Order-disorder in the super-sodalite Zn3Al6(PO4)12, 4tren, 17H2O (MIL-74):: A combined XRD-NMR assessment

A new mixed zinc-aluminum phosphate Zn3Al6(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, 36 h, autogenous pressure). The solid was characterized by a nonclassical method combining single-crystal X-ray diffraction and several solid-state NMR experiments, RFDR, C7 double quantum (P-31), and 3QMAS (Al-27). Its crystal structure is cubic, a = 16.7942(1) Angstrom, but the choice of the space group does not follow usual routes of structure determination, due to some "disorder" between Zn and Al. It can be assigned as well to I-43m or to P-43n. The open-framework is built up from an enneameric unit (T = Zn, Al) containing five TO4 and four PO4 tetrahedra (one of the P-O bonds is terminal). A central TO4 tetrahedral unit shares all of the corners with four phosphates groups. Two phosphate groups are connected to two other peripheral TO4 units. It 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-soclalite 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 bonds) through hydrogen bonds. A cluster of 17 water molecules occupies the center of the super-soclalite cage. The cationic (Zn, Al) occupancy is discussed for this specific topology.