Selectivity. Alkali Cations

Best overall selectivity for all M /Mf couples is shown by compound 30, [2.2.2], However a given selectivity may be increased by structural modifications. For instance the K+/Rb+ selectivity increases markedly from 30 to 36. Indeed the introduction of the benzo ring is expected to decrease the cavity size (shorter O. .. O distance) and to increase ligand rigidity (rotation about the central C—C bond in the benzo bridge is frozen). Both effects hinder the cavity dilatation required for Rb+ inclusion on the other hand, they also favour the smaller Na+ cation with respect to K+, thus diminishing the K+/Na+ selectivity of 36 as compared with 30. 

A comparison of the selectivities displayed by 15 and 19 is also instructive. Despite its higher number of O. . . K+ interactions the [19, K+] complex is less stable than the [15, K+] complex the K+/Na+ selectivity of 19 is, however, much higher than that of 15. This is clearly linked to the ability of 19 to change conformation and wrap around the K+ cation. 

The flexible macrobicyclie ligands 31—33 display high K+/Na+ selectivity (300—600), but weak K+/Rb+, Cs+ selectivities. On the other hand, the more rigid ligand 30 shows a K+/Na+ selectivity similar to those of the flexible ligands 19 and 31—33, but it also has appreciable K+/Rb+, Cs+ selectivity. 

For a given ligand and from one solvent to another (water to methanol for example), the selectivity for a given M.tjM.j couple generally increases as the stabilities increase. Thus only selectivities of different ligands in the same solvent can be compared. 

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The question of carrier design was first addressed for the transport of inorganic cations. In fact, selective alkali cation transport was one of the initial objectives of our work on cryptates [1.26a, 6.4]. Natural acyclic and macrocyclic ligands (such as monensin, valinomycin, enniatin, nonactin, etc.) were found early on to act as selective ion carriers, ionophores and have been extensively studied, in particular in view of their antibiotic properties [1.21, 6.5]. The discovery of the cation binding properties of crown ethers and of cryptates led to active investigations of the ionophoretic properties of these synthetic compounds [2.3c, 6.1,6.2,6.4-6.13], The first step resides in the ability of these substances to lipophilize cations by complexation and to extract them into an organic or membrane phase [6.14, 6.15]. 

A similar study was undertaken on the related crown ether systems 201 <2001PS29>. They all showed moderate extraction of both Ag(l) and Hg(ll) ions and so were less selective than compounds 184a and 184b from the previous study. However, the presence of the benzo-15-crown-5 substituent offered the simultaneous complexation of the hard alkali cation Na(l) as well as the thiophilic metals Ag(l) and Hg(n) by the thieno sulfur. Interestingly, this second extraction was not influenced by the presence of the other metal. 

The sequence of the selectivities towards cations is also solvent dependent for dibenzo-18-crown-6 [11] the sequence is K+ > Na+ > Rb+ > Cs+ in water, methanol, dimethylformamide and dimethyl sulfoxide (Dechter and Zink, 1976 Srivanavit et al., 1977), whereas it is Na+ > K+ > Rb+ > Cs+ in acetonitrile (Hofmanova et al., 1978). A reversal of the K+/Na+ selectivity on going to apolar aprotic solvents was also observed for fluorenyl salts (Wong et al., 1970). Whereas for alkali cations the sequence of binding constants and enthalpies are the same in water (Izatt et al., 1976a), they differ considerably in methanol/water mixtures (Izatt et al., 1976b), dimethyl sulfoxide and acetone (Arnett and Moriarity, 1971). 

The cation selectivity in extraction experiments is dependent on differences in both the distribution constants KD and the binding constants Klf) in the water-saturated organic solvent (3). The extractability of alkali cations into... 

Up till now anionic mercury clusters have only existed as clearly separable structural units in alloys obtained by highly exothermic reactions between electropositive metals (preferably alkali and alkaline earth metals) and mercury. There is, however, weak evidence that some of the clusters might exist as intermediate species in liquid ammonia [13]. Cationic mercury clusters on the other hand are exclusively synthesized and crystallized by solvent reactions. Figure 2.4-2 gives an overview of the shapes of small monomeric and oligomeric anionic mercury clusters found in alkali and alkaline earth amalgams in comparison with a selection of cationic clusters. For isolated single mercury anions and extended network structures of mercury see Section 2.4.2.4. 

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. 

Fig. 11. Comparison of the electrochemical alkali ion selectivity of neutral antibiotics in liquid membranes (log for macrotetrolides (85), log K lM for valinomycin (86)) with the stability of the complexes (log K from Table 2) between these antibiotics and alkali cations in methanol...

Comparing the effects of alkali cations of various sizes applied in reduction of C02 in HCOJ solution with a Cu cathode, Na+, K+, and Cs+ were shown to favor the formation of hydrocarbons.138 The selectivity of ethylene formation surpasses that of methane with increasing cation size. Deactivation of the Cu cathode... 

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. 

Although some scattered examples of binding of alkali cations (AC) were known (see [2.13,2.14]) and earlier observations had suggested that polyethers interact with them [2.15], the coordination chemistry of alkali cations developed only in the last 30 years with the discovery of several types of more or less powerful and selective cyclic or acyclic ligands. Three main classes may be distinguished 1) natural macrocycles displaying antibiotic properties such as valinomycin or the enniatins [1.21-1.23] 2) synthetic macrocyclic polyethers, the crown ethers, and their numerous derivatives [1.24,1.25, 2.16, A.l, A.13, A.21], followed by the spherands [2.9, 2.10] 3) synthetic macropolycyclic ligands, the cryptands [1.26, 1.27, 2.17, A.l, A.13], followed by other types such as the cryptospherands [2.9, 2.10]. 

Cation-anion cotransport was effected by an optically active macrotricyclic cryp-tand that carried simultaneously an alkali cation and a mandelate anion and displayed weak chiroselectivity [4.23a], as did also the transport of mandelate by an optically active acyclic ammonium cation [6.39]. Employing together a cation and an anion carrier should give rise to synergetic transport with double selection by facilitating the flow of both components of a salt (see the electron-cation symport below). Selective transport of amino acids is effected by a convergent dicarboxylic acid receptor [4.24b]. 

However, a careful study of the experimental data has led to some general trends. For instance, the nature of the final products depends heavily on the alkali cations used in the starting compounds sodium and lithium phenoxides reacting under similar experimental conditions yield the related salicylates as major products [18] (Scheme 5.1), whereas potassium, rubidium, and cesium phenoxides yield mixtures of 2-hydroxy-benzoic acid and 4-hydroxy-benzoic acid [1] (Scheme 5.2). As a rule of thumb, the yield of p-hydroxybenzoic acid generally increases with the increasing ionic radius of the alkali metal. Both, temperature and C02-pressure were also reported to be paramount in the selectivity of the carboxylation ... 

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... 

Wipff, G., Lauterbach, M. (1995), Complexation of Alkali Cations by Calix[4]crown Ionophores Conformation and Solvent Dependent Na+ / Cs+ Binding Selectivity A MD FEP study, Supramol. Chem. 6, 187-207. 

Kirch and Lehn have studied selective alkali metal transport through a liquid membrane using [2.2.2], [3.2.2], [3.3.3], and [2.2.C8] (146, 150). Various cryptated alkali metal picrates were transported from an in to an out aqueous phase through a bulk liquid chloroform membrane. While carrier cation pairs which form very stable complexes display efficient extraction of the salt into the organic phase, the relative rates of cation transport were not proportional to extraction efficiency and complex stability (in contrast to antibiotic-mediated transport across a bulk liquid membrane). Thus it is [2.2.Ca] which functions as a specific potassium ion carrier, while [2.2.2] is a specific potassium ion receptor (Table VI). 

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