Mn²⁺ cation-directed ionothermal synthesis of an open-framework fluorinated aluminium phosphite–phosphate

Abstract

An open-framework fluorinated aluminium phosphite–phosphate, H_3.2Mn_3.4[C₆N₂H₁₁]₂{Al₁₂(HPO₃)_15.0(HPO₄)_3.0F₁₂}·14H₂O (DNL-2), was ionothermally synthesized by employing the in situ released Mn²⁺ cations as structure-directing agent.

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Over the past few decades, phosphorus-based inorganic open-framework materials, i.e., metal phosphates/phosphites, have attracted strong interest owing to their fascinating structural topologies, various compositions, and broad potential applications.¹ Aiming at synthesizing inorganic open-framework materials with low framework density and extra-large pore openings, the construction of interrupted frameworks is often desired. Because H–PO₃²⁻ contains a terminal P–H group which excludes the formation of a complete framework, but has otherwise a similar charge and geometry as HO–PO₃²⁻, a huge effort has been devoted to the replacement of HO–PO₃²⁻ with H–PO₃²⁻ during the crystallization. However, fruitful results are mainly obtained with the synthesis of transition metal-containing phosphites.² In contrast to the various architectures of aluminium phosphates, only a few open-framework aluminium phosphites were captured previously.³ The rare achievement in synthesizing aluminium phosphites indicates the demand for new synthesis methods.

The ionothermal method, which can provide an ionic crystalline environment at very low pressure, has been widely applied to synthesize inorganic open-framework materials.⁴ The flexible chemical properties of ionic liquids have provided attractive possibilities for constructing phosphorus-based inorganic open-frameworks.⁵ Using this method, we synthesized a novel open-framework fluorinated aluminium phosphite–phosphate H_3.2–Mn_3.4[C₆N₂H₁₁]₂{Al₁₂(HPO₃)_15.0(HPO₄)_3.0F₁₂}·14H₂O (denoted as DNL-2). DNL-2 is one of the rare examples of three-dimensional metal phosphite–phosphates,⁶ and as far as we know, the first reported open-framework material constructed by aluminium-centred polyhedra and isomorphously mixed H–PO₃²⁻ and HO–PO₃²⁻ oxoanions. The successful synthesis of DNL-2 expands the understanding of isomorphous substitution of the oxoanions in phosphorus-based materials. More remarkably, the extra-framework Mn²⁺ cations are employed as structure-directing agent in the ionothermal synthesis of DNL-2. Herein, discussion of this unusual structure-directing effect is illustrated, following detailed presentation of the ionothermal synthesis and structural characterization of DNL-2.

Crystals of DNL-2 were ionothermally synthesized from a mixture containing H₃PO₃ (10 mmol, 50 wt% in H₂O), Al(OH)₃ (2.5 mmol), MnO₂ (2.5 mmol), HF (3 mmol, 40 wt% in H₂O) and 1-ethyl-3-methylimidazolium bromide ([EMIm]Br, 100 mmol) at 160 °C for 21 days (ESI†, S1). Single crystal X-ray diffraction analysis (ESI†, S2) shows that DNL-2 has a hexagonal space-group P6₃/m (no. 176) with a = 13.2544(2) Å and c = 15.6722(4) Å. Primary building units AlO_4bF_2b and H–PO_3b/HO–PO_3b alternate strictly through bridging O atoms to form 4.6.12-nets paralleling to the ab plane (Fig. 1a). In each 4-membered ring, the two Al atoms connect each other through the two bridging F atoms. Via primary building unit H–PO_2bO_t/HO–PO_2bO_t, the adjacent 4.6.12-nets are connected to form a pillared layer-like three-dimensional framework (12.6 nodal Al/P atoms per 1000 ų) with 12-membered ring main channels running along the c-axis (Fig. 1b).

Fig. 1 The 4.6.12-nets viewed along c axis (a) and the pillared layer-like structure viewed along b axis (b) of DNL-2. Octahedra, AlO_4bF_2b groups; black balls, P atoms. The P atoms with occupancies less than 0.5 and the terminal O atoms are omitted for clarify.

In the framework of DNL-2 exist two kinds of cages [6²8³] and [6²12³], which are filled with Mn²⁺ and [EMIm]⁺ cations respectively. Mn²⁺ cations occupy the two equivalent positions in each [6²8³] cage with occupancy 0.840(5) (Fig. 2a). Each Mn²⁺ cation is six-coordinated by framework Brønsted-basic O atoms, which originate partly from one of the two six-membered rings and from three additional H–PO₃²⁻/HO–PO₃²⁻ groups. This coordination mode is similar to that of the site I Mn²⁺ cations (at the centres of D6Rs) in Mn²⁺ exchanged zeolite X (Fig. 2b).⁷ Because the Mn site is not fully occupied, the [6²8³] cages may be filled with 0, 1, or 2 Mn²⁺ cations in principle. In the fully Mn²⁺ cation-filled [6²8³] cages, a short Mn⋯Mn distance (3.063(2) Å) is caused by the size and confinement effect of the [6²8³] cages. The two adjacent paramagnetic Mn²⁺ cations are superexchange-coupled through intervening O atoms. The ratio of the mononuclear/dinuclear Mn²⁺ cations can be determined from their different contributions to the magnetic susceptibility of DNL-2. The experimental temperature dependence of the magnetic susceptibility of DNL-2 was fitted as

where x = exp(−J(kT)⁻¹), χ is the molar magnetic susceptibility, Y and 1 − Y are the ratios of mononuclear and dinuclear Mn²⁺ cations respectively, N_α is the temperature-independent paramagnetism term, T is the absolute temperature, g is the Landé factor, J is the exchange integral, N, β and k are Avogadro's number, the Bohr magneton and the Boltzmann constant respectively (Fig. 3).⁸ As the fitting result of χ_exp ∼ T plot shows, 9.14% of the Mn²⁺ cations are mononuclear (Y = 0.0914) and the remaining 90.86% of the Mn²⁺ cations exist as dinuclear Mn²⁺⋯Mn²⁺ clusters with weak antiferromagnetic interaction (J = −2.40 cm⁻¹). Considering the ratio of mononuclear/dinuclear Mn²⁺ cations and the total number of them filled in each unit cell (3.4), the ratio of the [6²8³] cages filled with 0, 1, and 2 Mn²⁺ cations is calculated as approximately 0.15 : 0.30 : 1.55. The coexistence of these three filling conditions suggests that the extra-framework Mn²⁺ cations are inserted into the structure randomly during the framework formation, and can be removed from the [6²8³] cages of DNL-2 in some way.

Fig. 2 The coordination modes of Mn²⁺ cations in the [6²8³] cage of DNL-2 (a) and in the D6R of Mn²⁺ exchanged zeolite X (b, ref. 7). Mn and the coordinating O atoms are shown as large and small black balls respectively. The frameworks are shown in stick mode. Mn–O distances are labelled.

Fig. 3 The magnetic susceptibility data (○) for per mole of Mn²⁺ cations in DNL-2. The solid line was calculated from eqn (1) with g = 2.00, J = −2.40 cm⁻¹, Y = 0.0914, and N_α = −0.00405 cm³ mol⁻¹.

During the ionothermal synthesis of DNL-2, P^V species formed in the reaction between H₃PO₃ and MnO₂, as well as the pyrolysis of H₃PO₃.⁹ X-ray photoelectron spectra (XPS) of DNL-2 revealed the oxidation states of both P and Mn, and indicated the mixed-valence inhomogeneity of the surface P species (Fig. 4). The four clearly resolved peaks in the P 2p spectrum can be assigned to P^III (2p_3/2: 131.7 eV, 2p_1/2: 132.7 eV) and P^V (2p_3/2: 134.0 eV, 2p_1/2: 135.0 eV) species respectively. The semiquantitative XPS analysis shows that the surface P^III/P^V ratio is 1.5, which is much lower than the bulk P^III/P^V ratio (5.0) calculated from the structure refinement. This inhomogeneity suggests that the H–PO₃²⁻ groups within XPS detection depth (several unit cells thick) are oxidized gradually during the specimen preparation (grinding and pressing). The splitting values (both 6.2 eV) of the two exchange-splitting doublets in the Mn 3s spectrum support the oxidation state assignment as Mn^II rather than Mn^IV.¹⁰ For the Mn²⁺⋯Mn²⁺ clusters, the 3d electron delocalization of Mn²⁺ cations might affect the electrons in the 3s orbital and cause a difference in the chemical shift in the 3s level compared with the Mn²⁺ mononuclei. This could explain the existence of the two exchange-splitting doublets in the Mn 3s XPS of DNL-2.

Fig. 4 The X-ray photoelectron spectra of DNL-2. (a) P 2p. (b) Mn 3s.

In preceding studies of the ionothermal synthesis of aluminophosphates, it has been found that the structure of the product is always determined by the properties of the specific ionic liquid¹¹ and the specific organic molecule used for the synthesis.¹² In our experiment, however, products with DNL-2 structure were obtained by different ionic liquids ([EMIm]Br or 1-butyl-3-methylimidazolium bromide, ESI†, Table S2, entry 1) and also by adding different organic amines/ammonium cations (2-methylimidazole, triethylamine, or triethylamine hydrochloride, ESI†, Table S2, entry 2–4). An extended study indicated that, in this case, the Mn²⁺ cations released in situ play the role of structure-directing agent during the synthesis of DNL-2 (Fig. 5). In the absence of Mn species, only Al(OH)₃ could be finally obtained. However, isostructural product of DNL-2 could crystallize when MnO is used instead of MnO₂. Previous discussion on the structure-directing ability of metal cations derived from the hydrothermal synthesis of aluminium-rich zeolites in the presence of alkali/alkaline-earth metal cations. It was suggested that the structure-directing effect of alkali/alkaline-earth metal cations involves (i) the replacement of H₂O molecules around these cations with Si and Al species and (ii) the subsequent formation of the cage-like structures.¹³ However, for aqueous transition metal cations, above structure-directing process is always inhibited by their strong interaction with H₂O molecules.¹⁴ Therefore, the structure-directing effect of extra-framework transition metal cations has never been reported in practice. During the ionothermal synthesis of DNL-2, the in situ released Mn²⁺ cations should initially coordinate with Br⁻ anions. The intermediate strength of Mn–Br bond benefits the replacement of Br⁻ with framework Brønsted-basic O atoms and the following assembly of the [6²8³] cages. We even tried using MnCl₂·4H₂O instead of MnO₂ in the synthesis but failed in obtaining the isostructural product of DNL-2. This result suggests that once all the Mn²⁺ cations are coordinated with strong ligands such as H₂O and Cl⁻, the above mentioned structure-directing process of Mn²⁺ cations is inhibited. Moreover, it should be noted that a certain amount of H₂O added into the reacting mixture seems to have little influence on the structure-directing effect of Mn²⁺ cations. As both theoretical studies and spectroscopic experiments show, H₂O molecules dissolved in ionic liquids can be isolated via H-bonding with anions of the ionic liquids.¹⁵ The resultant deactivation of H₂O can stabilize the bonds which are sensitive to hydrolysis,¹⁶ and even modify the crystallization kinetics of inorganic open-framework materials.¹⁷ In the ionothermal synthesis of DNL-2, the insensitivity of Mn²⁺ cations to H₂O or other strong ligands is probably attributed to a similar mechanism.

Fig. 5 The X-ray powder diffraction patterns of products obtained from the reacting mixtures 2.5Al(OH)₃–10H₃PO₃–3HF–50H₂O–100[EMIm]Br–xMn at 160 °C. (a) 2.5 MnO₂, 7 d; ▼, the diffraction peak of Al(OH)₃. (b) Without Mn species, 14 d. (c) 2.5 MnO, 7 d. (d) 2.5 MnCl₂·4H₂O, 7 d.

A question raised by above discussion is whether the framework P^III can be further replaced by P^V during the synthesis. In order to answer this question, we used mixtures of H₃PO₃ and H₃PO₄ with different P^III/P^V ratios as P sources to synthesize DNL-2 (ESI†, Table S2, entry 5–8). With the P^III/P^V ratio reduced from 8/2 to 4/6, the solid product varied from the isostructure of DNL-2 to the CHA-type structure. This result indicates that there is a competition between the formation of interrupted structures and zeolitic structures. Unlike H–PO₃²⁻, the trigonal pyramidal HO–PO₃²⁻ can further deprotonate into four-connected PO₄³⁻ groups. Thus the zeolitic aluminophosphates with fully four-connected P sites tend to crystallize when the amount of H₃PO₄ in the reacting mixture reaches a fairly level.

In conclusion, an open-framework fluorinated aluminium phosphite-phosphate (DNL-2) was ionothermally synthesized. The Mn²⁺ cations released in situ play the role of structure-directing agent during the synthesis. This structure-directing effect of Mn²⁺ cations depends on the unique property of the ionic liquid [EMIm]Br, and is difficult to achieve under traditional synthesis conditions. This result improves our understanding of the structure-directing process of metal cations, and moreover, indicates a potential strategy to introduce extra-framework transition metal centres into inorganic open-framework materials.

We gratefully acknowledge Prof. Zhi-Dong Zhang and Dr Song Ma (IMR, CAS) for their help in the collection of magnetic susceptibility data and valuable discussions. This work was supported by NSFC (Grant no. 21373214).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, characterization results and the X-ray crystallographic file. CCDC 997982. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra05350h

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