K+-18-crown-6 complex

Its X-ray structure indicates the formation of a contact ion-pair where the complexed cation [K-18-crown-6 ] , shaped as "an overturned umbrella", is connected to the anion through three bridging iodine atoms. This arrangement allows the potassium ion to interact with other possible ligands in a direction perpendicular to the plane containing the crown-ether. 

Fig. 29. Formation of the phosphole complex [(CjPMe2Ph)4Na2(DME)2], 46, and the anion in the potassium compound tranj -[K([18]-crown-6)(THF)2]2[K(C4PMe2Ph)2THF]2. (Reprinted with permission from F. Paul et ai, Angew. Chem. Int. Ed. Engl. 1996, 35, 1125. Copyright 1996 Wiley VCH.)...

Hexacoordinate fluorosilicates were obtained from neutral pentacoordinate complexes by addition of K[18-crown-6]F191,192. In this way compounds 168-173 were prepared from their pentacoordinate precursors. These compounds, in addition to being fluorosilicates, contain in each chelate ring a dative Si—N bond. 

Crown ether Metal cation Ion-dipole Complex (cavitand) [K+([18] crown-6)]... 

Thus a large binding constant corresponds to a high equilibrium concentration of bound metal, and hence a more stable metal-macrocycle complex. Typical binding constants for crown ethers and alkali metal cations in water are in the range 101 102. In methanol, this increases up to 106 for [K([18] crown-6)]+. The binding constant for K+ and [2.2.2] cryptand is about 1010. Some other examples are given in Table 1.3. 

Fig. 3. Left Isoenergy contour diagram (energies in kJ/mol) for the interaction of K+ with [18] crwon-6 75). The conformation of the crown ether was fixed as found experimentally 172). Right Same view of the structure of the K.SCN [18] crown-6 complex as determined by X-ray crystallography 172. The distance between the position of the energy minimum (left) and the found position of K+ amounts to 0 pm...

Early in the development of the field, compounds were often prepared to define the limits of both the synthetic methods and the stable products that could be formed. To date, many thousands of heteromacrocycles have been prepared. The dominant application of the vast family of host or receptor molecules has been to bind or complex a guest structure. The guests can be metal cations, organic cations, neutral substrates, anions, or complementary molecules. The complexation process can be understood from the simple example of 18-crown-6 complexing K+Cl- in solution. Ignoring structural and solvation/desolvation issues, the process can be described simply as... 

More recently, o-bis(haloorganostannyl)benzenes were found to be powerful bidentate Lewis acids toward halide ions . Treatment of o-C6Fl4(SnMe2X)2, X = Cl, F (52i and 52j) with 1 molar equivalent of the halide ion afforded the corresponding anionic 1 1 complexes 52k, X = Y = F, M = Et4N, K 18-crown-6 X = Y = Cl, M = (Ph3P)2N X = Cl, Y = F, M = Et4, K dibenzo-18-crown-6 (equation 7). 

For the larger chloride ion, the final member in the series is the tetrachloride, An02CLi A The uranium, neptunium, and plutonium tetrachloride dianions have been isolated with a number of different cations, for example, Na+, NH4+, K+, Cs+, K+18-crown-6, and so on. The An=0 distances were found to be 1.768, 1.751, and 1.737A for An = U, Np, and Pu, respectively. The An-Cl distances in these complexes is virtually unchanged 2.675 A (U), 2.659 A (Np), and 2.656 A (Pu). These stmctural data suggest that the chloride hgands are bound in essentially a purely ionic manner while the covalent nature of the An=0 bond is preserved. 

A more stable open jr-system is provided by the pentadienyl ligand. The 2,4-dimethylpentadienyl ligand was first used in the generation of homoleptic ( -2,4-Me2C5H5)3U (equation 46). The mixed-ligand uranium(III) complex [K(18-crown-6)][(2,4-Me2C5H5)2U(BH4)2] has been prepared by reaction of (2,4-Me2C5H5)3U with... 

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