| Abstract
| - An iron−aluminum phosphate Fe3Al6(PO4)12·4tren·17H2O (MIL-74) has been hydrothermally synthesized with the tris(2-aminoethyl)amine (tren) as a structure-directing agent(453 K, 36 h, autogenous pressure). Single-crystal X-ray diffraction, magnetic measurements,and Mössbauer and solid-state NMR spectroscopies were carried out to characterize thecompound. Its crystal structure is cubic, a = 16.7705(1) Å and may be described as I4̄3m orP4̄3n depending on the degree of cationic ordering. The open framework is built up from anenneameric unit (T = Fe, Al) containing five TO4 and four PO4 tetrahedra (one of the P−Obonds is terminal). A central AlO4 tetrahedral unit shares all its corners with four phosphatesgroups. Two phosphate groups are connected to two other peripheral TO4 units. This resultsin 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 asuper-sodalite topology. The resulting cavities (diameter of 10 Å) are bound by 12-ringwindows 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 moleculesoccupies the center of the super-sodalite cage. The Mössbauer spectroscopy shows thatdivalent iron cations are incorporated into the framework with an unusual tetrahedralcoordination. Despite the existence of the paramagnetic cations Fe2+ in the inorganic network,31P, 27Al, and 1H solid state NMR signatures could be obtained with a relatively significantresolution. The spectroscopic methods indicate a certain degree of cationic ordering betweenAl and Fe occurs on the peripheral T sites of the enneameric building unit (correlated to alower symmetry, P4̄3n), although the X-ray diffraction analysis shows a disordering situation(space group I4̄3m).
- A mixed iron-aluminophosphate Fe3Al6(PO4)12·4tren·17H2O (MIL-74), was hydrothermally synthesized in the presence of tren molecule. The Fe(II) cations are incorporated into the super-sodalite AlPO4 framework. This unique feature was structurally characterized by single-crystal XRD analysis, Mössbauer spectroscopy, and 31P, 27Al, and 1H solid-state NMR.
|
