Crown ethers hydrogen bonding

There are several examples of crown ether complexes where yttrium is not bound to the crown ether. [Y(H20)8]C13 (15-crown-5) has a [Y(H20)8]3+ cation (distorted dodecahedron) with the crown ether and chloride hydrogen bonded to the coordinated water molecules.58,225 An aqueous solution containing yttrium triflate and 18-crown-6 produces crystals of [Y(CF3-COO)2(H20)6](CF3COO),(18-crown-6) which have the crown ether hydrogen bonded to the coordinated water molecules.226 The complex [Y(N03)3(H20)3] (dibenzo-24-crown-8) has a nine-coordinate yttrium formed by bonding to three bidentate nitrate groups and three water molecules. The crown ether is hydrogen bonded to the water molecules.227... 

Podates AcycHc analogues of crown ethers /coronands and cryptands (podands, eg, (11) (30) are also capable of forming inclusion compounds (podates) with cations and uncharged organic molecules, the latter being endowed with a hydrogen bond fiinctionahty. Podates normally are less stable than coronates and cryptates but have favorable kinetics. 

In media such as water and alcohols, fluoride ion is strongly solvated by hydrogen bonding and is neither very basic nor very nucleophilic. On the other hand, the poorly solvated, or naked, fluoride ions that aie present when potassium fluoride dissolves in benzene in the presence of a crown ether aie better able to express their anionic reactivity. Thus, alkyl halides react with potassium fluoride in benzene containing 18-crown-6, thereby providing a method for the preparation of otherwise difficultly accessible alkyl fluorides. 

Ethers are not very reactive. They are more volatile than alcohols with similar molar masses because their molecules cannot form hydrogen bonds with one another. Crown ethers adopt shapes that can enclose ions and carry them into nonpolar solvents. 

The crown-ether adduct shown in Scheme 13 exhibits both inter- and intramolecular C—H O hydrogen bonding in the solid state (Figure 3)7 ... 

A possible explanation comes from X-ray analyses of the sulfonic acids [45]. All X-rayed crown ether crystals contained water and the sulfonic acid moiety was dissociated. Therefore in crystals of [45], macrocyclic ben-zenesulfonate anions and hydronium ions (sometimes hydrated) are present. The ions are bound to each other by hydrogen bonds. The size of the included water-hydronium ion cluster (varying by the number of solvating water molecules) depends on the ring size. In the 15-membered ring, HsO" was found, whereas in a 21-membered ring HsO and in the 27-membered ring were present. This means the sulfonic acid functions in [45] are... 

X-ray analysis of the complex [67] between 2,6-pyridino-17-crown-9 [9b] (n = 6) and a guanidinium ion showed that one hydrogen bond is formed between the lone pair of the pyridine nitrogen atom and a hydrogen atom of Gu". The other five hydrogen atoms are bound to ether oxygens in their vicinity. The association constants for the reaction of some macrocycles of type [9b] (n = 6-8) with Gu" were comparable (logX 1.18-1.44). 

Triphenylsilanol also forms simple hydrogen-bonded adducts with ethers. For example, the crown ether 12-crown-4 forms a 2 1 adduct... 

Bi-0 2.54(l)-2.68(2) A], In contrast, the nine-coordinate capped square antiprism geometry for bismuth in [Bi(N03)3(H20)3] (18-crown-6) does not involve the expected multidentate ether coordination to bismuth, but rather a hydrogen-bonded interaction of the crown ether with the hydrated bismuth center chelated by bidentate nitrate groups [Bi-0 2.38(2)—2.56(2) A] 32, implying that the hexado-... 

Since Pedersen s original work on the use of cations to template the formation of crown ethers [18-20], a large number of different templating agents for macro-cyclization reactions have been reported. While the initial work concentrated on the use of metal cations, further developments demonstrated that species with hydrogen bonding donor or acceptor properties could be equally useful to template the synthesis of macrocyclic molecules. 

The strong hydrogen bonding interactions observed between the oxygen atoms of crown ethers and the N-H groups of ammonium groups can be successfully employed to prepare pseudorotaxanes and rotaxanes by templated processes. This approach has been extensively utilised by Stoddart, Busch and others to obtain a wide range of interlocked species. 

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