Cryst. Res. Technol. 44, No. 3, 331 – 335 (2009) / DOI 10.1002/crat.200800392

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A new photoluminescent supramolecular inorganic-organic

hybrid zincophosphate complex pillared by carboxylate ligand

through hydrogen bonding interactions

Li Li1,2

, Daofeng Sun1, Guangru Tian

1, Xinyu Song

1, and Sixiu Sun*

1

1 Department of Chemistry, Shandong University, Jinan, Shandong 250100, P. R. China 2 College of Chemistry and Chemical Engg., Ningxia University, Yinchuan, Ningxia 750021, P. R. China

Received 1 September 2008, revised 27 September 2008, accepted 1 October 2008

Published online 25 October 2008

Key words zincophosphate, pillared, inorganic-organic hybrid.

PACS 61.66.-f

A new inorganic-organic hybrid zincophosphate, [Zn(H2O)2(H2PO4)2]·2PABA (1) (PABA = p-aminobenzoic

acid), pillared by PABA ligands through hydrogen bonding interactions, has been synthesized and

characterized. Single-crystal X-ray diffraction reveals that 1 crystallizes in the monoclinic P21/c space group.

The geometric feature in 1 is the 3D supramolecular structure constructed from alternately arranged organic

and inorganic layers that consist of infinite parallel inorganic chains. These chains are extended to a 2D

inorganic layer through intermolecule hydrogen bonding interactions between N atoms (from PABA ligand)

and two O atoms (from adjacent inorganic chains). 1 was characterized by IR spectroscopy, XRD, DSC/TGA.

The fluorescent property for 1 in solid state was also investigated at room temperature.

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

Inorganic-organic hybrid compounds, which can combine the unique features of both the organic and inorganic substructures, have shown interesting properties in the areas of catalysis, biology, electrical conductivity, magnetism, and photochemistry owing to their fascinating structures and potential applications [1-4]. And the incorporation of organic substructures into inorganic portions provides a powerful method for structural modification and synthesis of novel organic-inorganic hybrid complex [5-8]. Weak interactions, such as hydrogen bonding and aromatic π-π stacking interactions may greatly affect the structures of these hybrid complexes [9,10]. P-aminobenzoic acid (PABA) is a simple but interesting organic compound, which containing both proton donor amino (-NH2) group and proton acceptor (or donor) carboxyl acid (-COOH) group. The combination of PABA and appropriate inorganic portions via hydrogen bonding and other interactions may give rise to some novel architectures. Zincophosphate were extensively studied in the past decades because of their potential applications in catalysis and separation processes [11-14]. These compounds are generally prepared under hydrothermal conditions in the presence of template amines [15-21]. In this paper, we introduce PABA to the synthesis of zincophosphate. And new supramolecular inorganic-organic hybrid zincophosphate [Zn(H2O)2(H2PO4)2]·2PABA (1) was obtained at room temperature. We have also measured the photoluminescence of 1 and free PABA ligand at room temperature. We found that the emission of 1 is blue-shifted by 19 nm compared with the free PABA ligand.

2 Experimental

Synthesis Compound 1 was synthesized under ambient conditions. All reagents were of analytical grade. In a typical synthesis, 1 mmol ZnSO4⋅7H2O was dissolved in 10 mL water. 0.5 mmol H3PO4 and 2 mmol p-____________________

* Corresponding author: e-mail: ssx@sdu.edu.cn

332 Li Li et al.: A new photoluminescent inorganic-organic hybrid zincophosphate complex

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

aminobenzoic acid dissolved in 10 mL ethanol were added to the above solution under stirring. Then, the whole solution was refluxed for 5 h, and filtered. The filtrate was sealed in a small beaker and left to crystallize at room temperature. Colorless and transparent crystals appeared after several days. The crystals were filtered, washed and dried at room temperature. Yield: 40% (based on Zn). The elemental analysis found (wt %): C, 28.98; N, 4.76; H, 3.55 (calculated value: C, 29.50; N, 4.92; H, 3.86). IR (KBr): 3402(s), 1707(s), 1614(s), 1527(m), 1508(s), 1429(w), 1407(m), 1257(s), 1093(s), 1257(s), 850(m), 754(s), 623(s), 480(s).

Characterizations The elemental analysis was performed on a Vario EL III element analyzer. FTIR spectrum was recorded in the 400-4000 cm-1 region, using KBr pellets on a Bruker vector-22 spectrometer. Differential scanning calorimetry and thermal gravity analysis (DSC/TGA) were carried out from room temperature to 900 °C by using a TA Instruments SDT Q600 under N2 atmosphere at a heating rate of 10 Kmin-1. A suitable size single crystal of the compound was carefully selected under a polarizing microscope and glued to a thin glass fiber with cyanoacrylate adhesive. The intensity data were collected on a Bruker-Nonius SMART APEX II CCD diffractometer (graphite-monochromated Mo Ka radiation, λ = 0.71073 Å). The structure of the crystal was solved by direct method and refined on F2 by the full matrix-least-squares technique using SHELXL program [22]. The hydrogen atoms except for those of water molecules were generated geometrically and allowed to ride on their parent atoms. The powder X-ray diffraction patterns were recorded on a Japan Rigaku D/max-γA 200 X-ray diffractometer by using Cu Ka radiation (λ=1.5418 Å).

Table 1 Crystallographic parameters for 1. (Crystallographic Data Center as supplementary publication

number CCDC-682531. Cambridge Crystallographic Data Center, 12 Union Road, Cambridge, CB2 1EZ,

UK; E-mail: deposit@ccdc.cam.ac.uk)

empirical formula C14H14N2O14P2Zn formula weight 561.58 crystal system monoclinic space group P21/c a (Å) 5.13840(10) b (Å) 25.3169(3) c (Å) 7.87530(10) beta(deg) 96.3430(10) volume (Å3) 1018.21(3) Z 4 Calculated density (mg.m-3) 1.832 Absorption coefficient (mm-1) 1.443 F(000) 568 Range of h, k, l -7/7, -36/24, -11/9 Total reflections 3005 Independent reflections 2670 Papameters 163 R indices [I >2σ(I)] 0.0331, 0.0926 R (all data) 0.0380, 0.0956 Goodness of fit on F2 1.028

3 Results and discussion

Crystal structure X-ray single crystal diffraction reveals that compound 1 crystallizes in monoclinic space group with centric P21/c symmetry. A summary of the crystallographic data is given in table 1. Selected bond distance and bond angle data are summarized in table 2. The asymmetric unit of 1 is presented in figure 1. Figure 2 shows the structure of 1D chain viewed along the c-axis direction. The 1D chain consists of strictly alternating ZnO6 octahedral and H2PO4 tetrahedra units. Two ZnO6 octahedrons and two H2PO4 tetrahedrons are connected through their oxygen vertices to form a four-membered ring. The four-membered rings are linked via O-Zn-O bridges to form the 1D inorganic chains propagating along a-axis. The Zn-O bond lengths are in the range of 2.0848-2.1202 Å with an average value of 2.0984 Å, which are in agreement with the reported ones in the reference [23]. The O-Zn-O bond angles are nearly perfect and in the range of 86.12 -180°. Notice that Zn atom is octahedrally coordinated to six oxygen atoms, and this ZnO6 unit is rarely mentioned in the synthesis of zincophosphate materials [23,24]. The Zn atoms are connected to the neighboring P atoms via Zn-O-P bridges (bond angle Zn-O1-P =134.35°, Zn-O4-P =129.27°). The PABA molecules, acting as pillars of the 1D chains, are located in the interchain spaces, and they are almost perpendicular to the inorganic chains (a-axis direction). As shown in figure 3, one side of the PABA ligand (-NH2 group) is hydrogen bonded to two

Cryst. Res. Technol. 44, No. 3 (2009) 333

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oxygen atoms from the adjacent inorganic chains with the corresponding N…O distance of 2.854 and 2.917 Å. Thus, these parallel 1D chains are extended to a 2D inorganic layer. Meanwhile, the other side of the PABA ligand (-COOH group) is also hydrogen bonded to the adjacent oxygen atoms of another inorganic chain in another inorganic layer, with the corresponding O…O distance of 2.673 and 2.762 Å. Adjacent PABA ligands adopt opposite direction to interact with the inorganic chains. Finally, 3D supramolecular hybrid complex pillared by PABA ligand is constructed through intermolecule hydrogen bonding interactions. It can be seen clearly from figure 3 that the 3D supramolecular structure is constructed from alternately arranged organic and inorganic layers. Additionally, the aromatic rings adopt offset face-to-face stacking. No π-π interactions occurred, as the centroid-to-centroid distance being 3.938 Å [25].

Table 2 Selected bond distances and angles for 1.

moiety distance (Å) moiety distance (Å) Zn-O5 Zn-O4 Zn-O1 P-O1

2.0848(14) 2.0903(13) 2.1202(14) 1.4710(15)

P-O2 P-O3 P-O4

1.4806(13) 1.4714(15) 1.4767(13)

moiety angle (deg) moiety angle (deg) O5-Zn-O5 O5-Zn-O4 O4-Zn-O4 O5-Zn-O1 O4-Zn-O1 O1-Zn-O1 P-O4-Zn

180.000(1) 87.67(6) 180.000(1) 93.88(5) 92.89(6) 180.0 129.27(9)

O2-P-O3 O2-P-O4 O3-P-O4 O2-P-O1 O3-P-O1 O4-P-O1 P-O1-Zn

111.18(10) 107.32(9) 109.94(9) 109.31(9) 109.27(9) 109.80(8) 134.35(8)

Fig. 1 The asymmetric unit of 1 (hydrogen atoms

omitted for clarity).

Fig. 2 Inorganic chain structure in 1 viewed along c-axis.

(Online color for both figures at www.crt-journal.org)

Fig. 3 The packing of complex 1 viewed along the a-axis. The red dashed lines represent the intermolecule

hydrogen bondings. (Online color at www.crt-journal.org)

334 Li Li et al.: A new photoluminescent inorganic-organic hybrid zincophosphate complex

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

XRD analyses We examined the structural homogeneity of bulk powder sample of 1 through comparison of experimental and simulated powder XRD patterns. The peak positions of the experimental patterns are in agreement with that of the simulated ones generated from single-crystal X-ray diffraction data (Fig. 4), which indicate the phase purity of compound 1. The intensities of the experimental XRD patterns are a little weak, due to the preferred orientation of the powder samples and the instrumental limitations.

Fig. 4 Powder XRD patterns for compound 1: (a) the

simulated XRD pattern calculated from the single-crystal

structure and (b) taken at room temperature.

Fig. 5 The emission spectrum of 1 (a) and pure PABA

(b) in the solid state at room temperature.

Thermogravimetic analysis Differential scanning calorimetry and thermogravimetic analysis

(DSC/TGA) were carried out from room temperature to 900 °C. TGA and DSC curve indicate that the weight losses occurred mainly in three mass losses. The first weight loss of 6.70% occurred around 215 °C was in agreement with the calculated amount of the removal of coordination water molecule. (calculated value of 6.32%). The second weight loss of 24.4% occurred around 373 °C was due to the removal of one PABA molecule (calculated value of 24.07%). The last weight loss of 24.2% occurred from 373 °C to 515 °C corresponds to the loss of another PABA molecule. The decomposed sample was found to be poorly crystalline and corresponds to dense zinc phosphate and some other phase, indicating destruction of the structure upon loss of the H2O and PABA molecules.

Luminescent properties The emission spectrum of compound 1 and pure PABA in solid state at room temperature are shown in figure 5. The emission band at 423 nm (λex = 370 nm) of PABA corresponds to the π-π* transition. Compound 1 exhibits a emission maximum at 404 nm (λex = 340 nm) which is blue-shifted by 19 nm compared with that of pure PABA. The shift of the luminescence is therefore attributed to the hybrid of the PABA to the 1D inorganic chain via intermolecule hydrogen bonding.

4 Conclusions

In conclusion, a new photoluminescent supramolecular hybrid zincophosphate with the formula [Zn(H2O)2(H2PO4)2]·2PABA has been synthesized at room temperature. The geometric feature in 1 is the 3D supramolecular structure constructed from PABA pillared inorganic layers. The structure of the new hybrid supramolecular is different from that of previous reported zincophosphates. Therefore, the new complex described in this paper is a compensation of zinc phosphate family.

References

[1] S. I. Stupp and P. V. Braun, Science 277, 1242 (1997).

[2] A. K. Cheetham, G. Férey, and T. Loiseau, Angew. Chem., Int. Ed. 38, 3268 (1999).

Cryst. Res. Technol. 44, No. 3 (2009) 335

www.crt-journal.org © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

[3] P. J. Hagrman, D. Hagrman and J. Zubieta, Angew. Chem. Int. Ed. 38, 2638 (1999).

[4] G. Férey, Chem. Mater. 13, 3084 (2001).

[5] P. J. Zapf, R. C. Haushalter, and J. Zubieta, Chem. Commun. 321 (1997).

[6] R. C. Finn and J. Zubieta, Inorg. Chem. 40, 2466 (2001).

[7] G. H. Li, Z. Shi, X. M. Liu, Z. M. Dai, L. Gao, and S. H. Feng, Inorg. Chem. 43, 8244 (2004).

[8] M. C. Hong, Cryst. Growth Des. 7, 10 (2007).

[9] A. M. Beatty, B. A. Helfrich, G. A. Hogan, and B. A. Reed, Cryst. Growth Des. 6, 122 (2006).

[10] A. M. Madalan, N. Avarvari, and M. Andruh, Cryst. Growth Des. 6, 1671 (2006).

[11] A. K. Cheetham, G. Ferry, and T. Loiseau, Angew. Chem. Int. Ed. 38, 3268 (1999).

[12] Z. Shi, S. Feng, L. Zhang, G. Yang, and J. Hua, Chem. Mater. 12, 2930 (2000).

[13] A. Clearfield, Chem. Mater. 10, 2801 (1998).

[14] M. J. Zaworotko, Angew. Chem. Int. Ed. Engl. 37, 1211 (1998).

[15] X. H. Bu, P. Y. Feng, and G. D. Stucky, J. Solid State Chem. 125, 243 (1996).

[16] W. T. A. Harrison and M. L. F. Phillips, Chem. Mater. 9, 1837 (1997).

[17] S. Neeraj, S. Natarajan, and C. N. R. Rao, Chem. Mater. 11, 1390 (1999).

[18] K. H. Lii, Y. F. Huang, V. Zima, C. Y. Huang, H. M. Lin, Y. C. Jiang, F.L. Liao, and S. L. Wang, Chem. Mater. 10,

2599 (1998).

[19] S. Neeraj and S. Natarajan, Cryst. Growth Des. 1, 491 (2001).

[20] S. Mandal and S. Natarajan, Cryst. Growth Des. 2, 665 (2002).

[21] S. Neeraj, S. Natarajan, and C. N. R. Rao, Chem. Commun. 165 (1999).

[22] G. M. Sheldrick, SHELXS and SHELXL, Institut für Anorganische Chemie der Universität Göttingen, Germany,

1998.

[23] D. Z. Zhang, H. J. Yue, Z. Shi, M. Guo, and S. H. Feng, Solid State Sci. 8, 192 (2006).

[24] L. Herschke, V. Enkelmann, I. Lieberwirth, and G. Wegner, Chem. Eur. J. 10, 2795 (2004).

[25] C. Janiak, J. Chem. Soc., Dalton Trans. 3885 (2000).