Post‐Metalation Modification of a Macrocyclic Dicobalt(III) Metallohost by Site‐Selective Ligand Exchange for Guest Recognition Control

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, [LCo 2 X 4 ] 2+ (X=axial amine ligand). Four piperidine ligands in [LCo 2 (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 [LCo 2 X 4 ] 2+ is primary amine > secondary amine > tertiary amine and that the piperidine‐coordinating complex, [LCo 2 (pip) 4 ] 2+ , was reasonably reactive among the isolable complexes. Indeed, two piperidine ligands at the diagonal positions in [LCo 2 (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 + .