Crown ethers complexes with alkali metals

Figure 4.11 Molecular structures of typical crown-ether complexes with alkali metal cations (a) sodium-water-benzo-I5-crown-5 showing pentagonal-pyramidal coordination of Na by 6 oxygen atoms (b) 18-crown-6-potassium-ethyl acetoacetate enolate showing unsymmelrical coordination of K by 8 oxygen atoms and (c) the RbNCS ion pair coordinated by dibenzo-I8-crown-6 to give seven-fold coordination about Rb.

In a seminal 1967 paper by Pedersen, the exceptional stability of macrocyclic crown ether complexes with alkali metal ions was reported." What followed was the birth of a new area of macrocyclic chemistry, one not involving transition metal complexes but focused more on organic chemistry. The field of supramolecular chemistry was just beginning to blossom. Perhaps the most studied of these macrocycles is the [18]crown-6. Nomenclature for the simple crown ethers derives from a simple notation that refers to the number of atoms in the ring enclosed in brackets,... 

Oksman, P. Lajunen, L. H. J. (2007). Stability of crown-ether complexes with alkali-metal ions in ionic liquid-water mixed solvents.. Inch Phenom. Macrocycl. Chem., 59, 377-381. 

Popov, K Ronkkomaki, H. Hannu-Kuure, M. Kuokkanen, T. Lajunen, M. Vendilo, A. Oksmann, P. Lajunen, L.H.J.(2007) Stability of Crown-Ether Complexes with Alkali-Metal Ions in Ionic Liquid-Water mixed Solvents, /. Incl. Phenom. Macrocycl. Chem., 59 377-381. 

Alkalides and electrides are effective reducing agents comparable to solvated electrons. Alkalides and electrides are crystalline salts consisting of crown ethers complexed with alkali ions or salts with alkali metals as anions, and consist of trapped electrons. They are of the type K+(15 — crown — 6)2Na [197-200]. The reduction is carried out in solvents such as THF under... 

Ozutsumi, K Natsuhara, M. Ohtaki H. (1989) An X-ray Diffraction Study on the Structure of 18-crown-6 Ether Complexes with Alkali Metal Ions in Aqueous Solutions, Bull. Chem. Soc. Jpn., 62 2807-2818. 

Polymeric pseudocrown ether networks have been generated in situ by the photopolymerization of poly(ethylene glycol) diacrylate transition metal complexes <00CM633>, and the effect of metal ion templation was evaluated. The 1,6,13,18-tetraoxa[6.6]paracyclophane-3,15-diyne (termed pyxophanes) was prepared from hydroquinone and l,4-dichlorobut-2-yne it forms size-selective 7i-complexes with alkali metal cations <00CC2377>. Dibenzo[ ]crown-m have been used in numerous elegant studies in which they were the needles that were threaded by diverse reagents the resultant... 

Two complexing agents that form stable complexes with alkali metal ions. The ethylenediamine derivative (left) contains eight potential bonding atoms and the crown ether (right) contains... 

Bis(crown ethers) connected by a flexible spacer are a source of intramolecular sandwich-type complexes with alkali metal ions <1992MI1>. A conformational analysis (based on a combination of semiempirical and ab initio methods) performed on 12-crown-3 38 and 12-crown-4 39 predicted that, in the case of sandwich-type complexation, the nucleophilic cavity of 12-crown-3 rather than that of 12-crown-4 is more prone to complexation with the Na+ ion. Accordingly, ion-selective electrodes based on bis(12-crown-3) derivatives with dialkylmalonate spacers displayed the highest selectivity for Na+ ions among the alkali and alkaline earths investigated, and superior to the Na+ selectivity reached with the bis( 12-crown-4) analogue <2003ANA(480)291>. 

Dialkyl-substituted anthracene-bridged bis-crown ethers 92 formed 1 1 and 1 2 (crown to metal) complexes with alkali metal ions <1999J(P2)1193>. The stability constants of the latter were suggestive of a negative cooperation effect between the two crown ether units < 0.25). The derivative with R = Et showed a decrease in... 

Kobuke et al. [40] demonstrated that the podand 28 only forms weak complexes with alkali metal ions. However, when a chiral macrocyclic crown ether structure is formed by complexation with boric acid, alkali metal ions are bound much more strongly (Scheme 10). Stiffening, provided by covalent B-O bonds, improves the preorganization of the ligands that is required for the complexation of metal ions. 

Molecular model of 18-crown-6. This crown ether can form strong complexes with alkali metal ions. The formation constants of the Na", and Rb+ complexes are in the 10 to 10 range. 

In contrast to conventional cation exchangers, a reversed elution order is observed with crown ether phases, which is mainly determined by the size ratio between crown ether ring and alkali metal ion. Due to the high affinity of poly(benzo-15-crown-5) toward potassium and rubidium ions, these are more strongly retained than lithium, sodium, and cesium ions, respectively. However, the complexing properties of crown ethers also depend on the counter ion being employed. Thus, in potassium salts, for example, an increase in retention in the order KC1 < KBr < KI is observed with an increasing size of the counter ion. 

Vhc and Vhn couplings have been applied by Grothjahn et al in order to obtain structural information on the bis(benzo crown ether)s and their complexes with alkali metal cations. 

A novel bis(crown ether) based on Kemp s triacid has recently been reported [16]. It has been shown that this molecule forms a sandwich-type complex with alkali metal ions. This crown ether (shown in Figure 6) was reported to exhibit a higher preference for potassium ions over sodium ions (85% K+ versus 46% Na" " extracted). 

Macrocycllc compounds (some crown ethers and cryptands) are selective reagents for extractive separation of alkali metals [22-27]. These ligands form cationic complexes with alkali metal ions, and these can be extracted as ion-pairs with suitable counter-ions e.g., picrate) [28], most often into chloroform. For potassium, p-nitrophenoxide was used as counter-ion [29]. In cases, where a coloured anionic complex is a counter-ion [30], the extract may serve as a basis for determining the alkali metal. The effect of the structure of the dibenzo-crown ether rings upon the selectivity and effectiveness of isolation of alkali metals has been studied in detail [31]. Chromogenic macrocyclic reagents applied for the isolation and separation of alkali metals have been discussed [32]. 

Crown ethers can be modified, e.g. 18—crown-6 gave a range of adducts [10] and, what is remarkable, each of these adducts forms complexes with alkali—metal ions. Indeed, in eluting alkali—metal salts over these adducts, adsorbed on nitrocellulose, and then looking at the systems by plasma-desorption mass-spectrometry, none of the uncomplexed adducts could be detected (Becker, J., Odense University, Denmark, unpublished results). The properties of these unusual complexes will be interesting to explore. 

Substances of the amphotericin D (a polyene), polyether (for example crown cyclic ethers), Antamanide (a peptide), and valinomycin (a depsi-peptide) represent structural types capable of complexing with alkali metal ions and thereby promoting their dissolution in fairly nonpolar solvents. Such compounds are known as ion carriers and some display antibiotic properties which may in part reside with activity in natural membranes. In order to evaluate structural changes upon such interesting functions, Gisin and Merri-field have synthesized a cyclododecapeptide (Chart 15) where the D-a-hy-droxyisovaleric acid and L-lactic acid units of valinomycin were replaced respectively with D-Pro and L-Pro. In MeCl-Aq the valinomycin analog was found to exhibit a seven times greater affinity for potassium picrate (to form a 1 1 hydrophobic complex) than that of the parent depsipeptide. 

Polyethylene glycols can be often alkylated without TAA catalyst. Being open-chain analogues of crown ethers, they form complexes with alkali metal cations that are soluble in organic media (eq. 24). 

One of the earliest applications of computational methods to supramolecular phenomena was a theoretical investigation into the widely acknowledged size selectivity of crown ethers for particular alkali metals (Figure 7). As early as 1975, ST0/3G quantum mechanical simulations of [Li(12-crown-4)]+ had been attempted, to be followed by 18-crown-6 complexes of Na+, K+, and NH4+ in 1979. Yanabe s calculations, using the semiempirical CNDO/2 method, gave a good correlation with experimental photoelectron spectra and, importantly noted that ... 

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