Macrobicyclic cryptate

Cryptands, 42 122-124, 46 175 nomenclature, 27 2-3 topological requirements, 27 3-4 Cryptate, see also Macrobicyclic cryptate 12.2.2], 27 7-10 applications of, 27 19-22 cylindrical dinuclear, 27 18-19 kinetics of formation in water, 27 14, 15 nomenclature, 27 2-3 spherical, 27 18 stability constants, 27 16, 17 Crystal faces, effect, ionic crystals, in water, 39 416... 

Macrobicyclic cryptate cation selectivity, 27 16-17 complex stability, 27 14-15 kinetic studies, 27 13-14... 

In macrobicyclic cryptate complexes where the cation is more efficiently encapsulated by the organic ligand these ion pair interactions are diminished and the reactivity of the anion is enhanced. This effect is seen in the higher dissociation constant, by a factor of 104, of Bu OK in Bu OH when K+ is complexed by [2.2.2]cryptand (12) compared to dibenzo[18]crown-6 (2). The enhanced anion reactivity is illustrated by the reaction of the hindered ester methyl mesitoate with powdered potassium hydroxide suspended in benzene. 

A similar effect occurs with the cryptates. Comparing the stability constants of c 2.2.2] and its one-strand opened analogue (54) (known as a lariat ether , see later) the stability increases by a factor of 10 (Figure 2.16), and this is known as the cryptate effect or as the macrobicyclic cryptate effect (47). After comparing the thermodynamic quantities associated with formation of the [K c 2.2.2] complex to those for the K -dicyclohexyl-18-crown-6 (2) complex, Kauffmann and co-workers concluded that the cryptate effect is of enthalpic origin (52). 

There is considerable interest in the properties of macrobicyclic cryptands, for example [78] to [81], and particularly in their ability to complex protons, metal ions, and small molecules (Lehn, 1978). In the proton cryptates there exists the possibility of intramolecular +N—H N hydrogen bonding as well as interaction of the proton with the oxygen atoms, and the properties are also strongly influenced by the size of the molecular cavity. In the [l.l.l]-cryptand [78] the molecular cavity is small (Cheney et al., 1978) and... 

In comparison, both the free ligand and the dinuclear Cu(I) cryptate of an analogous macrobicyclic structure possessing a diphenylmethane group as a central unit display only two resonances for the CH2CH2 fragment, as is the case here only for the complexes 91 and 92. This points to the special conformation features of the free macrobicycles 89 and 90. 

Fig. 7. Crystal structure and conformation of a) its RbSCN cryptate (from Ref. (103) and (125)) b) the free macrobicyclic ligand [2.2.2], 30...

As the results make immediately clear (Tables 7—10), it is among ligands of the macrobicyclic type G (27—44), which give complexes of the [2]-cryptate type, that by far the highest stability constants may be found for any cation. In particular, these optimum Ks values are several decades higher than those of the most stable complexes formed by natural ligands,... 

Finally, a macrobicyclic or [2]-cryptate effect is found by comparing the stability of the K+ complex of 30 with that of 22, where the solvation shell is completed by solvent molecules (a better model would be a ligand of type 22 bearing a — CH2CH2OCH2CH2OCH3 chain on one nitrogen) a stability increase of more than 105 (in methanol/water, 95/5) is found on introduction of the third bridge. 

Both macrocyclic and macrobicyclic ligands allow preparation of alkali metal solutions, but the former yield mainly M-, whereas solvated electrons are obtained with the [2]-cryptates (162,163). The enhancement of anion reactivity should also useful for activating anionic polymeriza-... 

In the previous section it was noted that hexacyclen 1 does not possess a macrocyclic cavity of sufficient size to encapsulate anionic guests such as Cl" and NO3. In contrast, the octaaza macrobicyclic analogue of hexacyclen (17) forms the fluoride cryptate [F" c 17-6H ] (Figure 7). Consistent with the observed high... 

Mono- and bimetallic lanthanide complexes of the tren-based macrobicyclic Schiff base ligand [L58]3- have been synthesized and structurally characterized (Fig. 15), and their photophysical properties studied (90,91). The bimetallic cryptates only form with the lanthanides from gadolinium to lutetium due to the lanthanide contraction. The triplet energy of the ligand (ca. 16,500 cm-1) is too low to populate the terbium excited state. The aqueous lifetime of the emission from the europium complex is less than 0.5 ms, due in part to the coordination of a solvent molecule in solution. A recent development is the study of d-f heterobimetallic complexes of this ligand (92) the Zn-Ln complexes show improved photophysical properties over the homobinuclear and mononuclear complexes, although only data in acetonitrile have been reported to date. 

Bkouche-Waksman, I. Guilhem, J. Pascard, C. Alpha, B. Deschenaux, R. Lehn, J.-M. 110. Crystal structures of the lanthanum(III), europium(III), and terbium(III) cryptates of tris(bipyridine) macrobicyclic ligands. Helv. Chim. Acta 1991, 74,1163-1170. 

Macropolycyclic ligands containing intramolecular cavities of a three-dimensional nature are referred to as cryptands. The bicyclic cryptands (73) exist in three conformations with respect to the terminal nitrogen atoms, exo-exo, endo-exo and endo-endo 6 these forms can rapidly interconvert via nitrogen inversion but only the endo-endo form has been found in the crystal structures of a variety of complexes372 and for the free ligand ([2.2.2], 73, m = n = / = l).449 In their complexes with alkali and alkaline earth cations, the cryptands exhibit an enhanced stability over the crown ethers and coronands dufe to the macrobicyclic, or cryptate, effect.33 202... 

Sepulchrates, the polyaza cage macrobicycles analogous to the cryptates, were first synthesized in 1977.178 The cobalt(lII) complex shown in Figure 3 (19) is the octaazasepulchrate analog of the [2.2.2] cryptand (Figure 3 16), and is commonly written [Co(sep)]3+ (sep = sepulchrate). 

Indeed, macrobicyclic ligands such as 7-9 form cryptates [Mn+ c cryptand], 10, by inclusion of a metal cation inside the molecule [1.26, 1.27, 2.17, 2.24-2.26]. The optimal cryptates of AC and AEC have stabilities several orders of magnitude higher than those of either the natural or synthetic macrocyclic ligands. They show pronounced selectivity as a function of the size complementarity between the cation... 

The macrobicycle 9 also binds NH4+, forming cryptate 24. The dynamic properties of 24 compared with 23 reflect the receptor-substrate binding complemen-... 

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