High-temperature, high-pressure hydrothermal synthesis, crystal structure and photoluminescent properties, of K₃[Gd_1−xTb_xGe₃O₈(OH)₂] (x = 0, 0.3, 0.1, 1)

Abstract

A family of 2D-layered lanthanide germanates K₃[Gd_1−xTb_xGe₃O₈(OH)₂] (x = 0, 0.3, 0.1, and 1), have been synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction, photoluminescence, IR spectra, and Energy-Dispersive Spectroscopy (EDS). The X-ray powder diffraction patterns of these compounds reveal that they are isostructural. The single-crystal X-ray diffraction analysis of K₃[GdGe₃O₈(OH)₂] reveals that it is a 2D-layered [LnGe₃O₈(OH)₂]_n³ⁿ⁻ anionic framework which is built up from GeO₄H/GeO₄ tetrahedra and GdO₆ octahedra by sharing vertex O atoms. K⁺ ions locate in the free void space to achieve the charge balance of the framework. A sample containing only Tb³⁺ emits mainly from one transition, ⁵D₄ → ⁷F₅ (552 nm). Mixed lanthanide samples, K₃[Gd_1−xTb_xGe₃O₈(OH)₂] (x = 0.3, and 0.1), have also been prepared and efficient Gd → Tb energy transfer has been observed.

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1. Introduction

In recent years, lanthanide-containing silicates and germanates have been attracting much interest because of their rich structural chemistry and potential applications in optical materials.^1,2 The use of silicates and germanates as host materials permits us to obtain phosphors with superior color richness and excellent chemical and thermal stability.^3–6 In the past decades, many lanthanide silicates that are built from SiO₄ tetrahedra and LnOn (n ≥ 6) polyhedral have been successfully synthesized under mild hydrothermal conditions in Teflon-lined stainless steel autoclaves in the temperature range of 100–240 °C. For instance, the first cerium silicate Na₄K₂Ce₂Si₁₆O₃₈·10H₂O (AV-5) was reported by Rocha and co-workers under mild hydrothermal conditions at 503 K in 2000.⁷ Since then, a series of lanthanide silicates AV-n (n = 9, 20, 22, 23) have been successfully prepared under mild hydrothermal conditions, their structures and fine-tuning luminescent properties derived from the multiple Ln³⁺ ions have also been investigated.^8–11 High-temperature and high-pressure hydrothermal synthetic method has also been used in exploratory syntheses of transition metals,^12,13 lanthanide elements,^14,15 and uranium-based^16–19 silicates because of the important roles of high pressure in the synthetic chemistry.²⁰

However, in contrast to the lanthanide silicates, much less work has been reported on the germanium analogues. Unlike the lanthanide silicates, most lanthanide germanates are dense phases and comprised of anionic groups^21–23 that are further connected via LnOn (n ≥ 6) polyhedra. In 2007, Pei-Lin Chen et al. reported a new Eu(III) germanate KEuGe₂O₆ with parallel zigzag chains of edge-sharing Eu–O polyhedra synthesized by both the flux-growth method and the high-temperature, high-pressure hydrothermal method.²⁴ Recently, Lii and co-workers reported a number of uranium germanates prepared by high-temperature and high-pressure hydrothermal synthetic method and investigated their crystal structure as well as luminescence properties.^25–28 Importantly, these compounds mentioned above can not be obtained under mild hydrothermal conditions.

In this work, we report a family of new lanthanide germanates K₃[Gd_1−xTb_xGe₃O₈(OH)₂] (x = 0, 0.3, 0.1, and 1), which are synthesized by high-temperature and high-pressure hydrothermal synthetic method at 400 °C and 100 MPa. Their structures are closely related to K₃LnSi₃O₈ (OH)₂ (Ln = Y³⁺, Eu³⁺, Tb³⁺, Er³⁺; denoted as AV-22)¹⁰ that were prepared under the mild hydrothermal conditions at 230 °C for 7 days. The luminescent property and energy transfer from Gd³⁺ to Tb³⁺ in K₃[Gd_1−xTb_xGe₃O₈(OH)₂] (x = 0.3, and 0.1) have been studied as well.

2. Experimental section

2.1. Materials and synthesis

High-temperature, high-pressure hydrothermal synthesis was carried out under autogenous pressure in silver tube contained in a hydrothermal research system (Model HR-1B-2, LECO Tem-Pres), where pressure was provided by water. Typically, a reaction mixture of 0.246 g of KOH (Beijing chemical plant), 0.132 g of GeO₂ (Alfa Aesar, 99.99%), 0.0788 g of Gd(NO₃)·6H₂O (Alfa Aesar, 99.995%) or 0.079 g of terbium nitrate hexahydrate (Alfa Aesar, 99.995%), and 0.1 ml of deionized water (molar ratio K : Gd : Ge : H₂O = 7 : 1 : 0.14 : 4.3) in a 5.0 cm long silver tube (inside diameter = 4.90 mm) was heated at 400 °C for 5 h. The pressure was initially increased to 120 MPa and maintained until the temperature was increased to 400 °C. Then the pressure was maintained at 100 MPa. After reaction, the autoclave was then fast cooled to room temperature by removing the autoclave from the furnace. The resulting colorless crystals were filtered, washed with deionized water, and dried at 353 K. The Gd³⁺/Tb³⁺ mixed samples were prepared by introducing the desired Gd³⁺ and Tb³⁺ contents in the initial mixture.

2.2. Characterizations

Powder X-ray diffraction (XRD) data were collected using a Rigaku D/Max 2550 V/PC X-ray diffractometer with graphite-monochromated Cu Kα radiation (λ = 0.15418 nm) at 50 kV and 200 mA at room temperature. IR spectra were recorded on a Nicolet Impact 410 FT-IR spectrometer using the KBr pellet technique. Energy-dispersive spectroscopy (EDS) analysis was carried out using an EDS system with a window attached to a JEOL JSM-6700F scanning electron microscope. The photoluminescence (PL) spectra were obtained on a FlouroMax-4 spectrophotometer with Xe 900 (150 W xenon arc lamp) as the light source. To eliminate the second-order emission from the source radiation, a cut-off filter was used during the measurement. The PL decay curves were measured on an FLS920 spectrophotometer (Edinburgh Instruments) with a μF920H flash lamp as the light source. Slit widths were 0.20 (excitation) and 0.20 (emission) nm. All spectra were recorded at room temperature.

2.3. Single-crystal structure determination

Suitable single crystals of K₃[GdGe₃O₈(OH)₂], and K₃[TbGe₃O₈(OH)₂] with dimensions of 0.12 × 0.06 × 0.04 mm and 0.16 × 0.08 × 0.05 mm, respectively, were selected for single-crystal X-ray diffraction analysis. Intensity data collection were collected on a Bruker SMART APEX 2 micro-focused diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 nm) at 50 kV and 0.6 mA at a temperature of 296 K. Data processing was accomplished with the APEX 2 processing program. The structures were solved by direct methods and refined by full-matrix least-squares techniques with the SHELXTL crystallographic software package.²⁹ All heaviest atoms, K, Gd, and Ge for K₃[GdGe₃O₈(OH)₂], K, Tb, and Ge for K₃[TbGe₃O₈(OH)₂], were unambiguously located in the Fourier maps, and then O atoms were found in the subsequent difference Fourier maps. The H atoms attached to the GeO₄ tetrahedron were placed geometrically. All atoms were refined with anisotropic displacement parameters, except for H atoms, which were refined with an isotropic thermal displacement parameter (U_iso) fixed at 1.5 U_eq of the parent O atoms. The final cycles of least-squares refinement including atomic coordinates and anisotropic thermal parameters for all atoms converged at R₁ = 0.0174, wR₂ = 0.0428, and S = 1.104 for K₃[GdGe₃O₈(OH)₂]; R₁ = 0.0195, wR₂ = 0.0444, and S = 1.064 for K₃[TbGe₃O₈(OH)₂]; A summary of the crystallographic data is presented in Table 1. The selected bond lengths [Å] and angles [deg] for K₃[GdGe₃O₈(OH)₂], and K₃[TbGe₃O₈(OH)₂] are presented in Table S1 and S2,† atomic coordinates and equivalent isotropic displacement parameters are presented in Table S3 and S4 (ESI†).

Table 1 Crystal data and structure refinement for K₃[LnGe₃O₈(OH)₂](Ln = Gd, Tb)

Empirical formula	K3GdGe3O8(OH)2	K3Tb Ge3O8(OH)2
Formula weight	654.34	656.01
Temperature (K)	293(2)	293(2)
Wavelength (Å)	0.71073	0.71073
Crystal system, space group	Orthorhombic, Pnma	Orthorhombic, Pnma
Unit cell dimensions	a = 13.6880(7) Å, α = 90 °C	a = 13.6552(15) Å, α = 90 °C
	b = 13.6891(7) Å, β = 90 °C	b = 13.6541(15) Å, β = 90 °C
	c = 6.0821(3) Å, γ = 90 °C	c = 6.0595(6) Å, γ = 90 °C
Volume (Å^3)	1139.64(10)	1129.8(2)
Z, calculated density (Mg m^−3)	4, 3.814	4, 3.857
Crystal size (mm^3)	0.12 × 0.06 × 0.04	0.16 × 0.08 × 0.05
Absorption coefficient (mm^−1)	14.712	15.230
F(000)	1196	1200
Theta range for data collection	2.98–26.35°	2.98–26.35
Completeness to theta = 26.35	100.0%	100.0%
Max. and min. transmission	0.555 and 0.498	0.737 and 0.701
Goodness-of-fit on F^2	1.104	1.064
Final R indices [I > 2sigma(I)]	R1 = 0.0162, wR2 = 0.0423	R1 = 0.0186, wR2 = 0.0439
R indices (all data)	R1 = 0.0174, wR2 = 0.0428	R1 = 0.0195, wR2 = 0.0444
Largest diff. peak and hole	1.069 and −1.063 e Å^−3	0.945 and −1.429 e Å^−3

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3. Results and discussion

3.1. Characterization

Unlike the lanthanide silicates hydrothermally prepared by using an excess amount of water as the solvent, single crystals of these lanthanides germanates could be only obtained in a concentrated gel system with a low H₂O/GeO₂ molar ratio. Interestingly, single crystals of Ln₂Ge₂O₇ were obtained when the H₂O/GeO₂ molar ratio exceeds the ratio used in the experiment.

The X-ray powder diffraction patterns of the four compounds are shown in Fig. 1, which are consistent with the simulated one on the basis of single-crystal structural analysis, indicating that they are isostructural. The energy-dispersive spectroscopy (EDS) analysis results of the four compounds are displayed in Table S5 and Fig S1 in the ESI,† which are in agreement with the values given by the theoretically calculated values.

Fig. 1 Simulated powder XRD pattern of K₃[GdGe₃O₈(OH)₂] and experimental XRD patterns of the as synthesized four compounds.

3.2. Description of the structure

The four compounds are isostructural, therefore, only the structure of K₃[GdGe₃O₈(OH)₂] is discussed. The structure of K₃[GdGe₃O₈(OH)₂] crystallizes in the Pnma space group (no. 62) with a = 13.6880(7) Å, b = 13.6891(7) Å, and c = 6.0821(3) Å, that is analogous to some lanthanide silicates K₃LnSi₃O₈(OH)₂ (Ln = Y³⁺, Eu³⁺, Tb³⁺, Er³⁺; denoted as AV-22). Fig. 2 shows the asymmetric units of K₃[GdGe₃O₈(OH)₂]. The structure is constructed from the following building units: 2 K⁺ cations, 1 GdO₆ octahedra, 1 GeO₄ tetrahedra, and 1 GeO₄H tetrahedra. GdO₆ is discrete and is distorted with the Gd–O bond lengths in the range from 2.226(4) to 2.332(3) Å.

Fig. 2 Thermal ellipsoid plot (50% probability) of the asymmetric unit of K₃[GdGe₃O₈(OH)₂].

The existence of Ge–O bonds and an –OH group is confirmed by IR analysis (Fig. S2†). The peaks at 810, 781, and 760 cm⁻¹ can be assigned to the asymmetric stretching vibrations of the Ge–O bonds, and the bands at about 3000 cm⁻¹ correspond to the –OH groups. On the basis of the maximum cation–anion distance by Donnay and Allmann,³⁰ a limit of 3.35 Å was set for K–O interactions, which gives the following coordination numbers: K(1), 5-coordinate [K(1)–O, 2.808(3)–3.063(2) Å]; K(2), 9-coordinate [K(2)–O, 2.694(3)–3.281(3) Å].

The crystal structure analysis of K₃[GdGe₃O₈(OH)₂] reveals that it is a 2D-layered [GdGe₃O₈(OH)₂]_n³ⁿ⁻ anionic framework, which is built up from GeO₄H/GeO₄ tetrahedra and GdO₆ octahedra by sharing vertex O atoms. As can be seen in Fig. 3a, The GeO₄ tetrahedra, GeO₄H tetrahedra and GdO₆ octahedra are connected together by sharing corners to form a single layer in the (010) plane with the composition GdGe₃O₈(OH)₂ containing 3, and 7-rings. Adjacent layers are interconnected through the O–H⋯O hydrogen bonds [O(5)–H(1), 0.820 Å; H(1)⋯O(2), 1.860 Å; O(6)⋯O(3), 2.671 Å] between the neighboring Ge–OH groups, and stacked in the sequence of [ABAB…] in the (001) plane (Fig. 3b). K(1) ions are arranged in the interlayer space, while K(2) ions are located in the seven-ring windows.

Fig. 3 (a) Single layer with three and seven rings viewed along the [010] direction. (b) Polyhedral view of K₃[GdGe₃O₈(OH)₂] along the [001] direction. Colour code: Gd, purple; Ge, bright green; K, green; O, red; H, orange.

3.3. Photoluminescence property

Fig. 4 shows the room temperature (RT) excitation spectra of K₃[Gd_1-xTb_xGe₃O₈ (OH)₂] for x = 0.1, 0.3 and 1 (pure Tb³⁺ sample), monitored within the ⁵D₄ → ⁷F₅ transition (542 nm). In the excitation spectrum of K₃[TbGe₃O₈ (OH)₂], the sharp lines between 300 and 500 nm are assigned to ⁷F₆ → ⁵D_0,1, ⁷F₆ → ⁵G_2–6, ⁵L₁₀, and ⁷F₆ → ⁵D_3,4 transitions of Tb³⁺. The broad band between 250 and 300 nm is ascribed to the spin-forbidden (high-spin) interconfigurational ⁴f₈ → ⁴f₇⁵d¹ transition.^31,32 The excitation spectra monitored at the main ⁵D₄ → ⁷F₅ transition (542 nm) of K₃[Gd_1−xTb_xGe₃O₈(OH)₂] for x = 0.1, 0.3 (Fig. 4) display the same Tb³⁺ lines. In addition, the excitation lines at 274 nm and 312 nm due to transitions from ⁸S_7/2 → ⁶I_9/2, and ⁸S_7/2 → ⁶P_7/2 of Gd³⁺ ion can be observed. This is clear evidence for Gd³⁺-to-Tb³⁺ energy transfer. Furthermore, the Gd³⁺-to-Tb³⁺ energy transfer is also detected in the emission of the K₃[Gd_1−xTb_xGe₃O₈ (OH)₂] (x = 0.3, and 0.1) samples (Fig. 5).^10,33–35

Fig. 4 RT excitation spectra of K₃[Gd_1−xTb_xGe₃O₈(OH)₂] for x = 0.1, 0.3 and 1 (λ_m = 542 nm).

Fig. 5 RT emission spectrum of K₃[TbGe₃O₈(OH)₂] (λ_ex = 377), and K₃[Gd_1−xTb_xGe₃O₈(OH)₂] for x = 0.1, and 0.3 (λ_ex = 274).

Fig. 5 shows the RT emission spectrum of K₃[Gd_1−xTb_xGe₃O₈ (OH)₂] for x = 0.1, 0.3 and 1. The emission spectrum of K₃[TbGe₃O₈ (OH)₂] (pure Tb³⁺ sample) excited at 377 nm (Tb^{3+ 5}D₃) displays a series of sharp lines from 475 to 675 nm, which are associated with the ⁵D₄ → ⁷F_3–6 transitions of Tb³⁺ with the strongest at about 542 and 552 nm (⁵D₄ → ⁷F₅). Luminescence from the higher (e.g., ⁵D₃) excited states is not detected, even for the samples with the lowest Tb³⁺ content, indicating very efficient nonradiative relaxation to the ⁵D₄ level. The emission spectra of K₃[Gd_1−xTb_xGe₃O₈(OH)₂] for x = 0.1, and 0.3 excited at the Gd^{3+ 6}I_9/2 level (274 nm) show the typical Tb³⁺ lines (present in the spectrum of the pure Tb³⁺ sample, Fig. 5). This further supports the above mentioned energy transfer between Gd³⁺ and Tb³⁺.^36,37

The RT luminescence decay curves detected at the ⁵D₄ → ⁷F₅ transition of K₃[Gd_1−xTb_xGe₃O₈(OH)₂] (x = 0.1, 0.3) are shown in Fig. 6. Both of them can be well fitted by the single exponential equation: I(t) = I₀ + A exp(−t/τ), where I and I₀ is the luminescence intensity, A the constant, t the time, τ the decay time, yielding the lifetime values of τ = 2.255 and 2.909 ms for x = 0.1 and x = 0.3, respectively. The results confirm the presence of one local Tn³⁺ environment, which is consistent with the asymmetric Tb³⁺ location according to structural analysis.^14,36

Fig. 6 RT fluorescence decay curves detected at 542 nm (λ_ex = 276 nm) for K₃[Gd_1−xTb_xGe₃O₈(OH)₂](x = 0.1, 0.3). The solid line represents the best fit to the data.

4. Conclusion

Four isostructural 2D-layered lanthanide germanates K₃[Gd_1−xTb_xGe₃O₈(OH)₂](x = 0, 0.3, 0.1, 1), have been prepared by a high-temperature, high-pressure hydrothermal method at 400 °C for 5 h. The crystal structure analyses reveal that they are built up from GeO₄H/GeO₄ tetrahedra and LnO₆ octahedra by sharing vertex O atoms, giving rise to a 2D-layered [LnGe₃O₈(OH)₂]_n³ⁿ⁻ anionic framework. The energy transfer from Gd³⁺-to-Tb³⁺ is observed in the excitation spectra of K₃[Gd_1−xTb_xGe₃O₈(OH)₂] (x = 0.1, 0.3) and confirmed by their emission spectra excited at the Gd^{3+ 6}I_9/2 level (276 nm). The successful high-temperature, high-pressure hydrothermal synthesis of K₃[LnGe₃O₈(OH)₂] will provide a new way to preparing many more novel lanthanide germanates.

Acknowledgements

This work was supported by the National Sciences Foundation of China (no. 21271082, 21301066 and 21371068).

Notes and references

1. C. Feldmann, T. Jüstel, C. R. Rond and P. J. Schmidt, Adv. Funct. Mater., 2003, 13, 511.
2. B. M. van der Ende, L. Aartsa and A. Meijerink, Phys. Chem. Chem. Phys., 2009, 11, 11081.
3. A. Dobrowolska and E. Zych, J. Solid State Chem., 2011, 184, 1707.
4. K. V. Ivanovskikh, A. Meijerink, F. Piccinelli, A. Speghini, E. I. Zinin, C. Ronda and M. Bettinelli, J. Lumin., 2010, 130, 603.
5. P. L. Chen, P. Y. Chiang, H. C. Yeh, B. C. Chang and K. H. Lii, Dalton Trans., 2008, 1721.
6. V. R. Bandi, Y. T. Nien and I. G. Chen, J. Appl. Phys., 2010, 108, 023111.
7. J. Rocha, P. Ferreira, L. D. Carlos and A. Ferreira, Angew. Chem., Int. Ed., 2000, 39, 2174.
8. D. Ananias, A. Ferreira, J. Rocha, P. Ferreira, J. P. Rainho, C. Morais and L. D. Carlos, J. Am. Chem. Soc., 2001, 123, 5735.
9. A. Ferreira, D. Ananias, L. D. Carlos, C. M. Morais and J. Rocha, J. Am. Chem. Soc., 2003, 125, 14573.
10. D. Ananias, M. Kostova, F. A. Almeida Paz, A. Ferreira, L. D. Carlos, J. Klinowski and J. Rocha, J. Am. Chem. Soc., 2004, 126, 10410.
11. M. H. Kostova, D. Ananias, F. A. Almeida Paz, A. Ferreira, J. Rocha and L. D. Carlos, J. Phys. Chem. B, 2007, 111, 3576.
12. C. Y. Li, C. Y. Hsieh, H. M. Lin, H. M. Kao and K. H. Lii, Inorg. Chem., 2002, 41, 4206.
13. L. I. Hung, S. Wang, S. P. Szu, C. Y. Hsieh, H. M. Kao and K. H. Lii, Chem. Mater., 2004, 16, 1660.
14. M. Y. Huang, Y. H. Chen, B. C. Chang and K. H. Lii, Chem. Mater., 2005, 17, 5743.
15. X. G. Zhao, J. Y. Li, P. Chen, Y. Li, Q. X. Chu, X. Y. Liu, J. H. Yu and R. R. Xu, Inorg. Chem., 2010, 49, 9833.
16. C. S. Chen, H. M. Kao and K. H. Lii, Inorg. Chem., 2005, 44, 935.
17. C. S. Chen, R. K. Chiang, H. M. Kao and K. H. Lii, Inorg. Chem., 2005, 44, 3914.
18. C. S. Chen, S. F. Lee and K. H. Lii, J. Am. Chem. Soc., 2005, 127, 12208.
19. H. K. Liu and K. H. Lii, Inorg. Chem., 2011, 50, 5870.
20. G. Demazeau, J. Phys.: Condens. Matter, 2002, 14, 11031.
21. X. G. Zhao, T. T. Yan, K. Wang, Y. Yan, B. Zou, J. H. Y and R. R. Xu, Eur. J. Inorg. Chem., 2012, 2527.
22. K. Kato, M. Sekita and S. Kimura, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1979, 35, 2201.
23. P. L. Chen, P. Y. Chiang, H. C. Yeh, B. C. Chang and K. H. Lii, Dalton Trans., 2008, 13, 1721.
24. P. L. Chen, P. Y. Chiang, H. C. Yeh, B. C. Chang and K. H. Lii, Dalton Trans., 2008, 51, 1721.
25. C. H. Lin, C. S. Chen, A. A. Shiryaev, Y. V. Zubavichus and K. H. Lii, Inorg. Chem., 2008, 47, 4445.
26. C. H. Lin, R. K. Chiang and K. H. Lii, J. Am. Chem. Soc., 2009, 131, 2068.
27. Q. B. Nguyen and K. H. Lii, Inorg. Chem., 2011, 50, 9936.
28. Q. B. Nguyen, H. K. Liu, W. J. Chang and K. H. Lii, Inorg. Chem., 2011, 50, 4241.
29. G. M. Sheldrick, SHELXTL Programs, Version 5.1, Bruker AXS GmbH, Karlsruhe, Germany, 1998.
30. G. Donnay and R. Allmann, Am. Mineral., 1970, 55, 1003.
31. R. C. Evans, D. Ananias, A. Douglas, P. Douglas, L. D. Carlos and J. Rocha, J. Phys. Chem. C, 2008, 112, 260.
32. D. J. Hou, H. B. Liang, M. B. Xie, X. M. Ding, J. P. Zhong, Q. Su, Y. Tao, Y. Huang and Z. H. Gao, Opt. Express., 2011, 19, 11071.
33. D. M. Fernandes, L. C. Silva, R. A. S. Ferreira, S. S. Balula, L. D. Carlos, B. D. Castro and C. Freire, RSC Adv., 2013, 3, 16697.
34. D. K. Cao, Y. H. Zhang, J. Huang, B. Liu and L. M. Zheng, RSC Adv., 2012, 2, 6680.
35. C. F. Guo, H. Jing and T. Li, RSC Adv., 2012, 2, 2119.
36. W. R. Liu, C. H. Huang, C. W. Yeh, Y. C. Chiu, Y. T. Yeh and R. S. Liu, RSC Adv., 2013, 3, 9023.
37. L. L. Han, L. Zhao, J. Zhang, Y. Z. Wang, L. N. Guo and Y. H. Wang, RSC Adv., 2013, 3, 21824.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 995425 and 995426. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra03048f

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