Characterisation and properties of the n = 3 and n = 4 members of the Al2[O3PCnH2nPO3](H2O)2F2 framework aluminium alkylenediphosphonate series

A combination of techniques has been used to fully determine the structure and properties of Al2[O3PC4H8PO3](H2O)2F2·2H2O (monoclinic, P21/m, a = 4.96926(6) Å, b = 12.0013(1) Å, c = 10.7440(2) Å, β = 94.058(1)°, Z = 2, Rp = 0.0732, Rwp = 0.0939), the n = 4 member of the framework aluminium alkylenediphosphonate Al2[O3PCnH2nPO3](H2O)2F2 series. The combination of thermogravimetric and thermodiffraction data and the structure determination of the partially dehydrated structure, Al2[O3PC4H8PO3](H2O)2F2·0.17(1)H2O (monoclinic, P21/m, a = 4.9392(3) Å, b = 12.1867(4) Å, c = 10.7830(5) Å, β = 94.767(3)°, Z = 2, Rp = 0.0839, Rwp = 0.1121), indicate that the framework structure is stable to, and remains unchanged, with respect to the loss of the extra-framework water molecules but not the framework water molecules. This behaviour is in stark contrast to the n = 3 member of the Al2[O3PCnH2nPO3](H2O)2F2 series, Al2[O3PC3H6PO3](H2O)2F2·H2O, for which the combination of thermogravimetric and thermodiffraction data indicates that the framework structure contracts during the loss of the extra-framework water molecules before collapsing after loss of the framework water molecules. The formation of the n = 4 member of the series indicates that the design of the alkyl chain can be used to successfully control aspects of the pore volume within this family of materials and comparison of the dehydration behaviour of the n = 3 and n = 4 members of the series indicates that related materials containing channels housing the same number of water molecules exhibit different dehydration behaviour depending on the stacking of the constituent inorganic layers within the material.

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