Charged Calixarenes

With the help of mass spectroscopy (ESI), self-complexation of the charged host n-16 is detectable even in the gas phase. Also the complexation between the hosts 11-15 and n-16 is monitored by the related mass peaks. 

The binding of iminium ions by neutral calixarene hosts in the gas phase has also been detected with the help of SIMS-mass spectroscopy, and even binding constants have been estimated. The peak intensities observed in mass spectrometry reflect the association in a matrix and the stability of the complexes in the gas phase. Thus, mass spectrometry is a suitable tool to detect charged complexes based on cation- r interaction. Because of 

However, the differences in host geometry are much smaller in complexes than in free 

---

C. A. Schalley, R, K. Castellano, M. S. Brody, D. M. Rudkevich, G. Siuzdak, J. Rebek, Jr, Investigating Molecular Recognition by Mass Spectrometry Characterization of Calixarene-Based Self-Assembling Capsule Hosts with Charged Guests , J. Am. Chern Soc, 1999, 121, 4568-4579. 

After the separation of the actinides from the high-level waste, it is desirable to remove certain other fission products from the nuclear wastes. Some Cs and Sr are low-charged cations that react well with macro-cyclic ligands (e.g., crown ethers, calixarenes). Research to synthesize and investigate the properties of macrocyclic ligands for application in nuclear waste treatment has been an active effort internationally. Some of the results obtained are discussed in section 12.7. 

Single-element Separation Extraction of Cs + ion is fairly difficult due to the small charge density of the atomic surface. Thus, calix-crowns were preferentially used for the extraction, because they trap Cs + ion not only by coordinating with the crown ring, but also by interaction with the n-electrons of the phenyl rings of the calixarene (382, 383). On the other hand, many reports appeared concerning extraction of Sr2+ from acidic solutions by crown ethers (384). 

Meanwhile, cation - crown complexes were fully optimized by ab initio QM techniques in the gas phase, allowing to investigate the effect of electronic reorganization (mainly charge transfer + polarization effects) on the recognition properties/15,161 QM/MM hybrid methods which combine a QM representation of the ion coordination sphere with a MM representation of the more remote species, have been tested on 18C6/ cation complexes/17, 18] Typical applications of "computational approaches in supramolecular chemistry" up to 1994 can be found in ref. [191 There are also reviews on more specialized fields like calixarenes/20,21]... 

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]. 

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. 

---