Tetrahedrally coordinated iron(II) incorporation in the super-sodalite aluminophosphate Fe3Al6(PO4)12•4tren•17H2O (MIL-74)

An iron-aluminum phosphate Fe3Al6(PO4)(12).4tren.17H(2)O (MIL-74) has been hydrothermally synthesized with the tris(2-aminoethyl)amine (tren) as a structure-directing agent (453 K, 6 h, autogenous pressure). Single-crystal X-ray diffraction, magnetic measurements, and Mossbauer and solid-state NMR spectroscopies were carried out to characterize the compound. Its crystal structure is cubic, a = 16.7705(1) Angstrom and may be described as I (4) over bar 3m or P (4) over bar 3n depending on the degree of cationic ordering. The open framework is built up from an enneameric unit (T = Fe, Al) containing five TO4 and four PO4 tetrahedra (one of the P-O bonds is terminal). A central AlO4 tetrahedral unit shares all its corners with four phosphates groups. Two phosphate groups are connected to two other peripheral TO4 units. This results in the formation of a "pseudo" planar building block T5P4 consisting of four square 4-rings. The connection of the T5P4 units generates a three-dimensional framework, which defines a super-sodalite topology. The resulting cavities (diameter of 10 Angstrom) are bound by 12-ring windows in which are located the tren species in interaction with the phosphate groups (mainly terminal P-O bonding) through hydrogen bonds. A cluster of 17 water molecules occupies the center of the super-sodalite cage. The Mossbauer spectroscopy shows that divalent iron cations are incorporated into the framework with an unusual tetrahedral coordination. Despite the existence of the paramagnetic cations Fe2+ in the inorganic network, P-31, Al-27, and H-1 solid state NMR signatures could be obtained with a relatively significant resolution. The spectroscopic methods indicate a certain degree of cationic ordering between Al and Fe occurs on the peripheral T sites of the enneameric building unit (correlated to a lower symmetry, P (4) over bar 3n), although the X-ray diffraction analysis shows a disordering situation (space group I (4) over bar 3m).