Crown ethers sandwich

With somewhat the same idea, Minoura [44] used the formation of a crown ether sandwich complex with K+ to force the association of helical dimers of C-terminal modified poly-benzylglutamate (52). Although no structural data were provided, he observed by viscosimetry the reversible formation of a K+ sandwich complex with two 15-crown-5 modified peptides. 

The crystal structure of the crown ether sandwich electride Cs (15C5)2C in Figure 8-8(a) shows both the coordination of two 15C5 rings to each Cs ion and the cavity occupied by the electron e. ... 

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. 

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules. 

In the cation [Pb(15-crown-5)2]2+ the lead atom is sandwich-like coordinated by the ten oxygen atoms of the two crown ether molecules. (Figure 13). 

MeCN)][SbCl6]3 (106) have also been isolated. The dication has bismuth encompassed by the crown ether and bonded to all ether oxygen atoms, and the chlorine atom [Bi-Cl 2.479(6) A] and the pair of acetonitrile nitrogen atoms [Bi-N 2.82(2) and 2.86(3) A] are bound from the opposite side to the crown ether. The trication adopts a sandwich structure in which bismuth is bound to all four oxygen atoms of each crown ether [Bi-0 2.436(5) and 2.545(5) A]. The acetonitrile nitrogen atom occupies a position between the two ligands [Bi-N 2.650(9) A],... 

Crown ethers whose cavity is too small to accommodate the anion overcome this handicap in an ingenious way by forming sandwich-type complexes (5). The formation of 2 1 complexes is very sensitive to steric... 

FABMS has been used as a semiquantitative indication of the selectivity of receptors for particular guest metal cations (Johnstone and Rose, 1983). The FABMS competition experiment on [7] with equimolar amounts of the nitrates of sodium, potassium, rubidium and caesium gave gas-phase complex ions of ([7] + K)+ ion (m/z 809) and a minor peak ([7] + Rb)+ ion (m/z 855) exclusively. The relative peak intensities therefore suggested a selectivity order of K+ Rb+ Na+, Cs+, indicative of the bis-crown effect, the ability of bis-crown ether ligands to complex a metal cation of size larger than the cavity of a single crown ether unit, forming a sandwich structure. 

The complexation ability of crown-ethers has been improved by the introduction of secondary donor sites covalently bonded to the macrocyclic ring through a flexible arm, e.g. "lariat ethers" (27). It is also known that in particular conditions crown-ethers can make 2 1 sandwich complexes with the cation (8). 

We synthesized bis crown-ethers 1 and 2 in which cations can be well accomodated between the two ligand units, thus giving rise to sandwich complexes ( ) which can be expected to have complexation constants higher than those of crown-ethers. 

The ability of the crown ether ligand, 12-crown-4, to separate the lithium cation from the organic moiety, thus stabilizing the SSIP structure, was also observed in the study dealing with cyclopentadienyllithium systems . As described in Section II.C.3, SSIP structures of these compounds are characterized by low x( Li) values. However, it is important to reahze that the variation found for x( Li) is basically caused by the local symmetry around the lithium cation and other highly symmetric situations will also lead to small x( Li) values. Examples are the sandwich compounds mentioned in Section n.C.3. It is thus necessary to consider also / Li and C chemical shift data in order to classify a certain complex as SSIP or CIP. 

The crown ethers and the related cryptates (cryptates were first reported by J-M Lehn (J ) of France in 1969 (Figure 3)), have been used in a variety of synthetic procedures primarily because of their ability to solvate ionic materials in organic solvents (1, 14). Recently, it has been shown that crown ethers in the solid state form "sandwich" like complexes with most metals and that the counterion is also tightly bound (1 ). It is this evidence that suggested their use as ion traps in silicone RTV formulations. 

Although the ligands incorporated are the same, the sandwich complexation of a cation with two molecules of crown ethers should be discussed separately. The sandwich complexes are formed in two successive processes consisting of usual stoichiometric 1 1 complexation followed by further ligation by another molecule... 

The bis-macrocyclic ring is positively charged because of the presence of Ni(II) and Cu(II). The crown ether and the bis-azamacrocyclic ring form a sandwich-like structure in such a way that one of the crown ether aromatic rings is located between the two metal-coordinated macrocyclic rings. The second aromatic ring is located almost parallel to the previous one outside the two linked macrocycles. 

In a potentiometric study in propylene carbonate, using Pb11 or Tl1 as auxiliary ions, stability constants have been determined for a variety of crown ethers. Some results464 are shown in Table 8. They show that the wrap-around ligand dibenzo-30-crown-10 is relatively quite effective, while the 2 1 complexes, presumably of the sandwich type, are favoured for larger lanthanides and smaller crowns. 

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