Flux synthesis of (3,4)-connected zinc phosphites with different framework topologies

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Abstract

Two three-dimensional open-framework zinc phosphites, H2aem·Zn3(HPO3)4·0.5H2O (1) and H2apm·Zn3(HPO3)4 (2), have been synthesized by a phosphorous acid flux method, where aem=4-(2-aminoethyl)morpholine and apm=4-(3-aminopropyl)morpholine. Compound 1 crystallizes in the monoclinic system, P21/c, a=9.5852(7) Å, b=20.3941(8) Å, c=10.5339(8) Å, β=94.125(9)°, V=2053.8(2) Å3, Z=4, R1=0.0319, wR2=0.0628. Compound 2 crystallizes in the monoclinic system, P21/n, a=8.589(2) Å, b=14.020(3) Å, c=16.606(3) Å, β=97.190(8)°, V=1983.9(7) Å3, Z=4, R1=0.0692, wR2=0.1479. Both compounds are based on (3,4)-connected networks with 8- and 12-ring channels, which are constructed from Zn3(HPO3)4 clusters as the same secondary building units. These inorganic clusters are spatially organized by different structure-directing agents into different three-dimensional frameworks.

Graphical abstract

Two three-dimensional open-framework zinc phosphites have been synthesized by a phosphorous acid flux method. The two compounds are constructed from Zn3(HPO3)4 clusters and have noz and pcu topologies, respectively.

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Introduction

Crystalline microporous materials have found widespread applications in catalysis, separation, gas storage and ion-exchange processes [1], [2], [3], [4], [5]. Of fundamental importance is to synthesize new zeolitic materials with novel topologies and pore structures. During the past decades, a great deal of attention has been paid to the use of tetrahedral anions, such as SiO44–, GeO44− and PO43− units, to construct open-framework structures [6], [7], [8]. The presence of dangling units in these building blocks such as the hydroxyl group in HPO42− anion, enable the formation of open structures with large pores and low framework density. Notable examples include an open-framework gallium phosphate cloverite and an aluminophosphate JDF-20 with extra-large 20-ring channels [9], [10]. As a structural analog of HPO42−, the HPO32− unit is of current interest in the construction of open-framework structures. A number of main group and transition metal elements, including Be, Al, Ga, In, V, Cr, Mn, Fe, Co, Ni, and Zn, have been successfully incorporated into phosphite frameworks [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32].

Open-framework metal phosphites are generally synthesized with water as the main solvent under hydrothermal conditions. The formation of these open structures is kinetically controlled and is sensitive to the reaction conditions, such as pH, temperature, and solvent. Recently, the H3PO3 flux method has been developed to prepare open-framework metal phosphites in an attempt to eliminate the influence of the solvent and to decrease the system pressure [33]. Two layered zinc phosphites were prepared under H3PO3 flux conditions in the presence of bulky diamines as the structure-directing agents [33], [34]. The use of small amines as the structure-directing agents may favor the formation of three-dimensional structures [35]. We report here the H3PO3 flux synthesis of two new three-dimensional open-framework zinc phosphites, H2aem·Zn3(HPO3)4·0.5H2O (1) and H2apm·Zn3(HPO3)4 (2), where aem=4-(2-aminoethyl)morpholine and apm=4-(3-aminopropyl)morpholine. The two compounds have (3,4)-connected frameworks with 8- and 12-ring channels. Although they are constructed from the same Zn3(HPO3)4 clusters, they adopt different framework topologies due to the spatial organization of these building units by different templating agents.

Section snippets

Synthesis

To prepare compound 1, a mixture of aem (0.325 g), Zn(OAc)2·2H2O (0.548 g), and H3PO3 (0.308 g) in a molar ratio of 2:2:3 was sealed in a Teflon-lined steel autoclave and heated at 130 °C for 3 days. After cooling to room temperature, the resulting colorless product was recovered by filtration, washed with distilled water and dried in air (68.8% yield based on zinc). Colorless crystals of compound 2 were obtained in a yield of 72.8% (based on zinc) under the same reaction conditions as used for 1,

Characterization

The powder XRD patterns of as-synthesized compounds 1 and 2 and the simulated patterns based on the single crystal structures were compared (Fig. 1). The diffraction peaks on simulated and experimental patterns corresponded well in position, indicating the phase purity of the as-synthesized compounds 1 and 2. CHN analysis results confirmed their stoichiometry (Anal. Found for 1: C, 10.66; H, 2.80; N, 4.15%. Calculated: C, 10.96; H, 3.22; N, 4.26%; Anal. Found for 2: C, 12.21; H, 2.87; N, 4.14%.

Conclusions

Two three-dimensional open-framework zinc phosphites, H2aem·Zn3(HPO3)4·0.5H2O (1) and H2apm·Zn3(HPO3)4 (2), were obtained as good quality single crystals under flux synthetic conditions. Structural analyses indicate that both compounds are constructed from the same Zn3(HPO3)4 clusters. For compound 1, each Zn3(HPO3)4 cluster is connected to five adjacent clusters, producing an unusual framework with noz topology. For compound 2, each Zn3(HPO3)4 cluster is linked to six adjacent clusters,

Acknowledgments

This work was supported by the NSF of China (Grant 20801037) and the A.v. Humboldt Foundation. The authors are grateful to M. Gerlach for performing TGA measurements, and G. Geiseler for collecting single-crystal X-ray diffraction data.

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