Anions, calixarene stabilization

It is not only p-substituted phenols that have been connected by methylene groups an extended calixarene based on naphthalene units [57] has also been synthesized and its acidity constants have been measured (see Table 25 Poh and Lim, 1989, 1990). In contrast to the calix[4]arenes, the pKa of the expanded calixarene [57] did not differ much from the of the analogue [58]. This may have two explanations (i) the conformation of the expanded calixarene is different from phenol-based calix[4]arenes and a special stabilization of the anion does not occur, (ii) this stabilization occurs but it is also present in the analogue. A stabilization by the neighbouring... 

Complexes of calixarenes with bipyridyl chromophores can be stabilized by the addition of anionic side arms, such as iminodiacetate units (85). Whilst the lanthanide complexes of ligands [L51]4- and [L52]4- are not soluble in water, their photophysical properties in... 

The corresponding dependence of cation complex stability on the anion differs profoundly from that of most other cation receptors such as cyclophanes or calix-arenes [16]. For these cation complex stability decreases on changing the anion from picrate through iodide to tosylate, a dependence that has been attributed to ion-pair aggregation in non-polar solvents. Because the interaction of quaternary ammonium ions with tosylate or iodide in chloroform is considerably stronger than with picrate, cation complexes in the presence of the latter anion are usually more stable. Only when iodide or tosylate cooperatively contributes to cation binding, as in 3 or in some recently described calixarene derivatives [17], is reversal of this order observed. 

The possible number of inherently chiral structures and conformers further increases if the calixarene contains both different phenolic units and different bridges in the macrocyclic skeleton. For example, two chiral monoethers 88a,b are available from dihomooxacalix[4]arenes (one -CH2-0-CH2- bridge instead of -CH2-).17188b is the preferred product of the mono-O-alkylation, since the negative charge of the respective phenoxide anion is better stabilized by intramolecular hydrogen bonds due to the smaller distance between the phenoxide anion and the hydroxy groups. Tetraketone derivatives (Y = CH,-C(0)-R) in the two possible partial cone conformations, have been prepared in moderate yields. 

Stabilization and crystallization of [Cr f/x-OI 0/0112)J(3j y>+ cations have been achieved by exploiting the supramolecular interactions with large anionic assemblies initially with mesitylene-2-sulfonate or its adduct with 18-crown-6,837-839 and subsequently with the adducts of calixarene sulfonates with N- or O-macrocycles.840,841 This approach, illustrated in Scheme 27, allowed the crystal structures of di-, tri- and tetrameric Cr111 complexes, (165)-(167),837,839-841 as well as that of a heteropolynuclear complex, [(H20)4Rh(/i-0H)2Cr(0H2)4]4+,838 to be determined. 

Another way to synthesize novolacs is under weak acidic conditions. Typical reaction conditions are a pH-range between 4 and 7 and the use of bivalent metal salts (e.g. Ca, Zn, Mg, Pb, Co, Cu, Ni) [56,57]. These conditions are highly selective for ortho,ortho -novolacs that are of increasing interest These compounds possess only a vacant para position for further reactions. Such materials have increased importance as molding compounds and foundry resins because ortho-linked novolacs can only react at the para position and they possess a hyperacidity [58]. The hyperacidity can be easily explained by the formation of intramolecular hydrogen bondings that stabilize the phenoxy anion. Complete methylene bridging at the ortho position of phenolic compounds can lead to calixarenes. 

Fig. 1. Structure of the calixarene 5— anion viewed from the top of the cavity. Hydrogen atoms OHl and OH2 stabilize the cone, while OH4 is used in hydrogen bonding to a water molecule of the inorganic layer.

The increase in stability constant can be ascribed to (i) an increase in acidity of the urea NH protons because of the electron withdrawing effect of the bound cation (ii) the rigidification of the calixarene units by cation complexation and (iii) an electrostatic attraction between the bound cation and anion. Thus, this receptor cooperatively binds a range of sodium and potassium halides with the bound group 1 metal cation effectively switching on anion complexation. Extraction experiments demonstrated that the host is capable of solubilizing sodium and potassium salts in chloroform. 

Multi-carbo grlated (or multi-thiolated and multi-carbonyl) compounds, as well as synthetic macrocyclic compounds, are always introduced to stabilize metal nanoparticles. For example, quantum dots (QDs) have been hybridized with cyclodextrin and calixarene derivatives to construct hybrid materials used as sensors for ions, organic molecules and chiral compounds. Thus, CP[5]A with 10 carbo qrl anion groups on both rims of the cavity has... 

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