Topological variety and self-sorting in homo- and heteroleptic PdnL2n metallo-supramolecular assemblies

Abstract

A plethora of nanoscale Pd_nL_2n-type architectures has been synthesized through the coordination-driven self-assembly of Pd(II) ions and organic bis-monodentate bridging ligands. While initially, the focus was on homoleptic structures, comprising one type of ligand per assembly, the field has recently shifted towards reducing symmetry in heteroleptic multicomponent assemblies, containing two or more distinct ligands in defined positions. In parallel, the incorporation of functional moieties such as binding and catalytic sites, photoswitches and redox units has seen a steep development. While empirical data have been gathered on the relationship between the ligand structure and assembly outcome for a limited number of cases, confidently forecasting the result of reacting a given ligand with Pd(II) cations often still remains challenging and has been mastered only for the simplest systems. Additionally, new Pd_nL_2n topologies – along with subtle factors driving their formation (such as counter anion or guest templation and solvation effects) – are discovered continuously. For designing metallosupramolecular assemblies for application, it is of pivotal importance to increase predictabilty and gain control over assembly topology, as the structure and properties are often closely connected. To raise awareness for the problem's complexity, we commence this review by exploring the surprising breadth of topological diversity among homoleptic Pd_nL_2n (n = 2–8) architectures that has so far been found experimentally. We next discuss strategies for increasing the structural complexity even further through the non-statistical self-assembly of heteroleptic cages, the orientational self-sorting of asymmetric ligands, and chiral self-sorting effects. Special emphasis will be placed on factors governing the particular self-assembly outcome as well as on rationalization approaches based on computations or geometrical considerations.