pubs.acs.org/crystal Published on Web 08/27/2009 r 2009 American Chemical Society

DOI: 10.1021/cg900734d

2009, Vol. 94258–4261

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

Wayne Ouellette,† Hongxue Liu,‡ Kelly Whitenack,† Charles J. O’Connor,‡ andJon Zubieta*,†

†Department of Chemistry, Syracuse University, Syracuse, New York 13244, and ‡Advanced MaterialsResearch 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 excessHClyielded 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 magneticproperties of the material were investigated.

The contemporary interest in organic-inorganic materials re-flects their applications to catalysis, optical materials, membranes,and sorption.1-10 This diversity of properties reflects a vast com-positional range, which is manifested in variations in covalency,geometry, and oxidation states, and a versatile crystalline architec-ture which provides materials with various dimensionalities, poresizes and geometries, coordination sites, or juxtapositions of func-tional groups. The unique characteristics of the organic andinorganic components are combined in a complementary fashionto provide materials with unusual solid-state structures, exhibitingcomposite or even novel properties, and providing access to a vastarea of complex and multifunctional materials.11,12

An important category of organic-inorganic hybridmaterials isthemetal-organic frameworks (MOFs) or coordinationpolymers,in whichmetal atoms or clusters are linked through polyfunctionalorganicmolecules.13-15Whilematerials incorporating carboxylateand pyridine-based ligands have been most extensively studied,16

more recently attention has focused on polyazoheteroaromaticligands, such as pyrazolate, imidazolate, triazolate, and tetrazolate,as ligand components.17-24 Polyazoheteroaromatic ligands havebeen shown tobridgemetal ions to afford polynuclear compounds,to possess superexchange capacity resulting in unusual magneticproperties of the complexes, and to be readily derivatized toincorporate additional functional and/or steric groups.

Aspart ofour investigationsof synthetic approaches fordesignofhybrid materials incorporating cluster building blocks,25 we studiedthe hydrothermal chemistry of triazole with transitionmetal cationsto provide a facile route to materials with sorptive, magnetic,luminescent, ormultifunctional properties.26-30 Simple ligandmod-ifications 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 bythe simple expedient of expansion of the ligand tether length inthe structurally analogous [Cu3(OH)3(4-pt)3(DMF)4] 3 5DMF 33MeOH (Scheme 1). Encouraged by these results and the well-documented tendency of carboxylate ligands to bridgemetal sites inMOFsand toparticipate in the formationof cluster building blocks,the dipodal multidentate ligand 4-(1H-tetrazol-5-yl)-benzoic acid(H2tba) was reactedwithCuCl2 toyield the3-D frameworkmaterial(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)-

(H2O)4}7þ clusters linked through the tba2- ligands.32 The tetra-

nuclear building block consists of four ‘4þ 2’ axially distortedCu(II)sites, sharingacommonvertexat theμ4-chloride residingat thecenterof the Cu4Cl plane. The {M4(μ4-X)}

nþ cluster has been observedpreviously 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] 3solvate (H3btt = 1,3,5-tris(tetrazol-5-yl)benzene).33,34

The Cu(II)polyhedra are defined by the bridging chloride and aterminal aqua ligand in the axial positions, and two carboxylateoxygen donors and two tetrazolate nitrogen atoms in the equatorialplane. Each carboxylate group bridges two Cu sites of a cluster, asdoes each tetrazolate group. The four carboxylate groups asso-ciated 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 clusteris 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. Theaqua ligands project into the void domain, which is also occupiedby two Me2NH2

þ cations and four molecules of solvation. Thestructure exhibits 57% void volume.

In order to study the sorptive properties of 1, desolvatedsamples of 1 were carefully prepared. The thermogravimetricprofile of 1 3 2DMF (Supporting Information, SupplementaryFigure S2) exhibits a gradual weight loss of ca. 11% betweenroom temperature and 320 �C, corresponding to the loss of theDMF of solvation (11.1% theoretical weight loss). This desolva-tion process is followed by the rapid loss of 68% by weight

Scheme 1

*To whom correspondence should be addressed. Address: Department ofChemistry, Syracuse University, Syracuse, NY 13244. E-mail: jazubiet@syr.edu; wouellet@syr.edu. Fax: þ35-443-2547.

Communication Crystal Growth & Design, Vol. 9, No. 10, 2009 4259

between 320 and 370 �C, consistent with loss of the organiccomponents and of the aqua ligands (66.8% theoretical). Over-night soaking of 1 3 2DMF inmethanol results in solvent substitu-tion to provide 1 3 4.5MeOH. The thermogravimetric (TGA)profile of 1 3 4.5MeOH shows loss of the methanol of solvationin the temperature range 25-50 �C (ca. 10.5% weight lossobserved; 10.2% theoretical), followed by a plateau of stabilitybetween 50 and 320 �C,whereupon ligand decomposition and lossof coordinated water are observed.

Overnight soaking of 1 3 4.5MeOH with dichloromethane,followed by evacuation under dynamic vacuum provided thedesolvated material 1 (Supporting Information, Figures S3 andS4). H2 and N2 isotherms of 1 were measured in liquid nitrogenbaths. The N2 sorption isotherm (Figure 2) exhibited type Ibehaviorwith aBET surface area of 354.8m2 g-1 andaLangmuirsurface area of 464.4 m2 g-1. The materials exhibited modest H2

uptake of 0.49% byweight (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 amagnetic field 1000 Oe in the 2-300 K temperature range afterzero field cooling using aQuantumDesignMPMS-XL-7 SQUIDspectrometer (Figure 4).At 300K, the effectivemagneticmoment(8χ0T)

1/2 is 3.41μB, consistent with four Cu(II) sites per formulaunit. The moment decreases with lowering temperature, reachinga minimum of 2.32μB at 25 K and then increasing rapidly. Theinitial decrease in moment on cooling implies the existence ofdominant antiferromagnetic interactions within the Cu tetramer.Consequently, themagnetic datawereanalyzedusing themodel ofa Cu tetranuclear rhombus (or puckered square) with interactionHamiltonianH=-2J1(S1S2þS2S3þS3S4þS1S4)- 2J2S1S3-2J3S2S4. The expression for the molar magnetic susceptibility canbe derived from the van Vleck equation using the solved energymatrix35 and is shown in eq 1.

χ ¼ χ0þχTI ¼ Ng2μB2

kT

A

BþχTI ð1Þ

where A=

10 exp2J 1kT

þ J 2kT

þ J 3kT

� �þ2 exp -

2J 1kT

þ J 2kT

þ J 3kT

� �

þ2 expJ 2kT

-J 3kT

� �þ2 exp -

J 2kT

þ J 3kT

� �

and B=

5 exp2J 1kT

þ J 2kT

þ J 3kT

� �þ3 exp -

2J 1kT

þ J 2kT

þ J 3kT

� �

þexp -4J 1kT

þ J 2kT

þ J 3kT

� �þ3 exp

J 2kT

-J 3kT

� �

þ3exp -J 2kT

þ J 3kT

� �þexp -

J 2kT

-J 3kT

� �

The calculated susceptibility was corrected for exchange inter-action zJ0 between spins as shown in eq 2.

χ0 ¼ χ0

1- 2zJ 0Ng2μ2

B

� �χ0

ð2Þ

The best fit gives g=2.06, J1/k=-44K, J2/k=-92.50K, J3/k= 4.48 K, zJ0/k = 0.14 K, and TIP = 8 � 10-4 emu/mol. Themagnetic model of a puckered square is consistent with thecrystallographic data which shows that the structure is distortedfrom the idealized tetragonal symmetry resulting in two inequi-valent Cu sites and inequivalent diagonal vectors with the centralCl atom displaced from the Cu4 best plane in the direction of thecarboxylate donors.

The tetrazolate derived ligand tba2- is effective in bridgingmetal sites to provide open framework materials and to effectmagnetic superexchange. A recurring theme of the polyazahe-terocyclic based materials is the presence of metal clusters asbuilding blocks. As noted previously,36 the identity of the

Figure 1. (a) A ball and stick representation of the three-dimen-sional structure of 1 3 2DMF in the ac plane. (b) The tetranuclearbuilding block {Cu4(μ4-Cl)(H2O)4(N4C-)4(O2C-)4}.

Figure 2. N2 sorption isotherm for desolvated 1.

4260 Crystal Growth & Design, Vol. 9, No. 10, 2009

building units and the details of their interconnectivity intoframework materials are dependent on additional functionalgroups and the presence of anionic components, such aschloride. While the microporosity of these materials can beactualized, careful sample preparation is required in order toavoid partial or total collapse of the framework structure. Thetitle compoundmay be described as amultifunctionalmaterial,displaying both sorptive properties and interesting magneticproperties. The magnetic behavior of 1 3 2DMF exhibits strongsuperexchange through the tetrazolate moieties in contrast to[Cu3(OH)3(4-pt)3(DMF)4] 3 5DMF 3 3MeOH where geometricfactors minimize superexchange.

Acknowledgment. This work was funded by a grant from theNational Science Foundation (Grant CHE-0907787).

Supporting Information Available: Description of ExperimentalDetails, including synthetic methods, crystallography, magnetismand sorption; supplementary figures, including TGA, infraredspectroscopy, powder XRD profiles, gas sorption isotherms, andanisotropic thermal ellipsoids for compound 1 3 2DMF; Tables of

Crystallographic Data and CIF files for compound 1 3 2DMF. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

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Figure 3. Methanol sorption isotherm for 1.

Communication Crystal Growth & Design, Vol. 9, No. 10, 2009 4261

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