Macrocyclic chemistry without solvents: Gas phase reaction rates

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Abstract
Both size selection and macrocyclic effects are observed for crown ethers reacting with alkali metal cations in the absence of solvent species. The efficiencies of the radiatively-stabilized 1 :1 1igand:metal complexation reactions scale directly with cation charge density, suggesting that ion-induced rearrangement of the ligand into a favorable binding conformation is more efficient for smaller cations. The 1 :1 reaction efficiencies are also greater for the macrocycles than for the corresponding acyclic ligands, reflecting preorganization of the binding cavity in the case of the crowns. The efficiencies of the subsequent reactions of the 1 :1 complexes to form 2:l 1igand:metal species are dominated by size effects: when the cations are small enough to enter the macrocycle cavity, reaction is very slow, but the reactions occur readily when the cations cannot enter the crown cavity. Acyclic ligands undergo the 2:l complexation reaction much more slowly and less selectively than do the macrocycles, again evidencing macrocyclic effects in the absence of solvation.

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