A Novel Five-Connected BN Topological Network Metal−Organic

A Novel Five-Connected BN Topological Network Metal-Organic Framework Mn(II) Cluster Complex Xian-Wen Wang,*,†,‡ You-Ren Dong,‡ Yue-Qing Zheng,*,‡ and Jing-Zhong Chen† Faculty of Materials Science and Chemical Engineering, China UniVersity of Geoscience, Wuhan 370007, P. R. China, The State Key Laboratory Base of NoVel Functional Materials and Preparation Science, Faculty of Materials Science and Chemical Engineering, Ningbo UniVersity, Ningbo 315211, P. R. China

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ReceiVed December 22, 2006; ReVised Manuscript ReceiVed March 8, 2007

ABSTRACT: Hydrothermal reaction of MnCl2‚4H2O with H3PTC (PTC ) pyridine-2,4,6-tricarboxylate) yields a novel three-dimensional manganese(II) coordination polymer, [Mn2(OH)(H2O)(PTC)] (1), constructed by Mn4 clusters and PTC linkers. Compound 1 displays fiveconnected BN topology, and there exists weak antiferromagnetic exchange interactions between the manganese(II) ions with g ) 2.02, Jwb/k ) -0.96 K, and Jbb/k ) -1.07 K. Metal-organic frameworks (MOFs) as functional solid materials have attracted great interest in recent years because of their potential applications in magnetism, electrical conductivity, catalysis, separation, gas storage, ion exchange, biology and molecular recognition, and nonlinear optical materials, as well as their fascinating architectures and topological networks.1-6 To date, a variety of MOFs with unusual topologies have been obtained by chemists via the use of crystal engineering concepts of the joints (metal centers) and linkers (appropriate organic ligands).7 A large number of simple examples of infinite n-connected (with the minimum number of three) three-dimensional nets described by Wells8 represent themselves as realistic targets of inorganic compounds or minerals for the crystal engineer: the R-polonium or NaCl net (six-connected); the diamond, Lonsdaleite, quartz, feldspar-related, and zeoliterelated nets (four-connected); the NbO net (four-connected) and the PtS net (four-connected). However, to the best of our knowledge, the five-connected MOFs networks are relatively rare, and no examples displaying the simplest five-connected BN topological net have been reported to date.9 On the other hand, many polynuclear manganese clusters with the metal in the oxidation state range of II-IV or mixed-valence clusters have been extensively investigated for these motives: (1) to elucidate the magnetic coupling between paramagnetic metal ions and build up magnetostructural correlations;1 (2) to mimic the active centers of some biological metalloenzymes, as Mn is prominent in the active sites of many metallobiomolecules;10 (3) to produce new functional materials such as molecular magnets below a critical temperature.11 Many of these clusters have large ground-state spin (S) together with a strong uniaxial anisotropy, which leads to a slow relaxation of their magnetization and functions as singlemolecule magnets (SMMs).11-12 However, there are only a few examples using manganese clusters as the building blocks to construct complicated and enchanting architectures.13 Pyridine-2,4,6tricarboxylatic acid (H3PTC), a preferred multifunctional nitrogenand oxygen-donor connector with diverse chelating and bridging mode for constructing coordination complex, has been recently utilized to generate zero-, one-, two-, and three-dimensional metalorganic frameworks.14 Remarkably, the chelating probability of the pyridyl 2- and 6-position carboxylate groups for generating metaloxygen cluster SBUs (secondary building units) and the bridging possibility of pyridyl 4-position carboxylate group for producing higher dimensionality coordination polymers give one good opportunity to yield novel MOFs. However, its coordination chemistry remains largely unexplored compared with those of the pyridine2,6-dicarboxylate15 and benzene polycarboxylate ligands.16 We are interested in assembling higher-dimensional networks on the basis of the metal-organic cluster core SBUs under hydrothermal conditions. Herein, we present the first example of novel three* Corresponding author. E-mail: [email protected] (W.X.W.). † China University of Geoscience. ‡ Ningbo University.

Figure 1. Ortep view of [Mn2(OH)(H2O)(PTC)] with displacement ellipsoids (60% probability) and atomic labeling, showing the coordination environment of Mn(II) atoms and PTC ligand together with an incompletely double-cuboidal Mn4 cluster.

dimensional manganese(II) coordination polymer [Mn2(OH)(H2O)(PTC)] 1, consisting of incompletely double-cuboidal tetranuclear Mn(II) clusters and PTC multifunctional ligands. Topological approach analysis shows compound 1 exhibits five-connected BN topology. The hydrothermal reaction of MnCl2‚4H2O with H3PTC and triethylamine in water at 170 °C for 6 days afforded yellow crystals of 1.17 Single-crystal X-ray analysis18 revealed that the asymmetric unit of compound 1 contains two Mn2+ cations (Mn1 and Mn2), one PTC anion (C8H2O6), one hydroxyl anion (OH-), and one coordinated water molecule. Each PTC anion links seven Mn atoms, and the OH- anion acts as a tridentate bridging ligand. Interestingly, in compound 1, the carboxylate O3 and O4 atoms deviate from the mean plane of the PTC anion ligand with a dihedral of 72.59° (Figure 1). The 3D polymer 1 is constructed by the Mn4 clusters building units arranged in an incompletely double-cuboidal fashion and PTC linkers. The distorted pentagonal bipyramidally coordinated Mn1 atoms are surrounded by one pyridyl nitrogen atom and six oxygen atoms, of which five are from four crystallographically different PTC anion ligands and one is from the hydroxyl. Each Mn2 atom is octahedrally coordinated by six oxygen atoms, three of which belong to crystallographically distinct three PTC anions ligands, two from OH- anions and one from water molecules. The edgeshared distorted pentagonal bipyramidally coordinated Mn1 atoms and octahedrally coordinated Mn2 atoms are connected by two µ3O7 atoms and four carboxylate O4 and O5 atoms into an incompletely double-cuboidal Mn4 cluster core. The Mn1-N bond length is 2.290(3) Å, and the Mn1-O and Mn2-O bond distances fall in the ranges of 2.171(3)-2.404(3) and 2.113(3)-2.450(3) Å, respectively. The Mn1-O1A distance (2.404(3) Å) is longer than

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Figure 2. Inorganic manganese-oxygen and carboxylato layer constructed by Mn4 cluster cores and carboxylato groups.

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Figure 4. BN topology: yellow atoms are Mn atoms, purple rods indicate Mn4 clusters, white rods denote PTC ligands, green linkers are Mn-oxygen dimers, and brown sticks are Mn-O-C-O-Mn jointers.

Figure 3. Mn4 cluster cores linked by PTC ligands into graphite (6,3) topological sheet.

the other Mn1-O distances, which are comparable with those of the reported compound composed of [MnII2MnIII2] building blocks.13c O4A resides on the axial apex shared by the pentagonal bipyramid and octahedron. The Mn2-O4A length (2.450(3) Å) is significantly longer than those in the equatorial plane of octahedron (average value of 2.135 Å). The tetranuclear Mn(II) clusters are centered at the crystallography 1f positions and linked by two carboxylate oxygen atoms into 1D chains along the [001] direction. The resulting chains are further interlinked by carboxylate groups and PTC linkers into inorganic layers parallel to (010) (Figure 2) and inorganic-organic hybrid layers parallel to (101) (Figure 3), respectively. The inorganic-organic hybrid layers feature as (6,3) topological sheets, and these sheets are then further linked via 3-carboxlyate groups into the three-dimensional structure. A better insight into the nature of this intricate framework can be achieved by the application of topological approach. The network topology can be simplified by considering just Mn4 clusters and the PTC ligands. The graphite (6,3) topological inorganic-organic layers are linked by the MnO-C-O-Mn jointers into a three-dimensional framework, and the resultant network can be thought of as the five-connected BN topology as shown in Figure 4. Although a large number of examples of three- or four-connected networks based on d-block metal ions have been reported, five-connected MOFs networks are relatively rare,9 and this is the first example of a MOF network exhibiting the simplest five-connected BN topological net. The magnetic susceptibilities were measured using a SQUID magnetometer on crystalline samples of 1 in the temperature range of 1.8 to 300 K under 2 kOe. Above 35 K, the 1/χm product (χm is the magnetic suscepitibility per Mn4 unit) obeys the Curie-Weiss law with C ) 4.60 cm3 K mol-1 and Θ ) -15.99 K (C is the Curie constant and Θ is the Weiss constant).19 The negative Weiss constant indicates that antiferromagnetic exchange interactions are dominant in the Mn4 units. With lowering temperature, the χmT product (χmT ) 17.48 cm3 K mol-1 for four S ) 5/2 spins at 300 K) decreases gradually in the temperature range of 300 to 35 K

Figure 5. (a) Temperature dependence of the magnetic susceptibilities of 1 (χm being the magnetic susceptibility per Mn4 unit). (b) Diagram showing the two dominant magnetic exchange pathways between metal centers in complex 1.

and then abruptly to reach a minimum of 1.96 cm3 K mol-1 at 1.8 K, indicating overall antiferromagnetic coupling of the S ) 5/2 spins of the MnII ions in 1 (Figure 5a). A model that considers only two dominant exchange pathways was used as shown in Figure 5b, where the exchange interactions of Mn1‚‚‚Mn2 (Jwb) and Mn2 atoms (Jbb) are dominated by the carboxylate O4 or O5 atoms and µ3-O7 atoms, respectively. This approximation equates J12 ) J1′2′ ) J12′ ) J1′2 ) Jwb, J22′ ) Jbb, and the Kambe vector coupling method can be used to solve the spin Hamiltonian that is given in eq 120 (see Figure 5b; J ) magnetic exchange interactions)

H ˆ ) -Jwb(Sˆ T2 - Sˆ A2 - Sˆ B2) - Jbb(Sˆ A2 - Sˆ 12 - Sˆ 12) (1) where Sˆ A ) Sˆ 1 + Sˆ 1′, Sˆ B ) Sˆ 2 + Sˆ 2′, and Sˆ T ) Sˆ A+ Sˆ B. The Kambe method gives the eigenvalue expression in eq 2

E(ST) ) -Jwb[ST(ST + 1) - SA(SA + 1) - SB(SB + 1)] Jbb[SA(SA + 1)] (2) χM )

Ng2µB2 3κT

∑[ST(ST + 1)(2ST + 1)]e-E(S )/κT ∑(2ST + 1)e-E(S )/κT T

(3)

T

there are 145 possible states with ST ranging from 0 to 10. This eigenvalue expression eq 2 and the corresponding Van Vleck eq 3 were then used to least-squares fit the experimental data measured in the 1.8-300 K range. The parameters g, Jwb, and Jbb were varied to fit the data. At each setting of the parameters, eq 2 is used to calculate the energy for each of the 145 spin states and these

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energies are then used in eq 3 to calculate the molar paramagnetic susceptibility. A good fit was achieved (solid line, Figure 5a) with the fitting parameters g ) 2.02, Jwb/k ) -0.96 K, Jbb/k ) -1.07 K, and the agreement factor defined by R [R ) Σ(χiobs - χicald)2/ Σ(χiobs)2] is equal to 8.25878 × 10-8. The small J values indicate that the antiferromagnetic exchange interactions between manganese(II) ions mediated by oxygen atoms are relatively weak. Cluster-cluster interactions via monatomic O-bridges connecting into one-dimension chains may exist in 1. We have attempted to model the interactions by introducing a Curie-Weiss temperature in the Van Vleck equation, but we failed to obtain the ideal results because the augment variants in the formula would consumedly enhance the fitting difficulty. The magnetic behaviors of 1 are similar to these of the tetra-manganese(II) complex,21 but are different from those of the mixed-valence tetra-Mn(II/III) complexes [Mn4(hmp)4(OH)2Mn(dcn)6]‚2MeCN‚2THF 13a and [Mn4(hmp)6Cl2]n(ClO4),13c both of which are composed of Mn4 SMMs bulding blocks. In conclusion, a novel three-dimensional manganese(II) coordination polymer was synthesized under hydrothermal conditions. The title compound consists of Mn4 clusters and PTC linkers. From the topological point of view, the network of compound 1 exhibits relatively rare five-connected BN topology. Magnetic studies indicate that there exist weak antiferromagnetic exchange interactions between manganese(II) atoms in the Mn4 units. Acknowledgment. The project was supported by the Expert Project of Key Basic Research of the Ministry of Science and Technology of China (2003CCA00800), the Zhejiang Provincial Natural Science Foundation (Z203067), Ningbo Municipal Natural Science Foundation (2006A610061), and the Scientific Research Fund of Ningbo University (XK200459, SS2004033). Supporting Information Available: X-ray crystallographic files for 1 in CIF format (CCDC no. 624281). This material is available free of charge via the Internet at http://pubs.acs.org.

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