Mixed-Metal d-f Phosphonate Frameworks - Photoluminescence and Magnetic Properties

The synthesis and structural characterisation of a novel family of highly symmetric (rhombohedral crystal system) frameworks, incorporating d- and f-block elements formulated as [LnFe(III)Fe(6)(II)(Hpmida)(6)]center dot nH(2)O [Ln = Nd3+ (1), Gd3+ (2), Tb3+ (3), Er3+ (4)] is reported. This is the second example of a family of porous mixed d-f MOFs containing high-spin Fe2+ centres. These materials were prepared from an eutectic mixture (1:1 molar ratio, choline chloride/malonic acid) with a small amount of added water and isolated directly in large amounts as single crystalline phases. The structural details were revealed by single-crystal X-ray diffraction, variable-temperature powder X-ray diffraction, thermogravimetry, vibrational and Mossbauer spectroscopy and electron microscopy (SEM and EDS). Networks are assembled by unusual [Fe-2(Hpmida)(2)](2-) dimeric units, which connect to each other and to the lanthanide centres to form a 3D porous framework. The empty voids of this framework accommodate charge-balancing Fe3+ cations and uncoordinated water molecules, which are readily released without structural collapse at temperatures below 250 degrees C. The photoluminescence and magnetic properties of the compounds were studied at ambient and low temperature.