The thermodynamic and crystallographic studies carried out on crown-ether based calix[4]arenes, in which one polyethylene chain (-CH2-CH2-O)n bridges two oxygen atoms of two opposite phenol units at the ‘narrow-rim’ (as shown in Fig. 11.4), have led to the following observations:

• p-tert-Butylcalix[4]arenes-dimethoxy-monocrown-n (n = 5 or 6) better complex large alkali cations, such as potassium and rubidium, because they can adopt a flattened partial cone conformation (Ghidini et al., 1990), which is impossible for larger alkoxy functions.

• As soon as the two remaining functions grafted onto the ‘narrow — rim’ of a calix[4]arene contain more than two carbon atoms, any of the four conformations encountered (Fig. 11.3) can be blocked in solution.

• Unlike a podant-based calix[4]arene, the selectivity of which favours sodium complexation only if it adopts the cone conformation, the pres­ence of a polyether bridge on the ‘narrow rim’ of a calix[4]arene, that has been blocked in the cone conformation, enhances its complexing and extracting properties toward alkali cations and offers additional control of the selectivity through the adjustment of the size of its coor­dinating cavity to the targeted metallic cation radius. As a result, a bridge presenting five oxygen atoms appears suitable for potassium complexation, whereas a bridge containing six O-atoms better fits the caesium cation radius (Ungaro and Pochini, 1991).

• The presence of a polyether bridge on the ‘narrow rim’ of a calix[4] arene, blocked in the 1,3-alternate conformation, strongly increases both its extraction efficiency toward caesium from acidic feeds and its selec­tivity versus other alkali cations, which is assumed to be due to a favour­able enthalpy contribution (Ungaro et al., 1994, Casnati et al., 1995, 1996, 2001, Sachleben et al., 1999, Talanov et al., 2000, 2002).

In reality, the benefit of the 1,3-alternate conformation for caesium selective extraction was first observed with the symmetrical doubly crowned calix[4]arenes (Fig. 11.5), synthesized by Vicens’ team who looked for easier manufactured bridged calix[4]arenes, obtainable in single-step syntheses avoiding the alkyl substitution of the two remaining phenol units of the calix[4]arenes-monocrown-n (Asfari et al., 1992, 1995). Like crown ethers and calix[4]arenes-monocrown-n, the calix[4]arenes — biscrown-n perfectly illustrate the benefit of matching the size of the coordinating cavity of the ligand with the ionic radius of the target cation. For instance, calix[4]arenes-biscrown-n, bearing five (n = 5) or seven (n = 7) oxygen atoms in their ether-crowns, show neither high extraction yields toward caesium, nor higher selectivity toward Cs+ (over other alkali cations) than di-(tert-butyl-benzo)-21-crown-7. As the caesium aqua complex possesses six water molecules, the six O-atoms of the ether-bridges of calix[4]arenes-biscrown-6 are consequently well pre-organized to displace the six water molecules of caesium inner co­ordination sphere.

Furthermore, outstanding Cs+/Na+ selectivity (SFCs/Na, exceeding 30 000) was obtained with calix[4]arenes-crown-6, the polyether bridges of which contain aryl rings such as benzyl or naphthyl (Fig. 11.5, Hill et al., 1994, Dozol et al., 1999). The selectivity of these ligands toward caesium is so high that they are better Cs+ sensors than any other functionalized calix[4]arenes

11.5 Examples of doubly bridged calix[4]arenes with functionalized crown ethers.

(Perez-Jimenez et al., 1998). Molecular Dynamics calculations as well as X-ray crystallographic data suggest that, provided no steric hindrance is introduced in the poly ether bridge (s), hydrophobic interactions exist between the extracted alkali cations and the n electrons of the aryl rings, hence favouring the binding of the less hydrated caesium cation as com­pared to harder alkali cations, such as sodium (Wipff and Lauterbach, 1995, Lauterbach and Wipff, 1996, Thuery et al., 1996, Lamare et al., 1997, 1998, 1999, 2001, Asfari et al., 1999, Jankowski et al., 2003).

As expected, although unusual in metallic complexation, the symmetrical arrangement of calix[4]arenes-biscrown-6 with two complexing cavities, is well adapted to the formation of both 1 : 1 (ligand : metal) and 1 : 2 com­plexes, as indicated by NMR, electro-spray ionization mass spectrometry (ESI-MS), and X-ray crystallographic studies (Arnaud-Neu et al., 1996, Allain et al., 2000).

The design of calix[4]arenes-crown-6, presenting one (or two) polyether chain(s) bridging two opposite phenol units of calix[4]arenes, blocked in the 1,3-alternate conformation, has therefore allowed both concepts (ligand pre-organization and host-guest complementarity through size fitting between substrate and receptor) to be tested in the search for caesium selective lipophilic extractants.