Three Novel Metal-Organic Frameworks with Different Topologies

Dec 7, 2009 - The medium and broadband centered at 3445 cm−1 for 3 is attributed to the O−H stretching vibration of the free water molecules, wher...
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DOI: 10.1021/cg9012262

Three Novel Metal-Organic Frameworks with Different Topologies Based on 3,30 -Dimethoxy-4,40 -biphenyldicarboxylic Acid: Syntheses, Structures, and Properties

2010, Vol. 10 887–894

Xiao-Zhu Wang,† Dun-Ru Zhu,*,†,‡ Yan Xu,† Jie Yang,† Xuan Shen,† Jun Zhou,† Na Fei,‡ Xiao-Kang Ke,‡ and Lu-Ming Peng‡ †

State Key Laboratory of Materials-oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, P. R. China and State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China

‡

Received October 6, 2009; Revised Manuscript Received November 10, 2009

ABSTRACT: A series of three novel metal-organic frameworks, [CdL]n (1), [ZnL]n (2), and [Cu2L2(DMF)2] 3 6H2O (3), were successfully prepared by a solvothermal method using 3,30 -dimethoxy-4,40 -biphenyldicarboxylic acid (H2L) as the ligands. X-ray crystallography analysis reveals that complex 1 exhibits a 3D framework with sra topology and a rare eight-coordinated Cd(II) with two bound methoxy groups as secondary building unit (SBU). For complex 2, a 2D coordination network with a four-coordinated Zn(II) as SBU was observed. Complex 3 also shows a 2D layered architecture with 44 square lattice topology and a paddle-wheel Cu2(-CO2)4 as SBU. 113Cd NMR spectrum of 1 showed that the chemical shift of eight-coordinated Cd(II) appeared at δ -108.9 ppm. In addition, both 1 and 2 are robust coordination polymers with high thermal stabilities (>380 C). Fluorescence measurements of 1 and 2 display medium strong emission peaks at 403 and 401 nm, respectively. Magnetic study of 3 exhibits a strong antiferromagnetic interaction between two Cu(II) ions with -2J = 286 cm-1.

Introduction Metal-organic frameworks (MOFs) built from transition metal ions and organic bridging ligands have been rapidly developed in recent years. The interest arising not only from their fascinating structural beauty and diversity but also from their promising applications as functional materials.1,2 Over the past decades, numerous MOFs have been successfully constructed, and crystal engineering has attained such a relatively mature level that some MOFs with specific topologies can be designed by the judicious selection of metal ions and organic ligands.3 However, precise control of the MOF structure from a self-assembly system is still exceedingly difficult because subtle factors may affect the assembly process. The nature of the ligand, metal ion, the metal-ligand ratio, temperature, and other factors can influence the final topology of the MOFs, leading to unexpected structural diversity. Among all the factors, the choice of the ligands is crucial because it determines the macrostructures of the final MOFs, as does the choice of the metal ions. So far, a large number of rigid multicarboxylic acids such as 1,4-benzenedicarboxylic acid (H2BDC),4 1,3,5-benzenetricarboxylic acid (H3BTC),5 4,40 -biphenyldicarboxylic acid (H2BPDC),6 and 3,30 ,4,40 -biphenyltetracarboxylic acid (H4BPTC)7 have been adopted as organic linkers. However, far less effort has been expended on substituted aromatic multicarboxylic acids.8 Our strategy for preparing MOFs is to use a symmetrically substituted ligand: 3,30 -dimethoxy-4,40 -biphenyldicarboxylic acid (H2L) as shown in Scheme 1. It is expected that the attachments of two methoxy groups on the H2BPDC would not only introduce additional coordinating sites or a variable structural chemistry into its complexes, but create more robust networks of high thermal stability with the help of the

groups’ space-filling effects because the high thermal stability is especially required for MOFs materials to be practical for optical device applications.1c It is noteworthy that the coordination polymers constructed directly from this ligand have not been reported so far. On the basis of these considerations, we report here the preparation of three novel MOFs with the H2L ligand, [CdL]n (1), [ZnL]n (2), and [Cu2L2(DMF)2] 3 6H2O (3). Their single-crystal structures, spectral properties, and thermal stabilities are systematically investigated. In addition, the magnetic property of 3 is reported. Experimental Section

*To whom correspondence should be addressed. Tel: 86 25 83587717. Fax: 86 25 83172261. E-mail: [email protected].

Materials and Methods. All chemicals purchased were of analytical grade and used as received unless noted otherwise. Cd(NO3)2 3 4H2O, Zn(NO3)2 3 6H2O, Cu(NO3)2 3 3H2O, and N,N0 -dimethylformamide (DMF) were obtained from Sinopharm Chemical Reagent Co., Ltd. Shanghai, China. Melting point was determined using an X4 digital microscopic melting point apparatus and is uncorrected. Elemental analyses (C, H, N) were carried out with a Thermo Finnigan Flash 1112A elemental analyzer. IR spectra were recorded in the range 4000-400 cm-1 using KBr pellets on a Bruker Vector 22 FT-IR spectrophotometer. 1H NMR spectra in solution were recorded on a Bruker AM 500 Hz spectrometer. Chemical shifts are given in ppm. Electrospray ionization mass spectrum (ESI-MS) was recorded with a Thermo Finnigan Fleet mass spectrometer. Thermogravimetric analyses (TGA) were performed on a NETZSCH STA 449C thermal analyzer under nitrogen atmosphere at a heating rate of 10 C min-1. Powder X-ray diffraction (XRD) data were collected on a Bruker D8 Advance diffractometer with Cu KR radiation (λ = 1.5406 A˚). Fluorescence spectroscopy data for 1 and 2 were recorded on a Perkin-Elmer LS-55 spectrophotometer. Temperature-dependent magnetic measurement for 3 was carried out on a Quantum Design MPMS-7 SQUID magnetometer. Diamagnetic correction was made with Pascal’s constants.9 Synthesis of 3,30 -Dimethoxy-4,40 -biphenyldicarboxylic Acid (H2L). H2L was obtained via hydrolysis of dimethyl 3,30 -dimethoxybiphenyl-4,40 -dicarboxylate,10 which was synthesized by a modified method provided by our group (see Scheme S1 in the Supporting

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Crystal Growth & Design, Vol. 10, No. 2, 2010 Scheme 1. 3,30 -Dimethoxy-4,40 -biphenyldicarboxylic Acid (H2L)

Information).11 Dimethyl 3,30 -dimethoxybiphenyl-4,40 -dicarboxylate (5.01 g, 15.2 mmol) in 50 mL MeOH was refluxed with KOH (6.74 g, 120 mmol) under stirring for 1 h. After the reaction mixture was cooled to room temperature, H2O (30 mL) was added and the resulting solution was extracted with diethyl ether (30  3 mL). Acidification of the aqueous layers using 6 M HCl aqueous solution yielded the target product H2L (4.36 g, 95.1%). Mp = 269-270 C. 1 H NMR (DMSO-d6, 500 MHz) δ 12.66 (br, 2H, OH), 7.74-7.76 (d, 2H, Ph-H), 7.36-7.39 (m, 4H, Ph-H), 3.94 (s, 6H, CH3). IR (cm-1): 3227(m), 2964(w), 1734(vs), 1610(s), 1558(m), 1450(m), 1408(s), 1363(s), 1238(m), 1195(m), 1012(m), 833(m), 770(m), 684(m). ESIMS m/z (%): 300.92 (HL, 100)-, 287 (12)-, 249 (60)-. Anal. Calcd for C16H14O6: C, 63.57; H, 4.67. Found: C, 63.32; H, 4.45. Synthesis of [CdL]n (1). A mixture of Cd(NO3)2 3 4H2O (0.0617 g, 0.2 mmol), H2L (0.0605 g, 0.2 mmol), DMF (2 mL), and H2O (0.1 mL) was heated in a 25 mL capacity stainless-steel reactor lined with Teflon (Jinan Henghua Sci. & Tech. Co., Ltd. Shandong, China) at 120 C for 2 days and then cooled to room temperature. Colorless block crystals of 1 were obtained. Yield, 83.1% (0.0686 g) based on ligand. IR (cm-1): 2948(w), 1607(s), 1575(s), 1521(m), 1444(m), 1397(s), 1340(s), 1226(s), 1012(m), 864(m), 777(m), 671(w). Anal. Calcd for C16H12CdO6: C, 46.56; H, 2.93. Found: C, 46.53; H, 3.04. Synthesis of [ZnL]n (2). This complex was obtained following the same method as for 1 by replacing Cd(NO3)2 3 4H2O with Zn(NO3)2 3 6H2O. Colorless block crystals of 2 were obtained. Yield, 70.1% (0.0256 g) based on ligand. IR (cm-1): 2926(w), 1604(s), 1567(vs), 1515(s), 1455(m), 1403(vs), 1365(s), 1232(s), 1015(m), 860(m), 790(m), 673(w). Anal. Calcd for C16H12O6Zn: C, 52.55; H, 3.31. Found: C, 52.38; H, 3.25. Synthesis of [Cu2L2(DMF)2] 3 6H2O (3). The mixture of Cu(NO3)2 3 3H2O (0.0248 g, 0.1 mmol), H2L (0.0306 g, 0.1 mmol), DMF (3.5 mL), and H2O (0.5 mL) was heated in a 25 mL capacity stainless-steel reactor lined with Teflon at 80 C for 1 day and then cooled to room temperature. Green plate crystals of 3 were obtained. Yield, 95.3% (0.0468 g) based on ligand. IR (cm-1): 3445(m, br), 2935(w), 1665(s), 1607(vs), 1487(m), 1419(vs), 1242(s), 1209(m), 1155(m), 1028(m), 854(s), 788(s), 674(s). Anal. Calcd for C19H25NO10Cu: C, 46.48; H, 5.13; N, 2.85. Found: C, 46.30; H, 4.82; N, 3.02. X-ray Crystallography. Diffraction data for 1, 2, and 3 were collected on a Bruker Smart APEX II CCD diffractometer with graphite-monochromated Mo KR radiation (λ = 0.71073 A˚). Empirical absorption corrections were applied by using the SADABS program. The structures were solved by direct methods and refined by the full-matrix least-squares based on F2 using SHELXTL-97 program.12 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms, except for those of the water molecules, were placed on calculated positions (C-H 0.96 A˚) and assigned isotropic thermal parameters riding on their parent atoms. There was no observable high-angle X-ray scattering of complex 2 because of the small crystal size. The data of 2 were truncated at 2θ = 45 for this reason. Hydrogen atoms could not be located and were not calculated for the disordered water in complex 3. Crystallographic data for complexes 1-3 have been deposited with the Cambridge Crystallograhic Data Centre as supplementary publications CCDC 728872 (1), 744615 (2), and 744614 (3). The crystal data and structure refinement of complexes 1-3 are summarized in Table 1. Selected bond lengths and angles of complexes 1-3 are listed in Table 2.

Results and Discussion Syntheses of the Ligand and Complexes 1-3. Although 3,30 -dimethoxy-4,40 -biphenyldicarboxylic acid (H2L) was first synthesized by Mudrovcic in 1913,13 its yields, spectral

Wang et al. Table 1. Crystal Data and Structure Refinement Details for Complexes 1-3 formula fw T (K) crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z F (000) Fcalcd (g cm-3) μ (mm-1) GOF on F2 R1/wR2 [I > 2σ(I )] R1/wR2 (all data)

1

2

3

C16H12CdO6 412.66 275(2) monoclinic C2/c 24.08(2) 7.976(7) 7.481(7) 90.000 100.551(11) 90.000 1413(2) 4 816 1.940 1.576 1.057 0.0391/0.0892 0.0470/0.0918

C16H12O6Zn 365.63 293(2) triclinic P1 7.897(5) 8.014(5) 11.804(7) 84.329(9) 77.657(10) 73.374(10) 698.6(7) 2 372 1.738 1.788 1.053 0.0731/0.1538 0.1236/0.1769

C19H25CuNO10 490.94 173(2) monoclinic C2/c 10.7521(2) 17.7597(4) 22.4268(5) 90.000 93.462(2) 90.000 4274.67(16) 8 2040 1.526 1.078 1.046 0.0439/0.1027 0.0632/0.1105

Table 2. Selected Bond Lengths (A˚) and Angles (deg) for Complexes 1-3a

Cd1-O1 Cd1-O2 O1-Cd1-O2 O2-Cd1-O1iii O1-Cd1-O1ii O1-Cd1-O1iii O1-Cd1-O3i

2.309(3) 2.595(4) 51.84(11) 84.51(12) 118.88(19) 135.38(16) 134.96(12)

Zn1-O4iv Zn1-O5vi Zn1-Zn1A O4iv-Zn1-O6v O6v-Zn1-O5vi O6v-Zn1-O2

1.929(7) 1.960(7) 4.057(7) 114.9(3) 105.7(3) 108.7(3)

Cu1-O1vii Cu1-O4viii Cu1-O7 O1vii-Cu1-O2 O2-Cu1-O4viii O2-Cu1-O3ix O1ix-Cu1-O7 O4viii-Cu1-O7

1.945(2) 1.970(2) 2.168(2) 168.89(9) 87.64(9) 91.31(9) 99.40(8) 95.57(9)

1

Cd1-O1i Cd1-O3i O1ii-Cd1-O2 O1-Cd1-O1i O1i-Cd1-O2 O2-Cd1-O2ii O1i-Cd1-O1iii

2.287(4) 2.652(3) 98.04(12) 70.42(15) 116.36(11) 123.91(16) 136.52(17)

Zn1-O6v Zn1-O2 Zn1-Zn1B O4iv-Zn1-O5vi O4iv-Zn1-O2 O5vi-Zn1-O2

1.938(7) 1.969(7) 12.722(7) 107.8(3) 107.0(3) 112.8(3)

Cu1-O2 Cu1-O3ix Cu1 3 3 3 Cu1vii O1vii-Cu1-O4viii O1vii-Cu1-O3ix O4viii-Cu1-O3ix O2-Cu1-O7 O3ix-Cu1-O7

1.959(2) 1.976(2) 2.6158(7) 91.10(9) 87.89(9) 169.33(9) 91.71(9) 95.08(8)

2

3

a Symmetry codes: (i) 1 - x, 1 - y, -z; (ii) 1 - x, y, -1/2 - z; (iii) x, 1 - y, z - 1/2; (iv) 1 - x, 1 - y, -z; (v) -x, 1 - y, 1 - z; (vi) x - 1, y þ 1, z þ 1; (vii) -x - 1, 1 - y, -z - 1; (viii) -x - 1/2, y þ 1/2, -z - 1/2; (ix) x -1/2, 1/2 - y, z - 1/2.

data, and complexes were not provided. By use of an improved synthetic method, ligand H2L was obtained in good yield and characterized fully by FT-IR, 1H NMR, MS spectra, and elemental analyses. The solvothermal reactions of H2L with M(NO3)2 3 xH2O (M=Cd, x=6; Zn, x=4; Cu, x=3) in DMF/H2O gave complexes 1, 2, and 3, respectively. In contrast to the reaction conditions of 1 and 2, complex 3 was obtained at lower temperature and with a shorter reaction time. It is well-known that the structural geometry of MOFs is related to the nature of the metal ion and could be controlled and modulated by selecting an appropriate temperature and time.14 Moreover, it is noteworthy that 3 was obtained in a high yield with a single phase as perfect crystals (see Figure S6 in the Supporting Information), which is rather different from the synthesis of a related Cu(II) complex MOF-115 (with a contaminant MOF-604) with

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Scheme 2. Coordination Modes of L Ligand in (a) 1, (b) 2, and (c) 3

Figure 1. (a) Ball-and-stick structure view of 1. (b) Eight-coordinated Cd(II) (SBU). (c) View of the dodecahedral coordination polyhedron of CdO8. All hydrogen atoms are omitted for clarity.

3,30 -dimethyl-4,40 -biphenyldicarboxylic acid.8c Single-crystal X-ray diffraction analyses reveal that complexes 1-3 exhibit different structural topologies, and the H2L ligands coordinate to M2þ (M = Cd, Zn, Cu) with three different types of coordination modes (Scheme 2). Crystal Structure of Complex 1. X-ray crystallographic analysis reveals that 1 crystallizes in a monoclinic space group C2/c and there are one Cd(II) ion and one L ligand in the asymmetric unit (see Figure S1 in the Supporting Information). The Cd(II) center is coordinated by eight oxygen atoms from four carboxylate groups of four L ligands and two methoxy groups (O3A and O3B) in the biphenyl ring (Figure 1a and 1b). Because of its orthoposition to the carboxylate group, each methoxy group in the biphenyl ring connects the Cd(II) ion (Cd-O=2.652(3) A˚) to complete a CdO8 coordination sphere (SBU), which is very rare for Cd(II) complexes.15 This CdO8 unit displaying a dodecahedron geometry (Figure 1c)16 is linked together by carboxylate groups of L to produce an 1D Cd-O-C chain along the c axis with the Cd 3 3 3 Cd distance of 3.755(3) A˚ (Figure 2a). These 1D chains are interconnected through the biphenyl groups of L to generate a 3D network which is further stabilized by weak intermolecular C-H 3 3 3 O hydrogen bonding interactions (C8 3 3 3 O2 = 3.434(7) A˚, — C8-H8C 3 3 3 O2 = 157.0(4); see Table S1 in the Supporting Information). Notably, 1D parallelogram channels of 24.08  7.98 A˚2 (measured between the atoms in opposite angles) exist in 1 along the c axis (Figure 2c). When van der Waals radii of the atoms are taken into account, the channels are 19.68  3.58 A˚2. They are further divided into two compartments by the coordinated methoxy groups extruding into the channels, and the actual pores shown as yellow spheres of diameter 1.3 A˚ in Figure 2c are too small to entrap any solvent molecules. To understand the topology of 1, it is necessary to simplify the building blocks from which the 3D network is built. To

Figure 2. (a) Ball-and-stick view of 1D Cd-O-C chain in 1. (b) SBUs with Cd shown as polyhedra, in which the carboxylate carbon atoms can be connected to form a zigzag ladder. (c) View of the sra net with inorganic SBUs linked together via biphenyl rings of L ligand (Pores, yellow).

derive the net of 1, it is instructive to connect all carboxylate C atoms in the same way as shown in Figure 2b. This gives a tetrahedral net with the C atoms at the vertices. The Cd-O-C rods in 1 are built from a homoleptic eightcoordinated Cd(II) center, where each Cd(II) is bonded to carboxyl groups of four L linkers. This connectivity pattern is repeated infinitely to create Cd-O-C rods along the c direction and to give edge-shared CdO8 dodecahedrons. The Cd-O-C rods are ladders, to which the biphenyl rings of L represent the lines that connect the rods together to give the sra (SrAl2) net topology, which is still seldom reported for Cd(II) complexes (Figure 2c).1f,17 Crystal Structure of Complex 2. X-ray crystallographic analysis reveals that 2 crystallizes in a triclinic space group P1 and there are one Zn(II) ion and one L ligand in the asymmetric unit (see Figure S2 in the Supporting Information). As shown in Figure 3a, each Zn atom adopts a distorted ZnO4 tetrahedral geometry (SBU) coordinated by four O atoms from four different L ligands because the O-Zn1-O angles are 105.7(3)-114.9(3). The bond lengths of Zn-O (1.929(7)-1.968(7) A˚) are similar to those reported

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Figure 3. (a) Ball-and-stick structure view of 2. (b) An infinite 1D [Zn-(μ-CO2)2-Zn]n chain. (c) A side view of 2D coordination network (hydrogen bonds: amaranth dotted lines).

in the related Zn(II) complexes [Zn(ip)]n (ip = isophthalate),18 [Zn(BPDC)]n,19 and [Zn(CPA)]n (CPA = 2-(4-carboxylatophenoxy)propionic acid).20 In the structure of 2, two carboxylic groups connect two adjacent Zn(II) centers, forming a dimer [(-CO2)2Zn2] of eight-member rings with the Zn1 3 3 3 Zn1A distance of 4.057(7) A˚. These dimers locating in a chair conformation are further bridged by the carboxylic groups to produce a 1D [Zn-(μ-CO2)2-Zn]n chain along the b axis (Figure 3b). Then two adjacent 1D chains are interconnected by L ligands to obtain a 2D coordination network with the shortest interchain Zn1 3 3 3 Zn1B distances of 12.722(7) A˚. There are weak intramolecular C-H 3 3 3 O hydrogen bonding interactions (C13 3 3 3 O6 = 3.229(13) A˚, — C13-H13A 3 3 3 O6 = 123.0(3)) in these 2D networks (see Table S1 in the Supporting Information and Figure 3c). Crystal Structure of Complex 3. X-ray structure analysis reveals that 3 crystallizes in a monoclinic space group C2/c and the asymmetric unit consists of one Cu(II) ion, one L ligand, one DMF molecule, and three highly disordered water molecules (The occupancy factors for O1W, O2W, O3W, O4W, O5W, O6W, O7W, and O8W are 0.429(12), 0.539(10), 0.308(7), 0.571(12), 0.385(14), 0.290(10), 0.420(19), and 0.461(10), respectively; see Figure S3 in the Supporting Information). As shown in Figure 4a, each Cu(II) ion is five coordinated in a square pyramidal CuO5 geometry by four oxygen atoms from four different L ligands at the equatorial sites and one oxygen atom of DMF at the axial position. Each carboxylic group bridges two Cu(II) centers to form the well-known Cu2(-CO2)4 paddle-wheel units (SBU, Figure 4b). The average Cu-O(CO2-) distance is 1.962(2) A˚,

Wang et al.

Cu-O(DMF) has a longer distance of 2.168(2) A˚, and Cu 3 3 3 Cu distance is 2.6158(7) A˚, which are comparable with those found in the related MOFs with Cu2(-CO2)4 paddle-wheel units.8b,c The Cu2(-CO2)4 unit is defined by the carboxylate carbon atoms as a 4-connected square SBU, and are connected through L linkers forming 2D layers (44 square lattice topology sql-a = fes)7c,21 with rhomboidal grids of 24.3  17.8 A˚2 (measured between the atoms in opposite angles). When van der Waals radii of the atoms are taken into account, the grids are 19.9  13.4 A˚2. They are further divided into four compartments, where two of them are filled with water molecules and the others are filled by the methoxy groups (Figure 4d) because of the layers stacking in an AB offset manner as shown in Figure 4c. These 2D layers are further stabilized by weak C-H 3 3 3 O hydrogen bonds (see Table S1 in the Supporting Information and Figure 4d). The 2D layer structure in 3 is similar to those of the related MOFs constructed by the Cu2(-CO2)4 paddle-wheel units with a variety of organic linkers, especially MOF-115 and MOF-604 reported recently by Furukawa and co-workers.7c However, the packing of their layers is quite different. The layers in MOF-115 are stacked in a staggered manner, and MOF-604 is a triple interpenetration of three discrete layers. In complex 3, the layers are alternating packed where each square SBU of one layer sits exactly over the center of each rhomboidal grid of the other, as shown in Figure 4c.7b,22 Coordination Modes of the Ligand. Organic polycarboxylates have been widely employed to prepare MOFs partly because of their diverse coordination modes. In complexes 1-3, all carboxylic groups of the ligand L are deprotonated and there are three types of different coordination modes as shown in Scheme 2. In 1, L ligand can link four Cd(II) ions in an unusual bis(tetradentate) coordination mode (Scheme 2a) in which each methoxy group involves in the coordination to Cd(II) ions because of its ortho-position to the carboxylate group and the Cd(II) ion with a larger radius. In contrast, the L ligands in both 2 and 3 behave bis(bridging bidentate) coordination modes. However, each carboxylate group in 2 exhibits a syn-anti fashion (Scheme 2b) but in 3 a syn-syn fashion (Scheme 2c). Although the methoxy groups in both 2 and 3 do not take part in the coordination, they play a spacefilling effect contributing to the stabilization of the final MOFs (see TGA analyses). Moreover, the presence of two methoxy groups make different affect on the dihedral angles of biphenyl rings in 1, 2, and 3, though the groups are all located in a trans conformation relative to the biphenyl rings. The biphenyl rings in 1 are coplanar but in 2 and 3 with some dihedral angles (30.91(3) and 32.47(6)). There are distinct dihedral angles (5.36(3)-50.46(6)) between the carboxylate group and the phenyl ring in complexes 1-3 (Table 3). On the other hand, it is well-known that in coordination to the same ligand, different metal ions adopt different coordination numbers, that is, eight-coordinated Cd(II) in 1, fourcoordinated Zn(II) in 2, and five-coordinated Cu(II) in 3. Therefore, in complexes 1-3, the great differences in the coordination modes of both the ligand and the metal ions should be responsible for the prominent discrepancy in their structural topologies. IR Spectra. IR spectra of complexes 1-3 displayed the characteristic asymmetric and symmetric stretching vibrations of carboxylate groups in the ranges from 1610 to 1558 cm-1, and from 1450 to 1363 cm-1, respectively. The absence of any strong absorption bands around 1734 cm-1 confirms complete deprotonation of the carboxyl groups of the H2L

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Figure 4. (a) Ball-and-stick structure view of 3. DMF except oxygen atom and all hydrogen atoms are omitted for clarity. (b) The paddlewheel unit is viewed as a 4-connected node (sky blue, square SBU); the organic linker L is viewed as a rod (gray). (c) The AB offset packing of the corrugated sheets with rhomboidal grids viewed along a axis. (d) The sheets are stabilized by weak hydrogen bonds (amaranth dotted lines). Table 3. Dihedral Angles for L Ligand in Complexes 1-3 dihedral angles (deg) complexes

MeO groups

SBUs

Ph/Ph ring

1 2 3

coordinate noncoordinate noncoordinate

CdO8 ZnO4 CuO5

0 30.91(3) 32.47(6)

CO22-/Ph ring 17.70(3) 5.36(3)/8.99(3) 26.87(6)/50.46(6)

ligands during the reactions. The bands at 1226 (for 1), 1232 (for 2), 1242 cm-1 (for 3) (s), and 1012 (for 1), 1015 (for 2), and 1028 cm-1 (for 3) (m) are ascribed to Ar-O-CH3 asymmetry and symmetry stretching vibrations, respectively. In addition, the peak at 1665(s) cm-1 is assigned to the CdO

stretching vibration of DMF coordinated to Cu(II) in 3. The medium and broadband centered at 3445 cm-1 for 3 is attributed to the O-H stretching vibration of the free water molecules, whereas the absence of similar high-frequency absorption for 1 and 2 suggests that there are no water molecules within the two structures. These features are in accordance with the results of the X-ray diffraction analyses. P-XRD and TG Analyses. The simulated and experimental P-XRD patterns of complexes 1-3 are shown in Figures S4-S6 in the Supporting Information. Their peak positions are in good agreement with each other, indicating the phase purity of the bulk products.

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TGA shows high thermal stabilities of both 1 and 2 because no guest molecules are present in the structures. For complex 1, an abrupt weight loss was only observed above 390 C due to the decomposition of the framework (see Figure S7 in the Supporting Information). The remaining weight of 31.1% after heating to 600 C is due to the final residue of CdO, in agreement with the calculated value of 31.1%. The thermal stability of 1 is higher than those of the related MOFs with unsubstituted aromatic polycarboxylic acids, [Cd3(BPDC)3(DMF)] 3 5DMF 3 18H2O6g and (NH4)(Me2NH2)[Cd(3,30 -AZDB)2] (3,30 -AZDB = 3,30 -azodibenzoate),23 of which the frameworks decomposed at 380 and 220 C, respectively. Two factors can contribute to the high thermal stability of 1. On one hand, the substituted methoxy groups may have played a space-filling effect on the structure of 1. The covalently bound methoxy groups are fixed on the phenyl ring backbone so that the total stability of 1 could be enhanced. On the other hand, the involvement of the oxygen atoms from the methoxy groups in the coordination to the Cd(II) centers further strengthen the stability of 1. Similar to 1, practically no weight loss was observed up to 380 C for complex 2 (see Figure S8 in the Supporting Information). The coordination network of 2 is more robust than that of a related MOF [Zn(bpy)(BPDC)]n (decomposed at 350 C).24 Once again, the methoxy groups’ space-filling effects are mainly contributed to the high thermal stability of 2. In contrast, the TGA curve of 3 exhibits several stages of weight loss (see Figure S9 in the Supporting Information). A weight loss of 11.0% during the first step between 30 and 135 C was observed owing to the loss of six H2O (calculated, 11.0%). A further weight loss of 31.1% during the second step between 135 and 319 C was ascribed to the loss of two DMF molecules and four MeO groups (calculated, 30.9%). The framework of 3 began to collapse only at a temperature above 319 C. Luminescence Properties. The solid-state excitationemission spectra of the free H2L ligand, complexes 1 and 2 were measured at room temperature (Figure 5). The emission spectrum for free H2L ligand showed a main peak at 404 nm with λex =354 nm. The emission peaks of 1 and 2 were found at 403 nm (λex = 360 nm) and 401 nm (λex = 364 nm), respectively. The emission peaks (403 and 401 nm) of complexes 1 and 2 are probably due to π*-π transitions of the ligand because similar peak also appears for the free ligand. However, the fluorescence intensity of 2 obviously decreased compared with 1. This feature is associated with the structure of 2 where the dihedral angle of two phenyl rings is 30.91(3). Such a large distortion of two phenyl rings is disadvantageous to the luminescence. This phenomenon is similar to those observed for the related Zn(II) complexes.25 Because of the high thermal stability both 1 and 2 could find application as potential violet-light-emitting materials. Magnetic Properties. The temperature dependence of the magnetic susceptibility in the rang of 1.8-300 K for 3 was measured at an applied field of 2000 Oe and the χMT value versus T plot is shown in Figure 6 (χM is the molar magnetic susceptibility per Cu(II) ion). At 300 K, the value of χMT is equal to 0.5709 cm3 mol-1 K, which is below the spin-only value for two noninteracting Cu(II) ions (0.75 cm3 mol-1 K at RT). Then the χMT value decreases sharply to a value of 0.0424 cm3 mol-1 K at 75 K, following gradually to 0.0018 cm3 mol-1 K at 1.8 K. This magnetic behavior agrees with the strong antiferromagnetic couplings between the Cu(II) centers. The magnetic susceptibility data were best fitted to the

Wang et al.

Figure 5. Solid-state excitation and emission spectra of the free H2L ligand (black), 1 (red), and 2 (blue).

Figure 6. Temperature dependence of the molar magnetic susceptibility of 3 in the form of the χM versus T (4) and χMT versus T (O). Solid line represents the best fitting curves.

modified Bleaney-Bowers eq 1 for S=1/2 dimers under spin Hamiltonian H = -JS1S2 χM ¼

2Ng2 β2 1 Ng2 β2 ð1 FÞ þ F KT 3 þ e -2J=KT 2KT

ð1Þ

in which N, g, β, and K have their usual meanings.9 The parameter F denotes the fraction of paramagnetic impurity in the sample. The best least-squares fitting parameters give g= 2.315, -2J=286 cm-1, F=1.17% with the agreement factor R = Σ[(χMT)obsd - (χMT)calcd]2/Σ[(χMT)obsd]2 = 6  10-5. The -2J value (286 cm-1) of 3 is in the normal range (250-330 cm-1) for the dinuclear copper(II)-carboxylate complexes.26 It is well-known that for the exchange interaction in these molecules, a superexchange mechanism prevails a direct metal-metal interaction, in which the electronic structure of the bridging O-C-O moiety determines the magnitude of the antiferromagnetic interaction. Very important here is the bending of the Cu-O-C-O-Cu bridge (φbend, the dihedral angle between the Cu-O-O-Cu and the carboxyl moiety). It is noteworthy that the larger φbend would lead to a larger decrease in -2J because of reduced overlap of the Cu dx2-y2 orbital with the 2px carboxylate oxygen orbital in the symmetric HOMO.27 In the case of 3, the φbend is 8.8(2), which is smaller than that of [Cu(DMB)2(H2O)2],28 and larger than those observed for

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Table 4. Comparison of φbend Angles and Magnetic Data for Dinuclear Cu(II)-Carboxylates φbend (deg) -2J (cm-1)

complexes a

[Cu(DMB)2(H2O)2] [Cu2L2(DMF)2] 3 6H2O [Cu2(mEP)2(H2O)2]2b [Cu2(5-msbdc)2(H2O)2] 3 3DMFc [Cu2(5-nbdc)2(DMF)2] 3 2DMFd

12.0(2) 8.8(2) 1.66(2) 1.56(2) 0.86(2)

250 286 315 317 324

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refs

space-filling agents and/or coordination center may present new opportunities in design and synthesis of more robust MOFs with tunable properties.

28 this work 29 8b 8b

Acknowledgment. Financial support from the National Natural Science Foundation of China (20771059, 20771050, and 20971068) and the Natural Science Foundation of Jiangsu Province (BK2008371) is gratefully acknowledged.

a

HDMB = 2,6-dimethoxybenzoic acid. b mEP = 1,2-benzenedicarboyxlate monoethyl ester. c 5-msbdc = 5-methylsulfanylmethyl-1,3-benzenedicarboxylic acid. d 5-nbdc = 5-nitro-1,3-benzenedicarboxylic acid.

Supporting Information Available: X-ray crystallographic information in CIF format for the structure determination of complexes 1-3; synthetic route of the ligand, hydrogen-bonding geometries, molecule structures, TGA curves, P-XRD patterns (PDF). This material is available free of charge via the Internet at http://pubs. acs.org.

References

Figure 7.

113

Cd CP-MAS NMR spectrum of 1.

[Cu2(mEP)2(H2O)2]2,29 [Cu2(5-msbdc)2(H2O)2] 3 3DMF, and [Cu2(5-nbdc)2(DMF)2] 3 2DMF.8b Thus the -2J value of 3 is comparable with those of similar dinuclear copper(II)carboxylate complexes (Table 4). 113 Cd NMR spectrum of 1. The solid-state l13Cd NMR spectrum of 1 was obtained from 0.1202 g of the crystalline sample on the Bruker AM 400 NMR spectrometer operating at 88.745 MHz using CP-MAS techniques. The contact time was 3.5 ms, the delay time was 2 s, and the rotor speed was set at 14 kHz. Chemical shift was quoted relative to 0.1 mol L-1 Cd(C1O4)2 aqueous (D2O) solution as a reference with positive chemical shift downfield. The spectrum of 1 is shown in Figure 7 and the only peak at δ -108.9 ppm is assigned to the eight-coordinated Cd(II) ion. In general, eight-coordinated Cd(II) has a l13Cd chemical shift ranging from δ 0 to -115 ppm.30 The upfield shift of 1 among the eight-coordinated Cd(II)-O complexes is unusual and may be due to the shielding influence of the coordinated methoxy groups. Conclusions In summary, by use of 3,30 -dimethoxy-4,40 -biphenyldicarboxylic acid (H2L) as the ligand, three novel MOFs [CdL]n (1), [ZnL]n (2), and [Cu2L2(DMF)2] 3 6H2O (3) have been successfully synthesized under solvothermal conditions. MOF 1 displays a 3D framework with sra topology and a rare eightcoordinated Cd(II) with two bound methoxy groups as SBU. MOFs 2 and 3 possess 2D coordination networks but with different SBUs. Both 1 and 2 are robust frameworks showing the high thermal stability. 3 exhibits a strong antiferromagnetic interaction between the two Cu(II) ions with -2J = 286 cm-1. This work has revealed that the topologies of MOFs and the geometries of SBUs can be finely tuned by substituted aromatic polycarboxylic acids. Furthermore, the successful utilization of the positioning functional groups as

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