Fused aza-heterocyclic ligands: expanding the MOF chemist's toolbox

Abstract

Azolate-containing ligands have played an important role in the development of water-stable metal-organic frameworks (MOFs) and related materials due to their particularly strong bonding with earth-abundant first row transition metal ions. Fused ring analogues of pyrazole, imidazole and triazole offer untapped potential to expand the scope of these systems as compact, robust anionic bridging ligands with much greater control over bridging geometries, pendant functionalities and electronic properties. The current design approaches in these systems have been directed by two separate methodologies. In one direction, the simple azoles imidazole, pyrazole and 1,2,4-triazole have been built up by adding fused ring functionality to increase complexity and allow new linker geometries. Meanwhile, in the search for biologically-compatible MOFs, purines such as adenine and hypoxanthine have been explored as linkers and their backbone functionalities optimised to prioritise stability and bridging geometry, leading independently to MOF linkers with similar key features. This highlight article surveys the convergence of these two approaches which both point to the benefits of fused 5-6 ring systems for their electronic and structural properties, and considers the key features that the ideal compact and stable heterocyclic linkers in functional metal-organic frameworks might contain.