Six New Co-Coordination Polymers Based on a Tripodal Carboxylate

May 24, 2012 - (HBCPBA)(bipy0.5)2·(H2O)]}n (bipy = 4,4′-bipyridine) (2), {[Co3(BCPBA)2(dpe)·(μ2-. H2O)4]·2H2O·2DMF}n (dpe = 1,2-di-4-pyridyleth...
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Six New Co-Coordination Polymers Based on a Tripodal Carboxylate Ligand Jiehu Cui,† Yizhi Li,† Zijian Guo,† and Hegen Zheng*,†,‡ †

State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, P. R. China ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China S Supporting Information *

ABSTRACT: Six new compounds of partially or wholly deprotonated 3,5-bi(4-carboxyphenoxy)-benzoic acid (H3BCPBA), namely, {[Co(H2BCPBA)2(H2O)4]}n (1), {[Co(HBCPBA)(bipy0.5)2·(H2O)]}n (bipy = 4,4′-bipyridine) (2), {[Co3(BCPBA)2(dpe)·(μ2H2O)4]·2H2O·2DMF}n (dpe = 1,2-di-4-pyridyleth-ene) (3), {[Co3(BCPBA)2(pdp)·(μ2H2O)4]·2DMF}n (pdp = 4-[(E)-4-pyridinylazo]pyridine) (4), {[Co3(BCPBA)2(bpe)·(μ2-H2O)4]·2DMF}n (bpe = 1,2-bis(4-pyridyl)ethane) (5), {[Co2(HBCPBA)2 (bpp)·(μ2-H2O)2]·H2O·2DMF}n (bpp = 1, 3-bis(4-pyridyl)propane) (6) were synthesized in the presence or absence of auxiliary ligand. Their structures have been determined by single-crystal X-ray diffraction analysis and further characterized by elemental analysis, IR spectroscopy, and thermogravimetric analysis. Compound 1 is a zero-dimensional structure. Compound 2 is a two-dimensional (2D) sheet structure with two 2D → 2D interpenetration frameworks. Compounds 3−5 are similar and possess three-dimensional (3D) networks with two 3D → 3D interpenetration frameworks. In compound 6, H3BCPBA and bpp ligands link Co centers to generate a 2D sheet structure which is further connected by intermolecular hydrogen bonds to form a 3D supramolecular structure. The photochemical properties are performed in the solid state at room temperature. Magnetic susceptibility measurements indicate that compounds 2−6 exhibit antiferromagnetic coupling between adjacent Co(II) ions.

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INTRODUCTION Coordination polymers have been a subject of intense research activity in many laboratories in recent years because these materials show potential applications in the areas of magnetism, sensors, gas adsorption, ion exchange, and catalysis.1−6 An effective and facile method for the design of two-dimensional (2D) and three-dimensional (3D) metallosupramolecular species are still the appropriate choice of well-designed organic ligands as bridges or terminal groups (building blocks) with metal ions or metal clusters as nodes.7 Among various organic ligands, multicarboxylate ligands are often selected as multifunctional organic linkers because of their abundant coordination modes to metal ions, allowing for various structural topologies and because of their ability to act as H-bond acceptors and donors to assemble supramolecular structures.8 Recently, nonrigid ligands are usually the typical building elements in the multidimensional networks. The conformational freedom nature of the flexible ligand may provide more possibility for the construction of unusual topology structures and microporous coordination polymers. Therefore, 3,5-bi(4carboxy-phenoxy)-benzoic acid (H3BCPBA) triangular flexible ligand can be considered to be excellent candidates for the preparation of functional coordination polymers. This ligand has three obvious characteristics: first, H3BCPBA adopts various coordination modes when it coordinates to metals © 2012 American Chemical Society

and thus may produce various structural topologies, owing to its tetradentate carboxylate arms and flexible structures; second, it can completely or partially deprotonate and act not only as a hydrogen bond acceptors but also as a hydrogen bond donor; third, the carboxylic groups can propagate magnetic super exchange between metal centers. To design homometallic coordination polymers showing interesting magnetic behavior, we have chosen the Co(II) ion and the new ligand H3BCPBA as the bridging ligand. In addition, metal carboxylate systems with high dimensionality are important because of the possibility of enhancement of the bulk magnetic interactions. Use of a second bridging ligand to extend the metal carboxylate systems is one of the common ways to obtain higher dimensional networks.9,10 Pyridyl donors as neutral ligands have been used for this purpose. Herein we report the synthesis and crystal structures of compounds 1−6, namely, {[Co(H2BCPBA)2(H2O)4]}n (1), {[Co (HBCPBA)(bipy 0 . 5 ) 2 ·(H 2 O)]} n (2), {[Co 3 (BCPBA) 2 (dpe)(μ 2 H 2 O) 4 ]·2H 2 O·2DMF} n (3), {[Co 3 (BCPBA) 2 (pdp)·(μ 2 H 2 O) 4 ]· 2DMF} n (4 ) , { [ C o 3 ( B C P B A ) 2 ( b p e ) ( μ 2 H 2 O) 4 ]·2DMF} n (5), {[Co 2 (HBCPBA) 2 (bpp)·(μ 2 Received: March 31, 2012 Revised: May 19, 2012 Published: May 24, 2012 3610

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mixture was placed in a Parr Teflon-lined stainless steel vessel (15 mL) under autogenous pressure and heated at 95 °C for 3 days, red crystals were obtained (yield: 60% based on Co). Anal. Calcd for C42H38CoO22: C, 52.89, H, 4.01; found C, 52.64, H, 3.95. IR (KBr, cm−1): 3410(w), 1691(w), 1605(m), 1551(s), 1408(m), 1196(m),1097(m), 1022(m), 842(m), 776(m). {[Co(HBCPBA)(bipy0.5)2·(H2O)]}n (2). Compound 2 was prepared by a procedure similar to that for the preparation of compound 1 by using Co(NO3)2·6H2O (29.1 mg, 0.1 mmol), H3BCPBA (39.4 mg, 0.1 mmol), and bipy (15.6 mg, 0.1 mmol) in 8 mL of DMF/H2O (1:3, v/ v). A large quanitity of red crystals was obtained (yield: 63% based on Co). Calcd for C31H22CoN2O9: C, 59.53, H, 3.54, N, 4.48; found C, 59.48, H, 3.56, N, 4.36. IR (KBr, cm−1): 3379(m), 1698(s), 1650, 1607(s), 1596(m), 1537(s), 1483(m), 1397(s), 1329(m), 1221(m), 1191(m), 1063(m), 1018(w), 851(m), 815(m), 783(s). {[Co3(BCPBA)2(dpe)(μ2-H2O)4]·2H2O·2DMF}n (3). Compound 3 was prepared by a procedure similar to that for the preparation of compound 1 by using Co(NO3)2·6H2O (29.1 mg, 0.1 mmol), H3BCPBA (39.4 mg, 0.1 mmol), and dpe (18.2 mg, 0.1 mmol) in 8 mL of DMF/H2O (1:1, v/v). Red crystals were obtained (yield: 66% based on Co). Anal. Calcd for C60H58Co3N4O24: C, 51.62, H, 4.18, N, 4.01; found C, 51.48, H, 4.11, N, 4.04. IR (KBr, cm−1): 3416(m), 1665(w), 1594(w), 1543(m), 1396(s), 1295(s), 1235(w), 1182(s), 1081(m), 1009(s), 839(w), 809(w), 779(m), 705(w), 659(s), 625(s), 516(m). {[Co3(BCPBA)2(pdp)·(μ2-H2O)4]·2DMF}n (4). Compound 4 was prepared by a procedure similar to that for the preparation of compound 1 by using Co(NO3)2·6H2O (29.1 mg, 0.1 mmol), H3BCPBA (39.4 mg, 0.1 mmol), and pdp (18.4 mg, 0.1 mmol) in 8 mL of DMF/H2O (1:3, v/v). A large quanitity of red crystals was obtained (yield: 53% based on Co). Calcd for C58H52Co3N6O22: C, 51.15, H, 3.84, N, 6.17; found C, 50.98, H, 3.56, N, 6.36. IR (KBr, cm−1): 3379(m), 1668(s), 1607(s), 1596(m), 1537(s), 1483(m), 1397(s), 1329(m), 1221(m), 1101(m), 1063(m), 1018(w), 851(m), 815(m), 783(s), 715(s), 673(m), 501(m). {[Co3(BCPBA)2(bpe)·(μ2-H2O)4]·2DMF}n (5). Compound 5 was prepared by a procedure similar to that for the preparation of compound 1 by using Co(NO3)2·6H2O (29.1 mg, 0.1 mmol), H3BCPBA (39.4 mg, 0.1 mmol), and bpe (18.4 mg, 0.1 mmol) in 8 mL of DMF/H2O (1:1, v/v). A large quanitity of red crystals was obtained (yield: 53% based on Co). Calcd for C60H58Co3N4O22: C, 52.83, H, 4.28, N, 4.11; found C, 52.48, H, 4.56, N, 4.26. IR (KBr, cm−1): 3379(m), 3045(s), 2925(m), 1669(s), 1606(s), 1596(m),

H2O)2]·H2O·2DMF}n (6). Six new compounds have been characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis (TGA) and X-ray crystallography. The crystal structures, topological analyses, magnetic properties, and thermal properties are studied in detail. Scheme 1. The Pyridine and Carboxylic-Substituted Tripodal Carboxylate Ligands

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EXPERIMENTAL SECTION

Materials and Methods. The reagents and solvents employed were commercially available and used as received. H3BCPBA was synthesized according to a similar reported method.11 IR absorption spectra of the complexes 1−6 were recorded in the range of 400−4000 cm−1 on a Nicolet (Impact 410) spectrometer with KBr pellets (5 mg of sample in 500 mg of KBr). C, H, and N analyses were carried out with a Perkin−Elmer 240C elemental analyzer. Powder X-ray diffraction (PXRD) measurements were performed on a Bruker D8 Advance X-ray diffractometer using Cu−Kα radiation (λ = 1.5418 Å), in which the X-ray tube was operated at 40 kV and 40 mA. Solid-state UV−vis diffuse reflectance spectra was obtained at room temperature using a Shimadzu UV-3600 double monochromator spectrophotometer, and BaSO4 was used as a 100% reflectance standard for all materials. The as-synthesized samples were characterized by TGA on a Perkin-Elmer thermogravimetric analyzer Pyris 1 TGA up to 1023 K using a heating rate of 10 K min−1 under N2 atmosphere. Syntheses of 1−6. {[Co(H2BCPBA)2(H2O)4]·2H2O}n (1). A mixture of Co(NO3)2·6H2O (29.1 mg, 0.1 mmol), H3BCPBA (39.4 mg, 0.1 mmol) was added to in 8 mL of DMF/H2O (1:3, v/v). The final

Table 1. Crystallographic Data and Structure Refinement Details for Complexes 1−6 compounds

1

2

3

4

5

6

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) Z V (Å3) Dcalcd (g cm−3) μ(Mo Kα) (mm−1) F(000) R(int) observed data [I > 2σ(I)] R1, wR2 (I > 2σ(I)) S

C42H34CoO20 917.62 triclinic P1̅ 5.8035(6) 13.6010(13) 13.8761(14) 109.513(1) 99.189(1) 98.403(1) 1 995.44(17) 1.531 0.519 473.0 0.0171 3462 0.0365/0.1157 1.010

C31H22CoN2O9 625.44 triclinic P1̅ 7.429(2) 12.191(4) 16.531(5) 71.659(6) 89.296(6) 78.190(6) 2 1388.8(7) 1.496 0.679 642.0 0.0255 4867 0.0459/0.1219 1.023

C54H46Co3N2O26 1247.69 monoclinic P21/n 17.403(3) 10.834(2) 17.981(3) 90.00 116.758 (4) 90.00 2 3027.2(9) 1.369 0.887 1274.0 0.1031 5296 0.0708/0.1963 1.029

C52H40Co3N4O22 1249.67 monoclinic P21/c 17.283(3) 10.835(2) 18.443(2) 90.00 119.250(11) 90.00 2 3013.3(9) 1.377 0.892 1274.0 0.1021 5270 0.0476/0.1295 1.068

C54H38Co3N2O22 1243.65 monoclinic P21/n 17.4870(19) 10.8823(11) 18.0449(19) 90.00 116.170(2) 90.00 2 3013.3(9) 1.340 0.871 1426.0 0.0644 7632 0.0488/0.1290 1.016

C55H44Co2N2O19 1154.78 monoclinic P2/c 16.833(2) 17.632(2) 21.364(3) 90.00 110.832(2)) 90.00 4 5926.3(13) 1.294 0.630 2376.0 0.0766 5817 0.0451/0.1039 1.017

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1538(s), 1485(m), 1398(s), 1324(m), 1225(m), 1105(m), 1064(m), 1011(w), 851(m), 812(m), 783(s), 715(s). {[Co2(HBCPBA)2(bpp)·(μ2-H2O)2]·H2O·2DMF}n (6). Compound 6 was prepared by a procedure similar to that for the preparation of compound 1 by using Co(NO3)2·6H2O (29.1 mg, 0.1 mmol), H3BCPBA (39.4 mg, 0.1 mmol), and bpp (19.8 mg, 0.1 mmol) in 8 mL of DMF/H2O (1:1, v/v). A large quanitity of red-plank crystals was obtained (yield: 53% based on Co). Calcd for C61H58Co2N4O21: C, 56.31, H, 4.49, N, 4.30; found C, 56.48, H, 4.59, N, 4.46. IR (KBr, cm−1): 3379(m), 3025(s), 2926(m), 1717, 1638(s), 1596(m), 1498(m), 1397(s), 1304(m), 1215(m), 1150(m), 1101(m), 996(w), 769(s), 615(s). X-ray Data Collection and Structure Determinations. Single crystals of 1−6 were prepared in single crystal form. X-ray crystallographic data of 1−6 were collected at room temperature using epoxy-coated crystals mounted on glass fiber. All measurements were made on a Bruker Apex Smart CCD diffractometer with graphitemonochromated Mo−Kα radiation (λ = 0.71073 Å). The structures of complexes 1−6 were solved by direct methods, and the non-hydrogen atoms were located from the trial structure and then refined anisotropically with SHELXTL using a full-matrix least-squares procedures based on F2 values.12a The hydrogen atom positions were fixed geometrically at calculated distances and allowed to ride on the parent atoms. The distribution of peaks in the channels of 1−3 was chemically featureless to refine using conventional discrete-atom models. To resolve these issues, the contribution of the electron density by the remaining water molecule was removed by the SQUEEZE routine in PLATON.12b The relevant crystallographic data are presented in Table 1, while the selected bond lengths and angles are given in Supporting Information, Table S1.

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RESULTS AND DISCUSSION Synthesis and Spectral Characterization. Compounds 1−6 were prepared in single crystalline through the hydrothermal reaction of cobalt nitrate, H3BCPBA, bipy, dpe, pdp, bpe, and bpp. It should be pointed out that repeated synthesis of compounds 1−6 is highly reproducible under the reaction conditions employed in this work. The infrared spectra of all the compounds are consistent with their crystal structures (Figures S1−S6, Supporting Information). For all complexes, the absorption bands in the 1400−1600 cm−1 region and in 1454−1348 cm−1 show the skeletal vibrations of the aromatic ring for asymmetric vibration and for the symmetric vibration, respectively. The vibrations bands at 3400 cm−1 and 1671− 1660, and 1607−1602 cm−1 indicate the presence of H2O, DMF, and −COO−, respectively. In compounds 2 and 6, 1699 and 1711 cm−1 shows the presence of −COOH which demonstrates that H3BCPBA is partially deprotonated. Phase purity is verified by elemental analysis and powder XRD (Figures S7−12). All compounds do not dissolve in water and common organic solvents. Crystal Structure of {[Co(H2BCPBA)2(H2O)4]}n (1). X-ray analysis reveals that 1 crystallized in triclinic space group P1̅. As shown in Figure 1a, 1 consists of one crystallographically independent Co(II) ion, one H2BCPBA− ligand, and two coordinated water molecules. The partially deprotonated H2BCPBA− anions adopt a monodentate coordination mode to bridge one Co center, while other two carboxylic acids are not coordinated (Scheme 2a). Co is six coordinated by two carboxylate oxygen atoms from two H2BCPBA− ligand and four O atoms from four water molecules to form an octahedral geometry. The distances of O and Co range from 2.0795 to 2.0929 Å, which falls in the usual range for similar compounds.13 Two H2BCPBA− and four water molecules is coordinated with Co(II) to form a zero-dimensional structure. In compound

Figure 1. (a) Coordination environment of the Co(II) ions in 1. The hydrogen atoms are omitted for clarity. (b) View of H-bond in 1. (c) View of the 3D network by H-bonding.

Scheme 2. Crystallographically Established Coordination Modes of Carboxylic Groups in Compounds 1−6

1, there are strong (O−H···O) hydrogen bonds (O···O distances are from 2.619 to 3.240 Å) between those sheets, which involve four coordinated water molecules from one sheet and four carboxyl groups from another sheet, and vice versa (Supporting Information Table S2). These hydrogen bonds 3612

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Figure 2. (a) Coordination environment of the Zn(II) ions in 2. The hydrogen atoms are omitted for clarity. (b) View of 0 D framework by HBCPBA2− and Co ions. (c) View of six-membered cycle structure by HBCPBA2−, bipy, and Co ions. (d) Schematic representation of the 2D → 2D framework by two identical sheets. (e) View of H-bond mode in 2. (f) View of the 3D network by H-bondings.

range of 2.012(4)−2.167(1) Å. The Co−N lengths are 2.098(9) and 2.130(2) Å. If the bipy ligand is omitted, as shown in Figure 2b, compound 2 changes into a zero-dimensional structure. Because two bipy are coordinated with four Co centers, compound 2 turns into 2D net structures (Figure 2c). In this sheet, each six-membered cycle is formed by four bipy ligands and two zero-dimensional structures from two Co and two HBCPBA2−. The Co(II) ions can be regarded as 4-connected nodes with all crystallographically independent bipy ligands acting as 2-connected linkers, and the HBCPBA2− ligand acting as a V-shaped linker. Therefore, the whole structure can thus be reduced as a (4, 4) sheet (Figure 2d). The large channel within each sheet allows them to interpenetrate between the adjacent sheets in a “parallel interpenetration” fashion, resulting in a finite polycatenation. There are strong (O−H···O) hydrogen bonds (O···O distances are from 2.577 to 3.018 Å) between the interlocked sheets, which involve the coordinated water from

change the framework from 0D to 3D (Figure 1c). This suggests that the carboxyl group and coordinated water molecules play an important role in increasing the dimension and sustaining and stabilizing the whole structure. Crystal Structure of {[Co(HBCPBA)(bipy0.5)2·(H2O)]}n (2). X-ray analysis reveals that 2 crystallizes in triclinic space group P1̅. The asymmetric unit of 2 contains one independent Co(II) cation, one HBCPBA2− anion, one coordinated H2O and two half bipy ligands. As shown in Figure 2a, Co center is five coordinated by two carboxylic O atoms from two HBCPBA2− ligands, two N atoms from two bipy ligands, and one water molecule to form a slight distorted trigonal biyramid geometry. The one carboxylic O atom of the HBCPBA2− molecule is coordinated with Co(II) by monodentate mode, another one carboxylic O atom is coordinated with Co(II) by bidentate mode, while the third carboxylic acid is not coordinated (Scheme 2b). The Co−O lengths are in the 3613

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Compound 3 reveals this structure possessing a 2-fold interpenetrating 3D → 3D network constructed with a unique building block, [Co3(BCPBA3−)6], as shown in Figure 3c,f. Each of the three Co(II) atoms were bridged by two water molecules to form an trinuclear cluster with a Co···Co separation of 3.751(4) Å. The [Co3(BCPBA3−)6] unit, which acts as a node, is connected to six adjacent nodes through six H3BCPBA ligands to form a 2D layer parallel to the ab plane. The remaining coordination sites of the two Co1 centers in each building block are occupied by nitrogen atoms of dpe ligands that act as pillars, and which bind the adjacent 2D layers to generate a pillared 3D framework. Such a pillared 3D structure composed of “double” metal-carboxylate layers is rare. Two of these pillared 3D motifs, identical in structure, interpenetrate to yield a new type of catenated network consisting of large, open, one-dimensional (1D) channels (Figure 3c,d). To fully appreciate the structure and its topology, it is necessary to consider that the Co2 atom can be regarded as one kind of linker because the Co2 atom is located in a symmetrical position to link four Co1 atoms, and the H3BCPBA and dpe ligands considered as two other kinds of linkers. Thus H3BCPBA ligands and two Co1 atoms surrounding a Co2 center constitute an eight-connected octahedral SBU. According to this simplification, the topology of this 3D network can be described as a (43)2(46.618.84), and the topological type is tfzd (Figure 3e). {[Co 3 (BCPBA) 2 (pdp)·(μ 2 -H 2 O) 4 ]·2DMF} n (4) and {[Co3(BCPBA)2 (bpe)·(μ2-H2O)4]·2DMF}n (5). To evaluate the role of ligand in the self-assembly of organic−inorganic hybrid frameworks, the reaction with pdp or bpe was carried out to afford compounds 4 and 5. X-ray crystallography reveals that the structures of three compounds are similar and possess similar cell parameters in Table 1. The most prominent characteristic of these compounds is that they all possess a 2fold interpenetrating 3D → 3D network. The bond lengths of Co−μ2-O in 4 and 5 are 2.196(3) (2.168(3) Å) and 2.190(2) (2.174(2) Å), and the bond lengths of Co−N are 134(3) and 2.122(3) Å, respectively. In this series, the distances between the Co and the coordinated N or μ2-O donors are getting longer with the changes from dpe to bpe. The angle of Co1−μ2-O−Co2 of 4 and 5 is a slightly different from 3. The angles are 118.373°, 118.547°, and 118.946°, respectively, which will result in the difference of property (detailed parameters are summarized in Table S1). Because of the structure is very similar to compound 3, we do not describe it any more in detail. {[Co2(HBCPBA)2(bpp)·(μ2-H2O)2]·H2O·2DMF}n (6). When bipy was replaced by bpp, a structurally different complex 6 is obtained. X-ray analysis reveals that 6 crystallizes in monoclinic space group P2/c. The asymmetric unit of 6 contains one independent Co(II) cation, one HBCPBA2− anion, one coordinated H2O, and half a bpp. As shown in Figure 4a, Co center is six coordinated by four carboxylic O atoms from four HBCPBA2− ligands, one N atom from one bpp ligand, and one water molecule to form a slight distorted octahedron geometry. The HBCPBA2− anions adopt two coordination modes: one carboxyl group adopts a bis(monodentate) coordination mode to bridge two Co centers, one carboxylate group adopts a monodentate coordination mode, while the third carboxylic acid is not coordinated (Scheme 2d). The Co−O lengths are in the range of 2.042(4)−2.167(1) Å, and the Co−N length is

one sheet and the carboxyl groups from another sheet, and vice versa. These hydrogen bonds change the framework from 2D to 3D (Figure 2e,2f). Obviously, these H-bonded interactions increase the stability of whole crystal structure of 2, so it does not decompose until 390 °C. Crystal Structure of {[Co 3 (BCPBA) 2 (dpe)(μ 2 H2O)4]·2H2O·2DMF}n (3). X-ray analysis reveals that compound 3 is solved in monoclinic space group P21/n. As shown in Figure 3a, compound 3 consists of one and a half

Figure 3. (a) Coordination environment of the Zn(II) ions in 3. The hydrogen atoms are omitted for clarity. (b) View of 3D framework by BCPBA3− and Co ions. (c) and (d) Schematic representation of the 3D → 3D framework by two identical sheets. (e) Schematic representation of the (3, 8) topological net. (f) Trinuclear cluster formed in compound 3.

crystallographically independent Co(II) ions, one H3BCPBA ligand, half a dpe ligand, one μ2-H2O, two coordinated water molecules, and two free water molecules. The wholly deprotonated H3BCPBA anion adopts one bis(monodentate) and two monodentate coordination modes to bridge Co centers (Scheme 2c). Co1 adopts octahedral coordination geometry by two carboxylate oxygen atoms from two H3BCPBA ligands (Co1−O: 2.0996 and 2.0761 Å) in the equatorial positions, one μ2-O atom from water molecule (Co1−O: 2.187(3) Å), two O atoms from two water molecules (Co1−O: 2.0881 and 2.1091 Å), and one N atom from dpe (Co1−N: 2.1118 Å). The Co2 is also octahedrally coordinated, bound to four oxygen atoms from four BCPBA3− anions (Co2−O: 2.0657 and 2.0864 Å) and two μ2-O atoms from two water molecules (Co2−O: 2.158(3) Å). The trinuclear cluster is formed between the Co and Co through the connection of water molecules, and the angle of Co1−μ2-O−Co2 is 118.373°. 3614

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Figure 4. (a) Coordination environment of the Co(II) ions in 6. The hydrogen atoms are omitted for clarity. (b) View of the 2D framework by HBCPBA2− and Co ions along the c axis. (c) View of framework by HBCPBA2− and Co ions along the a axis. (d) View of the 2D net structure of 6 along the c axis. (e) View of H-bond mode in 6. (f) View 3D structure in 6 by H-bond net.

2.137(2) Å. The Co and Co is connected by a water molecule, and the angle of Co1−μ2-O−Co1# is 110.936°. In compound 6, the two Co(II) atoms are bridged by a water molecule to form an binuclear cluster with Co···Co separation of 3.570(5) Å. Each binuclear Co(II) is connected by three HBCPBA2− anions and two bpp to form a 2D net structure (Figure 4b). Although a carboxylate group of H3BCPBA is also not coordinated, which is similar to 2, there is a large difference in structure. If omitting auxiliary ligands, 2 is a zerodimensional structure, while 6 is a 2D structure. In addition, a 2-fold interpenetrating net structure does not exist in compound 6, while it occurs in compounds 2−5, which is due to the difference in the coordination environment. It is interesting that the role of bpp is only divided into two parts which is different from other compounds. There are strong (O−H···O) hydrogen bonds (O···O distances are from 2.585 to 3.168 Å) in compound 6 which involve the coordinated water between one sheet and the carboxyl groups from another sheet, and vice versa. These hydrogen bonds change the framework from 2D to 3D (Figure 4e,f). From the structural descriptions above, it can be seen that the neutral ligands have an influence on the frameworks of the

complexes. When the neutral ligands is not coordinated, the zero-dimensional structure only occurs through the reaction of H3BCPBA with Co, while the addition of the neutral ligands change their structure from 2D to 3D. As shown in Scheme 2, the H3BCPBA with flexibilities in 1−6 display difference bridging coordination modes (in Scheme 2a−d, respectively) and play an important role in affecting the final structures. 2 exhibits a bis(monodentate) bridging and a monodentate bridging modes through two oxygen atoms of H3BCPBA, 3−5 possess a bis(monodentate) bridging, two monodentate bridging modes, while 6 shows a monodentate and a bis(monodentate) bridging modes. Comparing with 2, although possessing the same 2D structure, 6 is no-interpenetrating net structure. Compared with 2 and 6, 3−5 are a 3D structure with an interpenetrating net structure. Because of the different coordination modes from 1−6, they show different structural types, as expected. The Assembly Process of Layer-by-Layer Evolution through Neutral Ligands. To date, supramolecular PCPs were usually obtained by one-pot reactions, in which the assembly processes or mechanisms are extremely difficult to be experimentally observed and clarified because of the current 3615

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technological limitation. Although some assembly processes have been proposed in a step-by-step approach, the step-bystep deprotonated of three carboxylic in H3BCPBA is still rarely investigated by neutral auxiliary ligands.14 The structures of 1−6 may represent the stepwise evolution, it can be rationally speculated that, under the absence of neutral auxiliary ligands, the Co(II) ions in solution are originally coordinated through one deprotonated carboxylic (H2BCPBA−). But in the presence of bipy and bpp, the Co(II) ions are coordinated by two deprotonated carboxylic. If bipy and bpp are substituted by dpe, pdp, and bpe, it is interesting that the Co(II) ions are coordinated by three deprotonated carboxylic. This fact shows that neutral auxiliary ligands play a role in regulating the solution pH. Under the same conditions, bipy and bpp only enable two carboxyl deprotonations, while dpe, pdp, and bpe can fully make three carboxyl deprotonations. PXRD and TG Results. To confirm whether the crystal structures are truly representative of the bulk materials, PXRD experiments are carried out for 1−6. The PXRD experimental and computer-simulated patterns of the corresponding complexes are shown in Supporting Information (Figures S7−12). They show that the synthesized bulk materials and the measured single crystals are the same. To estimate the stability of the coordination architectures, 1−6 thermal behaviors were studied by TGA (Supporting Information, Figure S13). For complex 1, a weight loss of 8.08% (calcd 8.10%) is observed from 50 to 248 °C, which is attributed to the loss of the free and the coordinated water molecules (166−202 °C), and it decomposed quickly after 403 °C, suggesting that the framework is thermally stable. For complex 2, a weight loss of 2.91% (calcd 2.88%) is observed from 128.1 to 182 °C, which is attributed to the loss of the coordinated water, next a weight loss of 12.7% (calcd 12.8%) is observed from 293 to 319 °C, which is attributed to the loss of the coordinated BIPY ligand, and the structure is collapsed at 384 °C. For complex 3, a weight loss of 17.2% is observed from 129 to 162 °C, which is attributed to the loss of the coordinated water and free water, and then it decomposed quickly after 410 °C, suggesting that the framework is thermally stable. For complex 4, a weight loss of 15.1% (calcd 15.2%) is observed from 124 to 172 °C, which is attributed to the loss of coordinated water and the free water molecules, and next the structure collapsed at 385 °C. For complex 5, the TGA curve is similar to that of 4, a weight loss of 14.8% (calcd 14.9%) is observed from 124 to 172 °C, and next the structure collapsed at 416 °C. For complex 6, a weight loss of 15.7% (calcd 15.6%) is observed from 134 to 248 °C, which is attributed to the loss of coordinated water and the free DMF molecules, and next the structure collapsed at 352 °C. Photochemical Properties. All UV−vis absorption spectra for both free ligands and complexes 1−6 were recorded in reflectance mode in the solid state at room temperature as shown in Figure 5. The absorption maxima of 1−6 are redshifted as compared to those of the corresponding free ligands. The first bands at 441, 344, 356, 400, 337, and 318 nm in the spectra of 1−6, respectively, are assigned as π → π* transitions of ligands.15 The second band at 440, 539, 522, 519, 533, and 503 nm in the spectra of 1−6, respectively, is assigned as MLCT transitions of ligands. The third absorption bands profile for complexes 1−6 are 831, 687, 791, 834, 828, and 768 nm, respectively, which is owing to the electronic transitions from 4T1g(F) to 4T2g(F).16

Figure 5. The diffuse reflect solid-state UV/vis spectra of free ligands (a) and 1−6 (b).

Magnetic Properties. The frameworks of CoII centers (2− 6) provide a good opportunity for investigating effective magnetic properties through carboxylic oxygen and water oxygen bridge, although they are organized into dramatically different supramolecular structures. The temperature dependence of the magnetic susceptibility of 2 in the form of χMT and χM versus T are displayed in Figure 6a. At room temperature, χMT is equal to 2.72 cm3 mol−1 K, which is much higher than the spin-only value of 1.87 cm3 mol−1 K based on single Co(II) ions because of the prominent orbital contribution. Upon lowering the temperature, χMT continuously decreases and reaches 1.52 at 1.8 K. Above 120 K, the magnetic properties of 2 obey the Curie−Weiss law and give C = 2.80 cm3 mol−1 K and θ = −10.65 K. The negative θ values indicate antiferromagnetic interactions exist between neighboring Co(II) ions. While the distance among Co(II) centers was bridged through the long spacer ligands bipy, it should exclude an efficient direct exchange between Co(II) centers;17 the overall antiferromagnetic interaction should be mainly attributed to the significant spin−orbit coupling, which is remarkable for the 4T1g ground term of Co(II) in an octahedral ligand field. It is noted that the obtained Weiss constant of close to −20 K for noninteracting Co(II) ions indicates single-ion behavior of the Co(II) ion above 120 K in 2.17b The obtained Weiss constant θ = −10.65 K indicates little antiferromagnetic interactions exist through the bipy bridge which demonstrates it should exclude efficient direct exchange. According to the preceding structure description of 2, no appropriate model could be used for fitting the magnetic properties of such a system. 3616

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Figure 6. Temperature dependence of magnetic susceptibility in the form χmT, χm, and 1/χmT (inset) for compounds 2 (a), 3 (b), 4 (c), 5 (d), and 6 (e). The red line shows the best-fit curve to the Curie−Weiss fit law.

Comparative investigations on similar compounds 3−5 have also been performed, which may be help to understand the contributions of different auxiliary ligands to the magnetic properties. At room temperature, χMT is equal to 8.37 cm3 mol−1 K, which is much higher than the spin-only value of 3.75 cm3 mol−1 K based on two Co(II) ions (g = 2 and s = 3/2) because of the prominent orbital contribution. As illustrated in Figure 6, the χMT value of three polymers decreases as the temperature decreases. The plots of 1/χm versus T for compounds 3−5 give three straight lines over 50 K, respectively. Fitting the curves gives the parameters C = 9.12(4) cm3 mol−1 K and θ = −25.4(2) K for 3, C = 10.41(6) cm3 mol−1 K and θ = −22.8(3) for 4, C = 7.02(2) cm3 mol−1 K and θ = −23.74(3) for 5, respectively. The negative θ values indicate antiferromagnetic interactions exist between neighboring Co(II) ions. While separations were bridged through the long spacer ligands dpe, pdp, and bpe, it should exclude an efficient direct exchange between Co(II) centers; therefore, the overall antiferromagnetic interaction should be mainly attributed to the μ2-H2O bridge in trinuclear Co for 3−5. However, it should be noted that χMT versus T values of 6 decrease dramatically to 1.36 cm3 mol−1 K at 1.8 K with cooling below 7 K. The magnetic properties compound 6 is not similar to that of compounds 3−5. Fitting the curves gives the parameters C = 10.36(2) cm3 mol−1 K and θ = −24.1(4) K. In the lower temperature region (from 7 to 1.8 K), the characteristic of the weak antiferromagnetic coupling effect may be correlated with the zero-field splitting of the 3A2g state18

and structures (trinuclear or 3D and binuclear or 2D) in compounds 3−5 and 6.

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CONCLUSION Under hydrothermal conditions, the reactions of Co salt with H3BCPBA in the presence or absence of auxiliary ligands generate six novel coordination polymers with different structures. The photochemical properties in the solid state at room temperature were performed. Compound 1 is a 0D structure. Compound 2 is a 2D sheet structure with two 2D → 2D interpenetration frameworks. Compounds 3−5 are similar and possess 3D networks with two 3D → 3D interpenetration frameworks. In compound 6, H3BCPBA and bpp ligand link Co centers to generate a 2D sheet structure which is further connected by intermolecular hydrogen bonds to form a 3D supramolecular structure. Magnetic susceptibility measurements indicate that compounds 2−6 exhibit antiferromagnetic coupling between adjacent Co(II) ions.

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ASSOCIATED CONTENT

S Supporting Information *

Six X-ray crystallographic files in CIF format; selected bond lengths, angles, UV absorbance, TGA, PXRD and IR in PDF format. This information 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]. Fax: 86-25-83314502. 3617

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Notes

Inc.: Madison, WI. (b) Platon Program: Spek, A. L. Acta Crystallogr., Sect. A 1990, 46, 194. (13) (a) Wang, H. L.; Zhang, D. P.; Sun, D. F.; Chen, Y. T.; Zhang, L. F.; Tian, L. J.; Jiang, J. Z.; Ni, Z. H. Cryst. Growth Des. 2009, 9, 5273. (b) Huang, F. P.; Tian, J. L.; Gu, W.; Liu, X.; Yan, S. P.; Liao, D. Z.; Cheng, P. Cryst. Growth Des. 2010, 10, 1145. (14) (a) Wang, X. F.; Wang, Y.; Zhang, Y. B.; Xue, W.; Zhang, J. P.; Chen, X. M. Chem. Commun. 2012, 48, 133. (b) Niu, D.; Yang, J.; Guo, J.; Kan, W. Q.; Song, S. Y.; Du, P.; Ma, J. F. Cryst. Growth Des. 2012, 5, 2397. (15) Das, B. K.; Bora, S. J.; Chakrabortty, M.; Kalita, L.; Chakrabarty, R.; Barmana, R. J. Chem. Sci. 2006, 487. (16) Liu, S. Q.; Kuroda-Sowa, T.; Konaka, H.; Suenaga, Y.; Maekawa, M.; Mizutani, T.; Ning, G. L.; Munakata, M. Inorg. Chem. 2005, 44, 1031. (17) (a) Zheng, L. M.; Wang, X. Q.; Wang, Y. S.; Jacobson, A. J. J. Mater. Chem. 2001, 11, 1100. (b) Mabbs, F. E.; Machin, D. J. Magnetism and Transition Metal Complexes; Chapman and Hall: London, 1973. (18) (a) Calatayud, M. L.; Castro, I.; Sletten, J.; Cano, J.; Lloret, F.; Faus, J.; Julve, M.; Seitz, G.; Mann, K. Inorg. Chem. 1996, 35, 2858. (b) Lin, J. G.; Su, Y.; Tian, Z. F.; Qiu, L.; Wen, L. L.; Lu, Z. D.; Li, Y. Z.; Meng, Q. J. Cryst. Growth Des. 2007, 7, 2526.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (Nos. 91022011; 20971065; 21021062) and National Basic Research Program of China (2010CB923303).

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