Structure relationships between bis-monodentate ligands and coordination driven self-assemblies

Self-assembled coordination cages based on organic ligands and a variety of transition metal cations have emerged as promising supramolecular materials thanks to their applicability in fields such as drug delivery, molecular recognition and catalysis. This review focuses on the structural diversity of coordination cages of the MnL2n type where M represents a square-planar coordinated metal cation (mostly Pd(II)) and L, a bridging bis-monodentate ligand. While the geometrical input given by the coordinated metal is thus fixed, the ligands can come in a variety of shapes, lengths and degrees of flexibility. Small changes in the latter features can have tremendous outcome on the silhouette, nuclearity and topology of the resulting coordination cages and a selection of most prominent and instructional examples is given in this review. A focus is set on bent and banana-shaped ligands that assemble into lantern-shaped M2L4 cages, three- or four-membered rings, spheres of different diameters or architectures of more intricate shape and topology such as tetrahedra, knots and catenanes. Furthermore, some examples based on a sophisticated functionalization of the ligands (i.e. photoswitchable behavior) and mixtures of ligands giving heteroleptic structures are illustrated. Recent progress in understanding the basic building principles of such self-assembled nanostructures has allowed to expand the variety of discrete supramolecular shapes through a design approach, increase their functional complexity and bring them closer to application.