Ionothermal synthesis and magnetic study of a new manganese(II) phosphite with an unprecedented Mn/P ratio

Abstract

A new layered manganese(II) phosphite, |NH₄|₄[Mn₄(PO₃H)₆] (JIS-10), has been synthesised via the ionothermal method by using 1-pentyl-3-methylimidazolium bromide ([Pmim]Br) as the solvent. JIS-10 is the first manganese phosphite with a Mn/P ratio of 2/3, whose layered framework is built by the alternate connection of Mn₂O₉ dimers and PO₃H pseudo-tetrahedra. JIS-10 represents an antiferromagnetic dimer system with S = 5/2 for each Mn²⁺ ion.

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Inorganic oxides such as zeolites are of significant interest in a number of scientific and industrial fields,^1,2 particularly in those associated with catalysis and gas storage.^3–6 Conventional routes for the preparation of inorganic oxide materials are hydrothermal and solvothermal syntheses, where the precursor solution is either aqueous or not, respectively. In 2004, Morris and co-workers developed the ionothermal route for the synthesis of inorganic oxide materials, such as aluminophosphate zeolites.⁷ Ionothermal synthesis employs ionic liquids or eutectic mixtures as the solvent in the reaction, where the cations of the ionic liquid, in many cases, serve as the structure directing agent for the resultant compounds with two-dimensional layered or three-dimensional open frameworks.^8–14 The ionothermal synthesis possesses several advantages over the conventional hydro- and solvothermal routes for the preparation of inorganic oxide materials. First, the very large polarity of an ionic liquid provides a distinct ionic atmosphere, which can easily dissolve the inorganic raw materials and is crucial for the resultant structures. Second, the negligible vapour pressure makes the synthesis much safer. More importantly, a tremendous number of ionic liquids can be used as solvents for ionothermal synthesis, providing distinct chemical environment that results in a wide variety of resultant materials. Since the discovery of an ionothermal route, great efforts have been made to prepare new inorganic oxide compounds, such as aluminophosphates, borophosphates, and phosphites, which exhibit unique structural characteristics and interesting properties.^15–20 These successes prove that ionothermal synthesis is an efficient route to the discovery of novel inorganic materials.

To date, a number of manganese phosphites have been synthesised hydrothermally or solvothermally. However, the ionothermal synthesis of manganese phosphite has never been investigated. As listed in Table S1,† the reported manganese phosphites with diverse Mn/P ratios exhibit a rich variety of structures. The differences between these structures originate from the different coordination states of Mn and P atoms. So far, the reported Mn/P ratios include 1/1, 1/2, 1/4, 1/6, 2/5, 2/7, 3/4, 5/2 and 11/8 (ESI Table S1†).^21–35 Considering the large variety of combinations of Mn- and P-centred polyhedra, one may expect that more manganese phosphites with different Mn/P ratios could be found in the future. Here, we report the synthesis of a new manganese phosphite (NH₄)₄[Mn₄(HPO₃)₆] (JIS-10) by using 1-pentyl-3-methylimidazolium bromide ([Pmim]Br) as the ionic liquid solvent. Interestingly, JIS-10 possesses a Mn/P ratio of 2/3 that has never been observed among all the known manganese phosphites.

Pink hexagonal-prism-shaped crystals of JIS-10 were obtained from a reaction mixture of MnCl·4H₂O (0.07 g, 0.35 mmol), H₃PO₃ (0.30 g, 3.66 mmol), NH₄F (0.15 g, 4.05 mmol), and the ionic liquid of [Pmim]Br (1 mL, 5.09 mmol) heated at 423 K for 7 days with a yield of 70% (calculated on Mn). JIS-10 (|NH₄|₄[Mn₄(PO₃H)₆]) crystallises in the hexagonal space group P6₃/mmc with a = 5.4649(1) Å and c = 19.0227(8) Å. It consists of one crystallographically distinct Mn and two crystallographically distinct P atoms (Fig. 1a). Each Mn atom is octahedrally coordinated to three μ₂-O and three μ₃-O atoms, forming Mn–O–P and Mn–O–Mn linkages, respectively. The Mn–O bond lengths vary in the range of 2.148(4)–2.265(4) Å (ESI Table S2†), which is in agreement with those observed in other manganese phosphites.^21,33 Bond valence sum calculation shows that the Mn atom has an oxidation state of +2. Two adjacent MnO₆ octahedra are connected via three μ₃-O atoms to form a Mn₂O₉ dimer with a Mn⋯Mn distance of 3.057(3) Å (Fig. 1a). Both the two distinct P atoms have the oxidation state of +3, forming PO₃H pseudo-tetrahedra with three bridging O atoms and one terminal H atom. One of the P atoms, P1, is linked to three adjacent Mn₂O₉ dimers by sharing three μ₃-O atoms. Note that P1 is disordered over two crystallographically equivalent sites. The other P atom, P2, is linked to three adjacent Mn₂O₉ dimers by sharing three μ₂-O atoms. The P–O bond lengths in JIS-10 are in the range of 1.511(4)–1.516(6) Å (ESI Table S2†), which is in agreement with those observed in other metal phosphites.^27,31 The existence of the P–H with bond lengths of 1.33(5) Å (for P1) and 1.34(5) (for P2) is confirmed by the characteristic absorption bands of phosphite groups [ν(H–P) = 2453 cm⁻¹] in the IR spectrum (ESI Fig. S1†).²¹ Thus, each Mn₂O₉ dimer is connected with nine PO₃H pseudo-tetrahedra, and each PO₃H pseudo-tetrahedron is connected with three Mn₂O₉ dimers. The strictly alternate linkage of Mn₂O₉ dimers and PO₃H pseudo-tetrahedra leads to a two-dimensional layered framework structure with a Mn/P ratio of 2/3, which has never been observed among the known manganese phosphites.

Fig. 1 (a) Thermal ellipsoid plot of a Mn₂O₉ dimer and its nine neighbouring PO₃H pseudo-tetrahedra (50% probability). (b) Crystal structure of JIS-10 viewed along the [100] direction. Colour code: green, Mn; yellow, P; red, O; blue, N; grey, H.

These manganese phosphite layers are stacked together along the [001] direction (Fig. 1b). The interstitial spaces are occupied by ammonium cations, which not only balance the negative charges in manganese phosphite layers but also donate multiple H-bonds to adjacent layers. The N⋯O distances of these H-bonds vary in the range of 3.014(4) Å and 3.110(8) Å, respectively (ESI Table S3†). Due to these strong interactions, the interstitial ammonium cations could not be completely removed by calcination before the collapse of the layered structure. In situ temperature-dependent powder X-ray data show that the crystalline structure of JIS-10 is stable up to 483 K (ESI Fig. S2†). The thermogravimetric curve shown in ESI Fig. S3† exhibits a total weight loss of 9.3% from 440 K to 720 K (theoretical weight loss: 9.2%). An ion exchange experiment was carried out in a sealed autoclave heated at 353 K for 25 min by using a microwave. The results indicate that about 10% of the ammonium cations could be exchanged by potassium cations.

Because of the existence of Mn₂O₉ dimers in the structure of JIS-10, we wonder whether there will be magnetic interactions within the dimers. The magnetisation of JIS-10 was first measured at 5 T under a direct current (DC) magnetic field. As shown in ESI Fig. S4,† the χ_mT vs. T curve increases steadily from 2 K to 300 K and reaches a maximum of 3.76 cm³ K mol⁻¹ at room temperature, indicating the antiferromagnetic interaction between Mn atoms in the structure. As shown in Fig. 2, the 1/χ_mvs. T curve shows a linear dependence of the Curie–Weiss formula when the temperature is higher than 30 K, leading to a Curie constant (C) of 4.435 K emu mol⁻¹ for Mn²⁺ ions, which is very close to that estimated from isolated Mn²⁺ ions (C = 4.377 K emu mol⁻¹, g = 2.0, S = 5/2). The negative Weiss temperature of −44.9 K further suggests antiferromagnetic interactions. According to the crystal structure, the Mn²⁺ ions are dimerised along the crystallographic c-axis in the structure. Therefore a spin-dimer model with S = 5/2 for each Mn²⁺ ion was built up to elucidate the antiferromagnetic interactions in JIS-10. The Heisenberg Hamiltonian of this dimer model can be expressed as.

where J is the exchange coupling constant and the spin quantum numbers of S₁ and S₂ are 5/2. The system possesses six eigen states of S = 0, 1, 2, 3, 4 and 5, and the corresponding eigenvalues are −35J/4, −31J/4, −23J/4, −11J/4, 5J/4 and 25J/4, respectively. The temperature dependence of magnetic susceptibility of this model is then derived as follows:
where N_A is the Avogadro number and k_B is Boltzmann constant. As shown in Fig. 3, the model is quite reasonable for the antiferromagnetic interaction of JIS-10 at a higher temperature above 30 K, in which it fits very well with the experimental data; whereas it deviates obviously from the experiment data at a lower temperature. We speculate that this is due to the introduction of a very tiny amount of unbound spins and paramagnetic components obeying the Curie law. In this context, the measured magnetisation was re-elucidated by contributions from both parts of the dimers and paramagnetic components. As shown also in Fig. 3, the experimental data and the calculated one fit perfectly when the Curie constant belonging to the paramagnetic component is calculated to be 0.0594 ± 0.0005 K emu mol⁻¹. The result indicates that the contribution of magnetisation from paramagnetic impurities is very small, i.e. less than 1%. The calculated antiferromagnetic exchange coupling constant (J/k_B) is 12.20 ± 0.02 K. Consequently, JIS-10 represents an antiferromagnetic dimer system with S = 5/2 for each Mn²⁺ ion.^36,37

Fig. 2 1/χ_mvs. T curve of JIS-10 under 5 T.

Fig. 3 Magnetisation of JIS-10 and the corresponding spin-dimer calculation. χ_dimer and χ_para magnetisations are from spin-dimer and paramagnetic components, respectively.

Conclusions

A new layered manganese(II) phosphite, JIS-10, has been prepared in the ionic liquid of 1-pentyl-3-methylimidazolium bromide ([Pmim]Br). The as-prepared compound possesses a new Mn/P ratio of 2/3, which has not been observed earlier in the family of manganese phosphites. The structure of JIS-10 features Mn₂O₉ dimers, in which antiferromagnetic interactions have been revealed by magnetisation measurements. The calculation based on a spin-dimer model also reveals that JIS-10 represents an antiferromagnetic dimer system with S = 5/2 for each Mn²⁺ ion. The successful synthesis of JIS-10 demonstrates that more inorganic materials with new structures and properties could be prepared ionothermally.

Acknowledgements

We thank the National Natural Science Foundation of China (Grant No: 21273098, 21473024, and 21501062), the Program for New Century Excellent Talents in University (NCET-13-0246), and the Research Project of the Science and Technology of Jilin Province, China (Grant no: 201413001) for financial support.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis and characterisation details, IR spectrum, thermogravimetric curve, χ–T curve, temperature-dependent XRD, and selected bond distances and angles. CCDC 1441924. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qi00014b

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