Hexamethylenetetramine: An old new building block for design of coordination polymers

The design of new coordination polymers is nowadays a challenging research topic that attracts increasing interest due to the unique structural and functional properties of such metal-organic materials. In contrast to the recognized use of some N-donor ligands for the construction of coordination polymers, the application of hexamethylenetetramine (hmt) as a simple, commercially available, water-soluble and highly versatile cagelike building block has so far been explored to a lesser extent, although a considerable number of hmt-driven metal-organic networks have been reported in the last few years. Given the high potential of hmt for future developments of this research field, the present review summarizes the main structural and topological types of coordination polymers bearing hmt. These compounds feature a high diversity of topologies that include linear, zigzag, double, triple and quadruple 1D chains, rectangular grids, flat and undulating 2D layers, as well as layer-pillared, octahedral, zeolite-like, honeycomb-like and other complex 3D nets, in which hmt acts as a linker or spacer, pillar or connector, stabilizer and/or supporting ligand. The most common synthetic strategies are reviewed, showing that a diversity of metal-organic networks can be generated by facile self-assembly routes in aqueous medium and using rather simple chemicals. The main types of auxiliary ligands necessary for the construction of hmt-driven coordination networks are also identified. The additional structural features such as the formation of supramolecular networks and water clusters are described, and the selected properties and potential applications of hmt-containing coordination polymers as porous, magnetic or photoluminescent materials are highlighted.