Cryptands nitrogen

For the purposes of nomenclature, the simple cryptands are assumed to be macro-bicyclic and nitrogen is assumed to be the bridgehead atom. The different cryptands are designated by assigning numbers according to the number of heteroatoms in each ethylenoxy chain. The three cryptands shown below are designated [ 1.1.1 ]-cryptand (9), [2.2.1 ]-cryptand (10) and [3.2.2]-cryptand (11), respectively. 

An even more complicated nomenclature problem arises with the closely related all-oxygen cryptands. These compounds do not utilize nitrogen as the three-chain junction. Most examples of this class of compounds have utilized pentaerythritol or glycerol as the junction. This naturally imparts a somewhat lower flexibility to the molecule than would be present in the nitrogen-containing cases. Structures of two such molecules are illustrated below. 

Although the principal application for 9 has been in the synthesis of cryptands (see Chap. 8), this material has also served as precursor to a number of nitrogen based lariat ethers , sometimes referred to as crown complexanes . Binding constants for such compounds have been measured for a few examples in a few cases , but... 

Although a large number of such compounds have been prepared, many of the mixed nitrogen-sulfur macrocycles are included in other tables, most notably in Chap. 4. Some have also been recorded in Chap. 8 as monocyclic precursors to bi- or polycyclic ligands (i.e., cryptands). The reader is directed to the tables of other chapters where the desired compound, if known, may be reported. 

Synthesis of cryptands which contain nitrogen atoms in the bridges has also attracted Lehn s attention. In general, a similar synthetic strategy was utilized, but ni-... 

An interesting alternative approach to the synthesis of a cryptand having nitrogen atoms in the bridges was presented by Newkome and coworkers. This group condensed triethanolamine with 2,6-dichloropyridine in a relatively straightforward but low yield (5%) nucleophilic aromatic substitution to form 7, illustrated below in Eq. (8.6). The identity of the compound was confirmed by X-ray structural analysis. 

The similarity between the cryptands and the first of these molecules is obvious. Compound 7 7 is a urethane equivalent of [2.2.2]-cryptand. The synthesis of 7 7 was accomplished using a diacyl halide and l,10-diaza-18-crown-6 (shown in Eq. 8.13). Since amidic nitrogen inverts less rapidly than a tertiary amine nitrogen, Vogtle and his coworkers who prepared 7 7, analyzed the proton and carbon magnetic resonance spectra to discern differences in conformational preferences. Compound 7 7 was found to form a lithium perchlorate complex. 

The cryptands (490) coordinate by three sulfur and three nitrogen donors only, forming distorted octahedral Ni11 complexes.1336... 

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 equilibrium constants, obtained by NMR spectroscopic methods, showed a strong dependence on both the macrocycles and the diorganozinc compounds. Polydentate nitrogen donor ligands, particularly triazacyclononanes, form much stronger complexes than cryptands and crown ethers. 

Cryptands of this type are able to exist in three isomeric forms since each of the bridgehead nitrogens may be orientated inwards or outwards with respect to the molecular cavity - the three isomers are thus in-in , in-out , and out-out . In the solid state, 2.2.2 has been shown to have an in-in arrangement (Metz, Moras Weiss, 1976). 

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). 

The cation affinity of aza-crown ethers depends on the type of substituent attached to the nitrogen. Wester and Vogtle (1978) have determined the cation binding constants for substituted [2.2]-cryptands 138] and [89]—191 ] in... 

PET-5, PET-6 and PET-7 are examples of macrobicydic structures (cryptands) (Figure 10.12). The cavity of PET-6 and PET-7 fits well the size of K+. PET-6 has been successfully used for monitoring levels of potassium in blood and across biological membranes, but pH must be controlled because of pH sensitivity of this compound via protonation of the nitrogen atoms. This difficulty has been elegantly overcome in benzannelated cryptand PET-7, in which the aromatic nitrogens have lower pKa than those of aliphatic amines. 

F. Pages, J.-P. Desvergne, and H. Bouas-Laurent, Nonlinear triple exciplexes Thermodynamic and kinetic aspects of the intramolecular exciplex formation between anthracene and the two anchored nitrogens of an anthraceno-cryptand, /. Am, Chem. Soc. Ill, 96-102(1989). 

---