Ethers, crown structural features

The combined features of structural adaptation in a specific hybrid nanospace and of a dynamic supramolecular selection process make the dynamic-site membranes, presented in the third part, of general interest for the development of a specific approach toward nanomembranes of increasing structural selectivity. From the conceptual point of view these membranes express a synergistic adaptative behavior the addition of the most suitable alkali ion drives a constitutional evolution of the membrane toward the selection and amplification of a specific transport crown-ether superstructure in the presence of the solute that promoted its generation in the first place. It embodies a constitutional selfreorganization (self-adaptation) of the membrane configuration producing an adaptative response in the presence of its solute. This is the first example of dynamic smart membranes where a solute induces the preparation of its own selective membrane. 

Yakshin, V. V. (2002) Structural and Chemical Features of Extraction with Crown Ethers, Russ. J. Coord. Chem., 28(10), 697-705. 

The use of chiral stationary phases (CSP) in liquid chromatography continues to grow at an impressive rate. These CSPs contain natural materials such as cellulose and starch as well as totally synthetic materials, utilizing enantioselective and retentive mechanisms ranging from inclusion complexation to Ti-electron interactions. The major structural features found in chiral stationary phases include cellulose, starch, cyclodextrins, synthetic polymers, proteins, crown ethers, metal complexes, and aromatic w-electron systems. 

X-Ray crystallographic data on the interaction of alkali metal ions with mononuclear metal carbonyls are not very extensive. No data appear to be available for lithium salts of mononuclear carbonylates. The findings on sodium and potassium salts reveal interesting details in the variety of modes of interaction. The situation may be summarized as follows. An essentially end-on interaction (e.g., FeCO—Na) is seen in the majority of instances, but if the alkali metal ion is unencumbered with large ligands, such as crown ethers, an interaction also is seen with the carbonyl carbon atoms and with the metal. Thus the solid state species contain some of the same structural features which were discussed above for ion pairs in solution. Unfortunately, there are no detailed analyses of the vibrational spectra of the solids for which structures are available. This might provide one avenue to the refinement of our knowledge of the species present in solution. 

In the context of crown ether hosts, non-covalent bonds of pole-pole, pole-dipole, and dipole-dipole types can all be employed [3-6] in the formation of host-guest complexes. Where the guest species is an alkali metal (i.e. Li, Na", K", Rb, Cs ), alkaline earth metal (i.e. Mg, Ca, Sr, Ba ), or harder transition or post-transition metal (e.g. Ag", TT, Hg, Pb, La, Ce ) cation [3-6,14], an electrostatic (M" O) pole-dipole interaction binds the guest to the host whilst the (M" X ) pole-pole interaction with the counterion (X ) is often retained. The features are exemplified by the X-ray crystal structure [15] shown in Fig. la for the 1 2 complex (1) (NaPF jj formed between dibenzo-36-crown-12 (1) and NaPF. Molecular complexes involving metal cations have considerable strengths even in aqueous solution and a template effect involving the metal cation is often observed during the synthesis of crown ether derivatives. 

CpLi exists in ethers as a contact ion pair, in which the lithium ion is located above the cyclopentadienyl ring. The crystal structure of CpLi(12-crown-4) reveals analogous structural features. This complex has a sandwich-type structure in which the lithium ion is coordinated to the four oxygen atoms of the crown ether on one side and to the Cp ring on the other. Several X-ray structures of alkali metal cyclopentadienyl compounds containing silyl substitution of the Cp ligand have been reported. The structures of fluorenyl alkali metal complexes (Li-Cs) stabilized by diglyme have also been reported. The crown-ether-stabilized compounds of cyclopentadienyl, indenyl, and fluorenyl with the heavier metals, Rb and Cs, have also been reported. " A common structural feature is the rf coordination of... 

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