). To the best of our knowledge, the simultaneous presence of three different coordination modes only using one simple ligand in the coordination polymers is rather rare.

Fig. 1. View of the Pr(III) and K(I) ions on coordination environment. Symmetry codes:#1: –x + 1, –y + 1, –z; #2: –x + 1, –y + 2, –z; #3: x, y + 1, z; #4: x – 1/2, –y + 3/2, z + 1/2; #5: –x + 1,–y + 3, –z + 1; #6: –x+ 3/2, y + 1/2, –z + 1/2; #7: – x+ 3/2, y – 1/2, –z + 1/2; #8: x + 1/2, –y + 3/2, z – 1/2

Fig. 2. Coordination modes of the oxalate anion(a) μ5-?1, ?2, ?2, ?2. (b) μ4-?1, ?1, ?2, ?2. (c) μ4-?2, ?2, ?2, ?2

It is shown that oxalates [C(1)～C(2)] and oxalates [C(6)～(C7)] both adopt similarly complicated pentadentate chelating and bridging μ5-?1, ?2, ?2, ?2coordination modes to bond two Pr(III) cations and three K(I) cations, and form a zigzag 1D chain along the b axis, respectively (Fig. 3a and Fig. 3b). And then, these chains generate an infinite 2D polymeric layer along the ab plane by sharing K(I) cations (Fig.3c). Oxalates [C(3)～C(4)] also func- tion as μ5-?1,?2, ?2, ?2pentadentate ligands to bridge adjacent and parallel layers and produce an interesting 3D structure (Fig. 3d). While, oxalates [C(5)～C(8)]directly acting as μ4-auxiliary sup- porting bridges(μ4-?1, ?1, ?2, ?2vs μ4-?2, ?2, ?2, ?2) further connect and consolidate the 3D structure and finally form a dense polymeric network (Fig. 4). Furthermore,hydrogen bonds among coordinated water molecule and carboxylate groups of oxalate in complex 1 are formed: O(17)–H(1W)···O(3) and O(17)–H(2W)···O(12).

Fig. 3. (a) View of the infinite 1D chain built by oxalate [C(1)～C(2)]. (b) View of the infinite 1D chain constructed from oxalate [C(6)～C(7)]. (c) View of the 2D layer structure. (d) View of the 3D net

Fig. 4. 3D coordination network structure for complex 1

During the synthesis of 1, it is noteworthy that the addition of a small amount of 3,4-thiophenedicarboxylic acid is very important to obtain the heterometallic framework, although 3,4-thiophenedicarboxylic acid was not present in 1. When 3,4-thiophenedicarboxylic acid was removed from the reaction system, attempts to make 1 were unsuccessful under the same conditions. Complex 1 was insoluble in water and common organic solvents such as methanol, ethanol, toluene, acetonitrile and N,N-dimethylformamide. Thermogravimetric (TG) analyses of complex 1 were performed on polycrystalline samples under a N2atmosphere (Fig. 5a). The TG analysis of 1 exhibits the weight loss occurs from 50 to 110 ℃, corresponding to the release of coor-起dinated water molecule (2.47% calculated, 2.53%observed). No weight loss is observed between 110 and 397 ℃, indicating that during this temperature range, the composition of the dehy- drated product remains stable. Above 397 ℃, the whole framework begins to decompose. In addition, powder X-ray diffraction (XRD) technique has been used to check the phase purity of the bulky samples in the solid state. The measured XRD patterns closely match the simulated patterns generated from the results of single-crystal diffraction data (Fig. 5b), indicative of pure products. The dissimilarity in intensity may be due to the variation in the preferred orientation of the powder sample during the collection of experimental PXRD data.

Fig. 5. (a) TGA curve for 1.(b) PXRD patterns of 1 simulated from X-ray single-crystal diffraction data and experimental data

4 CONCLUSION

In conclusion, a new 3D heterometallic oxalate coordination polymer has been synthesized and characterized. The complex is the first example of Pr(III)–K(I) heterometallic coordination polymer constructed only from the simple organic oxalic acid ligand. Interestingly, oxalates exhibit three coordination behaviors, which further enrich the coordination diversity of oxalic acid. Furthermore, thermogravimetric analysis and powder X-ray diffraction properties of the complex are also investigated.

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