Anion cryptates

Spherical recognition of halide ions is displayed by protonated macropolycyclic polyamines. Thus, macrobicyclic diamines yield katapinates [3.9]. Anion cryptates are formed by the protonated macrobicyclic 16-6H+ [2.52] and macrotricyclic 21-4H+ [2.97] polyamines, with preferential binding of F and Cl- respectively in an octahedral and in a tetrahedral array of hydrogen bonds. 

Linear recognition is displayed by the hexaprotonated form of the ellipsoidal cryptand bis-tren 33, which binds various monoatomic and polyatomic anions and extends the recognition of anionic substrates beyond the spherical halides [3.11, 3.12]. The crystal structures of four such anion cryptates [3.11b] provide a unique series of anion coordination patterns (Fig. 4). The strong and selective binding of the linear, triatomic anion N3" results from its size, shape and site complementarity to the receptor 33-6H+. In the [N3 pyramidal arrays of +N-H "N- hydrogen bonds, each of which binds one of the two terminal nitrogens of N3-. 

Fig. 4. Crystal structures of the anion cryptates formed by the hexaprotonated receptor molecule 33-6H+ with fluoride (left), chloride (centre), and azide (right) anions.

Figure 4.20 19F NMR spectrum of 4.44- (n-Bu)4N+F 0.5 1 after heating at 150 °C for 1 h in DMSO-c/6 followed by storage at 25 °C for 10 d. The resonances from left to right correspond to anion cryptates with respectively 0, 1, 2, 3, 4, 5 and 6 NH protons replaced by deuterium. The observed multiplicity follows the standard formula multiplicity = 2n/+ 1 where n is the number of H nuclei remaining and /is the nuclear spin quantum number of H, i.e. xh. (Reproduced with permission from [38] 2004 American Chemical Society).

Graf, E., Lehn, J.-M., Anion cryptates-highly stable and selective macrotricyclic anion inclusion complexes. J. Am. Chem. Soc. 1976, 98, 6403-6405. 

Dietrich, B., Dilworth, B., Lehn, J.-M., etal, Anion cryptates Synthesis, crystal structures, and complexation constants of fluoride and chloride inclusion complexes of polyammonium macrobicyclic ligands. Helv. Chim. Acta 1996, 79, 569-587. 

Dietrich, B., Guilhem, J., Lehn, J.-M., Pascard, C., Sonveaux, E., 11. Molecular recognition in anion coordination chemistry. Structure, binding constants and receptor-substrate complementarity of a series of anion cryptates of a macrobicyclic receptor molecule. Helv. Chlm. Acta 1984, 67, 91-104. 

The series of azacryptands used in this study are relatively easily made via 2+3 Schiff-base condensation of the triamine, tren, with the appropriate dialdehyde, followed by borohydride reduction of the imine functions (Scheme 1). Adjustment of pH in the presence of the appropriate anion followed by slow evaporation of solvent yields the anion cryptate, often in a form suitable for X-ray diffraction analysis. 

Earlier discussion [12] of the structures of protonated cryptand/oxoanion assemblies was based on consideration of H-bonds between the encapsulated anion and the NH+ donors of the ciyptand. These interactions are assumed to be responsible for retention of the guest anion in the host ciyptand cavity, both in the sohd state and in solution. We have shown that, in all cases, anion cryptates exhibit at least three, and often more, direct H-bond NH+-Oanion contacts tethering the included oxoanion within the crypt. 

B. Dietrich, J. Guilhem, J.M. Lehn, C. Pascard, E. Sonveaux, Molecular Recognition in Anion Coordination Chemistry - Structure, Binding Constants and Receptor-Substrate Complimentarity of a Series of Anion Cryptates of a Macrobicyclic Receptor Molecule , Helv. Chim. Acta, 67,91 (1984)... 

Alkali metal and other cationic cryptates have been known for a number of years. More recently, anionic cryptates have been characterized. Suggest a structure for [ClNfCHoCHoNHCHjCHjNHCH.CHjljN] . (See Footnote 112.)... 

Chemistry in cages dinucleating azacryptand hosts and their cation and anion cryptates, M. Arthurs, V. McKee, J. Nelson and R. M. Town, J. Chem. Ed., 2001, 78, 1269. 

Anion cryptates are formed by macrotricycles like (5) in their tetraprotonated state with the spherical halide anions [8]. (5)-4H binds the chloride ion very strongly and very selectively, giving the [Cl" c (5)-4H J cryptate (7), but does not complex other types of anions. These properties are unique at present with respect to both synthetic and natural halide binding sites, very little being known about the latter. Non-complementarity between an ellipsoidal cryptand and the spherical halides results in appreciable ligand distortions in the cryptates formed and in lower binding constants [9, 10] (see also below). 

Figure 4 nmr spectra (at 39.2 MHz) of aqueous solutions of the crcomplex of ligand [1]-4H+ (20 mM) and uncomplexed eras a function of the ratio of free CFto creomplex. The shifts are given in ppm relative to external aqueous NaCI (0.1 M) pH 1.5 and 20 C. Reprinted with permission from Kintzinger JP, Lehn J-M, Kauffmann E, Dye JL and Popov Al (1983) Anion coordination chemistry - Cl NMR studies of chloride anion cryptates. Journal of the American Chemical Society 105 7549-7553. Copyright 1983 American Chemical Society. 

Kintzinger JP, Lehn J-M. Kauffmann E, Dye JL and Popov. A1 (1983) Anion co-ordination chemistry - C1 NMR studies of chloride anion cryptates. Journal of the American Chemical Society 105 7549-7553. 

J.-M. Lehn and coworkers are pioneers in the use of covalent capsule (so-called anion cryptates ) for anion encapsulation, separation, and detection. 

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