Thermodynamic Studies of Cation–Macrocycle Interactions in Nickel Pincer–Crown Ether Complexes Enable Switchable Ligation

Abstract

The thermochemistry of cation–macrocycle interactions in nickel pincer complexes bearing a hemilabile aza-15-crown-5 or aza-18-crown-6 macrocycle is investigated and applied to cation-controlled reversible ligand binding. Cation–crown interactions were examined in a noncoordinating, low polarity solvent (dichloromethane) and a coordinating, polar solvent (acetonitrile). Structural studies provide solid-state information on cation–crown interactions, whereas binding affinity studies in solution provide quantitative thermodynamic information. The different hemilabile ligand coordination modes have vastly different cation binding affinities, with the tridentate pincer coordination mode binding cations more than 100 000 times more strongly than the tetradentate coordination mode with a crown ether oxygen donating to nickel. Dichloromethane enforces strong cation–crown interactions without disrupting the hemilabile ether ligand, whereas acetonitrile disrupts hemilability by displacing the ethers from the nicke...