Lariat crown ether ligands

Gokel for reducible quinone (25, 26) and nitroaromatic (32, 33) lariat crown ethers with alkali metal guest cations discussed earlier. Support for the latter effect comes from the related ligand system (81), containing no amide carbonyl groups, and electrochemical results revealing... 

There has been considerable interest shown in the design and synthesis of functionalized crown ethers, and in whether the appended functional group can then further complex the cation. Examples include the so-called lariat ethers, which may be functionalized on either N or C bridgeheads in the ligand frameworks (89).427... 

The trends are noted to be relatively similar for both the carbon-pivot and nitrogen-pivot types of lariat ethers. Binding strengths and selectivities are dependent on ring size and in general increase as ligand size increases. Strong selectivities are noted for the potassium ion, as in the crown ethers. 

Abstract. The evolution of lariat ethers from relatively simple, substituted crown ethers into electrochem-ically sensitive ligands is presented. Although nitrogen-pivot lariats were observed to be better binders than the corresponding parent crowns and to retain considerable flexibility after complexation, overall stability constants were not favorable for cation transport applications. This led to the syntheses of nitrobenzene- and anthraquinone-substituted systems capable of reversible redox behavior and drastically enhanced cation binding abilities when reduced. Application of these in enhanced cation transport processes has been demonstrated. 

When acidic macrocycles are used as membrane carriers of cations, the ionized macrocycle serves as the counterion in the ttansport process. For example, Bartsch s group has synthesized and studied cation separations using a series of acidic lariat ethers in liquid membranes. Lariat ethers are crown ether derivatives with one or more side functional groups. The ligand 26 in Figure 23 was used as a carrier in BLMs to transport alkali metal cations. Several membrane solvents such as dichloromethane, chloroform, carbon tetrachloride, 1,2-dichloroethane, 1,1,1-trichloroehane,... 

Due to their ability to form complexes selectively with metal cations and thereby solubilize such cations in liquid media of low dielectric constant, macrocyclic multidentate ligands have been widely investigated as carriers in metal ion transport processes for the past two decades (2-P). Such macrocycles include crown ethers, lariat ethers (crown ethers with a side arm which contains potential binding sites) (JO), calixarenes, and some cryptands. Studies of structural variations within a given... 

A study of the selectivity of lariat aza[15] crown-5 (3.133) and BiBLE (bi-brachial lariat ether) diaza[18] crown-6 derived ligands (3.134) containing variously substituted arene rings showed that the Ag+ complex stability decreased as a function of the electron-withdrawing ability of the substituents, R, as measured by the substituents Taft o° inductive constant (a parameter measuring the electron-withdrawing... 

There are many other heterocycles containing not only oxygen but N and/or S. Crown-type groupings can also be attached to other ligand systems to give different types of binding sites, and in lariat ethers, which are crowns with side chains. 

---