Synthesis and Crystal Structureof aNew Charge-assisted Hydrogen-bonded Host Framework [Co(en)3]2[Zr2(C2O4)7]·2H2O①

HAN Yi-De ZHANG Jing LIU Ning-Ning WANG Yu ZHANG Xi②

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Synthesis and Crystal Structureof aNew Charge-assisted Hydrogen-bonded Host Framework [Co(en)3]2[Zr2(C2O4)7]·2H2O①

HAN Yi-DeaZHANG JingaLIU Ning-NingbWANG YucZHANG Xiaa②

a(110819)b(113001)c(130012)

Using the deep eutectic solvent formed of oxalic acid and choline chloride, a new charge-assisted hydrogen-bonded host framework[Co(en)3]2[Zr2(C2O4)7]·2H2O (1) has been obtained. The title complex crystallizes in the monoclinic, space group21/(No. 14) with= 7.7448(10),= 14.5683(19),= 19.375(3) ?,= 92.124(2)o,= 2184.5(5) ?3,= 4,D= 1.996 g.cm?3,(000) = 1332,=1.328 mm-1,= 0.0353 and= 0.0718 (> 2()). Single-crystal structure analysis reveals that the title complex possesses a 3D network assembled through a multitude of charge-assisted hydrogen bonds between the in situ generated anionic coordination complexes [Zr2(C2O4)7]6-and metal complexes Co(en)33+.

deep-eutectic solvent, metal complex, hydrogen-bonded host framework;

1 INTRODUCTION

Hydrogen-bond host frameworks have been widely investigated owing to their designable struc- tures, unusual flexibility, and tunable functions[1-9].But the rational design and construction of such structures are still a challenging task. One of the most powerful strategies available to design such architectures uses a preformed coordination com- pound (i.e., metal complex or anion coordination complex) as a molecular building block.Currently, [Co(en)3]3+complexes as templates were used for the synthesis of porous materials and supramolecular networks[5-11]. These architectures with flexible skeletons are held together by multiple hydrogen bonds and electrostatic interactions. On the other hand, the use of anion coordination complexes [Zr(C2O4)4]4-as molecular building blocks is also studied in the formation of charge-assisted hy- drogen-bond architectures. The outer oxygen atoms of tetrahedral [Zr(C2O4)4]4-units act as efficient hydrogen-bond acceptors on the construction of supramolecular architectures[12-15]. However, the assembly of [Co(en)3]3+and [Zr(C2O4)4]4-units has rarely been reported.

Ionothermal synthesis, using ionic liquids or eutectic mixtures as solvent and sometimes struc- ture-directing agent, is currently receiving great attention to prepare novel porous solids. Deep- eutectic solvent (DES), formed of quaternary ammo- nium salts (QAS) and oxalic acid, can be used to prepare hitherto unknown metal-organic frameworks (MOFs)[16-18]. With the aim of exploring new charge-assisted hydrogen-bond host frameworks, we report herein a new supramolecular network, [Co- (en)3]2[Zr2(C2O4)7]·2H2O (1), which consists of the in situ generatedanionic coordination complexes[Zr2(C2O4)7]6-and the cationic metal complexes assembled via hydrogen bonds.

2 EXPERIMENTAL

2. 1 Materials and instruments

The elemental analysis was performed by Perkin- Elmer 2400 elemental analyzer;The crystal data were collected by a Bruker AXS SMART APEX II diffractometer; IR spectra were taken on a Perkin- Elmer spectrum One FT-IR spectrometer in the 4000～400 cm-1region with KBr pellets. All chemicals purchased were of reagent grade and used without further purification.

2. 2 Synthesis of the title complex

A 20 mL vial was charged with DES made from oxalic acid dihydrate (1.0 g, 7.94 mmol) and choline chloride (1.0 g, 12.25 mmol), ZrOCl2·8H2O (0.322 g, 1mmol), and Co(en)3Cl3(0.175 g, 0.5mmol). The vial was heated at 100 ℃ for 3 days. The sample was cooled to room temperature, washed with water, and air-dried. Finally, the orange block-shaped crystals of the title complex were obtained. Anal. Calcd. (%) for 1: C, 23.76; H, 3.96; N, 12.79. Found (%): C, 22.92; H, 4.02; N, 11.83.

2. 3 Crystal structure analysis

A single crystal with dimensions of 0.30mm × 0.3mm × 0.2mm was chosen for data collection which was performed on a Bruker AXS SMART APEX II single-crystal diffractometer equipped with a graphite-monochromatic Mo-radiation (= 0.071073 nm) using an-scan mode at 296(2) K. In the range of 1.75≤≤28.34°, a total of 15671 reflections were collected, of which 5440 (int= 0.0475) were independent, and 4013 were observed with> 2() and used to solve the structure. The final refinement converged at= 0.0598,= 0.080776 (= 1/[2(F2) + (0.0335)2+ 0.3770], where= (F2+ 2F2)/3),= 1.002, (Δ)max= 0.718 and (Δ)min= –0.512 e/?3. Software SHELXL-97 was adopted to analyze and refine the crystal struc- ture[19]. Anisotropic thermal parameters were as- signned to all non-hydrogen atoms. The hydrogen atoms of cobalt complex and water molecule were located by difference Fourier map.

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of the title complex

The title complex 1 crystallizes in the monoclinic space group21/with= 4. The asymmetric unit contains half of a [Zr2(C2O4)7]6-anoion, an octahe- dral [Co(en)3]3+cation, and one water molecule. In the unit [Zr2(C2O4)7]6-withsymmetry, the two equivalent zirconium ions are connected to each other by a bridging oxalate, and the three remaining oxalate ligands are surrounding each zirconium (Fig. 1). The Zr–O bond lengths are 2.140(2)～2.259(2) ?, and the O–Zr–O angles are 70.89(7)～145.30(7)o. In the final architecture, there are 30 H-bonds between the amine groups of en ligands of metal complex cationsas hydrogen-bond donors andthe oxygen atoms of a Zr2(C2O4)76-unitas hydrogen-bond acceptors (N···O = 2.895(4)～3.454(4) ?). Water molecule forms 2 H-bonds to the oxygen atoms from two different Zr2(C2O4)76-units and one H-bond to amine groups from the en ligands. The selected bond lengths and bond angles are listed in Table 1and the related hydrogen bonds are given in Table 2.

Each Zr2(C2O4)76-is linked to ten [Co(en)3]3+cations (Fig. 2a), whereas each [Co(en)3]3+is con- nected to five bis-Zr units via hydrogen bonds. The structure can be more easily understood by taking into account five Zr2(C2O4)76-anionic and one [Co(en)3]3+cationic modules as elemental unit. Thus, the architecture can be taken as a continuation of such units in theplane leading to a 2D layer, and then these layers are further linked through hydrogen bonding to construct a 3D network. As shown in Fig. 2b, the [Co(en)3]3+cationic andZr2(C2O4)76-anionic modules are interconnected via hydrogen bonds inthe title complex1 developing a 3D network.This network shows narrower channels running along the [100] direction. However, the channels are occluded by the methylene groups of en ligands. Compared with the previous study, the [Co(en)3]3+cationic modules are introduced into the association. The [Co(en)3]3+cationic modules not only have the number of hydrogen available for bonding, but also offer more charges to balance the negative charges of Zr2(C2O4)76-anionic modules. These results illustrate that the overall structure is held together by multiple charge-assisted hydrogen bonds and elec- trostatic interactions.

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

Symmetry transformation: a: –, –, –

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°)

Symmetry codes: (a) –+2, –, –; (b),+1,; (c) –+1, –+1, –; (d)–+2, –+1, –; (e)–+5/2,+1/2, –+1/2; (f)–+3/2,+1/2, –+1/2

Fig. 1 . ORTEP view of the title complex (50% thermal ellipsoids)

Fig. 2 . (a) One [Zr2(C2O4)7]6- unit interacts with ten complex cations through hydrogen bonds. (b) Network of the title complex viewed along the [100] direction

3. 2 IR spectrum of the title complex

The bands of NH2and CH2groups are observed at 3134, 808, 1387 mm-1and 1269, 1152, 890 mm-1, respectively, and a C–N stretching band at 1070 cm-1. These bands were characteristic of cobalt ammine complex. The bands of 1686 and 1650 cm-1are characteristic of oxalic acid.

3. 3 Thermogravimetric analysis

Thermogravimetric analysis was carried out with a Perkin-Elmer TGA 7 unit instrument in air from 25 to 800 ℃ at a heating rate of 10 ℃·min–1, as shown in Fig. 3. A major weight loss of 68.23% occurred at 200～800 ℃, which is attributed to the decomposi- tion of the cobalt complexes and the loss of oxalate and water molecule.

3. 4 UV/vis spectrum

The band gap was investigated by UV/vis spec- trum measurement. As shown in Fig. 4, the value of Eg (band-gap energy) for the title complex 1 is 2.76 eV, which indicates that complex1 may have the degradation ability of organic dyes.

Fig. 3 . TG curve of the title complex

Fig. 4 . αhν2vs. hν curve of the title complex

4 CONCLUSION

In summary, using oxalic acid and choline chloride as the ionic deep-eutectic solvent, a new 3D hydrogen-bonded supramolecular framework Co(en)3]2[Zr2(C2O4)7]·2H2O (1), consisting of the in situ generated anionic coordination complexes [Zr2(C2O4)7]6-and metal complexes Co(en)33+, as- sembled through multiple hydrogen bonds. Intere- sting, compound 1may have the degradation ability of organic dyes. Further investigation of such kind of photocatalysts is ongoing.

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8 October 2014; accepted 18 December 2014 (CCDC 995428)

① This work was supported by the National Natural Science Foundation of China (Nos. 21301024, 21103017) and the Fundamental Research Funds for the Central Universities (N120305003)

. E-mail: xiazhang@mail.neu.edu.cn

10.14102/j.cnki.0254-5861.2011-0529