FULL PAPER DOI: 10.1002/ejic.201001145 Mixed-Metal d-f Phosphonate Frameworks – Photoluminescence and Magnetic Properties João Rocha,* [a] Filipe A. Almeida Paz,* [a] Fa-Nian Shi, [a] Duarte Ananias, [a] Nuno J. O. Silva, [b] Luís D. Carlos, [b] and Tito Trindade [a] Keywords: Metal–organic frameworks / Lanthanides / Iron / Photoluminescence / Magnetic properties The synthesis and structural characterisation of a novel family of highly symmetric (rhombohedral crystal system) frameworks, incorporating d- and f-block elements formu- lated as [LnFe III Fe II 6 (Hpmida) 6 ]·nH 2 O [Ln = Nd 3+ (1), Gd 3+ (2), Tb 3+ (3), Er 3+ (4)] is reported. This is the second example of a family of porous mixed d-f MOFs containing high-spin Fe 2+ 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- Introduction Metal–organic frameworks (MOFs) are one of the most active research areas in chemistry and materials science. [1] The concept behind their assembly lies in the use of organic molecules (linkers) possessing two or more coordinating- capable functional groups and metal centres (nodes) to pro- mote the formation of highly organised polymeric com- pounds, typically forming 3D networks. Because the combi- nation of linkers and nodes is, in essence, infinite the ex- ponential growth of the field was initially motivated by the intriguing architectures obtained. [2] More recently, with the advent of novel frameworks exhibiting interesting proper- ties with real potential for application, [3] research is increas- ingly focused on the design of functional materials for in- dustry, particularly for gas adsorption, [4] heterogeneous ca- talysis, [5] fabrication of thin films or membranes, [6] sensors (e.g., ethanol and pH), [7] photoluminescence or magnet- ism [8] and MRI contrast agents [9] among others. Traditionally, the functional groups of the organic linkers were mainly pyridine rings and carboxylate groups but, more recently, other moieties such as imidazole, sulfonates [a] Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal Fax: +351-234-370084 E-mail: rocha@ua.pt filipe.paz@ua.pt [b] Department of Physics, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201001145. Eur. J. Inorg. Chem. 2011, 2035–2044 © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2035 temperature powder X-ray diffraction, thermogravimetry, vi- brational and Mössbauer spectroscopy and electron micro- scopy (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 Fe 3+ cations and uncoordinated water molecules, which are readily released without structural collapse at tem- peratures below 250 °C. The photoluminescence and mag- netic properties of the compounds were studied at ambient and low temperature. and phosphonate groups have been used. In particular, tetra- hedral units are interesting anchoring sites to many types of metal centres, leading to MOFs with unusual architec- tures and potentially useful applications. [10] Indeed, three tetrahedrally arranged oxygen atoms emulate zeolite build- ing blocks, allowing straightforward coordination to transi- tion and lanthanide metal centres, and the presence of a carbon atom permits the inclusion of almost any organic group. Moreover, phosphonate groups exhibit high chemi- cal and thermal robustness and are therefore a viable route to the formation of highly stable frameworks. Many exam- ples of remarkable phosphonate-based frameworks have been reported: the Ni-STA-12 framework discovered by Miller and collaborators [11] exhibits permanent porosity and a remarkable uptake of CO 2 over methane at ambient temperature (ca. ten times larger); Evans and collabora- tors [12] prepared a chiral bisphosphonate molecule for the design of homochiral frameworks, which they used as heterogeneous catalysts in various organic transformations, such as cyanosilylation of aldehydes and ring opening of meso-carboxylic anhydrides; Plabst and Bein [10b] discovered a functional porous framework (based on a previously de- signed tetraphosphonic acid) for which the charge-balanc- ing Na + cations could be exchanged only by monovalent cations, excluding di- and trivalent ones; the group of Yue [10e] developed a new organic ligand based on S-proline for the construction of homochiral frameworks, which exhi- bit selective sorption abilities of nitrogen, water and meth- anol, over ethanol.