Torands torand complexation

Polypyridine Torand 1. Complexes of alkali metal triflates and picrates have been prepared from the monotriflate salt of torand 1, which is isolated directly from the macrocyclization reaction mixture. As shown in Figure 3, neutralization of the triflate salt with alkali metal hydroxides or carbonates gives the alkali metal complexes. With triflate or picrate counterions, torand complexes and salts partition selectively into chlorofonn rather than water. Thus 1 can be shuttled back and forth between the triflate salt and various complexes simply by washing the chloroform solution with aqueous acid or base. The free ligand is prepared by reaction of the triflate salt with tetra-n-butylammonium hydroxide in butanol/acetonitrile. Combustion microanalysis showed that the Li, Na, K, Rb and Cs complexes all have 1 1 host/guest stiochiome-try, despite the cavity/ion size mismatch at both ends of the series. The X-ray crystal... 

Inclusions of Other Grown Analogues. A variety of crown analogues and hybrid modifications (24—28) with other topological features (lariat ethers (31,32), octopus molecules (33), spherands (eg, (12) (34), torands (35)) including chiral derivatives (36) have been prepared and demonstrated to show particular inclusion properties such as chiroselective inclusion (Fig. 4) (37) or formation of extremely stable complexes (K ">(LR) for (12)... 

AC and AEC complexation is also effected efficiently by other macrocyclic ligands such as the spherands 13, cryptospherands 14 [2.9, 2.10], calixarenes [2.38, A.6, A.23], torands [2.39], etc., some of them, for instance the spherands displaying particularly high stabilities. A special case is represented by the endohedral complexes of fullerenes in which the cation (Sr2, Ba2+, lanthanides) is locked inside the closed carbon framework [2.40],... 

Numerous macrocyclic and macropolycyclic ligands featuring subheterocyclic rings such as pyridine, furan or thiophene have been investigated [2.70] among which one may, for instance, cite the cyclic hexapyridine torands (see 19) [2.39] and the cryptands containing pyridine, 2,2 -bipyridine (bipy), 9,10-phenanthroline (phen) etc. units [2.56,2.57,2.71-2.73]. The [Na+ c tris-bipy] cryptate 20 [2.71] and especially lanthanide complexes of the same class have been extensively studied [2.74, 2.75] (see also Sect. 8.2). 

Torand 1 binds a wide range of metal salts. The following 1 1 complexes have been prepared in methanol and characterized by microanalysis l Ca(CF3S03)2, l MnCl2(H20)3, l Ni(N03)2(H20)4, l Cu(N03)2(H20)2, l CdCl2(H20)4,... 

Expanded porphyrins are of current interest as ligands for multinuclear complexes and as cationic receptors for anions (17,18). Torand 4 (14) is the largest expanded porphyrin known and has potentially useful absorption and emission properties. Figure 7 shows the effects of protonation on the UV-visible spectrum of 4. Solutions of free base 4 in ethanol give broad, indistinct absorbtions, suggesting... 

Table 1. UV-visible absorption maxima (A, ax) methanol solutions of torand 1, its monotriflate salt and some metal complexes (br = broad s = strong sh = shoulder)...

Fig. 5 Typical cyclophane receptors for metal ion complexation (siderophore 31. torand 32, crown ether 36), onium ion-rt interactions (33, crpptophane 38). hydrogen bonding interactions (34-37), and chiral recognition (36-38). Self-assembled cyclophanes 39 and 40.

Torand 1 (R = butyl) forms remarkably strong complexes with alkali metal salts. Lithium, sodium, potassium, and... 

Fig. 4 Stability constants (logisT,) of sodium and potassium picrate complexes in H O-saturated CHCI3 (or D20-saturated CDCI3) for torand 1. cryptahemispherand 23, spherand 24. cryptand 25. terpyridyldiketone 18. crown ether 26, preorganized teipyridyl 27. and flexible terpyridyl 28 (R = butyl in 1. 18, and 27).

The x-ray crystal structures of the lithium, potassium, and rubidium picrate complexes of torand 1 (f = butyl) were determined.and the structures of the potassium and rubidium complexes are shown in Fig. 5. In ihese two complexes, the metal atoms rest in the center of the cavity of 1, which adopts the staggered pseudo-D3j conformation predicted by molecular modeling. Potassium is clearly an excellent fit to the torand cavity,... 

In contrast to and Rb", lithium picrate crystallizes with torand 1 as a 2 2 complex containing three water molecules.The two torands are coaxially stacked and threaded by a hydrated dilithium chain H20-Li -H20-Li -H20. Each lithium cation binds unsyinmetri-cally to two of the six nitrogens in each torand, and two water molecules complete the tetrahedral Li" coordination sphere. No anion coordination is observed, and hydrogen... 

Fig. 5 Crystal conformations of cationic complexes of torand 1 (R = H, = butyl) with potassium (top left, bottom left) and rubidium (top right, bottom right). (View this art in color at www.dekker.com.)...

Fig. 6 Complexation of guanidinium by expanded torand 2 and flexible analogue 29...

Bell. T.W. Guzzo. F. Drew. M.G.B. Molecular architecture 1. Sodium, potassium and strontium complexes of a hexaazamacrocycle an 18-crown-6/torand analogue. J. Am. Chem. Soc. 1991. 113. 3115-3122. 

Bell T.W. Firestone, A. Ludwig, R. Exceptionally stable alkali metal complexes of a torand. J. Chem. Soc.. Chem. Commun. 1989. 1902-1904. 

Bell, T.W. Cragg. P.J. Drew. M.G.B. Firestone, A. Kwok, D.-I.A. Conformational preference of the torand ligand in its complexes with potassium and mbidium picrate. Angew. Chem. 1992. 104. 319. 

Figure 13 Examples of host-guest complexes (from top) Bell s torand, Heirtzler s copper dimer and Steed s podand.

ABSTRACT. A series of macrocyclic ligands related to hexaaza[is]-annulene form stable complexes with alkali metal and alkaline earth ions. A planar, substituent-solubilized "torand", consisting of multiply fused pyridine rings, has been synthesized and has been found to sequester calcium from a dilute source. 

Since the discovery of crown ethers by Pedersen there have been many attempts to design and synthesize hosts whose ion affinities and selectivities surpass those of the original cyclic polyethers. Particularly important examples are the cryptands and spherands. These polycyclic receptors form stronger complexes and are generally more selective than crown ethers, however their complexes equilibrate more slowly as a consequence of their more rigid, encapsulating structures. This report is focused on the development of a new class of macrocyclic hosts, the torands, whose rigid toroidal structures should permit rapid equilibration of complexes. 

These torand model studies demonstrate that effective complexing agents for alkali metal and alkaline earth ions may be constructed using sp hybridized nitrogen binding sites. In addition, the low solubilities of the model compounds led us to incorporate flexible, solubilizing substituents in our synthetic approaches to fully fused torands (5) We have already reported the synthesis of a soluble heptacyclic terpyridyl... 

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