Tricyclic cryptands

The tricyclic cryptand 20 has 10 binding sites and a spherical cavity.Another molecule with a spherical cavity (though not a cryptand) is 21, which complexes... 

Figure 22 The molecular structure of the binuclear sodium complex of the tricyclic cryptand (110) (reproduced with...

Cylindrical and spherical tricyclic cryptands are made by progressive construction of the framework following the general strategies shown in Schemes 3 and 4.34-66-69... 

Adarsh NN, Tocher DA, Ribas J (2010) Metalla-macro-tricyclic cryptands anion encapsulation and selective separation of sulfate via in situ crystallization. New J Chem 34 2458-2469... 

The crown ethers form stable complexes with many cations, both inorganic and organic, as well as neutral species. As would be expected, the Macrocyclic Effect see Macrocycle) provides a significant contribution to this stability. For example, the stability constant of the K+ complex of 18-crown-6 in anhydrous methanol is 10 , whereas the complex of the open chain analog has a stability constant of 10. The tricyclic cryptands are even more preorganized, and generally form more stable complexes than the corresponding crown... 

The authors have also synthesized 16, a tricyclic cryptand-like receptor, and have studied its ability to bind anions. Proton NMR titration experiments in DMSO-de at 23 °C revealed that compoimd 16 was selective for bifluoride (FHF") with an association constant of 5500 M being calculated. Dihydro-genphosphate, azide and acetate were also found to bind to 16 with stabUity constants of 740 340 and 100 M, respectively [24]. 

In this chapter, the synthesis and properties of two further categories of synthetic macrocyclic ligand types, exhibiting different degrees of restricted conformational flexibihty, are discussed. These are the rigid spherands and the somewhat more flexible but stiU sterically restrained bi- or tricyclic cryptand ligands. 

The preparation of the tricyclic cryptand LI487 is carried out by a multi-step synthesis. The reaction of the bis(azacrown) L1486 with ethane-1,2-bis(toluene-p-sulphonate) on an Li+ matrix forms the last step of this synthesis (Eq. 6.58) [167]. 

The synthesis of unsymmetrical, cylindrical cryptands necessitates a stepwise construction of the tricycle. This is best achieved by initially mono-protecting the diaza crown ether (41) as its monobenzyloxycarbonyl derivative (77NJC77). 

By replacement of just one oxygen atom in 5 by a methylene group, affinity of the corresponding cryptand 15 to NH4+ decreases by a factor of ca. 100 (NH4+ c 15 log /Ca = 4.3). The macrobicycle 16 has almost completely lost the complexation ability and the selectivity of 5 (NH4+ c 16 log /Ca = 1.7). This dramatic effect results from the removal of one bridge of 5, i.e., from a decrease in cyclic order from the tricyclic to the bicyclic ring system, demonstrating the importance of the spherical macrotricyclic structure for the binding properties of 5. 

Polyammonium cryptands and tricyclic lactams probably represent the largest family of cryptand-type anion receptors known today and a detailed overview of the properties of these compounds lies outside the scope of this review. The interested reader is therefore referred to two recent reviews [11, 53]. Compound 35b in the following section is the only polyammonium cryptand besides 5 whose properties are described in this review in more detail to illustrate the effect of the incorporation of additional hydrogen bond acceptor sites on binding properties. 

The interesting complexone research, in the following years, spread to may laboratories, by constructions of macrobicyclic and -tricyclic (3-dimensional) cage-like ether- or amino cryptands mainly by J.M. Lehn and coworkers, efforts eventually leading to the Nobel Prize in 1987 [22]. 

Monocyclic Cl2Sn[(CH2) ]2SnCl2 (n = 8, 10, and 12) bind two Cl" anions rapidly but are poorly selective and there is little dependence on ring size [56]. The anionbinding capacity of tricyclic organotin cryptands depends on the tin-tin distance in the host [57, 58]. 

Following these preliminary studies, it was possible to design host molecules of the type shown in 15, based upon diaza-12-crown-4 and diaza-15-crown-5 receptors, which would form inclusion complexes with either one bisalkylammonium cation H3N (CH2) N H3 21 or two simple primary alkylammonium cations RN+H3 22. The use of tricyclic compounds of this type has been described elsewhere [23]. The cryptands 23, which are based upon two identical diaza-crown ether moieties, were synthesised by the simple one step procedure shown in Scheme 1, whereas it was necessary to use the stepwise procedure [24] summarised in Scheme 2 for the synthesis of analogous cryptands 24 in which the two diaza crown ether moieties are different. 

Figure 32 Chemdraw diagrams of the hosts, (a) and (d) and top, (b) and (e), and side, (c) and (f), views of the cyclophane cryptand and the octaaza tricycle with terephthalate, respectively. In (e), the water molecules are not shown for clarity, but to emphasize the similar positioning of the terephthalate.

Besides the popular bridgehead TREN for cryptands, substituted benzene rings provide an alternative for bicyclic and tricyclic receptors with a 3D cavity for anion recognition. Anslyn and co-workers prepared amide-linked Cs-symmetric cryptand 15 with a cavity of 78.3 A [47]. X-ray crystal structure of its chloride complex showed a 1 2 stoichiometry, and each chloride formed two N C1 H-bonds with... 

Due to the success on the TREN cryptands, Bowman-James, and co-workers synthesized bicyclic and tricyclic cyclophane capsules 16 and 17 which contain three or four flexible diethylamine chains and two phenyl groups as caps [48]. Although the cavity of 16 and 17 was too small to encapsulate anions, 16 displayed tight binding for F K = 21,000 M ), while 17 showed slightly less affinity for H2P04 = 21,000 M ). The single X-ray crystal structure of 16 [FHF] showed... 

---