CHEN Shui-Sheng QIAO Rui

SHENG Liang-Quan YANG Song

(College of Chemistry & Chemical Engineering, Fuyang Normal College, Fuyang 236041, China)

1 INTRODUCTION

The design and synthesis of functional metalorganic frameworks (MOFs)by the self-assembly of organic ligands with appropriate functional groups and metal ions acting as connecting nodes have attracted considerable interest, because of their intriguing variety of topologies and potential applications[1-4]. In the process of self-assembly of MOFs,the coordination preference of the nature of ligands and the coordination geometry of metal centers are generally the primary considerations, and intermolecular non-covalent bonding interactions including hydrogen bonds[5-6], π··π stacking[7-8]and C–H··π[9]are also important factors in the construction of supramolecular frameworks. In this regard, a series of difunctional organic linkers with N and O donors combining anionic carboxylates and neutral 1H-imidazol-1-yl donors like 4-(1H-imidazol-1- yl)benzoic acid and 3,5-di(imidazol-1-yl)benzoic acid, are employed as good candidates for the con- struction of novel MOFs in our previous study[10-12]. Taking the favorable coordination ability of N/O- donor difunctional groups into account, we design a novel 1H-imidazol-4-yl group and carboxylate groupscontaining ligand (L). It is obvious that the L ligand possesses favorable coordination ability due to the carboxylate and 1H-imidazol-4-yl groups; moreover,the NH, N and O atoms of the difunc- tional groups as well as aromatic rings can act as hydrogen bonding donor or acceptor, contributing to the construction of supramolecular structures[13-14]. In this paper, we report the synthesis and crystal structure of a new supramolecular polymer built from noncovalent bonding interactions.

2 EXPERIMENTAL

2. 1 Materials and measurements

All the commercially available chemicals and solvents were of reagent grade and used as received without further purification. The ligand L was synthesized according to our previously reported literature[15]. Elemental analyses were performed on a Perkin-Elmer 240C Elemental Analyzer. IR spectra were recorded on a Bruker Vector 22 FT-IR spectrophotometer using KBr pellets. Thermogravimetric analyses (TGA)were performed on a simultaneous SDT 2960 thermal analyzer under nitrogen at a heating rate of 10 ℃/min. Power X-ray diffraction(PXRD)patterns were measured on a Shimadzu XRD-6000 X-ray diffractometer with CuKα (λ =1.5418 ?)radiation at room temperature.

2. 2 Synthesis of complex [Ni2(L)4(H2O)8](1)

A mixture of L (0.0188 g, 0.1 mmol)and Ni(NO3)2·6H2O (0.0290 g, 0.1 mmol)in 10 mL H2O was sealed in a 16 mL Teflon-lined stainless steel container and heated at 140 ℃ for 3 d. Green block crystals of 1 were collected with a yield of 45% by filtration and washed with water and ethanol for several times. Anal. Calcd. (%)for C20H22N4O8Ni: C,47.56; H, 4.39; N, 11.09. Found (%): C, 47.43; H,4.29; N, 11.21. IR(KBr): 3650～2530(m), 1608(s),1583(s), 1525(vs), 1460(m), 1410(vs), 1376(vs),1174(m), 1149(m), 1096(m), 967(w), 855(s), 835(s),786(s), 657(w), 628(w), 525(w), 508(w).

2. 3 Crystal structure determination

The green crystals of complex 1 were selected for diffraction data collection at 296(2)K on a Bruker Smart Apex II CCD diffractometer equipped with a graphite-monochromatic Mo-Kα radiation (λ =0.71073 ?). A total of 35372 reflections were collected for 1, of which 9380 (Rint= 0.0469)were independent in the range of 0.91≤θ≤27.47o for 1 by using a φ-ω scan mode. The structure was solved by direct methods with SHELXS-97[16]program and refined by full-matrix least-squares techniques on F2with SHELXL-97[17]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms of 1 were generated geometrically, expect that those (H(1B),H(2B), H(3A), H(4B), H(9B), H(10B), H(15B)and H(16A))of water molecules (O(1), O(2), O(3), O(4),O(9), O(10), O(15)and O(16))were located directly.The final R = 0.0452, wR = 0.1152 (w = 1/[σ2(Fo2)+(0.0480P)2+ 4.8227P], where P = (Fo2+2Fc2)/3), Rint= 0.0469, (Δ/σ)max= 0.000, S = 1.080,(Δρ)max= 0.804 and (Δρ)min= –0.569 e/?3for 1. The selected bond distances and bond angles for complex 1 are listed in Table 1.

Table 1. Selected Bond Lengths (?)and Bond Angles (°)of [Ni2(L)4(H2O)8]

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of 1

Single-crystal X-ray diffraction analysis revealed that complex 1 crystallizes in the monoclinic system with space group P21/c. As shown in Fig. 1, there are three kinds of mononuclear molecules consisting of one and two halves of crystallographically unique Ni(II)ions, four L ligands, and eight coordinated water molecules in the asymmetric unit. The Ni1 ion in 1 is located at a slightly distorted octahedral centre relating two N-bonded L ligands and four coordinated water molecules, forming a mononuclear molecule of [Ni1(L)2(H2O)4]while both of Ni2 and Ni3 ions are sitting on the inversion centers and have octahedral coordination geometry defined by four oxygen atoms from four different water ligands and two nitrogen donors from two different L ligands. The Ni–N (2.065(2)～2.083(2)?)and Ni–O (2.034(2)～2.085(2)?)distances are within the ranges observed for other octahedral complexes[18]. The bond angles around the nickel ion range from 86.86(9)to 180.000(1)o (Table 1). The uncoordinated carboxylate group of L deprotonates to act as anionic component to balance the positive charge of the metal ions. Meanwhile, the carboxylate moieties serving as a hydrogen bonding acceptor can effectively benefit the construction of supramolecular structures. As a result, abundant hydrogen bonds are present between terminal coordinated water molecules and carboxylate groups, as exhibited in Table 2. Interestingly, three kinds of mononuclear molecules are stacked into similar two-dimensional (2D)layers {Ni1–(L)2, Ni2–(L)2and Ni3–(L)2layers} by hydrogen bonds and C–H··π stacking interactions, respectively. Taking the Ni3–(L)2layers for example, the mononuclear Ni3(L)2(H2O)4units are linked together by this set of non-covalent forces like O–H··O hydrogen bonding interactions(O(16)··O(13)j2.666(3)?, O(16)–H(16)··O(13)175.00°, O(15)··O(14)i2.882(3)?, O(15)–H(15A)··O(14)158.00°, O(15)··O(14)j2.843(3)?,O(15)–H(15B)··O(14)176(4)°)to give a 2D framework (Fig. 2). It should be noteworthy that the 2D layer also exhibits non-covalent forces like C–H··π interactions between the benzene rings with H··centroid distance of 2.86 (2)?[19], which contributes to the stabilization of crystal packing of the 2D layer (Fig. 2). For the overall framework of 1, it can be seen clearly that the 2D layers repeat in the unit ··Ni1-(L)2, Ni2-(L)2, Ni1-(L)2, Ni3-(L)2·· stacking sequence along the c axis, and N?H··O, C?H··O and O?H··O hydrogen bonds of adjacent layers further link the 2D layer into a threedimensional (3D)supramolecular polymer (Fig. 3).

Fig. 1. Three different kinds of independent Ni(II)centre mononuclear molecules in the asymmetric unit (Symmetry codes: (A)1?x, 2?y, ?z; (B)?x, ?y, 2?z)

Fig. 2. 2D Ni3–(L)2 layer of 1 linked by hydrogen bonds indicated by a pink dashed line and C–H··π stacking interactions highlighted by green dashed line

Fig. 3. 3D supramolecular structure constructed from 2D stacking units in sequence of ··Ni1–(L)2, Ni2–(L)2, Ni1–(L)2, Ni3–(L)2·· by hydrogen bonds along the c axis

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

Symmetry codes: (a)x, –1/2–y, 1/2+z; (b)x, –1/2–y, –1/2+z; (c)x, 1/2–y, 1/2+z; (d)x, 1/2–y, –1/2+z; (e)x, –1+y, z; (f)–x,1/2+y, 3/2–z; (g)1–x, 1–y, 1–z; (h)–x, –y, 1–z; (i)x, 3/2–y, –1/2+z; (j)1–x, –1/2+y, 1/2–z; (k)x, 1+y, z; (l)1–x, 2–y, –z

3. 2 IR spectrum, thermal stabilities and powder X-ray diffraction of 1

The infrared spectrum of the complex has been recorded between 4000 and 450 cm-1and some important assignments are shown in the experimental section. The IR spectra exhibit strong absorption centered at 2530～3650 cm?1for 1, corresponding to the N?H/O?H stretching vibration of ligand or water molecule (see experimental section)[20]. Strong characteristic bands of carboxylic group are observed in the range of 1570～1620cm?1for asymmetric vibrations and 1400～1480 cm?1for symmetric vibrations, respectively. Therefore, the complete deprotonation of the carboxylic acid to give the corresponding carboxylate ligand in 1 was confirmed by crystal structural analysis (vide post)as well as the IR spectral data since no vibrational bands in the range of 1680～1760 cm-1were observed in the IR spectra of complex 1. The strong absorption band at 1608 cm-1is attributed to the asymmetric stretching vibration of C=O group,which is significantly smaller than that of the protonated carboxylate group monomer (1760 cm-1),indicating the delocalization of C=O double bond[21].

Complex 1 was subjected to thermogravimetric analysis (TGA)to ascertain the stability of supramolecular architecture, and the result is shown in Fig. 4.A total weight loss of 14.04% was observed for 1 in the temperature range of 75～215 ℃, which is attributed to the loss of water molecules (calcd.14.25%), and the residue is stable up to about 450 ℃.Powder XRD experiment was carried out to confirm the phase purity of bulk sample, and the experimental pattern of the as-synthesized sample can be considered comparable to the corresponding simulated one, indicating the phase purity of the sample(Fig. 5).

Fig. 4. TG curve of 1

Fig. 5. Simulated and experimental XRPD patterns of complex 1

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