Chelation-Driven Self-Assembly of Luminescent Magnesium Coordination Cages

The construction of discrete coordination cages using s-block metal ions is challenging due to the weak and electrostatic nature of their coordination bonds, which can lead to the formation of mixtures of products that include intractable coordination polymers, rather than well-defined structures. The alkali and alkaline earth elements are also weaker templates for imines, as they are more oxophilic than transition metals. Here we describe a strategy to overcome these challenges by employing a chelating tris(pyridyl)aldehyde subcomponent to define the vertices of magnesium-templated cages. This subcomponent constrains the flexible coordination sphere of magnesium, enabling the assembly of three distinct coordination cage structure types: edge-bridged and face-capped tetrahedra, and a heteroleptic trigonal prism. These hosts displayed diverse binding properties for a range of guests. The two magnesium-based tetrahedral cages also luminesce upon illumination, a feature absent in their transition-metal counterparts. Our work thus provides a general strategy for accessing discrete s-block coordination cages and introduces magnesium coordination cages as a new class of luminescent supramolecular materials.