Supermolecular assembly of polyoxoanion and metal–organic cationic units towards a model for core–shell nanostructures

Abstract

Two novel core–shell-like molecular composites, [Ni(dap)₃]₄[HV^IV₁₂V^V₆O₄₂(PO₄)] (1) and [Ni(dap)₂]₄[HV^IV₁₂V^V₆O₄₂(PO₄)]·4H₂O (2) (dap = 1,2-diaminopropane), were obtained from the supermolecular assembly of polyoxoanions and metal–organic cationic units. These two inorganic–organic hybrids were prepared under hydrothermal conditions and characterized by elemental analysis, IR spectroscopy, PXRD and single crystal X-ray diffraction analysis. In the well defined molecular core–shell structure of composite 1, the sphere-like anion [PV₁₈O₄₆]⁹⁻ was surrounded by a shell composed of eight windmill-type [Ni(dap)₃]²⁺ cationic units. In 2, ten sheet-like [Ni(dap)₂]²⁺ metal–organic groups surrounded the sphere-like anion [PV₁₈O₄₆]⁹⁻, which was achieved by the easy alteration of the chemical composition of the core–shell structure by simply adjusting the feeding amount of dap. The resulting core–shell-like molecular composites represent a promising structural model toward core–shell nanostructures and exhibit electrocatalytic activity for the reduction of H₂O₂ and oxidation of NO₂⁻.

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Introduction

In the previous decades, inorganic–organic hybrids with great contribution to material science have attracted much attention owing to their special and enhanced physical and chemical properties achieved through the interaction between the inorganic and organic components.¹ Components with desirable physical and chemical properties can be combined for constructing the composite inorganic–organic hybrid molecular material. According to the interactions between the inorganic and organic components, the inorganic–organic hybrids can be simply classified into two groups, wherein the weak interactions (e.g. electrostatic and hydrogen bonding) and stronger interactions (such as covalent bonds) are dominant between two components, respectively.² Polyoxometalates (POMs), which are discrete early transition metal–oxo clusters with nanometer size and unrivalled structural diversity, represent a class of excellent modular building blocks for constructing the inorganic–organic hybrid multifunctional materials with wide applications in catalysis, electrochemistry, magnetism, biomedicine and photochemistry.³

Core–shell nanostructures are regarded as one of the hottest topics in current materials science because of their fascinating properties⁴ and the easy tuning of their chemical compositions and structures. According to their compositions, the core–shell nanostructures can be divided into homogeneous and heterogeneous materials with the same and different compositions for both the core and the shell.⁵ Molecular composites with well-defined structures have attracted more and more attention as they can serve as the structural model for the design and synthesis of functional materials. Recently, much effort has been paid to constructing single crystalline solid materials to obtain a model example of core–shell nanostructures, to study their structure-related properties.⁶ Polyoxoanions, with a lot of terminal oxygen atoms and high negative charges, are usually used to integrate functional moieties via the covalent bonds or weak interactions.⁷ However, molecular core–shell nanostructures based on POM units were rarely explored. Several typical core–shell nanostructures based on the polyoxotungstates, polyoxomolybdates, and polyoxoniobates were designed and synthesized: [Cu₃(C₂H₄N₄)₄][PW₁₂O₄₀], [Ag₈(C₂H₃N₄S)₄][SiW₁₂O₄₀]·2H₂O and [Ag₆(C₂H₃N₄S)₄S₂][H₃PW₁₂O₄₀]·2H₂O;⁸ polyoxoniobate {XNb₁₂V^IV₂O₄₂} (X = Ge or Si) surrounded by the [Cu(en)₂]²⁺ groups,^5b and a Keplerate-type {Mo₇₂Fe₃₀} shell encapsulating a Keggin-type polyoxomolybdate anion.⁹

Herein, we report the design and synthesis of two polyoxovanadates (POVs)-based coordination inorganic–organic hybrids, [Ni(dap)₃]₄[HV^IV₁₂V^V₆O₄₂(PO₄)] (1) and [Ni(dap)₂]₄[HV^IV₁₂V^V₆O₄₂(PO₄)]·4H₂O (2) (dap = 1,2-diaminopropane). Compounds 1 and 2 are both composed of the typical polyoxovanadate core, surrounded by shells consisting of eight [Ni(dap)₃]²⁺ or ten [Ni(dap)₂]²⁺ groups, respectively. They represent the first POV-based molecular core–shell nanostructure. Electrocatalytic study revealed that the title compounds exhibit electrocatalytic activity for the reduction of H₂O₂ and oxidation of NO₂⁻.

Results and discussion

Crystal structure

Single-crystal X-ray diffraction analysis revealed that compound 1 crystallized in the cubic space group Im3̄m and was composed of a spherical [PV₁₈O₄₆]⁹⁻ cluster (abbreviated as {PV₁₈}) and the windmill-type [Ni(dap)₃]²⁺ cationic units (Fig. 1). As shown in Fig. 1 and S1,† the tetra-coordinate {PO₄} tetrahedron, the hexa-coordinate {VO₆} octahedra and penta-coordinate {VO₅} pyramids were fused together to form the {PV₁₈} cluster. The {PV₁₈} cluster can also be divided into six {VO₅} caps and a Keggin-type {PV₁₂} core consisting of twelve {VO₆} octahedra and a {PO₄} tetrahedron. The {PO₄} with the P–O bond length of 1.549(18) Å is located in the center of the Keggin-type cage through four μ₄-O atoms on the top point of the four triplets, each composed of three VO₆ octahedra. In the {PV₁₈} core, six {VO₅} pyramids are capped on the Keggin-type {PV₁₂} via edge-shared patterns from six different directions onto the {V₁₈} cluster. The bond lengths of V–O in the {PV₁₈} cluster are in the range of 1.588(10)–2.476(11) Å. Bond valence calculations revealed that the V centers in the {PV₁₈} anion are in the mixed oxidation state of +4 and +5,¹⁰ which are usually observed in the literature, such as in the isolated {PV₁₈} and {VV₁₈} clusters.¹¹ It is well known that polyoxoanions with high negative charges can integrate metal–organic units as shells into the molecular core–shell system. The synthesis of the crystalline core–shell inorganic–organic hybrid materials might supply a potentially effective method to establish molecular models for core–shell nanostructure. As shown in Fig. S2,† the nickel cation in 1 was in a distorted octahedral coordination environment completed by six nitrogen donors from three bidentate dap ligands. It is noteworthy that the spherical-shell type {PV₁₈} cluster interacted with eight neighboring [Ni(dap)₃]²⁺ fragments in a core–shell composite (Fig. 1a), which was further connected into a three-dimensional hexagram-type supermolecular framework via the H-bonding interactions (Fig. 1b and S3†) (Table 1).

Fig. 1 (a) From left to right, the structure of {PV₁₈} in 1, the arrangement of the [Ni(dap)₃]²⁺ groups in the shell and [Ni(dap)₃]–{PV₁₈}; (b) the 3D hexagram-type supermolecular structure of 1. Color code: V (wine red), Ni (light blue), P (pink), O (red), N (blue), C (black).

Table 1 Crystal data and structure refinements for 1 and 2

Compounds	1	2
a R1 = ∑||F0| − |Fc||/∑|F0|; wR2 = ∑[w(F0^2 − Fc^2)^2]/∑[w(F0^2)^2]^1/2.
Empirical formula	C36H127N24Ni4O46PV18	C24H95N16Ni4O50PV18
λ/Å	0.71073	0.71073
Mr	2810.31	2584.84
T/K	293(2)	293(2)
Crystal dimensions/mm	0.26 × 0.12 × 0.08	0.33 × 0.12 × 0.10
Crystal system	Cubic	Orthorhombic
Space group	Im3̄m	Pnma
A/Å	17.5402(10)	11.7978(9)
B/Å	17.5402(10)	26.8599(19)
C/Å	17.5402(10)	23.7929(17)
V/Å^3	5396.4(5)	7539.7(10)
Z	2	4
Dc/mg m^−3	1.730	2.277
μ/mm^−1	2.251	3.213
F(000)	2830	5144
θ range/°	1.64–27.46	1.14–27.19
Data/restraints/parameters	645/14/51	8485/18/545
R1 (I > 2σ(I))^a	0.0740	0.0729
wR2 (all data)^a	0.2484	0.1666
Goodness-of-fit on F^2	1.082	1.086

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Compound 2 crystallized in the orthorhombic Pnma space group, constructed from a spherical [PV₁₈O₄₆]⁹⁻ cluster and sheet-like [Ni(dap)₂]²⁺. It is worth mentioning that the amount of organic amine has great influence on the coordination configuration of the nickel centers in the self-assembly process under hydrothermal conditions. In 2, a planar [Ni(dap)₂]²⁺ formed instead of the sphere-like [Ni(dap)₃]²⁺ group in 1 (Fig. S2†). Moreover, the structure of {PV₁₈} in 2 has some differences from that of 1. In 2, a {PV₈} combined with ten {VO₅} pyramids resulting in the {PV₁₈} cluster. The {PV₈} could be regarded as a tetra-vacant Keggin-type structure composed of eight {VO₆} octahedral and a central {PO₄} tetrahedron. In 1, the {PV₁₈} cluster is composed of the Keggin-type {PV₁₂} core, consisting of twelve {VO₆} octahedra and six {VO₅} pyramids capped on the {PV₁₂} core forming the {PV₁₈} cluster. As shown in Fig. 2, ten planar [Ni(dap)₂]²⁺ fragments surround the spherical-shell type {PV₁₈} cluster in a core–shell structure, where the ten [Ni(dap)₂]²⁺ fragments form a distorted dodecahedron encapsulating the {PV₁₈} cluster in its center (Fig. S4†).

Fig. 2 From left to right, structure of the spherical {PV₁₈} in 2 and the arrangement of the [Ni(dap)₂]²⁺ groups around the {PV₁₈} core. Color code: V (wine red), Ni (light blue), P (pink), O (red), N (blue), C (black).

Electrochemical and electrocatalytic properties

POMs with diverse electrochemical properties have been widely used to prepare chemically bulk-modified carbon paste electrodes (abbreviated as CPEs). The CPEs with the characteristics of low cost and high sensitivity have attracted much attention in electrochemical applications and detection in the food industry.¹² In this study, the electrochemical and electrocatalytic properties of compounds 1 and 2 were studied by cyclic voltammetry (CV) at a pH of 6.5 in 0.20 M Na₂HPO₄–NaH₂PO₄ buffer solution containing quantified concentrations of KCl. The reproducibility of the CV studies showed that 1-CPE and 2-CPE were stable in the 0.20 M Na₂HPO₄–NaH₂PO₄ buffer solution at pH 6.5 (Fig. S5b and d†). Fig. 3a and b show the unique CV curves of 1-CPE and 2-CPE in the region of −0.7 to 1.0 V at different scan rates from 25 to 250 mV s⁻¹. The {PV₁₈} clusters in these two compounds exhibit similar electrochemical behavior and four pairs of redox peaks were detected with the peak potentials E_1/2 = 650, 286.5, −45, −332 mV, and E_1/2 = 625, 297, −60, and −368.5 mV (where E_1/2 = (E_pc + E_pa)/2) for 1 and 2, respectively. This can be ascribed to the redox process of the V centers in the anionic clusters. These results further confirm the existence of the similar {PV₁₈} clusters in 1 and 2. Moreover, four redox couples were observed in the CV curve of the V₁₆-based organic–inorganic hybrid material [Cd(phen)₃]₂{[Cd(H₂O)(phen)₂](V₁₆O₃₈Cl)}²⁻.^13a The shift in scan rates (ν) (from 25 to 250 mV s⁻¹) has an impact on the peak currents (i_pa and i_pc). As illustrated in Fig. S5a and c,† the relationships between peak currents and scan rates (from 25 to 250 mV s⁻¹) of 1-CPE and 2-CPE were linear, manifesting the surface controlled electrochemical process.

Fig. 3 CV of 1-CPE (a) and 2-CPE (b) in 0.20 M Na₂HPO₄–NaH₂PO₄ buffer solution at pH 6.5, with 0.2 M KCl at different scan rates (from 25 to 250 mV s⁻¹); CV of 1-CPE (c) and 2-CPE (d) in 0.2 M Na₂HPO₄–NaH₂PO₄ buffer solution with 0.2 M KCl containing various concentrations of H₂O₂. Scan rate: 50 mV s⁻¹; CV of 1-CPE (e) and 2-CPE (f) in 0.20 M Na₂HPO₄–NaH₂PO₄ buffer solution at pH 6.5, with 0.2 M KCl containing various concentrations of NO₂⁻. Scan rate: 50 mV s⁻¹.

Hydrogen peroxide, ascorbic acid and nitrite widely exist in the food industry. The design and synthesis of electrocatalysts for the detection and elimination of these hazardous substances have attracted more and more attention all over the world. Electrocatalytic reduction of H₂O₂ by 1 was studied in 0.2 M Na₂HPO₄–NaH₂PO₄ buffer solution at pH 6.5. As shown in Fig. 3c, with addition of a slight amount of H₂O₂, the reduction peak currents of the vanadium center increased, while the opposite oxidation peak current obviously decreased, indicating an electrocatalytic reduction process occurred on the 1-CPE. The electrocatalytic oxidation of nitrite was carried out in the same medium. Fig. 3e shows that compound 1 has excellent electrocatalytic activity for the oxidation of the nitrite. As shown in Fig. 3d and f, compound 2 also exhibits electrocatalytic activity for the reduction of H₂O₂ and oxidation of NO₂⁻. Previous study has shown that the polyoxovanadates exhibit electrocatalytic activity towards the reduction of H₂O₂ and NO₂⁻; it was found that the title compounds exhibit higher catalytic activity than the As₈V₁₂ and V₁₆O₃₈Cl-based organic–inorganic hybrid materials.¹³ The relationships between the peak currents and the concentrations of H₂O₂ and NO²⁻ have been studied in detail. As shown in Fig. S6,† the linear relationship between the peak currents and concentrations of H₂O₂ and NO₂⁻ are maintained for concentrations lower than 30 mM for H₂O₂ and 25 mM for nitrite. In these concentration ranges, H₂O₂ and NO₂⁻ could be determined quantitatively. With an increase in the concentration, limiting catalytic currents were observed for both H₂O₂ and nitrite (Fig. S7†).

Experimental section

All the chemical reagents were commercially purchased and used without further purification. K₈[Ta₆O₁₉]·17H₂O was prepared according to the literature and identified by IR.¹⁴

Synthesis of 1

A mixture of NH₄VO₃ (0.20 g, 1.71 mmol), Ni(CH₃COO)₂·4H₂O (0.10 g, 0.12 mmol), benzenetricarboxylate (BTC), (0.033 g, 0.016 mmol), K₈[Ta₆O₁₉]·17H₂O (0.02 g, 0.01 mmol) and 10 mL distilled water was stirred for 30 min. 1,2-Diaminopropane (300 μL) and H₃PO₄ (100 μL, 7.50 mol L⁻¹) were then added to the mixture. The mixture was transferred to a Teflon-lined autoclave (25 mL) and maintained at 150 °C for 4 days. After being slowly cooled to room temperature at a rate of 10 °C h⁻¹, black crystals were filtered off, washed with distilled water, and dried at room temperature to obtain a yield of ca. 30% (based on V). Anal. calcd (%) for 1: Ni, 8.35; V, 32.66; P, 1.10; found: Ni, 8.08 V, 32.44; P, 1.02. IR (KBr pellet) for 1: ν (cm⁻¹) = 1639 (w), 1571 (s), 1459 (w), 1044 (m), 920 (s), 693 (s), 645 (s), 578 (s).

Synthesis of 2

A procedure similar to that of compound 1 was used for the isolation of 2, only the quantities of the Ni(CH₃COO)₂·4H₂O and 1,2-diaminopropane were changed to (0.03 g, 0.04 mmol) and (100 μL), respectively. Black crystals were filtered off, washed with distilled water, and dried at room temperature to obtain a yield of ca. 15% (based on V). Anal. calcd (%) for 2: Ni, 9.08; V, 35.51; P, 1.20; found: Ni, 8.92, V, 35.12; P, 1.08. IR (KBr pellet) for 2: ν (cm⁻¹) = 1653 (w), 1575 (m), 1457 (m), 1092 (m), 909 (s), 687 (s), 577 (s), 531 (s).

X-ray crystallography

Single crystal X-ray structure analysis was performed at 296 K on a Bruker Apex-II CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structures of 1 and 2 were solved by direct methods and further refined by full matrix least-squares refinements on F² using the SHELXL-97 software.¹⁵ In the refinement, the restraint command ‘ISOR’ was employed to restrain several C atoms so as to avoid the ADP and NPD problems in the crystal data. Such refinement led to the restraint value of 14 for 1 and 18 for 2. Crystallographic data in this study have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. 1445001 for 1 and 1445002 for 2.†

Conclusions

In conclusion, two novel core–shell structure molecular composites based on a polyoxovanadate core and a metal–organic shell were designed and synthesized. The mixed-valence clusters {PV₁₈} were first introduced into the core–shell structure for constructing the molecular composites, which supplied a molecular model with well-defined structure for researching the physicochemical properties of the core–shell nanomaterials. Electrocatalytic study revealed that compounds 1 and 2 exhibit electrocatalytic activity for the reduction of H₂O₂ and oxidation of NO₂⁻.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 21301020), Science and Technology Development Project Foundation of Jilin Province (20130522126JH/20150520001JH), and Ph. D. Station Foundation of Ministry of Education for New Teachers (No. 20122216120003).

Notes and references

1. (a) M. S. Wang, G. Xu, Z. J. Zhang and G. C. Guo, Chem. Commun., 2010, 46, 361; (b) J. Y. Yuan, Y. Y. Xu and A. E. Müller, Chem. Soc. Rev., 2011, 40, 640; (c) Z. M. Zhang, T. Zhang, C. Wang, Z. K. Lin, L. S. Long and W. B. Lin, J. Am. Chem. Soc., 2015, 137, 3197.
2. (a) Y. F. Song, D. L. Long, C. Ritchie and L. Cronin, Chem. Rec., 2011, 11, 158; (b) P. Gomez-Romero, Adv. Mater., 2001, 13, 163.
3. (a) N. Mizuno and M. Misono, Chem. Rev., 1998, 98, 199; (b) D. E. Katsoulis, Chem. Rev., 1998, 98, 359; (c) T. Ruether, V. M. Hultgren, B. P. Timko, A. M. Bond, W. R. Jackson and A. G. Wedd, J. Am. Chem. Soc., 2003, 125, 10133; (d) T. Yamase, Chem. Rev., 1998, 98, 307; (e) Z. M. Zhang, J. Liu, E. B. Wang, C. Qin, Y. G. Li, Y. F. Qi and X. L. Wang, Dalton Trans., 2008, 463; (f) R. N. N. Khan, C. L. Lv, J. Zhang, J. Hao and Y. G. Wei, Dalton Trans., 2015, 44, 4568; (g) J. W. Zhang, Z. H. Liu, Y. C. Huang, J. Zhang, J. Hao and Y. G. Wei, Chem. Commun., 2015, 51, 9097; (h) A. Proust, R. Thouvenot and P. Gouzerh, Chem. Commun., 2008, 1837; (i) A. Nisar, Y. Lu and X. Wang, Chem. Mater., 2010, 22, 3511; (j) A. Nisar, X. X. Xu, S. L. Shen, S. Hu and X. Wang, Adv. Funct. Mater., 2009, 19, 860.
4. (a) J. Qiao, K. Shi and Q. M. Wang, Angew. Chem., Int. Ed., 2010, 49, 1765; (b) P. F. Gao, S. Zhang, H. W. Li, T. Zhang, Y. Q. Wu and L. X. Wu, Langmuir, 2015, 31, 10888; (c) T. Zhang, H. W. Li, Y. Q. Wu, Y. Z. Wang and L. X. Wu, Chem.–Eur. J., 2015, 21, 9028.
5. (a) J. J. Gong, W. S. Zhang, Y. Liu, H. C. Zhang, H. L. Hu and Z. H. Kang, Dalton Trans., 2012, 41, 5468; (b) Y. Zhang, J. Q. Shen, L. H. Zheng, Z. M. Zhang, Y. X. Li and E. B. Wang, Cryst. Growth Des., 2014, 14, 110.
6. (a) A. Dolbecq, E. Dumas, C. R. Mayer and P. Mialane, Chem. Rev., 2010, 110, 6009; (b) D. L. Long, R. Tsunashima and L. Cronin, Angew. Chem., Int. Ed., 2010, 49, 1736; (c) S. Nlate, L. Plault and D. Astruc, Chem.–Eur. J., 2006, 12, 903; (d) Y. Wang, P. T. Ma and J. Y. Niu, Dalton Trans., 2015, 44, 4679; (e) H. J. Lv, Y. N. Chi, J. V. Leusen, P. Kögerler, Z. Y. Chen, J. Bacsa, Y. V. Geletii, W. W. Guo, T. Q. Lian and C. L. Hill, Chem.–Eur. J., 2015, 21, 17363; (f) J. Zhou, J. W. Zhao, Q. Wei, J. Zhang and G. Y. Yang, J. Am. Chem. Soc., 2014, 136, 5065; (g) S. T. Zheng, Q. X. Zeng, Z. E. Lin and G. Y. Yang, Chin. J. Inorg. Chem., 2002, 12, 1234.
7. (a) Y. Z. Wang, H. L. Li, C. Wu, Y. Yang, L. Shi and L. X. Wu, Angew. Chem., Int. Ed., 2013, 52, 4577; (b) T. Akutagawa, R. Jin, R. Tunashima, S. Noro, L. Cronin and T. Nakamura, Langmuir, 2008, 24, 231; (c) D. Volkmer, A. D. Chesne, D. G. Kurth, H. Schnablegger, P. Lehmann, M. J. Koop and A. Müller, J. Am. Chem. Soc., 2000, 122, 1995; (d) Y. K. Han, Y. Xiao, Z. J. Zhang, B. Liu, P. Zheng, S. J. He and W. Wang, Macromolecules, 2009, 42, 6543; (e) Y. Jia, H. Q. Tan, Z. M. Zhang and E. B. Wang, J. Mater. Chem. C, 2013, 1, 3681; (f) Y. Xiao, Y. K. Han, N. Xia, M. B. Hu, P. Zheng and W. Wang, Chem.–Eur. J., 2012, 18, 11325.
8. Y. A. Yang, J. J. Gong, W. S. Zhang, H. C. Zhang, L. L. Zhang, Y. Liu, H. Huang, H. L. Hu and Z. H. Kang, RSC Adv., 2012, 2, 6414.
9. A. M. Todea, J. szakács, S. Konar, H. Bögge, D. Crans, T. Glaser, H. Rousselière, R. Thouvenot, P. Gouzerh and A. Müller, Chem.–Eur. J., 2011, 17, 6635.
10. (a) W. T. Liu and H. H. Thorp, Inorg. Chem., 1993, 32, 4102; (b) I. D. Brown and D. Altermatt, Acta Crystallogr., 1985, B41, 244; (c) K. H. Tytko, J. Mehmke and D. Kurad, Struct. Bonding, 1999, 93, 1.
11. (a) W. B. Yang, C. Z. Lu, Q. Z. Zhang, S. M. Chen, X. P. Zhan and J. H. Liu, Inorg. Chem., 2003, 42, 7309; (b) C. L. Pan, J. Q. Xu, G. H. Li, X. B. Cui, L. Ye and G. D. Yang, Dalton Trans., 2003, 517; (c) T. Yamase, K. Ohtaka and M. Suzuki, J. Chem. Soc., Dalton Trans., 1996, 283; (d) M. I. Khan, E. Yohannes and D. Powell, Chem. Commun., 1999, 23.
12. (a) J. E. Toth and F. C. Anson, J. Am. Chem. Soc., 1989, 111, 2444; (b) X. L. Wang, Z. H. Kang, E. B. Wang and C. W. Hu, Mater. Lett., 2002, 56, 393; (c) Z. Zhang, Y. Qi, C. Qin, Y. Li, E. Wang, X. Wang, Z. Su and L. Xu, Inorg. Chem., 2007, 46, 8162; (d) J. W. Zhao, Y. Z. Li, F. Ji, J. Yuan, L. J. Chen and G. Y. Yang, Dalton Trans., 2014, 43, 5694.
13. (a) B. Dong, J. Peng, A. Tian, J. Sha, L. Li and H. Liu, Electrochim. Acta, 2007, 52, 3804; (b) Y. Qi, Y. Li, C. Qin, E. Wang, H. Jin, D. Xiao, X. Wang and S. Chang, Inorg. Chem., 2007, 46, 3217.
14. M. Filowitz, R. K. C. Ho, W. G. Klemperer and W. Shum, Inorg. Chem., 1979, 18, 93.
15. (a) G. M. Sheldrick, SHELXL97, Program for Crystal Structure Refinement, University of Göttingen, Göttingen, Germany, 1997; (b) G. M. Sheldrick, SHELXS97, Program for Crystal Structure Solution, University of Göttingen, Göttingen, Germany, 1997.

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Footnotes

† Electronic supplementary information (ESI) available: Supplementary structure figures, IR, crystal data in CIF files, PXRD. CCDC 1445001 and 1445002. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra00223d

‡ These authors contributed equally.

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