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A calix[6]arene wheel, whose cavity has been extended and rigidified by N-phenylureido groups on the upper rim, forms pseudorotaxane species with molecular axles containing the viologen (4,4'-bipyridinium) unit in CH(2)Cl(2) solution. In these conditions, the self-assembly process is very efficient, with associated DeltaG degrees values of around -8 kcal mol(-1). The counteranions of the bipyridinium-based threads play indeed an important role in the formation of the complex. The use of either tosylate or hexafluorophosphate salts of the guests affects both the stability of the complexes and the rate of the threading process. Such effects have been interpreted in terms of ion-pair recognition, suggesting that coordination of the counteranions of the viologen thread by the ureido groups of the calixarene wheel is crucial for the breaking of tight ion pairs prior to threading. The rate constants of the threading/dethreading reactions coupled with the redox processes of the viologen unit of the axle have been obtained by means of cyclic voltammetry. The pseudorotaxane species undergo fast dethreading (submicrosecond time scale) on electrochemical reduction of the guest. The heterogeneous electron-transfer kinetics for the reduction of the viologen unit is slowed upon encapsulation into the calixarene cavity.

Single-crystal X-ray analysis has shown the formaldehyde and para-t-butylphenol and its ability to tetrameric structure of the low melting point product obtained from the base catalysed condensation of form a stable cage-type clathrate with toluene.

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Exceptionally high selectivity for cesium ions (α(Cs/Na) > 33000) is displayed by calix[4]arenecrowns‐6 in the 1,3‐alternate conformation. They also remove 137Cs quantitatively (>96%) from radioactive waste that is 1 M in HNO3. The complexation properties result from the simultaneous operation of several effects: the size of the crown ether ring, the polarity of the calix conformation, and the strength of the cation/π‐electron interactions. The latter interaction was evident in the X‐ray crystal structure of the cesium complex (shown on the right).

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for his valuable help in obtaining the 500-MHz NMR and high-resolution mass spectra, molecular mechanics calculations, and elemental analysis determinations. We also thank CONICET (Consejo Nacional

Dynamic 'H NMR measurements of the tetramethyl ether of p-tert-butylcalix[4]arene (2) show for the first time that all four possible conformations of one particular calix[4]arene are present, including the 1 ,balternate conformation. The thermodynamically most stable partial cone conformation readily interconverts to a cone or to a 1,3-alternate conformation; the interconversion to a 1,2-alternate conformation is much slower. The 1,2-alternate conformation of 2 is the kinetically stable conformation at the I H NMR time scale. The 1,a-alternate conformation was confirmed by comparison of its 'H NMR spectrum with that of the newly synthesized tetraethyl ether of p-tert-butylcalix[4]arene in a fixed 1,2-alternate conformation (6), of which the X-ray structure was determined. Partial rigidification of the calix[4]arene moiety in four different ways was achieved by replacing two of the methoxy groups of the tetramethyl ether 2 by ethoxy groups. The relative thermodynamic stabilities of the conformations of the calix[4]arene are influenced strongly by this relatively small change; in particular the 1,2-alternate conformation becomes much more stable. For the anti-1,3-diethyl-2,4-dimethyl ether 7b the 1,2-alternate is even the thermodynamically most stable conformation. Molecular mechanics calculations indicate that this is caused by the combined favorable effects on the electrostatic energy of the inside orientation of the methoxy groups and the relative large distance between the two ethoxy groups. The tetraethyl ether of p-tert-butylcalix[4]arene is not flexible at room temperature, but it equilibrates in solution at temperatures above 100 "C to a mixture of also all the four possible conformations.