Selectivity, cation complex formation

Impetus was given to work in the field of selective cation complex-ation by the observation of Moore and Pressman (5) in 1964 that the macrocyclic antibiotic valinomycin is capable of actively transporting K+ across mitochondrial membranes. This observation has been confirmed and extended to numerous macrocyclic compounds. There is now an extensive literature on the selective complexation and transport of alkali metal ions by various macrocyclic compounds (e.g., valinomycin, mo-nactin, etc.) (2). From solution spectral (6) and crystal X-ray (7) studies we know that in these complexes the alkali metal cation is situated in the center of the inwardly oriented oxygen donor atoms. Similar results are found from X-ray studies of cyclic polyether complexes of alkali metal ions (8) and barium ion (9). These metal macrocyclic compound systems are especially noteworthy since they involve some of the few cases where alkali metal ions participate in complex ion formation in aqueous solution. 

In this context the ligands synthesized (3,5,7) were rather attractive as new acidic complexones for the extraction of divalent cations because of the carboxylic groups present on their lipophilic backbones. In particular for the crown ether (7) a double control of selectivity in complex formation could be expected, one deriving from charge interaction between the two carboxylate groups and cations and the second from the cavity size. 

Bradshaw and his coworkers have listed several motivations for their explorations in this area. One objective of [the] research program is to prepare and study a series of multi-dentate compounds which resemble naturally occurring macrocyclic compounds . Further, Bradshaw and his coworkers have said that it is our hope that we can prepare macrocycles to mimic the selectivities of the naturally occurring cyclic antibiotics and thereby make available models for the investigation of biological cation transportation and selectivity processes . These workers have presented a number of comparisons with valinomy-cin . The other expressly stated goal of their research is to prepare molecules which will allow us to systematically examine the parameters which affect complex stability and to understand that stability in terms of AH and TAS values for complex formation . 

The general, selective and specific formation of complexes by metallic cations. G. Schwarzenbaeh, Adv. Inorg. Chem. Radiochem., 1961, 3, 257-285 (70). 

Complex formation, selective precipitation, and control of the pH of a solution all play important roles in the qualitative analysis of the ions present in aqueous solutions. There are many different schemes of analysis, but they follow the same general principles. Let s think through a simple procedure for the identification of five cations by following the steps that might be used in the laboratory. We shall see how each step makes use of solubility equilibria. 

Each of the heterogeneous charge-transfer reactions (32.3) and (32.4) can be conpled to a homogeneons chemical reaction. Often, an ion association or complex formation occnrs, for example, transfer of the cation facilitated by the formation of the complex with valinomycine (Val) in the potassinm-selective electrode. 

Since poly(oxyethylene)-type nonionic surfactants have a capability of facilitating the transfer of cations [51,52], the above interphase complexation may be seen as an example of precomplex formation before the bulk transfer of ions, which is seen when Aq (p is sufficiently positive. The presence of such precomplex formation at the interface, which is detectable voltammetrically [53], may have significance in the rate of complex formation and the selectivity in the bulk facilitated transfer. 

Finally, the most complex synthetic reaction clearly catalysed by RNA molecules generated by in vitro selection is the formation of the C-N bond of a nucleoside (Scheme 7), from 4-thiouracil and most of the natural substrate for the natural (uracil phos-phoribotransferase) reaction.1461. (Thiouracil was used because it is easily tagged by alkylation on sulfur.) The catalytic RNAs produced by 11 rounds of selection required Mg++ cations and had kcat as high as 0.13 min-1,with kcaJKM at least 107 times greater than the (undetectable) uncatalyzed reaction. Once again these systems are convincing, rather efficient enzyme mimics. 

The enhanced selectivity of the complexed transition metal cation compared to the uncomplexed aqueous form can be expressed as a gain in the stability constant of the adsorbed complex with respect to its stability constant in the solution phase (80). The complex formation reaction and corresponding stability constants of a transition metal cation M with an uncharged ligand L in both the surface (indicated by bars) and solution phase are defined as... 

The selectivity-complex formation function (81) describes the variation of the selectivity coefficient of the transition metal ion versus a non complex forming reference cation with increasing ligand concentration as... 

The General, Selective, and Specific Formation of Complexes by Metallic Cations... 

Fig. 2. Schematic representation of selective ion transport through a lipophilic membrane by complex formation between cation 1+ and a lipophilic ligand S (Fig. 2a) or S (Fig. 2b)...

Independent of the assumptions A to C the cation selectivity of the membranes in the equilibrium domain is therefore controlled by the ratio of the complex formation constants (6) and should therefore be identical for different types of neutral carrier membranes.18 Figure 2 indicates that there is indeed a close parallelism between the selectivities of solvent polymeric membranes (SPM) and bilayer lipid membranes (BLM) modified with valinomycin 1, nonactin 2, trinactin 5, and tetranac-tin 6 (see also Ref. 18). This is in good agreement with findings from Eisenman s45 and Lev s15 research groups. 

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