LU Zhi-Qiang LI Yan-Zhi HAO Chu RU Yi YANG Shu-Jian ZHANG Ning-Dan FU Yu-Qin② WU Wei-Long ZHOU Yan

a (Hubei Key Laboratory of Natural Products Research and Development, Key Laboratory of Functional Yeast (China National Light Industry), College of Biological and Pharmaceutical Sciences, China Three Gorges University, Yichang 443002, China)b (Luoyang Key Laboratory of Organic Functional Molecules, School of Chemistry & Chemical Engineering,Luoyang Normal University, Luoyang 471934, China)

ABSTRACT Two new complexes, namely [Cd2(L)(pycy)(Cl)]·2H2O (1) and [Cu(HL)(H2btac)]·2H2O, have been synthesized by the reaction of the corresponding metal salts with mixed ligands including 1-(1H-imidazol-4-yl)-4-(4H-tetrazol-5-yl)benzene (HL), 2,6-pyridinedicarboxylic acid (H2pycy) and 1,2,4,5-benzenetetracarboxylic acid (H4btac),and characterized by single-crystal X-ray diffraction, IR spectroscopy, elemental analysis and powder X-ray diffraction(PXRD). Complex 1 crystallizes in triclinic, space group Pwith a = 9.8938(7), b = 10.1662(7), c = 12.4520(8) Å, α =92.7070(10), β = 108.1310(10), γ = 105.3230(10)°, V = 1136.44(13) Å3, Z = 2, C17H12N7O6ClCd2, Mr = 670.59, Dc = 1.960 g/cm3, μ = 2.037 mm-1, S = 1.062, F(000) = 648, the final R = 0.0532 and wR = 0.1651 for 12156 observed reflections (I >2σ(I)). Complex 2 crystallizes in triclinic, space group Pwith a = 7.3058(8), b = 7.4890(8), c = 10.8937(12) Å, α =71.3810(10), β = 84.134(2), γ = 65.745(2)°, V = 514.68(10) Å3, Z = 1, C20H16CuN6O10, Mr = 563.94, Dc = 1.819 g/cm3, μ =1.139 mm-1, S = 1.187, F(000) = 287, the final R = 0.0314 and wR = 0.0807 for 3564 observed reflections (I > 2σ(I)).Single-crystal X-ray structural analysis reveals that complex 1 features a trinodal (3,3,4)-connected two-dimensional (2D)net layer with Schläfli symbol (4·6·8)(4·62·83)(62·8) while 2 has a 2D network with 44 sql topology. Moreover, the photoluminescent property of 1 and magnetic property of 2 were also discussed.

Keywords: synthesis, complex, luminescence, magnetic property; DOI: 10.14102/j.cnki.0254-5861.2011-3206

1 INTRODUCTION

Metal-organic frameworks (MOFs), as a class of crystalline materials, have received great attention in recent years, not only due to their intriguing architectures and intricate topologies, but also because of their interesting properties and possible applications as functional materials in numerous areas in fluorescence, gas adsorption/separation, electrochemistry, magnetic properties, catalysis, and so on[1-5]. The crystalline materials of ZIFs and IRMOFs possess permanently porous structures with high surface area and can be used to store energy gas and capture exhaust gases or toxic pollutants. Generally, the organic linkers and inorganic metal units have great effect on the properties of MOFs[6,7].However, the appropriate selection of solvents, pH, organic ligands, metal ions, and reaction time as well as the reaction temperature may play an important role in the self-assembly process of MOFs[8,9]. The organic ligands including multi-N donor or carboxylate ligands are two most employed building units to construct desired MOFs. Particularly, the proper selection of organic ligands with N-heterocyle and carboxylate groups, or mixed multi-N donor and carboxylate ligands would be a suitable route to modulate novel structures with expected properties[10,11]. For example, the MOFs consisting ofd10metal centers and conjugated organic molecules have excellent luminescence emission properties to be employed as photoluminescent materials to sense guest molecules[12,13]. Meanwhile, the MOFs comprised from paramagnetic Cu2+, Co2+, Mn2+and Ni2+ions and bridging carboxylate organic ligands have magnetic properties[14,15].Recently, some interesting MOFs have been successfully generated by the employment of mixed multi-N donor and carboxylate ligands, and this has gradually become an effective method to construct supramolecular frameworks with specific and anticipated physical properties[16,17]. As an extension of our previous work[18,19], we use the multi-N donor organic compound 1-(1H-imidazol-4-yl)-4-(4H-tetrazol-5-yl)benzene (HL), 2,6-pyridinedicarboxylic acid (H2pycy)and 1,2,4,5-benzenetetracarboxylic acid (H4btac) as mixed ligand to react with metal salts, aiming to build a new metal-organic framework. Here, we report the synthesis and crystal structure of two new coordination polymers[Cd2(L)(pycy)(Cl)]·2H2O (1) and [Cu(HL)(H2btac)]·2H2O (2)obtained by the reaction of HL, H2pycy, H4btac and corresponding metal salts under hydrothermal condition.

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. 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. Power X-ray diffraction (PXRD) patterns were measured on a Shimadzu XRD-6000 X-ray diffractometer with CuKα(λ= 1.5418 Å) radiation at room temperature. The fluorescent spectra were measured using a Perkin Elmer LS-55B fluorescence spectrometer. A Quantum Design SQUID MPMS-5 magnetometer was used to test the temperature-dependent magnetic property.

2. 2 Synthesis of complex [Cd2(L)(pycy)(Cl)]·2H2O (1)

A mixture of HL (21.2 mg, 0.1 mmol), H2pycy (16.7 mg,0.1 mmol), CdCl2·2.5H2O (0.023 g, 0.1 mmol) and NaOH(0.008 g, 0.2 mmol) in 12 mL H2O was sealed in a 25 mL Teflon-lined stainless-steel container and heated at 140 ℃ for 3 d. Colorless block crystals of 1 were collected with a yield of 62% by filtration and washed with water and ethanol for several times. Anal. calcd. (%) for C17H12Cd2ClN7O6: C,30.45; H, 1.80; N, 14.62. Found (%): C, 30.23; H, 1.71; N,14.75. IR(KBr): 3445～2818(m), 1629(m), 1605(s), 1569(m),1532(vs), 1508(m), 1409(vs), 1192(w), 1115(w), 871(s),839(m), 861(s), 778(s), 719(m), 696(w), 618(m), 515(m).

2. 3 Synthesis of complex [Cu(HL)(H2btac)]·2H2O (2)

A mixture of L (21.2 mg, 0.1 mmol), H4btac (25.4 mg, 0.1 mmol), Cu(NO3)2·3H2O (24.1 mg, 0.1 mmol), and NaOH(16.0 mg, 0.4 mmol) in 10 mL H2O was sealed in a 16 mL Teflon-lined stainless-steel container and heated at 120 ℃ for 3 d. Green block crystals of 2 were collected in 62% yield.Anal. calcd. for C20H16CuN6O10(%): C, 42.60; H, 2.86; N,14.90. Found: C, 42.36; H, 2.93; N, 15.03. IR (KBr pellet,cm-1): 3403(m), 1609(s), 1541(s), 1450(s), 1371(vs), 1198(vs),1139(s), 1069(m), 1041(s), 948(m), 829(s), 810(m), 781(m),722(m), 650(s), 619(s), 523(w), 449(w), 419(w).

2. 4 Crystal structure determination

The colorless crystals of complexes 1 and green 2 were selected for diffraction data collection at 296(2) K on a Bruker Smart Apex II CCD diffractometer equipped with a graphite-monochromatic MoKαradiation (λ= 0.71073 Å). A total of 12156 reflections were collected for 1 and 3564 for 2,of which 4154 (Rint= 0.0209) and 2378 (Rint= 0.0125) were independent in the ranges of 1.74≤θ≤25.50º and 1.97≤θ≤27.64º for 1 and 2 by using aυ-ωscan mode. The structure was solved by direct methods with SHELXS-97[20]program and refined by full-matrix least-squares techniques onF2with SHELXL-97[21]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms of 1 and 2 were generated geometrically. The finalR= 0.0523,wR= 0.1651(w= 1/[σ2(Fo2) + (0.1138P)2+ 4.9551P], whereP= (Fo2+2Fc2)/3),Rint= 0.0209, (Δ/σ)max= 0.000,S= 1.062, (Δρ)max=2.907 and (Δρ)min= -0.733 e/Å3for 1. The finalR= 0.0314,wR= 0.0807 (w= 1/[σ2(Fo2) + (0.0172P)2+ 0.7359P], whereP= (Fo2+ 2Fc2)/3),Rint= 0.0125, (Δ/σ)max= 0.003,S= 1.187,(Δρ)max= 0.43 and (Δρ)min= -0.48 e/Å3for 2. The selected bond distances and bond angles for complexes 1 and 2 are listed in Tables 1 and 2.

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of 1

The result of X-ray diffraction analysis revealed that complex 1 crystallizes in triclinicPspace group with the asymmetric unit consisting of two Cd(II) atoms, one deprotonated L-ligand, one completely deprotonated pycy2-,one Cl-, one coordinated and free lattice water molecules. As shown in Fig. 1, Cd(1) with a N2O2Cl donor set is five-coordinated by two oxygen (O(2), O(3)) atoms and one nitrogen (N(7)) atom from one pycy2-ligand, another nitrogen(N(1C)) atom from L-ligand, and one chloridion (Cl-), with the Cd(1)-O average bond distance of 2.379(5) Å, Cd(1)-N of 2.22(6) Å and Cd(1)-Cl of 2.46(2) Å (Table 1). The central Cd(2) atom lies in a distorted octahedral coordination environment with N2O4donor set, in which the equatorial plane contains O(1), O(2) and O(4B) from two distinct pycy2-ligands and N(5A) from a coordinated L-ligand, and atoms N(4) and O(5) from L-and water ligand occupy the axial positions with a N(4)-Cd(2)-O(5) angle of 176.0(2)° (Table 1). In 1, the HL and H2pycy ligands are deprotonated to be Land pycy2-anions, while Cl-coordinated with Cd(II) atoms acts as a counteranion to balance the positive charges of framework. Each pycy2-in 1 works as aμ3-bridge to link three Cd(II) atoms with two carboxylate groups adoptingμ2-η1:η1-bis-monodentate andμ2-η2:η1-bridging coordination modes, and the N atom in this molecule also participated in coordination with the Cd(II) atom. In this connection, the carboxylate ligands bridged the Cd(II) atoms to form a one-dimensional (1D) chain (Fig. 2a). And the L-anions further connect adjacent 1D chains to form a two-dimensional(2D) double layer (Fig. 2b). To gain better insight into the 2D framework structure, topological analysis by reducing the structure to a simple node-and-linker net was performed on 1.Each L-anion acts as three nodes to connect three Cd(II)atoms, and pycy2-ligands are neighbored by two Cd(II) atoms,and one L-ligand. Thus both L-and pycy2-can be regarded as 3-connectors. Meanwhile, each Cd(2) atom links two Lligands and two pycy2-ligands; hence, Cd(2) atom can be regarded as a 4-connector, and every Cd(1) atom can be viewed as 2-connector through the connection of one Lligand and one pycy2-ligand. Thereby, this framework is ascribed to be a trinodal (3,3,4)-connected 2D net layer with Schläfli symbol (4·6·8)(4·62·83)(62·8) (Fig. 3). Furthermore,the carboxylate group and the NH or N atom from L-ligands can be effective hydrogen bonding donor or acceptor in the construction of supramolecular structures[22]. As a consequence, rich hydrogen bonding interactions (N(6)···O(6)a1.94 Å, N(6)-H(6A)···O(6) 149°; O(6)···Cl(1)b2.40 Å, O(6)-H(6B)···Cl(1) 166°; O(6)···N(3) 2.09 Å, O(6)-H(6C)···N(3)166°; C(10)···O(3) 2.54 Å, C(10)-H(10B)···O(3) 126°) exist in the 2D structure (Fig. 4, Table 2), further reinforcing the stability of this complex.

Table 1. Selected Bond Lengths (Å) and Bond Angles (°) of [Cd2(L)(pycy)(Cl)]·2H2O and [Cu(HL)(H2btac)]·2H2O

Table 2. Hydrogen Bond Lengths (Å) and Bond Angles (°) for [Cd2(L)(pycy)(Cl)]·2H2O and [Cu(HL)(H2btac)]·2H2O

Fig. 1. Coordination environment of Cd(II) in complex 1 with ellipsoids drawn at 30% probability level.Hydrogen atoms were omitted for clarity (Symmetry codes: (A) -x, 2-y, -z; (B) -1+x, y, z; (c) -x, 1-y, -z)

Fig. 2. (A) 1D chain built from [Cd(pycy)(Cl)]n. (B) 2D net layer of [Cd2(L)(pycy)(Cl)]

Fig. 3. (A) 3,3,4-nodes for L-, pycy2- and Cd(2) atom, respectively.(B) Schematic representation of the trinodal (3,3,4)-connected 2D net layer of 1 with Schläfli symbol (4·6·8)(4·62·83)(62·8)

Fig. 4. View of the 3D supramolecular structure of 1 formed by hydrogen-bonding interactions

3. 2 Crystal structure of 2

Complex 2 crystallizes in the same triclinicPspace group with similar cell parameters and the asymmetric unit of 2 contains a half of unique Cu(II) atom, a half of partly deprotonated H2btac2-ligand, and half of HL ligand. The Cu(II) atom is sitting on an inversion center and has octahedral coordination geometry defined by four carboxylate oxygen atoms from two different H2btac ligands and two coordinated water molecules and two nitrogen donors from two different HL ligands (Fig. 5). The Cu-O bond lengths are 1.984(1) and 2.5393(2) Å, and the Co-N ones are 2.000(2) Å(Table 1). The partly deprotonated H2btac2-ligand connects Cu(II) atoms to form 1D chains using its two opposite carboxylate groups with aμ1-η1:η0-monodentate coordination mode. And the linear HL ligands bridge Cu(II) atoms to form 1D chains. In this context, two kinds of 1D chains are interconnected into a 2D structure layer with (4, 4) sql topology (Fig. 6). Similarly, rich hydrogen bonding interactions exist in the 2D structure and connect the 2D layers into a 3D supramolecular structure (Fig. 7).

Fig. 5. Coordination environment of Cu(II) in complex 2 with ellipsoids drawn at 30% probability level.Hydrogen atoms were omitted for clarity (Symmetry codes: (A) 1-x, 2-y, -z; (B) 2-x, 2-y, 1-z; (C) 1-x, 1-y, 1-z)

Fig. 6. (A) 2D network of complex 2. (B) 2D sql network for 2

Fig. 7. View of the 3D supramolecular structure of 2 formed by hydrogen-bonding interactions

3. 3 Thermal stabilities and powder X-ray diffraction of complexes 1 and 2

Complex 1 was subjected to thermogravimetric analysis(TGA) to ascertain the stability of the supramolecular architecture, and the result is shown in Fig. 8. A total weight loss of 5.72% was observed for 1 in the temperature range of 55～105 ºC, which is attributed to the loss of coordinated and lattice water molecules (calcd. 5.41%), and the residue is stable up to about 405 ºC. A loss of 6.19% (calcd. 6.32%) was found for 2 ranging from 65 to 125 °C for the loss of coordinated water molecules and then it reaches a continual collapse. Powder XRD experiment was recorded to investigate the phase purity of bulk sample, and the experimental patterns of the as-synthesized samples are considered to be well consistent with the corresponding simulated one calculated from the single-crystal diffraction data, indicating the phase purity of the sample (Fig. 9).

Fig. 8. Thermal analysis curve of complexes 1 and 2

Fig. 9. Simulated and experimental XRPD patterns of complexes 1 and 2

3. 4 IR spectrum and photoluminescent property

The infrared spectra of complexes 1 and 2 have been recorded between 4000 and 450 cm-1and some important assignments are shown in the experimental section. IR spectra exhibit strong absorption centered at 3445～2818 or broad peak around 3403 cm-1for 1 and 2, corresponding to the N-H/O-H stretching vibration of ligand or water molecule(see experimental section). Strong characteristic bands of carboxylate group are respectively observed in the ranges of 1629～1532 and 1609～1541 cm-1for 1 and 2 arising from asymmetric vibrations. Therefore, the complete deprotonation of the carboxylic acid to give the corresponding carboxylate ligand for two complexes was confirmed by crystal structural analysis (vide post) as well as the IR spectral data since no vibrational bands in the range of 1760～1680 cm-1were observed in the IR spectra of 1 and 2.

As is well known, the coordination compounds with rational selection and design of conjugated organic spacers and metal centers can be employed as new luminescent materials for potential applications in photochemistry and chemical sensors[23,24]. In this study, the solid state photoluminescent properties of the free HL ligand and complex 1 have been investigated at ambient temperature (Fig. 10). The free ligand HL exhibits a blue fluorescence emission band at 385 nm upon excitation at 336 nm, which can be assigned to ligand-centered electronic transitions, that is, the n →π*orπ→π*transition in nature according to the literature report[25].Upon excitation with 330 nm light, complex 1 displays fluorescence emission bands at 384 nm. The result reveals that the peaks of the emission spectra for 1 are very close to that of the corresponding free ligand HL, which can probably be attributed to the intraligand fluorescence originating from ligand-centered emission (Fig. 10)[26,27]. The studies of corresponding quantum yield (QY) and decay lifetimes were investigated for the crystalline material of 1, in order to further study the luminescence properties (Figs. 11 and 12).The QY value of complex 1 is 4.08%. In addition, the exponential function asI(t) = Aexp(-t/τ) can be employed to fit the luminescence decay curve. The luminescence lifetime of complex 1 is 16.12 ns, much shorter than the ones resulting from a triplet state (> 10-3s), so the emissions should arise from a singlet state[23]. Therefore, the good photoluminescence property of 1 indicates that compound 1 could be potentially used as a luminescent material.

Fig. 10. Excitation and emission spectra in solid state for HL and complex 1

Fig. 11. QY of complex 1

Fig. 12. Decay curve of compound 1

3. 5 Magnetic property

The MOFs consisting of paramagnetic ions Cu(II) and carboxylate or N-heterocyclic ligands may exhibit meaningful magnetic coupling properties. Therefore, the magnetic susceptibility (χM) of complex 2 was measured at a field of 2 kOe, ranging from 1.8 to 300 K. As shown in Fig. 13, the experimental χMTvalue is 0.76 cm3·K·mol-1close to the normal temperature, which agrees well with the expected one(0.75 cm3·K·mol-1) calculated for two noninteracting Cu(II)spin carriers (S= 1/2) withg= 2, and remains almost constant as the temperature decreases until ca. 60 K, and then rapidly drops down to 0.36 cm3·K·mol-1[28]. The Curie-Weiss fitting for the 1/χMvs.Tdata in the full temperature range explored gives Curie constant (C) = 0.76 cm3·K·mol-1and Weiss constant (θ) = -2.71 K (Fig. 13, inset). Therefore, this demonstrates a weak antiferromagnetic interaction existing between the Cu(II) ions within the structure of 2 due to the small negative Weiss constant.

Fig. 13. Plot of χMT vs. T for 2. Inset shows 1/χM vs. T data, in which the red solid line represents a fit of the data to the Curie-Weiss expression