Cation complexation with crown ethers

Interactions of alkali metals cations (complexed with crown-ethers and kryptands) with aromatic Ji-systems 00EJO2967. 

RCO , RO, BH , Mno , Cio ) which is transferred into the apolar phase (benzene, toluene, pentane, CH2CI2, CHClj, CQ ) by the help of a catalytic cation C (R4N, R P", R4As, metal cation complexed with crown ethers or cryptates) suitable for forming an ion pair, [C" A ]°, in the apolar solvent. 

Binding interactions available to these ions are essentially electrostatic in nature, namely, cation-anion attractions responsible for cation-pairing to counter-anions, and ion-dipole interactions that provide the basis for the well-known complexation with crown ethers. 

Fig. 16 Rhenium(I) tricarbonyl diimine complexes with crown ether pendants as metal cation...

As far as cation complexation by crown ethers is concerned, N-(dithiocarbamato)-2-aminomethyl-15(18)-crown-5(6) ethers, obtained by reaction of the appropriate amino precursor with CS2 and alkali metal hydroxide <2006BCC473>, form highly stable cationic "mTc-nitrido complexes, which have been evaluated as radiopharmaceuticals for heart imaging <2006NMB419, 2007JCD1183>. 

Interest has been restricted this year to studies of photoelimination of nitrogen from arenediazonium salts and attempts to characterize the resulting aryl cations. Nanosecond laser photolysis techniques have been used to examine the photodecomposition of diazonium tetrafluoroborates. Aryl cations could not directly be detected, but evidence from bleaching experiments indicates that, in water, the phenyl cation has a lifetime of about 500 ps. The isolation of the ether (131) as the sole product of photodecomposition of the diazonium salt (132) in methanol demonstrates that rearrangement of the intermediate aryl cation does not occur. Both heterolytic and homolytic pathways have been described for photodecompositions of arenediazonium salts complexed with crown ethers. A useful synthetic application of this conversion is the formation in 53% yield of ethyl 2,4-difluoroimidazole-5-carboxylate (133) on irradiation of the diazonium fluoroborate (134). ... 

Ions of diameter significantly smaller than that of the corresponding macrocyclic cavity are not effective for construction of the compounds of interest [12, 17J. However, these products can be synthesised in reasonable yield by using cations larger than the corresponding cavity size [12, 18-20]. The effectiveness of cations in assembling macrocycles is in one to one correspondence with their ability to form complexes with crown ethers, including those with a cavity size smaller than the metal ion diameter. 

This caging ligand is also reported in [120] to be able to encapsulate not only alkali metal cations but also their complexes with crown ethers, thus forming the corresponding 1 1 Matreshka complex-in-complex assembhes by Scheme 4.122. As follows from the multinuclear NMR data, these... 

Besides, stability constants of metal ion complexes with crown ethers in IL aqueous solutions were obtained (Popov et al., 2007) and the solubilization of the ILs [Cnmim][Tf2N] by the presence of crown ether was studied (Rickert et al., 2007). It is also reported that the stability of cesium complexed with 18C6 in ILs is affected by the nature of both cation and anion of ILs (Vendilo et al., 2009 Vendilo et al., 2010b). Moreover, the complex stability is important for the extraction process (Vendilo et al., 2010a). 

For crown ethers in which the H-nmr probes Ha and Hb are diastereotopic in the free crown ether too, the cation exchange process (50) becomes more complicated. For 1 1 complexes the measured free energy of activation also involves conformational inversion components (AG = AG + AG ), whereas exchange between free and complexed crown ether (2 1 ratio of crown ether to salt) only involves the free energy of activation for decomplexation (AG ). Bradshaw et al. (1979b) have recently reported AG -values for cation exchange in RNH3X complexes of crown ethers [257]. The AGf term was estimated to be 0.7, 0.5, and 1.2 kcal mol-1 for compounds [257 n = 1] with... 

Juri and Bartsch (1979) have studied the coupling of 4-t-butylbenzene-diazonium tetrafluoroborate with N,N-dimethylaniline in 1,2-dichloroethane solution. The addition of one equivalent (based on diazonium salt) of 18-crown-6 caused the rate constant to drop by a factor of 10, indicating that complexed diazonium is less reactive than the free cation. Benzenediazonium tetrafluoroborate complexes of crown ethers are photochemically more stable than the free salt. The decomposition into fluorobenzene and boron trifluoride is strongly inhibited but no explanation has been given (Bartsch et al., 1977). 

An interesting question is whether the peroxo ligand in our [Fe (Porph)(02 )] complex is coordinated in a side-on or end-on fashion. Taking into consideration the DMSO coordination and the electrophilic potassium cation in the crown ether lying above the peroxo ligand, [Fe (Porph)(02 )] may in a way represent a model for the proposed (59) nucleophilic attack of the end-on peroxo form, with an axially coordinated solvent molecule, to an electron-deficient substrate (Scheme 14). 

With soft anions crown-ethers are more efficient than quaternary salts, the reverse being observed when less polarizable nucleophiles are used. This is explained by the different extent of complexation of crown-ethers which depends not only on the complexed cation, but also on the anionic counterpart. Swelling and hydration measurements of polymer-supported crown-ethers in toluene/aqueous KY showed that the content of water in the imbibed solvent increases with the loading. This leads to a progressive polarity increase within the polymers and to a better crown-ether complexing... 

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