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 nickel center and supports weaker cation-crown interactions. In dichloromethane, lithium binding provides at least 7 kcalmol(-1) of stabilization to assist in ligand binding, and the extent of stabilization can be tuned by the choice of cation. The newfound thermodynamic insight guided the development of in situ switchable ligand binding and release at nickel using cations. The nickel complex binds pentafluorophenylnitrile only upon addition of Li+ salts in dichloromethane, and the nitrile ligand is readily released upon sequestration of the Li+ with 12-crown-4.