Selectivity. Alkaline-Earth Cations

Control over Divalent/Monovalent Cation Selectivity 

It follows from the above discussion (sections II. 5,6,7), that the thicker the organic ligand shell and the lower the dielectric constant of the 

Comparing ligands 22 and 38 which contain the same number of binding sites, we note that the special complexation features of 38 result from the cryptate nature of its complexes. Indeed, whereas in complexes of 22 polar solvent molecules may approach the cation from top and bottom, it is much more shielded in complexes of 38. This difference in behaviour is reflected in the corresponding change in ligand thickness (Table 12). The results in Table 11 also display the expected decrease in M2+/M+ stability ratio as the dielectric constant decreases from water to methanol. 

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This ionophore is able to carry selectively alkaline-earth cations through membrane phases by an antiport mechanism (m W 2h ) with a transport efficiency in favour of Ca versus Mg in biological membranes(3).Owing to its specificity, calcimycin is universally used as a tool to investigate the role of the second messenger calcium in physiological processes. 

Reaction of Na2Se and Na2Sc2 with Se in the presence of ethanolic solutions of tetraalkyl-ammonium halides and catalytic amounts of I2 yields dark green or black crystalline polyselenides (jc = 3,5-9) depending on the conditions used and the particular cation selected. Tetraphenylphosphonium salts and crown ether complexes of alkali or alkaline earth cations in dimethylformamide solution can also be used. " )... 

Other recent works in this field, studies on the transport of alkali and alkaline earth cations with p-zerr-butyl calix[n]arene esters and amides, were carried out by Arnaud-Neu et al. [20] and Casnati et al. [21]. They prepared 1,3-alternate calix[4]arene-crown-6 as a new class of cesium selective ionophore. 

Cryptand-based PCT sensors The above-described chelators are well suited to the detection of alkaline earth cations but not alkali cations. In contrast, cryptands are very selective towards the latter. 

Diaza-18-crown-6 substituted with 5-chloro-8-hydroxyquinoline exhibits very interesting complexing properties. For instance, M-6 is very selective for Ba2+ over other alkaline earth cations and for K+ over Na+ in methanol. Unfortunately, investigation of fluorogenic effects with other cations has not been reported. On the other hand, the fluorescence intensity of M-7 was shown to increase by a factor of 1000 in the presence of Mg2+ (in a mixture of methanol/water 1 1 v/v) whereas other alkaline earth ions have no effect. 

Since the central topic of the present chapter is the design of ligands for the selective complexation of alkali and alkaline-earth cations, we shall first consider the most general features of such processes. 

Table 11. Selectivity of alkali and alkaline-earth cation complexationa)... 

Ligands Selective for Alkali and Alkaline Earth Cations. 115... 

As in the coronands, binding site identity plays an important role in the complexation preferences of cryptands. Replacement of oxygens by sulfur or nitrogen in the bridges (45b-d) results in decreased stability and selectivity for alkali metal and alkaline earth cations (as was also noted for the coronands).66,164 Selectivity is then shifted toward softer metal ions such as Ag+, Tl+, Cd2+ and Pb2+. 

Stability constants associated with complex formation correspond to two successive steps as shown in Scheme 6.190 The constants for the formation of 1 1 and 2 1 complexes of the smaller alkali cations are comparable to those of the lone macrocycles [1.1] for complexes of (52). Very stable complexes are formed by the alkaline earth cations with this ligand (log 7 in H20).,9° The larger macrocycles of (53a-d) form alkali metal and alkaline earth complexes in which the stabilities of the 1 1 complexes are similar to those of the model V-methylated monocycle. The stabilities and selectivities of the 2 1 complexes are also similar to those of the 1 1 complexes. In fact, KS2 for incorporation of a second Ba2+ into (44a) is as high as KS] for the same ligand.190 Such findings indicate two essentially independent macrocyclic units. 

Electrophoretic mobility and 31P-NMR measurements were made to investigate the binding of the alkaline earth cations to membranes formed from phosphatidyl choline molecules with either saturated or unsaturated hydrocarbon chains. Calcium and magnesium bind to the same degree to membranes formed from lipids with unsaturated chains. These phosphatidylcholine molecules are present in the liquid crystalline state at all temperatures under consideration. Calcium binds more strongly than magnesium to membranes formed from lipids with saturated chains, even when the lipids are in the liquid crystalline state. The selectivity is enhanced when the temperature is lowered and the saturated chain lipids are in the gel state. 

Several classes of synthesized calixarenes bearing several moieties (ether, ester, and amide derivatives), were tested for the extraction of strontium picrates (from aqueous solutions into dichloromethane).128 Only a few of them show appreciable extraction levels. The p-i-butyl calix[6]arene hexa(di-/V-ethyl)amide (CA4) shows a very high extraction level of alkaline earth cations with respect to alkali metal cations. Moreover, dealkylation of the calix[6]arene hcxa(di-/V-cthyl)amidc (CA5) decreases the extraction of both sodium and strontium. As this decrease is much more important for sodium than for strontium, the Sr/Na selectivity, which increases from 3.12 to 9.4, is better than that achieved for DC18 derivative under the same conditions (8.7). These results were confirmed by extraction of strontium (5 x 10 4 M) from 1 M HN03 solutions, where it was found that p-t-butyl calix[4]arene tetra(di-N-ethyl) amide (CA2) (10 2 M in NPOE) extracts only sodium (DNa = 12.3, DSl < 0.001). 

In the early 1990s, there existed several classes of extractants for actinides (CMPO), for cesium and more generally alkali cations, and for strontium and alkaline earth cations (crown ethers and cosan). The combination of these extractants and the grafting of these functions on calixarene platforms have led to new classes of extremely efficient and selective extractants, in particular calixarene-crown, which are presently applied in the United States to treat the huge amounts of waste at the SRS. Calixarenes/ CMPO, crown ethers/cosan, CMPO/cosan, and more recently calixarenes/CMPO/ cosan are promising compounds. It is desirable that these studies, conducted at the international level, continue in particular to obtain a better understanding of the complex mechanisms of extraction of these compounds.127187... 

D. M. Huryn, Carbanions of Alkali and Alkaline Earth Cations (ii) Selectivity of Carbonyl Addition Reactions, in Comprehensive Organic Synthesis (B. M. Trost, I. Fleming, Eds.), Vol. 1, 49, Pergamon Press, Oxford, 1991. 

The previous parts of Sect. 5 testify to the fact that development of sensors for a given chemical species is dependent on the availability of receptors with suitable selectivity, electroactivity and optical transparency. These criteria were most easily satisfied with amine derivatives with various structural modifications for the binding of protons, alkali and alkaline earth cations. Such is the ubiquity of the amine motif in fluorescent PET research that several of the examples to be discussed in this section also contain it within their receptors. 

Thermodynamic data in MeOH show that ligand 68 selectively binds Mg2+ (log K = 6.82) over other alkali and alkaline earth cations (log K=2.89-5.31).123 UV-vis... 

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