Characterisation and properties of the n = 3 and n = 4 members of the Al2[O3PCnH2nPO3](H2O)2F2 framework aluminium alkylenediphosphonate series
Graphical abstract
Introduction
Immense interest is currently being shown in the family of hybrid organic–inorganic framework materials due to their novel properties and great potential for applications, such as gas storage, gas separation and catalysis [1], [2], [3], [4]. The attraction of these hybrid materials stems from the benefits introduced by inclusion of both the organic and inorganic components in one product. In particular, the incorporation and modification of the organic groups within structures allow the possibility of rationally designing materials with specific chemical functionality, properties and structures. The desire to design the structure of a microporous hybrid material is fuelled by the enduring desire to synthesise crystalline extended solid state materials with the type of precision already practised in synthetic organic chemistry. Such an approach has been demonstrated by the groups of Yaghi and Férey in their formation of series of metal–organic frameworks constructed from octahedral Zn–O–C clusters or triangles of metal (III) centred octahedra linked by an array of bi- or tricarboxylate groups [5], [6], [7], [8].
Most of the recent work in this area of framework structure design has related to metal organic framework materials in which isolated inorganic clusters are linked together by the organic linkers. Less work has been reported concerning the rational design of extended inorganic hybrid type materials in which extended inorganic components, for instance chains, layers or frameworks of corner-, edge-, and face-sharing polyhedra, are present within the material [9]. Examples of systems in which the inorganic component of the hybrid material has remained constant but the organic component has been modified in incremental stages to form designed framework solids include work on a variety of metal carboxylate and diphosphonate materials [3], [10], [11], [12], [13], [14].
If the inorganic layers of the hybrid material are non-linear, for example corrugated, then it is possible to form materials with either one type of channel or two when such layers are linked together so that the inorganic corrugated layers are either aligned (in-phase) with an AAAA stacking arrangement or unaligned (out-of-phase) with an ABAB stacking arrangement. The control of the alignment of the corrugated inorganic chains or layers, through suitable choice of the nature of the organic linker, determines the number of types of channel, and their size, in the resultant framework material and provides a strategy to design the pore structure of such extended inorganic hybrid type materials.
Several exciting hybrid aluminium phosphonate and diphosphonate materials have been reported [15], [16], [17], [18], however relatively little has been reported on the design of the framework structures of these extended inorganic hybrid materials. We have reported the potential use of the above design strategy for the framework aluminium alkylenediphosphonate series, Al2[O3PCnH2nPO3](H2O)2F2, where it was shown that the number of types of channel, and their size, in the resultant materials could be controlled by increasing incrementally the length of the alkyl chains in the diphosphonate groups connecting the corrugated inorganic Al–O/F–P layers [19], [20]. We recently reported the synthesis and structure of Al2[O3PC4H8PO3](H2O)2F2·2H2O determined using a single-crystal diffraction data set collected from a small microcrystal [21]. However, owing to the small size of microcrystal used to collect the X-ray data for structure determination, the final residuals for the reported structure were relatively high leaving some uncertainty in the final refined structure of the material. In this paper, we use a combination of powder diffraction and spectroscopic techniques to fully determine the structure of Al2[O3PC4H8PO3](H2O)2F2·2H2O, and report the thermal stability/dehydration properties of Al2[O3PC4H8PO3](H2O)2F2·2H2O and Al2[O3PC3H6PO3](H2O)2F2·H2O. The latter comparison demonstrates how structures with channels containing the same water content but those consisting of different stacking arrangements of the component inorganic layers behave quite differently upon dehydration.
Section snippets
Experimental section
Polycrystalline samples of Al2[O3PC3H6PO3]F2(H2O)2·H2O and Al2[O3PC4H8PO3]F2(H2O)2·2H2O were prepared following the reported procedures [20], [21]. Microprobe EDXA analysis on the products showed that the particles contained Al:P in the ratio 1:1 and that fluorine was present.
The MAS SSNMR spectra were collected from Al2[O3PC4H8PO3]F2(H2O)2·2H2O using a Varian Unity Inova spectrometer with a 7.05 T Oxford Instruments magnet. The spectrum collected for 31P nuclei was referenced to an 85%
Results and discussion
The Rietveld refinement using the starting model obtained from the microcrystal structure solution [21] yielded low final residuals, an excellent fit between the observed and calculated diffraction profiles and a high quality, final refined crystal structure of Al2[O3PC4H8PO3]F2(H2O)2·2H2O. The structure of Al2[O3PC4H8PO3](H2O)2F2·2H2O is shown in Fig. 3. The structure is formed from corrugated chains of corner-sharing AlO4F2 octahedra similar to those found in Al2[O3PC2H4PO3](H2O)2F2·H2O [19],
Conclusions
The synthesis and structural characterisation of the extended inorganic hybrid type materials, Al2[O3PCnH2nPO3](H2O)2F2, n = 2, 3, 4, have now been reported. The adoption of an aligned or a misaligned arrangement of the corrugated inorganic layers in this system is proven to be dependent on the conformation the alkyl linker adopts to link the inorganic layers together. It is apparent that alkyl chains with an even number of C atoms, such as ethylene and butylene, give rise to an aligned inorganic
Acknowledgments
The authors thank Dr. P. Hill of the University of Manchester and Dr. D. Apperley of the EPSRC Solid State NMR Service, University of Durham, UK for collection of the microprobe data and the SS MAS NMR data, respectively. The authors thank the EPSRC and CCLRC for the beamtime allocation for the synchrotron component of this work. MPA thanks the Royal Society for provision of a University Research Fellowship, and ZY thanks ORS and EPSRC for financial support.
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