Picrates, alkali-metal

In Pedersen s early experiments, the relative binding of cations by crown ethers was assessed by extraction of alkali metal picrates into an organic phase. In these experiments, the crown ether served to draw into the organic phase a colored molecule which was ordinarily insoluble in this medium. An extension and elaboration of this notion has been developed by Dix and Vdgtle and Nakamura, Takagi, and Ueno In efforts by both of these groups, crown ether molecules were appended to chromophoric or colored residues. Ion-selective extraction and interaction with the crown and/or chromophore could produce changes in the absorption spectrum. Examples of molecules so constructed are illustrated below as 7 7 and 18 from refs. 32 and 131, respectively. 

Calestani et al. [8] prepared dietylamide of the tetra-carboxymethyl ether of p-tert-h W.y calix[4]arene and reported alkali metal picrate extraction constants of 1.9... 

Table 1. Extraction (%) of alkali metal picrates from the aqueous into the organic phase by 3a-d 3a-d 10 2 M (in dichloromethane), alkaline metal picrates 10 1 M (in water), picric acid 7xl0 5 M (in water).

MCAm is the ion-pair formed between the crown (C) complex containing the metal ion (Mm+) and A- is the counter ion. It needs to be noted that the degree of extraction is anion-dependent. For example, the extraction of an alkali metal into an organic phase is enhanced when the counter ion is a large anion such as picrate. Alkali metal picrates undergo extraction into benzene in the presence of 18-crown-6 in the order K+ > Rb+ > Cs+ > Na+ (Iwachido, Sadakane Toei, 1978). Divalent ions may also be extracted. For 15-crown-5 in benzene, the picrate extraction coefficients (from water) fall in the order Pb2+ > Sr2+ > Ba2+ > Ca2+... 

Kollig, H.P., Ellington, J.J., Weber, E.J., and Wolfe, N.L. Environmental research brief - Pathway analysis of chemical hydrolysis for 14 RCRA chemicals. Office of Research and Development. U.S. EPA Report 600/M-89/009, 1990, 6 p. Kolthoff, I.M. and Chantooni, M.K., Jr. Crown ether complexed alkali metal picrate ion pairs in water-saturated dichloro-methane as studied by electrolytic conductance and by partitioning into water. Effect of lithium chloride on partitioning, J. Chem. Eng. Data, 42(l) 49-53, 1997. 

Table 7. Photostationary cis/trans Percentages and Extractabilities of Alkali Metal Picrates with lonophores 88, 89, and 90 under Visible and UV Light Irradiation ...

Disc kerb off Powders. Mixtures of BkPdr with at least 15% of one/or several alkali metal picrates. The BkPdr could be substituted by a mixture of K nitrate and sulfur Ref Daniel (1902), 205... 

Many polypodands having terminal donor groups are also available, e.g. (39)-(41).33 These also exhibit cation complexation and phase transfer properties. KMn04 and aqueous alkali metal picrates are much more readily taken into organic phases in the presence of these podands than with dibenzo-18-crown-6.239 The KSCN complex of (39) exhibits a novel coordination geometry as all 10 donor atoms participate in coordination of the metal cation, and in order to do this the three arms wrap around the cation in a propeller-like fashion.240... 

Selectivity sequences in solvents such as water, methanol and ethanol do not guarantee a similar behaviour in the lipid membrane. Experiments have been carried out in attempts to investigate the selective transfer of cations across model membranes, and these are exemplified here by reference to an investigation concerning the cryptands [2.2.2], [3.2.2], [3.3.3] and [2.2.C8]. Two aqueous phases (IN and OUT) were bridged by a chloroform layer into which the carrier can be dissolved. Alkali metal picrate was dissolved in two aqueous layers such that the IN layer was 1000 times more concentrated than the OUT layer. All layers were stirred and the transport monitored via increase in picrate in the OUT layer (UV) and increase in potassium in the OUT layer (atomic absorption). The membrane phase was also analyzed at the end of the experiment.497... 

Table 3.8 Binding free energies (kcal mol 1 kcal = 4.184 kj) for alkali metal picrates under standard conditions. The values of the metal ions most complementary to each host are highlighted in bold.

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). 

Alkali metal picrates have been used to measure formation constants for crown ethers in solution, but the selectivity of benzo crown ethers for metal picrates, relative to the analogous chlorides, nitrates, perchlorates, and thiocyanates, may vary significantly. Apparently, it—it interactions between the picrate ions and the aromatic ring(s) on the crown are responsible for the difference. The importance of the picrate effect rises as the number of benzo groups in the crown ether is increased, and it varies with their location in the macrocycle. The dependence of the picrate NMR chemical shift on the metal cation and/or macrocycle identity has been used to study picrate-crown ether 7r-stacking in large crown ether (18, 21, and 24-membered) complexes. 

The distribution of alkali-metal picrate 1 1 complexes of dibenzo-18-crown-6 between water and benzene has been investigated. The formation of 2 1 complexes was recognized for Rb and Cs ions in a large excess of polyether. The extractability of complex cation-picrate ion pairs decreased in the order K > Rb > Cs>Na>Li. This order was confirmed in a parallel investigation, and bis-(3,5-di-t-butylbenzo)-18-crown-6 also has good extraction ability for alkali-metal ions. ... 

Marchand, A.P. Chong, H.-S. Synthesis and alkali metal picrate extraction capabilities of novel, cage-functionalized diaza(17-crown-5) ethers. Tetrahedron 1999. 55. 9697-9706. 

---