- The macrobicyclic cryptate effect in the gas phase
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The alkali cation (Li+, Na+, K+, Rb+, and Cs+) binding properties of cryptands [2.1.1], [2.2.1], and [2.2.2] were investigated under solvent-free, gas-phase conditions using Fourier transform ion cyclotron resonance mass spectrometry. The alkali cations serve as size probes for the cryptand cavities. All three cryptands readily form 1:1 alkali cation complexes. Ligand-metal (2:1) complexes of [2.1.1] with K+, Rb+, and Cs+, and of [2.2.1] with Rb+ and Cs+ were observed, but no 2:1 complexes of [2.2.2] were seen, consistent with formation of 'inclusive' rather than 'exclusive' complexes when the binding cavity of the ligand is large enough to accommodate the metal cation. Kinetics for 2:1 ligand-metal complexation, as well as molecular mechanics calculations and cation transfer equilibrium constant measurements, lead to estimates of the radii of the cation binding cavities of the cryptands under gas-phase conditions: [2.1.1], 1.25 ?; [2.2.1], 1.50 ?; [2.2.2], 1.65 ?. Cation transfer equilibrium studies comparing cryptands with crown ethers having the same number of donor atoms reveal that the cryptands have higher affinities than crowns for cations small enough to enter the cavity of the cryptand, while the crowns have the higher affinity for cations too large to enter the cryptand cavity. The results are interpreted in terms of the macrobicyclic cryptate effect: for cations small enough to fit inside the cryptand, the three-dimensional preorganization of the ligand leads to stronger binding than is possible for a floppier, pseudo-two-dimensional crown ether. The loss of binding by one ether oxygen which occurs as metal size increases for a given cryptand is worth approximately 25 kJ mol-1, and accounts for the higher cation affinities of the crowns for the larger metals. The Li+ affinity of 1,10-diaza-18-crown-6 is ~1 kJ mol-1 higher than that of 18-crown-6, while the latter has lower affinity than the former for all of the larger alkali cations (about 7 kJ mol-1 lower for Na+, and about 15 kJ mol-1 lower for K+, Rb+, and Cs+). The equilibrium measurements also allow the determination of relative free energies of cation binding for a number of crown ethers and cryptands. Molecular mechanics modeling with the AMBER force field is generally consistent with the experiments.
- Chen, Qizhu,Cannell, Kevin,Nicoll, Jeremy,Dearden, David V.
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p. 6335 - 6344
(2007/10/03)
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- Macrocyclic chemistry in the gas phase: intrinsic cation affinities and complexation rates for alkali metal cation complexes of crown ethers and glymes
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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.
- Chu, In-Hou,Zhang, Hong,Dearden, David V.
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p. 5736 - 5744
(2007/10/02)
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