Reactive extraction processes represent efficient and smart technologies for separation and concentration of metal ions in solution, which are frequently used in industry. Despite the importance of anions in biology, medicine, environment and industry, practical examples of anion extraction are relatively limited compared to metal ion separation. Anion extraction processes are mainly based on the nonspecific ion pair formation with hydrophobic ammonium cations. In this case the phase transfer of anions is dominated by their lipophilicity. The reasons for this situation are closely connected with the specific features of anions in contrast to cations. Novel approaches for specific binding and selective transport of anionic components are based both on the better understanding of the biological role of anions and on the possibilities of supramolecular chemistry to create receptor architectures with complementary binding modes for anions. In the given review the authors discuss present research tendencies and application possibilities of new extractant types for separation and concentration of anionic species in solution.
The reactions of MlC3 (M = Sb, Bi) with maleonitrile-dithia-15-crown-5 (mn-15S2O3) and maleonitrile-dithia-18-crown-6 (mn-18S2O4) in MeCN yielded the complexes [MCl3(mn-15S2O3)] {M = S b (l), Bi(2)} and [MCl3(mn-18S2O4)] {M = Sb(3), Bi(4)}, respectively. The pyramidal MCl3 units are coordinated very weakly to the three oxygen and two sulphur donor atoms of mn-15S2O3 in 1 and 2, and to the four oxygen donor atoms of mn- 18S2O4 in 3 and 4. Both mn-15S2O3 complexes, 1 and 2, crystallize isotypically in the m onoclinic space group P2(1)/n with four formula units per unit cell, while the isotypic mn-18S2O4 complexes, 3 and 4, are triclinic, space group P-1, with two formula units per unit cell. In the SbCl3 complexes, 1 and 3, the mean contact distances between the Sb centres and the macrocyclic donor atoms are longer than the corresponding distances in their isostructural BiCl3 analogues, 2 and 4, which may reflect a stereochemical activity of the Sb111 lone pair. Under the conditions of DEP-M S experiments with 1 and 3 the monocationic SbCl2+ complexes [SbCl2(L)]+ (L = mn-15S2O3, mn-18S2O4) were detected. NaSbCl6 and mn-18S2O4 in MeCN furnished the 2:1 complex [Na(mn-18S2O4)2]SbCl6 {(5)SbCl6). In the complex cation 5 the sodium atom is coordinated sandwich-like through the eight oxygen atoms of two mn-18S2O4 molecules. Compound (5)SbCl6 crystallize in the triclinic space group P -1 with two formula units per unit cell
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