Cryptands anion cryptates

Linear recognition is displayed by the hexaprotonated form of the ellipsoidal cryptand bis-tren 33, which binds various monoatomic and polyatomic anions and extends the recognition of anionic substrates beyond the spherical halides [3.11, 3.12]. The crystal structures of four such anion cryptates [3.11b] provide a unique series of anion coordination patterns (Fig. 4). The strong and selective binding of the linear, triatomic anion N3" results from its size, shape and site complementarity to the receptor 33-6H+. In the [N3 pyramidal arrays of +N-H "N- hydrogen bonds, each of which binds one of the two terminal nitrogens of N3-. 

Earlier discussion [12] of the structures of protonated cryptand/oxoanion assemblies was based on consideration of H-bonds between the encapsulated anion and the NH+ donors of the ciyptand. These interactions are assumed to be responsible for retention of the guest anion in the host ciyptand cavity, both in the sohd state and in solution. We have shown that, in all cases, anion cryptates exhibit at least three, and often more, direct H-bond NH+-Oanion contacts tethering the included oxoanion within the crypt. 

Anion cryptates are formed by macrotricycles like (5) in their tetraprotonated state with the spherical halide anions [8]. (5)-4H binds the chloride ion very strongly and very selectively, giving the [Cl" c (5)-4H J cryptate (7), but does not complex other types of anions. These properties are unique at present with respect to both synthetic and natural halide binding sites, very little being known about the latter. Non-complementarity between an ellipsoidal cryptand and the spherical halides results in appreciable ligand distortions in the cryptates formed and in lower binding constants [9, 10] (see also below). 

A D a-symmetric hydrogen-bonded capsule 420 with a cavity volume of approximately 950 is reported in [77] to encapsulate suitable cryptand and cryptate guests (including their ionic associates with SCN and CN" anions) having the volumes of 390-420 thus forming the host-homoguest and host-heteroguest 1 1 and 1 1 1 Matreshka complex-in-complex assemblies by Scheme 3.80. 

The proportion of the /rans-O-alkylated product [101] increases in the order no ligand < 18-crown-6 < [2.2.2]-cryptand. This difference was attributed to the fact that the enolate anion in a crown-ether complex is still capable of interacting with the cation, which stabilizes conformation [96]. For the cryptate, however, cation-anion interactions are less likely and electrostatic repulsion will force the anion to adopt conformation [99], which is the same as that of the free anion in DMSO. This explanation was substantiated by the fact that the anion was found to have structure [96] in the solid state of the potassium acetoacetate complex of 18-crown-6 (Cambillau et al., 1978). Using 23Na NMR, Cornelis et al. (1978) have recently concluded that the active nucleophilic species is the ion pair formed between 18-crown-6 and sodium ethyl acetoacetate, in which Na+ is co-ordinated to both the anion and the ligand. 

The X-ray crystal structures of the F", Cl , and Br" cryptates of 19-6H demonstrate the inclusion of one of the halide anions in an unsymmetrical fashion. In the case of the small fluoride ion complex a tetrahedral coordination environment is observed for the guest anion with a mean N(H) - P hydrogen-bonding distance of 2.72(8) A. The CP and Br" cryptates exhibit octahedrally coordinated halide ions situated more centrally within the host framework with N(H) - X" distances in the ranges 3.19-3.39 A (X = CP) and 3.33-3.47 A (X = Br ). It is noteworthy that the hydrogen-bonded distances for the anion within the cryptand host are longer by up to ca 0.15 A than those for the other anions in the lattice, suggesting a particularly... 

In macrobicyclic cryptate complexes where the cation is more efficiently encapsulated by the organic ligand these ion pair interactions are diminished and the reactivity of the anion is enhanced. This effect is seen in the higher dissociation constant, by a factor of 104, of Bu OK in Bu OH when K+ is complexed by [2.2.2]cryptand (12) compared to dibenzo[18]crown-6 (2). The enhanced anion reactivity is illustrated by the reaction of the hindered ester methyl mesitoate with powdered potassium hydroxide suspended in benzene. 

Alkali metal anions have also been generated as a result of cryptand stabilization of the corresponding cation. Cryptands were found to enhance the solubility of zerovalent alkali metals in various organic solvents.156-157 Initially, the solutions apparently contain the cryptate cation and solvated electrons together with free ligand. When more metal is dissolved, metal anions, M , are formed.158 Dye and co-workers have isolated gold-colored crystals of [Na+ c 2.2.2]Na 159160 and the crystal structure has been determined.161,162 Anion clusters such as Sb] , Pb2 and Sn," have been isolated as crystalline salts of the [2.2.2] cryptate counterion [2.2.2].162,163... 

Dietrich, B. Lehn, J.-M. Guilhem, J. Pascard, C. Anion receptor molecules synthesis of an octaaza-cryptand and structure of its fluoride cryptate, Tetrahedron Lett. 1989, 30, 4125-4128. 

In order to accommodate the small, highly charged calcium ion, the cryptand adopts an unsymmetrical conformation in which two of the chains are pushed apart to allow coordination of a water molecule and the N N distance is reduced to 5.44 A (42). In the corresponding barium complex, the structural unit consists of two cryptates, two water molecules, and four thiocyanate anions, two of which (9) are bound through nitrogen to the metal, (43). 

Cryptands were found to react with metal solutions in basic solvents to generate the alkali metal cryptate and an alkali anion (alkalide), for example (Na[2.2.2])+Na (62, 63). 23Na-NMR measurements of this salt in methylamine, tetrahydrofuran, and ethylamine solutions showed that the Na resonance is shifted strongly upheld from the Na resonance (free or complexed) as shown in Fig. 7. The anion resonates at approximately the same frequency as that calculated for the free... 

Cryptand was obtained as the corresponding sodium cryptate 20 in 27% yield. The cation-free cryptand was isolated by passing an acidic solution of the complex through cation- and anion-exchangers. However, the overall yield was not reported. When sodium carbonate was replaced by potassium carbonate, no detectable amount of cryptate K+ c [2.2.2] was observed this confirms the involvent of a template effect, although this method is rather limited to simple mononucleating cryptands. 

Fig. 4.19. (Top) Dimeric structure of a Pr(III) cryptate with (2.2.1) in which the two monomers are connected through two /r-OH links (redrawn from J. Rebizant et al., J. Incl. Phenom. Mol. Recogn. Chem. 5, 505, 1987. (Bottom) Structure of the [Eu(C104 )(2.2.2)]2+ cation showing the coordination of a perchlorate anion between two arms of the cryptand receptor. Redrawn after M. Ciampolini et al., J. Chem. Soc., Dalton Trans. 974, 1979.

Fig. 34 Cascade process for the formation of a ternary dicopper(II) bistren cryptate, including an ambidentate anion. The bistren cryptand 20 has been chosen as an example...

In general, discrete Zintl anions, whether they obey the simple octet rule or are electronically more complex, can often be obtained intact from the initial Zintl phase by substituting tetraalkylammonium ions for the alkali metal cations or by encapsulating the latter in a cryptand such as cryptate-222 (see 1-XXII).25... 

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