Pre-programmed self-assembly of polynuclear clusters

Abstract

This perspective reviews our recent efforts towards the self-assembly of polynuclear clusters with ditopic and tritopic multidentate ligands HL¹ (2-phenyl-4,5-bis{6-(3,5-dimethylpyrazol-1-yl)pyrid-2-yl}-1H-imidazole) and H₂L² (2,6-bis-[5-(2-pyridinyl)-1H-pyrazole-3-yl]pyridine), both of which are planar and rigid molecules. HL¹ was found to be an excellent support for tetranuclear [Fe₄] complexes, [Fe^II₄(L¹)₄](BF₄)₄ ([Fe^II₄]) and [Fe^III₂Fe^II₂(L¹)₄](BF₄)₆ ([Fe^III₂Fe^II₂]). The homovalent system was found to exhibit multistep spin crossover (SCO), while the mixed-valence [Fe^III₂Fe^II₂] complex shows wavelength-dependent tuneable light-induced excited spin state trapping (LIESST). For H₂L², a variety of polynuclear complexes were obtained through complexation with different transition metal ions, allowing the isolation of rings, grids, and helix structures. The rigidity of the ligand, difference in its coordination sites, and affinity for different metal ions dictates its coordination behaviour. In this paper, we summarise these ligand pre-programmed self-assembled clusters and their diverse physical properties.