Synthesis, Ion Recognition Ability, and Metal-Assisted Aggregation Behavior of Dinuclear Metallohosts Having a Bis(Saloph) Macrocyclic Ligand

Macrocyclic molecule <b>1</b> that has two saloph coordination sites was designed and synthesized. The macrocycle <b>1</b> was easily converted into the corresponding metallohosts <b>2</b> and <b>3</b> by the reaction with nickel­(II) and palladium­(II), respectively. As expected from the molecular structure of these metallohosts having an 18-crown-6-like cavity, the nickel­(II) metallohost <b>2</b> showed excellent binding affinity toward Na⁺, Ca²⁺, and Sr²⁺ to give 1:1 host–guest complexes. Preorganization effect due to the extremely rigid metal-containing macrocycle was suggested to be a major factor for the strong binding. Larger cations such as K⁺, Rb⁺, Cs⁺, and Ba²⁺ gave higher aggregated host–guest complexes such as <b>2</b>₂M, <b>2</b>₃M₂, and <b>2</b>₄M₃. Density functional theory calculations revealed that smaller metal ions do not occupy the center of each macrocycle in the sandwich structures <b>2</b>₂M, while larger Cs⁺ simultaneously interacts with all the 12 oxygen donor atoms. On the basis of the interaction energy calculations, the preference for <b>2</b>·Na over <b>2</b>₂Na can be explained by destabilization of <b>2</b>₂Na due to the elongated Na–O bonds and repulsive three-body interactions. When the ionic radius of the guest ion increases (K⁺, Rb⁺, Cs⁺), this destabilization becomes less significant and the formation of sandwich complexes <b>2</b>₂M is favored. Such aggregation would significantly affect the physical and chemical properties of the metal complexes due to the interplane interactions between the metal centers