Potassium cation complexes

By the potassium cation complexation with specific ligands, the positive charge is screened and as an immediate consequence the electrostatic interaction between the anion and cation decreases and the dissociation degree increases [14, 53] ... 

Figure 4.9 Some potassium cation complexes with different ligands...

Figure 1.3 Chemical and X-ray crystal structures of simple crown ether z> metal cation and cryptand z> potassium cation complexes.

FIGURE 8.1 Modeled structures of 18-crown-6 and potassium cation complex (left) and crytophane (right). 

Compound 5 was analyzed by NMR spectroscopy to gain information relative to conformation and complexation preferences. When complexation with potassium cations was attempted, the N—CHj signals were affected more than others. When the cation present was Ag , the protons adjacent to sulfur were more strongly affected. This observation may indicate the relative binding sites for soft versus hard cations in this system. ... 

With respect to the carrier mechanism, the phenomenology of the carrier transport of ions is discussed in terms of the criteria and kinetic scheme for the carrier mechanism the molecular structure of the Valinomycin-potassium ion complex is considered in terms of the polar core wherein the ion resides and comparison is made to the Enniatin B complexation of ions it is seen again that anion vs cation selectivity is the result of chemical structure and conformation lipid proximity and polar component of the polar core are discussed relative to monovalent vs multivalent cation selectivity and the dramatic monovalent cation selectivity of Valinomycin is demonstrated to be the result of the conformational energetics of forming polar cores of sizes suitable for different sized monovalent cations. 

Application of the reaction to the 2-azidobenzoyl derivative of diethylene glycol monomethyl ether 92, in a mixture of tetrahydrofuran and diethylene glycol monomethyl ether as the nucleophile, affords 2-(2-methoxyethoxy)ethyl 2-[2-(2-methoxyethoxy)ethoxy]-37/-azepine-3-carbo-xylate (93), which displays metal cation complexing properties towards lithium, potassium, and. to a lesser extent, barium and calcium cations.198... 

Early work by Strassner and co-workers showed that the chelating bis-NHC Pd complexes 32a and 32c were capable of promoting the oxidation of methane, whilst the iodo-analogues 32b and 32d were inactive under the same reaction condition [45], Indeed, in a mixture of TEA and TFAA, in the presence of potassium peroxodisulfate under 20-30 bar of methane, trifluoroactic acid methyl ester is produced, using 32a or 32c as catalyst (Scheme 10.14). hi a more recent work, the authors disclosed the use of pyrimidine-NHC Pd complexes for the same reaction. A shghtly better catalytic activity was obtained with the unexpected cationic complex 34 [46],... 

Figure 4.2. X-ray structure of the cationic complex of potassium with dibenzo-30-crown-10 (Busch Truter, 1972).

A typical phase transfer catalytic reaction of the liquid/liquid type is the cyanation of an alkyl halide in an organic phase using sodium or potassium cyanide in an aqueous phase. When these phases are stirred and heated together very little reaction occurs. However, addition of a small amount of crown ether (or cryptand) results in the reaction occurring to yield the required nitrile. The crown serves to transport the cyanide ion, as its ion pair with the complexed potassium cation, into the organic phase allowing the reaction to proceed. 

Complex 65 (Cardiolite), 99mTc(I)-sestamibi, is used for myocardial perfusion imaging. It was designed on the basis that lipophilic cationic complexes behave as potassium mimics and are taken up by the myocardium (281). The sequential metabolism of the six methoxy groups of 65 to hydroxyl groups in the liver leads to formation of 99mTc complexes with greater hydrophilicity which are not retained in myocardial tissues (282). 

The proportion of the /rans-O-alkylated product [101] increases in the order no ligand < 18-crown-6 < [2.2.2]-cryptand. This difference was attributed to the fact that the enolate anion in a crown-ether complex is still capable of interacting with the cation, which stabilizes conformation [96]. For the cryptate, however, cation-anion interactions are less likely and electrostatic repulsion will force the anion to adopt conformation [99], which is the same as that of the free anion in DMSO. This explanation was substantiated by the fact that the anion was found to have structure [96] in the solid state of the potassium acetoacetate complex of 18-crown-6 (Cambillau et al., 1978). Using 23Na NMR, Cornelis et al. (1978) have recently concluded that the active nucleophilic species is the ion pair formed between 18-crown-6 and sodium ethyl acetoacetate, in which Na+ is co-ordinated to both the anion and the ligand. 

X-ray crystallographic analysis of the potassium complex of [5] confirmed, as expected, that these receptors form intramolecular 1 1 sandwich complexes with potassium cations (Fig. 8). 

Electrochemical complexation studies of [5], disappointingly, revealed that the reversible ferrocenoyl oxidation wave was not perturbed on addition of either sodium or potassium cations, implying that the complexed group 1... 

Proton nmr titration experiments of [26] and [27] with KPF6 in acetonitrile revealed that in solution both compounds form 1 1 intramolecular sandwich complexes with the potassium cation. A number of alkyl-, vinyl- and azo-linked bis(benzo-15-crown-5) ligands are well known to exhibit this mode of K+ coordination. In the case of [26], a solid-state potassium complex was isolated whose elemental analysis and fast-atom bombardment mass spectrum ([26] K+ = 1083 complex ion) was in agreement with 1 1 complex stoichiometry (Fig. 20). 

Only Cram (36) has published a rationale for the very high (99%) enantiomeric excess achieved in the reaction of methyl vinyl ketone and the hydrindanone in the presence of the chiral crown ether. This mechanism envisions a bimolecular complex comprising the potassium cation and chiral host as one entity and the enolate anion of the hydrindanone as the counterion. Methyl vinyl ketone lies outside this complex. The quinine-catalyzed reaction appears to have a termo-lecular character, since the hydroxyl of the alkaloid probably hydrogen bonds with the methyl vinyl ketone—enhancing its acceptor properties—while the quin-uclidine nitrogen functions as the base forming the hydrindanone—alkaloid ion pair. 

An interesting question is whether the peroxo ligand in our [Fe (Porph)(02 )] complex is coordinated in a side-on or end-on fashion. Taking into consideration the DMSO coordination and the electrophilic potassium cation in the crown ether lying above the peroxo ligand, [Fe (Porph)(02 )] may in a way represent a model for the proposed (59) nucleophilic attack of the end-on peroxo form, with an axially coordinated solvent molecule, to an electron-deficient substrate (Scheme 14). 

In our initial studies of the polymerization of butyl acrylate by solid potassium persulfate in acetone solution (2), we attempted to relate the rate of polymerization to the ability of various crown ethers to complex the potassium cation. A reasonable correlation was discovered between log Rp and log K, where K represents the binding constant of the crown ether for in methanol solution (Figure 1). This finding provided some support for the idea that a typical phase transfer process was occurring in these reacti ons. 

Fanfois et al. studied the optimization of lactic acid and 18-crown-6 as complexing agents. She found that 0.5 mM 18-crown-6 in a 10 mM imidazole buffer (pH 4.5) could resolve sodium from lead as well as ammonium from potassium cations. 

The logical conclusion reached while considering these data is as follows. In liquid phase (THF), under the conditions of a regular volume continuum without gradients of concentration and potential, all anion-radicals of azoxybenzene can be stabilized just after formation due to their bonding with potassium cations. This yields the coordinative complex. The complex is diamagnetic and, therefore, azoxybenzene anion-radicals cannot be revealed by ESR spectroscopy (Scheme 2.15). 

The diamagnetic complex is not reduced further by the cyclooctatetraene dianion. This prevents the conversion of the azoxybenzene anion-radicals into azodianions. Potassium cation plays an important role in this limitation of the reduction process, which, generally, proceeds readily (the... 

The removal of potassium cations makes the results of the liquid-phase and electrode reactions similar. In the presence of crown ether, the eight-membered complex depicted in Scheme 2.16 is destroyed. The unprotected anion-radicals of azoxybenzene are further reduced by cyclooctatet-raene dianion, losing oxygen and transforming into azodianion. The same particle is formed in the electrode reaction shown in Scheme 2.13. In the chemical reduction, stabilization of azodianion is reached by protonation. Namely, addition of sulfuric acid to the reaction results in the formation of hydrazobenzene, which instantly rearranges into benzidine (4,4 -diamino-l,l"-diphenyl). The latter was isolated from the reaction, which proceeded in the presence of crown ether. 

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