Crown ether sandwich complex

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. 

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. 

Interest in this type of complex started in the mid-1970s. The design, by Smid and co-workers, of the first bis(crown ether) <75JA3462> proceeds from the observations that many crown ethers form complexes with two crown moieties per cation sandwich complexes it was postulated that the connection of the two crown units should improve the stability of the complex. The schematic representation in Figure 1 illustrates the bis(crown ether) complexation process the two macrocycles connected by a bridge surround the cation. It should be mentioned that the best sandwich complexes are obtained when the cation is too large to fit the cavity therefore to fulfill this requirement the best macrocycles are the 12-crown-4 for Na" ", the 15-crown-5 for K , and the 18-crown-6 for Cs+. This active field of research has been extensively reviewed by Izatt and co-workers <94CRV939>. 

Ionic supramolecular self-assembly will be discussed in Chapter 6, but some supramolecular systems based upon ionic interactions will be discussed earlier, e.g. the organocyclosiloxanolates, which form sandwich compounds by intercalating transition metal ions between two macrocyclic rings (held together by ionic interactions) and acting as endo receptors which concurrently have crown-ether-type complexing properties (as exo receptors) (see Section 2.1.2). 

This type of coordination stabilizes a crown conformation of the cyclohexa-siloxane ring. There are, in addition, two sodium cations attached at the top and the bottom of the sandwich in a crown ether-type complexation mode, with each sodium ion bonded to six oxygens of the rings, 9, and further solvated by alcohol molecules (not shown). The macrocyclic siloxane acts as both endo and exo receptors and the formation of the supramolecular architecture involves both ionic self-assembly and crown-ether-type ion recognition. 

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... 

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. 

The introduction at the C- or N-terminal position of a crown ether unit has been used as a strategy to control the aggregation of poly(benzyl glutamate) derivatives 19 The incorporation of the crown unit at the C-terminal position is performed using (benzo-15-crown-5)-4-amine as initiator of the polymerization of l-G1u(OBz1)-NCA. Physical properties of such crown derivatives can be modulated by the formation of sandwich 2 1 complexes driven by the addition of specific alkali metal ions. In the reported case, the formation of K+ sandwich complex between two C-terminal benzo-15-crown-5 modified helical polypeptides induced aggregation. In a similar approach,f20 addition of Cs+ to 18-crown-6 terminated helical peptides results in the formation of supramolecular assemblies having membrane ion conductivity activities. 

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