Calix arene, encapsulation

Calix[ ]arenes are a family of macrocycles prepared by condensation reactions between n /v/ra-substituted phenols and n formaldehyde molecules under either base or acid catalysis. Different sizes of the macrocycles can be obtained (n = 4-20) (Stewart and Gutsche, 1999) depending on the exact experimental conditions, which were mastered in the 1960 s (Gutsche, 1998), but the most common receptors are those with n =4,6,8 (macrocycles with an odd number of phenol units are more difficult to synthesize). We use here the simplified nomenclature in which the number of phenolic units is indicated between square brackets and para substituents are listed first.4 Calixarenes, which can be easily derivatized both on the para positions of the phenolic units and on the hydroxyl groups, have been primarily developed for catalytic processes and as biomimics, but it was soon realized that they can also easily encapsulate metal ions and the first complexes with d-transition metal ions were isolated in the mid-1980 s (Olmstead et al., 1985). Jack Harrowfield characterized the first lanthanide complex with a calixarene in 1987, a bimetallic europium complex with p-terf-butylcalix[8]arene (Furphy etal., 1987). 

Catalytic effect of calix[ ]arene based sol-gel encapsulated or covalent... 

Calix[5]arenes encapsulate groups of fullerenes into their cone-shaped cavity [48-59]. Atwood, Barbour, Raston and coworkers have reported that calix[5]arene 3, toluene and the [60]fullerene molecule form co-crystals one with a 1 1 1 composition with the [60]fullerene molecules arranged in a supramolecular zigzag and the other with a 4 2 5 composition, giving rise to a Z-array of the [60]fullerene... 

Figure 8.7 Encapsulation of palladium in a heterocapsule formed by ionic interactions between a concave Xantphos-based diphosphine ligand and a tetraanionic calix[4]arene.

The X-ray structure of the L-Sr(Picrate)2 (L = p-tert-butyl-calix[4]arene-tetra(diethylamide)) is reported, as well as MD simulations on the L M2+ complexes in vacuo, in water, and in acetonitrile solutions for alkaline earth cations with a comparison of converging and diverging conformers.130 In the simulated and solid-state structures of the L M2+ complex, the ligand wraps around the complexed cations M2+ (more than it does with alkaline cations), which are completely encapsulated within the polar pseudo-cavity of L, without coordination to its counterion in the crystal or to solvent molecules in solution. In contrast to alkali cation complexes, which display conformational flexibility in solution, computations show that the alkaline earth cation complexes are of the converging type in water and in acetonitrile. Subtle structural changes from Mg2+ to Ba2+ are observed in the gas phase and in solution. Based on FBP calculations, a binding sequence of alkaline earth cations was determined Mg2+ displays the weakest affinity for L, while Ca2+ and Sr2+ are the most stable complexes, which is in agreement with the experiment. 

Photoirradiation of a tetrahydrofuran (THF) solution of calix[4]arene 386 having two anthryl groups produces 387 (Scheme 101). Encapsulation of alkali metal ions into the ionophoric cavities of 386-387 was investigated by Shinkai and co-workers [358], Compound 386 itself showed the poor ion affinity and the poor ion selectivity, whereas 387 showed the much improved ion affinity and the sharp Na+ selectivity. 

Some other capsules have been described in which both complimentary functions, carboxylic acids and nitrogen-containing aromatic rings, are respectively attached to the rims of two bowl-shaped molecules. Reinhoudt has prepared selfassembling, multi-hydrogen bonding molecular capsules in which a calix[4]arene substituted with four carboxylic functions at the upper rim interacts with a complimentary calix[4]arene with four pyridines attached to the lower rim. These capsules have been identified by H-NiMR, IR and VPO measurements but encapsulation properties have not been reported [106]. 

The dimerisation of larger calix[6]arenes bearing three carboxylic acid groups in alternate positions on the upper rim has been reported. In this case the cavity is of sufficient size to encapsulate simple pyridinium salts. ... 

Calix[5]arenes have also been demonstrated to form crystalline capsule-like dimers. Specifically, a tetra-substituted calix[5]arene with four methyl and iodo groups attached at opposite rings along the upper rim formed a dimer that encapsulated buckminsterfullerene, or C60 (Fig. 18).36 Raston has also described dimers based on tetrabenzyl-derivatized calix[3]- and calix[5]arenes that encapsulate C60 in the solid state.37,38 The components assembled via van der Waals forces. 

Fig. 18 X-ray crystal structure of the encapsulation complex between two molecules of an iodo-derivatized calix[5]arene and C60.

Encapsulated potassium cation in a new calix[4]arene neutral ligand - synthesis and X-ray crystal structure, G. Calestani, F. Ugozzoli, A. Arduini, E. Ghidini and R. Ungaro, J. Chem. Soc., Chem. Commun., 1987, 344. 

Complementarity between different molecules has also been harnessed in the pursuit of capsular complexes. Schrader and Kraft have pioneered the design of complementary systems with three- and fourfold symmetry based on 1,3,5-tris(aminomethyl)benzene derivatives [18] and calix[4]arenes [19], respectively. This approach has also been taken by Reinhoudt whose vast output of calixarene chemistry includes work on calix[4]arene dimers held together by ionic interactions [20]. Other examples show how the presence of encapsulated guests templates the formation of self-complementary capsules [21], how two host molecules may be held together by metal coordination [22-24] or where hydrogen bonding by a second entity holds the assembly together [25,26],... 

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