Synthetic approaches for the incorporation of free amine functionalities in porous coordination polymers for enhanced CO₂ sorption

Porous coordination polymers, or metal–organic frameworks (MOFs), provide a unique materials platform due to their regular structure that allows for almost unlimited tunability in terms of pore size, shape, dimensionality and chemical functionalisation. The ability of this class of material to act as agents for the selective capture or separation of small molecules has driven much of the research in the field over the past two decades. One of the major driving forces in recent years has been the ability of porous coordination polymers to act as selective hosts for carbon dioxide, with obvious potential environmental implications for successful materials. Efforts to enhance the selectivity for CO₂ over other gases (particularly those present in exhaust flues) have explored chemical functionalisation of the interior pore space using amines, to give materials that to some extent mimic the standard solution-based processing methods of flue gas that are currently employed. This review examines the three major synthetic routes that are used in order to affect this chemical modification of porous frameworks; (i) post-synthetic grafting of amines to coordinatively unsaturated metal sites, (ii) incorporation of ligands that contain terminal amine groups, and (iii) incorporation of ligands that contain amine embedded within the core of the ligand. The relative merits and shortcomings of these different synthetic approaches are discussed alongside examples of the materials generated using each method.