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Abstract Spacious M₄L₆tetrahedra can act as catalytic inhibitors for base‐mediated reactions. Upon adding only 5 % of a self‐assembled Fe₄L₆cage complex, the conversion of the conjugate addition between ethylcyanoacetate and β‐nitrostyrene catalyzed by proton sponge can be reduced from 83 % after 75 mins at ambient temperature to <1 % under identical conditions. The mechanism of the catalytic inhibition is unusual: the octacationic Fe₄L₆cage increases the acidity of exogenous water in the acetonitrile reaction solvent by favorably binding the conjugate acid of the basic catalyst. The inhibition only occurs for Fe₄L₆hosts with spacious internal cavities: minimal inhibition is seen with smaller tetrahedra or Fe₂L₃helicates. The surprising tendency of the cationic cage to preferentially bind protonated, cationic ammonium guests is quantified via the comprehensive modeling of spectrophotometric titration datasets. 

Abstract Appending functional groups to the exterior of Zn₄L₄self‐assembled cages allows gated control of anion binding. While the unfunctionalized cages contain aryl groups in the ligand that can freely rotate, attaching inert functional groups creates a “doorstop”, preventing rotation and slowing the guest exchange rate, even though the interiors of the host cavities are identically structured. The effects on anion exchange are subtle and depend on multiple factors, including anion size, the nature of the leaving anion, and the electron‐withdrawing ability and steric bulk of the pendant groups. Multiple exchange mechanisms occur, and the nature of the external groups controls associative and dissociative exchange processes: these bulky groups affect both anion egress and ingress, introducing an extra layer of selectivity to the exchange. Small changes can have large effects: affinities for anions as similar as PF₆⁻and SbF₆⁻can vary by as much as 400‐fold between identically sized cavities.