DOI: 10.1021/cg900734d
A Three-Dimensional Porous and Magnetic Framework Constructed from {Cu4(μ4-Cl)}þ7 Clusters and 4-(1H-Tetrazol-5yl)-benzoic Acid, (Me2NH2)[Cu4Cl(tba)4(H2O)4] 3 2DMF
2009, Vol. 9 4258–4261
Wayne Ouellette,† Hongxue Liu,‡ Kelly Whitenack,† Charles J. O’Connor,‡ and Jon Zubieta*,† †
Department of Chemistry, Syracuse University, Syracuse, New York 13244, and ‡Advanced Materials Research Institute, College of Sciences, University of New Orleans, New Orleans, Louisiana 70148 Received June 30, 2009; Revised Manuscript Received August 17, 2009
ABSTRACT: The hydrothermal reaction of CuCl2 3 2H2O and 4-(1H tetrazol-5-yl)-benzoic acid (H2tba) in the presence of excess HCl yielded the three-dimensional material (Me2NH2)[Cu4Cl(tba)4(H2O)4] 3 2DMF. The structure is constructed from {Cu4(μ4-Cl)(H2O)4}7þ clusters linked through tba2- ligands into an open framework with 57% void volume. The microporosity and magnetic properties of the material were investigated. The contemporary interest in organic-inorganic materials reflects their applications to catalysis, optical materials, membranes, and sorption.1-10 This diversity of properties reflects a vast compositional range, which is manifested in variations in covalency, geometry, and oxidation states, and a versatile crystalline architecture which provides materials with various dimensionalities, pore sizes and geometries, coordination sites, or juxtapositions of functional groups. The unique characteristics of the organic and inorganic components are combined in a complementary fashion to provide materials with unusual solid-state structures, exhibiting composite or even novel properties, and providing access to a vast area of complex and multifunctional materials.11,12 An important category of organic-inorganic hybrid materials is the metal-organic frameworks (MOFs) or coordination polymers, in which metal atoms or clusters are linked through polyfunctional organic molecules.13-15 While materials incorporating carboxylate and pyridine-based ligands have been most extensively studied,16 more recently attention has focused on polyazoheteroaromatic ligands, such as pyrazolate, imidazolate, triazolate, and tetrazolate, as ligand components.17-24 Polyazoheteroaromatic ligands have been shown to bridge metal ions to afford polynuclear compounds, to possess superexchange capacity resulting in unusual magnetic properties of the complexes, and to be readily derivatized to incorporate additional functional and/or steric groups. As part of our investigations of synthetic approaches for design of hybrid materials incorporating cluster building blocks,25 we studied the hydrothermal chemistry of triazole with transition metal cations to provide a facile route to materials with sorptive, magnetic, luminescent, or multifunctional properties.26-30 Simple ligand modifications were also reflected in the properties of the materials. For example, the sorptive capacity of the three-dimensional (3-D) material [Cu3(OH)3(trz)3(H2O)4] 3 4H2O was enhanced by the simple expedient of expansion of the ligand tether length in the structurally analogous [Cu3(OH)3(4-pt)3(DMF)4] 3 5DMF 3 3MeOH (Scheme 1). Encouraged by these results and the welldocumented tendency of carboxylate ligands to bridge metal sites in MOFs and to participate in the formation of cluster building blocks, the dipodal multidentate ligand 4-(1H-tetrazol-5-yl)-benzoic acid (H2tba) was reacted with CuCl2 to yield the 3-D framework material (Me2NH2)[Cu4Cl(tba)4(H2O)4] 3 2DMF.31 As shown in Figure 1, the 3-D framework of (Me2NH2)[Cu4Cl(tba)4(H2O)4] 3 2DMF is constructed from {Cu4(μ4-Cl)*To whom correspondence should be addressed. Address: Department of Chemistry, Syracuse University, Syracuse, NY 13244. E-mail: jazubiet@syr. edu;
[email protected]. Fax: þ35-443-2547. pubs.acs.org/crystal
Published on Web 08/27/2009
Scheme 1
(H2O)4}7þ clusters linked through the tba2- ligands.32 The tetranuclear building block consists of four ‘4 þ 2’ axially distorted Cu(II) sites, sharing a common vertex at the μ4-chloride residing at the center of the Cu4Cl plane. The {M4(μ4-X)}nþ cluster has been observed previously as a building block of the MOFs [Mn(dmf)6]3[Mn4Cl)3(btt)8(H2O)12]2 3 solvate and H[Cu(dmf)6][(Cu4Cl)3(btt)8(H2O)12] 3 solvate (H3btt = 1,3,5-tris(tetrazol-5-yl)benzene).33,34 The Cu(II)polyhedra are defined by the bridging chloride and a terminal aqua ligand in the axial positions, and two carboxylate oxygen donors and two tetrazolate nitrogen atoms in the equatorial plane. Each carboxylate group bridges two Cu sites of a cluster, as does each tetrazolate group. The four carboxylate groups associated with a cluster are disposed off one face of the Cu4Cl plane, while the four tetrazolate groups are disposed about the other face. Each tba2- ligand links two tetranuclear clusters. Thus, each cluster is linked to eight adjacent clusters to provide the 3-D connectivity. This bonding pattern generates channels of dimensions 19.1 7.2 A˚ along the b direction and 7.2 7.2 A˚ along the c axis. The aqua ligands project into the void domain, which is also occupied by two Me2NH2þ cations and four molecules of solvation. The structure exhibits 57% void volume. In order to study the sorptive properties of 1, desolvated samples of 1 were carefully prepared. The thermogravimetric profile of 1 3 2DMF (Supporting Information, Supplementary Figure S2) exhibits a gradual weight loss of ca. 11% between room temperature and 320 °C, corresponding to the loss of the DMF of solvation (11.1% theoretical weight loss). This desolvation process is followed by the rapid loss of 68% by weight r 2009 American Chemical Society
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Overnight soaking of 1 3 4.5MeOH with dichloromethane, followed by evacuation under dynamic vacuum provided the desolvated material 1 (Supporting Information, Figures S3 and S4). H2 and N2 isotherms of 1 were measured in liquid nitrogen baths. The N2 sorption isotherm (Figure 2) exhibited type I behavior with a BET surface area of 354.8 m2 g-1 and a Langmuir surface area of 464.4 m2 g-1. The materials exhibited modest H2 uptake of 0.49% by weight (Supporting Information, Figure S5). Compound 1 also incorporates significant solvent volume: 9.73% by weight H2O (Supporting Information, Figure S6) and 8.56% by weight methanol, as shown in Figure 3. The temperature-dependent magnetic data were recorded at a magnetic field 1000 Oe in the 2-300 K temperature range after zero field cooling using a Quantum Design MPMS-XL-7 SQUID spectrometer (Figure 4). At 300 K, the effective magnetic moment (8χ0T)1/2 is 3.41μB, consistent with four Cu(II) sites per formula unit. The moment decreases with lowering temperature, reaching a minimum of 2.32μB at 25 K and then increasing rapidly. The initial decrease in moment on cooling implies the existence of dominant antiferromagnetic interactions within the Cu tetramer. Consequently, the magnetic data were analyzed using the model of a Cu tetranuclear rhombus (or puckered square) with interaction Hamiltonian H = -2J1(S1S2 þ S2S3 þ S3S4 þ S1S4) - 2J2S1S3 2J3S2S4. The expression for the molar magnetic susceptibility can be derived from the van Vleck equation using the solved energy matrix35 and is shown in eq 1. χ ¼ χ0 þχTI ¼
Ng2 μB 2 A þχ kT B TI
ð1Þ
where A =
Figure 1. (a) A ball and stick representation of the three-dimensional structure of 1 3 2DMF in the ac plane. (b) The tetranuclear building block {Cu4(μ4-Cl)(H2O)4(N4C-)4(O2C-)4}.
2J 1 J 2 J 3 2J 1 J 2 J 3 10 exp þ þ þ2 exp þ þ kT kT kT kT kT kT J2 J3 J2 J3 þ2 exp þ þ2 exp kT kT kT kT
and B =
2J 1 J 2 J 3 2J 1 J 2 J 3 þ þ þ þ þ3 exp kT kT kT kT kT kT 4J 1 J 2 J 3 J2 J3 þ þ þ3 exp þexp kT kT kT kT kT J2 J3 J2 J3 þ þexp þ3exp kT kT kT kT
5 exp
The calculated susceptibility was corrected for exchange interaction zJ0 between spins as shown in eq 2. χ0 ¼
1-
Figure 2. N2 sorption isotherm for desolvated 1.
between 320 and 370 °C, consistent with loss of the organic components and of the aqua ligands (66.8% theoretical). Overnight soaking of 1 3 2DMF in methanol results in solvent substitution to provide 1 3 4.5MeOH. The thermogravimetric (TGA) profile of 1 3 4.5MeOH shows loss of the methanol of solvation in the temperature range 25-50 °C (ca. 10.5% weight loss observed; 10.2% theoretical), followed by a plateau of stability between 50 and 320 °C, whereupon ligand decomposition and loss of coordinated water are observed.
χ0
2zJ 0 Ng2 μ2B
χ0
ð2Þ
The best fit gives g = 2.06, J1/k = -44 K, J2/k = -92.50 K, J3/k = 4.48 K, zJ0 /k = 0.14 K, and TIP = 8 10-4 emu/mol. The magnetic model of a puckered square is consistent with the crystallographic data which shows that the structure is distorted from the idealized tetragonal symmetry resulting in two inequivalent Cu sites and inequivalent diagonal vectors with the central Cl atom displaced from the Cu4 best plane in the direction of the carboxylate donors. The tetrazolate derived ligand tba2- is effective in bridging metal sites to provide open framework materials and to effect magnetic superexchange. A recurring theme of the polyazaheterocyclic based materials is the presence of metal clusters as building blocks. As noted previously,36 the identity of the
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Crystal Growth & Design, Vol. 9, No. 10, 2009 Crystallographic Data and CIF files for compound 1 3 2DMF. This material is available free of charge via the Internet at http://pubs. acs.org.
References
Figure 3. Methanol sorption isotherm for 1.
Figure 4. Temperature dependence of the magnetic susceptibility χ (red circles) and of the effective magnetic moment μeff (blue diamonds) for 1 3 2DMF in the 2-300 K range.
building units and the details of their interconnectivity into framework materials are dependent on additional functional groups and the presence of anionic components, such as chloride. While the microporosity of these materials can be actualized, careful sample preparation is required in order to avoid partial or total collapse of the framework structure. The title compound may be described as a multifunctional material, displaying both sorptive properties and interesting magnetic properties. The magnetic behavior of 1 3 2DMF exhibits strong superexchange through the tetrazolate moieties in contrast to [Cu3(OH)3(4-pt)3(DMF)4] 3 5DMF 3 3MeOH where geometric factors minimize superexchange. Acknowledgment. This work was funded by a grant from the National Science Foundation (Grant CHE-0907787). Supporting Information Available: Description of Experimental Details, including synthetic methods, crystallography, magnetism and sorption; supplementary figures, including TGA, infrared spectroscopy, powder XRD profiles, gas sorption isotherms, and anisotropic thermal ellipsoids for compound 1 3 2DMF; Tables of
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