Inorganic Chemistry Communications 15 (2012) 202–207

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Inorganic Chemistry Communications

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A multifunctional three-dimensional uninodal eight-connected metal–organicframework based on pentanuclear cadmium subunits: New topology, fluorescent andNLO properties

Lin Cheng a, Huayou Hu b, Liming Zhang a, Shaohua Gou a,⁎a School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, PR Chinab Jiangsu key Laboratory for Chemistry of Low-Dimensional Materials, School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huaian 223300, PR China

⁎ Corresponding author.E-mail address: sgou@seu.edu.cn (S. Gou).

1387-7003/$ – see front matter. Crown Copyright © 20doi:10.1016/j.inoche.2011.10.024

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 August 2011Accepted 17 October 2011Available online 25 October 2011

Keywords:Eight-connected34414510 topologyMultifunctional materialNLO

A new three-dimensional uninodal eight-connected metal–organic framework (MOF) based on pentanuclearcadmium subunits has been synthesized and characterized. To our knowledge, this is the first example ofMOFs with the eight-connected 34414510 topology. The framework exhibits high thermal stability until300 °C confirmed by thermogravimetric analysis and has a potential application as a multifunctional materialwith fluorescent and nonlinear optical (NLO) properties.

Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

The design and assembly of metal–organic frameworks (MOFs)have attracted great interest in the field of supramolecular chemistryand crystal engineering due to their potential applications as functionalmaterials, as well as their intriguing variety of architectures andmolecular topologies [1,2]. One of current efforts has been focusedon the synthesis of such frameworks with new topologies [3] ormultifunctional properties, such as homochiral and absorbentproperties [4], magnetic and electric properties [5,6], and fluorescentand nonlinear optical (NLO) properties [7].

A large number of MOFs with low connectivity topologies, such asthree-, four- and six-connected networks, have been reported. Incontrast, the seven-, eight-connected and highly connected topologiesare relatively rare, because the construction of such frameworks isseverely hampered by the number of available coordination sitesat the metal centers and the sterically demanding nature of mostcommonly used organic ligands [8]. Until now, there have beentwo potential and effective synthetic strategies to overcome these twodrawbacks. One is to use f-block metal centers with high coordinationnumbers and flexible coordination modes as nodes [8c,d,9]. The otherapproach, probably more general in practice, is to employ polynuclearmetal clusters as secondary building units (SBUs) [10–33]. The largesurface areas (sometimes even nanoscale) and more coordination sitesof polynuclear metal clusters can induce them readily accommodatingthe steric demands of organic ligands. Meanwhile, the introduction of

11 Published by Elsevier B.V. All rig

metal clusters into MOFs may result in new solid-state functionalmaterials that possess fascinating structures and special properties.

Compared with other high-connected topologies, eight-connectednetworks have attracted more attention due to their topologicaldiversity and moderate connected number. Most examples ofeight-connected topologies reported previously are mainly focusedon polynuclear metal clusters as building blocks, such as dinuclear(Cu2 [10], Cd2 [11], Co2 [11b,12], Cu2 [12b,13], Ni2 [14], Ln2 [15], LnCu[16]), trinuclear (Co3 [17], Cd3 [18], Cu3 [19], Mg3 [20]), tetranuclear(Co4 [21], Cu4 [22], Mn4 [23], Zn4 [24]), pentanuclear (Zn5 [25], Cd5 [26],Dy2Mn3 [17b]), hexanuclear (Pb6 [27], Co6 [28], Cu6 [29]), heptanuclear(Zn7 [30]), octanuclear (Pb8 [31], Cu4V4 [32]) and undecanuclear(Cd11)-based [33] SBUs. From the topological view, most of thesenetworks have a body-centered cubic net with the 42464 (CsCl-type)topology [11a,b,d,f, 12b, 13, 14b, 15, 17a,e,h, 18a, 19, 20, 21c, 23a,25–27, 30, 31, 33]. However,with recent advances in crystal engineering,a variety of new eight-connected networks with the 36422 [11e,15],3641656 [11c], 4165864 [17b], 334155862 [8d], 354115864 [13], 36414576[17f], 364145464 [21b] and 36418536 [11b,12a,16,17b,28] topologies, aswell as the self-penetrating 42464 [8d,11d,14b,29,32], 42068 [14a,17d,g],416612 [23b], 424563 [14c,22,25] and (46.614.88)(43)2 [18b] topologies,have been reported. From the view of the space groups, these reportedeight-connected topologies of MOFs, respectively, belong to triclinic,monoclinic, orthorhombic, tetragonal and trigonal crystal systems, andno eight-connected MOF with cubic system has been reported so far[8–33].

On the other hand, multidentate O-donor organic polycarboxylateligands have been extensively employed in the preparation of MOFswith interesting networks and attractive multifunctional properties,

hts reserved.

Scheme 1. Structure of H4bta.

203L. Cheng et al. / Inorganic Chemistry Communications 15 (2012) 202–207

not only due to their diverse coordination modes and high structuralstability, but also due to their potential functions as hydrogen-bonding acceptors and donors, depending upon the number ofdeprotonated carboxylic groups [34], in which the aromatic multi-carboxylates, such as 1,4-benzenedicarboxylate [35], 1,3,5-benzene-tricarboxylate [36] and 1,2,4,5-benzenetetracarboxylate [37], havebeen extensively studied. However, the biphenyl-2,2′,6,6′-tetracarboxylicacid (H4bta) ligand (Scheme 1), as a member of multidentate O-donorligands, is rarely used [38,39], though it has the following structurefeatures: (a) it has a rich variety of coordinationmodes and deprotonatedforms with four carboxylic groups, which can contribute to constructnovel MOFs; (b) it possesses D2d symmetry, which is easy to assemblehigher symmetrical topological networks; (c) it is a flexible ligand andtwo phenyl rings can be rotated around the C\C single bond, whichmaybe help the synthesis of noncentrosymmetric and/or chiral MOFsbecause of the non-coplanarity of the two phenyl rings.

As part of our ongoing study ofMOFs by using bta as amultidentateO-donor organic polycarboxylate ligand [39], we report here theassembly of CdBr2, H4bta and NMe3 in DMF under hydrothermalconditions [40], generating a multifunctional three-dimensional Cd(II)framework {[Cd5(bta)4]·6H2NMe2}n (1) with a new uninodal eight-connected 34414510 topology, in which the pentanuclear Cd(II) formedby five Cd(II) ions and four bta ligands can be defined as SBUs.

Single-crystal X-ray analyses revealed that compound 1 crystallizesin the cubic system with I-43d space group. The asymmetric unit of 1contains one and a quarter of crystallographic independent Cd(II)centers and one bta ligand, as well as one and a half of free H2NMe2cations. As shown in Fig. 1a and b, Cd1 in 1 displays a bicapped trigonal

Fig. 1. Local coordination environments of Cd1 (a) and Cd2 (b), and coordinationmode of bta (c5/4−z; d, 1/2−y, 1−x, 1/2+z; e, 3/4−y, 3/4+x, 5/4−z.

prismatic geometry, being surrounded by four equal chelating carboxyl-ates from four individual bta ligands;while the coordination environmentof Cd2 in 1 is a slightly distorted octahedron, being coordinated by threeunequal chelating carboxylates from three individual bta ligands.Meanwhile, each bta ligand in 1 is coordinated to one Cd1 andthree Cd2 ions, in a η8,μ4-octadentate (four chelating) mode (Fig. 1c),which is different from those of the reported bta complexes [38,39]. Ina bta ion, the two benzene rings is approximately perpendicular withan angle of 85.79(2)°.

Each Cd1 ion is coordinated by four chelating carboxylates comingfrom four bta ligands, respectively, and eachpair of these four bta ligandsis linked to one Cd2 ion by the carboxylates, resulting in a pentanuclear[Cd5(bta)4] (Cd5) SBU, which consists of one Cd1, four Cd2 and four btaligands (Fig. 2a). In a Cd5 SBU, Cd1 and Cd2 atoms form a distortedtetrahedron, in which Cd1 is located in the center and four Cd2atoms act as four vertices, with the Cd1···Cd2 distance of 6.887(2) Å,and the Cd2⋯Cd2 distances of 10.123(2) and 13.203(2) Å. The angles ofCd2\Cd1\Cd2 are 94.67(2) and 146.86°, the angles of Cd2\Cd2\Cd2are 49.33(2) and 81.35(2)°, and the angles of Cd1\Cd2\Cd2 are 16.57(1) and 42.67(1)° in the Cd5 SBU.

In a Cd5 SBU, the four Cd2 atoms, being chelated by two carboxylatesfrom a pair of bta ligands, are in an unsaturated coordination, which arecompleted by four bta ligands from four adjacent Cd5 SBUs, respectively.Meanwhile, there is one naked carboxylate of each bta ligand in a Cd5SBU, which is also coordinated to one adjacent Cd2 atom from anadjacent Cd5 SBU. Therefore, each Cd5 SBU is linked to eight adjacentCd5 SBUs by the coordination bonds between the chelating carboxylatesand Cd2 atoms (Fig. 2b), resulting in a three-dimensional anionicnetwork with the shortest Cd1⋯Cd1 distance of 14.593(1) Å.

A topologic analysis of 1 with the OLEX program [41] yields a34414510 topology with the long vertex symbols of (3.3.3.3.4.42.42.42.4.4.4.4.4.42.4.4.42.42.5.5.5.5.5.5.5.5) (Fig. 2c), which have thefollowing characteristics: (1) to the best of our knowledge, this 8-connected topology is unique and unprecedented [42,43]; (2) it'sthe first MOF reported with 8-connected topology belonging to cubiccrystal system, in which the views along the a, b and c directions areidentical [8–33]; (3) it presents a noncentrosymmetric 8-connectedtopology, which means that the MOF has potential application as aNLO material; (4) it consists of an unique anionic net.

On the other hand, the three-dimensional network can beregarded as an unique trinodal (3,4,4)-connected topology with thepoint symbol of (4.6.8)(42.62.82)(44.62) analyzed by TOPOS program[44] by considering six-coordinated Cd1, bta and eight-coordinatedCd2 units as three, four and four-connecting nodes, respectively(Fig. 3).

It is noted that there are “empty” spaces in the three-dimensionalnetwork, which are filled with [(CH3)2NH2]+ cations. These

) in1. Symmetry codes: a, 1/4+y, 3/4−x, 5/4−z; b, 1−x, 1/2−y, z; c, 3/4−y,−1/4+x,

Fig. 3. The trinodal (3,4,4)-connected (4.6.8)(42.62.82)(44.62) topology of 1, in whichpurple, blue and yellow balls represent six-coordinated Cd1, bta and eight-coordinatedCd2 units, respectively.

Fig. 4. Thermogravimetric analysis of 1.

Fig. 2. Structures of the pentanuclear [Cd5(bta)4] SBU(a), the linkages of a eight-connectedCd5 core (b) and the 34414510 topology (c) in 1. In c, the purple balls represent Cd5 SBUs.

204 L. Cheng et al. / Inorganic Chemistry Communications 15 (2012) 202–207

[(CH3)2NH2]+ cations, which maybe come from the decomposition ofDMF at the high temperature and high pressure under hydrothermalconditions [39b], are stabilized in the apertures of this three-dimensional construction by the N\H⋯O hydrogen bonds between[(CH3)2NH2]+ and bpt ligands. The cations-accessible void volume,

as determined with the PLATON program [45] is 14383.7 Å3, whichis 47.4% of the unit cell volume.

The thermogravimetric analysis of powder sample of 1was carriedout from 16 to 686 °C under a nitrogen atmosphere at a heating rateof 10 °C min−1, as shown in Fig. 4. The TGA curve for the compoundshows that there is no weight loss between 16 and 300 °C, whichindicates that the framework of 1 can remain stable up to 300 °C.Decomposition of the polymer began at 300 °C; in the temperaturerange of 300–393 °C the removal of [(CH3)2NH2]+ cations occurredwith a loss of 10.3% (calc. 12.9%). The second weight loss of 65.0%(calc. 60.7%) between 412 and 596 °C corresponds to the pyrolysis ofbta ligands and the final residual weight was 24.7% (calc. 30.2%)corresponding to CdO.

The solid-state luminescence of complex 1 and free H4bta ligandwere investigated at room temperature, as shown in Fig. 5. 1 exhibitsan intense radiation with λmax at 381 nm upon excitation at 238 nm.Since a weakly similar emission (λmax=380.5 nm) is also observed forH4bta, the luminescence of 1 is tentatively assigned to the intraligandfluorescent emissions. Compared with H4bta, the enhancement in 1 isprobably due to the unique coordination of bta to the Cd(II) centers

Fig. 5. Luminescent spectra of H4bta and 1 in the solid state at room temperature.

205L. Cheng et al. / Inorganic Chemistry Communications 15 (2012) 202–207

increasing the ligand conformational rigidity, thereby reducing thenon-radiative decay of the intraligand (π–π*) excited state [11d,46].

We conducted the quasi-Kurtz second harmonic generation (SHG)measurements on powdered sample to confirm its acentricity as wellas to evaluate its potential application as a second-order NLOmaterial[47]. The preliminary experimental result indicates that 1 displays aSHG efficiency that is approximately 0.8 times that of KDP. Themodestpowder SHG response of 1 may be attributed to a comparable shortdonor-acceptor system, which is essential for second-order opticalnonlinearity [48].

In summary, we have synthesized and characterized a three-dimensional uninodal eight-connected metal–organic frameworkbased on pentanuclear cadmium subunits with a new uninodaleight-connected 34414510 topology. This framework has a potentialapplication as a multifunctional material with both fluorescent andnonlinear optical (NLO) properties.

Acknowledgments

The authors are grateful to the financial support from NationalNatural Science Foundation of China (no. 21001024), the Fundingfrom Southeast University (no. 4007041121 and no. 9207040016)and the Natural Science Project of University of Jiangsu Province(no. 10KJB150002).

Appendix A. Supplementary material

Supplementary data to this article can be found online at doi:10.1016/j.inoche.2011.10.024.

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