Dynamic functions of bis- and tris(saloph) cobalt(III) structures based on axial coordination

This review article focuses on the functionalization and dynamic functional switching of low-spin d⁶ cobalt(III) complexes derived from various oligo(saloph) structures, such as bis(saloph) macrocycles and tris(saloph) cage complexes (H₂saloph = N,N'-disalicylidene-o-phenylenediamine). The bis(saloph) dicobalt(III) complexes with methylene- or phenylene-bridged ligands exhibit reversible redox-driven structural switching, in which the axial functional ligands dissociate and reassociate in response to the Co^III/Co^II interconversion. The ether-bridged macrocyclic bis(saloph) cobalt(III) complexes show excellent cation binding affinity at the central O₆ binding site, which is significantly influenced by the nature of the axial ligands at the cobalt centers. In particular, an anion-capped structure leads to the formation of a unique metastable host-guest complex, enabling stimuli-responsive behavior upon external triggering. Post-metalation ligand exchange with anionic ligands and bridging diamine ligands provides a versatile strategy for structural and functional tuning of these macrocyclic hosts. In some complexes, the ligand exchange reactivity and the guest binding affinity enhance each other. A helical tris(saloph) cobalt(III) cryptand exhibits dynamic P/M chirality interconversion via the axial ligand exchange involving achiral or chiral amines, allowing precise control over the chirality inversion rates and enabling a unique transient chirality inversion during racemization. Furthermore, closed-cage metallocryptands bearing bridging diamine ligands effectively suppress the guest uptake/release kinetics. Thus, the introduction, removal, and exchange of axial ligands (X) in the [Co(saloph)X₂]⁺-type units have been successfully employed for the functionalization and dynamic switching of metallohosts and metallo-supramolecular structures.