Crown ethers metal complexes

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. 

Figure 4.9 A crown ether-metaL complex simulation using explicit solvent...

Figure 4.10 Simulated binding modes for crown ether-metal complexes Na+-[15]crown-5 (top left), Na+-[18]crown-6 (top right), K+-[15]crown-5 (bottom left), K+-[18]crown-6 (bottom right)...

Perhaps because of inadequate or non-existent back-bonding (p. 923), the only neutral, binary carbonyl so far reported is Ti(CO)g which has been produced by condensation of titanium metal vapour with CO in a matrix of inert gases at 10-15 K, and identified spectroscopically. By contrast, if MCI4 (M = Ti, Zr) in dimethoxy-ethane is reduced with potassium naphthalenide in the presence of a crown ether (to complex the K+) under an atmosphere of CO, [M(CO)g] salts are produced. These not only involve the metals in the exceptionally low formal oxidation state of —2 but are thermally stable up to 200 and 130°C respectively. However, the majority of their carbonyl compounds are stabilized by n-bonded ligands, usually cyclopentadienyl, as in [M(/j5-C5H5)2(CO)2] (Fig. 21.8). 

In principle, mass spectrometry is not suitable to differentiate enantiomers. However, mass spectrometry is able to distinguish between diastereomers and has been applied to stereochemical problems in different areas of chemistry. In the field of chiral cluster chemistry, mass spectrometry, sometimes in combination with chiral chromatography, has been extensively applied to studies of proton- and metal-bound clusters, self-recognition processes, cyclodextrin and crown ethers inclusion complexes, carbohydrate complexes, and others. Several excellent reviews on this topic are nowadays available. A survey of the most relevant examples will be given in this section. Most of the studies was based on ion abundance analysis, often coupled with MIKE and CID ion fragmentation on MS " and FT-ICR mass spectrometric instruments, using Cl, MALDI, FAB, and ESI, and atmospheric pressure ionization (API) methods. 

Lariat ethers of structure 8 were found to be selective toward Li ion and the lariat crown ether-Li+ complexes are more stable than the corresponding complexes with Na or K+, in methanol. Nevertheless, experiments conducted in aqueous solution showed that Na+ had a better complexation ability than the other two alkali metal cations. Hence, selective complexation of lariat crown ethers with cations changes with the solvent system this may be due in part to the difference in solvation between solvent and cation (Figure 9 f. ... 

Reaction of 175 with Cgg yields a hydroxy-functionalized fullerene that can be further derivatized. This hydroxy-fullerene was coupled with a porphyrine unit via a polyethyleneglycol-Hnker. This linker can be arranged similarly to a crown-ether to complex metal cations. Complexation is used to tune the distance between the porphyrin imit and the Cgg-moiety and thus tune the donor-acceptor properties of this porphyrin-fuUerene hybrid [177]. 

While high polymers of /3-lactones can also be formed by cationic polymerization, most of the commercial production seems to be by the anionic route. Carboxylate salts such as sodium acetate or benzoate are commonly the initiators, but other nucleophiles, such as triethylamine, betaine, potassium f-butoxide, aluminum and zinc alkoxides, various metal oxides and tris(dimethylamino)benzylphosphonium chloride (the anion of which is the initiator), are of value. Addition of crown ethers to complex the counter cation increases the rate of reaction. When the reaction is carried out in inert but somewhat polar organic solvents, such as THF or ethyk acetate, or without solvent, chain propagation is very fast and proceeds without transfer reactions. 

It is well known that crown ethers can complex with metal ions through ion-dipole interaction and form hydrogen bonds with acidic protons such as NH, OH, and ammonium ions [9, 25, 26]. However, it was not until very recently that scientists started to utilize this interaction in the preparation of rotaxanes. 

Enolase type activity is displayed in the efficient supramolecular catalysis of H/D exchange in malonate and pyruvate bound to macrocyclic polyamines [5.32]. Other processes that have been studied comprise for instance the catalysis of nucleophilic aromatic substitution by macrotricyclic quaternary ammonium receptors of type 21 [5.33], the asymmetric catalysis of Michael additions [5.34], the selective functionalization of doubly bound dicarboxylic acids [5.35] or the activation of reactions on substituted crown ethers by complexed metal ions [5.36]. 

Chiral metal alkoxides and naphthoxides have been used as catalysts for asymmetric Michael reaction. An early successful example was reported by Cram et al., who used 4 mol % of KO Bu-chiral crown ether 8 complex as the catalyst to afford the Michael adduct with up to 99% ee (Scheme 8D.7) [16], In this case KO Bu complexed with chiral crown ether 8 plays two... 

Crown ether Metal cation Ion-dipole Complex (cavitand) [K+([18] crown-6)]... 

Crown ethers selectively complex various alkali metal cations and can be thus used as model systems to study interactions between a macrocycle-bound cation and the 7r-system of a sidearm arene. Alkali-metal cation-7i interactions have recently received considerable attention because of the biological importance [88, 89, 175]. These studies have focused on Na+ and K+ interacting with benzene, phenol, and indole, which are the side chain arenes of phenylalanine, tyrosine, and tryptophan, respectively. Recent work [177-180] has demonstrated the formation of stable complexes between, for example, K+... 

Molybdenum and tungsten complexes with three crown ether benzenedithio-lene ligands (21) have been reported (105) and the effect of alkali ion binding has been probed by CV (106). Upon binding with Li+, Na+, or K+, positive shifts in the redox potential have been observed for all complexes. This observation suggests that the tris(crown ether benzodithiolene) complexes of Mo and W may potentially be useful as sensors for alkali metal cations (106). 

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... 

An interesting recent development is the synthesis and sttuctural characterization of unusual cyclosiloxanolate metal complexes, " the crown ether-type complexation... 

The dynamics of excitons in isotopically mixed naphthalene crystals has been reviewed and the effects of orientation of metal ion perturbers in naphthalene-crown ether metal ion complexes on the external heavy atom examined . ... 

Crown ethers. Metal and crown ether complexes have to fulfill two requirements to form stable complexes. First, the coordination sphere of the metal must be stabilized by the crown ether and any complexing anions. A trivalent uranium coordination sphere is typically satisfied... 

---