Anion macrocyclic effect

A macrocyclic receptor (100) has also recently been prepared and its crystal structure was elucidated (220). In comparison with its acyclic analogue 101, an anion macrocyclic effect was observed, the stability constants for chloride complex formation [in DMSO] being K = 250 M (100) and K = 20 M (101). Receptor 102 was shown to act as a switchable cobaltocenium based chloridebinding host (221). The free receptor binds chloride anions, but on the addition of potassium ions, the binding is switched off. This effect is probably due to the ability of the potassium ion to form a sandwich complex with the two crown ether substituents, sterically hindering the anion-binding site. 

Chloride anion stability constant determinations with (18) and the related acyclic analogue (19) revealed the presence of an anion macrocyclic effect . The stability constant value for the chloride complex of (18) in dimethylsulfoxide (log Ks = 2.4) is at least an order of magnitude greater than the chloride complex of (19) (log Ks = 1.2). ... 

Dicarbonyl functions have been built into macrocyclic structures, and pKa values for the resulting macrocycles [60] have been determined (Alberts and Cram, 1979). When the open-chain model [62] is compared with the macrocycles [60], identical first pK values were found (pKa = 8.6). Thus for the diketones [60], no macrocyclic effect is noticeable. But for the dissociation of a second proton from the mono-aniorts of [60] much higher pKa values are found. To a certain extent. Coulomb repulsions (see Section 2) are probably the reason for this behaviour, but the large difference in the pKa values (ApKa = 2.9, see Table 26) argues for a special stabilization of the mono-anion. Again hydrogen bonds are not unreasonable. 

Thioethers lack the capacity to neutralize positive charge and display weak donor properties. Consequently, they do not readily displace strong donor solvents (water) or strongly bonding anions (such as halides) from the coordination sphere. As a consequence, many thioether complex syntheses employ aprotic or alcoholic solvents and precursor complexes with weakly bound solvents (such as DMSO or acetone) or anions (such as C+3S03 ). Despite the synthetic challenges, a wide range of complexes has been reported, particularly with the cyclic poly-thioethers, where the macrocyclic effect overcomes many of the above difficulties. 

Because earlier reviews deal explicitly with these template effects [4, 7, 12], the following discussion is restricted to a recently discovered anion template effect (Scheme 6.5.3). fn non-competitive solvents, the tetralactam macrocyde strongly binds anions such as chloride, bromide, or phenolate. Phenolate stoppers bound to the macrocycle can act as wheeled nudeophiles and react through the wheel with a semi-axle generated in situ to yield ether rotaxanes in yields of 57% to 95% [13]. Consequently, the macrocyde not only represents an anion receptor, but as a concave template simultaneously provides the correct orientation of the guest for threading the axle into the wheel. 

While the synthesis of macrocycles 9.24-9.27 can be considered as being conceptually straightforward in terms of experiment, one fact proved unexpected and interesting. This was the finding that nitric acid proved to be the most efficient catalyst for macrocycle formation. Indeed, attempts to prepare system 9.26, for instance, using other acid catalysts (e.g., HCl, HBr, H2SO4) led to considerably lower yields. This led Sessler and coworkers to suggest that the formation of this macrocycle was mediated via an anion template effect. This was a rather new conception at the time it was put forward. ... 

Macrocycles containing guanidinium moieties as binding sites (see 60) were synthesized by Lehn and coworkers (Dietrich et al., 1978). These macrocycles were expected to be effective and selective ligands for phosphate, di- and triphosphate, AMP, ADP, and ATP anions. However, the PO complex was found to have low stability and there was almost no macrocyclic effect (Dietrich, 1984 Dietrich et al., 1978). 

Whereas for anion binding, the charge and shape of the macrocycle are of paramount importance the stability and selectivity in metal complexation are governed by several other factors, which have been summarized into the term macrocyclic effect <88Mi 928-01 >. The higher stability of metal complexes with cyclic polyamines compared to that with open-chain polyamines is, however, only found for smaller rings such as 1,4,7-triazacyclononane and 1,4,8,11-tetraazacyclotetradecane, for which a favorable enthalpic contribution to the stability has been observed. 

H-bonding sites similar to cyclic receptor 9a, they interacted with anions much more weakly than the cyclic analogue due to the lack of the macrocyclic effect. 

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