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Solid State Sciences 39 (2015) 1e5

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Solid State Sciences

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Synthesis and characterization of a novel inorganiceorganic hybridopen-framework zinc phosphate with 16-ring channels

Guo-Ming Wang a, *, Jin-Hua Li a, Cui-Li Gao a, Jun-Cheng Zhang a, Xiao Zhang a,Zhen-Zhen Bao a, Ying-Xia Wang b, Jian-Hua Lin b

a Teachers College, College of Chemical Science and Engineering, College of Physics Science, Qingdao University, Shandong 266071, Chinab Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

a r t i c l e i n f o

Article history:Received 31 August 2014Received in revised form11 November 2014Accepted 14 November 2014Available online 15 November 2014

Keywords:Open frameworkZinc phosphate16-Ring channelsHybridLuminescence

* Corresponding author.E-mail address: gmwang_pub@163.com (G.-M. Wa

http://dx.doi.org/10.1016/j.solidstatesciences.2014.11.01293-2558/© 2014 Published by Elsevier Masson SAS

a b s t r a c t

A novel inorganiceorganic hybrid open-framework zinc phosphate, (NH4)[Zn5(PO4)3(HPO4)2(HA-PIM)2]$(H2O) (1), was solvothermally synthesized in the presence of 1-(3-aminopropyl)imidazole (APIM)as structure-directing agent. Its structure is built up by the alternate linkages between ZnO4, ZnO3N, PO4

and HPO4 tetrahedral moieties, forming an unusual three-dimensional framework with intersecting 8-,10-, and 16-ring channels. The arrangement of the monoprotonated APIM molecules in 1 is unique andthey play dual roles as both ligands and countercations. It is the first example with APIM as template inextra-large-channel structures in zincophosphates. The framework contains a Zn/P ratio of 1:1 and ex-hibits remarkable low framework density. Photoluminescence studies show that there is a ligand-to-metal charge transfer.

© 2014 Published by Elsevier Masson SAS.

1. Introduction

Microporous materials have attracted considerable researchinterest owing to their rich structural chemistry and potential ap-plications in gas storage, catalysis, adsorption and separation [1e3].Since the discovery of crystalline aluminophosphate molecularsieves in 1982 [4], considerable progress has been achieved in thesynthesis of microporousmetal phosphates/phosphites due to theircompositional diversity and rich structural chemistry [5,6]. Therein,open-framework zinc phosphates/phosphites occupy an apicalposition and a large number of such solids with zero-, one-, two-,and three-dimensional structures have been prepared and char-acterized [7e12]. These structures are typically realized by versatileconnections of tetrahedral (octahedral) metal oxide clusters withphosphate (phosphite) tetrahedra under hydrothermal conditions.One common structural feature in this system is that the inorganicframeworks are often anionic and a remarkable variety of proton-ated organoamine templates are introduced as charge compen-sating and space-filling constituents.

ng).

07.

By contrast, the analogous organic-inorganic hybrid materialsassembled from covalently bonded organic and inorganic moieties,have not been studied as extensively. Undoubtedly, the combina-tion of different organic and inorganic moieties may offer oppor-tunities for making new structural types and novel properties. Atypical example in this system is [H2N(CH2)2NH2]0.5$ZnHPO3, inwhich the template is neutral, bonding directly to Zn atoms toproduce an unprecedented architecture of two interpenetrated,independent mixed inorganiceorganic network [13]. A review ofthe literature shows that anionic oxalate and neutral 4,40-bipyr-idine (4,40-bpy) are the most common organic ligands used in thepreparation of inorganiceorganic hybrid metal phosphates andphosphites [14e18]. Further, it is also observed that multifunctionalorganic ligands with N-donor or anionic O-donor groups, such asterephthalic acid, isonicotinate, pyrazine, imidazole, piperazine andnatural amino acid etc., are good coordinates for the construction ofhybrid frameworks. Notable examples include [Co3(4,40-bpy)(HPO4)2F2]$xH2O and [BAEOE][Zn(HPO3)]2 containing neutralinorganic layers pillared through distinct rigid 4,40-bipyridine andflexible 1,2-bis(2-aminoethoxy)ethane (¼ BAEOE) ligands respec-tively [19,20], Zn(HINT)(HPO4) possessing neutral Zn(HPO4) sheetswith pendent dipolar isonicotinate ligand [21], (H2tmdp)[(ZnHPO4)2(1,4-bdc)] (tmdp ¼ 4,40-trimethylenedipyridine; 1,4-bdc ¼ 1,4-benzenedicarboxylate) with zeolite-like topology and

G.-M. Wang et al. / Solid State Sciences 39 (2015) 1e52

bimodal porosity [22], [(Co(4-pya))3PO4] consisting of paddlewheelCo3 clusters connected by PO4

3� and 4-pyridinecarboxylate (¼ 4-pya) [23], ZnHPO-CJ56 and ZnHPO-CJ57 exhibiting homochiralinorganiceorganic hybrid frameworks with chiral L-trypophan andL-histidine molecules acting as both templates and the ligands [24],as well as several hybrid zinc phosphates/phosphites with differentimidazole-types ligands [25e28]. Herein, we attempt to assembletetrahedral Zn2þ with phosphate group and 1-(3-aminopropyl)imidazole (APIM) ligand for the construction of novel hybrid openframeworks. Excitingly, a new inorganiceorganic zinc phosphate(NH4)[Zn5(PO4)3(HPO4)2(HAPIM)2]$(H2O) (1), has been sol-vothermally synthesized. In this paper, we describe the synthesis,structure and luminescent properties of this compound.

2. Experimental section

2.1. Materials and methods

All reagents were purchased commercially and used withoutfurther purification. The CHN analyses were carried out on anElemental Vario EL III CHNOS elemental analyzer. Infrared spectrawere obtained from sample powder pelletized with KBr on an ABBBomen MB 102 series FTIR spectrophotometer over a range400e4000 cm�1. The thermogravimetric analysis (TGA) was per-formed on a Mettler Toledo TGA/SDTA 851e analyzer in N2 atmo-sphere with a heating rate of 10 �C/min from 40 to 800 �C.Fluorescent spectra were measured on a Jasco FP-6500 fluores-cence spectrophotometer.

Table 1

2.2. Synthesis

The title compound was synthesized solvothermally underautogenous pressure. In a typical synthesis, a mixture of ZnO(0.162 g, 2.0 mmol), H3PO4 (85%, 0.20 mL, 3.1 mmol), 1-(3-aminopropyl)imidazole (0.24 mL, 2.0 mmol), methanol (3.50 mL,86.5 mmol) and H2O (2.50 mL, 139.0 mmol) was sealed in a Teflon-lined steel autoclave and heated at 160 �C for 6 days. After coolingto room temperature, the resulting product, containing colorlesschunk-shaped crystals was recovered by filtration, washed withdistilled water and dried in air (46.2% yield based on zinc). CHNelemental analysis confirmed its stoichiometry (Anal. Calcd: C13.19, H 2.93, N 8.97%; Found: C 13.02, H 2.66, N 8.86%). The X-raypowder diffraction profile of the resulting product is in agreementwith the simulated pattern from single-crystal X-ray structure data,

Fig. 1. Experimental and simulated XRD patterns of 1.

indicating the purity of the as-synthesized sample of the titlecompound (Fig. 1).

2.3. Determination of crystal structure

A suitable colorless single crystal of as-synthesized compoundwas carefully selected under an optical microscope and glued to athin glass fiber with epoxy resin. Crystal structure determination byX-ray diffraction was performed on a Siemens SMART CCDdiffractometer with graphite-monochromated Mo Ka(l¼ 0.71073 Å) in the u and 4 scanning mode at room temperature.An empirical absorption correction was applied using the SADABSprogram [29]. The structure was solved by direct methods andrefined by full-matrix least-squares methods on F2 with theSHELXL-97 program package [30]. The zinc and phosphorus atomswere first located, whereas the carbon, nitrogen, and oxygen atomswere found in the successive difference Fourier maps. The C(2),C(3), C(4), C(5), C(8), C(9) and N(5) atoms are found to be disor-dered over two positions with an equal occupancy. The hydrogenatoms associated with the hydroxyl groups (O(7) and O(17)) wereplaced geometrically and refined in a riding model. All of the non-hydrogen atoms were refined anisotropically. Crystallographic dataand structure refinements for this compound are listed in Table 1.CCDC 1009196 contains the supplementary crystallographic datafor this paper.

3. Results and discussion

3.1. Infrared (IR) spectra

The IR spectrum of the title compound is shown in Fig. 2. Thebroad bands at 3422e2870 cm�1 correspond to the stretchingbands of the OH, NH2 and CH2. The bending bands of NH2 and CH2are present at 1635e1433 cm�1. The bands at the region of1118e897 cm�1 can be assigned to the asymmetric stretching vi-brations of PO4 group, and the symmetric vibration of PO4 group isobserved at around 780 cm�1. The bending vibrations of PO4 groupscan be observed in the range 483e705 cm�1.

3.2. Structural description

Single crystal X-ray analysis reveals that 1 crystallizes in theorthorhombic space group Pna21 (No. 33). The asymmetric unit

Crystal data and structure refinement for 1.

Empirical formula C12H32N7O21P5Zn5

Formula weight 1092.15Crystal system OrthorhombicSpace group Pna21a/Å 9.7925(2)b/Å 22.9726(6)c/Å 14.4497(3)a/� 90b/� 90g/� 90V/Å3 3250.59(13)Z 4Dc/g cm�3 2.232m (Mo-Ka)/mm�1 3.978F(000) 2184Reflection collected 76525Independent reflections 5696 [R(int) ¼ 0.0544]Parameters refined 108Limiting indices �11 � h � 11, �27 � k � 27, �17 � l � 17Goodness-of-fit on F2 1.081Final R1, wR2[I > 2s(I)] 0.0437, 0.0903

Fig. 2. The IR spectrum of 1.

G.-M. Wang et al. / Solid State Sciences 39 (2015) 1e5 3

contains 50 non-hydrogen atoms, specifically five zinc atoms, fivephosphorus atoms, twenty oxygen atoms, two monoprotonatedAPIM ligands, one ammonium ion and one noncoordinated latticewater molecule (Fig. 3). Atoms Zn(2), Zn(3) and Zn(4) are tetrahe-drally coordinated by four oxygen atoms with an average ZneObond length of 1.941 Å. The Zn(1) and Zn(5) atoms are respectivelycoordinated by three oxygen atoms and one N atom from a APIMligand, forming a ZnO3N unit with an average ZneO/N bond lengthof 1.964 Å. The OeZneO/N bond angles are in the range of 97.9(3)e122.3(2)� (av. 109.4�). Of the five independent phosphorus atoms,P(1), P(3) and P(4) link to four adjacent Zn atoms via four PeOeZnlinkages, whereas P(2) and P(5) atoms only make three PeOeZnbonds and have one terminal PeO bonds. The PeO bond lengthsvary from 1.501(6) to 1.590(2) Å (av. 1.529 Å), and the OePeO bondangles are in the range 106.7(3)-112.5(3)� (av. 109.5). Bond valencesum values [31] indicate that the terminal P(2)-O(7) and P(3)-O(17)linkages with longer distances of 1.572(2) and 1.590(2) Å are ter-minal PeOH bonds, and this assignment also corresponds well withthe proton positions observed near the oxygen atoms in the dif-ference Fourier maps. Assuming the usual valence of Zn, P, O tobe þ2, þ3 and �2, respectively, the composition of[Zn5(PO4)3(HPO4)2] creates a net charge of �3, which could bebalanced by one ammonium ion and two monoprotoned APIMmolecules.

Fig. 3. ORTEP view of the asymmetric u

The framework structure of 1 is constructed from strictly alter-nating Zn-centered tetrahedra (ZnO4 and ZnO3N) and P-centeredtetrahedral (PO4 andHPO4) linked via vertexoxygen atoms to form a3D structure with intersecting channels. As shown in Fig. 4a, regularelliptical channels with 8-ring and 16-ring apertures are locatedalong the [100] direction. The 8-ring window, being composed offour PO4, two ZnO4 and two ZnO3N tetrahedral units, has a pore sizeof ca. 6.2 � 7.6 Å; the largest 16-ring window was defined by fourPO4, four HPO4, four ZnO4 and four ZnO3N tetrahedral units, with adiameter size of ca. 7.0 � 14.6 Å (corresponding to the interatomicO$$$Odistances, not including thevanderWaals radii). However, thefree space within each 16-ring is greatly reduced because the ZneNbonds attached to these four zinc atoms protrude into the center ofthe 16-ring (Fig. 4b). The arrangement of the APIM ligands in thestructure is unique: thehydrophobic imidazole rings of APIM ligandsserve to anchor the organic species to the inorganic framework andoccupy the middle of the 16-ring channels, while the hydrophilicamino groups exclusively extend to the center of the smaller 8-ringchannels (Fig. 4c). Similar behavior has also been observed in otherextra-large pore materials, such as 24-ring zinc phosphate ND-1templated by 1, 2-DACH, 20-ring aluminophosphate JDF-20 tem-plated by triethylamine and 16-ring gallophosphate ULM-16 tem-plated by cyclopentylamine, etc [32e34]. Intersecting the large 16-ring channels are those smaller channels, i.e. 8-ring channelsrunning along the [010] direction and 10-ring apertures along the[001] direction (Fig. S1,2). It is worthy to note that the monoproto-nated APIM molecules in 1 play dual roles as both ligands andcountercations, distinct from those zeolitic frameworks with extra-large-pores filled by organic ammonium templates. Strong andextensive NeH$$$O hydrogen bonds exist between the terminaleNH3 groups of the template molecules and the framework oxygenatoms with N$$$O distances in the range of 2.624(7)-3.176(9) Å(Table 2 for detailed H-bond information). A void space analysisusing the program PLATON indicates that these extra-frameworkorganic cations occupy 47.6% of the unit cell volume [35].

A search of the available literature reveals that less than twentyzinc phosphates containing 16-ring channels have been synthe-sized. With the exception of five examples with piperazine,piperidine, cyclohexylamine, N,N0-dimethylpiperazine and 1-(2-aminoethyl)piperazine as templates [36e39], most zinc phos-phates with 16-ring apertures were prepared by using chain-typepolyamines as template molecules, such as diethylenetriamine,triethylenetetramine, tetraethylenepentamine, dipropylenetri-amine andN-(2-aminoethyl)-1,3-diaminopropane, etc [40e44]. To

nit of 1 (50% probability ellipsoid).

Fig. 4. (a) Polyhedral view of the structure along the [100] direction with 8- and 16-ring windows. Color code: ZnO4 and ZnO3N tetrahedra, green; PO4 tetrahedra, purple; (b) Ball-and-stick representation of the 16-ring window with terminal ZneN bonds protrude into the center of the 16-ring; (c) The arrangement of organic APIM molecules in 1. All thehydrogen atoms are omitted for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 2Details of hydrogen bond interactions for 1.

D�H/A d(D�H) d(H/A) d(D/A) :(DHA)

(Å) (Å) (Å) (deg)

O(7)�H(7B)/O(10) 0.82 2.26 2.057(2) 165.1O(17)�H(17A)/O(5) 0.82 1.81 2.624(7) 175.2N(3)�H(3B)/O(2) 0.89 2.13 2.872(9) 140.2N(3)�H(3D)/O(20) 0.89 2.06 2.816(9) 141.6N(3)�H(3D)/O(12) 0.89 2.25 3.041(9) 147.3N(6)�H(6B)/O(19) 0.89 1.98 2.851(9) 167.8N(6)�H(6C)/O(3) 0.89 2.30 3.716(9) 167.6N(6)�H(6C)/O(14) 0.89 2.50 3.163(9) 131.2N(6)�H(6D)/O(15) 0.89 2.05 2.806(9) 142.7

Fig. 5. The TG curve of 1.

G.-M. Wang et al. / Solid State Sciences 39 (2015) 1e54

our knowledge, it is thefirst time that 1-(3-aminopropyl)imidazoleligand behaves as an effective template in generating a zinco-phosphate with 16-ring channels. In addition, the Zn/P ratio inQDU-7 is 1: 1 and quite different from 3:4 that commonlyencountered in existing zinc phosphates with the same channelapertures. The framework density (FD, defined as the number ofpolyhedra per 1000 Å3) of 1 is 12.3, which is very close to that forthe extra-large 24-ring channels porous materials ND-1 (12.1).

3.3. Thermal stability

Thermogravimetric (TG) analysis of 1was performed in a flow ofN2 to investigate its thermal stability. As shown in Fig. 5, the first

weight loss was observed between 52 and 188 �C, corresponding tothe loss of the free water molecule in the structure (observed:1.58%; calcd: 1.65%). On further heating, a continuous three-stepweight loss of 24.45% was observed between 188 and 876 �C. Thisis in good agreement with a process involving the removal ofammonia and the organic ligands (calculated 1.65% for one NH3 and

Fig. 6. The solid-state fluorescent spectrum of 1 (lex ¼ 294 nm) at room temperature.

G.-M. Wang et al. / Solid State Sciences 39 (2015) 1e5 5

22.92% for two APIM molecules per formula unit). The residue isamorphous after the calcination and its phase is unidentified.

3.4. Fluorescent spectrum

The photoluminescence spectrum of 1 was measured in thesolid state at room temperature (Fig. 6). On excitation at 294 nm,the emission spectrum of the compound exhibits a main peak at486 nm and a shoulder at about 367 nm. The emission at 367 nmmay be caused by the presence of the imidazole moiety because asimilar emission band at about 374 nm has also been observed onexcitation at 265 nm for the APIM organic ligand (Fig. S3). Similarfluorescent emission band from imidazole moiety of differentorganic molecules is also observed in zinc phosphates ZnPO-CJ36and ZnPO-CJ57, both of which exhibit one sharp emission band atabout 362 nm when excited at 220 nm [24,45]. Therefore, thestrong emission at 486 nm can be assigned as ligand-to-metalcharge transfer. This hybrid compounds may find widespread ap-plications in blue-light emitting devices, since it is thermally stableand insoluble in common polar and non-polar solvents.

4. Conclusion

In summary, a novel inorganiceorganic hybrid open-frameworkzinc phosphate was solvothermally synthesized and structurallycharacterized. Its structure features unusual three-dimensionalmicroporous framework with intersecting 8-, 10-, and 16-ringchannels. The presence of large 16-ring windows in the structureis noteworthy and, to the best of our knowledge, is firstly observedin zinc phosphates with APIM acting as an effective template. Thestructure is also evidently unique for its remarkable lower frame-work density compared to those existing 3D open-frameworkzincophosphates with channel apertures of 16 rings. This com-pound shows intense photoluminescence at room temperature.Further investigations on utilizing multifunctional imidazole-typeligands to synthesize inorganiceorganic hybrid metal phosphatesor phosphites and explore their valuable properties are still underway in our laboratory.

Acknowledgments

This work was supported by the NNSF (20901043), A Project ofShandong Province Higher Educational Science and TechnologyProgram (J13LD18), Beijing National Laboratory for Molecular

Sciences (BNLMS), the Young Scientist Foundation of ShandongProvince (BS2009CL041), the Development Project of QingdaoScience and Technology (13-1-4-187-jch) and Taishan ScholarProgram.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.solidstatesciences.2014.11.007.

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