In Situ: 2-D Inorganic Layer Lead-MOF Built from [(l 4 -O)Pb 4 ] Core

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1 J Clust Sci (2011) 22: DOI /s ORIGINAL PAPER In Situ: 2-D Inorganic Layer Lead-MOF Built from [(l 4 -O)Pb 4 ] Core Kui-Rong Ma Hua-You Hu Yu-Lan Zhu Yu Zhang Rong-Qing Li Feng Ma Received: 6 August 2010 / Published online: 10 April 2011 Ó Springer Science+Business Media, LLC 2011 Abstract In situ lead-mof derived from 2-Methyl-3-acetylnaphtho[2,3-b]furan- 4,9-dione (MAFD), [Pb 7 O 2 (OH) 2 (1,2-BDC) 4 (H 2 O)] 1, 1,2-BDC = C 6 H 4 (COOH) 2, phthalic acid, has been synthesized and characterized by elemental analysis, IR, TG- DTA, powder X-ray diffraction (XRD) and single crystal X-ray diffraction. Compound 1 possesses 2-D inorganic layer structure built from rare tetranuclear unit [(l 4 -O)Pb 4 ]. In 1, both crystallographic distinct Pb(1) and Pb(4) ions adopt sixcoordination geometry, and the other two crystallographic distinct Pb(2) and Pb(3) ions display eight-coordination geometry under the condition of Pb O bond length extended to 3.10 Å. A 3-D supramolecular network is also formed by hydrogen bonds (C HO). Result of photoluminescence measurement indicates an emission band at 385 nm (k excitation = 209 nm). Keywords In situ Lead(II)-MOF phthalate Crystal structure Photoluminescence Electronic supplementary material The online version of this article (doi: /s ) contains supplementary material, which is available to authorized users. K.-R. Ma (&) H.-Y. Hu Y.-L. Zhu Y. Zhang R.-Q. Li F. Ma Jiangsu Key Laboratory for Chemistry of Low-dimensional Materials, School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huai an , People s Republic of China kuirongma@163.com F. Ma Department of Chemistry, Science College, Yanbian University, Yanji , People s Republic of China

2 268 K.-R. Ma et al. Introduction Recently, the selection of the metal centers and organic spacers is considerable interest in the synthesis and characterization of metal organic frameworks (MOFs). In particular, Pb(II)-base MOF materials have attracted much effort due to a wide range of their potential application, such as catalysis, gas storage, gas absorption and ion exchange [1]. Lead (II) ion possesses a large radius, a variable stereochemical activity and a flexible coordination environment, which can provide unique opportunities for the construction of novel MOFs [2 6]. Moreover, people have invested a lot of energy to obtain suitable organic ligands for the synthesis of MOFs by in situ method [7]. In situ ligand synthesis usually generates unexpected structures. In this paper a MOF with aromatic carboxylate, [Pb 7 O 2 (OH) 2 (1,2- BDC) 4 (H 2 O)] 1, is prepared via in situ ligand synthesis under solvothermal conditions. We reported the synthesis, structure, and luminescent property of 1, which was generated through in situ C C bond-breaking reactions of MAFD under the catalysis of nitrate radical (Scheme 1). As everyone knows, the hydro-/solvothermal reaction is so complicated that it is difficult to get a reaction mechanism of in situ ligand synthesis at high temperature and pressure. The title compound can t be obtained by the direct reactions of 1,2-BDC with lead nitrate. In addition, the effect of different lead salts, including lead acetate and lead chloride, on the structure of the target product has been investigated. PXRD analysis showed that the obtained products are different from the target product, using lead acetate and lead chloride instead of lead nitrate under similar reaction conditions (see supplement Fig. S1). Therefore, nitrate radical may play a role of catalysts in this solvothermal reaction. Experimental Synthesis of [Pb 7 O 2 (OH) 2 (1,2-BDC) 4 (H 2 O)] 1 2-Methyl-3-acetylnaphtho[2,3-b]furan-4,9-dione (MAFD) was synthesized according to the literature methods [8]. A mixture of g (0.8 mmol) Pb(NO 3 ) 2, g (0.16 mmol) MAFD and 10.0 ml ethanol was sealed in a 15 ml Teflon-lined stainless steel autoclave, and then heated at 150 C for 120 h. The initial and final ph values are *6.2 and 5.5, respectively. The crystalline of 1(colorless block-shaped) was collected by vacuum filtration, washed thoroughly with deionized water and dried in air (yield 18% based Scheme 1 In situ ligand synthesis

3 Inorganic Layer Lead-MOF Built from [(l 4 -O)Pb 4 ] 269 on lead). Elemental analysis, C 32 H 20 O 21 Pb 7 : C, 17.65; H, Calcd.: C, 17.53; H, IR data (cm -1 ): 3420 (ms), 2925 (w), 2857 (w), 1631 (w), 1602 (w), 1579 (w), 1560 (w), 1522 (s), 1444 (ms), 1398 (w), 1385 (s), 1359 (w), 1259 (w), 1128 (w), 1084 (ms), 1003 (w), 984 (w), 858 (ms), 829 (w), 776 (w), 715 (w), 694 (ms), 618 (w), 473 (ms). (See supplement Fig. S2) Materials and General Methods All other chemical reagents were obtained from commercial sources and used without further purification. The elemental analysis was conducted on a Perkin Elmer 2400 LC II elemental analyzer. IR spectrum was carried out on a Nicolet AVATAR360 spectrometer with KBr pellets in the cm -1 region. Thermogravimetric analysis (TGA) was performed in an atmospheric environment with a heating rate of 10 C/min on a TGA/SDTA851e thermogravimetric analyzer. Emission and excitation spectra were recorded on a Perkin Elmer LS-55 photoluminescent spectrometer with a 500 W xenon lamp. The powder X-ray diffraction (XRD) patterns was collected on an ARL X TRA diffractometer using graphite-monochromated Cu Ka radiation (k = Å) in the angular range 2h = 4 40 with stepping size of 0.02 and counting time of 4 s per step. X-Ray Data Collection and Structure Determinations Intensity data were collected on a Bruker SMART CCD diffractometer equipped with a graphite-monochromated Mo Ka radiation (k = Å) at 296 K using the x-2h scan technique. The structure was solved by direct methods and refined by full-matrix least-squares fitting on F 2 by SHELXL-97. A total of reflections were collected, of which 3637 (R int = ) were unique. All non-hydrogen atoms were located from the initial solution and refined with anisotropic thermal parameters. The position of hydrogen atoms were located by calculated geometrically and their contributions in structural factor calculations were included. Crystallographic data and structural refinements are summarized in Table 1. Selected bond lengths [Å] and angles [ ] are list in Table 2. Hydrogen bonds [Å] and angles [ ] are given in Table 3. CCDC contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via Results and Discussion Description of Structure Single-crystal X-ray diffraction analysis reveals that compound 1 possesses a 2-D architecture, in which the asymmetric unit (Fig. 1) contains four crystallographic unique Pb(II) ions and two 1,2-BDC ligands.

4 270 K.-R. Ma et al. Table 1 Crystal data and structure refinements for 1 Empirical formula C 32 H 20 O 21 Pb 7 Formula weight Temperature 296(2) K Wavelength Å Crystal system Monoclinic Space group P21/c Unit cell dimensions a = (3) Å b = (2) Å c = (4) Å b = (2) Volume (7) Å 3 Z 2 Calculated density Mg m -3 Absorption coefficient mm -1 Theta range for data collection Limiting indices -16 B h B 15, -9 B k B 9, -22 B l B 22 Reflections collected/unique 13724/3637 [R(int) = ] Completeness to theta = % Refinement method Full-matrix least-squares on F 2 Data/restraints/parameters 3637/7/277 Goodness-of-fit on F Final R indices [I [ 2sigma(I)] R 1 = , wr 2 = R indices (all data) R 1 = , wr 2 = Largest diff. peak and hole and e Å -3 Pb(1) ion is a slightly deformed octahedral coordination with two oxygen atoms (l 2 -O(6), l 3 -O(8)) from two carboxyl groups, three bridge oxygen atoms (l 3 -O(9), l 3 -O(9A), l 4 -O(10)) and one lattice water molecule (O(1w)), in which the Pb(1) O(6) distance (2.99 Å) is longer than those of Pb(1) O distances ( Å). Pb(2) ion is eight-coordinated with six oxygen atoms (l 2 -O(3), l 2 -O(7), l 3 -O(8), l 2 -O(3A), l 2 -O(7A), l 3 -O(8A)) from four 1,2-BDC ligands and two bridge oxygen atoms (l 4 -O(10)), in which the Pb(2) O(8)/O(8A) distance (2.82 Å) is longer than those of Pb(2) O distances ( Å). Pb(3) ion adopts a rare pentagonal pyramid coordination in a normal bond length range of Å, including five subface oxygen atoms (l 2 -O(1), O(4), l 2 -O(5), l 3 -O(8), l 3 -O(9)) and one vertex oxygen atom (l 4 -O(10)). The coordination geometry around the Pb(3) ion can be also described as a {w-pbo 7 } pseudo-pentagonal bipyramid with the seventh coordination site occupied by the lone pair of the Pb(II) ion. Pb(4) ion is also eightcoordinated with seven oxygen atoms (l 2 -O(1), l 2 -O(2), l 2 -O(2A), l 2 -O(3), l 3 -O(5), l 2 -O(6), l 2 -O(7)) from four 1,2-BDC ligands and one bridge oxygen atom (l 4 -O(10)). Pb(4) O(1)/O(2) distances ( Å) are longer than those of Pb(4) O distances ( Å) and O Pb O angles are range from 50.2(3) to 153.5(3), resulting in a severe distorted dodecahedron.

5 Inorganic Layer Lead-MOF Built from [(l 4 -O)Pb 4 ] 271 Table 2 Selected bond lengths [Å] and angles [ ] for 1 Bond length Pb(1) O(10) 2.328(8) Pb(1) O(9) 2.353(8) Pb(1) O(9)# (8) Pb(1) O(8)# (8) Pb(1) O(1 W) 2.756(9) Pb(2) O(10)# (8) Pb(2) O(10) 2.630(8) Pb(2) O(3)# (8) Pb(2) O(3)# (8) Pb(2) O(7) 2.734(9) Pb(2) O(7)# (9) Pb(3) O(10) 2.170(8) Pb(3) O(9)# (8) Pb(3) O(1) 2.623(8) Pb(3) O(5) 2.645(9) Pb(3) O(8) 2.676(8) Pb(3) O(4)# (9) Pb(4) O(10) 2.342(8) Pb(4) O(6) 2.574(8) Pb(2) Pb(4)# (9) Pb(4) O(2)# (8) Pb(4) O(5) 2.660(9) Pb(4) O(7) 2.684(9) O(1) C(7) 1.261(1) O(2) C(7) 1.284(1) O(2) Pb(4)# (8) O(3) C(8) 1.274(1) O(3) Pb(2)# (8) O(4) C(8) 1.231(1) O(4) Pb(3)# (9) O(5) C(15) 1.253(1) O(6) C(15) 1.274(1) O(7) C(16) 1.261(1) O(8) C(16)# (1) O(8) Pb(1)# (8) O(9) Pb(1)# (8) O(9) Pb(3)# (8) C(16) O(8)# (1) Bond angle O(10) Pb(1) O(9) 96.9(3) O(10) Pb(1) O(9)#1 79.1(3) O(9) Pb(1) O(9)#1 69.9(3) O(10) Pb(1) O(8)# (3)

6 272 K.-R. Ma et al. Table 2 continued O(9) Pb(1) O(8)#1 71.3(3) O(9)#1 Pb(1) O(8)#1 82.2(3) O(10) Pb(1) O(1W) 94.3(4) O(9) Pb(1) O(1W) 87.8(4) O(9)#1 Pb(1) O(1W) 155.5(4) O(8)#1 Pb(1) O(1W) 100.5(4) O(10)#2 Pb(2) O(10) 180.0(1) O(10)#2 Pb(2) O(3)# (2) O(10) Pb(2) O(3)#3 74.0(2) O(10)#2 Pb(2) O(3)#1 74.0(2) O(10) Pb(2) O(3)# (2) O(3)#3 Pb(2) O(3)# (1) O(10)#2 Pb(2) O(7) 111.7(2) O(10) Pb(2) O(7) 68.3(2) O(3)#3 Pb(2) O(7) 83.6(3) O(3)#1 Pb(2) O(7) 96.4(3) O(10)#2 Pb(2) O(7)#2 68.3(2) O(10) Pb(2) O(7)# (2) O(3)#3 Pb(2) O(7)#2 96.4(3) O(3)#1 Pb(2) O(7)#2 83.6(3) O(7) Pb(2) O(7)# (1) O(10)#2 Pb(2) Pb(4)# (2) O(10) Pb(2) Pb(4)# (2) O(3)#3 Pb(2) Pb(4)# (2) O(3)#1 Pb(2) Pb(4)# (2) O(7) Pb(2) Pb(4)# (2) O(7)#2 Pb(2) Pb(4)# (2) O(10) Pb(3) O(9)#1 79.8(3) O(10) Pb(3) O(1) 81.3(3) O(9)#1 Pb(3) O(1) 69.5(3) O(9)#1 Pb(3) O(4)# (3) O(1) Pb(3) O(4)# (2) O(5) Pb(3) O(4)#3 69.6(3) O(8) Pb(3) O(4)#3 67.4(3) O(10) Pb(4) O(6) 100.7(3) O(10) Pb(4) O(2)#4 87.9(3) O(6) Pb(4) O(2)#4 76.3(3) O(10) Pb(4) O(5) 74.5(3) O(6) Pb(4) O(5) 50.2(3) O(2)#4 Pb(4) O(5) 117.1(3) O(10) Pb(4) O(7) 73.3(3) O(6) Pb(4) O(7) 153.5(3) O(2)#4 Pb(4) O(7) 77.7(3) O(5) Pb(4) O(7) 143.8(3)

7 Inorganic Layer Lead-MOF Built from [(l 4 -O)Pb 4 ] 273 Table 2 continued Symmetry transformations used to generate equivalent atoms: #1, -x? 1, -y? 1, -z? 1; #2, -x? 1, -y, -z? 1; #3, x, y - 1, z; #4, -x? 1, y - 1/2, -z? 3/2; #5, -x? 1, y? 1/2, -z? 3/2; #6, x, y? 1, z O(5) Pb(3) O(8) 131.6(3) O(10) Pb(3) O(4)#3 84.1(3) Pb(3) O(5) Pb(4) 92.1(3) Pb(4) O(7) Pb(2) 85.8(2) Pb(3) O(8) Pb(1)#1 99.1(3) Pb(1) O(9) Pb(1)# (3) Pb(1) O(9) Pb(3)# (3) Pb(1)#1 O(9) Pb(3)#1 92.3(3) Pb(3) O(10) Pb(1) 102.5(3) Pb(3) O(10) Pb(4) 115.7(3) Pb(1) O(10) Pb(4) 119.1(3) Pb(3) O(10) Pb(2) 110.4(3) Pb(1) O(10) Pb(2) 113.9(3) Pb(4) O(10) Pb(2) 95.6(3) O(10) Pb(3) O(5) 77.6(3) O(9)#1 Pb(3) O(5) 142.3(3) O(1) Pb(3) O(5) 77.6(3) O(10) Pb(3) O(8) 77.1(3) O(9)#1 Pb(3) O(8) 70.3(3) O(1) Pb(3) O(8) 136.9(3) Owing to stereochemical activity of the 6 s electrons, coordination sphere of Pb(II) ion in 1 changes from hemi-directed (Pb(3)) to holo-directed (Pb(1), Pb(2), Pb(4)) coordination geometry with the change of coordination number based on weak Pb O interactions ( Å) which will produce distinct voids or gaps in the coordination sphere [9]. It is noteworthy that the 1,2-BDC ligand exhibits two different coordination modes, and two carboxylate groups of each 1,2-BDC ligand act as various types: (1) tetra-dentate, two carboxyl groups of 1,2-BDC ligand in l 4 : g 3 g 2 mode connecting with four Pb(II) ions (Fig. 2a). (2) hexa-dentate, in l 6 : g 5 g 3 mode bonding to six Pb(II) ions (Fig. 2b). The two coordination styles of the 1,2-BDC ligand are entirely different those of reported compounds on the basis of normal Pb O bond (\2.9 Å). Ali A. Soudi [10] reported phthalate ligand acted as a tetradentate ligand in l 4 : g 4 g 2 mode (Fig. 2c), and Jian-Fang Ma [11] reported phthalate ligand displayed a tridentate mode in l 3 : g 1 g 3 (Fig. 2d). Yuan-Gen Yao [12] reported the 1,2-BDC ligand had been divided into two types: l 6 : g 4 g 4 and l 5 : g 4 g 3 (Fig. 2e, f). Tetrahedron [Pb 4 O] is corner-shared with each other through Pb(2) vertex to form a [Pb 7 O 2 ] polyhedron and the other two tetrahedra [Pb 4 O] share two edges Pb(1) O(9) to obtain a [Pb 8 O 4 ] polyhedron. And then polyhedron [Pb 8 O 4 ] is also corner-shared with polyhedron [Pb 7 O 2 ] by Pb(4) vertex to give rise to an alternative inorganic chain arranged in ABAB running along the c-axis (Fig. 3). Two adjacent inorganic chains are further joined together via three lead atoms Pb(1), Pb(2) and Pb(4) to form a 2-D inorganic framework in the bc plane (Fig. 4a). Regarded Pb(II) ion as a node, 2-D meshy topological structure can be observed in the bc plane

8 274 K.-R. Ma et al. Table 3 Hydrogen bonds [Å] and angles [ ] for 1 D HA d(d H) d(ha) d(da) \(DHA) O(9) H(9)O(3)# (1) O(9) H(9)O(1)# (1) O(1w) H(1w1)O(7) (2) C(13) H(7)O(1w) (1) C(11) H(5)O(4) # (4) Symmetry transformations used to generate equivalent atoms: #1, -x? 1, -y? 1, -z? 1 Fig. 1 ORTEP view of 1 showing the atom-labeling scheme (50% thermal ellipsoids) (Fig. 4b). The Pb(1) Pb(1), Pb(1) Pb(3), Pb(2) Pb(4) and Pb(3) Pb(4) distances are , , , and Å, respectively. In particular, the Pb(II) Pb(II) contact is within 4.0 Å that the shorter metal metal contacts further reinforce the inorganic chain structure. It should be noted that this 2-D structure is the first example of containing the two types of polyhedral [Pb 7 O 2 ] and [Pb 8 O 4 ] in Pb(II) BDC compounds, based on a Cambridge Structure Database (CSD) (May, 2010) search until now [13]. Except for those long Pb O(N) contacts, there are many hydrogen bond interactions (C HO) in 1. These hydrogen bonds are formed through 1,2-BDC ligand (O(4), C(11), C(13)) and the water molecule O(1w). Consequently, a 3-D supramolecular network is constructed by C(13) H(7)O(1w) and C(11) H(5)O(4) hydrogen bonds (Fig. 5).

9 Inorganic Layer Lead-MOF Built from [(l 4 -O)Pb 4 ] 275 Fig. 2 Coordination modes around Pb(II) ions Fig. 3 1-D inorganic chain structure constructed from two types of polyhedrons [Pb 8 O 4 ] and [Pb 7 O 2 ]in an ABAB array along the c-axis XRD and TG-DTA Study The powder XRD pattern of 1 indicates that as-synthesized product is a new material, and the pattern is entirely consistent with the simulated one from the single crystal X-ray diffraction (see supplement Fig. S1). The TG analysis shows three major weight losses (see supplement Fig. S3). The first mass loss of 0.96% from 250 to 280 C corresponds to the loss of lattice water molecule (calc. 0.82%). The second mass loss of about 24.4% (calc %), in the range of C, can be assigned to the pyrolysis of 1,2-BDC organic framework. The step shows a continuous weight loss in this region of temperature. The final thermal decomposition is from 880 to 1000 C, and the corresponding residue is PbO, which possesses a mass of 71.3% (calc. 71.9%). Photoluminescence Property According to the work reported previously, coordination polymers containing Pb(II) ion can exhibit photo luminescent properties [2, 4]. Here, we investigated the

10 276 K.-R. Ma et al. Fig. 4 a 2-D inorganic layer built from inorganic chains in the bc plane. b View of 2-D meshy topological structure Fig. 5 3-D supramolecular network created from 2-D cationic layers. Hydrogen bonds are drawn as dotted lines Fig. 6 Solid-state emission spectrum of 1

11 Inorganic Layer Lead-MOF Built from [(l 4 -O)Pb 4 ] 277 solid-state photo luminescent property of 1 at room temperature as shown in Fig. 6. For making clear the nature of emission bands, the luminescent properties of free ligand 1,2-BDC have also been examined. The free ligand 1,2-BDC displays emission at 330 nm (k excitation = 310 nm), which can probably be assigned to the p* p transitions. Whereas the title compound shows an emission band at 385 nm (k excitation = 209 nm). Therefore, the emission band should be attributed to ligandto-metal charge transfer between delocalized p bonds of this aromatic carboxylate groups and p orbital of Pb 2? centers [14]. As a sequential work, we still pay more attention to the lead-mof and investigate effect on the luminescent properties through structural modifications. Conclusions In summary, we have synthesized an unexpected 2-D Pb(II)-BDC MOF by in situ ligand synthesis under solvothermal condition. The compound 1 displays a 2-D inorganic layer architecture with 1-D inorganic chain, which includes two types of polyhedrons [Pb 7 O 2 ] and [Pb 8 O 4 ] arranged in an ABAB way based on [(l 4 -O)Pb 4 ]. Organic moieties point to gap between two adjacent layers. In addition, many hydrogen bonds (C HO) have led to the production of 3-D supramolecular structure. The solid-state fluorescence measurement of 1 at room temperature reveals an emission band at 385 nm under excitation of 209 nm, likely caused by ligand-to-metal charge transfer between delocalized p bonds of this aromatic carboxylate groups and p orbital of Pb 2? centers. Acknowledgments The authors are grateful to the financial support from the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province, China, Open Fund of Jiangsu Key Laboratory for Chemistry of Low-dimensional Materials and Huai an Scientific and technological support industrial projects (Projects Nos. 10KJD150001, 10KJB150002, JSKC09068 and HAG ). References 1. S. R. Fan and L. G. Zhu (2007). Inorg. Chem. 46, K. R. Ma, D. J. Zhang, and Y. L. Zhu (2010). Aust. J. Chem. 63, S. C. Chen, Z. H. Zhang, Q. Chen, H. B. Gao, Q. Liu, M. Y. He, and M. Du (2009). Inorg. Chem. Commun. 12, K. R. Ma, J. N. Xu, L. Wang, J. Shi, Y. Wang, J. Ha, D. K. Ning, Y. Fan, and T. Y. Song (2007). Chem. J. Chin. U. 28, K. L. Zhang, Z. C. Pan, Y. Chang, W. L. Liu, and S. W. Ng (2009). Mater. Lett. 63, Y. H. Zhao, H. B. Xu, K. Z. Shao, Y. Xing, Z. M. Su, and J. F. Ma (2007). Cryst. Growth Des. 7, J. Blake, N. R. Champness, S. S. M. Chung, W. S. Li, M. Schröder (1997). Chem. Commun H. Y. Hu, Y. Zhu, L. Wang, and J. H. Xu (2005). Synthesis 10, L. Shimoni-Livny, J. P. Glusker, and C. W. Bock (1998). Inorg. Chem. 37, F. Marandi, M. Ghorbanloo, and A. A. Soudi (2007). J. Coord. Chem. 60, J. Yang, J. F. Ma, Y. Y. Liu, J. C. Ma, and S. R. Batten (2009). Cryst. Growth Des. 9, L. Zhang, Z. J. Li, Q. P. Lin, Y. Y. Qin, J. Zhang, P. X. Yin, J. K. Cheng, and Y. G. Yao (2009). Inorg. Chem. 48, F. H. Allen (2002). Acta Crystallogr. Sect. B: Struct. Sci. 58, G. Blasse and B. C. Grabmaier Luminescent Materials (Springer Verlag, Berlin, 1994), p. 13.