Co(II) Coordination Polymers: Positional Isomeric Effect, Structural and

Feb 4, 2010 - Synopsis. We present here the synthesis, structural characterizations, and magnetic properties of ten Co(II) coordination polymers based...
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DOI: 10.1021/cg901014r

Co(II) Coordination Polymers: Positional Isomeric Effect, Structural and Magnetic Diversification

2010, Vol. 10 1145–1154

Fu-Ping Huang, Jin-Lei Tian, Wen Gu, Xin Liu, Shi-Ping Yan,* Dai-Zheng Liao, and Peng Cheng Department of Chemistry, Nankai University. Tianjin 300071, PR China. Received August 23, 2009; Revised Manuscript Received December 9, 2009

ABSTRACT: To systematically investigate the influence of the positional isomeric ligands on the structures and magnetic properties of their complexes, we synthesized ten Co(II) complexes with three positional isomeric dipyridyl ligands (4,40 -bpt, 3,40 -bpt and 3,30 -bpt), as well as the phenyl dicarboxylate anions, namely, [Co(o-BDC)(4,40 -bpt)(H2O)] (1), [Co(o-BDC)(3,30 bpt)(H2O)] (2), [Co(3-Cl-o-BDC)(4,40 -bpt)(H2O)] (3), [Co(3-Cl-o-BDC)(3,40 -bpt)] (4), [Co(m-BDC)(3,40 -bpt)] (5), [Co(mBDC)(3,30 -bpt)] 3 H2O (6), [Co2(5-NO2-m-BDC)2(3,40 -bpt)(H2O)4] (7), [Co(5-NO2-m-BDC)(3,30 -bpt)] 3 H2O (8), [Co(p-BDC)(3,40 -bpt)2(H2O)2] 3 2H2O (9), and [Co(p-BDC)(3,30 -bpt)2(H2O)2] 3 2H2O (10) (o-BDC = 1,2-benzenedicarboxylate anion, 3-Clo-BDC = 3-Cl-1,2-benzenedicarboxylate anion, m-BDC = 1,3-benzenedicarboxylate anion, 5-NO2-m-BDC = 5-NO2-1,3benzenedicarboxylate anion, p-BDC = 1,4-benzenedicarboxylate anion). Structural analysis reveals that the phenyl dicarboxylate anions display versatile coordination modes to manage the Co(II) ions to form 1-D chains or 2-D layers, which are further extended via the isomeric bpt connectors in different directions, to give rise to a variety of coordination polymers, such as 2-D square 44-sql layer (for 1-5), 3-D CsCl net (for 6), 2-D honeycomb 63-hcb layer (for 7), 1-D double chain (for 8), and 1-D decorated chain (for 9-10). This work indicates that the isomeric effects of the bpt ligands are significant in the construction of these network structures, based on same Co(II) centers. Furthermore, these compounds exhibit different magnetic behaviors. In 1-3, although Co(II) ions have the similar structural characteristic, the phenomenon of spin-canting in 2 can be observed more easily than 1 and 3, which may reveal that the different orientations of the pyridyl groups in bpt isomer have a profound impact on the magnetic properties of the solids. In 4, 5, and 7, which all are bridged by asymmetric 3,40 -bpt ligand, spin-canting in 7 can be observed more easily than 4 and 5 because the adjacent Co(II) ions are related to a inversion center in 4 and 5. Moveover, there are approximate isotropic Co(II) ions in 4. In addition, the magnetic behavior of compounds 6 and 8 was also studied and indicated the existence of antiferromagnetic interactions.

*To whom correspondence should be addressed. Tel.: þ86-022-23505063. Fax: þ86-022-23502779. E-mail: [email protected].

used as spacers to form open MOFs.11,12 A couple of bent N, N0 -donor ligands, introducing moieties between the two pyridyl groups, have been used as building blocks to extend coordination polymers.13-16 Here, we introduce the 1H-1,2,4triazole moiety between the two pyridyl groups to design and synthesize three positional isomeric N,N0 -donor ligands: 4,40 bpt, 3,40 -bpt, and 3,30 -bpt (Chart 1), which are chosen as bridging ligands base on the following considerations: (i) They can act as bridges between metal centers, thus mediating exchange coupling. (ii) The prototropy and conjugation between the 1H-1,2,4-triazole and pyridyl groups not only alter the electron density in different parts of the molecules, but also make the ligands more flexible.17 In addition, the different positions of the pyridyl N atoms in the positional isomeric ligands maybe benefit the formation of different topological structures. From a magnetic perspective, metal carboxylates (and in particular those Co(II) carboxylates) have shown to be potential candidates to construct molecular magnets, exhibiting various bulk magnetic behavior, such as antiferromagnetism, ferromagnetism, spin canting, ferrimagnetism, and so on.18 To explore how variation of the orientations of the pyridyl groups in bpt isomers, as well as the carboxylate groups in phenyl dicarboxylate anions, affect the structural and magnetic properties significantly, a series of Co(II) coordination polymers are synthesized successfully, namely, [Co(o-BDC)(4,40 -bpt)(H2O)] (1), [Co(o-BDC)(3,30 -bpt)(H2O)] (2), [Co(3Cl-o-BDC)(4,40 -bpt)(H2O)] (3), [Co(3-Cl-o-BDC)(3,40 -bpt)]

r 2010 American Chemical Society

Published on Web 02/04/2010

Introduction The design and synthesis of coordination polymers are of great interest not only because of their intriguing variety of architectures and topologies but also because of their tremendous potential applications in nonlinear optics, catalysis, gas adsorption, luminescence, magnetism, and medicine.1-5 Studies in this field have mainly focused on the design and preparation, as well as the structure-property relationships. Significant progress has been achieved;6 however, it is still a great challenge to rationally prepare and predict their exact structures. The resulting structures are determined by several factors, including the coordination geometry of the central metal ions, ligand structure, solvents, metal-ligand ratio, pH value, counterions, and so on.7-9 Reasonable design of ligands is usually a useful and important way of studying the controllable synthesis of the molecular architecture. With this understanding, one crucial aim of this work is to explore the essential factors of positional isomeric ligands for regulating the structural assembly, which may provide further insights in designing new hybrid crystalline materials. Because an accurate prediction of the final structure is impossible, the exploration of the positional isomeric factors for the ultimate structures is not trivial.10 In the decades, indeed, the linear 4,40 -bipyridine (4,40 -bpy) and 4,40 -bipyridine-like N,N0 -donor building blocks are often

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Figure 1. Structure of 1 showing the local coordination environments of the Co(II) atoms (left), the 1-D [Co(o-BDC)]n zigzag chain (top right) and the 2-D layer of 1 formed by the [Co(o-BDC)]n chain and 4,40 -bpt connector (bottom right).

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Figure 2. Structure of 2 showing the local coordination environments of the Co(II) atoms (left), the 1-D [Co(o-BDC)]n zigzag chain (top right), and the 2-D layer of 2 formed by the [Co(o-BDC)]n chain and 3,30 -bpt connector (bottom right).

Chart 1. Positional Isomeric Bridging Ligands (4,40 -bpt, 3,40 bpt, and 3,30 -bpt) Used in This Work

(4), [Co(m-BDC)(3,40 -bpt)] (5), [Co(m-BDC)(3,30 -bpt)] 3 H2O (6), [Co2(5-NO2-m-BDC)2(3,40 -bpt)(H2O)4] (7), [Co(5-NO2m-BDC)(3,30 -bpt)] 3 H2O (8), [Co(p-BDC)(3,40 -bpt)2(H2O)2] 3 2H2O (9), and [Co(p-BDC)(3,30 -bpt)2(H2O)2] 3 2H2O (10). Because of the low solubility of the ligands used and the resultant difficulty to grow crystals of coordination polymers, a hydrothermal technique was adopted in this paper to put the designed strategy into practice. Results and Discussion Description of the Crystal Structures. [Co(o-BDC)(4,40 bpt)(H2O)] (1). Compound 1 has a 2-D polymeric coordination layer structure as depicted in the reference reported recently (Figure 1).19 [Co(o-BDC)(3,30 -bpt)(H2O)] (2). Compound 2 has a 2-D polymeric coordination layer structure, which is similar to that of 1. Both crystallographically independent Co(II) ions lie on inversion centers and display a similar octahedral environment, which is provided by two pyridyl N donors and four carboxylate/water O atoms. As illustrated in Figure 2, the adjacent Co(II) centers are connected by syn-anti carboxylate bridge from o-BDC with a separation of 5.281(4) A˚ to furnish a polymeric chain. These 1-D arrays are further interlinked through 3,30 -bpt spacers (via syn-anti mode, as shown in Scheme 1) to generate a 2-D (4,4) coordination layer along the bc plane with a Co 3 3 3 Co separation of 12.092(3) A˚. [Co(3-Cl-o-BDC)(4,40 -bpt)(H2O)] (3). With a slight change in the structural carboxylate from o-BDC to a dicarboxylate with a substituting group, 3-Cl-o-BDC, a 2D polymeric coordination layer, similar to that of 1, was generated under similar reaction conditions. In 3, both crystallographically independent Co(II) ions lie on inversion centers and display a similar octahedral environment, which is provided by two pyridyl N donors and four carboxylate/

Figure 3. Structure of 3 showing the local coordination environments of the Co(II) atoms (left) and the 2-D layer of 3 (right).

Scheme 1. Coordination Modes of 3,30 -bpt Ligands Used in the Construction of Coordination Polymer Frameworks

water O atoms. As illustrated in Figure 3, the adjacent Co(II) centers are connected by syn-anti carboxylate bridge from o-BDC with a separation of 5.431(3) A˚ to furnish a polymeric chain. These 1-D arrays are further interlinked through 4,40 -bpt spacers to generate a 2-D (4,4) coordination layer along the bc plane with a Co 3 3 3 Co separation of 14.093(2) A˚. [Co(3-Cl-o-BDC)(3,40 -bpt)] (4). The asymmetric unit of 4 consists of one independent Co(II) cation, one independent 3,40 -bpt molecule, and one independent 3-Cl-o-BDC anion. The Co(II) ions exhibit a distorted five-coordinated trigonaldipyramid environment with three carboxylic O atoms from three 3-Cl-o-BDC anions and two N atoms from two 3,40 -bpt ligands. As shown in Figure 4, one carboxylate is monodentate whereas the other adopts a syn-syn bridging mode. As a consequence, the adjacent Co(II) atoms are bridged by three 3-Cl-o-BDC ligands to result in 1-D double chains along the a axis, which are further linked by 3,40 -bpt ligands to generated a sql net with the node defined by a Co(II)

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Figure 4. Structure of 4 showing the local coordination environments of the Co(II) atoms (top left), the 1-D [Co(3-Cl-o-BDC)]n chain (bottom left), and the 2D layer of 4 (right).

Figure 5. Structure of 5 showing the local coordination environments of the Co(II) atoms (top left), the 1-D [Co(m-BDC)]n chain (bottom left), and the 2-D layer of 5 (right).

dimer. The Co 3 3 3 Co distance separated by the 3,40 -bpt linker is 13.943(5) A˚, and it is 3.769(5) A˚ in the dimeric unit bridged by a pair of carboxylate groups. [Co(m-BDC)(3,40 -bpt)] (5). Compound 5 has a 2-D polymeric coordination layer structure, which is similar to that of 4. The asymmetric unit of 5 consists of one independent Co(II) cation, one independent 3,40 -bpt molecule, and one independent m-BDC anion. The Co(II) ions exhibit a distorted octahedral geometry, which is provided by two pyridyl N donors and four carboxylate O atoms. As shown in Figure 5, one carboxylate is chelate, whereas the other adopts a syn-anti bridging mode. As a consequence, the adjacent Co(II) atoms are bridged by three m-BDC ligands to result in 1-D double chains, which are further linked by 3,40 -bpt ligands to generated a sql net with the node defined by a Co(II) dimer. The Co 3 3 3 Co distance separated by the 3,40 -bpt inker is 13.925(3) A˚, and it is 4.330(4) A˚ in the dimeric unit bridged by a pair of carboxylate groups. [Co(m-BDC)(3,30 -bpt)] 3 H2O (6). As shown in Figure 6, the asymmetric unit of 6 has one crystallographically independent Co(II) center, displaying a slightly distorted octahedral coordination geometry. The Co(II) center is coordinated by four equatorial O atoms from three m-BDC ligands and two apical N atoms from two 3,30 -bpt ligands. Each m-BDC ligand links three Co(II) centers: one carboxylato group chelating to one Co atom, and the other carboxylate group coordinating to two Co atoms through a syn-syn mode, which bridge the Co atoms to give a [Co(CO2)2Co] dimeric unit. The adjacent dimeric units are linked though the coordination by two carboxylates donor in m-BDC ligands, resulting in the generation of a 2D (4,4)-sql layer architecture in the crystallographic bc plane, The layer is further connected by 3,30 -bpt pillars to generate a noninterpenetrating 3D open framework with 1-D solvent-filled

Figure 6. Structure of 6 showing the local coordination environments of the Co(II) atoms (top left), the 2D [Co(m-BDC)]n layer (top right), the overall 3D network (bottom left), and the schematic representation of the (42464)-CsCl topology of 6 (bottom right).

channel. The Co 3 3 3 Co distance separated by the 3,30 -bpt inker is 11.902(3) A˚, and it is 4.193(3) A˚ in the dimeric unit bridged by a pair of carboxylate groups. Because of the steric hindrance of the dicarboxylate, the entity charge balance, and coordination mode of the metal ion, the m-BDC ligand should find it difficult to form a 3D structure in the absence of bridge ligands.20 In fact, the appropriate degree of subsidiary ligand 3,30 -bpt is probably favorable for a self-filling structure (the syn-anti 3,30 -bpt bridge (Scheme 1) in the interlayer region bridges across the diagonal of a single window in the (4, 4)-sql net). A better insight into the nature of this intricate architecture can be achieved by topological analysis. In 6, each [Co(CO2)2Co] dimeric unit links eight nearest neighbors through m-BDC and 3,30 -bpt ligands, so we can define it as an eight-connected node, whereas m-BDC and 3,30 -bpt ligands as linkers. Thus, the structure can be described as a body-centered-cubic structure (bcu, normally called the CsCl net) with the Schl€ afli symbol of 42464. To our knowledge, this is the first example with CsCl-type topology based on bimeric cobalt units.21 [Co2(5-NO2-m-BDC)2(3,40 -bpt)(H2O)4] (7). Interestingly, with a slight change in the structural carboxylate from m-BDC to a dicarboxylate with a substituting group, 5-NO2m-BDC, a honeycomb net was generated upon reaction with the same metal salt. However, differences exist between the complexes 5 and 7. The asymmetric unit of 7 consists of two Co(II) cations, one 3,40 -bpt molecule, two 5-NO2-m-BDC anions, and four coordinated water molecules. Both crystallographically independent Co(II) ions display a similar octahedral environment, which is provided by one pyridyl N donors, three carboxylate O atoms, and two water O atoms. As shown in Figure 7, one carboxylate is monodentate, whereas the other is chelate. As a consequence, the adjacent Co(II) atoms are bridged by two 5-NO2-m-BDC ligands and one 3,40 -bpt ligands to generate a honeycomb

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Figure 10. Structure of 10 showing the local coordination environments of the Co(II) atoms (left) and the 1-D chain of 10 (right). Figure 7. Structure of 7 showing the local coordination environments of the Co(II) atoms (left) and the 2-D layer of 7 (right).

Figure 8. Structure of 8 showing the local coordination environments of the Co(II) atoms (left) and the 1-D chain of 8 (right).

Figure 9. Structure of 9 showing the local coordination environments of the Co(II) atoms (left) and the 1-D chain of 9 (right).

net. The Co 3 3 3 Co distance separated by the 3,40 -bpt inker is 11.392(2) A˚, and that separated by the 5-NO2-m-BDC is 8.772(3) A˚. [Co(5-NO2-m-BDC)(3,30 -bpt)] 3 H2O (8). The asymmetric unit of 8 consists of one independent Co(II) cation, one 3,30 bpt molecule, and one 5-NO2-m-BDC anion. The Co(II) ions exhibit a distorted five-coordinated trigonal-dipyramid environment with three carboxylic O atoms from three 5-NO2-m-BDC anions, two N atoms from two 3,30 -bpt ligands. As shown in Figure 8, one carboxylate is monodentate whereas the other adopts a syn-syn bridging mode, which bridges the Co atoms to give a [Co(CO2)2Co] dimeric unit. As a consequence, the adjacent dimers are bridged by 5-NO2-m-BDC ligands to result in 1-D double chains along the a axis, which are further stabled by 3,30 -bpt ligands (via syn-syn mode). The Co 3 3 3 Co distance separated by the 3,30 -bpt inker is 7.601(3) A˚, and it is 3.915(4) A˚ in the dimeric unit bridged by a pair of carboxylate groups. [Co(p-BDC)(3,40 -bpt)2(H2O)2] 3 2H2O (9). Compound 9 has a 1-D polymeric structure with neutral {[Co(p-BDC)(3,40 -bpt)2(H2O)2] 3 2H2O} as repeating units, As shown in Figure 9, Co1 in 9 was located on a crystallographic inversion center and coordinated octahedrally by two 3,40 -bpt N atoms, two p-BDC carboxylato O atoms, and two water molecules. The CoN2O4 octahedron around the metal center was completed. The p-BDC spacers bridge adjacent Co(II) atoms with the Co 3 3 3 Co separation of 11.443(3) A˚, leading to a 1-D chain along the crystallographic c axis. Both of the carboxylic groups in the p-BDC ligand in 9 adopt monodentate modes to connect with two Co atoms and the 3,40 -bpt ligand are in monodentate coordination but bridging fashions.

[Co(p-BDC)(3,30 -bpt)2(H2O)2] 3 2H2O (10). Compound 10 has a 1-D polymeric coordination layer structure, which is similar to that of 9, The Co(II) is coordinated by two carboxylato O atoms from two p-BDC ions, two N atoms from two 3,30 -bpt ligands and two water molecules (Figure 10). The geometry around the metal center is a slightly distorted octahedron. The p-BDC spacers bridge adjacent Co(II) atoms with the Co 3 3 3 Co separation of 11.383(2) A˚, leading to a 1-D chain along the crystallographic c axis. Both of the carboxylic groups in the p-BDC ligand in 10 adopt monodentate modes to connect with two Co atoms, and the 3,30 -bpt ligand are in monodentate coordination fashions. Structural Diversity of MOFs 1-10. It is noteworthy that a variety of framework structures can be achieved on the basis of the choice of the phenyl dicarboxylate anions with differently oriented carboxyl groups and triazole-containing dipyridine isomers with differently oriented pyridyl groups as building blocks, and the same cobalt ions. The phenomenon of structural diversification in 1-10 may mainly arise from two sources: (i) These dicarboxylate anions exhibit several coordination patterns, in which the carboxylate groups can adopt the unidentate, chelating, and bridging modes, respectively. ii) Although the bpt isomers always behave as the bidentate connectors in these coordination networks (partial monodentate in 9 and 10), the differently oriented pyridyl N atoms in these isomers may play significant roles in the formation of different topological structures. Fom the viewpoint of crystal engineering, we can generally discover that in all these structures the dicarboxylate isomers link the metal centers to form 1-D chains (for MOFs 1-5 and 7-10) or 2-D layer (for MOF 6), and their final structural discrepancies mainly come from the further extension of these arrays by the isomeric bpt connectors in different directions, which however cannot be accurately forecasted. Magnetic Properties. The magnetic properties of 1-8 were investigated in the 2.0-300.0 K range at 1000 Oe. For 1 (Figure 11a), the thermal evolution of χM-1 obeys CurieWeiss law, χM = C/(T - θ) over the whole temperature range from 300 to 40 K with Weiss constant, θ, of -21.14 K and the Curie constant, C, of 3.30 cm3 K mol-1. The χMT value at 300 K is 3.07 cm3 K mol-1 (4.96 μB), which is much higher than the expected value (1.88 cm3 K mol-1, 3.87 μB) of one magnetically isolated spin-only Co (II) ions (S = 3/2, g = 2.0) but close to the value expected when the spin momentum and orbital momentum exist independently [5.20 μB; μLS = [L(L þ 1) þ 4S(S þ 1)]1/2; L = 3, S = 3/2]. This indicates that an important contribution of the orbital angular momentum typical for the 4T1g ground term is involved. From 300 to 14 K, the magnetic moments decrease with the decreasing temperature, which basically corresponds to a single-ion behavior, which accounts for the splitting of the 4T1 term into six Kramers doublets as a consequence of the combined effect of spin-orbit coupling and distortion from ideal octahedral symmetry. When the temperature is decreased

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Figure 11. (a) Plots of χMT vs T (blue) and 1/χM vs T (black) for 1, the lines across χMT and 1/χM curves represent the best fit. (b) Field dependence of magnetization for 1 and the hysteresis loop at 2 K (inset).

below 14 K, there is a strange up-down-up behavior. This deviation below 14 K is mainly the result of the presence of intramolecular weak ferromagnetism caused by small uncompensated AF spin-canting or some impurity.22 The combined magnetic interactions can be approximated with the following simple phenomenological equation:23 χM T ¼ A expð -E1 =kTÞ þ B expð -E2 =kTÞ

ð1Þ

In the equation, A þ B equals the Curie constant (C ≈ 2.8-3.4 cm3 K mol-1 for octahedral Co(II) ions), and E1 and E2 represent the “activation energies” corresponding to the spin-orbit coupling and the super exchange interaction. 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 µ exp(RJ/2kT)]. The best fit of the experimental data of 1 in the temperature range about 12-300 K, gives A þ B = 3.31 cm3 K mol-1, E1/k = 61.37 K, and E2/k = -0.51 K (corresponding to J = 1.02 K). Notice that the negative E2/k is a very small number compared to E1/k, and this is in agreement with the much weaker ferromagnetic interaction at low temperatures. It should be pointed out that eq 1 was developed and is typically used for evaluation of spin-orbit and intrachain coupling in regular 1-D chains through the noncritical scaling approach.24 Although complex 1 can be roughly treated as a 1-D-like system (very weak or nonexistent magnetic coupling via the 4,40 -bpt ligand), the values derived for E1/k and E2/k are only very approximate estimations for the effective coupling. The M(H) curve at 2.0 K (Figure 11b) shows a typical paramagnetic behavior without long-range magnetic ordering, that is, a featureless increase with the field until reaching the saturation moment of Ms = 2.60 Nβ at 50 kOe for Co(II) (expected range of 2-3 Nβ).25 This was further confirmed by the observed featureless FC/ZFC curve obtained at a very low field of 50 Oe (Figure S11 in the Supporting Information). For 2, which has structural and magnetic properties similar to those of 1, the χMT value at 300 K is 3.15 cm3 K mol-1, which is much higher than the expected value (1.88 cm3 K mol-1) of one magnetically isolated spin-only Co(II) ion (Figure 12a). As T is lowered, χMT continuously decreases and reaches a local minimum of 2.32 cm3 K mol-1 at about 18 K, and then it increases to a value of 2.90 cm3 K mol-1 at 10 K, before dropping quickly to 2.35 cm3 K mol-1

at 2 K. The magnetic behavior of 2 is unusual and interesting, indicative of a strong single-ion behavior admixture with a weak ferromagnetic interaction caused by spin-canting. The fit of the curve for χM-1 versus T plot of 2 to the Curie-Weiss law gives a good result in the temperature range of 40-300 K with C = 3.30 cm3 K mol-1 and θ = -14.84 K. The combined magnetic interactions can be approximated with the eq 1 over the temperature range about 16-300 K, give A þ B = 3.30 cm3 K mol-1, E1/k = 50.82 K and E2/k = -0.82 K (corresponding to J = 1.64 K). The presence of the bifurcation point between the FC/ ZFC magnetization curves below 10 K (Figures 12b) indicates the occurrence of the phase transition near 10 K. The reduced molar magnetization (M/Nβ value per Co(II) ions) at 2 K tends to 2.56 Nβ. Indeed, there is a rapid and abrupt increase of M/Nβ at low fields (Figures 12c and S13, Supporting Information). This is the signature of long-range order commented on above when dealing with the FC and ZFC magnetization curves at low temperature. The magnetizations measured below 10 K show hysteresis loops with a small coercive field of 100 Oe and a remnant magnetization of 0.019 Nβ indicating a soft-magnetic behavior of 2. The zero-field ac magnetic susceptibility measurement (Figure 12d) only exhibits the characteristic of an antiferromagnet at TN = 10.0 K, where the in-phase part χM0 reaches a maximum, while no obvious out-of-phase χM00 reflection was observed at the same temperature (which clearly revealed the occurrence of a magnetic long-range ordering and agreed with the previous magnetic studies, though detailed information of the magnetic entropy could not be derived yet from the current specific heat data, with the lattice contribution unknown). Thus, we think that complex 2 should be a hidden weak ferromagnet because of spin canting.26 Furthermore, the absence of a frequency dependence of ac susceptibility precludes the possibility of the spin glass or the movement of domain-wall in 2. For 3, unlike 1 and 2, the magnetic properties of this compound are quite ordinary (Figure 13). Its χMT value at 300 K is 3.04 cm3 K mol-1, which is higher than the expected value of one magnetically isolated spin-only Co(II) ions. Upon cooling, the χMT value decreases slightly and reaches a shallow minimum of 1.57 cm3 K mol-1 at 2.5 K, accounting for strong single-ion behavior of the Co (II) ions, and then increases to a value 1.59 cm3 K mol-1 at 2 K, accounting for spin-canting. The fit of the curve for χM-1 versus T plot of 3 to the Curie-Weiss law gives a good result in the

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Figure 12. (a) Plots of χMT vs T (blue) and χM-1 vs T (black), (b) field-cooled (FC) and zero-field-cooled (ZFC) magnetization, (c) field dependence of magnetization and the hysteresis loop at 2 K (Inset), and (d) temperature dependence of ac susceptibility at various frequencies of 2.

Figure 13. Plots of χMT vs T (blue) and χM-1 vs T (black) for 3. The lines across χMT and χM-1 curves represent the best fit.

Figure 14. Plots of χMT vs T (blue) and χM-1 vs T (black) for 4.

temperature range of 40-300 K with C = 3.37 cm3 K mol-1 and θ = -34.98 K. And the combined magnetic interactions can be approximated with the eq 1, give A þ B = 3.37 cm3 K mol-1, E1/k = 70.08 K, and E2/k = -0.26 K (corresponding to J = 0.52 K). Interestingly, complexes 1-3, with similar structural characteristics, exhibit the phenomenon of spin-canting. The canting in these complexes may be caused by the acentrosymmetric syn-anti carboxylato bridge of the chains of [-Co-O-C-O-Co-], which together with the anisotropy of the Co(II) ions, leads to the generation of a net magnetization within the chain.27 However, from the point of the structural-magnetic relation, the canting in 2 can be observed more easily than 1 and 3, which suggests that the

different orientations of the pyridyl groups in bpt isomers not only can affect the formation of their complexes but also can affect the magnetic properties of the final complexes. For 4 (Figure 14), the χMT value at 300 K is 2.50 cm3 K mol-1, which is slightly higher than the expected value (1.88 cm3 K mol-1) of one magnetically isolated spin-only Co(II) ions. As T is lowered, χMT decreases continuously to a value of 0.22 cm3 K mol-1 at 2 K. This behavior indicates a dominant antiferromagnetic interaction between the Co(II) ions in the structures. In 4, the Co 3 3 3 Co distance through the 3,40 -bpt bridge is 13.943 A˚, and it is 5.099 A˚ through the 3-Cl-o-BDC bridge. The long Co 3 3 3 Co distances exclude an efficient direct exchange between the Co(II) ions. Therefore, the overall antiferromagnetic interaction should

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Figure 15. Plots of χMT vs T (blue) and χM-1 vs T (black) for 5.

be mainly attributed to the magnetic exchange coupling within the [Co(CO2)2Co] dimeric unit. The χM-1 versus T plot of 4 is in correspondence with the Curie-Weiss law in the range of 100-300 K with C = 2.824 cm3 K mol-1 and θ = -43.235 K. For 5 (Figure 15), χMT drops down very slowly from 3.00 cm3 K mol-1 at 300 K to a shallow minimum at 16 K and then increases to a value 2.91 cm3 K mol-1 at 3 K, before dropping to 2.85 cm3 K mol-1 at 2 K. The Curie-Weiss fit of χM-1 above 20 K results in a Curie constant C = 3.023 cm3 K mol-1 and θ = -2.565 K. As for many Co2þ compounds, substantial spin-orbit coupling contributes to the high χMT and C values. The decrease at high temperature is related to the depopulation of the higher energy Kramer’s doublets of the Co(II) centers with a 4T1 ground state. The increase in the values of χMT below 16 K must be attributed to the presence of intramolecular ferromagnetic interactions between the Co(II) ions mediated by the syn-anti carboxylate bridge. Similar magnetism behavior of reported complex [Co2(μO2CMe)2(L8)2][BPh4]2, (L8 = 1,3-bis[3-(2-pyridyl)pyrazol1-yl]propane) has been investigated in detail.28 Complex 5 exhabits a typical paramagnetic behavior without longrange magnetic ordering, which was further confirmed by the observed featureless FC/ZFC curve obtained at a very low field of 50 Oe (Figure S15, Supporting Information). For 6 (Figure 16), the χMT value at 300 K is 2.48 cm3 K mol-1, which is slightly higher than the expected value (1.88 cm3 K mol-1) of one magnetically isolated spin-only Co(II) ions. As T is lowered, χMT decreases continuously to a value of 0.68 cm3 K mol-1 at 2 K. This behavior indicates a dominant antiferromagnetic interaction between the Co(II) ions in the structures. In 6, the Co 3 3 3 Co distance through the 3,30 -bpt bridge is 11.902 A˚, and it is 7.707 A˚ through the m-BDC bridge. The long Co 3 3 3 Co distances exclude an efficient direct exchange between the Co(II) ions. Therefore, the overall antiferromagnetic interaction should be mainly attributed to the magnetic exchange coupling within the [Co(CO2)2Co] dimeric unit. The χM-1 versus T plot of 6 is in correspondence with the Curie-Weiss law in the range of 30-300 K with C = 3.22 cm3 K mol-1 and θ = -21.191 K. For 7, the χMT value at 300 K is 3.16 cm3 K mol-1, which is much higher than the expected value (1.88 cm3 K mol-1) of one magnetically isolated spin-only Co(II) ions (Figure 17). As T is lowered, χMT continuously decreases and reaches a local minimum of 2.16 cm3 K mol-1 at about 16 K and then increases to a value 2.62 cm3 K mol-1 at 9 K, before dropping quickly to 1.88 cm3 K mol-1 at 2 K. The fit of the curve for

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Figure 16. Plots of χMT vs T (blue) and χM-1 vs T (black) for 6.

Figure 17. Plots of χMT vs T (blue) and χM-1 vs T (black) for 7.

χM-1 versus T plot of 7 to the Curie-Weiss law gives a good result in the temperature range of 50-300 K with C = 3.38 cm3 K mol-1 and θ = -21.25 K. The magnetic behavior of 7 is unusual and interesting, substantial spin-orbit coupling contributes to the high χMT and C values. The combination of negative θ and increasing χMT below 16 K suggests a strong single-ion behavior admixture with a weak ferromagnetic interaction. In 7, the nearest Co 3 3 3 Co distance through the interlayer H-bonds is 5.052 A˚; the long Co 3 3 3 Co distances exclude an efficient direct exchange between the Co(II) ions. Therefore, we believe that the weak ferromagnetic interaction is caused by spin-canting, which arises from the antisymmetric 3,40 -bpt bridge. The presence of the bifurcation point between the FC/ZFC magnetization curves below 9 K (Figures S17) indicates the occurrence of the phase transition near 9 K. The reduced molar magnetization (M/ Nβ value per Co(II) ions) at 2 K tends to 2.41 Nβ. Indeed, there is a rapid and abrupt increase of M/Nβ at low fields (Figure S19, Supporting Information). This is the signature of long-range order commented on above when dealing with the FC and ZFC magnetization curves at low temperature. Generally, spin canting can arise from two contributions: (1) the presence of an antisymmetric exchange and (2) the existence of single-ion magnetic anisotropy.25 3,40 -bpt (Chart 1), having no inversion center, may be used as a asymmetric bridge ligand to tilt the adjacent Co(II) coordination geometry, thus resulting in spin canting. In 7, the canting may be caused by the disposition of the [-Co-3,40 bpt-Co-], which, together with the anisotropy of the Co(II)

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ions and the adjacent Co(II) ions are related to no inversion center (7), shows the phenomenon of spin canting. If the compound where have anisotropic Co(II) ions and the adjacent Co(II) ions are related to a crystallographic inversion center (5), the spin canting can be observed less easily than 7. If the compound has approximate isotropic five-coordinated Co(II) ions and the adjacent Co(II) ions are related to a crystallographic inversion center (4), the spin canting disappears. This proves that the different orientations of the pyridyl groups in bpt isomeric ligands, together with the coordinated geometry of the Co(II) ions, play a crucial role in the complicated system for the investigation of spin canting as a function of various subtle structural factors. Experimental Section Figure 18. Plots of χMT vs T (blue) and χM-1 vs T (black) for 8.

ions. While in 4 and 5, spin canting is not observed because of (1) the existence of an inversion center in the Co2(CO2)2 or (2) the existence of magnetic isotropy in five-coordinated Co(II) ions.29 For 8, the χMT value at 300 K is 3.13 cm3 K mol-1, which is higher than the expected value (1.88 cm3 K mol-1) of one magnetically isolated spin-only Co(II) ions (Figure 18). The χMT value of 8 remains almost constant from 300 to 80 K and then decreases on further cooling, reaching a value of 0.72 cm3 K mol-1 at 2 K. This behavior indicates a dominant antiferromagnetic interaction between the Co(II) ions in the structures. In 8, the Co 3 3 3 Co distances through the 3,30 -bpt bridge and the 5-NO2-m-BDC bridge are both 7.601 A˚. The long Co 3 3 3 Co distances exclude an efficient direct exchange between the Co(II) ions. Therefore, the overall antiferromagnetic interaction should be mainly attributed to the magnetic exchange coupling within the [Co(CO2)2Co] dimeric unit. The χM-1 versus T plot of 8 is in correspondence with the Curie-Weiss law in the range of 20-300 K with C = 3.15 cm3 K mol-1 and θ = -1.891 K. Conclusions We present here a new family of Co(II) MOFs generated from mixed-ligand systems of three positional isomeric dipyridyl bridging ligands (4,40 -bpt, 3,40 -bpt, and 3,30 -bpt) and phenyl dicarboxylate anions under the coordinative-driven assembly. In general, the phenyl dicarboxylate anions with different orientation of the carboxylate groups play dominating roles in the construction of these crystalline materials; that is, they behave as linear or angular building blocks, which are further extended via the isomeric bpt connectors in different directions, diverse and interesting coordination architectures upon metal complexation could be produced readily. These results discover the isomeric effect of the mixed bridging ligands in molecular tectonics of the metal-organic frameworks. Accordingly, our present findings will further enrich the crystal engineering strategy and offer the possibility of controlling the formation of the desired network structures. The study of the magnetic properties of eight of the polymers reveals that the different orientations of the pyridyl groups in bpt isomeric ligands have a profound impact on the magnetic properties of the solid, thst is, (i) in complexes 1, 2, and 3, although with the similar structural characteristic, the phenomenon of spin-canting in 2 can be observed more easily than 1 and 3, and (ii) based on the asymmetric bridge, 3,40 -bpt, the compound where have anisotropic six-coordinated Co(II)

Materials and Physical Measurements. With the exception of the ligands of 4,40 -bpt, 3,40 -bpt, and 3,30 -bpt, which were prepared according to the literature procedure,30 all reagents and solvents for synthesis and analysis were commercially available and used as received. IR spectra were taken on a Perkin-Elmer spectrum One FT-IR spectrometer in the 4000-400 cm-1 region with KBr pellets. Elemental analyses for C, H and N were carried out on a Model 2400 II, Perkin-Elmer elemental analyzer. The magnetic susceptibility measurements of the polycrystalline samples were measured over the temperature range of 2-300 K with a Quantum Design MPMSXL7 SQUID magnetometer using an applied magnetic field of 1000 Oe. Field dependences of magnetization were measured using a flux magnetometer in an applied field up to 50 kOe generated by a conventional pulsed technique. A diamagnetic correction to the observed susceptibilities was applied using Pascal’s constants. X-ray powder diffraction (XRPD) intensities were measured on a Rigaku D/max-IIIA diffractometer (Cu-KR, λ = 1.54056 A˚). The singlecrystalline powder samples were prepared by crushing the crystals and scanned from 3 to 60° with a step of 0.1°/s. Calculated patterns of 1-10 were generated with PowderCell. Preparation. [Co(o-BDC)(4,40 -bpt)(H2O)] (1). A mixture containing Co(NO3)2 3 6H2O (145 mg, 0.5 mmol), 4,40 -bpt (112 mg, 0.5 mmol), o-H2BDC (83 mg, 0.5 mmol), NaOH (40 mg, 1 mmol), water (10 mL) and ethanol (5 mL) was sealed in a Teflon-lined stainless steel vessel (23 mL), which was heated at 140 °C for 3 days and then cooled to room temperature at a rate of 5 °C/h. Red block crystals of 1 were obtained and picked out, washed with distilled water and dried in air. Yield: 42% (based on Co(II)). Elemental analysis for C20H15CoN5O5 (%) Calcd: C, 51.78; H, 3.26; N, 15.08. Found: C, 51.69; H, 3.72; N, 14.48. IR (KBr, cm-1): 3220s, 1573s, 1538s, 1415s, 1171m, 1012w, 993m, 849w, 727s. [Co(o-BDC)(3,30 -bpt)(H2O)] (2). The same synthetic procedure as that for 1 was used except that 4,40 -bpt was replaced by 3,30 -bpt, giving red block X-ray-quality crystals of 2 in a 38% yield (based on Co(II)). Anal. Calcd for C20H15CoN5O5: C, 51.78; H, 3.26; N, 15.08. Found: C, 51.04; H, 3.47; N, 15.68. IR (cm-1): 3596m, 3155m, 1540s, 1410s, 1324s, 1169m, 1098w, 986s, 817s, 702s, 646m. [Co(3-Cl-o-BDC)(4,40 -bpt)(H2O)] (3). The same synthetic procedure as that for 1 was used except that o-H2BDC was replaced by 3Cl-o-H2BDC, yielding red block X-ray-quality crystals of 3 in a 70% yield (based on Co(II)). Anal. Calcd for C20H14ClCoN5O5: C, 48.16, H, 2.83; N, 14.04. Found: C, 48.06; H, 2.55; N, 14.43. IR (cm-1): 3365s, 1625s, 1542m, 1359s, 1000w, 847w, 742s, 615 m, 433w. [Co(3-Cl-o-BDC)(3,40 -bpt)] (4). The same synthetic procedure as that for 3 was used except that 4,40 -bpt was replaced by 3,40 -bpt, giving red block X-ray-quality crystals of 4 in a 48% yield (based on Co(II)). Anal. Calcd for C20H12ClCoN5O4: C, 49.97, H, 2.52; N, 14.59. Found: C, 49.06; H, 2.55; N, 14.43. IR (cm-1): 3383m, 1573s, 1387s, 1145m, 1016w, 993m, 824w, 742s. [Co(m-BDC)(3,40 -bpt)] (5). The same synthetic procedure as that for 1 was used except that 4,40 -bpt and o-H2BDC were replaced by 3,40 -bpt and m-H2BDC, respectively, giving red block X-ray-quality crystals of 5 in a 27% yield (based on Co(II)). Anal. Calcd for C20H13CoN5O4: C, 53.83, H, 2.94; N, 15.69; found: C, 53.11; H, 2.55; N, 15.34. IR (cm-1): 3152s, 1616s, 1572s, 1396s, 1173m, 1034w, 996m, 816s, 751s.

8.211 (2) 9.121 (2) 11.388 (2) 84.10 (3) 73.24 (3) 78.46 (3) 799.2 (3) 1 1.541 1.00 0.063 0.171 8.362 (2) 8.739 (2) 11.440 (2) 73.68 (3) 83.72 (3) 78.61 (3) 785.2 (3) 1 1.568 1.15 0.065 0.118 10.018 (2) 10.386 (2) 10.667 (2) 104.04 (3) 100.69 (3) 107.10 (3) 988.8 (3) 2 1.710 1.03 0.063 0.164 11.269 (2) 11.987 (2) 12.723 (3) 112.95 (3) 90.96 (3) 102.69 (3) 1533.8 (5) 2 1.800 1.13 0.066 0.129 12.345 (3) 12.155 (2) 13.603 (3) 90 95.20 (3) 90 2032.8 4 1.517 1.19 0.061 0.096 8.761 (2) 9.999 (2) 11.646 (2) 89.80 (3) 84.81 (3) 68.48 (3) 944.7 (3) 2 1.569 1.14 0.056 0.091

Fw crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) volume (A˚3) Z Fcalcd (g/cm3) GOF on F2 R1 [I > 2σ(I)] wR2 (all data)

8.151 (2) 10.740 (2) 12.808 (3) 105.81 (3) 95.87 (3) 111.27 (3) 979.7 (4) 2 1.691 1.04 0.088 0.244 8.321 (2) 10.561 (2) 12.470 (3) 108.65 (3) 100.27 (3) 108.03 (3) 939.4 (3) 2 1.641 1.10 0.037 0.089 8.053 (2) 10.533 (2) 13.076 (3) 66.93 (3) 80.25 (3) 68.36 (3) 948.1 (3) 2 1.626 1.11 0.044 0.098

7.703 (2) 19.216 (4) 12.647 (3) 90 97.33 (3) 90 1856.7 (6) 4 1.720 1.08 0.073 0.120

P1 P1 P1 P1 P21/n P1 P1 P1 P1

P21/n

9

C32H30CoN10O8 741.59 triclinic C20H14CoN6O7 509.30 triclinic

8 7

C28H23Co2N7O16 831.39 triclinic C20H15CoN5O5 464.30 monoclinic

6 5

C20H13CoN5O4 446.28 triclinic

Table 1. Crystal Data and Structure Refinement for 1-10

4

C20H14ClCoN5O5 498.74 triclinic C20H15CoN5O5 464.30 triclinic C20H15CoN5O5 464.30 triclinic formula

3 2 1

C20H12ClCoN5O4 480.73 monoclinic

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[Co(m-BDC)(3,30 -bpt)] 3 H2O (6). The same synthetic procedure as that for 5 was used except that 3,40 -bpt was replaced by 3,30 -bpt, giving red block X-ray-quality crystals of 6 in a 35% yield (based on Co(II)). Anal. Calcd for C20H15CoN5O5: C, 51.74; H, 3.25; N, 15.08. Found: C, 51.32; H, 3.52; N, 15.15. IR (cm-1): 3364m, 1623s, 1546s, 1483m, 1403s, 1167m, 1056m, 989m, 832m, 751s, 647s. [Co2(5-NO2-m-BDC)2(3,40 -bpt)(H2O)4] (7). The same synthetic procedure as that for 5 was used except that m-H2BDC was replaced by 5-NO2-m-H2BDC, yielding red block X-ray-quality crystals of 7 in a 45% yield (based on Co(II)). Anal. Calcd for C28H23Co2N7O16: C, 40.45; H, 2.79; N, 11.79. Found: C, 40.56; H, 2.53; N, 11.15. IR (cm-1): 3303m, 1620s, 1353s, 1195m, 1085s, 984m, 843m, 734m, 529m. [Co(5-NO2-m-BDC)(3,30 -bpt)] 3 H2O (8). The same synthetic procedure as that for 7 was used except that 3,40 -bpt was replaced by 3,30 -bpt, giving red block X-ray-quality crystals of 8 in a 53% yield (based on Co(II)). Anal. Calcd for C20H14CoN6O7: C, 47.17; H, 2.77; N, 16.50. Found: C, 47.32; H, 2.52; N, 17.15. IR (cm-1): 3424m, 3061s, 2666s, 1621s, 1564s, 1447m, 1381s, 1145m, 1057m, 990m, 847m, 754s, 655s. [Co(p-BDC)(3,40 -bpt)2(H2O)2] 3 2H2O (9). The same synthetic procedure as that for 5 was used except that m-H2BDC was replaced by p-H2BDC, giving red block X-ray-quality crystals of 9 in a 53% yield (based on Co(II)). Anal. Calcd for C32H30CoN10O8: C, 51.83; H, 4.07; N, 18.89; found: C, 51.04; H, 4.74; N, 18.68. IR (cm-1): 3405m, 1565s, 1419m, 1370s, 1145m, 991m, 752s, 694w, 646m. [Co(p-BDC)(3,30 -bpt)2(H2O)2] 3 2H2O (10). The same synthetic procedure as that for 9 was used except that 3,40 -bpt was replaced by 3,30 -bpt, giving red block X-ray-quality crystals of 10 in a 38% yield (based on Co(II)). Anal. Calcd for C32H30CoN10O8: C, 51.83; H, 4.07; N, 18.89. Found: C, 51.87; H, 4.45; N, 19.18. IR (cm-1): 3596m, 3314s, 1561s, 1490m, 1388s, 1196s, 1153m, 987s, 751s, 700s, 544m. Crystal Structure Determination. X-ray single-crystal diffraction data for 1-10 were collected with a Bruker SMART CCD instrument by using graphite monochromatic Mo-KR radiation (λ = 0.71073 A˚). The data were collected at 293(2) K, and there was no evidence of crystal decay during data collection. A semiempirical absorption correction was applied using SADABS, and the program SAINT was used for integration of the diffraction profiles.31 The structure was solved by direct methods with the program SHELXS-9732 and refined by full-matrix least-squares methods on all F2 data with SHELXL-97.33 The non-hydrogen atoms were refined anisotropically. Hydrogen atoms of water molecules were located in a difference Fourier map and refined isotropically in the final refinement cycles. Other hydrogen atoms were placed in calculated positions and refined by using a riding model. The final cycle of full-matrix least-squares refinement was based on observed reflections and variable parameters. Further crystallographic data and structural refinement details are summarized in Table 1. Selected bond lengths and bond angles are given in Table S1, Supporting Information. XRPD Results. To confirm whether the crystal structures are truly representative of the bulk materials, X-ray powder diffraction (XRPD) experiments have also been carried out for 1-10. The XRPD experimental and computer-simulated patterns of the corresponding complexes are shown in ESI, Figure S1-S10, Supporting Information. Although the experimental patterns have a few unindexed diffraction lines and some are slightly broadened in comparison with those simulated from the single crystal models, it can still be considered favorably that the bulk synthesized materials and the as-grown crystals are homogeneous for 1-10. CCDC-739489 to -739497 for 2-10 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20771063 and 90922032). Supporting Information Available: Crystallographic data in CIF format for 1-10, tables for pertinent bonding parameters, additional information about physical characterizations (XRD), and

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more magnetic data for compounds 1, 2, 5, and 7. This material is available free of charge via the Internet at http://pubs.acs.org.

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