Coordination polymers with exo-coordinated unsubstituted macrocycles: Structure, properties, future perspectives and design guidelines

This article overviews the structures and properties of coordination polymers (CPs) with unsubstituted exo-macrocyclic ligands formed by coordinating inner macrocyclic ring donor atom(s) to the metal cation. The unique properties and applications of CPs are not solely dependent on their chemical composition, which includes metal ions, ligands, and anions. They are also influenced by dimensionality and motifs that can be creatively crafted to a certain extent. Due to the cyclic nature, designing macrocyclic ligands to bind specific metal cations, anions, or neutral species is possible if endo-cyclic coordination occurs. The exo-coordination of inner macrocyclic ring donor atoms to a metal ion(s) is a less common coordination mode that favors the formation of polymeric species. The primary focus of this article pertains to the correlation between designable macrocycle characteristics and the structural attributes of emerged CPs. A comprehensive review of crystal structures and structural motifs was done to set specific guidelines for designing macrocyclic ligands that tend to form exo-coordinated CPs. The coordination properties of ligands were analyzed concerning the donor atom set, cavity size, linkers between donor atoms, and anion influence to discuss and set applicable guidelines for designing macrocyclic CPs. The application of the investigated CPs as optical materials and biologically active substances was mentioned, and unfortunately, the broader application of these compounds is scarce. Therefore, future perspectives on these materials, such as gas sorption, conductivity, catalysis, and energetic materials, were conferred.