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DOI: 10.1021/cg9005696

Unique (3,13)-Connected Coordination Framework Based on Pentacobalt Clusters Constructed from the (3,12)-Connected Analogue and 4,40 -Bipyridyl Spacer: Structural and Magnetic Aspects

2009, Vol. 9 4239–4242

Ming-Hua Zeng,*,† Hua-Hong Zou,† Sheng Hu,† Yan-Ling Zhou,† Miao Du,*,‡ and Hao-Ling Sun§ † Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources (Ministry of Education), School of Chemistry & Chemical Engineering, Guangxi Normal University, Guilin 541004, P. R. China, ‡College of Chemistry and Life Science, Tianjin Key Laboratory of Structure and Performance for Functional Molecule, Tianjin Normal University, Tianjin 300387, P. R. China, and § Department of Chemistry, Beijing Normal University, Beijing 100875, P. R. China

Received May 27, 2009; Revised Manuscript Received August 17, 2009

ABSTRACT: Two related CoII 5-aminoisophthalate coordination frameworks [Co5(μ3-OH)2(5-NH2-bdc)4(H2O)2] 3 7.5H2O (1, 5NH2-bdc = 5-aminoisophthalate) and [Co5(μ3-OH)2(5-NH2-bdc)4(bpy)0.5(H2O)] 3 3H2O (2, bpy = 4,40 -bipyridine) based on [Co5(μ3-OH)2] clusters have been synthesized by hydrothermal reaction, structurally described, as well as properties characterized, that have unusual 3D networks with high connectivity (3,12) and (3,13)-connected, respectively. Notably, the unique (3,13)-connected framework is constructed from the (3,12)-connected network analogue and 4,40 -bipyridyl spacer. Magnetic studies show that a dominant antiferromagnetic coupling between Co(II) ions, and the generation of relatively effective noncompensated moments at very low temperature, mainly arises from the cooperative magnetic effect of the intercluster arrangement in the Td-SP-3Oh mixedgeometry-based pentamer.

*Corresponding author. E-mail: [email protected] (M.-H. Zeng); [email protected] (M. Du). Fax: 86 773 2120-958.

Perkin-Elmer PE Spectrum One FT/IR spectrometer using the KBr pellet. Elemental analyses (C, H, N) were performed on a Perkin-Elemer PE 2400 II CHN elemental analyzer. The TG analysis was performed on Pyris Diamond TG/DTA. Detailed ac and dc magnetic data were collected on a Quantum Design MPMS SQUID-XL-7 magnetometer using the crushed singlecrystal samples. Hydrothermal Synthesis. Preparation of 1. A mixture of Co(NO3)2 6H2O (0.447 g, 1.5 mmol), NaOH (0.060 g, 1.5 mmol), 5-NH2-bdc (0.182 g, 1.0 mmol), and H2O (15.0 mL) was stirred for 20 min in air. Then the mixture was placed in a 23 mL Teflon-lined autoclave and heated at 140 °C for 72 h. The autoclave was cooled over a period of 20 h at a rate of 5 °C h-1, and 1 as black-red block crystals were collected by filtration, washed with water, and dried at ambient temperature. Yield: 6.0 mg, 2% (based on 5-NH2-bdc). Elemental analyses for 1: C32H26Co5N4O27.5 (1201.22): Calc (%): C, 32.00; H, 2.18; N, 4.66. Found (%): C, 32.12; H, 2.36; N, 4.45. IR for 1 (KBr, cm-1): 3352s, 3278vs, 3169s, 1612s, 1560vs, 1541vs, 1483w, 1444m, 1381vs, 1330m, 1074m, 972w, 799w, 778m, 726m, 638w, 519w. Preparation of 2. A mixture of Co(NO3)6 6H2O (0.447 g, 1.5 mmol), 5-aminoisophthalic acid (0.182 g, 1.0 mmol), 4,40 -bipyridine (0.078 g, 0.5 mmol), NaOH (0.060 g, 1.5 mmol), triethylamine (0.3 mL) and distilled water (10 mL) was placed in a 23 mL Teflon-lined autoclave and heated at 150 °C for 144 h. Then, the autoclave was cooled over a period of 12 h at a rate of 10 °C h-1, and black-red block crystals of 2 were collected by filtration, washed with water, and dried at ambient condition. Yield: 148.7 mg, 50% (based on 5-NH2-bdc). Element analyses for 2: C37H28Co5N5O22 (1189.29): Calcd (%): C, 37.37; H, 2.37; N, 5.89. Found (%): C, 37.26; H, 2.68; N, 5.78. IR data (cm-1): 3418s, 1683vs, 1393vs, 1306s, 1219w, 1196w, 1152w, 1072w, 1028w, 963w, 905w, 842m, 712w, 695w, 669w, 525w, 495w. X-ray Crystallography. The diffraction data were collected on a Bruker Apex CCD diffractometer with graphite monochromated Mo KR radiation (λ=0.71073 A˚) at 173 K. Absorption corrections were applied by using SADABS. All the structures were solved by direct methods and refined with full-matrix leastsquares technique using SHELXTL.7 All non-hydrogen atoms

r 2009 American Chemical Society

Published on Web 08/20/2009

The research in cluster-based coordination polymers has attracted considerable attention for their intriguing architectures and potential applications as functional materials.1 The utilization of metal clusters as secondary building blocks (SBUs) has proved to be a feasible method to construct unusual coordination networks with high connectivity.1,2 In this regard, polynuclear (larger than four) CoII clusters have been rarely observed in such crystalline materials, which still represent a challenge and interest in magnetochemistry.3 Recently, Zaworotko et al. have reported a 3-D coordination framework based on {Co6} clusters, which shows a decorated 12-connected fcu net.2a Notably, most of the cluster-based coordination polymers are obtained by oxo-centered metal clusters with carboxylate and/or pyridyl ligands.4 Also, the incorporation of μ-hydroxyl group is likely to induce the aggregation of metal cores and thus affords clusters with interesting structural and magnetic features.3 Moreover, microporous magnets are current forefront of molecular-based materials, because the synergistic effect of magnetic and porous properties may develop a new route to multifunctional materials.5 In this context, the rigid aromatic polycarboxylates have been confirmed as suitable building blocks to assemble such coordination frameworks with interesting topological nets, while 5-NH2-bdc containing the -NH2 substituted group has been rarely used, which may offer different link modes, and orientation of the donor groups.6 Herein, we will present a 3D coordination polymer [Co5(μ3-OH)2(5-NH2-bdc)4(H2O)2] 3 7.5H2O (1) with (3,12)connected net constructed by pentacobalt clusters and 5-NH2bdc modules. Notably, a structural evolution can be well operated based on this complicated 3D framework by ligand replacement (from water to bridging 4,40 -bipyridine) to afford a unique (3,13)connected coordination framework [Co5(μ3-OH)2(5-NH2-bdc)4(bpy)0.5(H2O)] 3 3H2O (2) (bpy=4,40 -bipyridine), whose preliminary magnetic properties will also be reported.

Experimental Section. Materials and Physical Measurements. All chemicals were of reagent grade and used as received.

IR spectra were recorded in the range 4000-400 cm-1 on a

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were refined with anisotropic displacement parameters except the disordered solvent molecules of 1 and 2. The solvent molecules in two compounds were disordered and located from difference maps. It failed to add hydrogen to O atoms of solvent water. Further details for structural analysis for compounds 1 and 2 are summarized in Table 1. CCDC-727139 (1) and 727140 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/ retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ, UK; fax: (þ44) 1223-336-033; or [email protected]. ac.uk). Results and Discussion. Crystal Structures. X-ray singlecrystal diffraction analysis of 1 indicates the presence of five independent Co(II) ions (in general position), four 5-NH2-bdc, two μ3-OH-, two aqua ligands and seven and a half lattice water in the asymmetric unit. This structure is built around a [Co5(μ3OH)2] cluster, which can also be regarded as two [Co3(μ3-OH)] triangles sharing a common Co2 vertex. Each triangle has a central μ3-OH- core, and they are held together by bridging carboxylates around the periphery. Notably, such a pentanuclear Co(II) cluster is quite different from other known examples.3,8 Table 1. Crystal Data and Structure Refinement for 1-2 empirical formula fw cryst syst space group T (K) a (A˚) b (A˚) c (A˚) β (deg) V (A˚3) Z F(000) Fcalcd (g cm-3) abs coeff (mm-1) total no. of reflns no. of unique reflns GOF on F2 Rint R1a (I > 2σ/all data) wR2a (I > 2σ/all data) a

1

2

C32H26Co5N4O27.5 1201.22 monoclinic C2/c 173(2) 35.432(2) 11.1832(7) 21.475(1) 94.836(1) 8478.9(9) 8 4808 1.882 2.020 7364 5367 1.047 1.060 0.0655/0.0885 0.1541/0.1677

C37H28Co5N5O22 1189.29 monoclinic C2/c 173(2) 35.715(2) 11.1336(7) 22.146(2) 93.091(2) 8793.0(1) 8 4768 1.797 1.938 7739 4151 1.024 1.026 0.0674/0.1426 0.1874/0.2449

R1 = Σ||Fo| - |Fc||/ Σ|Fo|; wR2 = [Σw(Fo2 - Fc2)2/Σw(Fo2)2]1/2.

First, the two [Co3(μ3-OH)] triangles lack any symmetry and are not coplanar with a dihedral angle of 80.1(1)°. The Co 3 3 3 Co distance in the irregular triangles span in the range of 3.204(1)-3.989(1) A˚ (Table S1). Second, the pentanuclear cluster contains 4-, 5-, and 6-coordinated Co(II) ions which results from the various linking modes of 5-NH2-bdc (Figure 1a). Furthermore, the ligands adopt η1:η1:η1:η1:η1:μ5 and η1:η1:η1:η1:μ4 coordinated fashions. The first one is unique and the largest coordination mode ever seen for this 5-NH2-bdc ligand in Co(II) complexes (Scheme S1).6 In detail, the Co1 and Co5 ions have similar N2O4 octahedral (Oh) geometries surrounded by one μ3-OH-, three carboxylate oxygen atoms and two amino groups from five different 5-NH2-bdc ligands. Co2 also has an octahedral sphere that is completed by one terminal water ligand, two μ3-OH-, and three syn-syn bridging carboxylate groups from different 5-NH2-bdc ligands. Co4 is coordinated by one μ3-OHgroup, two syn-syn bridging and one monodentate carboxylates from three different 5-NH2-bdc ligands to complete a distorted tetrahedral (Td) geometry, whereas Co3 shows a square-pyramidal (SP) sphere composed by one μ3-OH-, three oxygen atoms from the bridging carboxylates and one terminal water ligand. The closest intercluster Co 3 3 3 Co distances is 7.31 A˚. Such pentacobalt units are further extended by the 5-NH2-bdc tectons to give a complicated 3D net (Figure 2). To fully understand the structure of 1, the topological approach is applied to simplify such a 3D coordination framework. Apparently, the pentanuclear Co(II) motifs could be regarded as the SBUs in the construction of this complicated network, each of which is connected by twelve 5-NH2-bdc ligands. On the other hand, each 5-NH2-bdc tecton is linked to three such Co5 SBUs and the resulting 3-connected nodes have the same Schlafli symbol of (43). Notably, the vertex symbols of four independent 5-NH2-bdc nodes are not equivalent, that is, (4.4.43) for the type (I) ligand, (4.42.42) for the type (II) ligand, and (4.4.42) for the type (III) ligand (see Figure 1c for the definition of the ligand type). Thus, each pentanuclear SBU is linked to twelve 5-NH2-bdc nodes [6type(I) þ 3type(II) þ 3type(III)] to serve as a 12-connecting node (Figure 3). Consequently, the 3D structure of 1 presents a novel (3,12)-connected 5-nodal network with the Schlafli symbol of (419.627.820)(43)4 (Figure S1). So far, only one (3,12)-connected coordination network based on nanosized octanuclear ZnII clusters has been reported but with different Schlafli symbol of (32.4)2(38.422.516.618.72).9

Figure 1. Perspective views of the coordination geometries of Co(II) for (a) 1 and (b) 2, and (c) coordination modes of 5-NH2-bdc. Color scheme: gray for C, white for H, sky blue for N, red for O, and brown for Co.

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Figure 2. View of the 3D network for 1 (left) and 2 (right) along the b axis, the orange molecules are better representation of bpy.

Figure 3. (a) Portion view of the coordination details of 1 showing 12 5-NH2-bdc ligands around each pentacobalt cluster. (b) Schematic presentation of the 12-connected node (purple).

Another interesting structural feature of 1 lies in its cageshape channel structure. In fact, this 3D network can be deconstructed into elaborate cages of ball and cylinder shapes with an alternate arrangement via sharing the edges and vertices. Each cage (Figure S2) consists of ten [Co5(μ3-OH)2] clusters (blue balls), twelve 5-NH2-bdc ligands (orange nodes). Interestingly, every two 5-NH2-bdc plus four pentacobalt clusters adopt a butterfly like geometry and are interconnected to constitute the circumjacent wall of the cage-like channel. As a result, the 3D framework occupies only 75.8% of the unit cell volume and the remaining space is filled by lattice water. As shown in Figure S3, the cage-like channel has a diameter of ca. 8.59 A˚ for the ball, taking into account the van der Waals radii, which are linked by the small cylinders (ca. 1.50  3.87 A˚) to constitute a 3D intersecting channel system. The yield of complex 1 is rather low (only 2%), which makes it difficult to further study the possible properties such as magnetizm. However, considering its special cluster-based network array and available voids, we attempt to introduce the familiar bpy coligand into the reaction system, which may serve as the guests in this open framework or replace the terminal water ligand of the clusters to induce a structural evolution. Fortunately, we succeed in the synthesis of a unique crystalline species 2 with relatively higher yield, in which the linear bpy spacers interestingly change the net nodes in 1 to form a (3,13)-connected topology. X-ray crystallography of 2 reveals a closely related structure to that of 1 (Figure 1b, Tables S1 to S3). Similar pentacobalt clusters are also found in 2 (Figure 4a), which are extended by the 5-NH2-bdc tectons. The only difference is that the site of coordination water binding to Co3 in 1 is replaced by a pyridyl nitrogen of bpy. As a result, each 5-NH2-bdc ligand in 2 still acts as a 3-connected node (the same as that in 1), whereas the pentacobalt motif is linked to twelve 5-NH2-bdc nodes plus one Co5 SBU via the bipy spacer to act as a unique 13-connected node (Figure 4b). Thus, the overall 3D structure of 2 represents the first topological paradigm for a (3,13)-connected net (Figure S1) with the Schlafli symbol of (419.515.624.712.88)(43)4. These results further definitely confirm that the use of cluster-based

Figure 4. (a) Portion view of the coordination details of 2 showing twelve 5-NH2-bdc ligands and one bpy spacer around each pentacobalt cluster. (b) Schematic presentation of the 13-connected node (purple).

SBUs is a rational and reliable strategy for the design and preparation of highly connected coordination frameworks with unusual network topology. A further structural comparison of 2 with 1 reveals three points: 1) the pentacobalt SBUs shows an increase of net connectivity; 2) the unit-cell volume increases by 3.6% and the free volume of the coordination framework decreases to 20.4% that is occupied by water guests; 3) the bpy ligands are located in the cage-like channels to link the pentacobalt units. The structural adjustment from 1 to 2 suggests the possibility to design higher-connected nets by decorate the nodes using additional spacers in the voids. Thermal Stability. Thermogravimetric analysis (TGA) was carried out to examine the thermal stability of the MOFs of 2 (Figure S4). The crushed single-crystal sample was heated up to 900 °C in N2 at a heating rate of 10 °C min-1. The DTG and TG curves for 2 show that the first weight loss of 6.2% between 20 and 260 °C corresponds to the loss of three lattice and one coordinated water (calc. 6.0%). The residue is stable up to 270 °C, where after pyrolysis of bpy and 5-NH2-bdc occurs and ends at 585 °C (weight loss: 68.9%; calc. 69.3%). The final residual weight of 24.2% (calc. 24.6%) corresponds to that of CoO. The above thermal behaviors may attribute to the structural features and the TGA results basically agree with the formula of 2. Magnetic Properties. The magnetic data were measured on the crushed single-crystal sample of 2. The dc magnetization data under 1 kOe reveal a room temperature χmT value of 13.44 cm3Kmol-1 (corresponding to per Co5 unit), which decreases monotonically upon cooling to a minimum of 1.75 cm3mol-1 at 2 K (Figure 5). The χm in the temperature range 53-300 K obeys the Curie-Weiss law (Figure S5), C = 9.53 cm3Kmol-1 and θ = -23.2 K, the large negative θ value suggests a dominant antiferromagnetic coupling between the Co(II) centers; and the single-ion behavior of octahedral Co(II) contributes somewhat to the θ value. The field-dependence of χmT vs T shows that the maximum at low temperatures decreases with increasing applied

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Crystal Growth & Design, Vol. 9, No. 10, 2009 the generation of relatively effective noncompensation moments at very low temperature. Increasing structural dimensionality from such isolated clusters to the final 3D coordination frameworks provides a possible opportunity to investigate novel and cooperative magnetic behaviors. Acknowledgment. This work was supported by NSFC (20871034), NSFGX (00832001Z), Program for New Century Excellent Talents in University (NCET-07-217), the Project of Ten, Hundred, Thousand Distinguished Talents in New Century of Guangxi (2006201) and Fok Ying Tung Education Foundation (111014). M.D. also thanks the support from Tianjin Normal University.

Figure 5. Plot of χmT vs T for complex 2. Inset: (a) the hysteresis loop and (b) its first-order derivative of 2 at 2 K.

field at 20 K (Figure S6), which indicated the source of an obvious spontaneous moment, probably spin canting behavior.6,10 Indeed, the AC magnetic susceptibility data confirm the presence of net magnetization as the data show a peak in the χ00 signals below 6.5 K (Figure S7). The position of the maxima shows very slight frequency dependence within the range 1-997 Hz that is also probable a spin canting-like behavior. At very low temperature χ0 curves undergo a slight increase which could be because of the presence of a paramagnetic contribution arising from defects in the crystal structure.11 The shape of the M/H plot at 2 K is similar to those of recently reported spin canted examples, and the first-order derivative of M-H curve does not overlap, with two peaks at about -100 and 100 Oe, clearly confirming the presence of small open loop (Figure 5 inset).10,11 increasing rapidly at low fields, with a maximum value of 3.5 Nβ per Co5 formula unit without obvious saturation observed up to 7 T. Obviously, the Td-SP-3Oh mixed-geometries-based pentanuclear Co(II) clusters have different configurations; and antiferromagnetic alignment will not result in a full cancellation of the magnetic moment.12 As an exact mathematical expression to evaluate the susceptibility of such a complicated 3D system has not been developed, we used an admittedly simple model that only considers the intracluster exchange based on the nature of the four exchange pathways in 2 of the Co(II) pentamer (Figure S9). The data can be fitted by a MAGPACK,13 using an exchange Hamiltonian of the equationS1 (see SI). However, the existence of the spin-orbit coupling that impedes the more accurate fit, further studies that are under way to do more complete study about the high-nuclear cluster-based systems. Compared with the structures of other known Co(II) coordination polymers, the magnetic behavior in 2 may be attributed mainly to the unusual mixed coordination geometries of Co(II) and the nature of the binding modes of Co(II) pentamer. All of the above phenomena mentioned are evidence of slightly spincanting for 2. Obviously, the single-ion anisotropy of Co(II) also gives some contribution. On the other hand, the uncompensated magnetic superexchange interaction of mixed geometries Co(II) ions, bridged by mixed μ3-OH, syn-syn and syn-anti carboxylate groups, leads to D-M interaction and then to the canting behavior.10,12 The bulk magnetic behaviors of 2 are consistent with the cooperation of pentamers in the 3D frameworks, although the long 5-NH2-bdc and bpy bridges do not likely have any importance in terms of the long-range order.6 Conclusions. In summary, two novel 3D cluster-based coordination polymers with unique network architectures, construted from the similar highly connected [Co5(μ3-OH)2] SBUs and 5-NH2-bdc, in which the close cages can be rationally interconnected by discarding the use of linear 4,40 -bipyridine linkers, has been designed, hydrothermally synthesized, as well as struturally and magnetically characterized. In spite of showing antiferromagnetic interaction within the cluster, the Td-SP3Oh mixed-geometry-based pentacobalt(II) cluster can result in

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