Azacryptand

Smaller macrobicycles and macrotricycles bearing less that six N-donors are also known. These ligands comprise tertiary or secondary amines and are derived from insertion of one or more straps between pairs of potentially bridgehead N-donors contained within a small ring macrocycle, and in this sense they may be considered analogs of the well-known azacryptands. The inability to offer six N-donors, and the presence of tertiary amines in these ligands leads to the preference for divalent over trivalent Co. 

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... 

Axle and collar molecular structure, 27 60-61 Axokine, 3 97 Azaboranes, 4 170, 204 Azacarbocyanine dyes, 9 257 Aza[18]crown-6, 24 41 Aza[18]crown-6 appended a-helical barrels / rods, 24 58 Azacrown ethers, 24 44 Azacrowns, 24 41 Azacryptands, 24 42 Azadirachtin, 24 474 Azafullerenes, 22 231, 232, 243 Azahomofullerenes, 22 243 Azaindene compounds, 29 197 Azalactone method (for Emphaze supports), for covalent ligand immobilization, 6 396t Azamethiphos (salmosan)... 

Anion binding by katapinand hosts such as diazabicyclo [9.9.9] nonacosane is much weaker than in the related azacryptand bis(tren) (4.4). What relevance does the phenomenon of in, out isomerism have on this observation ... 

Figure 2. Azacryptands and amidocryptands of varying size and flexibility.

A drawback to ciyptands with aliphatic spacers such as 3 is that the synthetic routes are long and tedious, requiring time-consuming protection and deprotection techniques in order to accomphsh the condensation reactions with reasonable yields. In the late 1980s however, a couple of groups realized that azacryptands could readily be obtained via Schiff base condensations between aromatic or heterocyclic dialdehydes and tren, followed by borohydride reductions [50,51], This simple pathway opened the door for a number of new anion ligands, examples of which include 5 and 6. 

Distances between the bridgehead amines vary according to the topicity of the receptors. For monotopic complexes, the bridgehead distances are between 6.5 and 7.9 A. In terms of incorporating multiple species, however, the azacryptands appear to be the most flexible, as seen additionally in the dinitrate structure with 5 [30]. When... 

Dussart, Y. Harding, C. Dalgaard, P. McKenzie, C. Kadirvelraj, R. McKee, V. Nelson, J. Cascade chemistry in azacryptand cages bridging carbonates and methylcarbonates, J. Chem. Soc., Dalton Trans. 2002, 1704-1713. 

Hossain, M. A. Morehouse, P. Powell, D. Bowman-James, K. Tritopic (cascade) and ditopic complexes of halides with an azacryptand, Inorg. Chem. in press. 

The pioneering work of Lehn established the fundamental strategy of using preformed three dimensional cage ligands, dubbed cryptands, to ensure enhanced thermodynamic and kinetic stability for cation complexation [1], Positively charged cryptands, typically the N-donor azacryptands, can also serve to encapsulate anions. 

The series of azacryptands used in this study are relatively easily made via 2+3 Schiff-base condensation of the triamine, tren, with the appropriate dialdehyde, followed by borohydride reduction of the imine functions (Scheme 1). Adjustment of pH in the presence of the appropriate anion followed by slow evaporation of solvent yields the anion cryptate, often in a form suitable for X-ray diffraction analysis. 

Chemistry in cages dinucleating azacryptand hosts and their cation and anion cryptates, M. Arthurs, V. McKee, J. Nelson and R. M. Town, J. Chem. Ed., 2001, 78, 1269. 

It is likely that the study of electrides would be more widespread if thermal stability at room temperature could be achieved. The recent success in synthesizing a sodide and potasside that are stable at room temperature provided the incentive to find aza-based complexants that will permit the crystallization of stable electrides. The cation of choice would be Li", because there would be no competition from Li. an alkalide that has never been made. A number of candidates were tried, from methylated cyclens to fully methylated azacryptands to ada-manzane-like complexants. Thus far none produced a crystalline electride of known structure. When the cation is kinetically able to leave the complexant. dissociation without decomposition occurs. The aza complexants used to date apparently do not form strong enough complexes to yield thermodynamically stable electrides. although aikalides can be made in some cases. The search for the ideal aza complexant continues. 

Nelson, J. McKee. V. Morgan. G. Coordination chemistry 20. of azacryptands. Prog. Inorg. Chem. 1998. 47. 167-316. 

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