Post-Metalation Modification of a Macrocyclic Dicobalt(III) Metallohost by Site-Selective Ligand Exchange for Guest...

We propose post-metalation modification as a useful strategy to control the guest recognition behavior of a metal-containing macrocyclic host. This is based on the ligand exchange of the axial ligands of a cobalt(III) dinuclear macrocyclic host, [LCo₂ X₄ ]²⁺ (X=axial amine ligand). Four piperidine ligands in [LCo₂ (pip)₄ ]²⁺ (pip=piperidine) were site-selectively replaced with primary amines. The competitive experiments revealed that the order of the affinity toward the cobalt centers in [LCo₂ X₄ ]²⁺ is primary amine > secondary amine > tertiary amine and that the piperidine-coordinating complex, [LCo₂ (pip)₄ ]²⁺ , was reasonably reactive among the isolable complexes. Indeed, two piperidine ligands at the diagonal positions in [LCo₂ (pip)₄ ]²⁺ were site-selectively replaced with pyridine or acetate ion. The replacement of piperidine with acetate ion significantly enhanced the recognition ability towards Na⁺ .