Synthesis, Structure, and Characterization of Keggin-Type Germanate

Abstract

A novel Keggin-type germanate, (NH₄)₉[Ge₇O₁₄F₃]₃·1.75H₂O (I), is hydrothermally synthesized and structurally characterized by X-ray single-crystal diffraction, elemental analysis, XRD, and TG. (I) is tetragonal system with space group I4/mmm and unit cell: 𝑎=28.280 (4) Å, 𝑐=24.881(5) Å, 𝑉=19899(6) ų, 𝐷𝑐=1.632g/cm³, 𝜇(MoK𝛼)=6.309mm⁻¹, 𝑍=2 , 𝑅₁=0.1141 for 2576 reflections with Fo>2(Fo). Ge₇O₁₄F₃ entry is defined as the cluster including one octahedron, two edge-sharing triganol bipyramids, and four tetrahedra. Every Ge₇O₁₄F₃ entry links adjacent four Ge₇O₁₄F₃ entries by four tetrahedra. Twelve Ge₇O₁₄F₃ entries construct a cage with all octahedra of Ge₇O₁₄F₃ pointing inside, which can be simplified into Keggin-type cage through Ge₇O₁₄F₃ as the node. The solvent experiment proves that (I) is stable in the water and sensitive to base and acid. The result of XRD shows that the structural water of (I) is easily lost to drop the crystalline. The thermal study indicates that the Keggin-type cage of (I) begins to partly collapse at 200°C and finally changes into GeO₂.

1. Introduction

Over the two decades, more efforts have been focused on synthesis of germanates because germanates could not only form the zeolites or molecular sieves [1, 2] but also achieve more openness than silicates owing to smaller rings and lower framework density based on the flexible Ge–O–Ge of ~130° [3–5]. It is hard for germanium to condense itself into zeolite due to the limitation of synthesis method. Only several examples have been found as BEC, ASV, and UOZ [6–8], and more instances can be accessible by germanium and the other tetrahedral elements—B, Al, Ga, Si, and so on [5, 9–17]. As a result of the large atomic radii of germanium conforms higher five- and six-coordination rather than four-coordination, the clusters comprised of the mixed coordinations give rise to large porous and extra porous frameworks [18–23]. The mixed 4-, 5- and 6-coordination Ge₇O₁₄F₃ cluster is discussed in detail as robust building unit of 2D and 3D nets [24–27]. In recently reported tubular germanate [22], [(C₅N₂H₁₄)₄(C₅N₂H₁₃)(H₂O)₄] [Ge₇O₁₂O_4/2(OH)F₂] [Ge₇O₁₂O_5/2(OH)F]₂[GeO_2/2(OH)₂], twelve Ge₇O₁₄F₃ clusters form a cage which mimics the Keggin structure with respect to Ge₇O₁₄F₃ cluster as the node. Interestingly, this Keggin-type germanate cage possesses 5.3 Å aperture and 8.3 Å cavity, which is bigger than Keggin-type POM-PMo₁₂O₄₀³⁻. In this work, we have aimed to crystallize Keggin-type germanate, (NH₄)₉[Ge₇O₁₄F₃]₃·1.75H₂O—CCUT-8 (denoted as the Changchun University of Technology) and furthermore studied the solvent and thermal stabilities of Keggin-type cage.

2. Experimental Section

2.1. Materials and Instrument

All of the reagents were of analytical grade and used as received. The deionized water was used in all the experiments.

The infrared (IR) spectra were recorded within the 400~4000 cm⁻¹ region on a BRUKER Vertex 70 FTIR spectrometer using KBr pellets. The elemental analyses were performed on a PerkinElmer 2400 element analyzer. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX PC2200 diffractometer for Cu Kα radiation (𝜆=1.5406 Å), with a scan speed of 5°/min⁻¹. The thermal gravimetric analyses (TG) were performed on Pyris Diamond TG/DTA instrument used in an atmospheric environment with a heating rate of 10°C/min.

2.2. Synthesis

The colorless crystals of (NH₄)₉[Ge₇O₁₄F₃]₃·1.75H₂O were obtained under solvothermal condition. GeO₂ (0.25 g) was firstly dispersed in H₂O (1 mL). Then pyridine (4 mL), 2-methylpiperazine (0.95 g), and hydrofluric acid (0.5 mL) were successively added under vigorous stirring. The clear solution with molar ratio of 1 GeO₂ : 4 (2-methylpiperazine) : 50 pyridine : 58.3 H₂O : 4.8 HF was stirring for 4 hours and then it was transferred into 15 mL stainless steel autoclave and heated at 438 K for 14 days. After naturally cooled to room temperature, colorless product was collected as a single phase. All the crystals were washed by water and alcohol. The resultant crystals were dried naturally before single-crystal X-ray diffraction. H₁₅₈F₃₆Ge₈₄N₃₆O₁₇₅ (10245.60): H 1.54, N 4.92; found H 1.74, N 5.32.

2.3. Spectra of FTIR

The peaks at 3254 and 1590 cm⁻¹ were attributed to the stretching and bending vibrations of NH₄⁺; the peaks at 3446 and 1454 cm⁻¹ were assigned to stretching and bending vibrations of H₂O; the peaks at 860, 831, 578, 460 cm⁻¹ were due to vibrations of Ge–O or Ge–F.

2.4. Thermal Stability

The results of thermal gravimetric analyses showed that the total weight loss was 15.3% from room temperature to 600°C, corresponding to structural water and decomposition of ammonia and fluoride (calculated value: 15.6%), respectively, (Figure 1). CCUT-8 experienced the two weight losses from room temperature to 600°C and gave rise to final residues (GeO₂). The gradual part from 100°C to 250°C was assigned to structural water, and the sharp part from 300°C to 350°C was attributed to the decomposition of ammonia and fluoride.

Figure 1 

TG analyses of I.

The crystalline and thermal stability were investigated through the XRD (Figure 2). The broad and weak peak of (I) in XRD pattern of room temperature indicated that the crystalline of (I) decreased owing to fast loss of the structure water. Furthermore, we made an attempt to fuse the separated Keggin-type cage of (I) by condensing the oxygen of adjacent cage to build the solid structure. However, the XRD of the samples treated at 200°C and 300°C for 3 hrs in the air showed that Keggin-type cage of (I) began to partly collapse at 200°C and basically changed into the GeO₂ at 300°C, which consisted with the results of TG.

Figure 2 

The XRD patterns of I. (a) simulated; (b) experimental; (c) treated at 200°C for 3 hrs; (d) treated at 300°C for 3 hrs.

2.5. X-Ray Single-Crystal Diffraction

X-ray diffraction data were collected at 293 K from a suitable single crystal sealed in glass capillary on a Rigaku R-AXIS RAPID diffractometer equipped with graphite-monochromatized Mo Kα radiation (𝜆=0.71073 Å). The structure was solved by the direct-method routine of SHELXS-97 and refined by full-matrix least-squares on 𝐹² using SHELXL-97. The heavy atoms (Ge) were firstly located by the direct method, and then the light atoms (N, O, and F) were indentified from the different Fourier maps. All nonhydrogen atoms were refined anisotropically. CCDC 903319 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

A summary of experiment data and refinement parameters of I is given in Table 1. The atomic coordinates and selected bond distances and angles of I are given in Tables 2 and 3.

Table 1 

The crystallographic data and structural refinement for I.

Table 2 

Atomic coordinates (×10⁴) and equivalent isotropic displacement parameters (Ų×10³) for I.

Table 3 

Selected bond lengths (Å) and bond angles (°) for I.

2.6. The Solvent Experiments

10 mg of I was added to 10 mL water, 10 mL base (1 M NaOH), and 10 mL acid (1 M HCl) overnight, respectively. The experimental results show that I was unsoluble in the water and soluble in the base and acid.

3. Results and Discussion

I may be designated to the Keggin-type cage of which the MO₆ octahedron in POM-PMo₁₂O₄₀³⁻ is replaced with Ge₇O₁₄F₃ cluster. The mixed-coordination Ge₇O₁₄F₃ cluster has been considerably studied [24–27], which consists of one octahedron, two edge-sharing trigonal bipyramids, and four tetrahedra (Figure 3). The distances of Ge–O (1.703(9) Å~2.168(13) Å) and Ge–F (1.736(14) Å~1.797(15) Å) and bond angles of O–Ge–O(88.6(6)°~178.0(6)°) and Ge–O–Ge(87.5(7)°~ 136.3(3)°) are consistent to the reported results [24].

Figure 3 

The Ge7O14F3 cluster containing one octahedron, two edge-sharing trigonal bipyramids, and four tetrahedra.

In Keggin-type PMo₁₂O₄₀³⁻ anion cage, twelve MoO₆ octahedra can be falled into 4 entities in which three edge-sharing MoO₆ octahedra are integrated by an oxygen in PO₄ tetrahedron. The Keggin cage is formed by vertex-sharingly connecting 4 entities, showing the 𝑇_𝑑 molecular symmetry. In cage of I, twelve Ge₇O₁₄F₃ clusters surround the cavity of 8.3 Å with two kinds of apertures defined as 6 MR and 12 MR. The larger 12 MR aperture gives rise to 5.3 Å pore. Like Keggin PMo₁₂O₄₀³⁻ anion, when three Ge₇O₁₄F₃ clusters by which 6 MR aperture is surrounded are looked as an entity, I mimics the 𝑇_𝑑 Keggin structure (Figure 4). In the case of JLG-5 in which the tetrahedral GeO₂ units link the adjacent Keggin-type cage into the infinite structure, I becomes the 0D separated cage because the excessive HF impedes the formation of GeO₂ unit.

(a)

(b)

Figure 4 

(a) Keggin-type I with the cavity bearing two kinds of apertures defined as 6 MR and 12 MR; (b) the simplified I with Ge7O14F3 cluster as node.

4. Conclusions

Keggin-type I has been solvothermally obtained. The structural determination shows that in cage of I, twelve Ge₇O₁₄F₃ clusters surround the cavity of 8.3 Å with two kinds of apertures defined as 6 MR and 12 MR. The thermal studies show that Keggin-type cage of I begins to partly collapse from 200°C to finally change into GeO₂.

Acknowledgment

This work was supported by the Scientific Research Foundation for the Returned Overseas Team, Chinese Education Ministry.