LI Jie SHENG Tin-Lu HUANG Yi-Hui

FU Rui-Biaoa HU Sheng-Mina

WEN Yue-Honga WU Xin-Taoa②

a (State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

b (Graduate University of Chinese Academy of Sciences, Beijing 100049, China)

1 INTRODUCTION

Over the past decade, the rational design and assembly of coordination polymers have received remarkable attention due to their potential applications in the fields of luminescence, adsorption,magnetism, catalysis, ion exchange, conductivity,nonlinear optics, and so on[1-7]. In particular, the design and construction of three-dimensional (3D)coordination polymers are of great interest, not only for their potential applications as functional materials, but also for their structural beauty, often with more complicated architectures and gorgeous topologies than the 0D, 1D and 2D frameworks[8-11]. To date, a large number of coordination polymers have been prepared, and usually possess low-connected networks with three-, four- and six-connected topologies[12-15]. However, highly connected networks are still less investigated, owing to the limited coordination sites of single metal ion and the steric hindrance of organic ligands[16-21]. Recently, a successful approach to obtain highly connected frameworks is to build polynuclear metal clusters with the aid of dicarboxylate and/or exobidentate bridged ligands. Moreover, both the carboxylate and pyridyl ligands are widely investigated in coordination chemistry and compared to the single species because the combination of different ligands can better satisfy the coordination needs of metal centers.Lastly, unlike rigid ligands, the flexible molecules are easier to tune their spacial conformation to reduce the packing steric hindrance.

Taking all the above discussions into account, by introducing flexible H2OBA and DPA mixed-ligands,a rare highly 8-connected 3D Cd(Ⅱ)coordination polymer 1 has been obtained. The crystal structure,topological analyses and thermostability are reported in this paper.

2 EXPERIMENTAL

2. 1 Materials and methods

All the reagents and solvents were commercially purchased and used as received without further purification. Elemental analyses (C, H and N)were performed with a vario MICRO CHNOS elemental analyzer. The infrared spectra of KBr pellets were recorded in the range of 4000～400 cm-1on a Perkin-Elmer Spectrum One FT-IR spectrometer.Thermal analyses were performed on a NETZSCH STA 449C instrument from room temperature to 800℃ with a heating rate of 10 K·min-1under nitrogen flow.

2. 2 Syntheses

{[Cd(DPA)(OBA)]·(H2O)}nA reaction mixture of 4,4?-dipyridylamine (34.2 mg, 0.2 mmol),4,4?-oxybisbenzoic acid (51.6 mg, 0.2 mmol),Cd(NO3)2·4H2O (92.5 mg, 0.3 mmol), H2O (4.0 mL)and N,N-dimethylformamide (DMF, 10 mL)was stirred for 30 min at room temperature and then heated at 90 ℃ for 2 days. Colorless crystals of 1 were filtered off, washed with DMF, dried in air at room temperature, and collected with high yield of 64% based on the ligands. Anal. Calcd. (%)for CdC24N3O6H19: C, 51.67; H, 3.43; N, 7.53. Found(%): C, 51.39; H, 3.51; N, 7.82. IR (KBr, cm-1):3422(m, br), 3245(m), 3156(m), 3065(m), 1594(vs),1518(vs), 1399(vs), 1341(m), 1238(m), 1155(m),1010(m), 882(w), 846(w), 817(w), 781(w), 537(m).

2. 3 Structure determination

A colorless single crystal with dimensions of 0.60mm × 0.30mm × 0.25mm was collected on a Rigaku Saturn724+ diffractometer equipped with a graphite-monochromatic MoKα radiation (λ =0.71073 ?)at 293(2)K using an ω scan mode. A total of 24902 reflections were collected in the range of 2.14≤θ≤27.53°, of which 5140 (Rint= 0.0179)were independent and 4802 were observed with I ＞2σ(I). The structure was solved by direct methods with SHELXS-97[22]and refined with SHEXL-97[23].Non-hydrogen atoms were refined anisotropically,and all hydrogen atoms were placed at the ideal positions and allowed to ride. The structure was refined by full-matrix least-squares techniques. The final R = 0.0307 and wR = 0.0867 (w = 1/[σ2(Fo2)+(0.0479P)2+ 8.5966P], where P = (Fo2+ 2Fc2)/3), S= 1.058, (Δρ)max= 1.799, (Δρ)min= –0.438 and(Δ/σ)max= 0.075 e/?3. The selected bond lengths and bond angles for the complex are listed in Tables 1 and 2.

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

Table 2. Hydrogen Bond Lengths (?)and Bond Angles (°)for Complex 1

3 RESULTS AND DISCUSSION

3. 1 Crystal structure

{[Cd(DPA)(OBA)]·(H2O)}nSingle-crystal structure analysis reveals that complex 1 crystallizes in the monoclinic space group C2/c. As shown in Fig. 1a,the asymmetrical unit contains one crystallographically independent Cd(Ⅱ)ion, one DPA ligand,one OBA2-ligand and one-fourth free water molecule.The Cd(Ⅱ)atom is seven-coordinated in a distorted pentagonal bipyramidal geometry by five oxygen atoms from three OBA2-ligands with two carboxyl groups in a chelating model and one carboxyl goup in a bridging model, and two nitrogen atoms from two DPA ligands. The Cd–O distances range from 2.2676(17)to 2.4782(19)?, while the apical Cd(1)–O(2)cbond has a longer distance of 2.7383(17)? for Cd(1)and the Cd–N distances vary from 2.2719(17)to 2.3977(18)?. Notablely, the free water molecules,oxygen atoms of OBA2-and nitrogen atoms of DPA are involved in hydrogen bonding as donors or acceptors (N(2)–H(2A)··O(1)d; O(6)–H(6B)··O(5);O(6)–H(6C)··O(5)e, symmetry codes: d: 0.5–x,0.5–y, –z; e: 1.5–x, 1.5–y, 1–z, as listed in Table 2(Fig. 2a)). Two kinds of weak π-π interactions exist in the framework in offset face-to-face model with the centroid distances of 3.6717(8)and 3.8750(8)?,respectively, as shown in Fig. 2b. Herein, the two carboxyl groups of OBA2-liangds show different coordination models with a μ1-η1:η1chelating model(O(4)–C(14)–O(5))and a μ2-η1:η2chelating and bridging model (O(1)–C(1)–O(2)), forming a dinuclear (Cd2O2)SBU (secondary building unit)with a separation of 4.1492(13)? between the metal ions,as depicted in Fig. 1b.

Fig. 1. (a)Coordination environment of the Cd(II)ion in complex 1; (b)Dinuclear (Cd2O2)SBU. All hydrogen atoms have been omitted for clarity. Symmetry codes: (a)–x+1, y–1, –z+1/2; (b)–x+1/2, y–1/2, –z–1/2; (c)–x+1, –y, –z

Fig. 2. Black dotted lines showing the hydrogen bonds (a)and π-π interactions (b)of compound 1

Ignoring the OBA2-ligands, the DPA molecules connected to the Cd(Ⅱ)ions generated 1D left-and right-handed helixes with a pitch of 12.192 ? along the b axis (Fig. 3a). Inversely, the coordination of OBA2-groups to Cd(Ⅱ)atoms forms a two-fold interpenetrating 2D framework (Fig. 3b). On the whole, each dimeric SBU is surrounded by four OBA2-ligands and four DPA molecules. Meanwhile,each OBA2-and DPA ligands are connected to two dimmers, respectively. In accordance with the simplification principle, the dinuclear SBUs can be considered as eight-connected nodes and both the OBA2-and DPA groups can be simplified as lines.The overall structure of 1 can be described as a rare 3D 8-connected uninodal LOMFOI[24-25]network with a point symbol of {424·64} (Fig. 4). Therefore,the framework of compound 1 can be viewed as two interpenetrating (4, 4)-nets cross-linked by the 1D left-and right-handed helixes (1D + 2D → 3D).

Fig. 3. (a)Right- and left-handed helixes of DPA,(b)Two-fold interpenetrating (4,4)-nets of OBA2-

Fig. 4. Three-dimensional LOMFOI topology of compound 1

Fig. 5. TGA curve for the complex under N2 flow

3. 2 IR spectrum and thermal analysis

The infrared spectrum of the complex was recorded and some important assignments are shown in the experimental section. The strong broad peak at 3422 cm-1in complex 1 was assigned to the stretching vibration of hydroxyl, indicating the presence of water molecule in the compound. It is well-known that a band which is active in the IR spectra appears at ca.1700 cm-1(ν(C=O))when the carbonyl is hydrogen-bonded. The absence of IR spectral bands between 1680 and 1760 cm-1in complex 1 confirms no hydrogen atom located on the carboxyl group.The very strong band at 1594 cm-1in 1 was assigned to the asymmetrical stretching mode of COO-groups,while the shoulder at 1399 cm-1in complex 1 corresponded to the symmetrical COO-stretching modes. These IR results are coincident with the crystallographic structural analyses.

Thermal stability of complex 1 was examined by TG analysis under N2atmosphere in the temperature range of 30～800 ℃ (Fig. 5). TG curve reveals that the host framework of the complex begins to decompose after 400 ℃ with the weight loss of 3.5% (calcd. 3.2%), corresponding to the release of guest water molecules. In the range of 400～700 ℃,there is a quick weight loss course and the remaining weight (21%)indicates that the final residue may be CdO (calcd. 23%). Owing to the sublimation of CdO around 700 ℃, the TG curve continues to decrease after 700 ℃.

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