Calixarene-derived ligands

Kinetic analyses allowed the experimental results to be related to the dependence of transport rates and selectivities on carrier properties [6.9, 6.17, 6.18]. Detailed studies of 8 and 9 in vesicles bore on the efficiency, the selectivity and the mechanism of the processes [6.19]. The rates of transport by proton ionizable macrocyclic carriers are pH dependent [6.12]. Diverse other ligands have been used as carriers, such as acyclic polyethers or calixarene derivatives [6.20, A.6]. 

Both the lower (phenolic) rim and the upper rim of the calixarene can be functionalized. A huge range of calixarene derivatives have been synthesized in this way, and only a few can be mentioned here. In addition, the linking methylene groups can be modified to give another group of useful ligands. ... 

The first bisphosphine calixarenes that have been used in catalysis are di(amide)-phosphine hybrids calix[4]ar-ene. Reaction of [RhCl(norbornadiene)]2 with these calixarene derivatives gave an organometallic complex whose norbornadiene-rhodium moiety lies above the cavity defined by the four substituents of the calixarene and between the two amide functionalities. This complex was applied in the hydroformylation reaction of styrene. The rather low reaction rate observed (7.5 turnovers per Rh per hour) has been attributed to a partial encapsulation of the metal center preventing the approach of the substrate. Indeed, the metal center may be viewed as located in a hemispherical ligand environment. 

The main classes of calixarene derivatives for the complexation and extraction of alkali and alkali earth cations are based on the total subsitution at the lower rim by alkoxyester. alkoxyamide. or alkoxyketone functions. Fig. 2. In this way. suitable donor ligand functions for cation complexation are introduced. Such derivatives were used for the complexation and solvent-to-solvent... 

Fig. 1 Molecular structures of the calixarenes, where, commonly, n = 4. 6. and 8 and the calixresorcinarene (A). The strategy for cation complexation by the introduction of ligand groups at either the lower or upper rims of calixarene derivatives (B). Metal complex with...

Our first simulations involving interfaces concerned ionophores L, free or complexed by cations As these molecules are not soluble in water, but are soluble in organic liquids or membranes, it was anticipated that, when placed initially on the border between the two liquids, they would diffuse to chloroform. This turned out not to be the case. Whatever the nature or conformation of L, it adsorbed on the chloroform side of the interface, anchored via the polar moities solvated by a few water molecules ("water fingers" ). Some of the ligands studied (calixarene derivatives podants CMPO s alkyl phosphates. .) are... 

Complexation studies were carried out by electrospray ionization mass spectrometry (ESI-MS) with three lanthanides (La, Eu, and Yr) at a concentration of 10 4 M, while the ratio of concentration of CPw2 versus calixarene (r = [calix]org/[Ln]aq) was varied between 0.1 and 10.165 The behavior of the three lanthanides is the same the 1 1 Ln calix-CMPO complex is always predominant in the range of concentration studied the 2 1 Ln calix-CMPO appears in the case of metal excess with a maximum for r = 0.5, while the percent of 1 2 Ln calix-CMPO increases as the concentration of ligand increases, but does not exceed 10% for r = 10. These studies show that two factors play a key role in the selectivity of CMPO derivatives, firstly the presence of phenyl groups on the phosphorus atom, which confers the selectivity to CMPO ligands, secondly the calixarene structure, which amplifies this selectivity due to its preorganization. Indeed the noncyclic derivative (Os2), in spite of the presence of phenyl units, does not display noticeable selectivity. 

Calixarenes are potential platforms on which specific binding arms can be grafted. The extractive properties of these molecules for metallic ions depend on the cavity size, the conformation, and the nature of the ligating groups. Different calix[4] arene-crown-6 derivatives in the 1,3-alternate conformation have been studied for Cs recovery from both basic and acidic solutions (257-262). Calixarene-based picol-inamide ligands have been proposed as candidates for separating actinides from lanthanides (263, 264). 

Therefore, another strategy was developed, based on the induced-fit concept, which uses flexible receptors in order to optimize the interactions between the donor atoms and the metal ion. In fact, the coordination environment is built upon complexation thanks to the flexibility introduced into the complexation agent, which is now termed predisposed ligand . These receptors are either large macrocycles able to wrap around the guest or small macrocycles fitted with pendant arms. The latter approach has proved to be very successful, particularly with calixarene (Asfari et al., 2001) and cyclen (1,4,7,10-tetraaza-dodecane) (Lukes et al., 2001) derivatives. 

The area covered is very widespread and the role of the calixarene molecules reach from a simple platform or skeleton on which to assemble chiral centers to an inherent part of the chiral structure. Biologically active molecules or derivatives are involved as well as artificial ligands and their metal complexes. Chiral calixarenes have been used as stationary phases in analytical separations or as host molecules in sensors. Basic properties of calixarenes, such as their conformational stabilities, have been studied with chiral derivatives as well as more... 

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