Complexes cryptates

The study of Lehn s cryptands has shown that a three-dimensional arrangement of binding sites leads to very stable inclusion complexes (cryptates) with many cations. For example, the stability constant for K+ in methanol/water (95/5) is five orders of magnitude higher with [2.2.2]-cryptand [37] (log K 9.75 Lehn and Sauvage, 1975) than with [2.2]-cryptand [38] (log... 

For potassium zeolites, cryptofix 222 and cryptofix 222BB, for example, can be used. The structures together with the stability constants Ks of the complexes (cryptates) of cryptofix 222 and cryptofix 222BB with potassium are shown in... 

From various observations it has been inferred that most AC and AEC complexes formed by the ligands of type 6—45 are 1 1 inclusion complexes, cryptates 34), in which the cation is held in the central cavity of the ligand molecule 34, 61, 106). This has been amply confirmed by several crystal structure determinations which also provided fundamental information about the shape of the ligand in the complex. 

The subject matter of this chapter will be subdivided into sections concerning template synthesis of the complexes structural and thermodynamic properties of the complexes with synthetic cyclic polyamines complexes with mixed-donor macrocycles reactivity of the complexes cryptates and complexes with phthalocyanines and porphyrins. 

Soft-ionization mass spectrometry techniques such as matrix-assisted laser desorption ionization (MALDI) and fast atom bombardment (FAB) allow the molecular ions of cryptands and their complexes (cryptates) to be observed. The... 

The cryptands were first prepared in 1969 and form a series of well-defined complexes (cryptates) with alkali and alkaline-earth cations. In this chapter the synthesis of the first cryptand, 8, a macrobicyclic ligand, will be described1,2. The schematic representation (Fig. 5.1) shows that one deals with a multi-step synthesis. The major drawback of this approach is the rather large number of synthetic steps, but the route offers the advantage of being able to construct unsymmetrical compounds (A B C). 

Cryptands (Greek krypte = vault, crypt) are polycyclic compounds containing oxygen and nitrogen atoms, forming extractable cationic complexes (cryptates) with metal ions (in the presence of a suitable anion). 

Azamacrobicyclic ethers such as 5 form stable inclusion complexes (cryptates) in which the metal cation is fully sequestered inside a 10 A spherical cavity taking the place of the solvation sphere. The resulting solvent separated ion pairs are particularly reactive in low-polarity media due to their scarce stabilization by the solvent and the cation. 

The alkaline-earth metal salts form complex cryptate-type compounds analogous to those formed by the alkali-metal salts (see Chapter 1 complexes of alkali metals). The bivalent metal ions, however, can take both water molecules and anions into the cryptate cage. Two compounds containing Ba and one containing Ca have been reported. The structures of these compounds are detailed in Table 2. In (8) and (9), the cryptate molecule is of the... 

A supramolecular species (host-guest compound) results from the interaction of a substrate (the guest) with its receptor molecule (the host). Normally one finds enclosure of the guest molecule in the cavity formed by a host framework (cf. crown complexes, cryptates). The host-guest association is not established by covalent and ionic bonds, but is caused by H-bonds andjor van der Waals interactions. With reference to the chemistry of weak intermolecular bonds, supramolecular association has contributed to the fundamental understanding of the elementary interactions on which mol ular recr nition and binding is bas l and represents an interface between chemistry and biology. 

Figure 9-4 The binding of a cation by a polycyclic ether (cryptand) to form a complex (cryptate). The system shown selectively binds the potassium ion, with a binding constant of K = 10 °. The order of selectivity is > Rb+ > Na" > Cs > Li. The binding constant for lithium is about 100. Thus, the total range within the series of alkali metals spans eight orders of magnitude.

The macrocychc hexaimine stmcture of Figure 19a forms a homodinuclear cryptate with Cu(I) (122), whereas crown ether boron receptors (Fig. 19b) have been appHed for the simultaneous and selective recognition of complementary cation—anion species such as potassium and fluoride (123) or ammonium and alkoxide ions (124) to yield a heterodinuclear complex (120). 

Ca can be complexed by crown ethers and cryptate ligands and ia this form can be transported across natural and artificial membranes. 

Other complexing agents sometimes advocated are cryptates, especially the compound dubbed [2.2.2] (Kryptofix 222) [23978-09-8] (see Chelating agents). Crown ethers were originally advocated for reactions in the presence of soHd reagents (Uquid-soHd PTC). It is now known, however, that onium salts are equally suitable in many cases. 

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use. 

Complexes Tlie term cryptate is now accepted to mean the complex formed between a cryptand and a substrate. Tlie corresponding complex with a coronand would be a coronate, a term suggested some years ago by the same authors . Presumably, a complex between a podand and some substrate would be a podate . 

Cryptates the chemistry of macropolycyclic inclusion complexes. J. M. Lehn, Acc. Chem. Res.,... 

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