Lanthanide phosphonate coordination polymers

In recent years, coordination polymers (CPs) have emerged as versatile scaffolding materials built from various metal ions and organic ligands, and these materials are highly investigated for their use in numerous applications. In particular, coordination polymers including lanthanide ions (Ln) are very promising because these ions transfer some interesting luminescence features to the frameworks, such as sharp emission, long lifetimes, large Stokes' shift and high color purity. Although much research focusses on carboxylate-type ligands for the preparation of coordination polymers owing to their ability to form porous structures, organophosphonates appear to be promising ligands as well. In fact they display higher coordination versatility and they are able to bridge a large number of metal cations enhancing the metal density and allowing for the introduction of multiple functionalities in the heterometallic structures. Moreover, thanks to the proton exchange properties of the phosphonate groups, these materials can display remarkable proton conduction properties. In the current review, lanthanide phosphonates assembling 1-, 2- and 3-dimensional (D) coordination polymers are described, and their value in different applications will be outlined. From the literature survey, it emerges that 1D structures with an extended hydrogen-bonding network were the most promising materials for proton conduction, while 2D structures were mainly investigated for their luminescent and magnetic properties. In many examples the luminescence of 3D lanthanide coordination polymers could be changed by incorporating small molecules or metal ions, which opens up new possibilities for their use as straightforward sensor materials.