Crown alkaline cation complexes

Since the discovery in 1964 that the antibiotic valinomydn exhibited alkali cation specificity in rat liver mitochondria, a new area of research has developed, based not only on biological systems but also on model systems such as crown ethers.484 The ability of neutral compounds to form lipid-soluble alkali and alkaline earth complexes was observed in 1951. The structure of the corresponding ligand, the anion of the antibiotic nigericin (78), was characterized as its silver salt in 1968.488 486 Silver was used as a heavy atom crystaUographically, since the Ag+ cation had a radius between that of Na+ and K+, which were the two alkali cations with which nigericin was most active. 

Complexation of inorganic cations such as alkaline or alkaline earth metals by macrocyclic polyethers produces large, lipophilic cationic metal-macrocycle complexes that are readily soluble in nonpolar solvents such as benzene, toluene and haloalkanes. In order to maintain charge balance, the cationic complex has an associated counter anion. In an immiscible two-phase liquid system, such as a mixture of chloroform and water, the anion is necessarily pulled into the organic phase as the cationic complex crosses the phase boundary. A simple illustration of this principle is obtained by addition of a chloroform solution of [18]crown-6 to an aqueous solution of potassium picrate (potassium 2,4,6-trinitrophenolate). The yellow colour of the picrate anion is transported rapidly into the contiguous (physically in contact) chloroform phase upon agitation (Figure 3.43). 

Crown ethers selectively complex alkaline ions [6,41], and the complexation of different sized cations leads to coronates [5,42] of various structures. The structural analogy between crown ethers and their topologically equivalent metallacrown ethers... 

There are thousands of discoveries in molecular science reported every year but very few of these are destined to promote a new generation of research activity. The serendipitous preparation of di-benzo-18-crown-6 1 by Pedersen in 1967 [1] and the subsequent discovery [1,2] that 1 and other crown ethers selectively complex biologically relevant alkali and alkaline earth cations was, however, the catalyst for a huge explosion of activity in the field of host-guest or supramolecular chemistry. The resulting inspired and innovative work by Lehn [3,4] on, in particular, the 3-dimensional bicyclic cryptands (e.g., 2) and by Cram [5] on chiral crown ethers and rigid spherands (e.g., 3) was recognised by the award to Pedersen... 

Since the ionic radii of alkaline and alkaline earth metal cations differ significantly in size within each group, the inclusion of small cations such as Na" or Ca " produces metalla-coronates with 1 1 metal to metalla-crown stoichiometry. In contrast, encapsulation of the larger cation results in the formation of metaUa-crown ether sandwich complexes with 1 2 metal to metalla-crown stoichiometry. 

Laser microprobe mass spectrometry (LMMS) confirms the existence of true interlayer complexes between cations and macrocyclic ligands (47). This technique, which provides mass spectra of fragmented solids after irradiation with a laser beam, was first used to characterize the intercalation compounds of crown ether- and cryptand-smectite complexes (47,48). One of the most interesting results derived from LMMS of these materials is the ability to corroborate the macrocycle-interlayer cation complexation, such as cryptand C(222)-Na-montmorillonite vs. Cu-montmorillonite. Alkaline-cryptand complexes are clearly assigned since the m/e values correspond to the sum of both the sodium and cryptand atomic mass (i.e., 399 Daltons for Na /C(222)). Transition metal... 

Ever since Petersen reported the complexing ability of the crown ether with alkali, alkaline earth and other cations, crown ether became a major subject for researchers. This is due to the fact that crown ether possesses the ability to form complexes with a variety of inorganic salts and also the ability to solubilize these salts in aprotic solvents The complexation between metal cation and crown ether is believed to involve ion-dipole interactions and therefore is similar in nature to ordinary solvation. In addition, crown ether/metal cation complexes can serve as catalysts in reactions involving ionic intermediates. Polymerization of diene with crown ether/metal cation complexes is a typical example of this subject, since this reaction involves ionic intermediates. The detailed information including a brief history, chemical properties of crown ether and its application in the anionic polymerization and copolymerization have been discussed. 

Simplest examples are prepared by the cyclic oligomerization of ethylene oxide. They act as complexing agents which solubilize alkali metal ions in non-polar solvents, complex alkaline earth cations, transition metal cations and ammonium cations, e.g. 12—crown —4 is specific for the lithium cation. Used in phase-transfer chemistry. ... 

Reaction of Na2Se and Na2Sc2 with Se in the presence of ethanolic solutions of tetraalkyl-ammonium halides and catalytic amounts of I2 yields dark green or black crystalline polyselenides (jc = 3,5-9) depending on the conditions used and the particular cation selected. Tetraphenylphosphonium salts and crown ether complexes of alkali or alkaline earth cations in dimethylformamide solution can also be used. " )... 

Crown polyethers. Macrocyclic effects involving complexes of crown polyethers have been well-recognized. As for the all-sulfur donor systems, the study of the macrocyclic effect tends to be more straightforward for complexes of cyclic polyethers especially when simple alkali and alkaline earth cations are involved (Haymore, Lamb, Izatt Christensen, 1982). The advantages include (i) the cyclic polyethers are weak, uncharged bases and metal complexation is not pH dependent (ii) these ligands readily form complexes with the alkali and alkaline earth cations... 

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