- 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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- KINETIC AND EQUILIBRIUM STUDIES OF THE REACTION OF 1,3,5-TRINITROBENZENE WITH CRYPTANDS IN ACETONITRILE
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Kinetic and equilibrium data for the formation of the adduct from 1,3,5-trinitrobenzene (TNB) with cryptands (221, 222, 21, 22 and di-(Ph-CH2)22) in acetonitrile are reported.The influence of structure and pKa of cryptands on the rate constants, activation and reaction parameters are discussed.A reaction mechanism between TNB and different cryptands has been proposed.
- Leska, Boguslawa,Hes, Marzanna,Schroeder, G.
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p. 671 - 680
(2007/10/02)
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- The Complexation of Alkaline Cations by Crown Ethers and Cryptand in Acetone
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Stability constants and thermodynamic values for the complex formation of alkali ions by crown ethers, diaza crown ethers and cryptands have been measured by means of potentiometric and calorimetric titrations in acetone as solvent.The interactions between the ligands and solvent molecules play an important role for the complex formation.Cryptands form the most stable complexes with alkali ions if inclusion complexes are formed.Even in the case that the salts are not completely dissociated in acetone the presence of ion pairs does not influence the calculated values of the stability constants.
- Buschmann, H.-J.,Cleve, E.,Schollmeyer, E.
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p. 569 - 578
(2007/10/02)
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- Thermodynamic and Electrochemical Behaviour of Lanthanide Cryptates in N,N-Dimethylformamide
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The stability of 222 (L1), 221 (L2), 211 (L3) cryptates of samarium(III), europium(III), and ytterbium(III) and the corresponding complexes with the related monocycle, 22 (L4) have been determined in N,N-dimethylformamide by potentiometry using a competitive method with auxiliary alkali cations.Their electrochemical behaviour has also been investigated in the same solvent by polarography and cyclic voltammetry.The results clearly show the low stability of the trivalent cryptates.The electrochemical study of Ln(3+) ions with increasing amounts of ligand shows two different behaviours: (i) with L3, Ln(3+) ions are complexed as shown by the decrease of the reduction waves of Ln(3+) but no stabilization of Ln(2+) is observed; (ii) with L1, L2, and L4 the height of the reduction wave of Ln(3+) is not affected by the ligand concentration and the reduction occurs according to the scheme: Ln(3+) + e(1-) Ln(2+) + L -> (2+).The differences δ between the redox potentials of the free and the complexed lanthanide(III) cations show an important stabilization of the divalent cryptates.Their stability constants have been calculated from the values of δ and the stability constants of trivalent complexes.While no particular ligand or cation selectivity is observed for the trivalent cryptates, this is not so for the reduced lanthanides which form the strongest complexes with L1.The results are interpreted by size and solvation considerations.
- Marolleau, Isabelle,Gisselbrecht, Jean-Paul,Gross, Maurice,Arnaud-Neu, Francoise,Schwing-Weill, Marie-Jose
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p. 367 - 370
(2007/10/02)
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- Solvent Dependence of Kinetics and Equilibria of Thallium(I) Cryptates in relation to the Free Energies of Solvation of Thallium(I)
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Stability constant and dissociation rate constants of thallium(I) cryptates have been measured in several solvents at 25 deg C.The Tl+ cryptates are more stable and less sensitive to ligand cavity size than the corresponding complexes of the al
- Cox, Brian G.,Stroka, Jadwiga,Schneider, Irmgard,Schneider, Hermann
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p. 187 - 198
(2007/10/02)
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- Proton NMR study of the dissociation of the lanthanum cryptate of 4,7,13,16,21-pentaoxa-1,10-diazabicyclo[8.8.5]tricosane
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We have used proton NMR spectroscopy to study the rate of dissociation of La[2.2.1](NO3)3 in D2O solutions with I = 1.00 M (NaCl). The dissociation obeys pseudo-first-order kinetics over the pH range from 1 to 13. The dissociation is independent of acid concentration but has a first-power dependence on hydroxide concentration. The variation in the experimental rate constant with hydroxide concentration has the form kobs = k1(OH-) + k0, with k1 = (1.05 ± 0.01) × 10-2 M-1 s-1 and k0 = (1.49 ± 0.05) × 10-5 s-1. Changes in the NMR spectra that occur when the pH is increased above pH 9 suggest the formation of hydrolyzed La[2.2.1](OH)x species at high pH.
- Torres, Richard A.,Baisden, Patricia A.
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p. 2807 - 2810
(2008/10/08)
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- Complex Formation of Alkaline-Earth Cations with Crown Ethers and Cryptands in Methanol Solutions
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The complexation of alkaline-earth cations by different crown ethers, azacrown ethers, and cryptands has been studied in methanol solutions by means of calorimetric and potentiometric titratios.The smallest monocyclic ligands examined form 2:1 complexes (ratio of ligand to cation) with cations which are too large to fit into the ligand cavity.With the smallest cryptand, only Sr2+ and Ba2+ ions are able to form exclusive complexes.In the case of the reaction of cryptand (211) with Ca2+, a separate estimation of stability constants for the formation of exclusive and inclusive complexes was possible for the first time.Higher values for stability constants are found for the reaction of alkaline-earth cations with cryptands compared to the reaction with alkali ions.This increase is only caused by favorable entropic contributions.
- Buschmann, Hans-Jurgen
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p. 453 - 462
(2007/10/02)
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- Synthesis of Simple Cryptands under High Pressure
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Simple N,N'-dimethyl diaza-crown ethers react with bis(2-iodoethyl)-ether (2a) and 1,2-bis(2-iodoethoxy)ethane (2b) under high pressure (10 kbar) to give bis-quaternary salts which are demethylated in good yield by triphenylphosphine in boiling dimethylformamide yielding simple cryptands.
- Pietraszkiewicz, Marek,Salanski, Piotr,Jurczak, Janusz
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p. 433 - 436
(2007/10/02)
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- Synthesis of +(1.1.1)>X- Cryptates Assisted by Intramolecular Hydrogen Bonding
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Condensation of diazacoronand and diethylene glycol bis(methanesulfonate)as well as with its alkylsubstituted derivatives in the presence of 1 molar equiv of BuLi or KH gives the corresponding +(1.1.1)>X- cryptates in 31-43percent yields.Experimental results indicate that in the intermediate monosubstituted diazacoronand, the NH hydrogen plays a templating role, thus favoring the bicyclic ring closure by intramolecular hydrogen bonding.Similar base-promoted condensations of and diazacoronands with triethylene glycol bis(methanesulfonates) afford and cryptates in 20-35percent yields.
- Anelli, Pier Lucio,Montanari, Fernando,Quici, Silvio
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p. 3453 - 3457
(2007/10/02)
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- Solvent Dependence of the Kinetics of Formation and Dissociation of Cryptate Complexes
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The rates of dissociation of a variety of alkali metal cations and Ca2+ cryptates have been measured in several solvents.These have been combined with measured stability constants to give the corresponding formation rates.The dissociation rates are very sensitive to solvent variation, covering a range of more than 9 orders of magnitude.Except for (2,1,1) cryptates, formation rates are all within the range 106 - 109 M-1 s-1.Changes in stability constants, whether from a change in the cation, ligand, or solvent, are largely reflected in changes in dissociation rates.The properties of the transition state, particularly with the respect to solvent variation, most closely resemble those of the reactants, suggesting that the transition state lies close to the reactants.The dissociation rates increase sharply with increasing donor number of the solvent, whereas the formation rates decrease but are much less sensitive to solvent variation.On the basis of these correlations, formation rates in water are much lower than expected and dissociation rates much higher than expected.It is suggested that this is due to the H-bonded interactions between water and the electronegative atoms (O and N) of the ligands.
- Cox, B. G.,Garcia-Rosas, J.,Schneider, H.
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p. 1054 - 1059
(2007/10/02)
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- Monocyclic macrocyclic compounds and complexes thereof
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Novel monocyclic macrocyclic compounds having nitrogen bridgehead atoms and having in the hydrocarbon bridging chains at least two additional hetero atoms selected from the group consisting of sulfur, oxygen, and nitrogen, when admixed with a compatible cation-donor compound form stable cation-containing macrocyclic complexes which, in turn, can be conveniently dissociated by addition of acid or a quaternizing agent. The novel macrocyclics are valuable for use in the same way and for the same purposes as chelating agents.
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