Structural Diversity, Luminescence, and Magnetic Property - American

Article pubs.acs.org/crystal

Structural Diversity, Luminescence, and Magnetic Property: Series of Coordination Polymers with 2,2′-Bipyridyl-4,4′-Dicarboxylic Acid Ya Zuo,†,‡ Ming Fang,† Gang Xiong,†,§ Peng-Fei Shi,† Bin Zhao,*,† Jian-Zhong Cui,§ and Peng Cheng† †

Department of Chemistry, and Key Laboratory of Advanced Energy Materials Chemistry, Ministry of Education, Nankai University, Tianjin 300071, PR China ‡ College of Sciences, Inner Mongolia Agricultural University, Inner Mongolia 010018, PR China § Department of Chemistry, Tianjin University, Tianjin 300072, PR China S Supporting Information *

ABSTRACT: Eight coordination polymers associated with the organic ligand 2,2′-bipyridyl-4,4′-dicarboxylic acid (abbreviated H2BPDC): {[Zn(BPDC)(H2O)3]·3H2O}n (1), [Zn(BPDC)(H2O)2]n (2), [Co(BPDC)(H2O)3]n (3), [Co(BPDC)(H2O)2]n (3a), [Cu(BPDC)(H2O)]n (4), {[Cu(BPDC)(H2O)2]·2H2O}n (4a), [Mn(BPDC)(H2O)2]n (5), and {[Mn(BPDC)]·2H2O}n (5a) were prepared by hydrothermal methods and structurally characterized. The structure analyses reveal that 1 exhibits a one-dimension chain, and 2, 3a, 5, and 5a are complicated 3D structures. 2 displays a 2-fold interpenetrating chiral 3D framework with the rare (12,3) topology and 5 is a chiral 3D framework. 4 and 4a are twodimensional networks, and 3 is a chiral 1D chain. The structural contrasts between 1 and 2, 3 and 3a, 4 and 4a, and 5 and 5a display the transformations from low to high dimensional motif, and/or from achiral to chiral structures. Interestingly, the large structure divergences mainly originated from the different reaction temperature (It should be noted that 3a, 4a, and 5a were also obtained independently by us, although they had been reported. Herein, they were only used to discuss the structural comparison investigations). The luminescent properties of 1 and 2 have been explored and compared with that of the ligand. The Cotton effect in solid circular dichroism (CD) spectra of 2 was significantly observed, indicative of the chirality of 2. Magnetic properties analyses for 3 and 5 were performed.

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INTRODUCTION Recently, considerable effort has been concentrated not only on the synthetic strategy to obtain variously unique coordination polymers (CPs) but also on their versatile applications such as sorption,1 catalysis,2 optical properties,3 chirality,4 magnetism,5 and so on. Some recent excellent reviews6 deeply discussed all aspects of this research hotspot, and witnessed the great progress of this field in the last two decades. Among these investigations, the structural diversities were exhibited by changing the synthetic strategy of CPs, ligand substituent, ligand to metal salts ratio, solvent, the anions of the metal salts, the reaction temperature, and concentrations of the reactants, etc., and the cases will provide a platform to tune the structures and functions of CPs. Few investigations on tuning the structures of CPs by changing the number of coordinated H2O resulted from different reaction temperature have been carried out so far.7,8 Theoretically, when decreasing or increasing the number of coordinated H2O on metal center by controlling temperature, the coordination geometry of metal ions will be destroyed, and the ligand connected to metal ions mayhave to change its coordination modes to finish the new coordination sphere of metal ions, resulting in the generation of new CPs. From the entropic point of view, the terminal ancillary ligands © 2012 American Chemical Society

like water or other solvent molecules may be removed under higher synthesis temperature. In our previous work, the strategy has been employed in fabricating 3d−4f heterometallic MOFs, realizing the transformation from 1D chain structure to nanotubular 3D frameworks through only reducing ancillary coordinated H2O on Mn2+ under higher temperature.7 Subsequently, the strategy was also used in the construction of 4f-based MOFs.8 Inspired by these results, the further investigation to utilize the strategy in the 3d-based CPs systems would be done in this work. Additionally, chiral CPs attract currently intense attention due to the unique functions like asymmetric heterogeneous catalysis,9 chiral separations,10 biomimetic chemistry11 and nonlinear optical materials.12 Generally, the chiral CPs may be obtained through employing either chiral organic ligand or the direct synthesis templated by chiral cations,13 and occasionally, through achiral ligands based on spontaneous.14 Nevertheless, the fabrication of chiral CPs still confront a great challenge in material science and synthetic chemistry, mainly originating Received: March 8, 2012 Revised: June 21, 2012 Published: July 10, 2012 3917

dx.doi.org/10.1021/cg300323u | Cryst. Growth Des. 2012, 12, 3917−3926

Crystal Growth & Design

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Table 1. Crystallographic Data for Compounds 1−5 empirical form form wt cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd/g cm−3 F(000) λ (Å) μ (mm−1) GOF Rint R1/wR2 Flack value

1

2

3

4

5

C12H18ZnN2O10 415.65 monoclinic P21/c 7.3319(15) 12.459(3) 18.157(4) 90 100.93(3) 90 1628.5(6) 4 1.695 856 0.71073 1.565 1.086 0.0404 0.0324/0.0704

C12H10ZnN2O6 343.58 hexagonal P3221 11.846(2) 11.846(2) 8.3276(17) 90 90 120 1012.0(3) 3 1.691 522 0.71073 1.849 1.002 0.2103 0.0876/0.1749 −0.01(3)

C12H12CoN2O7 355.17 orthorhombic P212121 7.3326(15) 13.460(3) 14.190(3) 90 90 90 1400.4(5) 4 1.685 724 0.71073 1.263 1.094 0.0380 0.0320/0.0812 0.52(2)

C12H8CuN2O5 323.74 monoclinic C2/c 21.695(4) 8.8627(18) 13.476(3) 90 125.77(3) 90 2102.4(7) 8 2.046 1304 0.71073 2.102 1.082 0.0367 0.0384/0.0921

C12H10MnN2O6 333.16 orthorhombic P212121 6.7519(14) 13.246(3 13.270(3) 90 90 90 1186.9(4) 4 1.864 676 0.71073 1.145 1.056 0.0965 0.0650/0.1599 0.53(4)

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from preferring formation of crystalline racemates to homochiral molecule crystalline, especially for the construction of 1D chiral chain associated with symmetrical ligands because of the lack of control in other two dimensions.15 Therefore, the explorations on the factors that cause the transformation between achiral and chiral CPs will be an exciting issue to prepare more desired chiral CPs. In this contribution, to construct novel CPs, the organic ligand 2,2′-bipyridyl-4,4′-dicarboxylic acid (abbreviated H2BPDC) was selected on the basis of the following reasons: (i) it is a polydentate ligands of up to six donor atoms-N2O4, displaying various coordination modes; (ii) as a rigid and symmetry ligand, it favors generating multidimensional CPs; (iii) importantly, the achiral free H2BPDC itself may induce axial chirality due to the stereoselective configuration after the coordination with the metal ions.16 As a result, eight CPs: {[Zn(BPDC)(H2O)3]·3H2O}n (1), [Zn(BPDC)(H2O)2]n (2), [Co(BPDC)(H2O)3]n (3), [Co(BPDC)(H2O)2]n (3a), [Cu(BPDC)(H2O)]n (4), {[Cu(BPDC)(H2O)2]·2H2O}n (4a), [Mn(BPDC)(H2O)2]n (5) and {[Mn(BPDC)]·2H2O}n (5a) were obtained and structurally characterized (complexes 3a,16a,b 4a,16c and 5a16d were also isolated independently by us, although they had been reported. Herein, they were only used to discuss the structural comparison investigations). The structure studies reveal that 1 exhibits one-dimension chain; 2 exhibits a rare 2-fold interpenetrating chiral (12,3) net; 3 is a chiral 1D chain; 4 and 4a are two-dimensional networks; 3a, 5, and 5a are complicated 3D structures, of which 3a and 5 are chiral 3D frameworks. The structural comparison between 1 and 2, 3 and 3a, 4 and 4a, and 5 and 5a, display the transformations from low to high dimensional motif, and/or from achiral to chiral structures. The significant structure divergences were tuned only by changing the reaction temperature. The luminescent properties of 1 and 2 have been explored and compared with that of the ligand. The Cotton effect in solid circular dichroism (CD) spectra of 2 was significantly observed, indicative of the chirality of 2. Magnetic properties analyses for 3 and 5 were performed.

EXPERIMENTAL SECTION

Materials. Reagents including the ligand 2,2′-bipyridine-4,4′dicarboxylic acid (H2BPDC) purchased from Chemzam pharmtech Co., Ltd. were used as received without further purification. Elemental analyses were taken on a Perkin-Elmer 240C analyzer. The solid-state circular dichroism (CD) spectra were carried out by using JASCO Corporation J-715 Spectrometer using KBr disks. Powder-XRD measurements were recorded on a D/Max-2500 X-ray diffractometer using Cu Kα radiation. The magnetic susceptibilities in the temperature range of 1.8 to 300 K were measured on a Quantum Design MPMS7 SQUID magnetometer in a field of 1 kOe. Diamagnetic corrections were made with Pascal’s constants for all samples. The fluorescent spectra were measured on a F-4500 FL Spectrophotometer. Synthesis of Complexes 1−5. The syntheses of 1−5 was described as following, while those of 3a, 4a, and 5a were provided in the Supporting Information. {[Zn(BPDC)(H2O)3]·3H2O}n (1). A 10 mL aqueous solution of Zn(NO3)2·6H2O (59.4 mg, 0.2 mmol) and LiOH (28.8 mg, 1.2 mmol), were added to H2BPDC (73.2 mg, 0.3 mmol) within a 25 mL Teflon-lined stainless steel container and heated at 100 °C for 3 days. The sample was cooled to room temperature in 3 days, and colorless block crystals of 1 were collected with a yield of 67%. Anal. Calcd (%) for C12H18N2O10Zn: C 34.70, H 4.34, N 6.75. Found: C 34.66, H 4.25, N 6.67. [Zn(BPDC)(H2O)2]n (2). A mixture of Zn(NO3)2·6H2O (59.4 mg, 0.2 mmol), H2BPDC (73.2 mg, 0.3 mmol), and LiOH (28.8 mg, 1.2 mmol) in H2O (8 mL) was sealed in a 25 mL Teflon-lined stainless steel container and heated at 160 °C for 3 days and then slowly cooled to room temperature in 3 days. The colorless prism crystals of 2 were collected with a yield of 85%. Anal. Calcd for C12H10Zn2N2O6: C 41.98, H 2.92, N 8.16. Found: C 41.79, H 2.81, N 8.37. [Co(BPDC)(H2O)3]n(3). Co(CH3COO)2·4H2O (24.9 mg, 0.10 mmol), H2BPDC (73.2 mg, 0.3 mmol) and LiOH (14.4 mg, 0.6 mmol) in H2O (5 mL), were sealed in a 25 mL Teflon-lined stainless steel container, then heated to 160 °C for 3 days and slowly cooled to room temperature in 3 days. The orange block crystals of 3 suitable for a single-crystal X-ray diffraction study were obtained with a yield of 74%. Anal. Calcd for C12H12CoN2O7: C 40.54, H 3.38, N 7.88. Found: C 40.46, H 3.30, N 7.79. [Cu(BPDC)(H2O)]n (4). CuSO4·5H2O (50.0 mg, 0.20 mmol), H2BPDC (73.2 mg, 0.3 mmol) and LiOH (14.4 mg, 0.6 mmol) in H2O (8 mL), were sealed in a 25 mL Teflon-lined stainless steel 3918



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Scheme 1. synthetic Strategy for 1−5 and 3a, 4a, and 5a

container and heated to 160 °C for 3 days and slowly cooled to room temperature for 3 days. The blue block crystals of 4 suitable for a single-crystal X-ray diffraction study were obtained with a yield of 58%. Anal. Calcd For C12H8CuN2O5: C 44.48, H 2.47, N 8.65. Found: C 44.40, H 2.56, N 8.69. [Mn(BPDC)(H2O)2]n (5). An aqueous solution of MnSO4·H2O (33.8 mg, 0.2 mmol), LiOH (21.6 mg, 0.9 mmol) and H2BPDC (73.2 mg, 0.3 mmol) in H2O (10 mL) was sealed in a 25 mL Teflon-lined stainless steel container and heated at 140 °C for 3 days. The sample was cooled to room temperature in 3 days, and pale-yellow block crystals of 5 were collected by filtration and washed by water for several times with a yield of 66%. Anal. Calcd for C12H10MnN2O6: C 43.22, H 3.00, N 8.40. Found: C 43.29, H 2.91, N 8.33. Crystallographic Studies. Crystallographic data of 1−5, 3a, 4a, and 5a was collected on a Bruker SMART CCD system equipped with monochromated Mo-Ka radiation (λ = 0.71073 Å) using the ω−φ scan technique. The data integration and empirical absorption corrections were carried out by SAINT programs. The structures were solved by direct methods (SHELXS 97).17a All the non-hydrogen atoms were refined anisotropically on F2 by full-matrix least-squares techniques (SHELXL 97).17b All the hydrogen atoms except for those of the uncoordinated water molecules in these coordination polymers were generated geometrically and refined isotropically using the riding model. Details of the crystal parameters, data collection, and refinements for 1−5 are summarized in Table 1, whereas those of 3a, 4a, and 5a are provided in the Supporting Information.

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Figure 1. Coordination environments of ZnII ions in 1. Symmetry operations A: x, 1/2 − y; −1/2 + z.

plane, forming the vertexes of the octahedron. Thus, each BPDC ligand chelates to one Zn(II) center through the pyridyl nitrogen donors, and the carboxylate group links adjacent Zn(II) ions to obtain 1D chain. As shown in Figure 2, the neighbor Zn ions distances are 9.081(2) Å and the angles of three adjacent Zn(II) are 177.12(0)°. The Zn−Ocoo‑ bond length is 2.035(0) Å, which is shorter than that of Zn−Owater from 2.123(8) to 2.152(2) Å, and the average distance of Zn− N is 2.132(2) Å. Crystal Structure of 2. Compound 2 crystallizes in hexagonal system, chiral space group P3221. A slightly distorted octahedron of the Zn(II) center is furnished by N atoms (N1 and N1A) of a chelating BPDC ligand, two oxygen donors (O2A and O2B) from carboxylate groups and two water molecules (O3 and O3A), as shown in Figure 3. Each Zn(II) center connects with three BPDC ligands, and each BPDC ligand links three Zn(II) cores, giving rise to an interesting 3D framework viewed along c directions, as shown in Figure 3. There exist two kinds of triangle channels A and B in the 3D framework, of which each A channel has six B channels as its nearest neighbor, while each B channel bordered with three A channels. The cross-section of the channel A comprises six Zn

RESULTS AND DISCUSSION

Syntheses. According to the synthetic strategy in scheme 1, the compounds 1, 3, 4a, and 5 were first prepared, and the number of coordinated H2O on the corresponding metal ions is 3, 3, 2, and 2, respectively. Through higher synthetic temperature, coordinated H2O in 1, 3a, 4, and 5a were successfully decreased to 2, 2, 1 and 0, respectively, and the corresponding CPs 2, 3a, 4, and 5a were obtained. Crystal Structure of 1. X-ray crystallography reveals that compound 1 crystallizes in monoclinic system, space group P2(1)/c. As shown in Figure 1, the six-coordinate geometry of the zinc is defined by the chelating nitrogen donors (N1 and N2) of BPDC ligand, three water molecules (O5−O7), and the oxygen atom (O3) of another BPDC ligand to form a slightly distorted octahedron with the N2O4 donors set. N1, O5, O6 and O7 are almost coplanar with the mean deviation of 0.1018 Å, whereas N2 and O3A atoms lie at the both sides of the 3919



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Figure 2. 1D ZnII-chain in 1 along c direction. Color codes: cyan, Zn; blue, N; red, O. other atoms are omitted for clarity.

Figure 3. (Left) Coordination environments of ZnII ions in 2. (Right) 3D framework in 2, containing two kinds of channels: A and B. Color codes: cyan, Zn; blue, N; red, O; gray, C. H atoms are omitted for clarity. Symmetry operations A: x − y, −y, −z + 4/3; B: −x, −x + y, −z + 2/3 ;C: −x, y − x, z − 2/3.

Figure 4. (a) Wall of channel A was a left-handed, double-stranded helices. (b) Wall of channel B was a right-handed, single-stranded helix.

Another structure feature of compound 2 displays 2-fold interpenetrating 3D networks, which causes the porosity of 2 to disappear. The topology structure was indicated in Figure 4. Both the Zn(II) centers and the BPDC ligands may be defined as 3-connecting nodes, and then the 3D frameworks produces the rare (12,3) topology (Figure 5),16a,b,18 which is similar with the topology structure of 4a. The net (12,3) has a unique nature among all the uniform nets with self-entanglement, and some of its shortest circuits have other shortest circuits passing

atoms, and that of the channel B contains three Zn atoms. The further investigation reveals left-hand double helix with a pitch of 33.31 Å interweaves to form the wall of channel A, possessing 31 screw axes along c directions (Figure 4), and the wall of channel B consists of right-hand single helix with a pitch of 16.66 Å and 31 screw axes. It should be noted that the repeat unit in double and single helix is linear [Zn3(BPDC)3] and [Zn2(BPDC)2] motif, respectively. 3920



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Figure 5. (Left) 2-fold interpenetrating 3D topological structure in 2. Color codes: yellow, the ligand BPDC as 3-connecting node; red, Zn. (Right) The 3D frameworks of 2 produces the rare (12, 3) topology. Color codes: white line, the beginning of the nets; red, green, blue, different nets; cyan, border of the neighboring nets.

Figure 6. Coordination geometry of Co center in 3 and 1D CoII-chain motif along the c direction. Color codes, green, Co; blue, N; red, O. Other atoms are omitted for clarity.

through them (Figure 5), namely “catenation”.16b It should be noted that the chiral (12,3) net possesses the largest value of n for the uniform (n,3) nets provided by Wells,19,20 and few examples of 2-fold interpenetrating (12,3) net have been reported hitherto.16a,b Crystal Structure of 3. Compound 3 crystallizes in orthorhombic system, space group P212121. As shown in Figure 6, the coordination geometry of cobalt center is defined by the chelating nitrogen donors (N1 and N2) of BPDC ligand, three aqua molecules (O5−O7), and the oxygen atom (O2) of BPDC ligand to form a slightly distorted octahedron. N1, N2, O2, and O6 are almost coplanar with the mean deviation of 0.0321 Å, while O5 and O7 atoms lie at the both sides of the plane, occupying the vertexes of the octahedron. Co(II) ions are bridged through BPDC ligands to generate a 1D helical chain. The Co−Co distance is 9.140(1) Å and the angle of three adjacent Co(II) is 101.84(0)°. The average distance of Co−N is 2.116(1) Å and the Co−Ocoo‑ bond length is 2.142(1) Å, which are longer than that of Co−Owater from 2.043(1) to 2.081(1) Å. The interesting feature of 3 is the chirality induced by the achiral free BPDC ligands. In 3, the left-hand helical chain is formed along the 21 screw axis in the b direction with a pitch of 14.190(3) Å. Although 3 and 3a have the same ligand-to-metal ratio (1:1), it is the different number of the coordinated water molecules (2 and 3, respectively) that results in the significantly structural divergency. The two oxygen atoms of coordinated water molecules in 3a occupy cis positions, whereas in 3, two of the three coordinated water molecules are located on the vertex of the axial, and the remaining one is in the equatorial plane of the octahedron. Moreover, the carboxyl groups of the BPDC ligand bound to the metal center are in an opposite orientation. In 3a, it is each BPDC anion that coordinated to two further Co centers through carboxyl group bridges, whereas in 3, the

only one carboxyl group coordinated to another Co center by a monodentate mode. Crystal Structure of 4. Compound 4 crystallizes in monoclinic system, space group C2/c. The Cu(II) core has a slightly distorted square-pyramid geometry defined by chelating bipyridyl nitrogen atoms, one water molecule and two carboxylic oxygen atoms from the BPDC ligands (Figure 7). It should be noted that one water molecule occupied the vertex of the pyramid, and the N2O2 donors are almost coplanar with the mean deviation of 0.059 Å. Each BPDC ligand chelates to

Figure 7. Extended sheet of (4,4) net topology along the ab plane. Color codes: cyan, Cu; blue, N; red, O. H atoms are omitted for clarity. 3921



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Figure 8. (Left) Coordination geometry of Mn(II) ions in 5. (Right)3D framework in 5. Symmetry operations A: −x, −1/2 + y, 1/2 − z; B: −1/2 − x, −y, 1/2 + z.

Figure 9. (Left) Left-handed helical motif constructed by [Mn−O−C−O−Mn] repeat unit in the [100] direction. (Right) (10, 3) topology nets is with 4-fold helices parallel to each of the axes. Color codes: cyan, the ligand BPDC as 3-connecting node; purple, Mn.

comprise ten nodes. As a result, the (10,3) topology net was obtained in Figure 9, showing 4-fold helices parallel to each of the crystallographic axes. It should be noted that seven uniform (10,3) connected nets were summed up by Wells,19,20 and the (10,3)-a net is chiral and possesses 4-fold screw axis, exhibiting the most high symmetry in all the possible 3-connected nets. Therefore, the framework for 5 belongs to (10,3)-a topology. Coordination Modes of BPDC. Single-crystal X-ray diffraction analyses reveal that compounds 1−5, 3a, 4a, and 5a exhibit structural diversity from 1D chain, 2D layer to 3D framework, as well as from achirality to chirality. From the structural point of view, the rich structures maybe directly benefit from the versatile coordination modes of BPDC anions, which may coordinate to metal center by up to twelve coordination modes (Scheme 2). Among these modes, mode B appears in 1 and 3, while mode C, D and E is observed in 2, 4, and 5, respectively. It should be noted that, the large structural difference between 1 and 3 occurs, although the same coordination mode B exists in them, of which 1 is an achiral 1D chain and 4 is a chiral 1D zig chain. Structural Diversity Tuned by Changing the Reaction Temperature. Compared compound 1 with 2, because of the different reaction temperature, three-coordinated H2O coordinated to one Zn2+ in 1, whereas only two-coordinated H2O located on Zn2+ in 2. The subtle change resulted in dramatically structural divergence from an achiral 1D chain of 1 to a chiral 3D 2-fold interpenetrating framework in 2. Actually, just mentioned above, after the coordinated H2O on Zn2+ was

one Cu(II) site through the pyridyl nitrogen donors and link two adjacent Cu(II) ions by carboxyl groups, resulting in a 2D grid structure of 4. As shown in Figure 9, the (4,4) net topology was observed in the ab-plane, and the Cu−Cu distances are 8.86 and 8.88 Å along the b- and a-axes, respectively. The change of coordinated water molecules from 2 in compound 4a to 1 in 4 leads to the different coordination modes of the metal cores, from octahedron to square-pyramid geometry. Crystal Structure of 5. Compound 5 crystallizes in orthorhombic system, chiral space group P212121. The Mn(II) center has a slightly distorted octahedron defined by chelating bipyridyl nitrogen atoms, two water molecules and two carboxylic oxygen atoms from the adjacent BPDC ligands (Figure 8). Both water molecules and the carboxylic oxygen atoms occupy cis-positions, which is different to that in 5a where carboxyl groups coordinate with trans-orientation.16d Each deprotonated BPDC ligand chelates to a Mn(II) center by the nitrogen atoms of bipyridine and its carboxyl groups bridge two adjacent Mn(II) centers as monodentate donors, and by the connection mode, a single 3D networks in 5 is generated, as shown in Figure 8. Interestingly, the structure of 5 possesses a left-handed helical motif constructed by [Mn−O−C−O−Mn] repeat unit in the [100] direction with the pitch of 6.752(1) Å (Figure 9), which is much shorter than those in 2 and 3 because the Mn2+ in 5 are bridged by two oxygen atoms of the same carboxyl group. From the topological point of view, both Mn2+ and BPDC ligand may be considered as 3-connecting nodes, and all of the three shortest circuits for each node type 3922



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Scheme 2. Coordination Modes of BPDC Anions

Figure 10. Solid-state CD spectra of 2 on excited at 268 nm.

Magnetic Properties. The magnetic susceptibilities of 3 were measured in the range from 2 to 300 K under an applied field of 1000 Oe, as illustrated in Figure 11. The χMT value

partly removed, the coordination modes of BPDC anions must be adjusted to retain the coordination geometry of Zn2+ under no participation of other ligand, which is confirmed by the change from coordination mode B in 1 to C in 2. As a result, the different coordination modes directly endow the significant structure difference between 1 and 2. Similarly, the number of coordinated H2O on Co2+ in 3 and 3a is 3 and 2, respectively, and the corresponding structure change displays from the chiral 1D chain to the chiral 3D framework. Compared with 4a, the number of coordinated H2O on Cu2+ in 4 decreased to 1, caused the different coordination number of Cu2+. As for 5a and 5, two coordinated H2O locate on Mn2+ in 5, but in 5a, coordinated H2O completely disappear. The little divergence should be responsible for large structure transformation from the chiral 3D framework of 5 to the achiral 3D framework of 5a. The structural changes between 1 and 2, 3 and 3a, 4 and 4a, 5 and 5a, from low to high dimensional, and/or from achiral to chiral structures, have been realized only through changing the reaction temperature, which gave rise to the number of coordinated H2O on corresponding metal centers. Circular Dichroism (CD) Spectra. Circular dichroism (CD) spectroscopy is a relatively new and powerful technique to obtain conformational information of chiral molecules. The analyses of the single-crystal X-ray diffraction revealed that compounds 2, 3, and 5 possess chiral space group. On the basis of the above experiment results, it may be necessary to identify whether the crystallization of their bulk crystals is racemic or enantiomeric excess. The bulk crystals for 2, 3, and 5 were crushed to powder and their solid-state CD properties were measured by employing a KBr disk. The results reveal that only 2 displays strong Cotton effects at 315 nm (strong, negative), 325 nm (strong, positive) and 335 nm (weak, negative), as shown in Figure10, indicative of that compound 2 is enantiomeric excess rather than racemic, while 3 and 5 may be the latter.

Figure 11. Plots of χMT (□) and χM−1 (■) vs T for 3. The solid line corresponds to fit curve.

continuously decreases smoothly from 2.67 cm3 mol−1 K at 300 K to 2.14 cm3 mol−1 K at 90 K upon cooling from room temperature, and then drops rapidly to reach 1.16 cm3 mol−1 K at 2 K. Phenomenologically, the curves of the χMT versus T exhibit the existence of antiferromagnetic exchange interactions between adjacent CoII ions. As well-known the Co(II) ions in a distorted octahedron exist large unquenched orbital angular momentum, and it may contribute to the decrease of χMT upon cooling, which means that the decay of χMT could not entirely attribute to antiferromagnetic coupling. In fact, the spin-only value of an uncoupled high-spin CoII ion (S = 3/2, g = 2) is 1.88 cm3 mol−1 K, lower than the χMT value of 2.67 cm3 mol−1 K at 300 K, which proves the existence of orbital contribution of the octahedral CoII ion.21,22 According to the fitting by Curie−Weiss law χM = C/(T − θ) above 25K, the value of the Curie constant, C = 2.87 cm3 K mol−1, is consistent with the reported value of hexa-coordinated high-spin Co(II) ions (C = 2.8−3.4 cm3 K mol−1),23 and the negative sign of θ = −27.34 K 3923



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fitted by using the infinite-chain model of classical spins derived by Fisher with H = −2J∑SiSi+1. The corresponding analytical expression is below27

maybe originates from a significant orbital contribution and/or antiferromagnetic coupling between Co2+. To obtain an estimate of the strength of the magnetic coupling, a simple phenomenological equation is selected23

χM =

χM T = A exp( −E1/kT ) + Bexp(−E2 /kT )

In this equation, A + B is equal to the Curie constant, E1 and E2 represent the “activation energy” corresponding to the spin− orbit coupling and the antiferromagnetic exchange interaction, respectively. This equation adequately describes the spin−orbit coupling, which results in a splitting between discrete levels, and the exponential low-temperature divergence of the susceptibility [χT ∝ exp(J/2kT)]. The fitted parameters for complex 3 is A + B = 2.67 cm3 mol−1 K, E1/k = 41.76 K, E2/k = 0.36 K. The A + B and E1/k values of 3 are in agreement with those values in previously reported cobalt complexes.23,24 The -E2/k value corresponds to J = −0.72 K according to the Ising chain approximation [χT ∝ exp(J/2kT)]. Such a weak coupling interaction of −0.72 K obtained by the experimental data most likely occurs through the carboxylate bridges in cobalt(II) compounds.25 Generally, various magnetic interactions between adjacent metal ions derive from different M−O−C−O−M geometries, especially relying on the connection conformation. Namely, strongly antiferromagnetic exchange coupling often occurs in a syn−syn conformation,26a and weak to moderately antiferromagnetic interaction corresponds to an anti−anti conformation,26b whereas very weak ferro- or antiferromagnetic coupling can usually be observed in syn−anti conformation.26c,d In present work, the CoII ions were bridged by carboxyl groups in syn−anti mode, and very weak antiferromagnetic interaction occurred. The variable temperature magnetic susceptibilities of 5 were measured in the temperature range from 2 to 300 K under 1000 Oe field, as shown in Figure 12. As the temperature decreases,

Nβ 2g 2 A + Bx 2 kT 1 + Cx + Dx 3 A = 2.9167, B = 208.04, C = 15.543, D = 2707.2

x = |J | /kT

where N, g, β, k have their usual meanings, and J is the exchange coupling constant between adjacent Mn(II) ions. For 5, a good fit could be achieved over the whole experimental curve with the parameters J = −0.27 cm−1 and g = 2.01, and the agreement factor defined by R = ∑[(χMT)obs − (χMT)cal]2/ ∑[(χMT)obs]2 is equal to 4.51 × 10−3. The fitting of the magnetic susceptibility values of 5 to the Curie−Weiss equation χM = C/(T − θ) gives values of C = 4.48 and θ = −7.08 K, which indicates an antiferromagnetic nature between the Mn(II) ions. Luminescent Properties. As shown in Figure 13, solidstate luminescent spectra of 1, 2 and L at room temperature has

Figure 13. Solid-state luminescent spectra of 1, 2, and L.

been determined. The free ligand L has weak luminescent emission bands centered at ca. 475 nm (λex = 272 nm), assigned to the intraligand π*−n transitions. For 1, the relatively wide emission band was observed around 500 nm upon excited at λex = 380 nm. The luminescent band of 1 was ascribed to electron transition intraligand. Comparing with that of the free ligand, the emission bands of 2 at 470 nm (λex = 290 nm), which is also mainly due to an intraligand luminescent emissions state, as reported for Zn(II) or other d10 metal complexes with Ndonor ligands.28 The effective enhancement of emission bands in 1 and 2 should be due to the coordination of L to Zn center increasing the rigidity of ligand conformation, thereby reducing the nonradiative decay of the intraligand excited state. It should be noted that, compared with the luminescent peak of free ligand L, that of compound 1 and 2 takes place slightly red-shift and blue-shift, respectively. A tentative explanation was provided here: From the molecular orbit theoretical point of view, the HOMO and LUMO energy levels of BPDC may suffer from perturbation of Zn(II) ions, and the energy difference between HOMO and LUMO maybe depends on the coordination modes of BPDC and Zn(II) ions. For 1, Zn(II)

Figure 12. (Left) Plots of χMT (□) and χM−1 (■) vs T for 5.

the χMT value for 5 drops off from 4.35 cm3 mol−1 K at 300K to 3.28 cm3 mol−1K at 16K and then slumps to a value of 0.70 cm3 mol−1 K at 2 K. The χMT value of 4.35 cm3 mol−1 K at room temperature is close to the expected value 4.38 cm3 mol−1 K for an isolated S = 5/2 Mn(II) ion. Phenomenologically, the curve of the χMT versus T indicated the existence of weak antiferromagnetic exchange interactions between the adjacent MnII ions. To determine the exchange coupling, χMT data were 3924



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ions coordinate to BPDC in B mode, and the energy difference between HOMO and LUMO maybe reduces, resulting in a redshift luminescence. On the contrary, the corresponding energy difference possibly enhances in 2 when Zn(II) ions coordinate to BPDC in C mode, generating a blue-shift luminescence.

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CONCLUSION In summary, eight coordination polymers used by H2BPDC as a versatile ligand were obtained successfully by changing reaction temperature, exhibiting various topologies from 1D chains, 2D layers to 3D networks, together with from achirality to chirality. Interestingly, 2 exhibits a rare 2-fold interpenetrating chiral (12, 3) net. Four chiral coordination polymers 2, 3, 3a, and 5 were constructed by the achiral ligand and the chirality of 2 was demonstrated by the strong signals of the CD spectrum. The magnetic susceptibilities reveal that the antiferromagnetic interactions between adjacent Mn(II) ions in compound 5, meanwhile the significant orbital contribution to the susceptibility and weak antiferromagnetic coupling between Co2+ existed in compound 3. Luminescence properties of compounds 1, 2 and ligand at room temperature were explored, and the results suggest that the coordination of Zn2+ enhance the luminescence density of 1 and 2.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files in CIF format for compounds 1−5; powder-XRD patterns for compounds 1−5 are depicted in Figure S7−11; syntheses and crystal data of 3a, 4a, and 5a. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-22-23494630; fax: +86-22-23502458. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Program (2012CB821702 and 2011CB935902), NSFC (20971074 and 91122004), FANEDD (200732), and NSF of Tianjin (Grant 10JCZDJC21700).

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