Alkali and alkaline earth metal ions

Tabushi, I. Yamamura, K. Water Soluble Cyclophanes as Hosts and Catalysts, 113,145-182 (1983). Takagi, M., and Ueno, K. Crown Compounds as Alkali and Alkaline Earth Metal Ion Selective Chromogenic Reagents. 121, 39-65 (1984). 

Crown compounds as alkali and alkaline earth metal ion selective chromogenic reagents. M. Takagi and K. Ueno, Top. Curr. Chem., 1984,121, 39-65 (37),... 

Foreign cations can increasingly lower the yield in the order Fe, Co " < Ca " < Mn < Pb " [22]. This is possibly due to the formation of oxide layers at the anode [42], Alkali and alkaline earth metal ions, alkylammonium ions and also zinc or nickel cations do not effect the Kolbe reaction [40] and are therefore the counterions of choice in preparative applications. Methanol is the best suited solvent for Kolbe electrolysis [7, 43]. Its oxidation is extensively inhibited by the formation of the carboxylate layer. The following electrolytes with methanol as solvent have been used MeOH-sodium carboxylate [44], MeOH—MeONa [45, 46], MeOH—NaOH [47], MeOH—EtsN-pyridine [48]. The yield of the Kolbe dimer decreases in media that contain more than 4% water. 

Selective Ion Transfer of Alkali and Alkaline Earth Metal Ions Facilitated by Naphtho-15-Crown-5 Across Liquid-Liquid Interfaces and a Bilayer Lipid Membrane... 

In Table 3 are also shown the hydrated radii (r ) which are evaluated with n and r by Eq. (25). A good correlation of with the Stokes radius [60] (r ) has been observed for hydrated cations (alkali and alkaline earth metal ions) [46] ... 

Bachmartn, K., Boden, J., and Haumann, I., Indirect fluorimetric detection of alkali and alkaline earth metal ions in capillary zone electrophoresis with cerium (III) as carrier electrolyte, /. Chromatogr., 626, 259, 1992. 

The insertion of the oxygen atoms widens the silicon lattice considerably. A relatively large void remains in each of the four vacant octants of the unit cell. In natural cristobalite they usually contain foreign ions (mainly alkali and alkaline earth metal ions) that probably stabilize the structure and allow the crystallization of this modification at temperatures far below the stability range of pure cristobalite. To conserve electrical neutrality, probably one Si atom per alkali metal ion is substituted by an A1 atom. The substitution of Si... 

Systems of the above type incorporating larger rings form binuclear complexes with a number of alkali and alkaline earth metal ions as well as with ions such as Ag(i) and Pb(n). For example, (212) forms a symmetrical dinuclear Na+ species in which the Na+ ions are associated with the respective azacrown and crown cavities and are separated from each other by 6.40 A in the solid (Fisher, Mellinger Weiss, 1977). 

Exchange in zeolites of alkali, alkaline earth, transition metal ions and small organic ammonium ions, has been reviewed (111), and in general, the exchange is characterized by small AG values comparable to those found in clay minerals. Althoufft identical selectivity orders for alkali and alkaline earth metal ions are obtained, as in montmorillonite, the opposite variation of AG with charge density is found. 

In the case of PCT-23 (Figure 10.25) containing four appended naphthalenic fluorophores, similar photophysical effects induced by complexation of alkali and alkaline earth metal ions (i.e. red-shift of the absorption and the emission... 

The importance of solvent effects has been outlined in Section 2.2.1. An illustration with some of the fluoroionophores described in this section is given in Table 2.2. For alkali and alkaline-earth metal ions, the stability constants are higher in acetonitrile than in methanol these cations are indeed hard and have a stronger affinity for oxygen atoms (hard) than for nitrogen atoms (soft). In contrast, the soft silver atom has a strong affinity for nitrogen atoms and no complexation is observed in acetonitrile, whereas complexes in methanol, ether, and 1,2-dichloromethane are formed. 

The use of metals for prearranging reaction centers as neighboring groups has a special value in the production of macrocycles (template effect). Although these ligands can be sometimes prepared directly, the addition of metal ion during the synthesis will often increase the yield, modify the stereochemical nature of the product, or even be essential in the buildup of the macrocycle. There have been few mechanistic studies of these processes. The alkali and alkaline-earth metal ions can promote the formation of benzo[18]crown-6 in methanol ... 

The electrodes modified by hexacyano-ferrates compounds were also used as voltammetric sensors [409-412]. The cadmium hexacyanoferrate-based composite ion-sensitive electrode for voltammetry was explored by Scholz and coworkers [409]. The potential of such electrode depends linearly on the logarithm of concentration of alkali and alkaline-earth metals ions in the solutions. Bo and fin have studied [410] Prussian blue (PB)/Pt modified electrode in GdGh electrolyte by cyclic voltammetry and in situ Fourier transform IR spectroscopy (FTIR) spectro-electrochemistry. Cadmium hexacyanofer-rates were formed on a PB film. 

These results clearly indicate that the chelate ligation is driven primarily by the enthalpic factor and the entropy plays merely a trivial role in determining the complex stability. This is quite reasonable since the structures of these chelate complexes are strictly defined by the number and direction of the coordination sites of given heavy/transition metal ions, and therefore there is little room for the entropic term to adjust flexibly the complex structure and stability. On the contrary, alkali and alkaline earth metal ions also have the formal coordination numbers, but the actual number and direction of ligand coordination are highly flexible in the weak interaction-driven ligation by hard donors like glyme and crown ether. 

In spite of the overwhelming importance of the channel mechanism for the transport of alkali and alkaline earth metal ions in biological systems, only carrier transport has been studied extensively by chemists. Studies on ion channel mimics of simple structures have long been limited to antibiotic families of gramicidin, amphotericin B, and others. Several pioneers have reported successful preparation of non-peptide artificial channels. However, their claims have been based on kinetic characteristics observed for the release of metal ions through liposomal membrane and lacked the very critical proofs of channel formation. Such a situation was... 

Ercolani, G. and Mandolini. L. (1990) Alkali and alkaline-earth metal ion catalysis in the reaction of aryl acetates with methoxide ion. Effect of a poly (oxyethylene) side arm. J. Am. Chem. Soc., 112, 423. 

Equation (4.5) is also valid in this case. Reactions of this type are realized in polarography at a dropping mercury electrode, and the standard potentials can be obtained from the polarographic half-wave potentials ( 1/2)- Polarographic studies of metal ion solvation are dealt with in Section 8.2.1. Here, only the results obtained by Gritzner [3] are outlined. He was interested in the role of the HSAB concept in metal ion solvation (Section 2.2.2) and measured, in 22 different solvents, half-wave potentials for the reductions of alkali and alkaline earth metal ions, Tl+, Cu+, Ag+, Zn2+, Cd2, Cu2+ and Pb2+. He used the half-wave potential of the BCr+/BCr couple as a solvent-independent potential reference. As typical examples of the hard and soft acids, he chose K+ and Ag+, respectively, and plotted the half-wave potentials of metal ions against the half-wave potentials of K+ or against the potentials of the 0.01 M Ag+/Ag electrode. The results were as follows ... 

Similar effects of the cation of the supporting electrolyte occur, to a greater or lesser extent, in the reductions of alkali and alkaline earth metal ions in basic aprotic solvents [26a]. In dimethylacetamide (DMA), the reductions of alkaline earth metal ions are electrochemically masked by Et4N+. In DMF and DMSO, the reversibility of the reductions of alkali and alkaline earth metal ions decreases with the decrease in the cationic size of the supporting electrolyte. This effect is apparent from the kinetic data in Table 8.3, which were obtained by Baranski and Fawcett [23 b] for the reductions of alkali metal ions in DMF. 

Correction for the double-layer effect is not always easy. Especially, there are cases when the difference between the distance of the nearest approach of the reactant (xr) and that of the OHP (%ohp) must be taken into account. Then, ohp in the above relations should be replaced by (f>r, which is the potential at r, In the reductions of alkali and alkaline earth metal ions at very negative potentials, the increase in %OHP with the increase in the cationic size of the supporting electrolyte is expected to make (f>x more negative and accelerate the reaction and vice versa. 

Table 6 Correlation of Cation and Cavity Radii (A) of Alkali and Alkaline Earth Metal Ions and some Crown Ethers...

With some catalyst systems, selectivity to primary metathesis products is near 100%, but side reactions (double-bond migration, dimerization, cyclopropanation, polymerization) often reduce selectivity. Such side reactions, such as oligomerization and double-bond shift over oxide catalysts, may be eliminated by treatment with alkali and alkaline-earth metal ions.26... 

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