Crown ethers, complexation with

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.

An enormous variety of solvates associated with many different kinds of compounds is reported in the literature. In most cases this aspect of the structure deserved little attention as it had no effect on other properties of the compound under investigation. Suitable examples include a dihydrate of a diphosphabieyclo[3.3.1]nonane derivative 29), benzene and chloroform solvates of crown ether complexes with alkyl-ammonium ions 30 54>, and acetonitrile (Fig. 4) and toluene (Fig. 5) solvates of organo-metallic derivatives of cyclotetraphosphazene 31. In most of these structures the solvent entities are rather loosely held in the lattice (as is reflected in relatively high thermal parameters of the corresponding atoms), and are classified as solvent of crystallization or a space filler 31a). However, if the geometric definition set at the outset is used to describe clathrates as crystalline solids in which guest molecules... 

S. Aoki, S. Sasaki, K. Koga, Simple Chiral Crown Ethers Complexed with Potassium tert>Butoxide as Efficient Catalysts for Asymmetric Michael Additions , Tetrahedron Lett. 1989, 30, 7229-7230. 

Reactivity of crown ether complexes with arenediazonium salts 420... 

Both the absolute and the relative values of the association constants of crown-ether complexes with cations are dependent on the type of solvent used. 

In the preceding section it was shown that the stability of crown-ether complexes with alkylammonium salts depends on the relationship between the structures of the crown ethers and the ammonium ions. How critically this relationship determines the complex stability will become clear in this section, which deals with the discrimination between the two enantiomers of racemic salts by chiral macrocyclic ligands. 

REACTIVITY OF CROWN ETHER COMPLEXES WITH ARENEDIAZONIUM SALTS... 

The chemical reactivity of crown-ether complexes with neutral molecules has received little attention. Nakabayashi et al. (1976) have reported crown-ether catalysis in the reaction of thiols with l-chloro-2,4-dinitrobenzene. The catalytic activity was attributed to deprotonation of thiols by dicyclohexyl-18-crown-6 in acetonitrile solution. Blackmer et al. (1978) found that the rate of aquation of the cobalt(III) complex [333] increases on addition of... 

Table 11 Chiral selectivity factors, RPI /RPI = [U M ]" /[U Ms]4 (equation (25)) for chiral crown ethers complexed with enantiomeric alkylammonium ions (Ur .f = 6 in Scheme 5)...

Figure 5 Crystal structures of (a) [18]crown-6 complex with two malonodinitrile molecules (b) binaphthol crown ether complex with water (reproduced with permission from 77CB2249 and 78AX(B)3387)...

C. J. Pedersen Crown ethers (complexes with planar, macrocyclic ligands)... 

The first crown ether complexes to be made were usually with lanthanide nitrates when crown ethers complex with lanthanide nitrates, the two ligands generally combine to saturate the coordination sphere, but since chloride is a... 

Relative thermodynamic stabilities of crown ether complexes with several cations can also be evaluated by means of Ti competition experiments. (20) Addition of 18C6 to a solution of caesium iodide affects... 

Photoisomerization of azobenzene moieties has been used also to trigger ion transport in photoresponsive membranes.161 A schematic representation of the photoinduced membrane transport of potassium ion is given in Table 2.10 . The bis(crown ether) (16) can form a 1 2 metal/crown ether complex with large metal ions which cannot be accommodated within simple crown ethers. This photoresponsive function of the azobenzene bis(crown ether) accelerates the transport of potassium and calcium cations across liquid membranes under photoirradiation. 

Monserrat et al. have discussed the properties of surfactant aza-crown ethers complexed with silver. Photoinduced electron transfer from a surfactant Ru complex was found to lead to the formation of zerovalent silver stabilized by the microenvironment of the vesicles and again the electron-transfer reaction was extraordinarily rapid, competing efficiently with other pathways for the deactivation of the excited singlet state. 

Finally, it should be mentioned that sandwich formation is not a privilege of crown ether complexes with metal ions displaying ionic radii which are too large to fit into the ligand cavity. Infrared spectroscopic studies revealed thatbenzo[15]crown-5 forms a 2 1 sandwich with Na (which could smoothly fit into the cavity), if the anion is tetraphenyl borate This is due to the inability of the anion to provide donor atoms for the Na ion which thus requires a second crown for a sufficient coordination. 

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... 

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,... 

Hetero-Crown Ether Complexes with Transition... 

HETERO-CROWN ETHER COMPLEXES WITH P-BLOCK ACCEPTORS... 

Alanine tosylate-18-crown-6 complex Lyophilize alanine (1 Eq) from 5 mL of water containing p-toluenesulfonic acid monohydrate (1.1 Eq). The alanine-tosylate salt is added to a chloroform solution of 18-crown-6 (1 Eq) and the mixture stirred until homogeneous. Evaporation of chloroform and crystallization of the residue from methanol-ethyl acetate (see Note 6) yields the solid alanine-crown ether complex with a melting point of 123-125°C. 

Crown ether complexes with amines, A -amino acids, peptides of varying size, and side chain amino group of Lys and Arg have been prepared (15-23). The examples given here are only representative. 

Vignier, M., Abadie, M., Schu6, E, and Kaempf, B., Preparation and UV characterization of crown-ether complexes with alkaline metals, Eur. Polymer J., 13, 213,1977. Vignier, M., Collet, A., Schue, E, and Kaempf, B., Preparation and characterization of alkaline radical anions in THE, /. Phys. Chem., 82, 1578, 1978. 

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