Cryptands Cryptates

CRYOENZYMOLOGY CRYPTANDS CRYPTATE EFFECT CRYPTIC CATALYSIS CRYPTIC STEREOCHEMISTRY CRYSTAL FIELD SPLITTING LIGAND FIELD SPLITTING CRYSTAL FIELD THEORY Crystal growth,... 

Cryptands Cryptates) (Figure 16), leading to a tetrahedrally surrounded zinc atom which remains outside the cavity of the crown ether see Crown Ethers). 

Podates AcycHc analogues of crown ethers /coronands and cryptands (podands, eg, (11) (30) are also capable of forming inclusion compounds (podates) with cations and uncharged organic molecules, the latter being endowed with a hydrogen bond fiinctionahty. Podates normally are less stable than coronates and cryptates but have favorable kinetics. 

Complexes Tlie term cryptate is now accepted to mean the complex formed between a cryptand and a substrate. Tlie corresponding complex with a coronand would be a coronate, a term suggested some years ago by the same authors . Presumably, a complex between a podand and some substrate would be a podate . 

Of these terms, the names crown ether, cryptand and cryptate are in general usage. 

Similarly, by Schiff-base condensation reactions have been used to generate free cryptands from triamines and dicarbonyls in [2+3] condensation mode. These ligands react with silver(I) compounds to give dinuclear or trinuclear macrocyclic compounds where Ag Ag interactions may be present. Thus, with a small azacryptand a dinuclear complex with a short Ag- Ag distance (55) is found.498 With bigger azacryptand ligands also dinuclear complexes as (56) are achieved but without silver-silver interaction. 65,499-501 A heterobinuclear Ag1—Cu1 cryptate has also been... 

The stability of cryptate complexes. The cage topology of the cryptands results in them yielding complexes with considerably enhanced stabilities relative to the corresponding crown species. Thus the K+ complex of 2.2.2 is 105 times more stable than the complex of the corresponding diaza-crown derivative - such enhancement has been designated by Lehn to reflect the operation of the cryptate effect this effect may be considered to be a special case of the macrocyclic effect mentioned previously. 

The dissociation rates for a number of alkali metal cryptates have been obtained in methanol and the values combined with measured stability constants to yield the corresponding formation rates. The latter increase monotonically with increasing cation size (with cryptand selectivity for these ions being reflected entirely in the dissociation rates - see later) (Cox, Schneider Stroka, 1978). 

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). 

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... 

For cryptands in which the molecular cavity is larger than in the case of the [l.l.l]-species [78], proton transfer in and out of the cavity can be observed more conveniently. Proton transfer from the inside-monoprotonated cryptands [2.1.1] [79], [2.2.1] [80], and [2.2.2] [81 ] to hydroxide ion in aqueous solution has been studied by the pressure-jump technique, using the conductance change accompanying the shift in equilibrium position after a pressure jump to follow the reaction (Cox et al., 1978). The temperature-jump technique has also been used to study the reactions. If an equilibrium, such as that given in equation (80), can be coupled with the faster acid-base equilibrium of an indicator, then proton transfer from the proton cryptate to hydroxide ion... 

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