Cryptand complexing

Until very recently, studies of the use of luminescent lanthanide complexes as biological probes concentrated on the use of terbium and europium complexes. These have emission lines in the visible region of the spectrum, and have long-lived (millisecond timescale) metal-centered emission. The first examples to be studied in detail were complexes of the Lehn cryptand (complexes (20) and (26) in Figure 7),48,50,88 whose luminescence properties have also been applied to bioassay (vide infra). In this case, the europium and terbium ions both have two water molecules... 

Further discussion of the stability of cryptand complexes is presented in Chapter 6. 

Rate constants for reaction of Ca2+aq with macrocycles and with cryptands (281,282,291) reflect the need for conformational changes, considerably more difficult for cryptands than for crown ethers, which may be considerably slower than formation of the first Ca2+-ligand bond. Ca2+aq reacts with crown ethers such as 18-crown-6 with rate constants of the order of 5 x 107M 1 s, with diaza crown ethers more slowly (286,326). The more demanding cryptands complex Ca2+ more slowly than crown ethers (kfslow reaction for cryptands with benzene rings fused to the macrocycle. The dominance of kA over kt in determining stability constants is well illustrated by the cryptates included in Table X. Whereas for formation of the [2,1,1], [2,2,1], and [2,2,2] cryptates kf values increase in order smoothly and gently, the k( sequence Ca[2,l,l]2+ Ca[2,2,l]2+ Ca[2,2,2]2+ determines the very marked preference of Ca2+ for the cryptand [2,2,1] (290). 

As well as increasing anion nucleophilicity, crown or cryptand complexation can enhance the basicity of the anion. Table 3 exemplifies this effect with 1-bromooctane where base-promoted elimination to 1-octene competes with nucleophilic substitution. Being small and poorly solvated, naked fluoride is a strong and hard base which causes, in the case of certain substrates (e.g. Scheme 6), the elimination product to predominate. As the naked anions increase in size they display less basic characteristics but retain high nucleophilic reactivity (74JA2250). 

As stated above, systematic names of macrocyclic host molecules were absurdly complicated for routine discussions [22]. Therefore Vogtle proposed the name coronand for crown ethers, and that of coronates for their complexes while cryptand complexes were called cryptates . The corresponding noncyclic analogues are podands such as 64 [23] and podates, respectively. The cumbersome name podando-coronands (and correspondingly podando-coronates ) was proposed for lariat ethers [24] having at least one sidearm like 65. Examples of hemispherands 66 [25], cavitands 25 [26] and those of some other hosts are discussed in Chapter 7 in some detail, whilst the exceptional stability of fragile guests 4 [2a] and 67 [27] in the hemicarcerand 5 cavity are discussed in Chapters 1 and Section 7.3. 

It was not possible to isolate this from liquid ammonia solution after formation by an exchange reaction with caesium ozonide. It was possible to isolate a cryptand complex, which was violently explosive but permitted diffraction studies. Sodium ozonide behaved similarly, though the other three alkali ozonides do not oxidise ammonia during isolation attempts. 

The elegant match of H30+ for [18] crown-6 suggests that the crystallisation of crown ether and cryptand complexes from acidic liquid clathrate media might represent a straightforward... 

A comparison of the binding geometries of the three halide complexes with 6 is shown in Figure 13. Here the two most common geometries that are repeated in other cryptand complexes are observed, tetrahedral (Figure 13A - C) and octahedral (Figure 13D). 

Coordination to a crown ether or a crypfand can also indnce ionization. For example, an X-ray crystal structure of the complex between Np2Mg and 2,1,1 -cryptand showed it to consist of RsMg and cryptand-complexed RMg+ ions. ... 

Addition can also occur to the formal ring double bonds of a nitrogen heteroaromatic compound. The usual preference is for addition adjacent to the nitrogen (equation 41), but this may be modified by substitution, cryptand complexation, quatemization, or copper salt catalysis. Pyridine N-oxides may yield deoxygenated product. 

Crown ether and cryptand complexation has been studied by Li, (15,61) Na, (62-66) K, (67) "Rb, (67) and Cs (20, 65, 68-72) NMR. Naturally occurring ionophores and their interactions with sodium and lithium ions were investigated by means of several physicochemical methods including Li (71) and Na (71,72) NMR. 

DMSO and 181 cm in pyridine (72JA5247). Similar results have been obtained with the [2.1.1]cryptand which shows a complexed-Li" vibration at 348 cm and [2.2.2]cryptand with a complexed-Na vibration at 234 cm (75MI52100). Raman spectroscopy has also been used, although not widely, to study crown ether and cryptand complexation and the ion pair interaction of the complexed species (B-79MI52103). 

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