Structures halides with cryptands

In both the chloride and bromide structures, the halide was also found to be encapsulated, but was positioned more centrally within the cavity, as explained by a better size match between the ligand and halide (Figure 7B) [48], In both cases the cryptand is hexaprotonated and all six of the protonated amines exhibit hydrogen bonding with the internal halide, with average NH—X distances of 3.30 A for chloride and 3.39 A for bromide. The distance between the bridgehead amines is 7.40 A and 7.50 A for the chloride and bromide complexes, respectively. 

The X-ray crystal structures of the F", Cl , and Br" cryptates of 19-6H demonstrate the inclusion of one of the halide anions in an unsymmetrical fashion. In the case of the small fluoride ion complex a tetrahedral coordination environment is observed for the guest anion with a mean N(H) - P hydrogen-bonding distance of 2.72(8) A. The CP and Br" cryptates exhibit octahedrally coordinated halide ions situated more centrally within the host framework with N(H) - X" distances in the ranges 3.19-3.39 A (X = CP) and 3.33-3.47 A (X = Br ). It is noteworthy that the hydrogen-bonded distances for the anion within the cryptand host are longer by up to ca 0.15 A than those for the other anions in the lattice, suggesting a particularly... 

Figure 3.10. Estimation of difference in affinity (AAG) of the two anions Cl and Br for the cryptand SC24 [(a) structural formula (b) schematic of complex formed with halide ion] as the parameters for Cl are slowly mutated into those for Brin water (- - -) as well as in the complex —). Used with permission (138).

Successful anion complexation was also achieved with larger macrobicyclic ligands. Thus, the macrobicycle 17 (Fig. 17). SI binds halide anions and the linear triatomic azide anion with a very high stability constant. The x-ray structure revealed an efficient shape and size complementarity between N3 and the cavity. A whole series of polyazacryptands was obtained in high yields by a simple Schiff base [2 + 3] condensation between the triaminotriethylamine (tren) and various dicarboxalde-hydes. The hexaimine macrobicycles prepared by this way lead, after reduction, to the polyaza cryptands, which in... 

A number of other metal-based redox-active centers have been incorporated into supramolecular receptors, representative examples of which are displayed in Fig. 5 (Compounds 24-28). Many of these receptors electro-chemically respond to cations. but species that respond to anions and neutral molecules are also known. A number of the cation binders are organometallic crown ether and metallocrown or metallothiacrown derivatives, for example. Compound 24. Flow-ever, in many cases, the redox processes are not particularly reversible, and relatively small anodic shifts in the metal-centered redox couples are observed. A series of self-assembled [12]metallocrown-3 complexes, two of which are 25 and 26, were found by Severin to bind halide salts of small Group 1 metals strongly in organic solvents, with affinities similar to those of the cryptands. X-ray crystal structures revealed that the metal cation was... 

Fig. 4 Complex formation of podands (a)-(c)log Kj of the Na and complexes of (a) pentaglyme, (b) 18-crovvn-6. and (c) [2.2.2] cryptand demonstrating the macrocyclic and cryptate effects (d)-(f) Rb" coordination spheres in the solid-state. structures of the Rbl complexes of bisquinolino podands of different chain length (cf Fig. 2j) (g)-(i) podand anion complexation (g) podand HPO4" complex, and podands for (h) halide (Cl , Br ) and (i) carboxylate (benzene-1.3.5-tricarboxylate) complexation and (j,k) complexes of podands with uncharged molecules involving (j) thiourea and (k) adenine.

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