Tuning of Guest Uptake/Release Kinetics of a Dinuclear Cobalt(III) Metallohost by Auxiliary Amine Ligands

A series of dicobalt(III) metallohosts, [LCo₂A₄](OTf)₂, which have different auxiliary amine ligands A (=EtNH₂, PhCH₂NH₂, PhC₂H₄NH₂, PhC₃H₆NH₂), were synthesized in order to investigate the tuning of the guest uptake/release kinetics without significantly affecting the guest binding affinity. The metallohosts were characterized by spectroscopic methods and X-ray crystallography. All the metallohosts showed a Na⁺ selectivity among the investigated alkali meal ions with a clear selectivity trend of Na⁺>K⁺>Rb⁺>Cs⁺. The difference in the auxiliary amine ligands A had a more significant influence on the guest uptake rates than on the release rates. This can be mainly explained by the structural features of the guest-free form, [LCo₂A₄]²⁺, in which the phenyl C-H groups in the auxiliary amine ligands A interacted with the oxygen atoms of the O₆ binding cavity via the C-H⋅⋅⋅O interactions.