Cryptand complexes with

Crown ethers and cryptands, complexes with metal cations 79AG613,... 

The acceptor and donor strength of many functions besides fhose of fhe amide type have been characterized by the analysis of associations between simple molecules, such as, e.g., phenols and anilines, for which fhousands of experimental data exist, mosfly measured in chloroform or in carbon tetrachloride [94, 95]. Although these data are hampered by less well-defined structures compared to supramolecular complexes, they not only give a fairly consistent basis for the prediction of hydrogen-bonded associations but also can be used, e.g., for crown efher and cryptand complexes with alkali or ammonium ligands [32]. 

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 . 

The spherand prepared by Cram and coworkers was designed to have a relatively small molecular cavity and appeared to prefer complexation with Li and Na over larger cations like K", Rb, etc. Tlie spheroidal cryptand prepared by Lehn ° involved strategy employed previously but the spherand 24 was prepared by quite a different approach. 

Poly (macrocyclic) compounds. The analytical application of compounds such as crown polyethers and cryptands is based on their ability to function as ligands and form stable stoichiometric complexes with certain cations. Special importance is due to their preference for alkali metal ions which do not form complexes with many other ligands. A number of these compounds are commercially available and their properties and analytical applications have been described by Cheng et a/.11... 

Compounds which produce a complex with Li+ ions have been investigated. The compounds examined were N,N,N, N tetramethylethylenediamine (TMEDA), eth-ylenediamine, crown ethers, cryptand [211], diglyme, triglyme, tetraglyme, eth-ylenediamine tetraacetic acid (EDTA) and EDTA-Li+ (n=l, 2, 3) complexes [59]. The cycling efficiency was improved by adding TMEDA, but the other additives did not show distinct effects. 

Crown ethers (Fig. 3.57) and cryptands (Fig. 3.58) can solubilize organic and inorganic alkali metal salts even in nonpolar organic solvents they form a complex with the cation (see Fig. 3.57c), and thus act as an organic mask (Gates, 1992). 

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... 

Sargeson and co-workers have structurally characterized encapsulated zinc in hexaaza cryptands.742 743 Related cryptands (l-methyl-8-amino-3,13-dithia-6,10,16,19-tetraazabicyclo[6.6.6]-icosane and l-methyl-8-amino-3-thia-6,10,13,16,19-pentaazabicyclo[6.6.6]icosane) incorporating thioether donors also formed complexes with zinc which were structurally characterized. In both cases the zinc ion was encapsulated in the macrobicyclic cavity and the octahedral coordination geometry distorted to the mixed nitrogen and thioether donor atoms.744... 

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. 

Cryptands of the type (217)-(220) tend to form stable complexes with a number of heavy metal ions. Of particular interest is the selectivity of (219) for Cd(n) the complex of this metal is approximately 106-107 times more stable than its complexes with either Zn(n) or Ca(n). This reagent may prove useful for removing toxic Cd(n) from biological systems as well as for other applications involving sequestration of this ion (for example, in antipollution systems). The selectivity observed in the above case appears to arise because (i) the nitrogen sites favour coordination to Zn(n) and Cd(n) relative to Ca(n) and (ii) the cavity size favours coordination of Cd(n) relative to Zn(n). 

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). 

For a more extensive discussion of the kinetics of complexation with cryptands and natural antibiotics, the reader is referred to a recent review of Liesegang and Eyring (1978). 

The fluorescence spectrum of the tris-acridine cryptand A-13 shows the characteristic monomer and excimer bands. Upon complexation with various organic anions (carboxylates, sulfonates, phosphates), the monomer band increases at the expense of the excimer band. The stability of the complexes depends on the contribution of the electrostatic and hydrophobic forces and on the structural complementarity. Stability constants of the complexes ranging from 103 to 107 have been measured. In particular, A-13 binds tightly to mono- and oligonucleotides, and it can discriminate by its optical response between a pyridimic and a purinic sequence. 

As regards other coordination compounds of silver, electrochemical synthesis of metallic (e.g. Ag and Cu) complexes of bidentate thiolates containing nitrogen as an additional donor atom has been described by Garcia-Vasquez etal. [390]. Also Marquez and Anacona [391] have prepared and electrochemically studied sil-ver(I) complex of heptaaza quinquedentate macrocyclic ligand. It has been shown that the reversible one-electron oxidation wave at -1-0.75 V (versus Ag AgBF4) corresponds to the formation of a ligand-radical cation. Other applications of coordination silver compounds in electrochemistry include, for example, a reference electrode for aprotic media based on Ag(I) complex with cryptand 222, proposed by Lewandowski etal. [392]. Potential of this electrode was less sensitive to the impurities and the solvent than the conventional Ag/Ag+ electrode. 

The 1 1 1 complex [54] is obtained by reaction of the cryptand 15 with 2,3-naphtha-lenediol in the presence of water. The water molecule is bound by bifurcated hydrogen bonds to four oxygen atoms of the cryptand, one binds symmetrically (HO = 2.33 and 2,56 A) while the other bind assymmetrically (2.03 and 2.56 A). The 2,3-naph-thalenediol forms one intramolecular hydrogen bond while the second diol is coordinated to the water molecule (H- O = 1.79 A). 

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