Macrocyclic chemistry in the gas phase: intrinsic cation affinities and complexation rates for alkali metal cation complexes of crown ethers and glymes

Article abstract of DOI:10.1021/ja00066a045

Reactions of 12-crown-4, 15-crown-5, 18-crown-6, and 21-crown-7, as well as the acyclic analogs triglyme, tetraglyme, and penta(ethylene glycol), with Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺, are observed and characterized using Fourier transform ion cyclotron resonance mass spectrometry (FTICR/MS) and tandem quadrupole mass spectrometry in the gas phase to obtain information on intrinsic host-guest interactions in the absence of the complicating effects of solvation. Radiatively stabilized attachment of the cations to the ligands is a rapid process, with rates in some cases a factor of 2 or more times the Langevin collision rate. The attachment efficiencies increase linearly with cation charge density, suggesting that attachment involves charge-induced rearrangement of the ligands to adopt favorable binding conformations. Attachment is more efficient, and more strongly dependent on charge density, for the cyclic ligands than for their acyclic counterparts. Metal-ligand undergo reaction with a second ligand to form 1:2 metal-ligand complexes, or "sandwiches". The efficiencies of crown sandwich formation are strongly dependent on the ratio of cation radius to binding cavity radius; when the ratio is than one, the efficiencies are too low to measure, but they become measurable at a ratio of 1:1 and increase by about 4 orders of magnitude as the ratio incrrases to about 1.25:1, At higher ratio values, efficiencies fall off slowly, probably due to decreasing cation density. The relative cation affinities of the various ligands are compared both collision-induced dissociation "kinetic" methods, with the tandem quadrupole, and using "bracketing" cation reactions in the FTICR. The tandem quadrupole results are in some cases dependent on the means of producing the 1;2 metal-ligand complexes, and in some cases they do not agree with the FTICR results. The two methods are compared and reasons for the discrepancies are discussed. We favor the FTICR results, which indicate that proton and alkali cation affinities increase with an increase in the number of oxygen donor atoms in the crowns. Equilibria observed in metal exchange reactions between 18-crown-6 and 21-crown-7 were found to always lie on the side of the cation bound to the larger ligand, but K⁺ has the smallest equilibrium constant of any of the alkali metals, reflecting the excellent size match between K⁺ and 18-crown-6.