Calixarenes direct complexation

Electrospray Ionization/Mass Spectrometry (ES/MS), a soft-ionization desorption technique using polar solvents such as water, methanol, or acetonitrile, was used for direct measurement of cations in solution. The first measurements carried out with mono or bis(crown-6) calix[4]arenes from an equimolar cation-extractant solution confirm that the calixarenes mono(crown-6) extract only one cesium cation. On the contrary, in the same conditions, bis(crown-6) calix[4]arenes can extract two cesium cations for a ratio Cs/BC6 equal to 2.5. The binuclear complex (composed of two cesium cations) is the major species. Cesium/sodium selectivity measurements implementing various mono or bis(crown-6) calix[4]arenes were in agreement with liquid/liquid results.42... 

Another class of metal-employing anion receptors is represented by structure 24 [23]. Its function is based on the incorporation of positively charged transition metal complexes directly into the calixarene skeleton. Such calixarenes with enhanced electron deficiency of the aromatic walls provide well-preor-ganised cavities suitable for anion inclusion. The corresponding rhenium [24], ruthenium, rhodium or iridium complexes of this type were prepared and studied for anion recognition [25,26]. 

Calixarenes are formed by condensation of a p-substituted phenol with formaldehyde [8]. These macrocycles are conformationally quite flexible but, by introducing suitable substituents in the aromatic subunits, the so-called cone conformation, in which all aromatic subunits point into the same direction, can be stabilized. This conformation is usually best suited to complex guest molecules because it has a well defined hydrophobic cavity. An inclusion of cations such as ammonium ions or quaternary ammonium ions into this cavity can be demonstrated, for example, by the characteristic upfield shifts of guest signals in the NMR, an effect that is a consequence of the close proximity of the corresponding protons to the surfaces of the aromatic receptor subunits in the complex. 

Because of expected electrical, electrochemical, optical, chemo-sensor, and other properties of the ball-type Pcs, several metallo and metal-free ball-type Pcs with different linkers have been reported. Metallo and metal free ball-type Pcs containing four calyx[4]arene units as linkers have been synthesized, Fig. 1 [37], The structures of compounds 1-3 were confirmed by UV-vis, IR, 1II-NMR, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and elemental analysis. Because of their unique structure and versatile complexation properties [38], calixarenes were found to be one type of interesting compounds to be incorporated into Pc. The cone conformation of the t-butylcalix[4]arene direct... 

The relatively low thermodynamic stability of complexes of hemicarcerands or other container-type hosts is a direct consequence of structural aspects of the walls that make up the inner surface of such compounds. These walls are lined by aromatic subunits while free electron pairs of heteroatoms such as those of the ether oxygen atoms are preferentially oriented to the outside. Complexes are therefore enthalpically stabilized only by weak dispersive interactions. In the case of positively charged guests cation-re interactions can contribute to binding enthalpy as in a self-assembled calixarene-derived capsule [9], but directed interactions such as hydrogen-bonding interactions are usually absent. 

The complex was characterized by thermogravimetric analysis (TGA) and solid state NMR. Low temperature X-ray crystal structure analysis of the compound indicated the inclusion of CS2 in the calixarene cavity to be highly symmetrically oriented directly on the C4 axis of the host. The CP/MAS experiment showed significant complexation-induced chemical shift (CIS=... 

One of the important applications of butylcalix[4]arenes arises from their ability to trap alkali metal ions. In particular, Cs+-calixarene complexes have received much attention because of the need to remove the Cs radionucleotide from nuclear wastes. Benevelli et al. have used one-pulse solid state NMR experiments to directly observe Li, Na and Cs ions in the host cavity [52]. More advanced experiments, which allow the investigation of metal lattice interactions were also reported. Rotational-echo double resonance (REDOR) NMR is a useful tool for obtaining structural details of butylcalix[4]arene [53]. Gullion and coworkers used REDOR to determine the position of the... 

The higher energy of the triplet states in calixarenes 39a-f (fig. 37), 20600-23 500 cm , allows sizeable sensitization of Eu and Tb° luminescence. Compared with calix[4]resorcin-arenes, better quantum yields are also obtained for the Nd and Yb 1 1 complexes (table 7), 0.06-0.24%, and 0.09-0.61%, respectively. However, the solvent is not the same, dmf as compared to ethanol, which may explain the difference. As for the calix[4]resorcinarene complexes, the speciation in solution has not been established, but in this case, the 1 1 complexes have been isolated and gave satisfactory elemental analyses (Korovin and Rusakova, 2001 Shevchuk et al., 1998a). The best quantum yields are obtained with H239f which bears two phenylamide chromophores on the narrower rim and these substituents are directly coordinated onto the lanthanide ion, which understandably increases the efficiency of the energy transfer process. 

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